A System of Natural Philosophy: Designed for the Use of Schools and Academies, on the Basis of ...

Google

This is a digital copy of a book that was preserved for generations on library shelves before it was carefully scanned by Google as part of a project

to make the world's books discoverable online.

It has survived long enough for the copyright to expire and the book to enter the public domain. A public domain book is one that was never subject

to copyright or whose legal copyright term has expired. Whether a book is in the public domain may vary country to country. Public domain books

are our gateways to the past, representing a wealth of history, culture and knowledge that's often difficult to discover.

Marks, notations and other maiginalia present in the original volume will appear in this file - a reminder of this book's long journey from the

publisher to a library and finally to you.

Usage guidelines

Google is proud to partner with libraries to digitize public domain materials and make them widely accessible. Public domain books belong to the
public and we are merely their custodians. Nevertheless, this work is expensive, so in order to keep providing tliis resource, we liave taken steps to
prevent abuse by commercial parties, including placing technical restrictions on automated querying.
We also ask that you:

+ Make non-commercial use of the files We designed Google Book Search for use by individuals, and we request that you use these files for
personal, non-commercial purposes.

+ Refrain fivm automated querying Do not send automated queries of any sort to Google's system: If you are conducting research on machine
translation, optical character recognition or other areas where access to a large amount of text is helpful, please contact us. We encourage the
use of public domain materials for these purposes and may be able to help.

+ Maintain attributionTht GoogXt "watermark" you see on each file is essential for in forming people about this project and helping them find
additional materials through Google Book Search. Please do not remove it.

+ Keep it legal Whatever your use, remember that you are responsible for ensuring that what you are doing is legal. Do not assume that just
because we believe a book is in the public domain for users in the United States, that the work is also in the public domain for users in other
countries. Whether a book is still in copyright varies from country to country, and we can't offer guidance on whether any specific use of
any specific book is allowed. Please do not assume that a book's appearance in Google Book Search means it can be used in any manner
anywhere in the world. Copyright infringement liabili^ can be quite severe.

About Google Book Search

Google's mission is to organize the world's information and to make it universally accessible and useful. Google Book Search helps readers
discover the world's books while helping authors and publishers reach new audiences. You can search through the full text of this book on the web

at |http: //books .google .com/I

HARVARD COLLEGE
LIBRARY

GIFT OF THE

GRADUATE SCHOOL
OF EDUCATION

3 2044 097 016 935

A SYSTEM

OF

NATURAL PHILOSOPHY,

DESIGNED FOR

THE USE OF SCHOOLS AND ACADEMIES,

ON THE BASIS OF

THE BOOK OF SCIENCE BY MR. J. M. MOFFAT.

COMPBISINO

MECHANICS,

HYDROSTATICS,

HYDRAULICS,

PNEUMATICS,

ACOUSTICS,

M

PYRONOMICB,

OPTICS, ,— .

ELECTRICITY, \^ ^ O

GALVANISM, ^ k^ ^

MAGNETISM.

WITH EMENDATIONS, NOTES, QUESTIONS FOB EXAMINATION, LISTS

OF WORKS FOB BEFERENCE, SOME ADDITIONAL

ILLUSTRATIONS, AND AN INDEX.

BY WALTER R. JOHNSON, A.M.

' Profesior of Chemistry in the Pennsylvania Colleife, Philadelphia, and Iat0
Professor of Mechanics and Natural Philosophy in the Franklin
Institute of the State of Pennsylvania, ice. ice.

ILLUSTBATED BY
MORE THAN TWO HUNDRED ENGRAVINGS.

EIGHTH EDITION.

PHILADELPHIA:
PUBLISHED BY p. C. BIDDLE.

1842.

/

a. I V I 5^^*^ H^ r*^'"

HARVARD COLLEGE LIBRARY

GIFT OF THE

GRADUATE SCHOOL OF EDUCATMII

JUN 20 1932

PRINTED BY T. K. & p. e. COLLINS, PBZL4.

CONTENTS.

Pbxtjlcz.

IVTBOSVOTIOir, 9.

MECHANICS.

Preliminary Observations, 17 — ^Mobility, 18 — ^Elasticity, 23 — ^Velo-
city of Motion, 25— Different Kinds of Motion, 27 — ^Parallelogram of
Forces, 31 — Gravitation, 36 — ^Accelerated Motion, 47 — Curvilinear Mo-
tion, 53 — Oscillation of the Pendalum, 59 — Centre of Gravity, 65—
Mechanic Powers, 72 — ^Lever, 74 — Wheel and Axle, 83— Machine of
Obliqae Action, 87— Pulley, 89— Inclined Plane, 91— Wedge, 93—
Screw, 94 — ^^Compound Machinery, 98~-Friction, 108 — Rigidity of Cor-
dage, 111 — Strength of Materials, 112-^Moving Powers, 113— Woika
on Mechanics, 121.

HYDROSTATICS.

Preliminary Remarks, 122 — ^Properties of Liquids, 123 — ^Hydrostatic
Pressure, 129 — Specific Gravity, 147 — Capillary Attraction, 162—
Works on Hydrostatics, 165— -Htdbaulics, 166— Hydraulic Machines,
172— Works on Hydraulics, 1 79.

PNEUMATICS.

General Observations, 179 — Common Properties of aerial Fluids, 181
—Different Kinds of Aurs, 183— Elasticity of Air, 185— Weight of
Air, 196 — Pneumatic Instruments and Experiments, 199 — ^Ashosta-
Tics, 218— Air-balloon, 221— Parachute, 223— Paper-kite, 226— Diving-
bell, 227— Torpedo, 230— Works on Pneumatics, 230.

ACOUSTICS

Cause of Sound, 231 — Sonorous Vibrations, 233 — Velocity of Sound,
238 — ^Musical Sounds, 242 — ^Musical Scale, 246 — ^Musical Instruments,
254— Human Voice, 262— Reflection of Sound, 266— Echoes, 267—
Ventriloquism, 273 — Works on Acoustics, 276.

CONTENTS,

PYR0N0MIC8.

Causes of Heat and Cold, 277— Sources of Heat, 280— Effects of
Heat, 288 — ^Instruments for measuring Temperature, 292->Latent Heat»
303— Ebullition, 309— Steam-engine, 313 — Propagation of Heat, 320
-Caloric Engine, 323— Works on Heat, 332.

OPTICS. •

^ Observations on Light and Tision, 333 — Catoptbics, 347 — "EfEsttB
of Plain Mirrors, 348 — Atmospheric Reflection, 351 — Convex Minon,
354 — Concave Mirrors, 358— Dioftrigs, 362 — Optical LenwM^ 8fl8 —
Organs of Vision, 373 — Theory of Vision, 375 — Ch&oxatigs, 384—
Chromatic Refraction, 387^-Rainbow, 895— Varieties of optical Instni-
ments, 407— Spectacles, 407 — Microeoopes, 411 — ^Telesoopes, 412—
Double Refraction and Polariaation of Light, 413^ WorkflO&0|ltio% 417

ELECTRicrry.

Miscelkmeous Observations, 418— Theoiy of Eleetridty, 421— 'Con-
ductors and Non«conductoT8, 425— ^Induction, 427 — ^Elecbical Instru-
ments and Experiments, 429*— Ekctro-galvanism — ^Electro-magnetism,
451 — ^Teenestrial Magnetism, 459 — VariaUon of the Compass, 461 —
Works on Electricity, 462— Index, 463.

PREFACE.

.1

The extensive adoption in the United States of that
Bjrstem of education which regards useful knowledge
as indispensable to a uatful life, necessarily calls lor
the preparation of treatises adapted to the requuitions
of those who are to impart as well as to the wants of
those who are to receive such knowledge. Not only are
elementary works demanded, but they must occasionally
be remodelled with a view to the advancement of the
"^ sciences to which they relate. To adhere pertinaciously
v^ to the text books of the last century would be to d[o
^^ eqoal injustice to the state of education and to the pro**
gross of human knowledge. To suppose that the
r Z labours of the learned have, within the last quarter of
a century, resulted in no modifications of elementary
I laws, as enunciated at the beginning of that period,
would be to contradict the plainest evidence of daily
O observation. The physical sciences have,^ithin this
"f^ period, enjoyed a most vigorous growth, r^ot content
t^p with tiie hare discovery of a law and the enunciation
of it as among the maxims hereafter to be received as
^ truthj the views of men of science have become habi-
^ — toally directed to the harmonizing of the known and
received laws, into more general and comprehensive
expr!»»ions of nature's vast designs. By ;this course of
proceeding, the mind is not only enabled to embrace
wider ranges of thought, and more rational speculative
views in each department or branch of science, but in
several instances to include under one department the
facts sni phenomena previously regarded as constitut-
ing several distinct sciences. Thus while the inductive
labours of Galvani and Volta laid the foundation, as
tAey (»id as all the world) supposed, for a new science^
tlie 2«cfiat researches of philosophers have wellnigh

i^^ 5

6 PBEFACE.

resolved not only that, but two or three other sciences
into one general principle of action, variously mani*
fested according to the circumstances of each particular
case. While this process of enlarging and clearing the
views of men in regard to general truths in science has
been advancing, there has been a constant and zealous
vigilance displayed in reference to the practical applica-
tions of kno\yledge to the great physical and social
purposes of man. These purposes can be fully sub-*
served, only by keeping the public mind apprized of
all the important steps taken in the advancement of
those sciences which are susceptible of a practical
application. Natural Philosophy, Chemistry, and seve-
ral branches of Natural History, are, of all departments
of human knowledge, those which have most engaged
the attention of modern philosophers ; and it is with a
view to the present state and the useful employment of
these sciences, that the publishers of the Scientific Class
Book have sought to furnish the schools and academies,
no less than the private students of the tJnited States,
with an appropriate and eligible manual.

In selecting for the basis of such a manual the text
of Mr. Moifatt, it was with the full understanding
that in't)rd«r to be adapted to the purposes of instruc-
tion here it must be somewhat varied from its English
dress.^ Some inaccuracies in the statement of facts and
principles were easily perceived ; some grave errors
in regard to persons, places, and things, especially in
this country, were at once discovered, a number of im-
portant discoveries and inventions due to tUe citizens
of the United States were wholly overlooked ; several
long notes in I^atin not likely to interest the young
reader had b^en introduced ; the allusion^s to local
objects and occurrences which pervaded the work
seemed to require more oral explanation than the
student in any other place than London was likely to
receive ; the puerile cuts which headed the chapters
in the English editions, and which have conveyed to
many persons the idea that the compilation was intended for
very young children^ were conceived to be less useful

PREFACE. 7

than some additional figures illustrating the topics
treated in the work. In the above-mentioned particu-
lars, the work was thought to require emendation.

But as the several treatises appeared in the main to
have been compiled with a view to the best authorities,
as well as in a style sufficiently simple and perspicuous
to warrant an attempt to adapt it to the purposes here
intended, the publishers were induced to believe that
they could not offer to teachers and students a more
acceptable addition to the means of instruction than the
work now presented for their consideration. '

The general practice of introducing into class-books
questions for examination is so well established as to
need little comment Yet not every kind of questions
can render a text-book more valuable than it would be
without them. It has been the aim of the editor so to
execute this part of his duty, as to lead the student into
habits of reflection on the true nature and bearings of
the subject before him, not to confine his attention while
answering the queries merely to certain words of the
text ; — to excite the industry of the pupil, rather than
increase the labour of ^ the teacher ; — to enable the stu-
dent to rise from his task with clearer conceptions of
things, than he enjoyed before attempting to answer
these questions, not to deceive either himself, his
teacher, or others, by a show of knowledge which he
does not possess. In some cases hints and suggestions
are conveyed by the same means, and the application
of certain terms not contained in the body of the work,
will be easily understood from the manner in which
they are introduced into the questions. The answers
will occasionally be inferences and deductions, gene-
rally easy to be made, from the facts and principles
contained in the text In these cases the mind will, it
is conceived, find a more pleasing and profitable exer-
cise than in the mere repetition of statements found on
the page to which these questions refer.

At the end of every important division of the subject
is presented a list of such works as may be found
useful to those who desire to prosecute particular

9 PRSFACE«

inquiries relafins to the part immediately preceding.
A few works in foreign languages are included in the
number, and as the French language in particular is
now so extensively read at least among teachers, it is
believed that references to them will by no means
prove useless. The index will, it is hoped,, prove
entirely adequate to the purpose which it is designed
to subserve.

In conclusion it may be observed that, whateve^,
merit may be claimed for other treatises on the same
departments of science, it is confidently anticipated that
this will be found to embrace as full and satisfactory a
view of the subject which it proposes to treat, as any
similar compilation, which has hitherto been dedicated
to the service of American youth. In this hope the
publishers respectfully submit it to the consideration
of the reader.

t0* The publisher has deemed it advisable to adopt a new title
for this book, setting forth more clearly the particular branches
of science of which it treats, and this change is rendered expedient
by the further consideration that there is another work of a totally
different character published in this country, called the Scientific*
Class Book.

INTEODUCTION.

1. AvoNO the flerenl diatinetioxiB which hare been made ia
iramaaJcnowiedge, thoee of most importuiee to be noticed in the
.oommencement of the psese«it woiek, regard the discriminatioa
between the mathematical and the physical sciences. The latter
axe so far dependent on the fbnner* that seme knowledge of map
<thematac8 10 afasolntely necessary, preriously to entering to any
ezteDt on the at»d.jr of physical or natural philosQphy. A geii»*
ml acqwuaatanee with at least the dejBiefitary branches of nwthfr-
anBtiea, .as arithmetic, geomefery, asid trigonometry, may be ev
. pected to have been aequired by all tolerably well edncated pe»-
•Bons, as usually forming a part of a common school education.
Physical science, or .natural philosophy, constitutes the exdusiTe
object of the present yolame, in mer to the perusal of which,
with profit and advantage, it will be requisite that the reader
shoula not be ignorant of the names and ^nwal properties of
gBometrical lines and figures. The foUowmg diagram and eik-
-planatoiry obserTaHom are therefore introduc^, as they may be
useful to thasse who aje but slightly acquainted with mathema-
tics, and invf sometimes save the better informed student the
trouble of retenri^g to other books for information, respectiDg the
signifiearion of particular terms.

A a Ciirle.

A /I its Piajneter.

B the Badiaa.

C a Chord.
. D an Arc.

EaTaogieat

Jra Dccaiit.
a Ga'Cootangent.

H the Siae of Are a ^.

XtheCo-siQe.

KtheTersed Sine.

L the Sine of jBoioplemcDtal Are b ۥ

2. Erery circle is supposed to be divided into 360 degrees,
and the line which bounds the circle, and on which therefore
these degrees may be marked, is called its circumference. Any
lines equidistant from each other throughout their whole extent

Describe the sereral parts of the diag^ram represented in the mxepi^
Into how maof degrees is a. circle supposed to be divided ?
What are paraUellines ?

9

10 INTRODUCTION.

are termed parallel lines. Two lines not parallel but in the same
plane must, if sufficiently produced, meet in a point, which is
called an angle. Angles are principally distinguished by the re-
lative inclinations of the lines by which they are formed. When
one line meets or crosses another perpendicularly, the angles they
form are called right angles ; and any angle smaller than a right
angle, is styled an acute anffle, and any greater an obtuse angle.
But angles are more prccis^y measured by reference to the num-
ber of degrees contained in an arc of a circle joining the two
lines by which an angle is formed. Thus a right angle must be
included within an arc of a circle equal to a quadrant, or the
fourth part of 360 degrees, namely, 90 degrees ; and an acute
angle included within an arc only half the extent of a quadrant
will, of course, be an angle of 45 degrees.

3. A space or flat surface, inclosed by three lines, is the most
simple of all definite figures, and is called a triangle. Among
the varieties of these figures are the rectangular* triangle, so
named because it has one right angle; the ecjuilateral triangle,
which has three sides of equal extent; the isosceles triangle,
which has only two equal sides ; and the scalene triangle, all the
sides of which are of different lengths. Any space inclosed by
four lines is called a quadrilateral, or four-sided fi^re. Among
such are included the square, having four equal sides and right
angles ; the rectangle, or oblong square, having only the opposite
sides equal ; the lozenge, which has equal sides and unequal an-
gles ; and the trapezium, which has only two of its ^des parallel.
When the sides of a quadrilateral figure are parallel, it is termed
a parallelogram. A line joining two opposite or alternate angles,
is called a diagonal. Any figure having several angles, and con-
sequently several sides, is named a polygon.

4. Solid figures include the tetraedron, or four-sided solid,
which is the most simple figure of the kind, as no solid can have
less than four sides; and when the number of sides is greater^ the
figure is called either a hexaedron, an octaedron,an icosaedron, era
polyedron, according to "the number of its sides.

5. Among polyedrons may be distinguished the prism, formed

What constitutes an angle f

By what names are different angles distinguished ? ,
How are they accurately roeasured ?

How many degrees of a circle are <;ontained in a right angle ?
What is tiie most simple definite %gure ?

What is a rectangular triangle ? an equilateral triangle ? an isoscelei
trianele ? a scalene triangle ?
What is a four-sided figure called ?

Describe some of the figures that come under this denomiuatioiu
What is a diagonal ?
What is a polygon ?

What is the most simple of solid figures ?
How many sides has a hexaeon ?
How many an octagon ? aa icosaedron and a polyedroa f
Of what form is a prism f

INTRODUCTION. 1 1

of.paTallelograms only, or of parallelograms and two polygons of
any number of sides. Among* the pnsms may be specified the
parallelopiped, formed of six parallelograms only; ana amon^rthe

?arallelopipeds may be noticed the cube, having six square sides.
'he pyramid is a polyedron, formed by a polygon of any kind as
its base, and as many triangular planes as the polygon has sides :
the point where all the triangular planes unite is called the sum-
mit of the pyramid. The most smiple solid of this kind is the
tetraedron, or four-sided pyramid, including tiie base.

6. The terms sphere, cylinder, and cone, designate solid figures,
having either entirely or partially curved surfaces; and the ex*
pressions spheriod, cylindroid, and conoid, are used to denote
solid figures, more or less resembling a sphere, a cylinder, or a
cone, respectively.

Natural Philosophy is the science which explains the causes of
the various properties of bodies in general, as shown by the
changes which tiiey undergo in any particular circumstances, or
the changes which lliey may occasion in other bodies, under cer-
tain circumstances. 'Fhe province of natural philosophy does not
extend to the explanation of the doctrine of final causes, or tiie
immediate and positive reasons why particular effects take place,
or why certain bodies possess the peculiar properties with which
they are endowed ; but it enables us to appreciate the conse-
quences of any body being placed in a given situation, or to fore-
tel what will be result of any body acting on another in a certain
manner.

. 7. Thus, we. know nothing of the absolute cause of gravity or
weight, which is that property of bodies in consequence of which
they fall towards the surface of the earth, if raised in the air by
any force and then dropped; but natural philosophy, while it
leaves us in ignorance olthe final cause of gravity, enables us to
determine a vast variety of curious circumstances with respect to
falling bodies. Thus, it is found that a heavy body, as for in-
stance a marble or a musket-ball, dropped from a high tower,
would fall faster as it approached near to the ground than it
would in passing through the former part of its descent; and the
rate at which a body falls through a given space has been ascer-

How is the parallelopiped formed ?

How many sides has a cube ?
- How is a pyramid formed ?

Where is the summit of a pyramid ?

What is the most simple solid of this kind ?

What kind of figures are designated by the terms sphere, cylinder,
and cone ?

How are those figures denominated that more or less resemble these
figures ?

What is Natural Philosophy }

What are some of the doctrines beyond the explanation of Natural
Philosophy.

What is there in the subject of gradty which is inexplicable by na-
tural philosophy, and what does this science enable ns to determine res-
pecting it ?

13 xnrsoBvcTSOir.'

tnnod by ezperimeiit, vod can be ealeidftted wi& Ifae^ utmost ex*
aetnesB. So- as to the final cause of electric and magnetic attnc-
tidn varions opinions hsre been advanced, and it is still inTohrei
in obscnritj; out we know by experience tbat a magnet attracts
kon with considerable force^ and Inat a thin bar of ma^pietic iron,
accurately poised on its centre, will, when left free, pouat towards'
the north with one end, and towards the south with the other ;
and on the latter property depends the action of the mariner's
compass, by means of which the sailor, crossing the pathless sea,
b able to ascertain in what direction his vessel is steering; and
to this little instrument, which was unknown to the ancients, W9
are in a great degree indebted for the important discoveries of
modem navigators.

8. Whether light and heat are owing to matter or motion has
been left amon^ the questioas which philosophy has lotherto
been unable satisfactorily to decide; but the effect of light on bo-
dies, whether opacpie, transpai^^t, or semi-transparent, the velo-
ct^ with which it passes through space, and the manner in which
it IS modified by optical glasses of various forms, are among the
iramerous interesting and GUrpriscAg properties of light, vniich
natural philosophy has laid open to our investigation, and which
we are enabled to verify and illustrate by means of mathematical
calculation; and the phenomena of heat and cdd, with which wo
are so intimately familiar, from the seimaticms they occasion, are
equally hidden as to theii final cause, and equally wonderful and
curious as to their efiects, the latter of which alone afibrd an as»*
pie field for the experiments and deduetfcms of the philosophical
inquirer.

9. Astronomy presents a boundless field for research, and not-
withstanding it has been explored with Mgnal suecess in modem
times, yet the most importent discoveries that have beon made
only serve most distinctly to evince that the wisest and m»st suc-
cessful investigators of the phenomena of the science have merely
entered on the confines of^ knowledge, and enabled us to form
some imperfect estimate of those boundless regions which display
an inexhaustible field for future speculation nid inquiry. It has
indeed been ascertained that the sun and the planetsury and other
bodies which constitute the solar system, are infiuenced by the
same moving power as that which causes the fall of an acom to
the ground, when detached from the oak on which it was pro-

What has experienee taught respecting iiMgiieti» and dectrie attrac-
tion }

What question respecting^ the nature of light and heat has beea li-'
tberto undecided by philosophy ?

What are some of the properties of light which natural philosophy
has laid open to our investigation, and how arethese to be verified ?

I» regard to heat, what points are fcnovD and what unknown f

What conclusions have been drawn from the most successfsl investi-
gations in astronomical science f

What has been ascertained to be the mof iag power that inflaeneei thf
Aodies of the solar system ?

IlfTaOi^UCTIOKi. II

daced; and that'the attcaotire force which retains the moon In her
oibit, and causes her leactioa on the fluid parts of the terrestria]
globe we inhabit, producing the tides, may be estimated with ae-
curacy, and subjected to mathonatical calculation. But thei9
are numberless topics of inquiry — ^with regard to the eonstitution
of the sun, the nature of comets, and the caftses of their peculiar
motions, the kind of medium which occupies the space beymid
the atmospheres iji the earth and planets, and the ralattoos that
may exist between ov solar system and the numberless other
systems, the existence of which maybe inferred firom the appear-
ance of the starry heavens— which may for an indefinite period
serre to exercise the talents of men of genius and learning, but
concerning which we can hardly hope to attain an^ Jcnowledge
approaching to certainty, till discoveries and inventions in other
sciences provide us with means for investigating the works
of nature, as mueh superior to those which we at present possess,
ia our instruments of research surpass those employed by the aiw
cients.

10. << The proper business of philosophical inquiry,'* says Les-
lie, ^* is to study carefully the appearances that successively
emerge, and trace their mutual relations. All our knowledge ot
external objects being derived through the medium of the senkas,
there are only two ways of investigating physical facts— <by o^
urvatton or experiment. Observation is confined to Uie close inves-
tigation and attentive examination of the phenomena which arise
in the course of nature; but experiment coasists in a sort of ar*
tificial selection and combination of circumstances, for the purpess
of searching minutely after the difierent results.

11. **The range of observation is limited by the position of the
spectator, who can seldom expect to follow nature through her wind-
ing and intricate paths. Those observations sreof the most value
which include the relations of time and space, and derive greater
nicety from their comprising a multiplied recurrence of the same
events. Hence Astronomy nas attained a much higher degree of
perfection than the other physical sciences.

12. '* Experiment is a more efficient mean than observatioa for
exploring the secrets of nature. It requires no constant fatigue of
watching, but comes in a great measure under the control of the
inquirer, who may often at will either hasten or delay the expected
event. Though the peculiar boast of modem times, yet the method

What is the effect of this force upon the moon, and indirectlj upon
the earth ^ How are these effects to he estimated }

What subjects of inquiry in astronomical science, are sappoted to lie
beyond our present means of investigation ?

What does Leslie alBrm to be the proper business of philosophical in-
quiry?

What are the only two methods of inveitigatiog pbyneal laeti^ and
what is the province of each }

What circumstance limits the range of observation ?

What obserraUons are of most raiae ?

Which is the more- efficient means of exploring the secrets of natnce f

B

14 ^STTRODtfCnOfK.

of proceeding by experiment was not "wholly unknown to tlie am
eients, who seem to have concealed their notions of itnnder the veil
of allegory. Proiew signified the mutable and changing forms of
material objects ; and me inqmaitive philosopher was counselled
by the poets to watch that slippery d«mon when slnmbering on
Ihe shore, to bind him, and compel the reluctant captive to reveal
his secrets.* This gives a lively picture of the cautious and intre*
pid advances of the skilfnl experimenter. He tries to confine
the working of natoTe — ^he endeavours to# distinguish the save*
ral principles of action—- he seeks to concentrate the predominant
agent— and labours to exclude as much as possible every disturb**
hig influence. By all these united precautions, a conclusion is
obtained nearly unmixed, and not confused, as in the ordinary
^rain of' circumstances, by a variety of intermingled effects* The
Operation of each distinct cause is hence severcdly developed." t

13. The object of Natural Philosophy may be stated to be the
study ef the general properties of unorganized bodies, or inert
substances in the state of solids, liquids, airs, or gases, and those
Which have been termed ineoerdhle or ethereal fluids. It is also
Within the province of the physical sciences to examine the me*
ohanical action which bodies, in their different states, may exer-
cise' on each other, and the different circumstances connected with
^eir movements.

14. The various effects of the motions and operations of bo-
dies depending on their general' pro|>ertie9 have hence been made
the foundation of several distinct sciences or branches of know*
ledge, which have been usually classed with reference to the so*
veral forms of matter called sdiids, liquids, and airs, or to oettain
kinds of phenomena^ supposed to depend respectively on the pre-
sence and action of some imponderable modification of matter or
ethereal fluid, to which have been referred thermometrical, optical,
electrical, and magnetic phenomena. Hence a treatise on Natural
I%ilosophy may be conveniently arranged under the different de«
partments of (1.) Mkchanics, or the doctrine of equilibrium and
motion as respects solids, including Statics and Phoronomics or
Dynamics; (a.) Hvdrostatics, including Hydrodynamics or Hy-
draulics, relating to the equilibrium and motion of lic^^uids ; (3.)
Fnevmatics, including Aerostatics, and Aerodynsmics, or the
effect of forces on air and other gaseous fluids ; (4.) Acoustics, or
the theory of sound, comprehending observations on musical and
vocal sounds; (5.) Pfronomics, or the investigation of the causes
and effects of heat, or more generally of change of temperature \

What 18 the object of Natural Fhilosopliy ?
How have, its divisions been formed ?

What are the different departments wider which a treatise on Natural
I%iloSophy may be properljr ftrranged ?

Of what does each or these departments treat }

* V. Virgil. Georgic. lib. iv.

f ^ tntrodnetion to Elements ef Natural Philosophy.

(6.) PaoTOHOMios or Opncsy iodadiBsr the dieoiT <^ light and
Vision; (7.) £i«BGTRO-MAGNSTisif, which treata of the caueee of
ekctrie and magnetic attraction and repulsion.

15. The idea of absolute or indefinite epaee is obtained by ab-
^Btraction, or conceiving in imagination the abseaoe of all bodieSy
or of all the properties of matter. Every part of tjiis apace* or
nther of this imaginary void or Tacunm, which -ean be conceiTed
to be included in any way between limits, is called relative epaee,
The term bodif is used to designate limited extension, to which are
attached any of the properties of matter. That which distinguishea
in general a simply extended body ficom a void space or vacuum, is
jhe property of impenetrability, oat is, the quality inconsequence
of which a body occupies a certain ^ace, and exdades from it all
other bodies.

1 6. We acquire a knowled^ of the properties oi matter through
our senses, either by immediate observation, or by experimental
inquiiT with the aid of instruments. The senses of sight and fe^
lag auord us abundance of information concerning the properties
of bodies around us, but our knowledge may be vastly extended
.when we assist the former by means of optical glasses, which open
new worlds to our view, or when by means cf delicate instruments
we measure degrees of temperature, electricity, or raagoetic power.

17. Solid b^ies are those which, like stone or wood, present a
•ensible resistance when touched, pressed, or handled. They
may he cut into various forms, and preserve without difficulty im
figures which are given to them, or which they possess natarally.
Sand, powders, and similar substances consist of smali particlet
not umted together; yet though, coUeetively, masses of sand present
but little lesifitance to pressure, the indiv^ual minute particles
have all the characteristies of solid matter, and though readily
dispersed by force, they may be assembled in heaps more or less
tBoiisiderable.

18. Liquid substances are those which, like wat«r, manifest
immediately to the touch but a very feeble resistance, but quite
aufficient to indicate their presence, even when in a state of repose.
They cannot be grasped b^ween the fingers like solid bodies, nor
can they be collected in heaps, or made to take any partieidar
figure, except that of the vessel in which they may be incladed.

19. Aerii&rm fluids are in general invisible bodi^ nHiiek like
the air surrounding us cannot be felt, and afford mo evidence of their
presence to the a&oBe of touch when in repose. But their existence
IS ascertained with abundant certainty when th^ are in motion:
thus no one can doubt the materiality of atmospheric .air after

Ho V do ypn iobtsih ah adch sf MiJMlMie or tfi(^Wto «p«oe f

What is relative space f

Wbit M namu by the I6mn bodt^f

What is the property, of itnpenetnibility ^
> Bv what meMM.do we aoqaine a JuMarledge ef the properties tt mat«
ter ? '

What are fob'tf bodies ? liquid sabstanees ? aeriform fluids f

16 iNTRomrcnoK.

f'xpetieneing the yiolent . exertion necessary in walkip? against a
high wind. AiBrifbrm bodies maybe confined in yesseis, whence
they exclude liquids or other bodies, demonstrating their impe-
netrability, though thej readily become compressible to a great
extent, but there are limits beyond which it is impossible to
reduce them.

90. Ineoeroible or imponderable fluids do not manifest their
existence by the exhibition of impenetrability or weight, which
hare usually been regarded as essential properties of matter; and
they must, uerefore, be considered as hypotheticUly admitted, in
order to account for certain phenomena, which appear to depend
on the presence and action of one or more ethereal media.^-
That light is such an imponderable fluid, emanatinfif from' the sun,
was one of the generally received doctrines of &e Newtonian
Philosophy; ^e caloric or matter of heat of the French chemists
was supposed to be a fluid of a similar nature ; and men of science
who have written concerning magnetism and electricity hare
Tag^ely employed the terms magnetic fluid and electric fluid to
designate the unkbown causes of the phenomena they describe. '

21. At present it is perhaps the more pieyalent opinion of phi-
losophic inauirers that there exists at least one kind of ethereal,
imponderable medium, the different modifications and modes of
action of which give rise to the various phenomena of light, heat,
and electroHnagnetic attraction and repulsion. Thus it may be
"supposed that as sound is conveyed to our ears by the vibrations of
the air, so light affects our eyes through the immensely more
rapid vibrations of the electro-luminous ether. The existence of
such a medium, manifesting neither weight nor impenetrability
capable of being appreciated hy the most delicate instruments, may
be fiiirly inferred mm the movements which take place in bodies
nndor certain circumstances when all the ponderable and coercible
kinds of matter have been carefidly excluded, and these move^
ments therefore must be ascribed to the presence of an ethereal
influence, which can penetrate glass and other dense substances
which are impervious to the rarest gases or most attenuated and
subtile vapours with the existence of which we are acquainted.

33. But such speculations, if not rather curious than useful,
would, if extended, be incompatible with the plan and objects of
the present work. Therefore, tiiough it would have been improper
to have omitted all mention of them, they must be dismissed for
the present, with the preceding short notice ; especially as oppor-
tunities for resuming them will oceur |n some of the ensuing
treatises.

* How is their esiitenee atoertaioed, aod how it their impenctrmbility
demoDBtrated ?

How do the tncoerdble or imponderable flnidi differ from these ?

What hay* hitherto been eoniidered imponderable fluids ?

What is the present more preralcot opinion requesting the impender*
able medium ?

MECHANICS

1. Thzrb is perhaps no department of Natural Philosophy of
finch extensive importance as Mediahics, since its pnncipiee are
founded on those properties of matter which are among the most
obvious and essential,— namely. Mobility and Weight ; and the
effects produced by the operation of these properties are so die*
tinet and certain, ihat they can be subjected lo mathematical oal-
enkition. Hence Dr. Wailis has described Mechanics, wiHi
aome degree of |>roipriQty, as the **> Greometry of Motion/^

3. The designation of t^s braaeh of kiiowledee, like moal
other ecienti^c terms, is derived from a Greek word,* signifying
a Machine ; and Mechanics may be considered as the Phuosophy
of Machinery, or the Theory of Moving Powers. Many writers
have treated of this science under two heads, regarding those
principles which relate to the gravity or weight and to the equi-
libriiun of bodies, or the powers which preserve bodies in the
state of rest, as the subject of tite doctrine of Statics ;f and tha
priiHULples relating to the causes of movemeltt, or the forces pro*
aucing motion, acting by means of solids, as forming the snlgeel
of ^e doctrine of Dynamics.^ But, as the respeottve stales of
bodies at rest, and bodies in motion, may be most correctly cott*
sidered as the consequences of different modes of action of tiia
same causes, they may be instmctively illustrated by ahowing
their relations to eadi other, for which reason it will be proper to
treat of them in conjunction, rather than separately.

3. From this statement of the nature and objects of Mecha*
nics, it will at once appear that we have bj do means overrated
the importance of an acquaintance with this science to the Stn^
dent of Natural Philosophy. For all motions are mora or leas
subject to the laws of Mechanics, and without a knowledge of
those laws, it is impossible to appreciate the effects, or calculate
the consequences, of those motions of the celestial bodies which
occasion the phenomena of Astronomy ; or of those properties of
fluids, whether liquid or gaseous, on which depend tne principles
of Pneumatics, Hydrostatics, and Hydraulics ; or indeed of any
circumstances affecting the ponderable forms of matter. And
those sciences which relate to Heat, Light, Electro^magnetism,

Upon what properties of matter are the priaeiplea of meehaniea,
founded ?

What definition is eiven of mechanics ? •

Under what heads has this science generally been treated ?

Mow extensive is the application of mechanical principles to other de-
partments of science }

t From the Greek verb &:«»•, to i^nd, or be fixed; or from £T«rif»
the act of standing.
% From the Greek word A«vt»/>i$, power or force.

b3 17

18 MECHANICS

Vital Power, either in Animals or Vegetables, or any other phe-
nomena which appear to be independent of the force of sravitap
tion, yet derive most important aid from Mechanics; for it is
chiefly by means of mechanical instruments that the influence of
heat, liffht, electricity, magnetism, or the effects of vitality, as in
the motion of the blood in animals, or of the sap or other fluids
in vegetables, can be estimated. Mechanics may, therefore, be
considered as the basis or groundwork of the other Physical
Sciences, or branches of Natural Philosophy.

4. Previously to entering on the consideration of the Theory
of Mechanical Powers, it will be necessary to show the nature
and effects of Mobili^, or the capacity for motion, and of Weight,
or the gravitation of bodies,— «s these are the general properties
of matter on which, as already stated, the phenomena of Mecha-
nics depend* ,

Mobility. '

5. Every individual body, or portion of matt^, must take up
a certain space. This may be considered as the absolute place
of the body, in reference to its situation simply and singly ; or
as its relative place, or situation with respect to other bodies.
The relative situation of a body may be changed either by its
own motion, or by the motion of the bodies around it. A body
may exhibit the appearance of actual motion, or absolute change
of place, while it remains at rest, its change of place being only
relative. Thus, the Moon, when a train of thin fleecy clouds is
passing over its face, if we attentively fix our eyes on it, seems
to move, and the clouds to stand still, though this is only an ap-
parent motion of the Moon, in a direction contrary to ihzt m
which the clouds are really moving. And if we hold a common
eyeglass, or any transparent substance, a few inches before the
eyes, and move it backwards and forwards, looking through it at
any object, as an inkstand or knife, which remains unmoved, it
wiUf as in the former case, exhibit an apparent motion, arising
from tiie actual movement of the glass.

6. Mobility is the capacity of a body for change of place by
its own motion, it therefore infers the capability of real or actual*
inotion, and not of relative motion only. Yet this change of
place may sometimes be most readily estimated by thevconse^
quent relative motion which accompanies it. Thus, a person
sailing in a boat on a smooth stream, or going swiftly in a coach
along an even road, would hardly perceive the motion of the ve-
hicle except by the change of scene, and trees or buildings on
the hanks of the stream, or by the road-side, would seem to move
in an opposite direction from that of the real motion of the boat
or cams^e. £very tolerably well-informed person now admits

What is meant by the abmthOe plaee of a body ?
What by the relative f
What it momty?

IKOTIOK* ' 19

th&t the earih motres, snd the sun stands still ; bot the motion of
the former is not perceptible, and the apparent daily motion of
the latter, beinff so obvious to onr senses, was, till within the
last three centuries, considered as a real motion, the existence of
which could not even be questioned with impunity.

7. Without some active cause motion can neither eommencv
nor cease ; since a body in the state of rest would always remaiA
unmoved, if never subjected to the influence of a moving force,
and on the contrary, a body when set in motion would* go on t
move for ever, if it met with no opposition to its progress* I
may seem inconsistent witii this doctrine, that any body set it
motion, within the range of onr observation, will continue fo
move without a fresh impnlse for a time, but at length will slacken
its speed, and finally resume the stSite of rest. Thus, a cannon-
ball will pass a certain distance when discharged from the mouth
of a cannon, but if it does not strike a solid body, still it will
ultimately fall to the ground; and a marble or a cricket-ball
thrown forwards with the hand, if it meet no obstacle, will reach
only a certain distance, proportioned to the force used in throw-
ing it.

8. In both these, and all similar cases, the termination of the
motion of the moving body is owing chieHy to two causes. The
iirst of these is gravitation towards the earth's centre, common
to all bodies, and which constantly tends to keep them aV rest,
pressing on the surface of the earth with a degree of force pro-
portioned to their weight and bulk ; or, if, as m the ease of the
cannon-ball, thc^ pass through the air, the force of gravitation
then tends to draw them continually nearer to the earth, till at
length they fall and rest upon it. But the second and more ob-
vious cause of the decay of motion is the resistance of the me-
dium through which the moving body taj^es its course ; and thus,
a body moving through the air, like the cannon-ball, gradually
becomes less and less able to pass forward till its moving force
is destroyed. It will be. readily perceived, that the resistance of
the medium to the body which passes through it, must depend
much on its density or consistence ; thus, a ball driven by a cer-
tain force would pass further through the air than through water,
and further through the latter than through a denser fluid, as
brine or syrup, or through solids, as sand or clay.

9. Another circumstance which will affect the motion of a
body, with relation to the medium through which it trarels, must
be taken into t}ie account, and this is the form of the moving
body. A small body will meet with less resistance than a large
one of the same weight; and a body which presents an extensive

State somje familiar examples, and show how real and apparent motioa
may best be distinguished.

How is a body at rest to be put in motion, and when in motion, how
brotight to rest ?

What other ciroumstanees go to retard or accelerate the motiQa of
bodies ?

20 auecHAKics.

surfeoe to the mediam trough which it mores, will be retarded
in its passage much more than one with a small siur&ce. A
sheet of paper stretched out to its full extent^ and suffered to &11
a few feet, and then folded up into a small compass, and again
suffered to fall from the same height, will afford an exemplxlca*
tion of the resistance of the atmosphere to felling bodies ; and
an illustration of a different kind, but to the same purpose, may
be drawn from the advantage which sharp-edged and pointed in-
struments have over blunt ones in penetrating hard or tough sub*
stances. A body moving in contact with a solid substance, as
when it is rolled or dragged along the ground, is also affected by
friction. This obstacle to motion is proportioned to the rough*
ness or smoothness of the surface ov» which the body passes :
thus, a marble thrown with any given force will run much fur-
ther along an even pavement, than along an equally level gravel
walk ; and still further along smooth ice. Here again the form
of the moving body has much influence on the velocity and ex*
tent of motion ; for the fewer the points of contact between the
surface and that which passes over it, the more freely will motion
take place.*

10. All bodies subject to our control ate exposed to the operas
tion of gravity, in various degrees, and from tiiis cause, mde^
pendent of the resistance of the medium which they traverse, or
of the effect of friction, their motions cannot be indefinitely con-
tinued, but must decline and terminate in a given time, according
to the circumstances in which they are placed. But though per-
petual motion cannot be exhibited by any methods whic^ human
skill or industry can contrive, yet we have continually before us
the display of bodies which have been moving with undiminished
velocity for ages past, and which no power but that which go-
verns all nature can preyent from moving in the same manner for
innumerable ages to come. , The bodies to which we refer, as
will probably be anticipated, are those whose motions are the ob-
jects of the science of Astronomy ; and though that subject will
not come under our immediate discussion, yet the general nature
of the forces which occasion the revolution of the celestial bodies
will be explained, and the causes of their uniform and uninter-
rupted motion will be illustrated.

11. That state of bodies just described, in which motion or
the cessation of motion can take place only in consequence of an
extraneous cause, has been termed Inertia, which signifies inac^

Whftt are some of the examples which illustrate this point ?

What other cause is there, independent of these, which operates upon
all bodies, limiting their motion and precluding the possibility of perpe-
tual motion by human skill f

What h inertia?

* This statement is to be understood as limited by the greater or less
difficulty with which the surface can be abraded.

• MOTIOK* 21

tivity, equally opposed to motion when at rest, and to rest when
in motion ; so tnat if a ^ven force ie required to make a body
moTe with a certain Telocity, the same force will be required to
•destroy its motion. When a garden roller is being drawn along
fn level surface, the exertion necessary to stop it suddenly, at any

f'vsen point, would be precisely the same as would be required
move it backward, if it were at rest, and of course the same
that was applied to set it in motion at first.

13. Any force applied to produce motion may be called Power
or impulse, which may be either continaed, as in the case of
pressure, or intermitting, as in the case of impact or percussion*
Whatever opposes m'otion so as to retard the moving body, destroy
its motion, or drive it in a contrary direction, may be termed Re-
sistance, and its effect, reaction or counteraction. It is one of
the laws of motion that action and reaction are always equal and
contrary. Thus^ in pressing down the empty scale of a balance,
while the other scale held a five pound weight, it is obvious that
the force exerted must be equal to five pounds ; but if one scale
•had been loaded with fifteen pounds, and the other with only ten,
the equilibrium might still be preserved by pressing on the latter
Iwith a force equal to five pounds only. And if a man, sitting in
a boat on a canal, draws towariis him, by means of a r^»e^
tino&er boat of equal weighty liiey will meet at a point half-way
firom the places whence they began to move. Suppo^e^ however,
tiie second boat to be sa laden as to be twice the ^feijgfat^^f the
first, it must move tiie slower of the two, and coiuleqtiently the
point of meeting would be nearer the second boat Knan the first.
If a body in motion strikes another body of equal mas» at rest,
the two bodies vrill move together, but with only hsdf .tUe arigiuai
velocity of the first, the other half having been expended in over-
-coming the inertia of the second body. Corresponding effects
will take place, whatever difference Uiere may be between the
-masses of the two bodies ; for if the second body should be dou-
ble the mass of the first body, the common velocity afler the im-
pact of the two bodies would be one-third that of the first ; and
if the mass of the first body be to that of the second, as 5 to 7,
the common mass after impact vrill be 12 ; and as the second wiiU
deduct from the motion of the first in proportion to its mass, the
motion lost by the first body will be seven-twelfths, and the mo-
tion retained would be five-twelfths.

13. If two bodies are both in motion in the same direction, and
one overtake and impinge on the other, suppose the masses of the
two bodies to be the same, and the velocity of the first to be 7,
and that of the second to be 5, their common velocity after im-
pact will be 6, or half the sum of the two velocities. But if the
•joasses are unequal, the mass of each must be multiplied seoa-

Whntisjjower.?

What resistanee ?

What is one of the laws of motion i

Give some of the illustrations.

22

MSCHAMICS.

rately hj its Velocity, and the prodacts added together, and their
sum divided by the sum of the two masses will give the commoB
velocity. When two bodies are moving in opposite directions;!
with the same velocity, and having equal masses, action and re*
action being equal, both motions will be destroyed. Suppose
however, the masses to be alike, and the velocity of the first
body to be 10, and that of the otiier to be 6, the first body will
lose 6 parts of its velocity, which will be requisite to neubraliie
or destroy the opposite velocity of the second body, and the re-
maining 4 parts of the velocity of the first body being divided
between the two, they will move together in the direction taken
by the first body with a common velocity equal to 3.

14. When the masses, as well as the velocities, ase unequal,
the common velocity of two bodies after impact may be found
by multiplying the numbers denoting the masses by those ex-

Sressing the velocities respectively, subtracting the less prc^
uct firom the greater, and dividing the remainder by the sum of
the numbers denoting the masses : the quotient wiU then show
the velocity with wmch the bodies will move together, in the di-
rection of the body having the ^atest quantity of motion.

15. An experimental illustration of the equality of action and
xeaction in the collision of bodies may bb thus enibited :

Suppose a and ^ to be two inelastic balls,^
suspended together at e, by threads of equkl
lengths, so that Ihey may be in contact when at
rest ; and let tf e be a graduated arc, over which
the balls may oscillate freely ; then, if the ball
b be moved a certain number of degrees towards
e, and let fiill so that it ma^ impinge on the ball
a, bo& together will move towards d, throogh u
number of degrees proportioned to their com-
mon velocitv.

Since it appears from the n>regoing observatioBs to be an esta-
blished principle of Mechanics, that the force or impetus of a body
in motion is to be estimated by its mass and velocity, it must be
concluded that a body, the mass of which is very inconsiderable,
may be made to act with the same force as andther body the mass

How do you find the eommon velocity of two £qual bodies which im«
nioge against each other ? state separately the cases where one of them
is at rest before impact, when they move m the same and when in oppo-
site directions.

How do you find the common velocity after impact when the masses
fts well as velocity difier ?

Describe the experiments which demonstrate the eqsali^ of sethn
and reaction.

* No sabstance in nature is wholly destitute of elasticity, hat soft elay,
which is among the least elastic «r solid bodied, may be used to make

the balls for the above experiment

XLA8T1CITV. 23

%t wMch is much greater, provided the smaller body has a Telocity
communicated to it greater than the velocity of the larger body
in the same proportion that the mass of the latter surpasses that
of the former. Thus, a pincushion weighing half an ounce might
produce as great an effect as a cannon-bafl weighing thirty-six
pounds, provided the pincushion had 1 153 times tiie velocity of the
eannon-ball; for 116^ half ounces being equal to 36 pounds, it
must be obvious that the velocity of the pincushion would be just
so much greater than the velocity of the cannon-ball, as tiie mass
of the latter would be greater than that of the former.

BS. Hence as the momentum or effect of moving ibrce is to be
estimated by the velocity of the motion and the weight or mass
of tiie moving body taken together, it may be perceived how it
happens that a small mass may produce an extraordinary effect
when moving with great velocity. Thus, a tallow candle fired
from a gun will pierce a deal board. On the other hand a great
effect may be produced by a small velocity if the moving mass is
extremely great As for Instance, a heavily laden ship of great
burden, afloat near a pier wall, may approach it with a velocity so
small as to be scarcely observable, yet its force will be sufficient
to crush a small boat.

17. When two bodies meet in consequence of moving from
opposite directions, each body will sustain a shock as ereat as if
one body at rest had been struck by the other with a force equal
to the sum of both their forces. Sappose two persons of equal
weight walking in opposite directions, one at the rate of two miles
an hour, and the other at the rate of four miles, if they should sud-
denly come in contact^ each would receive a shock as ffreat as if he
had been standing still, and another had run against him moving
at the rate of six miles an hour. In the ancient tournaments when
mailed knights met in full career, prodi^ous must have been the
shock when the collision vras direct, and both would often be
overthrown with a force proportioned to their joint weights and
velocities. So when two vessels under sail run foul of each other,
suppose one of them eight hundred tons burden, and the other
twelve hundred tons, their velocities or rates of sailing being
equal, each would sustain a shock equal to that which a vessel
would receive if at andhor, and struck by another vessel of two
thousand tons burden, sailing at the same rate with the vessels in
question. Yet though the shock would be the same, the conse-
quences would be most disastrous to the smaller vessel, the other
being protected in a greater degree from injury by its superior
strengtn and bulk.

18. Elasticity bein? a common property of matter, and many
substances employed for a variety of purposes, as several kipds of
wood and metal, possessing that property in a high degree, its

What remarkable examples can be cited of the effeot of momentum
on bodies at rest ?

What practical illttstrationi can be given of the effect of bodies en*
eoontering each other when moring in opposite directions ?

24 jnCHAMlCSb

mflnence In modifying the operation of m«Tiiig f(riCe» must not h9
a^ected.

The different effects exhibited b j bodies almoBt inelastic and those
which are highly elastic maj be iUoetrated by the simple experi-
ment of dropping a ball of aoficlay or wax from any given height
on a solid pavenieDt, and then letting fall from the same height a
ball of box-wood or irory of equal weight with the clay. The
first ball will give way to uie pressure of iSe pavement, and become
dented or flattened on the side on which it rest*, while the lattar
ball will reboand from the pavement with a force pioportioned (e
the height from which it fell. This resiliency or rebound, in an
ivory ball, ia partly occasioned by its giving way to the pressure
of the pavement, but iiiilil"° the clay it recovers its shape almoBt
iostaotaneouely, its surface thus acting as a spring againat the pave-
ment. Thatahaid substance Ukeivory is compressed by striking
against a similaT substance, may be shown by making a small dot
with ink on the surface of one ball, and then bringing it gently in
contact with anoUier ball at that point, when a sm^l mark wilt
also appear on the latter bal^; butijf the balls, one being marked ae
before, be brought into contact with considerable force, as by

Eressure or collision, a much larger mark will be found on the lattar
all Aan before ; proving that, though both have recovered their
shape, they must have undergone compression.

_■_ 19. Leltwoivoryballsof equal weight,ai,

/ \ be suspended by threads, as in the annexed

/ \ figure, if the former be then drawn aside to

/ \ e, and suffered to Ml gainst the latter, it

/ \ will drive it to fJ, or a distance equal to tiiat

.... A through ifbich the first ball fell; hut it will

* W^ y itself rest at a, having given un all its own

^ • moving power to the second ball.

If six ivory balls of equal wei^t

be hung by threads of the same
length, and ^e ball a be drawn out
from the perpendicular, and then let
fall against the second, that and the
' other four, t iJ </, will continue sta-

tionary ; but die last ball b will fly
oj-ta-ir ofrtofl,being the same distanceas

diat Arou^ which the first ball fell. Here the motion or rather the
moring force of ^e ball a is propagated through the whole train to
the bul b, which finding no resistance is acted on by the whole
fiirce. This experiment repeated with any number of balls would

WhatM

Wfaat ii (lie nature and ciaie of retiKmes?
Ho* m»j its exiatence in iwrj be made lenMble ?
DKHrlbe the ciperimeDti whish itlaMrate Ihe law of oolli^on in elai-
tie bodlea. .

VELOCITT OF MOTION. tt

f rve^d Bame result. It is proper to obsetYe iJiat in statmg the.
effect of the collision of the balls in these experiments, diey sm-
supposed to be perfectly elastic bodies ; such however do not exist
among the substances with which we are acquainted; the pheno*
mena exhibited by ivoiy balls would therefore be nearly, but not
exactly, such as are stated,

20. The effect of elasticity in modifying the propagatioii of
motion ia curiously displayed in those exhibitions of human
strength, which have occasionally taken place, and of which
lema^able instances are related by some authors. Yopiscns^ the
ftoman historian, mentions- a. circumstance of this kind* m his Life
of Firmus, who, in the reign of Aurelian endeavoured to make
himself emperor in Egypt, and wiia has therefore been reckoned
one of the Thirty Tyrants. He was a native of Seleucia, in S^priat
who espoused the cause of the famous^nobia, Queen of Palnma;
and having been taken prisoner, he was execnted by order of the
emperor Aurelian. The historian says of Firmus, that he was
able to bear an anvil on his breast, while others were hammerinff
G^ it : he lying alon^, with his body in a curved position* And
iPeckmaun, in his History of Inventions, notices the extraordinaiy
feats of John Charles von Eckebprg, a German, who travelle4<
over Europe about the beginning of £e l^st century. After menr,
tioning other feats, he adds, '' But what excited the greatest asstoa*.
ishment was, that he suffered large stones to be broken on his
breast with a hammer, or a smith to forge iron on an anvil
p^laced upon it.'^* A part of the mysterious effect produced in
these cases is to be accounted for by the position of the exhibiter,
which may be thus, described. He must place himself with hiS'
shoulders resting on one chair, and hi»feet upon another, bodt
chairs being fixed so as to yield firm supoQrt; and thus hisbackbone,
thighs, and legs would form an arch, ot which the chairs would be:
l^e abutments. The anvil also must be so large as by its inertia
f^d elasticity, nearly to counterbalance the force of the hammw ;
and thus the strokes would be scarcely omot at all felt; besides
which the elasticity of the man's, boay, as well as his position^
would contribute, to his secnri^ against the effect of the blo^e.

Velocity of Moving Bodies,

21. Communication of motion, however rapid, must take up
spme portion of time; for as there can be no such thin^ as inktann
taneous motion, much less can motion be propagated instantane^,
qusly from onp body to another. Hence motione performed with,

What property of matter is assomed in stating, these experiments,
and how is it to oe applied ?

What Eemarkable. ea^ainple of th^ e^trof el^stioity does the hnm^n
body afford ?

Wbi^t (^plu^atioo caji be g^yen of the. explQilY of. Fimnu and E«|cp
berg?

» I I II ■■ I I ..I I I II 1,1 I I ^»^— — p— ^H^^^M*.

^ HUL at hatoL, Eag. Traa*. VfW, Vol. iii. p, 9M.

C

26 MECHANICS

gnat velocity sometimes produce peculiar effects, as may be
shown by tbe following experiments.

EXPERIMENT I.

22. A long hollow stalk or reed, suspended horizontally by two
loops of single hairs, may, by a sharp quick stroke at a point
nearly in the centre, between the hairs, be cut through, without
breaking either of them. The hairs in this case would have been
ruptured, if they had partaken of the force applied to the stalk ;
but the division of the latter being effected before the impulse
could be propagated to the hairs, they must consequently remain
vnbroken.

EXPERIMENT II.

23. A smart blow, with a slight wand, or hollow reed, on the
edge of a beer-glass, would break the wand, without injuring the
glass.

EXPERIMENT III.

24. A shilling, or any small piece of money, being laid upon a
card placed over the mouth of a tumbler glass, and resting upon the
rim of the glass, the card may be withdrawn with such speed and
dexterity that the piece of money will not be removed laterally,
but will drop into the glass.

EXPERIMENT IV.

25. A bullet discharged from a pistol, striking the panel of a
door half open, will pass through the board, wimout moving the
door; for the velocity of the bullet will be so great that the aper-
ture is completed in a space of time too limited to admit of the
momentum of the moving body being communicated to the sub-
stance against which it is impelled.

26. It is an effect of the principle just illustrated, that the iron
head of a hammer may be driven down on its wooden handle,
by striking the opposite end of the handle against any hard sub-
stance wim force and speed. In this very simple operation, more
easily conceived than described, the motion is propagated so sud-
denly through the wood that it is over before it can reach the iron
head, which therefore, by its own weight, sinks lower on the han-
dle at every blow. Which drives the latter up.

27. The velocity of motion is measured by time and space
taken conjointly or relatively. Thus, a body moving through a
given space, in a certain time, and supposed to pass through
every part of that space at a uniform rate, is said to move with a
velocity denoted by the ratio of the time to the space ; and thefe^

Stiite the four experiments which exemplify the peculiar effects ot
rapidly communicated motions.

How do vou explain the operation of driving a handle into the ejt ot
a hammer r

How is velocity of motion measured ?

How are the relatiTe veloeitieB of different bodlei estimated ?

KINDS OF MOTION. 97

fore a unifonnly moving body will describe eqaal spaces in equal
times, and diflerent bodies relative spaces in relative times.
Hence a horse that will trot eight miles in an hour, would trot
sixteen miles in two hoars, and twenty-four miles in^three hoursp
if he could traverse the distance with unabated speed. If in this
case the three distances mentioned be considered as tiiree distinci
journeys, it will readily be perceived that the horse must have
passed through the same distance, in each of the two hours of the
second journey, and each of the Ihree hours of the third journey,
as in the single hour of the first; and this is what is meant by the
statement that equal spaces are passed over in equal times ; so thai
when the distance travelled is doubled or tripled, the time will
be doubled or tripled also ; and if the distance is reduced to one*
half or one-fourth, the time will be reduced in the same propor-
tion. The relative velocities of different bodies must be estimated
in a similar manner. A man walking three miles in an hour would
require double the time to perform a journey of eighteen miles^
that would be taken up by another man running six miles an
hour ; and a horse galloping twelve miles an hour would complete
the journey in one-fourth of the time of the first man, and one-
half the time of the second man. The minute-hand of a com-
mon clock or watch has twelve times the velocity of the hour-haiid«
since the former passes through a whole circle, while the latter
is passing through the twelfth part of it.

27. The velocity of a uniformly moving body may be disco-
vered by dividing the space passed through by tiie time consum-
ed : thus, the velocity of a steam-boat, going eighteen miles in
two hours, will be found to be nine miles an hour. The veloci^
being known, the distance passed over in a given time may be
discovered, by the contrary operation of multiply ing the velocity by
the time : thus, the steam-boat, with a velocity of nine miles an
hour, will of course run twice nine miles in two hours, and forty-
eight times nine miles in forty-eight hours.

Different Kinds of Motion,

28. Motion may be uiuform or variable with respect to its rate
or relative velocity. The nature of uniform motion has been jast
pointed out: and that of variable motion will be subsequentiy
mvestigated. But motion may be different in one case from what
it is in others, when considered with regard to the manner in
which a body moves : as whether in a straight line, in a circle,
or in any other curve. The line described by a body, in passing
from one point to another, is called its direction, or hne of motion.
The direction of a moving body mav be either a right line, across
a level Surface, or plane ; a curved line, passing over a similar
plane ; or a curved line, the different parts of which are not o|l
one plane.

What is the method of discoTering the yelocitj of a aDiformlj movtag
body ? of coroputin^ its distance passed over ?
What distinctions of motion are founded on its direcUon?

69. bixmlinear mottoh is bf a xnorb. c6inplii6at^ iifatoto fhiem
•tnofion in ia atraiffht Iine,ihe^itcum8tancei8 felatin^*to'it fheirefoite
^'cimiiot be properly eiiphiiiied without a prenotm iinrestigation df
•^eciiiihear motion.

Sir Isaac Newton, in his great "Work etititlisd *< PrincipJa Phi-
'losophiifc Naturalis," "Principles of NatuM Philosophy," hah
laid down three gfeneral positions, isfyled Look of Motion^ which
have been considered as the fonhdattoh t>f mechanical science.
These laws are the following:

I.

"Every body niust coittihne in its irtate of -restt, or of unrfonh
^nu^tion in a straight line, unless it is t^cmipelled to alter its state
*of test or motion, by some 'force or^fbnses mipressed tipon h."

II.

"Every change of motion mnst be proportioned \o the impress-
t^ force or forces, and must bie in the -direction of that force."

in.

"Action ahd ieacftioia tfie always equial s!nd dofttrary to each other."

30. Both ihe first and the last of theiae laws br positions, relat-
"iiig to moving bodies, have been already discussed, and their conse-
'({uences pointed oiit: they may therefore be admitted as propo-
^si^ons not jequiring further demonstration.

^e secona law of motion is of the highest Importance, 'as it re-
late9to compound motion, and the direction d^ a hody abted on by
't^ ibrces in different but not Contraoir directions. The effect of
'forceis thus applied ^ill be most reaotify imilerstood after a short
'explanation of the nature of reflected motion, which affords 'fL
ifWiliar example of acttion an^ reaction, the subjisct of thie tiurd of
ihe preceding laws.

^1. If a cricket-ball, dr atty iSitriilarly sliaipeid elastic body, "be
dropped perpendicularly Oh a dmdoth 'pavement,'it will rebound io
"a certain point in the dame dtraiglit line ill whfch it descehded;
but if it be impelled obliquely against *tiie pavement, it will not
rise in a perpendicular line, but in a line having the same degree
of obliqmty as that in which it struck the pavement.

^ Thus, if the ball were dropped from a, to the

o ^ pavement =at 6, its upward course Would be in

\ / the same line, h as but if it 'be tl^rown in tiie

\ / line c 6, it will rebound in the line h d. In this

\ / case the angle formed hjr the line c 6, Vith thte

\ / line a b, is called the "ai^le of incidence," and

^^^^\^^^ that formed by the Hne o i, with the line a 6,

fl^HHIHIil " the angle of reflection ;^ aind it is to be observ-

^ ^ed that these angles uHll always be precisely

'^^al. For it signifies not Whether the obKcprity of the line of

4Bc&denee be great or small, since theline of teflectioti will in ever]^

What three laws of motion were laid down by Kewtoo ? Which of
^theve is of the greatest importance, and irhy ?

How is the principle of epmpoand and refleeted motion iHuitrated ib
the motions of crioket and balbard baUa ?

EQUILIBRnnf.

ease kare the same obliquity, and conaeqoently fohn a similar
gle with the surface from which the body rebounds.

^ 33, Suppose the parallelogram in the

margin to represent a billia^-table, if m
ball standing on it be impelled in the di-
rection a by It will strike against the ead
5 cushion and return in the line b r, and
either of those lines ¥rould form a similar
and very acute angle with a line drawn
between them parallel to the sides of the table ; but if the ball
were driven from a against the side cushion at d^ it would letura
tn the corresponding Tine d e;

33. Equal weights, or equal forces of any kind, acting on a
body, in a similar manner, but in opposite directions, will keep it
in a state of rest or equilibrium, like the scales of a commoa
balance, each loaded with a weight of one pound. But when the
arms of a balance are of unequal lengtlis, as in the steelyard, a
small weight fixed at ^e end of the longest arm will coonter-
poise a much greater weight at the end of the short arm.

^ 34. Let a represent a

^ >^ ^ 9 globe of lead resting on a

level surface, and naving
an iron rod passing exactly
throu|Th its centre, the ex«
tremities of which e and/
are equidistant from the
ball; if threads of equal
lengths be fixed at those points^ with hooks at the lower ends for
the suspension of weights, the globe and rod will be kept in eqoi*
librium so long as the weights b and t are equal ; but if a longer
rod be passed through the ball projecting further from it towards
g than towards £, a smaller weight d win then counterbalance the
weight 6, and the relative number of ounces or pounds contained
in these weights will always bear certain proportions to the num-
ber of inches or feet in the respective parts ol the rod e/,and e^.
Here the ec^uilibrium is maintained by equal forces acting m
opposite directions; and the illustration of this simple principle is
deserving of attention, as it leads to the consideration of the case
of equilibrium maintained by the application of tliree forces.

35. In the annexed figure the weight a
being attached to the centre of a cord passing
over two small wheels, and the weights 7
and c to either end of the cord, the equilibri*
urn will be maintained only while the central
weight counterbalances those at the ends,
in order to which, exclusive of the effect of

•<

i' i

d

f Vi

How tiiay the equilibrium of a body be preserved, and how is ihU
•object exemplified ?

How may an equilibrium be maintained by the application of thref
forces to a'flexible cord ?

c 2

W> VBCHlUflCB*

I

'IHbfion, the wtfiglit a muBt be less thitn the sum of the two emial
weights b and c taken together. For if the weight a be equal to
<the ftnm of b and e, there can be an equilibrium only when the
two ends of the cord which sustain it becotne perfectly parallel
to each other and to the parts which support b and k. This catse
$8 familiarly illustrated by the manner onen adopted of suspend-
ing lamps from ceilings by means of a weight, to which the twb
ends of a chain or cora are attached, which having passed over
two puUies at the ceiling very near each oliier, comes down
through a hole in the centre of ue weigh^t, and receives the lamp
M the middle part of the chain. By ^s means free motion ill
allowed to the lamp to ascend and descend through a convenient
distance, and the equilibrium is maintained in all positions. K
the weight a be greater than the sum of b and e the cord will ob*-
Viously sink in the centre, and the weight b and e be drawn up to
the wheels ; and weight added on either side will drag down the
^ord on the side of the additional load and raise the central and
tipposite side-weight,

d6. Suppose ft cord, as in the mamnai
figrure, stretched over the wheels E F, at-
tached to an upright board, and having 6xed
to its extremities the weights B C. From
any part of the cord, between the wheels, as
at H, let a weight A be suspended, it will
then draw down the cord so as to form an

till angle, E H F, and the weights will remain
H c in equilibrium. It is obvious that in this

A case the weight A, acting in the direction

ft A, will counterbalance the weights B and C, acting in the
airection H E and H F, and their joint forces must be equivalent
to a force equal to A, acting in the direction H G. To ascertain
Utte relative effect of the weights thus operating, it will be neces-
fcary to complete the fiffure, by drawing on the board the dotted line
H^, in the direction of the cord A H ; and lines under the cords H E
and H F. Then on the line H G mark the point a, and H a must
be supposed to represent as many inches as the number of ounces
^contained in the weight A. From a draw ^e dotted line a by
)mrallel to H F, and the dotted line a c, parallel to H E ; then if
the diagram were in the proportion just described, the line H b
Would contain as many inches as there were ounces in the weight
B ; and the line H c as many inches as the number of ounces in the
Weight C. A moment's reflection will show that the relative
^Iveights and lengths might consist of any denominations of weight
^nd longitudinal measure ; so that feet and pounds, or any greater
^ smaller denominations might have been substituted for inches
and ounces; only in every case the same denomination of longi-

What familiar illustration can be giveo of an equilibrium of*' thit
"wrtr
Describe the apparatus for exemplifying the ^trtiIldos;niu ttf ioftiBi,'

PARALLBLbORAM •¥ F0BCE8. Si

tudinal measure nmst be applied to all the lines, and the saittl^
denomination of weight to all the gravitating forces.*

37. The case just considered affords an experimental illustration
of what is called the parallelogram of forces, a principle of the
utmost importaj^ce in mechanics, since it enables us to estimate
the joint operation of moving powers, as well as thek relativt)
effect or influence.

In the preceding diagram, the parallelogram of forces is repre-
sented by the lines ah^h H^He^ and e o, and the line H a, joiaine^
the opposite angles, which is called the diagonal. The sides of
the parallelogram, a 6, and a e, will represent me quantity and direct
tion of the two forces acting together, and the diagonal Ha will
denote the equivalent or counteroalanctng force. This last force is
styled the resultant, and the two forces opposed to it sre its com-
ponents.

38. In the preceding examples, the object has been to show the
effect of opposing forces in producing oauilibrium ; but precisely
the same method may be taken to explain the operation of forces
tipplied in different directions, when their effect is to produce mo-
tion, instead of restraining it.

If a Dody A be impelled at tfie same time
by two forces, which would separately cause it
to describe the lines A B and A C of the paral-
lelogram A B D C, the body will, by their joint
action, describe in the same time the diagonal
A D. For if the body had been previously
moving with the velocity, and in the direction A B, and had been
acted on at A by the force A C, it would have described A D in
the same time. So that, whether the forces begin to act simulta-
neously or successively, their effects may be cuculated on simi-
lar principles.

A ;»B 39. When the angle at which

^^^^s^-^ . \^ tiie different forces meet is very

> v """"'' — --^ acute,they act with greater power on

C ^ the moving body ;£us, as the angle

CAB, made by the directions of the composing forces, decreases,

the effect arising from their joint impression will be increased ;

Whence does this principle derive its importance in roecbftnies ?

How is the parallelogram of forces applied to explain the laws of mo-
tion ? - .

Under what circumstances will the effects of two forees co-operate in
producinfc motion in the same direction ? How may they destroy each
sther's effects }

• On this subject, see a description, in the Journal of the Franklin
Institute, vol. 3. p. 354, of the incardo, showing under what circum-
-atances three forces may produee a stable, and iu what eases an unstable,
equilibrium. — ^Es.

92 MSCHANIGS.

and hence the diag^onal A D, which expie^ses that effect, will
likewise be increased. Therefore, when the angle CAB van-
ishes, or in other words, when the sides A C and A B coincide
with the diagonal, the joint forces will have their full effect; but
this would no longer be a case of the composition of forces, but
of the junction or union of two. forces.

C 40. When the angle BAG, made by
the directions of the two forces, is very
obtuse, their effiect is diminished, and
the diagonal, representing the resultant
of the forces, is consequently contracted.
It will be obvious, therefore, that when the sides A B and AC
meet without forming any angle, the forces will act in opposite
directions ; and provided they were equal forces they would de-
stroy each other, no motion taking place ; but if one force be su-
perior to the other, the body will move on, not in a diagonal line,
but in the direction of the greater force.

41. The combined effect of three or more forces acting on a
body in different directions, may be discovered by means of
the parallelogram of forces ; and a single force may be thus as-
signed which will be the resultant of those forces. This may be
done by obtaining first the diagonal representing the resultant of
the combination of two forces, and considering that diagonal
as the side of a parallelogram, of which a line representing a
third force will form one of the other sides, and the parallelogram
being completed, the diagonal will be the resultant of the first
three forces ; and the operatioi^ itiay be extended in the same man-
ner, so as to discover the ultimate resultant of any given number
of forces. ^

Let the point A be impelled by
forces in the directions A B, A C,
A D, and A E ; then, to find out the
resultant of these combined forces,
complete the parallelogram C A B F,
ijj. and the diagonal A F will exhibit the
result of the forces, A B and A C.
Complete the parallelogram D A F G,
and its diagonal A G will denote the
eW-^ result of the three forces A B, AC,

and A D. In the same manner, complete the parallelogram E A
G H, and the diagonal A if will represent the force compounded
of all the four forces, A B, A C, A D, and A E. But the con-
struction may be simplified by merely drawing the lines B F,
equal and parallel to A C ; F G, corresponding with A D ; and
G H, bearing the same relation to A E ; then, the line joining A
and H, which as before will express the resulting force.

How may the combined effect of several forces be determined ? Con-
stjuict and explain the diagram relating to this subject.

COXPOBltiO* dT tORCES.

9i

A

■

/

/

^

/

'/

..^uJLdi^

■/9, .1

49. It may 1>e demonstrat-
ed by meteis of the parallelo^
gram of forces, that from
Siree ferces acting in the
^tectionB A B, A O, and A
i>, in the pronortiona of the
length, breadth and depth of
a parallelepiped,* will result
a motion in the diagonal A F of that parallelopiped ; for A 6 and
A C compose A £, and A E and A D compose A F ; which last
is the tesoltant of itae moying forces in the directions of the thre^
«ides of the parallelopiped.

43. The effect of me composition of forces, when a body inn
pelled in diffefrent directions takes its coarse in a diagonal line
'Det#e€fn the two impelling forces, may be thus experimentally
iei:«»iplified :

„ ^ On a ^illiard-table, A B C D,

dhite « ball at G, equally distant
from the side B C, and the end
C D, then let two spring guns^
capable of communicating equal
impulses, be placed so that when
the ball is impelled by £, it will
mote along ihe side A D, and
1> fhiait when the ball is impelled by
F only, it will more in the line
tJ H: thten if the ball be str- ck
'b^ bbl9i iiie gp^s fftthe ssHfhe fiaiBi^ml, it wHl be found to move in
the diagonal line G C, in the same time in which it would have
ntoii^ frbm G to B, impelled by the gun E 'tione ; or from G to
H, if acted on Only by the guh 'F. From &e obsenrations which
liaVe been alreiady ihade ^n the y«tationS betireen the extent of the
^nes desbribefd by InoYing l>odies, and the amount of the forces
by iTtrhieh they are impelled, it Will be apparent that this experi-
ment may be so modified as to show what would be the direction
of the ball, when the impelling forces, or the angles af which they
acted, were variously a^sted.

44. The operation of the pnncipl^ called the composition of
forceis may be perceived in numerous teases of ifrequent occurrence,
indeed there are no motions with which we are acquainted that
can be eonsidered, strictly speaking, as instances of simple mo-
tion ; for the effects of gravitation and the diurnal motion of the
earth are alone sufficient to occasion some degree of complexity

What will be the direction and amount of a motion produced by three
forces proportionate to the length, breadth, aind depih of a parallelo-
piped ?

What -experimental illustration exemplifies the composition of forces ?

How extensive is the application of this principle?

\1^

* See IntrodoctioD, 5.

34 HSOHANICS.

in all motiima takjnff place on the earth's Bnrfiice. Simple modon

therefore is only relative.

45. Suppose two persons to be seated on the opposite sides of
an omnibus, or any other oblong earrisfie, and to pass a ball for-
waids and backwards, iiom one 1« the other, in a level line.
Now, if the cairiage were four feet wide, and the ball were passed
across that space in precisely the same time that the caniago
would be going four feet along an even road, the real motion of
the ball though the air would be in a zigzag line.

46. A stone dropped on the deck, from the mast-head of a ship
under sail, would be affected by the motion of the vessel, as well
as by the force of gravitation, and would therefore fall, not in a
perpendicular, but in a diagonal line.

g Let A represent the mast, C the stone, D the
deck, and the line C E will be the distance that the
mast-head will have moved, while the stone-would
have fallen, by the force of gravity alone, from C to
the point under it on the deck ; the mast being fixed
is carried forward by the ship, and therefore ^e
foot of the mast will have moved equally with the
head, and will have reached the point vertically be-
neath E when the stone touches the deck; the stone
wilt also be found at the foot of the mast, having
jl_ taken a diagonal direction, in consequence of its
a being impelled at the same time by the ship's mo-

tion and by its own weight. For, if it bad not been affected by
the former as well as the latter, it would have &llen where the
f-f^i of ilie rnast was when it began to fail, and not at the actual
fort "( (iio masi..

47. Any one who has witnessed ^e common feala of equestrian
exhibilers at a circus, or elsewhere, may have seen a man leap
from tlie back of a horse over a garter or handkerchief stretched
horizonlaliy across the track in which the horse was ^loping,
round the border of a circular area, and the horse passing under

the garter, the man comes down again
on Uie saddle, alter hnisbing his leap.
To do this, it is only necessary for the
pder to spring upright from the sad-
> die, on wluch he was previously stand'

._ ing, and suffer himself to sink by his

eight on the saddle again; for as his body would partake
of the motion of the horse, that force would be sufficient to carry
him forwards, and his motion in rising, by an impulse which
would carrj'him from A to B if the horse were standing still.

What is the re«l molion ccmmnnioiited to ■ body thrown froin oiie
tide of » carriage to another when in molion ,'

niugtrale thia principle in ibe falling of a bod; from the nuitt-bad
of n vested under way ?

Wlial enable! an equeatrian perfornier, after leaping opvudfromA
borse in motion, la alight again on tlie nddle .'

RESOLUTION OF FORCES. 35

would be nearly in the line E, while he would descend in the
corresponding line F, through the joint effect of the force derived
from the horse, and his own weight, the latter of which alone
would occasion him to sink in the direction O D, or G H.

48. As it has been observed that all motions are really of a
compound nature, resulting in a greater or less degree from com-
binea forces, it may sometimes be requisite to ascertain the sepa-
rate effects of acting forces ; or to determine what portion of any
given force acts in some direction different from that in whicn
motion takes place. The operation requisite for this purpose is
called the resolution of forces, the object not being as before, to
discover the resultant from the combining forces, but to discover
one or both of those forces £rom the resultant.

If a compound force, acting upon a body, pro-
duces motion in the direction A B, and it is re-
quired to find the part of that force which affects
this body in any other direction, as D C ; by
drawing A D perpendicular to the direction D C,
3> will be found the proportion which the absolute
force b^s to that part, which acting alone would produce mo-
tion in the proposed direction.

49. A boat may be moved across a river by the cunent passing
in a direction parallel to its banks. To effect this the boat musi
have a rope fastened to it, the other end of which is connected
with another rope extended directly across the stream, a noose or
ring being fixed to the first or boat-rope, through which the
stretched rope is passed in such a manner that the ring may slide
freely in either direction. Then the rudder of the boat being pro-
perly turned to receive the impulse of the current, it will pass
across the river, for the ropes will prevent it from being carried
down the stream, while it glides with ease transversely as the
ring of the boat-rope slides from one extremity to the other of the
extended rope. Part of the force of the current in this case is
destroyed, and the remainder is made to produce a motion in a
direction different from that in which th^lwater is flowing. The
velocity of the current and that of the boat being ascertained, it
would be easy to calculate what proportion of the moving force
acted on the boat.

50. When the impulse of air or water is employed as a moving
power, either can seldom act directly and with full force, some
portion being lost, and the effect consequently diminished. A ship
sailing with a side wind has the sails set obliquely with respect

By what oiieration may the eeparate effects of acting forces be ascer-
tained ?

What is the precise object to be discovered in this case? construct and
explain the diagram.

How is the resolution of forces applied in the rope ferry ? How might
we calculate what proportion of the moying force acted on the boat?

What examples are afforded* in which the impulse of air and of water
produces a resolution of forces ? What becomes of the ineffectiye part
of the force in these instances ?

30 X^CQA^KSk

to the course pursued ; so tke vanes oi^ a windo^ill, and tb» float*,
boards of an undershot water-wheel are moved in general by a
ibrce applied in a slanting direction. Indeed the motion of a
windmill would be prevented^ by setting the surface of the sa^ls.
perpendicular to the direction of the wind. In these and many
other oase^, only part of a moving force is brought into action, the
other part being aissipated and lost, because it cannpt be made tp^
act in tl\e rec^uired direction.

GrwfiUUum,

51. Among the causes of motion, or moving forces, these axe
some, the effects of which are simple and uniform, producing move*
ment in a single direction or straight Jine, and for a given ttme^
proportioned to the degree of impulse. Others act in more than
one direction, but with combined effect, so as still to produce
uniform motion. Nature, however, presents to our notice motions
which are not uniform, the velocity of the moving body varying
in different parts of its course, so that the velocity or rate of motion
may gradually increase to a certain point, and be suddenly termi-
nated ; or first increase, and then decrease till it ceases altogether.
Motion with a perpetually increasing velocity is called accelerated
motion. The phenomena of simple and compound rectilineal mo-
tions have been already described ; but those of accelerated motion,
which come next to be considered, cannot be fully understood
without a previous acquaintance with the laws of gravitation, with
which they are intimately connected. So general indeed is the
effect of Ihe property of gravity or weight on all bodies, within the
reach of our observation, that its influence is perpetually interfering
with our operations and experiments ; and hence references have
necessarily been made to it in the preceding pages, as in explain-
ing the cause of the decay of motion, and elsewhere ; but it will be
requisite here to take a more extensive view of the nature and
effects of this important principle.

52. Gravitation or Gravity has been noticed in ihe Introduction,
under the appellation df gravitative attraction, ns distinguished
from cohesive attraction, capillary attraction, magnetic attraction,
and other forces which tend to brmg bodies into contact. Most of
these forces or kinds of attraction are perceived only under par-
ticular circumstances ; as cohesive attraction, which seems to act
on solid and liquid substances alone, and not on gases; and capil-
lary attraction, which only takes place between certain fluids and
solids. But the attraction of gravitation differs from other attrac-
tive forces in being a common property of all bodies, since every
thing to which we can attach the idea of materiality is affecte4
more or less by gravitation.

53. It is by no means inconsistent with this statement that som^

What is the distinctive character of vi^ri^ble motion ? "What is ap«e?>
lerated motion ?

How is gravitation distins:ni8hed from other specie! of attraction 2
Kow extendive is its influence over materia things?

GRATITATIOIC 87

bodies, possessing all the characteristics of solid matter, capable of
being seen and felt, yet in certain circamstances, instead of exhi-
biting the common effect of gravity, in falling towards the earth
or pressing on it, display the contrary phenomenon of ascending^
from it. Thus, smokis will be seen, in some states of the atmo-
sphere, rising in a column to a considerable height. Even solid
masses of no small bulk and weight may be made to ascend to a
great height, as by means of an air-balloon. Bnt all these and
similar phenomena are in feet so many instances of the effect of
gravitation ; for the ascending bodies are driven upward solely by
the force of the medium through which they pass; since the parti-
cles of smoke, or the balloon with its car and contents, cannot
advance upward in the most minute degree without displacing,
or thrusting downward, portions of the atmosphere equal to then
own bulk. Hence it will be perceived that aerostatical bodies
do not ascend because they possess absolute levity, but simply
because, bulk for bulk, they are lighter than the air. A cork or a
piece of deal, for the same reason, will float on water, and if
pressed down in it will rise again to the surface, by the effect of
relative levity.

54. All substances, then, gravitate towards the earth ; that is,
they have weight, which occasions them to fall to the earth whea
dropped from a height above it; to rest upon it with a certain
degree of pressure, according to circumstances ; or if rendered
buoyant, to rise in the atmosphere surrounding the earth, till they
reach a part of it where it is less dense ^an near the surface, so
that a portion of it, precisely equal to their bulk, would exactly
counterpoise them, and there of course they could neither rise nor
fall, without an alteration of their own weight taking place. In
the case of an air-balloon, the aeronauts have the means for les-
sening its buoyancy whenever they may find it convenient, by
opening a valve, and letting out a part of the gas, or light air,
to which it owes its ascending force ; thus they can, at any time,
render the weight of the whole apparatus much greater than
that of an equal bulk of atmospheric air, and then it must fall to
the ground. Smoke only remains suspended till its particles
unite, and thus becoming heavier than the air, they descend in
the form of small flakes of soot, covering with a dingy coat or
incrustation all buildings, after a time, in large and populous
places.

55. Let us suppose for a moment that while a mass of ^noke
and an air-balloon were hovering in the air near together, and at

How is the universal prevalence of gravitation to be reconciled with
the appearance of light substances rising from the sarface of the earth ?
What is the true explanation of these phenomena ?

What expedient enables the aeronaut to descend from a higher to a
lover level in the air ?

In what manner is smoke finally deposited fVom the nir P

What would be the effect on suddenly removing the air from beneatU
a mass of smoke and an aij>balloon hovering near each other ?'

D

88

MECHANICS.

f

precisely the same height, it were possible to withdraw from
under them the support of the atmosphere, it will be immediately
perceived that they must fall ; but probably the young reader will
be surprised to learn, that they would not only fall, but likewise
that they would both fa\\ through the same space in the same time ;
so that if their common height had been five hundred feet, the
smoke would have reached the surface of the earth at the same
instant with the balloon, though the latter might in weight far
exceed the other body. It must not be imagined that the cir-
cumstance just stated IS a mere philosophical conjecture, or that it
cannot be confirmed by the test of experiment; for, though it is
impossible to annihilate the atmosphere, or effectually remore it
from beneath an air-balloon, or any oliier body suspended in it, yet
on a small Scale appearances precisely similar to those just de-
scribed may be easily exhibitea. >

CT 56. Let A represent a tall bell-glass,

■EISB open at the bottom, and having the top

iS ** r1^ closed, so as to be air-tight, by a brass

N V cap or cover, B, through whicli passes the

wire C, fitting close, but capable of being
turned without admitting the air. The
lower end of the wire must be made to
support a small sta^e, the two sides of
which, D D, will fall and separate, when
the wire is turned in a transverse direc-
tion. Then, the stage being fixed, a gold
coin and 'a feather, E and F, or any
two small bodies differing greatly in their
comparative weight, may be laid on the
stage, and the bell-glass, or as it is called,
receiver, being placed on the plate, G, of an air-pump, must be
exhausted of the air it contained. This being done, if the two
bodies E and.F are made to fall by turning the wire, it will inva-
riably be found that they will both strike the plate of the air-
pump beneath them at the same point of time,

57. The influence of gravitation is not only extended to all bo-
dies on or near the surface of the earth, but likewise, as we have
the utmost reason to believe, to all bodies in the universe. This is
not the proper place to describe the nature and operation of those
forces which regulate the orbits of the moon, the planets, and the
comets belonging to the solar systenn ; but it may be here observ-
ed that Sir Isaac Newton discovered gravity to be the cause of all
the motions of the heavenly bodies ; and that the laws of gravita-
tion displayed in the monthly revolution of the Moon round the
Earth, the annual circuit of the Earth round the Sun, and the

In what manner can we prove, experimentally, that light and heaTy
bodies would fall with equal velocity if the air were suddenly annihi-
lated ?

How extensively is g;ravitation applicable to the works of nature ?

What discovery did Sir Isaac Newton make on this subject ?

ORAYITATIVE ATTRACTION OF MAMES. t9

corresponding motions of the other planets and their satellites are

capable of the strictest mathematical demonstration.

58. Gravitative attraction acts upon all bodies, with forces pro-
portioned to their masses. Thus suppose two bodies so situated
as to be wholly exempt from the influence of any attraction except
that resulting from their ^yitation towards each other, they will
l^en approach with velocities corresponding with their respective
forces. If the larger of the two boaies be double the size of the
smaller, the former will act with twice the force of the latter; and
therefore while the small body will move two feet in consequence
of the double power of the larger one, the larger will move bat
one foot drawn by the single power of the smaller. If the larger
body be four times the size of the other it will exert four times
as much attractive force, or m^e the smaller body move with
four times as great velocity as it would if the masses of the bo-
dies were equal.

59. Hence it may be regarded as a general law of gravitation,
that while the distance between two bodies remains unaltered,
they will attract and be attracted by each other, in proportion to
their respective masses ; and therefore any increase or decrease
of the mass must occasion a corresponding increase or decrease of
the amount of attractive force, as measured by the velocity.

60. Since gravitative attraction is a common property of all
bodies, it may naturally be inquired why all bodies not fastened
to the earth's surface do not, by their mutual attraction, come in
contact; or by what means the force which they derive from gra-
vitation is prevented from appearing in their relations to each
other. A little reflection will show that the cause of this seeming
inactivity of bodies at rest is the overpowering influence of the
earth's attraction. If a small particle of matter were placed at
the surface of a solid sphere or globe of gold, one foot m diame-
ter, its gravitation to the earth would be more than ten millions
of times greater than its gravitation to the gold. For the diame-
ter of the earth is nearly forty millions of feet, and the density of
gold is nearly four times the medium density of the earth ; there-
fore in a second, the particle would approach the gold less than
the ten millioneth part of sixteen feet, a space utterly impercepti-
ble. It is also owing to the immense difference in the mass of
the earth and that of any one body on its surface, that the attrac-
tive influence of bodies idling towards the earth produces an
effect in drawing the earth upwards so insignificant as to be infi-
nitely beyond the reach of our observation.

61. Though we cannot institute direct investigations of the

In what proportion does gravitation affect different bodies P
'What would be the relative velocities of two unequal bodies actuated
solely b^ each others gravitative attraction ] State the general law on
this subject.

Why do not all unoonfined bodies rash together by their mutual at-
traction ?

Why do not falliog bodies draw op the earth instead of descending to
its surface ?

40

XECHAKIC8.

eomparative effect of grayitation, by making experiments on de-
tached masses whose magnitude bears any considerable propor-
tion to that of the earth, yet it may be shown that partially iso-
lated portions of the earth^s sarface exhibit a sensible degree of
gravitative attraction, when small bodies are brought near them.
A mountain two miles in height and of an hemispherical figure,
risin? in a level country, would cause a plummet suspended be-
side It to deviate one minute of a degree from the perpendicular
direction which gravitation towards the earth would otherwise
produce. Observations of this nature have been actually made
on more than one occasion. The French Academicians, Bouguer,
De la Condamine, and others, when employed in measuring a
degree of the meridian, in Peru, towards the middle of the last
century, having placed their observatories on the north and south
sides of the vast mountain of Chimborazo, found that the pluin-
mets of their quadrants were deflected towards the mountain. The
manner in which these philosophers ascertained the amount of
the deflection of their plummets may be thus concisely explained.

Their object being to determine
the zenith distance of a star, I, it
was necessary to regulate the posi-
tion of a telescope by means of a
quadrant, the plummet of which, in-
stead of hansing in the vertical
lines A F, and C H, on the oppo-
site sides of the mountain, were
found to take the positions A B,
and C D, and thus the star seemed
to have the zenith distances e I, and
g I, instead of E. I, and 6 I, which
it ought to have had : hence it is
obvious that the plummet was
drawn aside, by the attractive force
of the mountain, from its proper
direction perpendicular to the earth's
sur^e, through a space capable of
being estimated by the differences perceived in making observa-
tions on the star I from the opposite sides of the attracting mass.
63. The phenomena thus observed by the French philosophers
having given rise, to discussion among men of science in different
countries, it was thought desirable to ascertain, by experiments
made for that particular purpose, the validity of the cause assign-
ed. King George III. therefore was induced to send the Astro-
nomer Royal, Dr. Maskelyne, to Scotland, in 1772, to make

Hov may a comparison be made between the whole mass of the earth
and an isolated portion projecting above its general level ?

Describe and illustrate the experiments which have been instituted on
this subject.

What amount of deviation did Dr. Maskelyne find in his plummet on
(he sides of Schehallien f

RAVrrATIVS ATTRACTION OF MA88E8.

41

Bimilar experiments on the north and tenth sides of Sehehallien,
a lofty and solid monntain in Perthshire, well adapted for the
purpose. The deviation towards the mountain on each side, was
found, after the most accurate observations, to exceed seven se-
conds ; thus confirming the inferences of preceding observers, and
proving the universal operation of gravitative attraction.

63. rhe influence of general gravitation was also experiment-
ally demonstrated in a different manner, by Mr. Henry Caven*
dish, in 1788.

TwosmaH metallic balls, C and
D, were fixed to the opposite ends
of a very light deal rod, which
was suspend^ horizontally, at its
centre £, by a fine wire. This
arm, after oscillating some time horizontally by the twisting and
mitwisting of the wire, came to^ rest in a certain position. Two

great spherical masses, or globes of lead, A and B, were then
rought into such a position, that the attraction of either globe
would turn the rod C D on its centre £, in the same direction.
By observing the extent of the space throng which the end of
the rod moved, and the times of the oscillations when the globes
were withdTawn,^the proportion was discovered between the
effect of the elasticity of the wire, and the gravitation of the balls
towards the leaden globes; and a medium of all the observations
being taken, the experimentalist was enabled to ascertain not only
the actual influence of gravitation on terrestrial bodies in general,
but likewise its, relative influence as depending on the density of
the attracting body.

64. As gravitative attraction draws bodies towards the centre
of the attracting mass, it miffht be expected that bodies under the
influence of gravitation would, diverge somewhat from a line per-
pendicular to an horizontal plane beneath them.

This indeed is precisely what takes place;
and if we imagine a pair of scales, as m the
marginal figure, to be formed in such a manner
as to bear a certain proportion to a sphere to-
wards the centre of which each scale was at-
tracted, the effect would be obvious. But the
magnitudes of any bodies which we can make
the subjects of experiment are so extreniely in-
considerable when compared with that of the
earth, as to render the deviation from the per-
pendicular, in lines which are actually conver-
gent, quite imperceptible.

65. It must also be considered that though the grand and pre-
ponderating force of gravitation is directed towards the centre

Describe the method adopted by C&Tendish to demonstrate the iDfia-<
eooe of grravitation, and the mean density of the earth ?

What are the directions in regard to a horizontal plane of two bodies
remote from each other, and obeying the foree of gravitation ?

d3

42 VBCHANICS.

(and all bodies, like those just mentioned, are attracted towards
the earth's centred yet eyery particle likewise has an attract^
ive power, therefore the grayitation of bodies on the earth's
surface is the effect of the attraction of its entire mass. Hence
in the investigation of the phenomena of falling bodies, it may
be assumed that all the particles of the same bcray are attracted
in parallel directions, perpendicularly to an horizontal plane; for
the spaces through which bodies fall, while under our obserra-^
tion, are not of sufficient extent to render it necessary that any
allowance should be* made for the effect of direct attraction to-
wards the centre.

6^6. The slightest observation will enable any one to ascertain
that the force of a falling body increases in proportion to the
height from which it has fallen. When bodies are precipitated
from a great height, they will strike with violence against a re-
sisting surface, or penetrate deeply into a yielding mass. Aero-
lites or meteoric stones, which are heavy bodies, resembling iron
ore, several of which have feUen at different periods, have some-
times been found to sink deeply into the earth ; as was observed
with regard to a meteoric stone, fifty-six pounds in weighty which
fell in a ploughed field in Yorkshire, England, in 1795.

67. Expenments serving to illustrate the effect of accelerated
Telocity on falling bodies may be made by observing liie rebound
of an elastic body, when dropped from different heights. A mar-
ble or a cricket^ball successively suffered to fall on a pavement,
from the respective heights of a foot, a yard, and double or treble
that height, would rise higher and hiffher at each trial, according
to the extent of the space through which it had fallen. More ex-
act experiments might be instituted by forming three or four balls
of soft wax or moist clay, exactly of equal weight, as one pound
each, and letting them drop from different heights on some smooth'
hard surface ; when it would be perceived that each ball waff
indented or flattened, on the. side on which it had fallen, more or
less deeply in proportion to the extent of the space it had fallen
through.

68. Having thus ascertained that the velocity of a falling body,
as denoted by its final force, is increased by the augmentation of the
distance passed through, it becomes an interesting speculation to
determine what are the relative degrees of velocity produced by
given distances of descent. In oSier words, it is desirable to
know whether a body falling through a space during two seconds,
or two minutes, would fall as fast again in the second period as

t did in the first, or three times as fast, ten times as fast, or in

Whenoe resalts the gravitation of bodies on the earth's sarfaee ?

How does the force of falling bodies vary with the heights from whieh
lliey fall ?

Exemplify this in the ease of aerolites.

What familiar experiments with elastic and with soft bodies prove the'
relation between velocity and extent of fall ?

What relation does the velocity of a falling body aetoaUy measure ? ■'

LAWS or OR^yiTATION. 4S

iirhat othtsr ratio of acceleration. This is obTiously a quesiioii of the
relation between time and space, for velocity is the measure of
that relation. Now the motion produced by graritatiTe attraction
is a continually increasing motion, so that a body under the influ-
ence of gravitation will not fall through exactly the same space
in any two consecutive periods of time, however inconsiderable.
For if we could suppose a single second to be divided into a thou-
sand parts, a falling body would' pass through a greater space in
the second thousandth part of the second, than in the first thou-
sandth part, and so on in like manner throu|[hout its course. How-
ever, in order to find out the rate or ratio of the increasing velocity
of &lling bodies, it»will be sufiicient to know what is the distance
passed tiirough by a descending body in each succeeding second,
minute, hour, or any other equal portion of the time of its whole
descent.

69. When we consider the various circumstances which inter-
fere with the motion of falling bodies, some arising from the re-
sistance of the medium through which they pass, and other inci-
dental sources of irresularify, others from the varying force of
gpravitation itself, at different distanoes from the centre of attrac-
tion, it will be at once perceived that the inquiry before us is
surrounded with difficulties. It is no wonder then that very con-
fused and erroneous notions concerning this subject prevailed till
a comparatively recent period.

70. Arisiotle, whose opinions were long regarded as indisputa-
ble, states, in his philosophical writings, that if one body has ten
times the density of another, it will move with ten times the ve-
locity ; and that both bodies being let fall together, the first will
&11 through ten times the space that the other will in the same time ;
besides other erroneous doctrines, which were generally received
till his theory was overturned by the discoveries of the celebrated
Italian philosopher Galileo, towards the end of the sixteenth cen-
tury. He showed that bodies, under the influence of cavitation
alone, would fall through spaces as the squares of the times of de-
scent: that is, that a body, which would ^1 through one inch in
one instant, would fall through fisur inches in two instants, and nino
inches in three instants; for the square of any number is the pro-
duct of that number multiplied by itself, so four is the scjuare of
two, nine the square of three, &;c. The principle thus laid down
by Galileo, though disputed by some later philosophers,* has not

Will m?itatien alone ever prodace a uniform velocity of motion ?
exemplify this point.

How may the rate of increasing yelooit^r be determined ?

What prevented the early philosophers from obtaining exact notloni
of this subject ?

What was Aristotle's opinion on the subject of falling bodies ?

* The autboritj of Galileo was questioned, and different opinions were
mainlRined by philosophera ooneeming the ratio of the acceleration of

44 MECHANICS

only been triumphantly established as a positive law of natnre,
with regard to falling bodies, but, as already mentioned, it has
been shown by Sir Isaac Newton that it is a general law of na-
ture, extending to the motions of the celestial bodies composing
the solar system.

71. In order to apply this principle to the purpose of ascertain-
in? the precise ratio of the accelerating velocity of falling bodies,
it is necessary to fix on some measure of time as the unit from
which calculations must commence, and to determine what space
a body will fall through in that portion of time ; and these data
being furnished, the application may be readily exnlained.

72. But before we proceed to the further consideration of the
velocity of idling bodies, as the effect of a uniformly accelerating
force, it will be proper to observe that it can only be thus strictly
estimated with respect to bodies falling through limited spaces,
as short distances nrom the surface of the earth, where the inten-
sity of the gravitating force may be regarded as continuing the
same during tl^e whole period of descent. For not only does the
velocity of gravitating bodies in descent become accelerated as
they approach the centre of attraction, but the intensity of the ac-
celerating force is also continually increasing. And on the con-
trary, the intensity of the force diminishes as the distance in-
creases. Hence the velocity of a body falling from a ^at height,
as fifty miles from the earth's surface, would increase in a ampler
ratio at the beginning of its descent, and in a much greater ratio
towards the end of its descent, than that of a body falling through
only as many feet.

73. The force of gravitation is to be estimated by the same rule
that has been already stated as applicable to the velocity of fall-
ing bodies. It increases as the squares of the distances of bodies
decrease, and decreases as the squares of their distances increase.
Thus, if one body attracts another with a certain force at the dis-
tance of one mile, it will attract with four times the force at half
a mile, nine times the force at one-third of a mile, and' so on in
proportion ; and on the contrary, it will attract with but one-fourth
the force at two miles, one-ninth the force at three miles, one-six-
teenth of the force at four miles, and so on as the distance in-
creases. Applying this principle to the gravitative attraction of
the earth, it follows that its force must be four times greater at

the earth's surface than at double that distance from its centre ;

•

What truth in regard to gravitation was first established by Gralileo ?

What measure must we adopt previously to applying the principles of
gravitation ?

Does the rate of acceleration by gravity continue the same at all dis-
tances above the surface ? State the law applicable to this subject.

the velocity of falling bodies, even till the time of Newton's discoveries.
— Yid. Regis Physic, lib. ii. cap. 23 ; also, Annotations of Dr. Samuel
Clarke, on Rohault*s Treatise on Natural Philosophy, a work which was
considered as of standard authority in the beginning of the last centar> -

GRAYITATIVE ATTSACTIOK OF THE EARTH. 45

and as the weight of bodies is estimated by the piessuTe or gravi
tating force with which thej tend towards the earth, a body
weighing one pound at the esurth's surface wonld have only one-
fourth of that weight, if it could be remoyed as far from the snr*
face of tHe earth as the surface is from the centre. And at the
distance of the moon from the earth, which is 240,000 miles, the
weight or graTitatin? force of the same body, as affected by the
attraction of the earth, would be equal to only the 3600th part of
a pound. For reckoning the distance of the earth's surface from
its centre to be 4000 miles, that is, half its diameter,* the dis«
tance of the moon would be sixty times as great, and the squajre
of that number, or 3600, would, as just stated, indicate the de-
crease of gravity^ at the distance of 240,000 miles from the surface
of the earth.

74. This decrease of weight, in proportion to the squares of in-
creasing distances; might in some situations be made the subject
of experiment. A ball of iron, weij|rhing a thousand pounds at
the level of the sea, would be perceived to have lost two pounds
of its weight, as ascertained by a spring^balance, if taken to the
top of a mountain four miles high. The same body removed
from Edinburgh to the north pole would gain the addition of three
pounds ; and if conveyed to the equator, xt would suflfer a loss of
four pounds and a quarter. To account for the loss of weight in
the last-mentioned situation, it must be recollected that the earth
is not a perfect sphere, but that its figure is spheroidal, the diame-
ter of the earth from pole to pole bein^ somewhat less than in the
line of the equator; the equatorial regions therefore must be more
distant from the centre oi attraction than the polar regions, and
the force of gravitation at the former consequently less than at
the latter. Hence the point of rreatest attraction must be at
either of the poles; for if the iron ball, just mentioned, could be
conveyed to the depth of four miles within the bowels of the earth,
it would be found to be lighter by one pound than at tiie surface;
since it would be attractea on every side, and the force of gravi-
tation upwards would in some degree counteract the preponderat-
ing force with which it would press downwards. If it were pos-
sible for the iron ball to reach the centre of the earth, it would
necessarily there lose the whole of its weight, for the attraction
of gravitation acting equally in every direction, no effect would

How much greater is the foree of gravitation at the earth's surface,
than at a semi-diameter above it ? How much woold a pound weigh if
carried to the distance of the moon ?

How might the decrease of weight in bodies removed to a distance
above the surface of the earth be experimentally proved ? '

How is difference of weights in different latitudes to be explained ?

What effect upon its weight would arise from carrying a body far be-
neath the surface ?

What would be the weight of a body carried to the centre of the earth ?

* The mean semi-diameter of the earth may be estimated more ex-
actly at 3956 miles.

46 MECHANICS.

be produced, and the ball would be fixed, as if encircled by an
iuiinite number of magnetic points.

75. Connected with this part of the subject there are some cu*
rious problems, the solution of which requires mathematical cal-
culations, but the results alone are here introduced, as furnishing
interesting illustrations of the power of ffravitation.

Suppose the axis of the earth were penbrated from p(de to pole :
a bodv falling through the perpendicular hole, being attracted on
all. sides, would be urged downwards only by a predominating
force, proportional to its distance from the centre. The velocity
acquired at this centre, reckoning the length of the axis 7900
miles, would be equal to 25,834 leet each second. The time of
descent would be 1268i seconds, or 21' 8''i ; and the whole time
of passing to the opposite pole 42' 16"i.*

76. Conceive a body, under the mere influence of terrestrial
attraction, to fell from the orbit of the moon to the earth's surface.
At the mean distance of sixty semi-diameters of the earth firom its
surface, the initial force would be diminished 3600 times : with
the same continued acceleration, therefore, it would consume a
period of 526,578 seconds, or six days, two hours, sixteen minutes,
and eighteen seconds, in performing the whole descent. The
final velocity, on this supposition being 4680.69 feet each se-
cond. Such would be the time of descent under the influence of
uniform acceleration; but the time required with an acceleration
inversely as the square of the distance from the centre would be
only 414,645 seconds, or four days, nine hours, ten minutes, and
forty-five seconds. And in this case the final velocity would be
36,256.45 feet, or about seven ^iles each second. Abstract-
ing, then, from the resistance of the atmosphere, a body propelled
directly upwards, with this last velocity of 36,256.45 feet in
a second, would mount to the Orbit of the moo9; but with the ad-
dition of one hundred and twentieth part more, or 305 feet to every
second, it would reach the sun ; and with the further acceleration
of less than one foot, amounting to 36,562.43 feet each second, the
body would be enabled to continue its flight into the regions of
boundless space.f

What would be the velocity and the time of a body descending'
through a perpendicular hole along the axis to the earth's centre ?

How long would it take a body to fall from the moon to the earth ?
and what would be its velocity on reaching the surface ?

With what velocity must a body be shot upwards, in order to pass be-
yond the solar system.

* In the hypothetical case here propounded, it must be admitted that
the acauired velocity of the body at the centre of the earth would over-
eome the obstacles to its ascent, and enable it to complete* its passage.

t Leslie's Elements of Natural Philosophy, Snd edit Ecunb. 18S9.
VoL i. p. 106, 7.

ACCELERATED MOTION. 47

Jicceleraied Motion,

77. The increase or acceleration of velocity, from the force of
graTitative attraction, has been stated to be as the squares of the
numbers representing equal portions of the time during which a
body falls. It has been found convenient to consider the time of
descent of filing bodies as divided into seconds, so that if a body,
under the influence of gravitation alone, falls one foot in one se-
cond, it must fall four feet in two seconds, nine in three seconds,
sizteeen in four seconds, and so on, in progression ; the squares
of the numbers of the seconds showing the number of feet passed
through by the falling body at the end of each second. In order
to discover the distance passed through in each particular second
of the time, it is merely requisite to subtract, from the whole dis-
tance completed at the end of that second, the number of feet at
the end of the preceding second. Thus, fi[t)m 4 feet, the distance
in two seconds, take 1 foot, the distance in the first second, and
3 the remainder, will be the number of feet passed through in the
second second only ; from 9, the distance in three seconds, take
4, the preceding distance in the first two seconds, and the remain-
der 5 will be the distance in the third second ; so from 16, the
distance in four seconds, the preceding distance of 9 being sub-
tracted, will leave 7, the distance in the fourth second.

78. Gravitation being a continually acting force, a body falling
through its influence alone would in every mstant of its descent
move faster than in the preceding instant, and consequently, at
the end of any given time, it would be impelled by a force be-
yond that which carried it through the preceding space. This
force may be estimated in the following manner. Suppose a body,
after having fallen daring one second, bv the impulse of gravita-
tion,' to be no longer acted on by an accelerating force, but to con-
tinue its motion with the velocity already acquired, describing
through the remainder of its descent equal spaces in equal times.
In such a case it would W found that the falling body, in every
successive second of its descent, after the first, would pass
through twice the space through which it had fallen in the first
second by the fOTce of gravitation. And the velocity being esti-
mated by the space described uniformly in one second, it follows
that the velocity acquired in one second must be equal to double
the space through which a body would fall freely by the action
of gravity in one decond. . Since then the velocity increases in
the same proportion as the time, it would be twice as great at the
end of the second second, as at the end of the first, thrice as
great at the end of the third second, and so on.

Mow can we disoover the diitanee passed through in each separate
second of the descent of a body ? Exemplify this by a particular case.

With what UBiform velocity per second would a body move, after hav-
ing fallen for one second, supposing the force of gravitation to be then
suspended ?

■ECBANICB.

79. The folloning table, t
body would fall throagh one

as furniBhing the moet Btmple resutta, will afford a
luBtraliona of the poaitionH laid down.

Space
fallen I

. 17

10 . . 100 . . 20 . .19
80. It will at onee appeu from the inspection of this table t
Ihe time of descent of fallinj bodiei iDcieaaing as the numb
I, 2, 3, Sic, and Ihe entire apacei paased through as the squa
of those numhera, the angmentation of velocity will be rep
eent«d by theereaauoiben, inregularpro?TeBsion, and the spac
passed trough in each second by the odd numbers. The si
of Ike number of feet in the fourth column wilt of conrae gi
the number of feet fallen throagh in the whole time f and the di
tanee fallen through in any part of the time may bo tband in tl
same manner. Thus, l-!-3-f.5, &c. to 19 inclusive will amou
to 100. So the apace fallen through in any number of seconi
may be ascertained by adding the correspond ing- numbers in tl
•jy seeocd and third columns, tog
ther with the number represenlin
the space fellen through in th
first second of descent Thus
+4=8+1=9; 13+36=48 + 1=
49; 18 +81 =99+- 1— 100. An
the same reeults may be ablain&
in any similar cases.

81. The nature of accelerating
velocity, as exhibited in fallingbo
dies, may, perhaps, be somewha
C elucidated by reference to the se
lies of triangles in the annexed diagram. Let the line A B dft
Bote the time of the descent of a falling body, divided into equal
portions, aa aeconda ; then the small nnmbeied triangles may re-

Eifriai 11 the relslion, » exhibited in Ihe Uble, between the time, the
entire apace falien throogh, leqaired Teloeilj, sni) tpace deiorib«l in
Mch Kcnnd. What leriei of number! repreKnti the mognieDtilian gl
*t1a«ltT ^

la vbat geometdeal Ggare may thii relation be exhibited i

LAWS OF ACCELERATED MOTION. 4i

present the space fallen throngfh, under the inHnenee of gMvitlii-
tion : the number of the triangles in each line showinar the num-
ber of teet passed through in each second, and the entire number
the whole space described in five seconds. By completing th#
square, as with the dotted lines, it may be peroeired now it hap-
pens that the Telocity, acquired by a falling body at the €nd of
each second, is more than is expended in its passage thfough the
next second ; and also it will appear that a body, moring uni-
formly with the velocity acquired at the end of any pven second
of time, will describe double the space described m the same
time by a body falling under the influence of grayitation alone*
For suppose the triangles a, 6, e, <£, e, to denote the surplus Telo-
city at uie end of each second, which must be sufficient to carry
the falling body through one foot, they will, if added successively
to the numbered triangles in each line, show the Telocity acquired
in each succeeding second ; and therefore the triangles 17, 18, 19,
80, 21, 33, 33, 24, 35, and e will be ten in number, the amount
of the Telocity acquired at the end of fiTe seconds. Now a body
moving with the uniform Telocity of ten feet in a second would
pass through the distance of fifty feet in fiTe seconds; while a
body fallinff through gravitation only would pass through but
twenty-five feet in me same time: and the space described by tfie
uniformly moving body, at the rate of ten feet in a second, may be
represented by the square A B C D ; while the triangle ABC
would represent the space described by a body moving with ao-
celerated Telocity, in tne same time ; and as the square is equal
to the doubled triangle, so the former space would be double the
latter.

83. Hence likewise a body moTing uniformly, vnth half tiia
Telocity it would acquire at the end of any giTen time, would pass
through a space exactly equal to that which it would describe
moTing with accelerating Telocity during the same time. Ac-
cording to the preceding table, the Telocity of a body at the end
often seconds would be equal to twenty fiset; now half that Telo-
city, or ten feet in a second, would carry a body through one hui^
dred feet in ten seconds, which is precisely the space it would
baTO fallen through in that time, by the effect of graTitation.

83. Thus, the velocity acquired at the end of any given time
being sufficient to have carried a body twice the distance it would
reach with gradually accelerated Telocity, it follows that the Telo-
city actually expended in the latter case is only half the Telocity
that has been acquired ; and since the final Telocity in each se-
eond is represented by a number double that denoting the time,
the real amount of accelerating Telocity may be expressed by
a number equal to the time. Hence as the space fidlen throogh

With what velocity mast a body move uniformly, in order to dewribe
a given ipaoe in the same time. as when uniformly accelerated by gravi-
tation ?

■ How may the real amoant-of aceeleratinr velocity be expressed f "Bf
what product may the 9pac€ be represented ?

If mCHAKlCS.

bgr a gfaTitadng body is ecjual to the sjiuare of the time, that is the
numl^r representing the time multiplied by itself, so the time and
tiie velocity being equal, the space must be as the square of the
velociu*, or as the time multiplied hj the velocity.

84. We have already taken occasion to observe that the force
of gravitation varies at different distances from the centre of attrac-
tion; and hence the absolute effect of gravitative influence must
vary also. The consequence of this principle, a8( exemplified in
the augmentation or reduction of the weight of bodies in different
situations, has been pointed out. And since bodies in motion are
^ted on by gravitation in the same manner aa bodies at rest, it
follows that filing bodies will describe greater spaces in equal
times, according to the increased intensity of gravitation, as oeca*
•ioned by the diminution of the distance through which it acts.

85. In order therefore to discover by experimeni; the force of
gravitation, as measured by the space through which a body woula
fall, in a given time, as one second, we must know what is the
distance of the gravitating body from the centre of attraction. If,
as abpeady remarked, the earth were a ]>erfect sphere, every part
of its surmoe would be equidistant from its centre; but, since it is
an oblate spheroid, or globe flattened at the poles, the attraction
must there be strongest, and must decrease in the intensity of its
force, in the direction of a line from either of the poles to the
equator. Such a line would be a meridian of longitude, and the
deg[ree8 of latitude measured on it would be so many points at
which the intensity of gravitation was progressively diminishing.
, 86. Hence, in experiments made to ascertain directly the
amount of gravitative force as measured by the space a body
would fisill through in one second of time, regard must be had to
the latitude of the place where the experiment might be made,
and if the utmost accuracy were required, the height of the spot
above the level of the sea must also be taken into the account.
These observations will be sufficient to show that iio email degree
of skill and attention would be requisite in order to ensure the
perfect exactness of such experiments. Instead therefore of pup*
suing this train of investigation further at present, we shall pro*
eeed to state that numerous and very accurate experiments have
been made, whence it appears that in the latitude of London,
which is near the level of the sea, a heavy body falls, from the
action of gravity, in the first second of its descent, through the
space of sixteen feet and one inch, or 193 inches.

87. In making calculations relative to the phenomena of falling
bodies, when extreme accuracy is not required, the space passed

What will Mable us to difooTcr by experiment the force of grtvita*
tion ?

How does, the figure of the earth affect its force of attraction at the
difllerent parts of its surface ? Through what space will a body fall ia
Uie first second in the latitude of London?

What maj irenerallir be sisumed for the space deteribed in one tecond
If a body fiafing freely ?

RATS OF TSLOGITT <V TALLIKO BODIXf •

throtigh in one seeond of time ms^ be estimsted il 16 ftet; and
faking tiliis as the common multiple of distances ind yelocities, m
table similar to that already given may be constracted, by means
of which the spaces fiillen through in any ffiren time may be
ascertained with sufficient exactness. The following short speci-
men of such a table may be easily extended by the ^onng stiH
dent, so as to afford data for the resolution of seTeral interesting
questions.

•cei

of F««t pasted
*^ throagh at the end
of each second.

Final telocitj
in each second.

Feetnassed

thfoogn in each

■eeond.

1

16 .

33

16

2

64

64

48

3

... 144

96

80

4

. 356

. 128

. 113

5

.400

. 160

. 144

88. Suppose now we wish to discover the height of an emt«
nence, or the depth of a well ; by dropping a leadenbuUet from the
top of either, and observing how many sec#lids elapsed before il
reached the bottom, a table like the above would show by inspee*
tion how many feet the snace amounted to in eithei case. No
notice, however, is here taken of the resistance of the air, which
would ffreatly affect the motion of bodies falling from a consider*
able height. Several years ago a man droppea from the balcony
of the Monument, near London Bridge, a height of about 200 ieet:
be would therefore have fallen to the pavement below in nearlf
three seconds and a half, but for the resistance of the atmosphers)
notwithstanding which he must have been whirled downwards
with a velocity, which perhaps rendered the miserable being in*
aensible of the appalling catastrophe that awaited him. Sotio*
times aerolites have exfdoded in the air, and &llen in showers of
meteoric stones, as happened near Sienna, in Ital^, in 1794 $ and
at L'Aiffle, in France, in 1803. If the moment of such an explo^
slon coud be observed, and also that at which the stoncH, or any
pne of them, came to the ground, the height at which the pheao*
menon took place might be estimated with tolerable accuracy.

89. The obstacles which occur in the experimental investigation
of the laws of gravitation are partly owing to the very extensive
space that womd be required for direct experiments on falling
bodies, even for a few seconds; and to these would be added the
variable effect of atmospheric pressure against bodies moving with
ffireat velocity, llie consideration of these difficulties led Mr«
George Attwood, an ingenious philosopher who died in the early
part of the present century, to contrive a machine in which th^
influence of gravitative force mi^ht be moderated without destroy*
ing its chankcteristtc efficiency, tn Che production of aaacceleialed

*

How might the height of an exploding meteor be estimated ? •

What obstacles oecnr in the direct experimental investigation sf fbe
kari of likUing bodies ?

/

MSCRANICf.

ri

*inotioii. This pieee of machinery wss rerj elabo*
Tately constructed, and some parts of it could not be
correctly described without enteringf into extensire
details, and pying delineations en a large scale 9
but the principle on which it acted may be concise*
ly explained. Equal weights A and fi, being sus-
pended by a fine silken cord, passing over a wheei
moying with the least possible degree of friction :
then by adding a certain quantity to one of the
weights, as by placing on it a small bar C, de-
I scending motion may be produced, differing in in-
tensity from that caused by the uitrestrained power
of gravitation, but obeyingr the same law of accelerating velocity ;
so (hat, though the loaded weight might be made toilescend only
one inch in one second, its continued motion would be found to
proceed in the regular ratio of the squares of the times of descent.
•. 90. It might be imagined, that as the large weights counterba-
lance each other, the small bar ought to descend as freely as if
they were removed ; but the gravitating force expended in n;cH
ducing motion is pamy consumed in overcoming the inertia of Uie
large weigh s, and therefore the portion of it which acts as a mov-
ing power IK Ui bear the same proportion to the whole force, as
the weight of the bar alone bears to the entire moving mass, /or
it is expended in dra^ring down the loaded weight A on one side,
and raising the weight S on the other side, at the same time*
Thus if the weights were two pounds each, and the bar weigiied
9ut half a pound, the force expended would be but 6ne-nintii part
of the whole force ; and the loaded weight A would descend but
one-ninth part of sixteen feet in the first second of time, and with
tiie same leduoed velocity, as the squares of the times, throughout
its descent. By means of this machine a variety of most interesting
and important experiments may be performed, and the laws o?
gravitation satisfactorily demonstrated.

91. Bodies projected directly upwards will be influenced by
gravitation in their ascent as well as in their descent; but its force
must be calculated inversely, producing continually retarded mo-
tion while they are rising, and contlnuuly increasing motion dur-
hig their fell. So that a body propelled perpendicularly through
the air, leaving out of the question the resistance of the medium
through which it passed, would rise to a height exactly e^ual to
that nom which it must have fallen to acquire a final velocity the
same as it had at the first instant of its ascent.. And the Telocity
would be the same in the corresponding parts of the ascent and
descent. The time likewise which the propelled body required
to attain its utmost height would be just equal to that during

Beteribe the prineiple of Attirood*8 maehine. What portion of the
^vitating force of the bar added to one of his equal weights is employed
m producing motion ?

What laws til motion apply to bodies proieeted directlj upwards ?

What relation eilstt between the times of their ascent and descent *

MOTION OV BOMBS 01^ IKCLIKSD KJOffiS. tt

tp^icli it woTild be fidling* to tbe ntniiid. Hence the Isws wUek
regulate imiforml y accelerated Telocities will apply equally to mA»
fbniily retarded velocities : that is, the Teloei^ loet m any fflTett'
time, by the influence of a uniformly retarding force, vifl be W9
the time; the space passed through as the sonare of the time, or
the square of the Telocity; and so on, as in toe ease of accelenA*
faigforces.

Motion of BMu on in^ned Phawa and Curveg,

92. Among the yarietiea of aocelemted motioa depending on
the iniiuence of jgr^vitation, that of bodies passii^ along incfined
planes requiies to be noticed, as exhibiting the modified eflfect of
a most extensiyely acting force. When pfessnre is applied in a
vertical direction to a body supported by a horiaontai plane, it is
manifest that no motion can ensue; and the force of gravitatioB
thus acting can be measured only by the direct weight of ths
body so situated. But if the plane snr&ce on which the body;
rests be inclined in any degree, the efficient weight will be pnv
portionally diminished; ana if the inclination of me plane be sa^
ilcient to enable the body to overcome the resistance to its motion,
arising from friction and similar causes, the body will move dow»
the plane with a velocity so much the greater as the sprfiice ovei
which it moves approaches to a vertical direction. The motioa*
in this case wiU be a continually accelerated motion, diflfering ift
degree of relative velocity from that caused by the direct influencs
of gravitation, but subject to the same law of acceleration.

93. In order to estimate the force with which bodies are im»
pelled down inclined planes, we omit for the present all conside*
ration of the i^sistanoe occasioned by friction ; and therefore sup*
pose a plane to l^ave a perfectly smooth surface, and the figure of .
the moving body to be globular, and of the same density in ererf
part, so as to be capable of motion in any direction.

94. Let A C represent the declivity of
an inclined ^ane, A B its perpendicular
height, and D E the absolute weight of
an ivory ball on its surface; now this
weight, by the parallelogram of forces,
will be found to act in two directions ;
D F, or G E, denoting the direct ptes-
sure perpendicular to the declivity of th»
plane, and D G, or F E, in the direction
'^ of that declivity : the former force it is^

How can the foree of mTitntioii in a body pretsing a horizontal plane
be measttred ? What eneet on the pressure of the plane will result from
its becoming inclined f When will motion commence on the inclineil
plane ?

Of what nature will be the motion otct the inclined plane ?

What circumstances are we to omit in first estimating the foree of
motion on inclined planes ? DeftsHbe the diagraiti relating to this «ib-
jeoL • -

qfaftovi will be doatrojcd bj the Teeiitanee of the plane, aai the
botl will coneequently move down the plena with a force besrinff
the eeme relation to the foiee of gTavitj tbet D G does to D £,
that ia, it would more down the plane throogh a space eqaal to
G, while it would tail thioogh a epaoe equal to D E by the
face of gravilation.

95. Whaterer may be tiie declivitj or inclina*
lion of the plane, the forceof abod^moringdown
it maj be estimated on the asme pnociple. Thoa
BDppoae the obliquity of the plane to be very eon-
bidenble, as represented in the margin, the line
D G would be nearly equal to O E ; and the force

of thebody moTinD'on such apian
festly be little inferior to that of the eame body
, fiitling freely.

Aa the force of a body moTincf on an inclined

plane ie lesa than that of a body movii^ by the

)ll£uence of graTitalion,itB final Telocity in a given time most also
be Imsj end Uie distance through which it must more on a decli-
vity to acquire a certain final velocity mnit be greater than tiiat
ttroiigh which it mnat fall &eely by the effect of graxity to ac-
^re the aame Telocity,

96, It may be demonetiated that a
body moTing down any inclined plane
will acquire the same final veloci^, in
passing tVom A to C, that it would haTe
gained in felliog through the relatJTe
distance A B. For let A D be the
apace thiongh which the body would
Bore down the plane in the same time that it would fall from A
to B, it'followi that, in order to acquire the same Telocity Chat it
Would ^in by falling from A to B, it must pass through a space
bearing the same proportion to A B that A B does to A D ; and
as the triangles A D B and A B C are similar, their correspond-
ing sides mast have the same relaUmis to each other; therefore
AD will be to A B, as A B to A C. Hence the proposition will
vniTersally hold good, that a body rolling down an inclined plane
of any extent or obliquin, but for the effect of friction or similai
causes, woald acquire the eame final Telocity, as if it had fallen
directly through a space equal to the perpendicular height of the
•Bmmit of the plane.

97. Bodiea moTing on curved surfaces would not exhibit uni-
fimnly accelerated Telodty,]ike those moving on inclined planes}

What relation wiU the find vrlocity of ■ bodj moTini; on an inclined
^ue, bear to that whish it would aeqaira in falling perpendicularly
ttrodgh the aime dlMance?

What nJation vill the nloelty ofa body fillinK fnely, and of one de-
aaandtng an indincd plane, bear la thr length andlieight of the plane t

With what urt of Tcloeitiet will i body nore down ■ curved lurlaBel

Why woold tiot the loation be nairormly aeoderalcd ?

MOTION OF BODIES ON CVllTED BTTRrACBf. 5S

for the Tesistanoe occasioned by the peculiar form of tiie cnrre ia
which any such body mi^ht more would be continually changing,
and the result of that resistance wonld be a consequent change in
both the velocity and the direction of the moTing body. Some
idea of the nature of this perpetual chanse may be obtained from
eonaideringr what would be the effect of presenting to a moTing

body a succession of inclined planes, either ascending or descend*
ing, the outline of which would form a rude resemblance to a
curved surface. From the mere inspection of the preceding
figures, it may be comprehended that a body passing over a con-
vex surface, as from A to B, would encounter a perpetually dimi«
nishing resistance ; and in passing over a concave surface, as from
C to D, the resistance would progressively increase. For in the
former instance, the effect would be as if the moving body rolled
down a number of declivities, each one more oblique man the
preceding ; and in the latter, it would be as if the body passed
over a series of declivities, each of which approached nearer than
the preceding to the figure of a horizontal plane.

98. Having thus endeavoured to explain the manner in which
curvilinear motions are produced by the constant action of variable
forces, we can now proceed to investigate the phenomena of cur-
vilinear motions in general. When a body moves through an
entire circle, with uniform velocity, as it must be impelled by
forces continually varying in intensity and direction, those varia-
tions must be supposed to take place momentarily, or in incon-
ceivably minute portions of time and space. So that such a body
might be considered as moving in the circumference of a polygon
having an infinite number of sides.

99. In the case of a body moving over a curved sur&ce and in
contact with it, there must be a certain pressure of the body on
the surface over which it passes, and a corresponding resistance,
or pressure on the body, m every instant of its progress. Now
this pressure shows the degree offeree to which the continual va-
riation of direction, or deflection of the moving body is to be attri-
buted. Suppose a leaden bullet, or a billiard-ball to be made to
move round within a hoop laid fiat on a table or any level surface,
it would obviously presa against the inside of the hoop, thus ma-^

How are the foreet whieh impel a rerolving body rappoted to vary f

Into what fi|;are may we eonceive the eirele to be resolved ?

How. would a body moving wlthia a earva^ rav&ee be affeetad by itf

66 MECHANICS.

nifesting a constant tendency to escape from the circle in which
it was moving, and only witiiheld by the connterpressure, or re-
sistance of the hoon. If then the hoop were suddenly lifted wMle^
the bcdl was passed round within it, tne circnlsi motion wonld no
longer be continued; but the )>all would fly off in a right line
from the point where it was set at liberty. The force operating
on the moving body in this case would be precisely similar to that
which would propel forwards a stone discharged trom a sling, on
letting go the cord which retained it during Uie previous circular
motion or whirling, whence it would acquire its subsequent ve-
locity.

100. The forces which act on bodies revolving in circles or
other orbits may be regarded as antagonist powers, one of which
perpetually impels the moving body m a riffht line from the cen«
tre of motion, and the other draws it towards that centre ; and by
the joint action of these forces curvilinear motion is produced.
*nie former, or the repellant power, is named centrifugal force, or
force causin? bodies to fly from a centre ; and the latter is styled
centripetal force, or that which attracts moving bodies tovnirds the
centre of motion.

101. These opposing forces have also received the common ap*
pellation of central forces. It may be here observed that the line
in which a body will move, on escaping from the circle around
which it must n^ve been previously whirled, will always form a
tangent to that circle, or in other words, it will extend in a direc*
tion perpendicular to another line drawn from the centre of the
circle to the ^oint of escape. Hence this force has been sometimes
called a tangential force ; but its usual appellation is that of cen-
trifugal force.

102. These forces must necessarily differ in degree according
to circum8tances,---8uch as the mass of the moving body, the ex-
tent of the circle in which it may move, and the velocity of its
motion.

Thus a ball, B, of two pounds weight,
would require a greater centrifugal force to
make it revolve round the circle ,A» in any
given time, than another ball weighing only
one pound. The extent of a circle is to be
estimated by its radius, or the line C B,
passing from its centre to some point in its
circumference, and conseouently always
equal to half the diameter. Now the centrifugal force or pres*

What line would &ach a bodj describe, if soddenlj relieved from the
confinement of the ciirred sarfiice ?

How may we explain the motion Of a stone discharged from a aline?

What is meant by the terms centrifiig'altin^ ceiUnpeUd^ as applied to
forces } What common appellation is applied to them ? When a body
escapes from the influence of its centripetal force, what will be the line
of its subsequent path ? What is signified by the term tangential Jorce?

By what circumstances are central forces caused to vary their inten-
•i^ ) Exemplify the ftftDCiplea applicable to iblt variation.

CENTBtFVOAL MOTION. 57

sore must increase, as the radius of the eorre in which a hody
moves increases. In a circle the same radins will apply to eTerr
part ; but if a body should moye in any other enrre, as an el*
lipse, the depree of curvature, and ccmseouently the lengtii of
ihe radius, wSl difier in different parts. Hence the expresmon,
radius of curvature, has been usea to denote the line which may
be drawn from the centre of motion to any given point of the
curve described by a revolving body. The velocity of revolving
bodies may be estimated by the actual space passed through in a
given time, or by reference to the time m which any such body
would pass from one point in the circuit in which it moved to an-
other point. Tfaeso distances, being measured by the an^e form-
ed by lines drawn from the centre of motion to the points jnat
mentioned, the velocity indicated may be styled the angular ve-
locity of the moving body.

103. The amount of centrifu^l force in difl^nt circumstances
may be experimentally determined by means of a machine called
a whirling table, which is So constructed that different weighti
may be wnirled at any given distance from the centre of mo^on,
and with any required degrees of velocity; and the measure of
the centrifugal force expended is obtained by causing the revolving
weights, by their rotatory motion, to draw up other weidits, which
are suspended freely; and thus the effect of centrifugal force maybe
ascertained 'in a satisfactory manner. From the rMulta of experi
-ments with the whirling table, it appears, that the oentriragBd
force will increase as die mass of the moving body increases)
that the centrifugal force will be doubled, other circumstances ro-
•mainih^ the same, if the radius or curvature be doubled ; that if
•tiie radius of curvature remain the same, and llie angular velocity
be doubled, the centrifugal force will be (|uadrupled ; and that if
jounal masses be made to revolve withm ^nues, the radii of
.wnich are as 9 to 3, and with angular velocities as 1 to S, the
centrifugal force will be as S to 13, or as 1 to 6. Hence it ap-
pears that the centrifogal force increases in direct proportion to
-the mass of the moving body, and to the distance nrom tiie ceiw
tre of motion, and also as the square of the angular velocity.
Thus :-^the radius of the circle being 3— the angular velocity 1,
the square of which is 1— -the centritugal force will be the pro-
duct, 3x 1=»3 ; the radios of the circle beini^ 3— the angular ve-
locity 3, the square of which is 4— the centrifugal force will bo
the product, 3x43^13; thus, as above, the centrifugal force in
the aifferent cases would be as 3 to 13.

Whut is meant hj radius of ennratare ^

In how many ways may the velocity of a reToWing body be estimated?

What Is meant hj angular vebcUyf

What apparatus is employed to demonstrate the laws of eeatrifvgal
forces?

What relation hare these forees to the masses of the reToWinrbodiea?
What relation to the radius of cunratnre ? What to the ancidar Tela*
eity?

56 mcHAincs.

104. In order to obtain the amoant of centrifogal foree at aoy
g^yen point, the square of the number of feet exnressing the an*
gular velocity in one second of time must be diviaed by Sie num*
ber of feet denoting the radius of curvature, and the quotient will

give the centrifu^ force, as' estimated by ibe number of feet a
ody impelled by it would describe in one second. Thus, a sling,
two feet long, circling vertically, with the velocity of ei^t feet
esu^h second, would communicate to a stone a centrifogal force
equal to thirty-two feet in a second, which would be the final ve«
locity of a body falling during one second, and the centrifugal
force therefore would be just sufficient to counteract the influence
of gravitation, and enable the sling to support its load. If the
motion of the sling were accelerated so as to perform a complete
revolution in one second, the tension of the string would uphold
the stone with a force 2^ times greater than the attraction of gra*-
▼itation.

105. An amusing experiment, illustrative t)f the influence of
centrifugal force in overcoming that of gravitation, may be per*
formed by placing a tumbler filled with water, in a sling, or fiz»
ing it upright in the bottom of a net, when it may be whirled
round with such velocity that not a drop of the water will be
spilled, though the mouti^ of the glass will be turned downwards
during a part of each resolution.

106. The centrifugal force at the equator may be computed by
taking the time of one diurnal revolution=B86,164 seconds, tm
equatorial radius of the earth«30,931,i8S feet, and the ratio of
the earth's circumference to ito diametei^3.141d9;l. ThoA
4x3.141593x20,931,185-r-86,1642«0.1,112,259, which is the
centrifugal force at the equator. Now as the actual force of roh
vitation, determined by experimente, the nature of which will be
subsequently described, is, 32.08818 ; and therefore, it the eartk
were at rest, it would be 32.08818+0.1,112;259»32.1,994,059,
it follows that the centrifu^ force at the equator is to the force
of gravity in the proportion of the numbers 0.1,112,259 to
32.1,994,059, or nearly as 1 to 289. So that the force of gravita-
tion is 289 times ffreater than the centrifugal force, at those parte
of the earth's surrace where the action of ^e latter is most pow-
erful.

107. Now sinde 289 is ^ square of 17, it will follow that if
the diurnal revolution of the earth had been completed in one-
seventeenth part of the time, which it now takes up ; that is, had

Ho«r may we obtain the amount of centrifagal foree at any given point }

How may we compare centrifugal force with that of gravitation ?

How may it be familiarly shown, that this foroe it mien aupeHor to
that of gravitation ?

How may we compute the centrifugal foree of the earth at the eqoator ?

What is the actual foree of gravitation there, as determined by experi-
ment ? What is the amount of centrifugal foree, and by how many timet
does Uie former exceed the latter ?

How much must the velocity of the earth's revolatioD be inereated, ki
•rder that bodies at the equator should lose all their weight?

▼niULTIOM or PSMP17L173I8. 09

tile earth revolired on her axis m eiffhty-four minatesy instead of
nearly twenty*fbar hoars, the ce^trifagal force would hare coun-
teracted that of grantation, and all hodies would haye heen abso*
lutely destitute of weight ; and if the centrifugal force were further
augmented, the earth reyolying in less time than eighty-four
minuteSi grayitation would he completely oyerpowere^ and all
fluids and loose 8ubstance9 near the equinoctial une would fly off
from' the surface.

108. Among the abundant examples of the effects of centrifu-
pi forces that might easily be adduced, a few may here be no-
ticed, in addition to those already giyen. The astonishing power
of tfais force, eyen when exerted on a small scale, appears from
its destructiye influence on hard solid bodies $ as when grindstones
are whirled about with extraordinary yelociw in our manufacto-
ries, they will sometimes split, and pieces fly off witii amaxing
force. The moire regulated, but no less powerful operation of
centrifugal force may be obseryed in some parts of the machinery
employe in certain branches of the arts: as in the fly-wheel
which regulates the motion of a steam-engine, and in the coininip

gross ; but these and other modifications of mechanical power wul
e noticed elsewhere. Semifluid and soft but tenacious sub-
stances, under the influence of centrifugal force, assume in a
greater or less degree the form of a compressed globe ; and thus
a rudely-shaped ball of clay, placed on a potter's wheel, with the
assistance of gentle pressure while in the state of reyolution, gradu-
ally acquires a symmetrical form; and globular glass yessels owe
their figure to the analogous manipulations of the glass-blower.
Liquids exposed to a wttirling motion are similarly affected ; as
may be perceiyed if a glass of water be 8U8])ended by thieadsy
and made to turn with great yelocity by the twisting and untwist-
ing of the threads, when the water would sink in we c^itre, and
rise on the sides so as to escape in part oyer the edge of the glass.
In all cases centrifugal force tends to make bodies under its influ-
ence recede from a central point, and when it acts in conjunction
with a centripetal force, the effect will be reyolying motion,
whether those powers be exerted in keeping a peg-top, or a tee-
totum spinning on a floor or table, for h few minutes ; or in caus-
ing the yast globe which we inhabit to reyolye with undiminished
energy through countless ages.

OseiilaHtm of the Penduhm,

109. Oscillation or yibration is a peculiar kind of curyilinear
motion, depending on tiie influence of ^yitative attraction, and
it not only affords the means for ascertaining the yariation of the
force of grayitation in different latitudes, but likewise furnishes

GiTc some examples of the effects obserTed to result from eentrifogal
foree.
Whstis meant bj oscillation ?
To what purposes in science and arts ia it appUeable i

60 MECRAmCfl.

&e in69t aeeorate method for measuring time, and leads to Taxi*
ous important results in the investigation of manjr natural pheno-
mena.

110. When any heayy hod^ is suspended hy a strin|r or small
wire, it will take a direction m a line vertical to that point of the
earth's surface oyer which it hangs, as in the case of the plumb-line
ot a mason's level when placed on a horizonlal plane. Now the
laws of oscillation are those which would regulate the motion of a
body thus suspended, if drawn aside from the yertical line in which
it would rest, and then let go and suffered to oscillate or swing for*
wards and backwards undisturbed. In treating this subject it will
be most conyenient to consider the phenomena of oscillatory mo*
tion simply and independently of the effects of tiie resistance of
the air, ihe friction of the suspending line on the point of suspeiH
sion, and the varying extension of that line ; all which it is obvious
would affect the results of actual experiments, and would therefore
require attention in making calculations founded on them.

111. Suppose A B to represent a pendu-
lum at rest m the vertical position, if it be
then drawn from B to C and let fall, it will
return to B, with an accelerated motion,
which, however, will not be tmifonnly ao->
celerated, since it must depend, partly on
the gravitation of the pendulum towards
the earth, which acting alone would cause
it to fell perpendiGularT3^ from the point C,
but which being modified by the tension of
B the line, it is hrced to describe the arc C

B. Now at B the direct power of gravitation will be not merely
modified but destroyed, for the line being stretched to its full
extent would prevent any further descending motion; but when
arrived at B, Uie pendulum would have acquired a certain degree
of velocity during its previous descent, which would be just suffi«
cient to overcome the force of gravity tending to retain it at the
point B, and ixake it move forward from that point to D, with a
retarding velocity, which would there be Entirely expended ; and
since the pendulum at D would be in a situation exactly correspond-
ing wiUi that in which it was placed at C, it must again describe
the same arc D B C, but in a retrograde direction, first with a
gradually accelerated veloci^, and then with a velocity progres-
sively retarded. 'Rms, but tor the obstacles already mentioned,
and the wear and tear of materials, a pendulum, once put in a
state of vibration, would go on regularly oscillating for ever.

112. Tlie vibrations of any one pendulum will be described in
equal times whatever be the extent of the arc through which it
moves, provided that arc do not exceed a certain limit.

What eircumitanees affect the results of experiments oo oscillation f
What^ forces combine to produce oscillatory motion ? What causes
the asccndiog part of an oscillation i

KATVRE OF OSCILLATORY MOTION.

61

Thus when the vibration of a pendolnm
is progressiyely weakened by the resist*
ance of the air, every sacceeding arc
passed through will be less than the fore-
going; and yet it will be found that though
uie pendulum moves slower and slower
continually, there will be but little diC"
ference in the time taken up by the ball
in moving from 5 to 5, 4 to 4, ^., on
each side of the line A B, till it stops
entirely. It is this remarkable property
of the pendulum that makes it so useful
as a measure of time ; and clocks, or time-keepers, regulated by a
pendulum, are nothing more than trains of wheel-work kept in
motion by weights, and so arranged as to register the beats of
pendulums which oscillate seconds. This equality of vibration
of bodies in certain curves was discovered by Galileo, whose
attention is said to have been excited by remarking the motion of
a chandelier hanging from the ceiling of a church at Pisa ; for,
noticing that it moved with uniformity as to time, independent of
the space passed through, he was induced to make experiments,
which established what has been termed the law of Isochronism,
or equality of time.*

11 3. As it is only when oscillating in very small arcs of circles that
pendulums preserve this regularity of vibration, it became a sub-
ject of inqmry among philosophers whether a curve could not be
found in which the isoohronism of a pendulum would be perfect;
and such a curve was discovered by the celebrated Dutch mathe-
matician, Huy^ens, the contemporary of Newton. It has been
named a cycloid,f and from its property an isochronal curve, and
it differs little from an arc of a circle, except in rising somewhat
more abmptly at each extremity. But it is the less necessary to
enter into any further description of its nature and properties, as
it has been found after all to be less adapted for practical purposes
than small circular arcs, in which therefore the pendulums of time-
keepers 2xe made to oscillate.

114. The vibrating weight of a pendulum does not influence its
motion ^ for whether a great or a small weight be affixed to a
vibrating line, its oscillations will be similar, provided the length
of the line, measured froln the point of suspension to the centre
of oscillation, remains the same. Sir Isaac Newton made experi-

fl

What is meant by the isochrontsm of oscillations ? By whom was this
character discovered ?

In what form of curve must oscillations be performed, in order to be
isochronnus ?

What influence hat the weight of a pendulum on the time of its Qseil«
lation ?

* From the Greek i^o^, equal^ and Xpovo(, time,
t From |he Greek x»K^os, a circle, and Et^sf , a resemblance.

F

^ MECHANICS.

ments on a great yariety of substances, as metals, stones, woods,
salts, portions of flesh, &c., whence he ascertained that how
greatly soever they might differ in weight, the addition of anjr of
5iem to a pendalum would not interfere with its rate of oscillation,
so long aa its len^h remained unaltered. Thus, as heavy bodies
and light ones would fall to the earth, through a given space, in
the same time, but for the resistance of the air, so they would be
found to vibrate in equal times at the end of a line of a given
length, provided atmospherical resistance could be made to act on
them in the same manner, or be entirely excluded, as by inclosing
the vibrating bodies in an exhausted receiver.

115. It is on the length of the pendulum that the rate of oscilla-
tion principally depends ; that is, the greater the distance between
the point of suspension and the point ^ oscillation, the longer will
be the period of each vibration ; and on the contrary, the shorter
that distance, the quicker will the vibrations take place. Now,
as gravitation is the power on which oscillatory motion (J^pcnds,
So the same law that regulates its operation on falling bodies is
observable in its action on oscillating bodies : for as the intensity
of gravitative force decreases as the squares of the increasing dis-
tances of bodies, thus the time of a vibration will increase as the
square root of the length of the pendulum, or the distance from
the point of suspension to the point of oscillation, increases. If
then a pendulum 1 yard in length, would make one vibration in
one second, a pendulum i of a yard long would vibrate half
seconds, one 4 yards long, would vibrate once in two seconds,
one 9 yards long, in three seconds, and so on ; for i is the square
root of i, 2 of 4, 3 of 9, &c.

116. But in order to obtain the absolute length of a pendulum
^at would swing seconds, it is necessary to take into considera-
tion the intensity of gravitation, which, as already stated, varies
at different parts of the earth^s surface, depending on their rela-
tive distance from the centre of gravitative attraction. The
greater the intensity of gravitation at any place, so much the
quicker will be the vibrations of a pendulum of a ffiven length :
so that a pendulum which would oscillate seconds at Loq4on would
perform each of its oscillations in somewhat less than a second, if
It could be removed to the north pole ; and on the contrary, would
take up more than a second in one vibration under the equinoctial
line.

117. The intensity of gravitation at any given point of the
earth's surface thus corresponding with the vibrations of a pendu-
lum of a given length, it follows that if the intensity of gravita-

Whftt resemblance in this respect has the pendolam to bodies falling
freely ?

On what circnmstance in a pendulum does the time of its oscillations
depend ? Between wliat two points is the true length of a pendulum to
be taken ? Illustrate the law of its motion by an example.

What local circumstance must be taken into view in obtaining the ab-
solute length of a pendulum ?

RELATIVE LEINOTHS OF PENDULUMS. 6S

tion at any place^ as estimated by the space which a body falling
freely would describe in any time, as one second, be known, the
length of a pendulum, which would yibrate seconds at that place,
may be ascertained by computation. For since the time of vibra-
tion is to the time of descent through half the length of the pendu-
lum, as the circumference of a circle to its diameter, that is, as
3.14159 to If .let the time of vibration be 1 second, then the length
of the pendulum may be thus found : the time of descent of a
body during 1 second, in the latitude of London, by the influence
of gravitation, has been already stated to be about 16 1-12 feet, or 1 93
inches ; and since the spaces of descent axe as the squares of the
times, therefore 3.141692 : l^ : : 193 : 19.0625^19 1-1 6=half the
length of the pendulum, which must therefore be 19 l-16x2sB39i
inches.

118. In order to determine the length of a second's pendulum
by experiment, a pendulum of a known length must be made to
oscillate for a certain time, as one hour; then the square root of
its length will be to the square root of the length of the required
pendulum, inversely, as tiie number of vibrations performed in
an hour, by the pendulum which has been the subject of the
experiment, to the number of seconds in one hour. Thus, if in
any latitude it could be ascertained that a pendulum 9 yards in
length oscillated 1200 times in an hour, then as the number of
oscillations, 1200, to the Square root of the pendulum, 3, the
square root of 9, so inversely would 3600, the number of oscilla-
tions required to be performed by the seconds pendulum, be to
the square root of its length : that is, as 3600 : 3 : : 1200 : 1 ;
since 1200 x 3-^-3600 »1, the square of which would be 1;
therefore a pendulum 1 yard long would swing seconds in any
place where a pendulum 9 yards in length would make but 1200
vibrations in an hour.

119. It will be obvious, from what has been already stated,
relative to the effect of friction, atmospheric resistance, and the
extensibility of the line of suspension of a pendulum, that a mul-
titude of precautions would be requisite in making direct experi-
ments on the lengths of pendulums, with reference to the times of
vibration at any given place. Dr. Halley, in the early part of
the last century, estimated the length of a second's pendulum at
39.125 inches, :=> 39i inches ; and that estimate has been general-
ly adopted, as sufficiently correct for practical purposes. From
uie most recent and accurate researches of men of science, it
appears that the length of a pendulum which oscillates seconds,
in vaeuo^ at the mean temperature of 62 degrees of Fahrenheit's
thermometer, in the latitude of London, 51** 31' 8" N., must bo
39.13929 inches : and as the further result of experimental inves*

By whal proportion may we find the length of a secoDd^s pendulam,.
when wc know the intensity of gravitation ?

How may that length be ascertained by experiment ?

State some of the lengths actually found neceasarv in different parts
of the eartli, ia order to produee the same number oJT beats per hour.

64 MECHANICS. •

ligation, it may be added that at MelviHe Island, in the Polar Sea,
Lat. 74° 47' 13" N., the length must be 39.207 inches ; at the
Galapagos Islands, Lat. 32^ N., 39.01719 inches; and at Rio
Janeiro, Lat. 22° 55' S., 39.01206 inches.*

120. As the force or intensity of gravitation decreases as the
distance from the earth's centre increases, it follows that a pendu-
lum, which would oscillate seconds at the bottom oPa mountain
one mile in perpendicular height, would not perform so many
complete oscillations as there are seconds in an hour, if removed
to the top of the mountain. Suppose the radius of the earth's
circumference to be 4000 miles, as a second's pendulum would at
that distance from the centre of attraction vibrate 3600 times in
an hour, and therefore 86400 = 3600 X 24 in a day, it follows that
it would lose the 4000th part of 86400 seconds in a day, at the
distance of 4001 miles from the earth's centre. Now 86400-r-
4000 = 21.6, that is, the loss would be 21.6 seconds in a day.

121. The length of a pendulum vibrating seconds being known,
that of one which will vibrate half seconds, like those in most
table clocks, or any other portion of time, may be readily calcu-
lated. For the times of vibration being as the square roots of the
length of the pendulum, hence, as one second to 6.255, the square
root of 39.125, so will half a second be to the square root of the
pendulum required ; that is as 1 : 6.255 : : 0.5 : 3.1275, the square
of which will be 9.78. But the length 9f the half seconds, or any
other pendulum, may be also found by taking the squares of the
times, which will be directly as the lengths of the pendulums ;
thus a8(l = 12)sec. : 39.125 : :(.25 = J'^^ec,: 9.78, as before, or
9.7 inches, the length of a half second's pendulum.

122. A. pendulum may be so constructed as to have its centre
of oscillation far beyond the limits of its actual dimensions ; and

thus a pendulum only one foot in length, may be
made to oscillate ^as slowly as another 12 feet
long. Suppose a rod of iron, A B, to be loaded
at both ends, and suspended at C, so that it
jc . might vibrate freely, it is manifest that though
the arc described in each vibration would be
limited by the length measured from the point
of suspension, the velocity of the ball B, would
be checked by the counterweight of the ball A,
and the latter being moveable on the rod, the
rate of vibration might be regulated at pleasure,

Wimt would be the effect on the rate of a clock, of carrying it to tlie
top of a high mountain ? Why ?

How may we calculate the true length of pendulums to vibrate in
Other tiroes than aeconda^ when that of the latter is known ?

How mav the centre of oscillation be carried beyond the limits of a
pendulum r Will this increase or diminish the number of its oscilla-
tions in a given time ?

• See Abstracts of Papers printed in the Philosophical Transacti')n8,
irom 1800 to 1830, vol. ii. p. 144, and p. 194.

^

CEKTRS OF OIULVITT. 6i

An instniment of this kind, called a Metronome, is ased to marki
by it9 oscillations, the time in perfonning pieces of music.

123. A rod of uniform diilaensions might be made to vibrate as
a pendulum, without any ball or appenda^ whateyer ; but in that
case the centre of oscillation would be raised, and such a pendu*
lum must consequently be longer than one of the usual form. In
a uniformly shaped rod or bar suspended at one extremity so that
it might vibrate freely, the centre of oscillation would be at two-
thirds of the distance between the point of suspension and the
other extremity of the rod. Force applied at that part to arrest
the motion of the rod would take complete efil^t, but at any other
part a stroke would cause a tremour or irre^lar action of the
moving body. Hence this point has been called the centre of
percussion. In using a weapon of considerable length and nearly
the same size throughout, as a cudgel or a sabre, the most effective
stroke would be when the point of impact coincided with the cen-
tre of percussion ; the situation of wnich must be at about two-
thirds of the length of the weapon, its exact place depending
chiefly on the relative weight of that extremity with which the
blow is inflicted.

Centre of Gramty.

124. In every body or mass oi matter at rest, there must be a
certain point, in the direction of which, any force acting parallel
to the surface on which the body is placed, will either be resisted
by the weight and friction of the mass, so as to produce no effect,
or if it be su&ciently powerful to overcome the resistance, the
body will move in the direction of the force applied ; but the same
force acting against any part of the surface of the mass, not hori-
zontally nor perpendicularly opposite to the point already men-
tioned, may cause the body to vibrate or be overturned, according
to circumstances. This point is commonly called the centre of
gravity, and sometimes the centre of inertia ; and from the pro-
perty just stated it might be termed the point of greatest resist-
ance.

125. The annexed fi^re will serve to exemplify the phenome-
non now described. Let A be the centre of gravity of a solid

bodj with a hemispherical oase resting on a
horizontal plane, then if pressure be applied
vertically at E, it is manifest that it can pro-
duce no motion ; but if applied at B, directly
opposite to the centre of gravity, its effect will
depend on the de^ee of rorce, as a small force
will be destroyed by the inertia of the solid

What substitute might be employed as a peodulam instead of the rod
and balls ?

Where would the point of oscillation of such a pendalam be found ?

When such a pendulum is to be suddenly arrested, where must the re*
sistanoe be applied ? Describe and illustrate the centre of percussion ?

What is meant by the terms " centre of gravity," centre of inertia,**
aod '* point of greatest retistaaoe,** when applied to bodies ? Whatitatai

r 3

60 MECHANICS.

mass, while a great force will be partly employed in counteracting
the inertia, and partly in propelling the mass steadily along the
level plane. Now if force be applied at C or D, or any other
point above or below B, it will have some effect, however incon-
siderable, causing the body to rock or vibrate, if the force be
small, and to be overturned if the force be great.

126. The centre of gravity in all bodies is that point at which
the influence of gravitation seems to be concentrated ; and hence,
in any 'body, unless that point be supported, motion will tike
place, and be continued till the body settles in a position in which
the centre of gravity cannot sink lower. Therelbre when no ob-
stacle is opposed to the motion of a body, either by its peculiar
figure or that of the surface beneath, it will always take such a
position that a line drawn from the centre of gravity to the point
where the body comes in contact with the surface below it will
be the shortest that can be drawn from the centre to any part of
its superfices. Thus an oviform body, placed as in the annexed
figure, would not stand in the position represented, but would turn
till the shorter line, A C, became perpendicular to the supporting
surface, instead of the longer line A B.

127. If a, body be supported from above, that
is, if it be suspended from a fixed point, hang-
ing freely, the centre of gravity will always
settle in a vertical line beneatn the point of
suspension. *

128. The exact situation of the centre of
gravity must depend partly on the figure and

partly on the uniform or varying density of the whole mass of any
Dody. Suppose a body to be of uniform density throughout, its
centre of gravity may be experimentally ascertained by balancing
it on the edge of a square table, in two positions, when the lines
of equilibrium will interl^ect each otheT at a point over the centra
of gravity, which manifestly must be in the centre of the mass.
A body of small dimensions may be more accurately balanced on
the edge of a knife ; or if the body can be conveniently suspended,
and a plumb-line let fall from the point of suspension, its direc-
tion being traced from two such pomts, will be found to intersect
each other as before, at a point on the superfices which will indi*
cate the situation of the centre of grravity. If a body varied in its
density in different parts, and possessed considerable thickness in
proportion to its length and breadth, holes bored through the mass
in directions vertical to difierent points of suspension, would meet
at the centre of gravity of such a body.

of bodies result respectively from the sapport, and from the want of
■apport to their centres of gravity ?

Of what comparative length will a line be found, between the centre of
gravity of a body and the point of its superfices on which it rests?

What relative positions will be found between the centre of gravity
of a body and its point of suspension }

On what two circumstances must the position of the centre of gravilgr
depend ? How can its sitoatioti be meehanteally determined f

CENTRE OF oEAvmr. 67

139. When the density of a body is nnifonn and its figfnre rega*
lar, the centre of gravity ^11 be the central point of the mass ; as
in a glbbe, an elliptical or oyiform spheroid, or a parallelepiped.
The surface of a triangle, its three sides, and its angular points,
will all have the same centre of gravity, situated at two-thirds of
the length of a riffht line passinff from the vertex of the triangle
to the middle of the base line. The centre of gravity of a cone
will be at three-fourths of the len^ of its axis ; and that of a

hemispherical solid at five-eights of the radius.
A p3rramid and its four terminating points will
have the same centre of gravity. The figure of
a body may be such that the centre of gravity
will not be included within the mass. Thus a
hollow cone, as a common extinguisher, or any
body of similar shape,«would obviously have
its centre of gravity in the void space within
it ; and so womd a basinnshaped body or hollow
hemisphere. A piece of wire twisted into the form of a hoise-
ahoe, or of a hoop, would also have its centre of gravity, not in
the wire, but in the open space within it.

130. The manner in which the centre of gravity of a body,
when unsupported, tends towards the lowest point it can reach,
may be illustrated by an amusing experiment, made with a piece
of wood or any suitable substance turned in the shape of a double
cone united at the base, then if a jointed two-foot rule be opened
a little way, and raised at the opei\ end, so as to form a sort of

inclined plane, the piece of wood on
being placed at the bottom of the
plane, will roll along to the raised
extremity of the rule, seeming to
ascend the inclined plane, passing
as in the annexed figure, from A to B. This is, however, merelv
an op^cal deception, for the centre of the double cone, which
must be its centre of gravity, really sinks lower and lower be-
tween the sides of the rule as it advances to the open end.

131. A somewhat similar experiment with an inclined plane,
serves to show the effect of the different distribution of density or
weight, in different parts of the moving body. Suppose a cylinder

to be made of light wood or cork, and to have a
plu&r of lead passed through it from end to end,
so Uiat its centre of gravity would be near its
surface : if then it were placed on a moderately
inclined plane with the loaded side towards the ascent, it would ne-
cessarily turn till that side rested on the plane ; but it could plainly
move no further, unless replaced, as in the marginal figure.

Where will it be found in bodies of uniform density and regular fignre ?

How can you ascertain the centre of gravity in a triangle f a cone ?
a hemisphere ? a pyramid ? Does the centre of gravity necessarily fall
within tne mass of every figure ?

Wh^t experiment illustrates the descent of the centre of gravity when
wimpported? What one esbihits the inflnenee of distribution of density/

68 KXCHAKICR.

133. Tlienecessitjof iapporting the evaUe otgn-vitj io erirj
aituBtioD appcBTB fiom the manner in which we move m the act
of rising from a seat. When a person is ftittiog the centre of
gravity of the body will be Bupported by the seat, from which it
will be impossible to rise without bending the body forward ea
as to bring the centre of gravity over the feet, previouily to as-
aaming the erect position ; or else iifUng the body by reatin? the
handa on the back or sides of the seat or some other point orsup-
port The ntter incapability of locomotion that taiies place when
an animal is so situated that it cannot by its own efforts raise the
centre of gravity of its body, is strongly exemplified in the case
of a fat sheep, or ewe with lamb, which has been so unlucky as
to lie down on the border of a shallow ditch or trench in a field,
and roll over on iia back into the hollow, where, in spite of its
ntmost efforts, it would lie with its feet in the air till it perished
with hunger, if not assisted to rise. A toitoJBe thrown on its
back affords another striking example of the same kind ; in this
manner sea turtle are captured on shore.

133. From what has been staled it is evident that the stability
of a body must be increased by lowering its centre of graviW,
A cylindrical vessel A B, suspended by a handle
tuminr on pivots fixed near the bottom, would in-
evitably overset when empty, as the centre of gravity
C would then be above the points of auspension ;
but if a very heavy substance as quicksilver, or
steel-tilings, wpre poured iato it, so as to fill it to
the line D E, the centre of gravity would be re-
duced to F, and the vessel might be suspended with
safety. Hence it may be perceived why vans and stage-coaches,
if heavily loaded at Uie top, will be very liable to be overturned,
while a similar or greater weight placed tow down will prove a

constructed, vridt receptacles for heavy
luggige nndei the bodies of the vehi-
cles.

134, "Die effect of placing tha

' centre of ^vity of a body in a Tery

low situation is shown in vibrating
fignres, such as that represented in
the margin, and other toys for the

; amusement of children, lornied on

' similar principles. 'Hius likewise a

long stick or ruler, placed loosely on
a Ifencb or table, with more than half

its length projecting beyond the edge of the board, may be Biade

HowinhepDiUionof eenlreof grarily iUuWnited io Ihe manner of
niing from ■ «« > How ii jta impartanee (hoitn in the paiitioni of
'aainili ? Wh«t effeot on die tiability of ■ body hu the depreMian of
Iti sentrc of |r*T>(7 -' Whst fkmiliir ipplimti<mi eso b< • ^^•^y^ >

OF SUPPORTING THE CENTRE OF GRATITT.

69

to support a backet of water or a half hundred weight suspended

on it. The manner in which this is ef-
fected will be easily comprehended from
the annexed figure, in which let A B
represent the stick, which must have a
notch or noose at the end B, against which
rests another stick or prop, and the handle
of the backet bein^ suspended by a string from the first stick, the
prop pressing against the string at its junction with the handle
C, fixes the bucket in such a position that the greater part of its
weight and consequently the centre of gravity of the whole ap-
paratus is supported by the table ; and Qierefore, so long as the
parts remain connected, the equilibrium will be presenred, for the
end of the stick B cannot be depressed without raising the centre
of gravity. A common tobacco-pipe, in the same manner, may be
miaSe to sustain any weight short of that which woald completely
crush it.

135. As a body of any kind cannot retain its position unless its
centre-of gravity be supported, it follows that stability may be
preserved so long as a line directed from that centre vertically
towards the surface below falls within the polygon formed by the
base of the body in question. Hence the broader the base of any
body the more securely will it stand ; and on the contrary when
the base is extremely narrow a body will easily be thrown down.
If a portion of any mass overhangs its base, it may still remain
standing so long as the vertical Tine from the centre of gravity
fells within the base. Thus a colunm, an obelisk, or a steeple

might incline somewhat from the perpendicu-
lar, and yet stand firm. From the inspection
of the annexed figures it wUl appear that tho
inclination of a column might be greater
than is represented in the first figure, where
the line A B &II3 within the base, without
endangering the stability of the body ; but
it must be less than that in the second figure,
where the corresponding line C D fells with-
out the base.

136. Most very lofly buildings swerve in
some degree from the perpendicular after a time, yet there can be
no hazard of their destruction if properly erected. The monument
built by Sir Cluristopher Wren, near London Bridge, to comme-
morate the great fire in 1666, and the elevated spire of Salisbury
Cathedral, have both become slightly inclined, but they will pro-
bably long remain to afford standing evidence of the consummate
skill of their respective founders. Travellers have frequently no-
How do vibrating figures exemplify the position of eeotres of gra-
vity ?

What advantage does an extended base afford for preserving the sta-
bility of bodies r What examples prove that leaning bodies may some-
times have a stable position ?

70 XECHAKICfi.

ticed the leaning towers of Bologna and Pisa, especially the latter
which is one hundred and thirty feet high, and inclines so mucl\
that the summit overhangs the base fifteen or sixteen feet ; yet
the line of direction from the centre of gravity dropping within
the base, the structure has continued to stand or rather to lean for
some centuries, and will probably endure centuries longer.

137. A change of the position of a body which leaves its centre
of gravity unsupported, must necessarily destroy its stability.
Hence a high carriage is liable to be overset when one side is
raised more than the other by the wheels passing over a bank or
by the sloping direction of the road ; and an over-freighted boat
may be capsized somewhat in the same manner, by a sudden lurch
throwing the weight on one side. Such an accident may like-
wise happen in consequence of a person incautiously rising when
a boat inclines to one side, the situation of the centre of gravity
being thus altered, so as to swamp or upset the boat.

138. The impossibility of preserving any position without
keeping the line of direction of the centre of gravity within what
may be termed the area of stability, or polygonal surface by
which the body is supported, may be experimentally illustrated
by observing the effect of placing a person to stand with his heels
close together and in contact with a perpendicular wall ; for with
such a position of the feet it would be found that he was unable
to stoop sufficiently to touch t^e floor with one hand. The act of
Stooping is performed by bendin? the lower part of the body
backward while the upper part is inclined forward, and thus
though the situation of the centre of gravity is lowered, its line of
direction still falls vertically between the feet. Now a person
with his heels and of course his back also against a perpenaicular
wall could not possibly bend backward, and in attempting to lean
forward he would inevitably lose his balance and fall down. So
that one might scatter a handful of silver or gold on the floor be-
fore a person stationed as just described, and offer him all that he
could pick up, while he kept his feet unmoved, without the
slightest risk of losing one's money. For the sake of any one
who might choose to try the experiment it should be remarked
that the terms specified must be strictly adhered to ; for if the
heels are raised so that the body is supported by the toes, it will
no longer be impossible to stoop sufficiently to touch the floor
without falling: the requisite condition therefore should be that
the heels must remain in contact with both the wall and the floor.

139. A body will remain at rest, or in the state of equilibrium
only in two cases, namely, when the centre of gravity is either
as near as possible to the point of support, or as far from it ae pos-

What facts prove Ihe importance of pireserTing the line of directicm
of a body within its base ? What is meant by polygonal surface or area
of stability? What experiment shows the applicatioD of the priociplea
of stability to the haman body ?

Under what two circumstances can the equilibrium of a body be pre*
•erved ? ,

FEATS OF DEXTERITY. 71

sible. In the fonner case, the stability of the bo4y will be secure ;
in the latter, extremely insecure ; thus a heavy elliptical solid laid
len^hwise would require a considerable force to remove it from
its place, but poised endwise the slightest impulse would cause it
to roll over. When the centre of gravity is at the lowest point,
a body is said to be in a state of stable equilibrium; and wnen it
is at the highest pomtj in the state of instable equilibrium.

140. Many feats of dexterity, as walkin? on stilts, dancin? on
the tight rope, standing on a slack wire, and balancing bodies either
in motion or at rest, depend cliiefly on the power of maintaining
the state of instable equilibrium. Walking on stilts, sometimes
practised by school-boys, as an amusement, is adopted as a mat-
ter of convenience by the shepherds in a district called the Landes,
in the south-western part of France. The country there being a
sandy level sometimes covered with water, the shepherds on
leaving home take their lofty stilts, and may be often seen strid*
ing along, on their artificial supports, at an immense rate. The
art of rope-dancing is facilitated by holding in the hands a lonff
pole in a transverse direction; for a trifling elevation of one end
of the pole and consequent depression of the other may be made
at any time, to prevent the lateral deviation of the centre of gra-
vity from its proper position vertically above the rope. Standing
or walking on the slack wire appears to be a more arduous feat
than moving on the tisht rope ; yet it is practised merely by keep-
ing tlie arms extended to preserve the equilibrium ; and some-
times in that attitude the performer will make a further display
of skill, by balancing bodies, one above another, on his chin.
Occasionally an exhibition of dexterity on the slzck wire is made
to appear more difficult, by the performer having handed to him a
chair, and a small table which he fixes ^across the wire, by rest-

< ing on it the rails which connect the legs of the chair and of the
table, then seating himself in the chair and placing his feet above
the front rail of uie table, he keeps the whole accurately equi-
poised even when the wire is made to swing frcm side to side.
But though this feat has a more imposing effect than standing
alone on the wire, there is no doubt that it may be performed with
greater facility ; for the table, and in a less degree the chair also
serve, like the pole in the hands of the rope-dancer, to assist in
maintaining the centre of gravity in its proper place.

141. These feats, curious as they are, appear much less won-
derful than the exhibitions described by some ancient writers
of respectability , in which elephants are represented as walking
on a tight rope. The difficulty of preserving the centre of gravity
of so unwieldly an animal, moving on such a narrow line seems

How do we distinguish the two states of stable and unstable eqnili-
briara ? What feats of dexterity refer to the conditions of equiUbrium
for their explanation ?

What remarkable feats are related to hare been executed on tiie same
principle by quadrupeds ?

72 UECHANICS.

nearly to approach impossibility; but the evidence of the fact ap-
pears to be deserving of credit.*

Mechanic Powers,

142. Nature presents to onr notice force capable of producing^
motion, under vanous modifications. The weight of solid bodies,
the impulse of flowing water, the pressure of currents of air, the
muscular exertions of men or brute animals afford familiar exam*
pies of different kinds of forces or means of originating motion ;
and it is the peculiar province of mechanical science to supply
rules for the accumulation, distribution, application, and expendi-
ture of these or any other forces, in the most advantageous man-
ner, by means of machinery.

143. In investigating the effect produced by any machine, there
are three things to be considered : 1. The nature of the force ap-
plied, generally styled the power ; 2. The force opposed to it,
called tlie resistance ; and 3. The point or points of connexion
between the power and the resistance, which when there is only
one point, as in the most simple machines, may be termed the
centre of action, and where there are two o^- more such points, the
action of the antagonist forces must be distributed over those
points.

144. Weight being in itself one of the most efficient kinds of
force, and at the same time a common property of all bodies to
which force can be applied, it has been very properly adopted as
a convenient measure or medium of comparison of moving forces
in general. But as the mere weight of a body in motion can
afford no just indication of its impulsive force, the term moment
or momentum has been adopted to denote the absolute force of a
moving body with reference to the effect it is capable of produc-
ing. The difference between the force of a body at rest and that
of the same body in motion, that is between its weight and its
momentum, in different circumstances, will be obvious on the
slightest consideration. Thus a musket-ball which might not be
heavy enough to break through a sheet of tissue-paper, when laid
gently on it, would perforate a much firmer substance, if dropped
on it from a considerable height, and fired from a gan it would
penetrate a thick deal board. The momentum of a b(3y then must
be estunated by its weight and velocity taken together.

Enumerate some of the natural forces capable of producing motion.
What has mechanical science to do with these forces }
How many and what things require to he considered ia examining the
eTect of a machine ?
What is meant by the term centre of action?
What is the difference between force and momentum ?

* " Notissimus Eques Romanus elephanto supersedens per catadro-
mura, id est funem, decurrit." — Sueionitu in Vita JVeronis, References
to other writers, aucient and modern, who have noticed the exhibitions
of elephants on the tiglit rope, are given in Beckmann't JSUt. ofbivenl,
Eng", Tr, vol. iii. p. 311.

THEORY OF THE MECHANIC POWERS. 78

•

145. From what has been stated elsewhen, it may be infeired
that any force which would drive a body weighing two pounds a
given distance in one minute, would drive a body weighing bnt
one pound twice as far in the same time ; and hence the veloci^
of the latter body would be double that of the former, though both
impelled by the same force. Both bodies also would have the
same momentum, as will appear on multiplying the velocity o^
each body respectively by its weight : for the velocity of the first-
mentioned body may be represented by 1, and that of the last-
mentioned, being double the other, by 2 ; then 21b. x 1 *=> 2, and
lib. X 2 »= 2. And the same result will be obtained if we take
the whole distance passed through by each body in a given time
as the measure of its velocity ; for suppose the body weighing
two pounds to run a quarter of a mile in a minute, and that weigh-
ing one pound half a mile in the same time ; then i m. ^^s .25 X S
«=50, ^ m. s= .50 X 1 «= 50 ; the sum expressing the momentum
of either body being the same. Since the momentum of a moving
body is to be estimated by its weight multiplied into its velocity,
it follows that a comparatively small body may by the celerity of
its motion produce a much greater effect than a body of fiur supe-
rior bulk moving slowly. Suppose the weight of a battering ram
(such as was anciently used in war), to be 20,000 pounds, and
that it moved at the rate of one foot in a second ; and the weight
of a cannon-ball to be 32 pounds, and that it moved 1000 feet in
a second, then the momentum of the former would be 20,000 x
1 = 20,000, and that of the latter 1000 X 32 = 32,000 ; and con-
sequently the effective force of the cannon-ball would be men
than half as great again as that of the ram, notwithstanding its
imaiense superiority of weight.

146. The mechanic powers are simple machines, or instru-
ments, by means of which the acting force technically styled the
power, is to be applied to the force which must be overcome, or
that called the resistance. The advantage which is obtained by
using these mechanical agents arises from the distribution of the
resisting force among the different parts of the machine, so that
the portion of it which is directly sustained or counterbalanced
by the power bears but a small proportion to the whole ; and thns
a power insufficient to communicate motion to a body or support
its pressure, without mechanical assistance, may effect the pur*
pose for which it is employed, by transferring a part of the weight
to one or more of those points ^ready noticed, whether it be the
fulcrum of a lever, the wheels of a pulley, or the surface of an
inclined plane.

147. Different authors have varied considerably in the enume-
ration of the simple machines or mechanic powers, Ifrom the
combination of which and their several modifications all other
machines, including those of the most complicated nature, are pro-
Give some examples to illustrate this difference.

What is the nature and what are the objects of the mechamcpofwen?
Uader how man/ general divisions maj all mcehaniG powers be elasted '

Q

74 MECHANICS.

daced. Considered as modes of the application of impolse to
overcome resistance, all the mechanic powers may perhaps be
most correctly arranged under three divisions: 1. The Lever;
S. The Multiplied Cord ; 3. The Inclined Plane. To these some
have added the Wheel and Axle, the Pulley, the Wedge, and the
Screw. But the wheel and axle is onl v a variety of the lever, the
principle which regulates the action of both machines being pre-
cisely the same. The pulley, so far as it possesses any distin-
guishing property, must be considered as a multiplied cord ; bat
in practice it is always used with wheels, and consequentiy it
partakes in some degree of the nature of the lever. The wedge
is nothing more than a double inclined plane applied in a peculiar
manner, and acting exactly as a single inclined plane, but with
twice the effect. The screw is a modification of the inclined
plane, usually operating through the assistance of a lever. All
these instruments have l>een commonly regarded as so many sim-
ple machines; it may therefore be as well to describe them
separately, and in such order that the developemeitt of their
xespective properties may illustrate the analogies among them
which have been just pointed out.

TTie Lever,

148. The principle of action of all the mechanic powers is
founded on the doctrine of equilibration, and is therefore intimate-
ly connected with the theory of the centre of gravity^ which has
been already explained. As no single mass of matter can remain
in the state of equilibrium unless its centre of gravity be support-
ed, so any number of bodies connected together must have some
common centre of |^vity on which they will rest securely, if
undisturbed, or oscillate round that centre, when impulse is ap-
plied on either side of it.

ji, 3 149. Suppose two balls of

^ iron. A, weighing three pounds,

and B,weighing but one pound,
to be fixed to the opposite ends
t>f an iron bar ; then whatever might be the length of that bar,
(provided it was of equal diameter throughout,) the centre of

Savity of the three connected bodies would be situated at a part of
e bar just three times as far from the lip^hter ball as from th*
l^eavier, the weight of the latter being three times as great as that of
the former; and the bar being supported at that point the equilibrium
would be maintained. Such a bar would be a kind of lever, with
respect to which the large ball might represeilt the resistance, or
force to be overcome ; the small ball the power applied ; and the

To which of these does the wheel and axle belong ? to which the wedge
and screw ? .

With what theory is the action of all mechanic powers connected ?

What namerical relation exists between the length of the two arms of
a IcTcr and the forces applied at their extrcmiUei when in equUibriom /

#

«

TBS LETER. 76

sapporting point the pr<m or centre of action, technically styled
the fulcrum, which is a Latin word, sigpoifying a prop.

150. The mode of action of the lever may be further illoBtrated
by observing what takes place when two or more boys amuse
themselves with a see-saw, or vertical swing.

Here the plank A B forms
a lever, of which the blocik
C is the fulcrum, and in o>
der for the plank to be eooir
poised, it must be shifted
mto such a position that the
greater weight of the boy D

nearest the fulcrum, may be compensated by the greater distance

from that fulcrum of the boy E.

151. Any number of boys might be placed at either side of the
fulcrum, provided that the sum of the weights of all the boys oa
one side, multiplied by their respective distances firom the fulcrum«.
were equal to tne sum of the weights of the boys on the other side*
multiplied by their distances respectively from the same point*,
*niu8, suppose the nlank to be twelve feet long, and the 'fulcrum
to be placed four fern from the end A, then a boy weighing thirty
pounds at the end B would counterpoise another weighing sixty
pounds at A; or the same boy at B would support two boys weigh-
ing forty pounds each, one being placed at A, and the other two
feet nearer the fulcrum. This wiU appear from calculation, foQ
the weight of the boy £, 30 X 8, his distance from the fulcrum,
gives for the product 240 ; the weight of the boy D, 60 X i, his
distance from the fulcrum, also gives 240 ; and tiie weight of one
boy at A, 40 X 4 B3 160, and another at two feet from the fulcrum^
40 X 2 » 80, will by the addition of the products make 340. The
plank being thus brought to a state of equilibration must, in order
to make it vibrate, have some impulse given to it, either by the
boys moving simultaneously upward on one side and downward
on the other, and so on ; or oy pressing alternately with theii
feet against the surface below, as either end preponderates ; or bj
any conesponding motion.

152. It has been proposed to adopt the principle of the see-saw
in the construction of machinery for economical purposes. In the
Journal des Savans, June 13, 1678, ain engine is described, bj
means of which cripples, if even deprived of their Hmbpt, bei^g
placed on the extremities of a lon^ lever, might, by the alternate
inclination of their bodies in opposite directions, produce sufficient
effect to work the pistons of pumps for raising water. And in
the same journal a description is given of a vibrating quadrangu-
lar frame, at one end of which four persons standing or sitting,
might by their regulated efforts in depressing and raising the

From what «pecie« of amasemeoi may a familiar illasCratien of this
trmh be derived ?

In what maDoer has it been pressed to apply the prinfiiple of the i
•aw to useful purposes ?

76 MECHANICS,

frame, communicate a vertical motion to a saw for cutting timbex :
horizontal motion to surfaces for polishing marble, or levigatin
powders ; force to a pair of shears for cutting through plates o
metal ; or rotatory motion to a wheel for any purpose. *

1 53. Since the momentum of a body
is always to be estimated by its weight
and velocity multiplied together, and
I the velocity by the space described by
a moving body in a given time, it will
follow that the momentum of bodies in

^V ^Ch ^ ^^^ ^^ equilibration must be the

^^ ^ same. For let A B represent a lever

kept in equilibrium by two leaden balls, the larger weighing two
pounils, and the smaller one pound ; then suppose the weights
were removed, the lever would take the direction E D, the ex-
tremity A would describe the small arc *A E, and the extremity
B the arc B D, and those arcs would denote the spaces moved
through by the respective ends of the lever. Hence the momen-
tum of the two weiffhts necessary to preserm the equilibrium of
the lever may be lound by multiplying the absolute weight of
each by the number representing the velocity, or space described ;
if therefore the arc B D be two inches, and A E one inch, it must
be obvious that the products of the respective weights and veloci-
ties n^ttltiplied together will in each case be two, which would
express the momentum or moviner force exerted by each weight
to preserve the equipoise of the lever. It must also be noticed
that the are B D, or F D, will always be in a direct proportion
to the line G B, and tiie arc A C, or E C, will bear the same pro-
portion to the line G A ; so that, whether the number of pounds
m each weight be multiplied by the number of inches in its
corresponding arc, or by the number expressing its distance from
the fulcrum, the result will show the momentum of both weights
to be the same. For let G B be 12 inches, and G A 6, th^en 13 X
1^6X2»12.

154. A lever theoretically considered must be an inflexible r6d,
of uniform weight in every part, turning freely on a fixed point or
lulcrum. There are three kinds or orders of levers : 1. That in
which the power P and the resistance R act in the same direction,
having the fulcrum F between them ; 2. That in which Uie power
and Tesistance are in opposite directions, the latter beiug between

What is meant by momentam, irhen applied to bodies at rest ? Ex*
einplify this in tlie case df the lever.

What are the three characters of a lever auamed in theoretical inves*
tigations ?

How many orders of levers may be enumerated ? How are the seve*
ral orders usually distinguished ?

* This simple kind of machinery, known in French by the npme of
bofcule, has been proposed for various other objects.— See JBorput
TraUe det Machina,

J

THS LSVSR* 77

the falcTum and the power; 3. That in which the power and d&«
resistance axe also opposed, the former occupying the intermedial
position, and being opposed to the fulcrum.

l/^

I

155. In a lever of the first kind, those parts on each side of the
fulcrum are termed the arms of the lever ; and the greater the
relative len^ of that arm with which the power is connected
compaied with that to which the weight or resistance is attached^
with so much stronger efifect will the power be enabled to act.
As the power will retain the lever in equilibrium when its mo-
mentum is barely equal to that of the resistance, it must have a
greater momentum in order to produce motion. Now its momen-
tum or acting force, so far as it depends on the lever, is derived
from the superior length of the arm with which it is connected ;
and therefore in order to raise the weight or resistance, it must
descend through a space as much greater than that through whieli
the weight rises, as the length of the arm to which the power i^
applied is greater than the length of that arm fb which the weight
is appended. Thus by means of the lever, a small power can
move a great weight ; but in this case the space passed through
by the power MrilT alvrays be greater than that through which w
weight moves ; and the greater the advantage which the power
derives from the lever, the greater must be Uie difierence of the
lengths of its arms, and consequently the less vnll be the motion
of the weight.

156. A long lever tum-
ing on a strong iron pin,
as shown in the margin, is
used by artillery-men to
raise pieces of ordnanee.or
other great weights. Wheelwrights and coachmakers employ a
lever of similar construction, but having a shorter handle, and a
higher fulcrum, and with this they raise a carriage on one Mde,
when they want to remove a wheel. Crowbars and handspikes
are levers of a similar kind, as also is the instrument called a
jemmy, used by thieves, in breakinff open doors or wrenching off
locks or other festenings. A pair of scissors, snuffers, or pincers,
consists of two levers turning in a rivet, which serves as the ful-

What are signified by the amu of a lever ? From what is the mecha-
nical efficiency of the /owcr derived ? ,.-^»

In what ordioaiy implements is the first order of levert cxemplilledr
In what familiar example do the force and resistance act at right aoglei
to each other ?

od

78

MECHANICS.

'0

cruxD, on one side of which power is applied to oTereome resist*
anee on the other side;

A common claw-hammer may be employed as a
^r tA jL lerer, acting with considerable effect in drawing out
'jBff nails. In Siis case the line of direction of the power

will be perpendicular to that of the resistance, as
appears from the marginal figure.

157. Here the advantage obtained by the power is
to be estimated by ita vertical distance from the ful-
crum A C, compared with the horizontal distance C
D, between the fulcrum and the resistance, represented by the
weight B. When the power, or resistance, or both act ob-
liquely, their effect will be diminished, according to the degree
ot obliquity.

Suppose A B to represent a lever
turning on a fulcrum at F, and let
A R be the direction of the power P,
and B S that of the ly^eight W ; then
if the line R A be continued to C,
and the line S B to D, and the per-
pendiculars F C and F D be drawn
from the fulcrum to meet the lines of direction in the points C
and D, the momentum of the power will be as its weight multi-
plied by the number denoting the length of C F, and the momen-
tum of the resistance will be as its weight multiplied by tne num-
ber denoting the length of D F.

158. Let B E be a curved
lever supported at F, and having
the power suspended at E, and
the weight at B ; then the mo*
mentum of the former will be
found by multiplying its weighi
by the line F G, or D E, and
that of the latter by multiplying
its weight by the line A F, or B
0. These lines A F, an& F G, are both shorter than the curve
arms of the lever. , If the fulcrum F be in a straight line between
B and E,this lever will possess the same character as if the lever
were straight ; but if the fulcrum be situated out of a straight line
while the force and resistance continue parallel, the lever will be
progressive. This is the character of the bent steelyard, in which
the poise being uniform, the weight is estimated by the height to
which it will elevate the poise.

159. Whatever may be the nature of thie lever employed, as
whether it be of the first, second, or third kind, its mode of action
is in every case to be explained according to the principles already

How is the ndvantRge obtained by the )>9ic«r estimated when the direc-
tions of the foree and resistance are not parallel ?

Desoribe the bent lever. How may the mode of action of all levers
be expUined ?

^■■■■■J <jr

THE LETER. 70

laid down. Thus in a lever of the second kind, in which the
resistance, or weight to be overcome, is placed between the ful-
crum and the power (see 154), the aid vantage of the latter
will be increased in the same ratio, as that of the distance or
space between the power and the fUlcrum to the space between
the resistance and the fulcrum.

160. The annexed figure (H represents the manner of using a
handspike or bar as a fever ot the first kind : (2) shows how a
similar bar may be employed as a lever of the second kind ; the
point of the lever here bemg fixed against the ground or surface
below the body to be moved, and the power applied to the oppo-
site end of the lever. Among the various examples which might
be adduced of levers of the second order, may be mentioned the
knife used by druggists for chipping sassafras, quassia, and other
medicinal woods; one end being connected with a table by a
hinge on which it moves as its mlcrum, the power is applied to
the handle at the opposite extremity, and the substance to be
chipped, forming the resistsmce, is placed between them, and is
cut through by the edge of the knife pressing it against the table.
The cutting blade used by chaff-cutters, and those of coopers, and
last makers are likewise made to act on the principle of a lever of
the second order.

161. In rowing a boat, regarding it as the weight or resistance
to be moved, the water must be considered as the fulcrum, against
which the pressure of the blade of the oar, acting as a lever of
the, second kind, moved by the hand of the waterman, as the
power, at the opposite extremity, produces the motion of the boat.
A pair of nut-crackers is formed by two levers of the kind just
described, moving on a hin|re as a mlcrum ; and so likewise is a
lemon-squeezer. When two men bear a weight on a handrbar-
row, one of them may be considered as occupying the place of
the power, and the other that of the fulcrum. If they have both
the same degree of strength, and can support the barrow in a
horizonUd direction, the weight or burden should be exactly be-
tween them ; for if it be placed nearer to one than to the other,
an advantage will be given to the man stationed furthest from it.

162. In a lever of the third kind, (see 154) the power being nearer
the fulcrum than the weight or resistance, the advantage lies on the

What implement illastnrtea the second order of levers ?

In what manner may we explain the effect of oars in rowing ?

How are we to compote the relative portions of a g^iven weight borne
by two persons on a pole ?

How are the power, fulcrum, and weight arranged in a lever of the
third order?

80 HECHANIC8.

side of the latter ; and therefore a greater deme of force would
be requisite to support or move the weight oy means of such a
lever than that which would suffice to produce the same effect
without the aid of any machine. But in this case the power will
raise the weight through a srreater space than that through which
the power itself passes, and will consequently cause the weight

to move with a velocity beyond its own.
-•v^ This will appear from the mspection of
'-^' the marginal figure, in which the power
P, acting over a pulley, from the point of
I the lever p, will, in moving the lever to
=*=® the position F p W, raise the weight or
^ ' resistance from w to W, while the power
only passes through the space from ptop; or more accurately
the line described by the weight will be the arc u^ W, and that
described by the point from which the power acts, will be the
very small arc p p.

163. This kind of lever therefore is not used to overcome great
resistance, but either to move a weight with great speed, or from
its peculiar adaptation to some particular purposes. Thus, a
builder in raising a long ladder from the horizontal position, to

J>]ace it against a wall, finds it convenient to fix the foot of the
adder against a block or stone, as a fulcrum, and laying hold of
the ladder at half or three-fourths of its length, he supports at first
the greater part of its weight, but gradually bringing it nearer
and nearer to a perpendicular position, he shifts his hands accord-
ingly from the point where he first grasped it, till he can bring
them low enough to keep the h dder upright, and then it may be
removed to the required situation. The treadle of a turning lathe,
or grinding machine, affords a familiar example of a lever of the
third order, in which the pressure of the foot becomes the power,
which, acting between the fulcrum and the resistaince, sets the
machine in motion. In a pair of tongs, or shears used in clip-
ping the wool from sheep, two such levers are connected so feis ty
have the fulcrum at the point of junction, and the hand in using
one or the other, acts as the power 0etween the fulcrum and the
resistance. .

164. But the most interesting examples of the application of
such levers may be found in the structure of animals. Thus the
fore-arm, connected with the upper part of the arm by the elbow-
joint, moves on that joint as a fulcrum, the power that lifts or
bends it being supplied by the contraction of muscles, acting from
points between the elbow and the vmst. The whole arm is raised
from the side of the body to a horizontal position in the same
manner, chiefly by the action of a strong muscle called the Del-

What sort of mechanical advantage is it the purpose of levers of the

thini order to attain ? ' •

What practical applications of this order of levers can be named }
What parts in the stracture of animals exemplify the third order of

levers ?

8T8TE3f OF LEVERS. 81

toid, forming the fleshy part of the shoulder, and stretching down
on the outside of the arm, with the bone of which it is firmly
connected. The bending of the knee-joint and the hip-joint in
walking, is performed by the corresponding action of strong mus*
cles ; and in Tarious parts of the human frame motion takes place
in a similar manner. In the lower orders of animals an analo*

fous kind of machinery may be discovered, as in the wings of
irds, which are thus made to move with exuaordinary velocity^
that they may be enabled to act on a medium having so inconsi
derable a degree of density as the air.

165« Any number of levers may be connected together, so as
to constitute a composition or system of levers, the power actin? .
on the end of the first lever raising the end of the second, and
that depressing the end of the third, so as to raise a weight at the
opposite extremity; or the alternate action may be continued
through a great number of levers, the effect of which would be to
augment vastly the momentum of the power, and to diminish in
the same proportion the velocity of the weight, or resistance, so
that the space through which that resistance would be moved would
in general soon become very insignificant. The effect of such*a
system of levers must be estimated according to the relative dis*
tances of the power and the weight respectively from the fulcrum«
whether the levers were all of one kiim, or some of one kind and
some o£ another.

166. Among the various applications of the lever, one of the
most useful and important is m the construction of the common
balance, styled, from its adventitious appendages, a pair of scales.
The beam, which is the essential part of the machine, is notliing
more than a lever of the first order, having equal arms, and turn-
ing freely on its fulcrum, or centre of action. It is hardly neces*
sary to add, that its use is to ascertain the weight of bodies by
equipoising them with an authorized standard ; and the principle
on which this is effected has been already amply illustrated.
There are however some circumstances requisite to insure the
accuracy of a balance, which Reserve to be noticed.

167. The beam of the balance should be so formed that its
centre of gravity may be placefi just below the axis or centre of
motion ; for if the centre of gravity and centre of motion coincided,
it must be obvious that the beam would rest in any position in
stead of assuming the horizontal direction necessary to indicats
the equality of weights on each side. However, when a very
delicate balance is required, its beam must be so constructed that
the centre of motion may be as near as possible to the centre of
gravity, but somewhat above it. The extremities of the arms of
a balance are named the points of suspension, to which are fixed
the scales ; and those pomts should be so situated that a straight

In what roftnoer may levers be eombioed together for the production
of Any desired efTeot ?
How is the effect of such a system of levers to be estimated ?
What cireumstaaoet are requisite to insure the accuracy of a balance?

82 KECBANICS.

line extending from one to the other would touch the point on
which the beam turns. The sensibility of the balance is likewise
influenced by the form of the fulcrum ; and in the most accurate
balances the beam rests on a knife-edge moving on agate, polish-
ed steel, or some very dense and smooth surface. Equsil nicety
is required in the suspension of the scales, which should hang
from thin edges.

168. Having thus stated the method of rendering a balance as
exact as possible, it may be proper to notice some of the imperfec-
tions of common balances, caused as they are too frequently by
design, for the purpose of fraudulent deception. If the two arms
be not precisely of the same length, the scale appended to the
longer arm will turn with a less weight than that hanging from
the shorter arm, and the purchaser of goods may thus be cheated :
so also if one arm of the l^ver be heavier than the other, the scale
on that side must preponderate. But deceptions of this kind may
be discovered by changing the places of the weight and the arti-
cle to be weighed ; for the lightest scale would no longer keep
equipoised. And yet with such a pair of scales the true weight
of a substance mignt be ascertained ; since by weighing it first in
one scale and then in the other, multiplying together th^ two
weights, and extracting the square root of the product, we should
obtam the true weight.*

169. 'Fhe steelyard is another well-known kind of balance,
more directly involving the principle of the lever in its eonstruo*
tion than the common balance. It consists of a lever with un-
equal arms, turning on its fulcrum, and having on the longer arm
a moveable weiorht, so that the body, whose weight is required,
bein? suspendea from the shorter arm, the equilibnum is attained
by shifting the weight to the necessary distance ^m the fulcrum,
and the longer arm being graduated and numbered, the weight
appears from inspection. This is sometimes called the Roman
balance, as alleged from its resemblance to the Roman sCatera ;
though it has been stated that the original term w;s6 Ronmian,
and that it wa^ so called in the East, f^om the sh?pe of the
weight, resembling a pomegranate.:|- Such a balance as the steel-
Tard, but of small dimensions, and made of ivory or wood, is used
by the Chinese for weighing pearls, precious stones, and other
small objects.

170. The Danish balance is a straight bar or lever, having a
heavy weight fixed at one end, and a hook or scale at the other.

What are some of the defects liable to be found in balances ?
How may a false balance be detected ?

How may the true weight of an article be obtained by means of such
a balance ?

How is the action of the steelyard to be explained ?
What is the construction of the Danish balance ?

* See Leslie's Elements of Natural Philosophy, vol. i. p. 186.
t Idem, p. 187.

THE WHEEL AND AXLE. 68

with amoreable fulcrum, the situation of which indicates the
weight of any substance which may be tried by it. The bar of
course is graauated, and thus the weight may be determined, but
the divisions becoming smaller in proportion as the weight in-
creases, inconvenience occurs in ascertaining the exact amount ot
the wei^t of very heavy bodies.

171. The weighing-machine used at toll-gates on turnpike-
roads, to discover the weight of loaded carriages, consists of a
system of levers supporting a quadrangular floor. Four levers
turning on their fulcrums extend from the angles of a box beneath
the floor towards its centre lirhere they are connected together,
and also with another lever extending across the middle of the
box, and passing beyond its limits ; this last lever acts on a third
which presses on a spring or is connected with the ann of a ba-
lance, by means of which the amount of pressure on the whole
system may be ascertained.

The Wheel and Jxle.

172. Though the lever may be considered as the most generally
applicable, and consequently the most useful of all simple ma-
chines, yet from the limited effect and intermitting action of power
employed to overcome resistance by means of the lever, its grand
utility must ever be confined to cases in which a momentary effort
is required to change the place or position of a body of a great
weight, by the application of comparatively small power, "nius,
if it be necessary to remove a heavy block of marble or granite
from one place to another, and a lever can be applied in such a
manner to one side of its base as to shift the position of its centre
of OTavity sufficiently to make the block turn over, it may thus be
rolled to any given distance: but supposing the utmost effect of
.the lever be to raise the mass but one inch, or any space through
which it would fall back to its first.position, the lever alone would
manifestly be quite useless. Hence different methods have been
contrived for rendering the lever more effective, as by employing
a German machine, called a Hebstock, by which the weight is
propped or supported during the intervals beti^^een the successive
operations of the lever; by the French machine, termed Roue de
la Garosse, from the nam*^ of the inventor, and by means of which
a lever is kept in a raised position by a ratchet wheel ; or by
using the Universal Lever, which also acts by means of a ratchet
wheel,*

To what objection is it liable f

What is the construction of weighing machines for carriages ?
What circumstaoce limits the ttility of the simple lever 7 How hat
it been proposed to obviate this defect ?

* This kind of wheel can only move forwards or in one direction, be-
ing prevented from turning the other way, by a spring detent falling be-
tween teeth on its periphery.

84 MECHANICS.

173. But these modes of operation must be nearly useless
where it is requisite to raise a body to a great height, or move it
through a considerable space, and for such purposes may be ad-
vantageously employed the wheel and axle, sometimes called
Axis in Peritrochio,* which has generally been ranged among the
the simple machines, or mechanic powers, though it is in fact only
a more complicated form of the lever, and it might with propriety
be styled a perpetual lever.

174. It consists of a wheel or la^e flat cylinder, with a smaller
cylinder passing through its centre, as an axle, to which it may
be fixed so as for both to move together about the same centre,
or the wheel may turn on its axld, in which case the effect will
be different from that where the parts of the machine are con-
nected.

In investigating the operation of the wheel
and axle both parts must be considered as
turning on a common centre. Let the an-
nexed figure represent a horizontal axle, rest-
ing at its extremities on pivots, or supported
by giidgeons, so that it may revolve freely,
carrying round with it the attached wheel.
On die axis is coiled a rope which sustains
the weight ; and round the periphery of the wheel is coiled ano-
ther rope, in a contrary direction, to which is suspended the
power. Then supposing the machine to be put in motion, the
velocity of the power will be to that of the weight, as the circum-
ference of the wheel to that of the axle ; fot it will be perceived
that the power must sink through a space equal to the circumfe-
rence of the wheel, in order to raise the weight through & space
equal to the circumference of the axle. And as the momentum
of any body may be found bjr multiplying together its weight and
its velocity, it fellows that if the number of inches in the cir-
cuit of the wheel multiplied by the number of pounds in the
power, produce a sum equal to the product of the measure of the
axle multiplied by the number of pounds in the weight, then the
power and weight will remain in equilibrium.

1 75. As before stated, the momentum
of bodies moving in circles will be as the
products of their weights and the radii
of the circles they respectively describe,
therefore when the power bears the
same proportion to the weight as the ra-
dius CD or the diameter F D of the axle does to the radius A B or
the diameter £ B, of the wheel, the machine will preserve the equi-

Wliat name mieht properly be Applied to the wheel and axle ? Ot
what does it consist ? In what ratio are the power and weight to each
other when this roachine is at rest ?

* -From the Greek A(«r, an axis, and ni^ tt^ixM, to turn rouad.

THE WBBSL ANIT AXLE.

Itbrium ; so that the effect of this machine will depend on the rape*
riority of the radios, or diameter of the wheel to that of the axle.

176. The wheel may be moved by a weight acting on its peri-
phery, as dready described; by projecting pins, or by a bent nan*
die, such as is used for the common draw-well ; but whether the
power be applied directly to the circumference of the wheel, to
the extremities of the projectingr pins, or to the handle, its effect
must be estimated by tne extent of the circle described.

177. That the wheel and axle differs not
in principle from the leyer ma^ be demon-
strated from considering the efiect of a sin-
gle wheel used not for the purpose of ia*
creasing power, but merely m order that a
power may be enabled to act in some re-
quired direction. For let C be any weight,
as ten pounds, suspended oyer a wheel by
a line held at D, it will be obTious that
setting aside the effect of friction, a power
equal to ten pounds must be applied to keep
the wei^t equipoised. Now the pivot on
which the wheel turns will manifestly be the centre of motion or
fulcrum, supporting thejoint action of the power and the weight;
and the lines A E and B E will represent the equal arms of a !••
ver held in equilibrium, like a balance loaded with eoual weighttb
178. A Venetian window-blind is usually suspendea in this man*
ner, by an endless line passing round two wheels; and while both
sides of the line are equally stitched, the blind will remain at any
height, but destroying the equilibrium, by pulling the line on one
side or the other, wilfraise or lower tiie blind at pleasure. In the
wheel and axle the radius of the wheel represents the loneer arm of
a lever, and the radius of the axle the shorter arm ; and hence the
advantage this machine affords. And as its action may be con-
tinued indefinitely, each revoluticm producing an uninterrupted
effect, the power may be regularly applied till the object in view

be attained.

179. One of the most efficient forms
of the wheel and axle is displayed IB
the capstan used on board ships and in
dock-yards. It consists of a vertical
spindle fixed firmly as in the deck of the
vessel, but turning on its axis, and suf^
porting a drum, or solid cylinder conr
n^ted with it, and having its periphery
pierced with holes directed towaras its

On what will the effi^et of the wheel and axle depend ? How is the
effect of the wheel to be estimated, when the cord is not applied direetly
to its periphery ? How can you prove the identity of the wheel and
axle, and the si mvdr lever f Of what practical applications is this machine
aasceptiblc f What iv the conttraction of the capstan, aad how is its tU
feet to be soapated f

H

8S MECHAMICB.

centre. It is then worked by long levers, inserted in the hdea
bj men who walk in eucceHSion round the capstan, and thus mak«
it levolve, while a rope or cable wound about the spindle ma; act
with force suflicient to weigh » ponderoas anchor, or warp a heary-
Uden vessel into harbour.

180. The tieadwheel is another modification of the wheel and

axle, in which the weight of several per-
I sons stepping constantly at tiie circamfer-

|< ence of a long wheel make it revolve by

: their weight; as may be readily compre-
: hended from the annexed li^re. A some-
; what similar wheel turned by the weight
of one man is used in Persia and some
other oriental countries, for raising water.
One or more horses may be made to work a mill, by harness-
ing them to the extremity of shafts or long levers 6ied to an axis,
which they turn round by walking in a circle ; as in a machine
for triturating clay for brick-making, and in some malt-mills. A
Ireadwheel of a peculiar form is used in some parts of the United
States acted on by horses, oxen, oi other animals.

181. The axle of a wheel sometimes has a conical or tapered
shaps, which affords an advantage when a varying force is to be
overcome. Tlie rnaJnspring of a watch, the power of which is
employed to uncoil a chain, acts thus on an axis, called the fiisee,
on the surface of which is cut a spiral groove to receive the chain;
and when the watch is newly wound up, the spring acts with ita

Seatest intensity to turn the (usee while the chain passing round
at part where the diameter is shortest, affords but a small lever-
age ( and as the elastic force of the spring gradually diminishea
by ita relaxation, it obtains greater and greater purchase by the
increasing diameter of the fiisee as the chain is uncoiled ; so that
by this means an equability of action is maintained, without
which the watch would be useless. A similar contrivance is
adopted to equalize the effect of power applied in raising ore Iram
a deep mine ; for the rope, when at its greatest length, (and con-
sequently when the resistance of the weight is gTeaCest), is coiled
•boat the narrow end of the axle, and the successive coils advance
towards the wider extremity, as the resistance diminishes by the
riiortening of the rope.

183. As ^e efficiency of the wheel and axle, whatever may
be its peculiar construction, is to be estimated by the ratio of ths
diameter of the wheel to that of the axle, it follows that increas-
ing the former or diminishing the latter will augment the effect.
Either method may be adopted to a certain extent; but if the wheel
be extremely large it may be inconvenient and unmanageable; and
on the other hand, if the axle he very slender, it wm be weak
■ '' "Joth these evils are avoided in '

For what purpose ii the (retdwheel u>ed in
What ii ihe conuruution *nd sdiantnge of i
How i* Ihc priDGiple of thefoice sppued in

THE MACHINB OF OBLIQVS ACTION.

87

of the doable capstan, an ingenfons
contrivance, said to have been brought
from China. It consists of two cylin-
ders differing in diameter, connect-
ed, as in the marginal figure, turn*
ing about the same axis, while the
weight is suspended by the loop of
a long cord, one end of which un-
coils progressively from the smaller
cylinder, as the other laps round the
larger: thus the weight is elevated at
each revolution through a space equal
to half the difference between the cir-
cumferences of the two cylinders. So that the mechanical ad-
vantage of the machine, with its pulley, will be in the ratio of
the diameter of the larger cylinder to half its excess above that
of the smaller one ; and therefore the equilibrium will be preserved,
when the product of the power multiplied by the former is equal
to that of the weight multiplied by the latter. This is true when
the machine is moved by a hand rope applied to the larger cylin-
der ; but when the crank is employed, twice ita length must be
substituted for the diameter of the larger cylinder.

183. The efficiency of wheel-work may also be indefinitely
augmented by a system or composition of wheels and axles, as in
the case of the lever Thus the effect of the power that acts at
the circumference of toe first wheel may be transmitted 4o the
circumference of its axle, with which a second wheel .being con-
nected may act through its axle on a third wheel, and so on to any

fiven extent. One wheel may be made to turn another merely
y the friction of their surfoces, when but little force is required ;
but the most direct and accurate method of connecting trains of
wheel-work is by teeth or cogs, on the peripheries of the wheels ;
and on this principle a great variety of complex machines arecon-
utructed. Different wheels may also be connected by a strap or
band, as is the case with spinning-wheels and the wheels of turn-
ing-lathes.

The Machine of Oblique Jetion, or MuUipUed Cord,

184. To this kind of mechanic nower may be referred all those
cases in which force is transmitted by means of flexible cords or
chains, from one point to another. It has also been styled the
funicular system, but as including a .variety of modes in which
power can be applied by means of inflexible rods or bars, as well
as by flexible lines, to produce an equilibrium depending on the

How is the double capstan, or differential axle^ formed ?
How much is a weight elevated by each turn of this machine ?
How may the efficiency of wheel'work be augmented ?
In how many methods may motion be transmitted from one wheel to
another ?

■SCHAHICfl.

eompoBition of forees, it might be, perhaps most properly, design
nated the machine of oblique action. From the theory of the
composition of forces, which has been elsewhere illustrated, it may
be assumed that a force applied in the proper direction will balance
any two forces ; but if one of these be sustained by some fixed
point, the first force may be considered as acting only against the
other ; and power may thus be indefinitely augmented.

185. Suppose B N, N C, C 0, and
O 6, to be four bars connected by
joints or hinges at B, N and O, and by
a spiral spring passing from the joint '
B, so as to unite it with the ends of
the bars N C and O C at C. Pres-
sure applied in the direction O N
would elongate the spring with an effect which would increase
in proportion to the decrease of the angle N C O, so that at the
eollapse of the bars B and C O into a rectilineal position, the
effect would be incalculably great.

186. If the end B of a pair of jointed rods be firmly fixed, and
the extremity C made to act by pressure, as by a
man pushing at A, the force at O, when the bars are
brought nearly into a straight line may be equal to
the weight of many tons. On this principle tiiat
part of the Russel printing-press is oontnved by
means of which the paper is applied to the types to
take off impressions ; instead of using a'screw turn-
ed by a leyer, as in the common printing-press.
The same kind of mechanic power ia employed for
extracting the steel core firom the hollow brass cy*
Under used as a roller in the printing of cottons;
and various modifications of it have been adoptedt

with great advantage in several operations of art, where a vast
momentary effort is requisite to produce a given effect.

187. The theory of the machine of oblique action, as it applies
to flexible cords, has been sufficiently explained in treating of the
composition of forces. (See 35 & 36.) It may, however, be hers
stated, that if a cord be acted on by equal forces in opposite direc-
tions, its tension will be measured by one of those forces or
weights, and must of course be uniform throughout ; and what-
ever flexures the cord may undergo, and however numerous be the
fixed points it passes over, provided its motion be unimpeded, the
weights required to keep it in equilibrium must be equal. But .
if a cord be fastened at one extremity and variously deflected, the
effect of weights suspended to different parts of it will be modi*

On what theoretical principle is the machine of oblique action founded?
Illustrate its apptication in the hineed apparatus or toggle^oint.

How is this machine applied in the printing press ?

For what species of effort is it peculiar!;^ adapted ?

What measures the tension of a cord stretched by equal weights at die
extremities ?

m POLLXT. 60

fied according to their dtnation; so that a great weieht acting
near the point of suspension may be counterbalanced by a com-
paratively small foice at the opposite extremity of the cord.

7%ePuiky.

188. This is rather a compound than a simple machine ; for
from the investigation of its nature and properties it will be eri
dent that it is merely a combination of the wheel and axle with
the multiplied cord ; and as the wheel, though a very useful, is
not an essential part of the pulley, this machme may be regarded
as a variety of tiie funicular system, or multiplied cord.
p .|. . . .. .M......mu.in».a. i 189. The effect of a single pulley, or moveable

wheel suspended by a cord from a hook at a fixed
point, as in the annexed figure, will be to dimi-
nish the resistance by one-half, so that a power
equal to one pound will su}>port a w^ght of two
pounds. This must be manifest from considering
that half the weight is supported by the hook,
consequently the other half only is opposed to the
power. The same conclusion will be derived
from attending to the result of the action of the
power in raising the weight; for double the length of rope must
pass over the fixed pulley on the side of the power compared
with that which passes over it from the weight; so that the power
must descend two inches in order to raise the weight one inch.
*rhus the power- will move as fast again as the weight, therefore
its velocity must be double that of the weight, and its effect must
be increased by such a pulley in the same ratio.

190. The fixed wheel or pulley here, has no other effect than
that of altering the direction of the power. (See 177.) Though
a puHey might obviously be made to act wiuiout wheels, and the
cord might be deflected by passing through rings or by other
means, so that the wheel must be considered as a sort of adventi-
tious appendage to the pulley, yet, as already observed, it is an
extremely useful one. For the wheel enables the cord to move
freely, by destroying in a great measure the friction which would
otherwise take place between the cord and the surface over which
it passes, and which would weaken, and in some cases interrupt,
the action of the pulley. The wheels also serve the important
purpose of keeping the deflected parts of the cord stretched in pa-
rallel lines ; for the effect of the power would be diminished in

When one extremity of a cord is fastened to an immoveabl€^tioint|
faov will weights applied to interniediaVe points affect the coi-d ?
How may the pulley be regarded in a theoretical view ?
How are we to coropute the effect of a single moveable pulley f
What is the effect of a single fixed pulley f
What it the advantage of the wheel in the eonatniBtion of tut ma*

89

Ti

■Dj Other poBition of the end. "nms «1ie*
the deflections of the cord form «n angle, as
lepresenied in the margin, the power mnat be
equal to more than half the weight, in order to
keep the latter suspended ; the machine will
become lest and leas effictcions as the an^o
formed by the side* of the cord iacreases ;
and when the two parts of the cord sapport-

— ■■ '- ' straight line, the power must

t to preserre the eqnilibiiun"
191. In the pullejs just described, is eL
hibited the emct of the power when the
weight is pertly supported from one fixed
point ; bnt that effect may be Tasily augmenl'
ed by such a system of pulleys as that in the
annexed figure, in which the wei^t is sus-
pended from the lowest of a series of wheels,
each having its own cord attacJied to a fixed
point. Here the resistance is diminished by
the distribnlion of ^he weight over five hxed
points ; so thai soppoeing the weight to ba
thirty-two pounds, the wheel A, with its cord,
^ will sapport the whole of that weight; the
wheel B, with its cord, half the weight or
sixteen pounds ; C, on^-fourth of the weight
or eight ponndBj D, one-eigfa& or fear ponnde;
E, one-sixteenth or two ponnds, which being
divided by the two sides of its cord, learea
fant one pound to be sapported by that side
which is extended orer the fixed pulley P ; ^uid
thus a power equal to one pound will eonater-
balance a weight of thirty-two pounds.

199. When one cord only is ttsed, whick

tiasses over two or more fixed and moveable ])ul-
eys, the ^ower will be to the weight, as unity,
or the s' ' - - ■■

^ the cord la paasing OTsr ui me mm anu iau*t^
B able pQlle;rs. Heneeif the powerbeangmented,
BO as to rsise the weight, the former must de-
scend through as many inches more than the
latterascends,Bsthenumherof bends in the cord
supporting the lower block exceeds nnity: that
is, the power must sink four inches or feet to
elevate the weight one inch or foot; and each
will he the ratio of its efficiency with encb ptiL-
leys as that shown in the marginal figure, the
advantage gained dependiuK on the number of
whMiU Biul ceiwqaSQt defiactions of toe cor£
Hev doM Ih* oUi^iuCy of llw wirdi afioet lbs reUlion b«t*em tta

THE IMCUmn PLANS.

M

tt_jg

nmmnia

193. A great TBriety of systemSt or, as they
are commonly tenned tackles of pulleys, have
been contrived ; but the advantages they respec-
tively afford may always be estimated by refer*
ence to the spaces relatively described by the
power and the weight or resistance. The greatr
est inconvenience occurring in the practical ap-
plication of the pulley, is owing to friction, and
consequent irregiilarity of action. Various plans
have been adopted to remedy this defect ; one of
the most ingenious of which consists in cutting
a proper number of concentric grooves on the
face of a solid wheel, with diameters, as the odd
numbers, 1, 3, 5, &c., for the lower block, and
corresponding grooves on another such wheel,
with diameters, as the even numbers, 2, 4, 6,
&c., for the upper block. Then the cord being
passed in succession over the ffrooves, as repre«
sented in the margin, it will be thrown off by
the action of the power, in the same manner as
if every groove formed a separate and independently revolving
wheel. A machine of this construction is called White's puUeyt
from the name of the inventor, Mr. James White, who obtained a
patent for it.

194. Tackles of pulleys are used on board ships, where the
wheels are fixed in blocks, by means of which the sailors can
raise the masts, hoist the sails, and conveniently perform other
necessary operations. Various combinations of pulleys are like-
wise used on land, as by builders, in raising or lowering great
weights ; and in removing from one level to another heavy balep
of goods, or other merchandize.

7%« Indined Plane.

1 95. This is the least com- *
plicated of all the simple
machines. It is, as the name
implies, a plane surface, sup-
posed to be perfectly smooth
and unyieldmg, inclined ob-

pover and the weight? Hour many tiroes is the power mQltiplied by
Beans of the system of attached eords and moTeable pollies combined f
In what manner is the weight distributed among the cords in this ar-
rangement ? How is the relation between the power and weight to be
discovered when a sinele cord is combined witn a system composed of
fixed and moveable pollies ?

In what general manner may the advantage of a tackle be computed ?

What practical difficulty is encountered in the use of pollies with sepa-
rate wheels? How does White's pulley obviate this dlflieiillyf Sute
some of the useful applications of the pulley.

What theoretissl charaeter it asmoed in treating of the indiind
plane?

M MECHANICS.

liquely to a horizontal plane ; and its effect, as commonly nsed, is
to diminish resistance, and thus enable a moderate power to sus-
tain or overbalance a ^eat weight. The mode of action of the
inclined plane has been already rally explained (see 93 to 96), and
the method of estimating its efficiency, in any given case, may be
readily comprehended by reference to the relative velocities of
two bodies, one falling through a space equal to the vertical
height of the inclined plane, and the other passing down its de-
clivity. Suppose the height A 6 to be one loot, and the inclined
surface A C to be four leet, then a weight of four pounds, W,
resting on the plane, will be equipoised by a weight of one pound,
P, hanging freely over a pulley. And as the inclined plane is
commonly employed to facilitate the rolling or shifting of ponder-
ous bodies from a lower to a higher level through a moderate
space, its efficiency will be in the ratio of the length of the inclin-
ed plane to its vertical height; thus with the machine just de-
scribed, one-fourth of the force necessary to lift a great weight
through the space A B, or the vertical height, would be sufficient
to impel it up the declivity, from C to A.

196. In this more than in most other machines great allowance
must be made for the effect of friction, which must materially
modify any calculation as to the advantage it affords. Instances
of the application of the inclined plane to practical purposes so
frequently occur, that it can scarcely be necessary to advert to
them. Roads formed on declivities are a kind of inclined planes ;
and railways are sometimes thus constructed, in such a manner
that any weight, as a loaded sledge, may be made to ascend one
plane or inclined railroad by the impulse of another carriage with
which it is connected, and which passes simultaneously down
an adjoining railroad.

197. The very simple nature of the inclined plane renders it
probable that it was the earliest of the mechanic powers known
and brought into use. It has been conjectured that it was em-
ployed by the Egyptians in raising the immense blocks of stone
which form the pyramids, and in executing other gigantic works,
which have excited the astonishment of successive ages. Mr*
Warltire, a gentleman who delivered lectures on naturd philoso-
phy, in the latter part of the last century, endeavoured to prove
that the ancient British Druids were the founders of Stonehenge,
on Salisbuiy Plain ; and that they erected the massive trilithons,
which partly compose that curious structure, by rolling or rather
by shifting the transverse blocks into their places by means of
temporary inclined planes of earth or rubbish, forming a sort of
road-ways for the passage of the several bl ock. The annexed figure

How is its mechanical efHciency estimated ?

What familiar applications of the inclined plane.majr be enumerated ?
What conjectures have beeq formed respecting its use among the anr
tuots/

THS WB90E.

will affbid a soffieieiitly tMorate idea
of one of the trilithons of Stonehenge,
and when the straetore was perfect,
seTeral of these were arranged in a cir*
cnlar figure. It will not be difficult to
conceiye that a sloping bank or decliYi*
ty, having but a small degree of inclU
nation, might be formed, np which any mass might be impelled
or dragged, with a force not much greater than would be required
to draw or push it forward on level ground.

The Wedge.

198. A wedge is the solid figure called by Geometricians a
triaiigular prism, bounded on two sides by equal and similar tri*
angles, and on the other three sides by rectangular parallelograms.
It IS composed of two inclined planes united at their bases ; as

will appear from the annexed representation. Its
use is to divide solid bodies, the edge £ F being
impelled against them by pressure or some other
force applied at the sur&ce A B C D ; and if the
force be estimated by its weight, its effect will be
in the ratio of the line D F to the line G D, that
is as the sides of the wedge to its breadth. So
that the advantage deriy^ from using this ma>
chine increases in proportion as the angle which forms its edee
diminishes. But the wedge is generally used for cleaving blocks
of wood or other hard substances, and the force applied to it is
that of percussion, wi& a heavy hammer or mallet, the effects of
which are so different from those of direct pressure, and are so
much modified by circumstances, as to render any theoretical cal*
culation utterly inaccurate and useless.

199. It appeara from the results of some experiments made in
the Dock-yard at Portsmouth, England, on the comparative efiqft
of driving and pressing in large iron and copper bolts, that a man
of medium strength striking with a mall weigmng eighteen pounds,
and having a handle forty-four inches in length, could start or
drive a bolt about one-eignth of an inch at each blow ; and that it
required tiie direct pressure of 107 tons to press the same bolt
through that space, but it was found &at a small additional weight
would press the bolt completely home.*

200. But numerous and varied experiments would be requi-
site to obtain any results which might afford data for computing

What 18 the geometrical forin of the w^dge ? What relation has the
advantage of this machine to the angk formed at its edge ? Of what
nature are the forces usaally applied to the wedge ?

What has experiment proved in regard to the difference between pret-
■nre and percussion ?

* Encyolop. Metropol.- Mixed Seieoees, vol. i. p. 58.

94 MSCHANICS^

ihe effect of impact or percassion on wedge-shaped bodies;
and if that effect could be exactly estimated, further difficulties
would arise from considering the very heterogeneous nature
of the resistance, depending on the relative hardness, tenacity,
and other properties of those substances on which the wedge is
made to act. This instrument must therefore be regarded as
one the effect of which can seldom be precisely determined ; but
which notwithstanding may be often very advantageously employ-
ed in certain circumstances.

201. Among the less frequent modes of application of the wedge
may be mentioned its having been used to restore to the perpendi-
cular position a building which declined slightly in consequence
of some defect in the foundation. The voussoirs of arches are so
many wedges ; and piles used for the foundation of the piers of
bridges may be considered as wedges, <driven into the bed of a
river by the percussion of a powerful machine. Sharp-edged and
pointed instruments in general act as wedges ; thus chisels, planes,
and axes used by carpenters manifestly produce the effect of
wedges ; and knives, razors, awls, pins, and needles, and indeed
all cutting and piercing instruments display an obvious analogy
to the common lorms of this mechanic power.

7%e Serew,

202. The sci'ew, though commonly reckoned among the me-
chanic powers or simple machines, cannot be considered as such
when applied to any practical purpose, as it would be found almost
wholly ineffective without the assistance of the lever, which is
therefore usually combined with it, and thus it becomes a most
powerful machine, applicable to a variety of important purposes.
The general form of the screw must be too well known to require
descnption : it may however be stated, that it consists of two
Wts, namely a solid cylinder, sometimes called the male screw,
aK a corresponding cylindrical cavity, to receive the former part,
and therefore styled a female screw ; round the surface of the cy-
linder passes what is termed the thread of the screw, describing
from one end to the other a curve sometimes inaccurately represents
ed as a spiral, but which is really a helix, precisely resembling
a common corkscrew, which, in fact, is nothing more than the
helical thread of a screw without the core. 'Hie hollow screw
has a similar helical thread winding within it, exactly adapted to
the interval between the turns of me thread of the solid screw ;

Why is the actual effect of the wedge more difficult to be computed
(ban that of other machines }

Of what applications is the wedge susceptible in the art of architee«
ture ?

Name some of the familiar applications of the wedge in ordinary io«
•truments.

What is the nature of the screw in its practical stmetore ?

• THE flCSBW. 06

and thus either part being made to revolTe while the other is kept
steady, motion or pressure may he produced to any extent.*

203. In order to obtain a correct estimate of the mechanical
effect of the screw, it will be necessary to develope its construe*
tion, from which it will appear that it is, in principle, identical
with the inclined plane ; and it mi^t be conceived to act as a
system of revolvinff inclined planes. This will appear from refer-
ence to the annexed figure. Let A B C D represent a cylinder

divided longitudinally into a number of
equal parts, B a, a 6, &c., and let lines a
'^ «, 6/, &c., be drawn perpendicular to the
-^ side A B, each equal to the circumference
'ff of the base ; then by joining B e^af, bg^
~ e hj will he formed so many right-angled
triangles Ji ae, a bf, b cg^ e ^ A, as the
number of equal parts into which the cy«
linder has been divided. Now suppose
these triangles to be rolled upon the cylin-
der, so that the point e should coincide with
the point a, /with b, g with e, h with <f, and so on, the hypothe-
nuses or longest lines of the triangles, Be^af,bgj eh, &c. would
form on the surface of the cylinder one continued helical line, re-
presenting the thread of a screw* These triangles might be con-
sidered as a series of inclined planes ; and therefore if such a
screw were fitted to a hollow or female screw, fixed so that- the
former might act vertically, it will be obvious that one revolution
of the male screw would raise or depress it through a space equal
to the height of one of the inclined planes, and the enect of the
screw, independent of friction, would be as the length of its
base to its height, or as the line a e to B a. If then B a be f of
an inch, and a e l^in. or 12-8, a powec equal to one pound acting
by means of the screw would balance a resistance equal to twelve
pounds. The power must here be supposed to act parallel to the
base.

What is the distinction between a helix and a spiral ?

How is an accurate estimate of the effect of the screw to be obtained ?

With what other simple machine is its principle of action to be coiJ!-
pared ? How ntuch does one turn of the screw raise the weight or re-
move the resistance ?

* A spiral or volute is a line which can be described on a plane ; but
no two points of a helix are in the same plane, and therefore it cannot
be correctly described on a plane surface.

Spiral Line. Helical Line.

i

U KKCSANICS.

204. But the reiiatance ariBing from Mction between the paila
of the solid and the hollow screw would in most cases i^^^
■great additional power to produce any considerable effect. Thi«
Uierefore renders the application of a lever oeoesBary to constitute
the screw an effective machine. The lever may be added to the
Bolid screw, to turn it within a fixed hollow screw; or to the
hollow screw, to turn it round the solid screw. The manner in
which the lever is applied in either case will appear from th«
following figures ; the former of which shows how pressure may

be produced bj a solid ter«w acting within a hollow screw in ■
fixed beam ; and the latter exhibits uie similar effect of a hollow
screw pierced in a block turning by means of a lever on a fixed
screw ; ^e pierced block thus adapted to a solid screw is called

SOS. As the effect of the screw is always to be estimated bj'
the proportion between the space described by the power, in one
revolution of the screw, and the space between any two of ita
contiguous threads, it must follow that when the power is appli-
ed lo a long lever instead of being made to act directly on Che
circumference of the screw, the effect must be vastly augmented.
Thus if die threads of a screw be as much as half an inch apart,
and it be turned by means of a lever extending three feet from
the centre of the screw, the effect or advantage of such a machine
will be as the number of half inches in the space described by the
extremity of the lever lo unity. Now reckoning the circumfer
ence of a circle in round numbers to be three times its diameter,
the circumference described with a radius of three feet wilt be
3Hx2 = T3 X 3 = 316 inches, and double that number, or 433
to 1 will be the measure of the advantage afforded by the ma-
SOS. Hence it will be apparent that tiie efficiency of the screw
acted on by the lever might be indefinitely increased by extending

■ neoeiurjr in Ihii msebiDE f la whtt

Vfhj i< the addition
two model muj the Ibvei- ok ■[■pjitru r

How ii ihe elTiiEt of the Krew to be eilimated >

Ho« Tar might the efficiency of the Kreir, iheoretlciUy M

■idend.

THE CONTOUKD 8CREW. M

the iengdi of the lever, or by dimiBiBhing the ifttteTal between
the thieads of the screw. But a Tery long lever would be awJc«
ward and inconveaieat, and extremely tain thieada would b(ft
broken by the preesore when any considerable force was applied
to turn the screw ; so that either method of improving ita actioB
could be practically serviceable only to a limited extent. Them
ie, however, a kind of double or eompound screw, invented hj
John Hunter, the celebrated surffeon, bearing much analogy to tlJa
double capstan or axle, already described^ (see 182) by means of
which the mechwpal eAcaey of th9 maehine may be augmented to
any extent without at aU diminishing ite atrengta or compaotnMa*

207. The marginal figure^ which will show how this object is
attained, repisesents a larger screw turning in a hollow screw or

nut in the fixed beam, and having vrithin it •
concave screw adapted to the lower or smaUer
screw, and so arranged that while the lu«t
screw passes forwardlhe smaller one wilTW
retracted ; hence aa both screws must ievolv»
together, in each revolutioii, the moveable bean
wul be pressed downvrard through a spae^
equal to me diffeiience of the distances b et w e e n
the threads of the lai^r and the smaller screws.
Therefore such a machine, in which the threads
of the upper screw were 1-20 of an inch ajiart,
and those of the lower screw 1-21 of an mch^
would have the same effect as a simple screw«
the threads of which were only 1-420 of an inch apart ; for 1-20
— 1-21 = 1-420, the difference between the distances of the
threads of the double screw just described.

208. A solid screw revolving on fixed axes, and having ito
thread adapted to teeth on the periphery of a wheel, is called aa
endless screw ; forming a part of a compound machine of consi-
derable power and utility. Fly-wheels, as that of a common jack
for roasting meat, are sometimes turned' by the action of a toothed
wheel on an endless screw.

209. Besides its usual application to &e purpose of producing
a high degree of compression, as ik the cider-mill, the common
printing-press, and a variety of sim^arly acting machines, the
screw is likewise employed to measure extremely minute intei^
vals of space. The manner in which this object is attained vdll be
best understood by referring to the theory of the screw, (see 205)
where it is demonstrated that any circle described by an arm or index

By what two expedients miij^bt tbit inweiiie be effected f

Wliat practieal dlffieolties prevent the unlimited aagmeotation ol the

power ox the lerew ?
What is the oonstruetion of Hunter's differential screw I
Through wl^at exteot doet a iipgUttUra of thiaiserew move the placeii

of the press ? v

.. In wMt maniier-ia^tbeeadleaaoF taagtntial- icreir applied for meelnr

nical ptirpoata ?

09 MECHANICS.

MTolriflg paTBllel to the circamference of the screw will have a
certain relation to the space between any two conti^ous threads ;
and therefore a small arc of such a circle may be concei*^^ to
measure the indefinitely minute space throa^h which the point of
the screw would advance or retreat in any given portion of one
eomplete revolntion of the screw. Suppose the threads to be i
i^an inch apart, and a circle fixed to the head of the screw to be
divided on its border into 100 equal parts, then on turning the
screw, the index would show the motion of the point of the screw
through as small a space as 1-400 part of an inch. The interval
between the threads of a screw for such a purpose might be ex-
tremely minute, or Hunter's screw might be adopted ; and the
circle of equal parts might be of sufficient extent to be divided
into 360 degrees, or any larger number of < parts; and thus the
means would be afforded for measuring with perfect accuracy the
almost invisible fibre of a spider's web, or for taking the dimen-
sions of- the capillary vessels throu^ which circulate the juices
of plants and animals, or for discovering the size of microscopic in-
sects or other objects too minute to be perceived by the naked
•ye. An instrument adapted to a microscope for such purposes
is called a micrometer,* and its screw a micrometer screw.

Compound Maehmery,

210. The advantage derived from combining together two of
the mechanic powers, as the lever with the wheel and axle, or
with the screw, has been already detailed ; and it is by means of
combinations of the simple machines, under their various modifi-
cations, liiat a vast multitude of complex machines are produced^
which are adapted to facilitate the numerous operations required
in the several departments of the arts, manufactures, and domes-
tic economy.

311. Among all the simple machines there is no one so gene-
rally useful, and therefore so frequently making a part of com-
pound machinery as that modification of the lever called the wheel
and axle. Its advantageous adaptation to the purposes of the
mechanist is partly owing to the^nature of the motion to which
it gives rise, namely rotation, which is capable of being uninter-
ruptedly continued through a period of indefinite extent ; and to
this advantage may be added the extreme facility with which
wheels may be connected in various modes with other kinds of
machinery. Hence there are few complex machines of which

What is the construction and use of the microineter screw }
On what are the divisions of a thread measured in a icrew of this de«
■oription ?

In what manner are the simple machines commonly adapted to the
mechanic arts ?

By what peculiarity is the wheel and axle rendered more serviceable
to the mechanist than the other simple machines ?

* ITrom the Greek m«x|»o(, little ; and Mi rper, a measure

COVPOVKD MACBIKXftT.

wheels do not constitate the most effectiTe or enential parts.
Thus are formed a vast variety of mills, from the cofifee-mill to the
powerful and complicated engine called a rolling'*mill, for eom-
pressing plates of iron and cutting them into rods or ban ; all the
multifanous kinds of wheel-carriages ; turning-lathes, and grind*
ingwmachines ; clocks, watches, and timekeepers, in general;
spmning-jennies, and many other machines nsed in the cotton,
imen, woollen, and silk manufactures ; and steao^-eoginea under
many of their modifications, to tiMscommodate them to we purposes
to which they are devoted.

212. The peculiar methods in which the parts of machinery
are connected, or the modes of action of one mechanic power upon
another, or upon a different form of the same power, are varionsly
diversified to suit particular purposes. The wheel and pinion,

represented in the margin, consists properly of
two wheels of unequal dimensions, the larger
havmg teeth on its circumference which are adapU
ed to correspondent teeth, or as they are some-
times called leaves, in the smaller wheel or pinion:
thus a pinion may be made to act on a crown
wheel, that is a wheel with teeth placed at right angles to its
circumference ; as may be observed in a watch, or tmiekeeper.
The endless screw is connected with the teeth of a wheel in the
manner represented in the annexed figure.

213. A little attention to the mode
of action of many machines in con-
stant use will afiford opportunities £(»
observing numerous instances of the
different ways in which trains of
wheel-work are combined together,
or made to aid the effect of the other
mechanic powers. These are, how-
ever, generally reducible to two me-
thods of proceeding, namely, either
by teeth, cogs, or some similar parts,
acting against each other, as just
described; or by bands, as cords,
chains, or other flexible lines passing wholly or in part round one
or more wheels and axles, so as to produce simultaneous motion.
214. With respect to the use of either of these methods, it is of
importance to observe the peculiar nature of rotatory motion, which
differs most essentially from what is termed a motion of translatioh,
or passage from one place to another, though it may or may not
aocompany such a motion. Suppose any body, as a billiard-ball,

Enumerate some of the applications of wheel-work. Detcribe the
wheel and pinion.

By what two methocls U motion commanicated from one part to an-
other in a system of wheel-worlc?

How does a raotloa of simple rotation differ fi'om one of transla-
tion ?

)%0 MECHANICS.

j^ Tgi to be pushed frcm A to B, every particle of

^A (j the bdl must hare partaken of the motion ;

but if it be made to spin round in one place,
th6 centre of ihe ball will remain unmored ; for imagine such a
"baU, or a large globular bead to be pierced centneally, and have
fit wire passed through it, the ball might be made to revolve with
any degree of* velocity, while the wire was held perfectly steady.
316. Let a circular disk of paper or any thin suostance be made

to revolve in this manner, on a pin, it will be
perceived that the exterior surface of such a
miniature wheel must move with greater ve-
locity tiian any other part ; so that the point
A will pass over more space in each revolu*-
tion of the wheel than the point B ; and tfa6
latter over mOiC than the interior point C.
Hence it must follow that every circle within
the circumference of a rev(^ving wheel will
iiave a relative velocity corresponding with its diameter ; so that
llie degree of velocity Icommunicated by a wheel in motion to some
other part of a compound machine must depend not merely on the
actual velocitv of the wheel, but on that taken in conjunction with
•the relative distance from its centre at which the communication
takes place; whether it be by means of teedi, projecting pins, or
cords running in grooved cavities.

316. When teeth are made the medium for the communication
of impulse, their peculiar form requires attention; but it can hei^
only oe generally stated that the teeth should be so constructed
as to act upon each other steadily, without ierking or rubbing'^
which would soon derange the machine ; and that Sie teeth most
accurately adapted to prcrauce the required effect, are such as have
their corresponding surfeces forming peculiar curves, the exact
figure of wnich in any case may be ascertained by geometrical
construction.*

21 7« It is likewise desirable that the teeth of one wheel should
work successively in those of the corresponding wheel, and that
tiie same teeth should not meet in each consecutive revolution of
4he larger wheel ; as they will thus act more uniformly, and wear
avray more slowly than if the same teeth came in contact more
frequently. This object is effected by making the numbers of
the teeth of wheels acting together, or of a wheel and its pinion,

How may this difference be illustrated la the motions of a billiard
ball ?

By what will the velocity^ of motion of every etrele in a revolving
wheel be determined ?

On what two ciroamstances in a driving wheel will the degree of ve-
loeiUr in the driven machinery depend }

What circumstance requires pariicolar attention ia the constntction of
toothed wheels ?

* See Leslie's Elemento of Nat Philosophy, vol. i. p. 199— 207*

WHBEI.-WORK. l^t

•8 discordant as possible ^ so that the number of the teeth in the.
small wheel may never be an aliquot part of the number in the
lar^r wheel, llius, if a wheel of sixty teeth be turned by m
pinion having but ten, each of the latter would come in contnci
with the same teeth of the former in each of its rerolutions^ or
in every sixth revolution of the pinion ; but if the larger wheel
have sixty-one teeth, it must be manifest that no two corresponding
teeth of the wheel antl pinion respectively can meet more than
once in every sixty-one revolutions of the pinion, during which
the wheel will have revolved ten times. The odd tooth or cog
by which this effect is produced is called by millwrights the
hunting-cog*

218. In the construction of complex machines, it is not merely
requisite that they should afford the means of communication be*
tween the power and the resistance, and enable the former to
overcome the latter by the combined assistance of two or more
of the mechanic powers, or simple machines ; but it also often
becornes an object of the highest importance to change the di-
rection of any given moving power or acting force, without which
it may be utterly inapplicable to the intended purpose, and there*
fore quite useless.

319. Reciprocating rectilinear
motion may be changed into
circular motion, by a crank ap-
plied to turn a wheel, as may
be seen in. the common knife-
grinding machine, and in the
turning-lathe ; and the same ef-
fect is produced by what has
been fancifully styled the sun
and planet wheel, represented
in the margin ; one wheel fixed
at the extremity of a vertical
rod which rises and falls alternately, acting by teeth on its peri-

fthery on a similar wheel to which it communicates a double ve-
ocity ; and thus the fly-wheel of a steam-engine was formerly *
made to revolve, but this method is now generally superseded by
the crank. •

220. The opposite effect of curvilinear motion producing alter-
nate rectilinear motion may be observed in the manner of working
the pistons of an air-pump, or a fire-engine, as in the marginal
figure below. A very ingenious contrivance for the conversion of
rectilinear into curvilinear motion, or rather for producing an ac-
curate correspondence between such motions, is displaycMd in the

How is the irregularity of wear, from the frequent meeting of the
same teeth in a wheel and pinion to be avoided ? \Vhat is meant by a
** hunUng cog ?*\

How may reciprocating rectilinear motion be changed into circahur
motion ?

How is curvilinear converted into rrctilinear reciprocating motion ?

I 2

IM HEC1IAM1C8;

gyBtem of jointed bars used to connect the piston-rods of the
steam-engine and its air-pump with the great beam, whose reci-
^ocating motion transmits the necessary force to the fly-wheel
and other parts of the machine. A much clearer idea of the nature
of this contrivance, termed the parallel motion, may be attained
horn inspecting a steam-engine at work, than fiom a detailed de-
scription, even with the aid of a figure represent-
ing Its construction.

231. The universal joint, invented by the cele-
brated Dr. Robert Hooke, affords a simple and
efficient mode of transferring rotatory motion from
one axis to another in an angular direction ; bnt
this may be done with greater accjiracy by means
of beveled wheels, which, as will be understood

from the foregoing figures may be made to act on each other at
any angle whatsoever.

. 222. The regulation of the velocity or rate of motion is of the
highest consequence to insure the efficiency of compound machine-
ry. When two or more of the mechanic powers are made
to act in concert, they must necessarily have certain points of
contact; and the material substances of which macnines are
eottstructed, bein^ subject to variations of density and dimen-
pions from the action of heat and cold, or other causes, regularity
of action cannot be perfectly attained, unless some mode can be
adopted to prevent the changes just mentioned from taking place,
or to counteract their efiects ; so that there may be such a stabili-
ty in the points of contact of the mechanic powers, as to produce
uniformity of combined action. Thus, in a clock or timepiece,
uniform motion is propa^ted throughout trains of wheel-work, by
means of a pendulum oscillating seconds ; and the pendulum there-
fore acts the part of a regulator to the clock.
.^ 223. In describing the pendulum and its peculiar kind of motion,
}t has been stated that to beat seconds it must have a certain

What was the purpose of Watt's jointed bars, used in the construction
•f his steam-engines ?

For what purpose are beveled wheels applied in the construction of
Isachines ?

To what great purpose are regulators applied in the movementt of
maoWaety ?

REGULATION OF 3UCHIKERY.

103

ieiifth, correfiBotulinff to the latitude of the place of obserratioiif
or more strictly speaking to the distance of that place from 'the
earth's centre. Now it has been discovered from obseryation that
a pendulum-rod of brass, steel, or in ^t of any substance adapt-
ed for such a use, will be elongated by heat, and contracted by
cold ; and that to such an extent by the conmion changes of tern*
perature in the atmosphere at different periods, that a pendulum
which would vibrate once in a second in the winter, would take up
more than a second in performing one vibration in the summer;
and hence it would require to be shortened at the latter period,
and lengthened again at the former to make it act with any tole-
rable degree of uniformity. To refirnlate a clock in this manner It
is obvious that the error must be observed before it could be cor-
Irected, and therefore this method though it might serve for com-
mon purposes, would be nearly useless to the astronomer or the
navigator, requiring a uniform and accurate measurement of a con-
siderable period of time, by means of an instrument more or less
exposed to alternations of temperature. The construction of a
pendulum which should preserve its len^ unalterably in all
situations, thhs became an object highly mt^esfting both to phi-
losophers and mechanics ; and the co\itrivances which different
individuals have adopted or proposed have 1i>een numerous and
diversified.

324. The general principle on which compensation pendulums,
BS they are termed, act, may be comprehended from ue annexed
£gure and description.

Suppose C D E F to represent a steel frame, and
G H a bar of metal connected by the copper rods 6
I and H K vnth the bar D E, to which they are firm-
ly fixed. The rod O P being fastened by a pin to
the bar 6 H, descends from it through an aperture in
the bar D E, hangin? freely from me point 0, and
supporting the pendulum-bob P : the pendulum turn-
ing on the suspension-spring A B. Now when the
longitudinal rods are dilated by heat, the elongation
of Sie rods G I and H K, will tend to raise the bar
G H to which the rod O P is attached ; but the cor-
responding elongation of the latter will tend to lower
'* the point F ; and if the apparatus be properly arrang-
ed the lengthening of one set of rods will compensate
that of the other, as they must take place in opposite
directions. On similar principles are constructed
Harrison's gridiron pendulum and the numerous sub-
sequent inventions, the common object of which has
been to obtain a pendulum-rod, the point of contact

What eharacter in the pendulum is indispensable in order to make it
beat seeonds ?

By what circumstances is Jt prevented from acting in its simple form
as a perfect regulator ?

At what season of the year would a dock with a sunple pendulura
mov« most rapidly ?

104 MECHANICS..

or axis of saspension of which shall be at a certain and invariable

distance from the centre of oscillation.

225. Thus it has been shown how the effect of a singrle cause
of irregularity of action in machinery may. be obviated ; but in the
greater number of the complex machines employed for various
purposes connected with arts and manufactures, there are often
several different circumstances contributing more or less to pre-
vent regular or uniform action. Besides uie difficulty of main-
taining certain poiats of contact between the moving parts of
machines, owing to inequality of temperature and consequent con-
traction and expansion of solid bodies, there are additional diffi-
culties arising from the gradual wearing away of surfaces by
friction and from other causes.

226. But admitting the possibilitv of preserving the points of
contact of the parts of a machine mvanable for a certain period,
abundant causes of irregular action might still exist ; among which
may be mentioned, as one of the most important, the irregular
effect of -the moving power. A familiar example of such a case
will occur in the common handmill, used by grocers to grind
coffee or cocoa ; for a greater degree of strength mhst be exerted
to turn the winch or handfle of sueh a mill at the lowest point of
the circle which it forms, in turning, than at the hi^est point ;
and thus the machine could not be made to act with an equable
motion, but for the heavy fly-wheel, connected with the axis of
the mill, which equalizes the effect, and enables the man to turn
the mill with any required velocity, working without interruption
or extraordinary efforts.

227. The variable inciting forces are, by the intervention of a
heavy wheel, blended together in creating one great momentum,
which afterwards maintains a nearly uniform action. The use of
the fly in mechanics hence resembles that of a reservoir, which
collects the intermitting currents, and sends forth a regular
stream.* That distinguished philosopher has given a description
of a machine called the concentrator of force, by means of which
an inconsiderable power, acting on a fly-wheel, may be made to
produce a vast momentary effect. On this principle of the effect
of the concentration of force depends the action of the coining-
press used for striking pieces of money. The momentum com-
municated to the machme by a man whirling round for a few
seconds the balls at the ex^mities of a horizontal bar, will cause

How does the compound pendulum obviate the irregiilaritj of a clock's
movement ?

What other difficulties besides those already enumerated interfere with
the action of machines ?

Wliat is one of the most important sources of irregularity in a ma-
chine?

What familiar illustration of this irregularity ?

By what means can force be concenUated ?

How is the coining-press enabled to produce its intense pressure?

• Leslie's Elements of Natural Philosophy, vol. i. p. 177.

COMPOirMD MACfllKERT.

106

<■>

the screw to descend with such force, carry-
ing the di^ aranst a circular disk of metal,
as to give it Uie required impression at one
stroke. This machine is said to have been
invented by Nicholas Briot, mint^master
{taiUeur^enemldu moimoiui) to Lonis XIII.
of France ; and by using it one man may do
as much work as twenty, striking coins with
a hammer, which was the old method of
coining.* .

2d8. A complicated machine, such as the steam-engine, requires
various modifications or adaptations of its essential parts, and tlie
addition of some peculiar parts to equalize or compensate irregu-
lar movements, and enable ^e engine to work with due accuracy
and effect. Besides the fiy-wheel, which is a necessary appen-
dage to the common low-pressure steam-engine, there is another
very ingenious and important contrivance, called the governor. It

consists of two heavy balls, connected by jointed
rods with a revolving axis, by any increase in
the velocity of which they diverge or separate
from each other, and draw downwards the jointed
rods ; while a slower motion of the axis causes
the balls to approach each other, and the system
of rods to be contracted laterally and be extended
upward. The grand effect produced by this
means depends on making the ascending and
descending extremity of the johited rods raise or
lower the end of a bar which aetis as a lever, and
moves a valve which regulate8.the supply of steam from the prin-
cipal steam-pipe. A similar method of controlling the effect of
moving power is applicable to wind and water imlls, and other
kinds of machinery.

229. Whatever may be the complexity of a machine, or however
Taried its action, its effect, theoretically considered, is to be esti-
mated according to the principles already laid down relative to
simple machines. There must be in every case an equality of ef-
fective action in the power and the resistance in order to produce
equilibrium; and consequently the efficient force of the power
must, with the assistance of the machine, exceed that of the weight
or resistance, before motion can take place.

230. It may be generally stated that a power can counterbalance
any given resistance, when the momentum of the former is rendered
equal to that of the latter. This has been repeatedly demonstrat-

In whfit manner whb the proeen of eoining performed before the pe-
riod of Briot's invention ?
On wbftt principle of motion is the mill gorernor eonctraeted ?
How is the effect of a compound machine to be estimated ?
When will motion succeed to a state of rest in any giTen machine 1

* y. Sigand de la Fond Elemens de Physique, 1787, t ii. p. 184.

106 MECHANICS.

ed in treating of the several simple machines. Thus, it has been
shown that a lever can be kept in e(|uilibrium only when the nutn-
ber of ponnds in the power, multiplied by the number of feet it
would describe, if put in motion, gives a product exactly equal to
that of the number of pounds in the resistance multiplied by the
number of feet in the space relatively described b^ it ; so that the
spaces passed through by the power and the resistance must al-
ways be in the inverse ratio of their respective weights, or actual
independent forces. Hence it follows that whatever advantage
is afforded by a machine, so as to enable a small weight or other
weak power to overcome a great weight or resistance, must de-
pend on communicatinflr to the power a degree of velocity, or
causing it to act through a space which shall more than equalize
the momentums of the antagonist forces.

231, It may perhaps be remarked that machines, regarded in
this point of view, give no additional force; since in order to raise
a weight of 500 pounds, a power must be made to act with an
effect superior to 500 pounds, either weight or pressure. The
object of machinery certainly is not to create force, which is im-
possible, but to accumulate, distribute and applj it, so as to pro-
duce certain effects ; and the advantage thus afforded is often of
the highest importance. Thus, a man, with a crow-bar, may be
able to turn over a log of wood, or a block of stone, which unas-
sisted he could no more move than he could one of die Egyptian
pyramids. But to raise such a mass of wood, or stone with a
crow-bar or lever, he must make the end of the bar to which he
applies his strength jnove through a space, probably fifty or sixty
times as great as that through which he would move the log or
block. So likewise if a man, who could pull with a force only
equal to 50 pounds, wanted to raise a bale of goods weighing 500
pounds through the space of 12 feet, he might do it by means of a
tackle of pulleys, but if it afforded him the assistance precisely
necessary to supply his deficiency of strength, it must be so con-
structed that he would have to pull down 120 feet of rope, in order
to make the bale ascend 12 feet. These examples will probably
suffice to illustrate the nature of the equilibrium of action resulting
from the application of machinery; and hence it will be apparent that
whatever be the moving power employed for any purpose, though
its actual force cannot be increased by any machine, as such an
increase would involve physical impossibility, yet its effective force
may often be indefinitely augmented ; that is, its actual force may
be made by a machine to overcome an actual resistance, to which,
alone, it would be utterly inadequate.

In whftt proportion to tlie power and resiitance mast be the spaces
which they respectivelv describe at the commeneement of motion ?

On what must the acfvantage of a machine for overcoming a great re-
sistance always depend ?

What is the time object of machinery in regard to mechanical foree ?

How may this be illustrated in the raising of a weight by the aid of a
lever?

THE VEDOS. 107

332. The action of machinery necessarily requires time to pro-
duce any given effect* Motion can in no case be instantaneous,
however rapid; and when it is the result of the operation of com-
plicated machinery, it must be relatively slow. It may indeed
be the real object of a piece of mechanism to extend a series of
consecutive movements through a certain period ; and v>f such an
arrangement examples may be found in common clocks and
watches. In an eight^day dock, for instance, a couple of weights
are wound up to a certain height, and left suspended to act by
their own gravity in setting in motion trains of wheel-work which
shall cause the indexes, or hour and minute hands, to describe
given circles in certain spaces of time, so as to famish a me-
thod for the e^ual measurement of time ; and the gravitating pow-
ers of the weights are so opposed by the resistance distributed
over the numerous wheels and pinions, that though the weights
may each amount to several pounds, they may descend so slowly
as to be more than a week in passing through the space of five or
six feet.

233. A story is told by an ancient writer, relative to the cele-
brated Archimedes, from which may be drawn a most pointed
illustration of the immensity of time and space required to pro-
duce mechanical effect, where the disproportion between the pow-
er and the resistance is extremely great. In relating the history
of the siege of Syracuse by the Komans, Plutarch, m his Life of
Marcellus, the Roman general, says that Archimedes told Hiero,
king of Syracuse, whose confidence he possessed, as being re-
lated to that prince and highly esteemed by him, that by his me-
chanic skill, he could, if there was another earth for him to stand
on, move the solid globe which we inhabit. Hiero, astonished at
this assertion, requested the philosopher to afford him some de-
monstrative evidence of its truth, by letting him behold a very
large body moved by a small force ; and the historian adds, this
effect was exhibited by Archimedes, who sitting on the sea-shore
drew into port, with one hand, a lars^ ship heavily laden, and
having a number of men on board. Iliis he is stated to have
done by ^ntly moving the handle of a machine called polyspas-
ton, a pulley.

234. It has been remarked, that if Archimedes had proposed to
move the earth by means of a lever, and had obtained not only the
place he required to stand on, but also another whereon to fix his
fulcrum, with an hypothetical lever of requisite length and strength,
and had also been endowed vrith muscular power sufficient to
enable him to act on the end of his lever so as to move it with the

With what are the action and effect of machinery neeettarily con-
nected ?

How are mechanical forces made capable of nipplying a measure of
time ?

How is the importance of time to mechanical actions exemplified in
the celebrated assertion of Archimedes ?

How did that philosopher illusU'ate the truth of his statement?

168 moiiAifies.

Telocity of a eannon-^iall, he would not have shifted the earth more
than the twenty-seven millionth of an inch in a million of yean;
and supposing him to have had hut the average power of a strong
man, it would have taken him 3,653,745,176,803 centuries to have
moved the earth with the machine he had in view in his address
to his royal relative.*

235. Faradoxical as these statements may appear, it may be
easily shown that the^ are founded on mathematicnu evidence. To
comprehend this it will be only necessary to consider how fur into
boundl^s space such a theoretical lever as that imagined for Ar->
chimedes must have extended, and the consequent ineomj^hensi^
ble immensity of the arc which such an imaginaiy- lever must be
supposed to describe.

236. Those who have leisure and inclination for making such
computations lAay ascertain what length of theoretical rope must
be drawn over imaginary pnUeys, to raise through the space of
one inch, by means of a power equal to seventy-two pounds, a
spherical mass 8000 miles in diameter, having a mean density
five times that of water, and taking the weight of a cubic foot of
that fluid to be 1000 ounces avoirdupois. The result of such a
calculation would afford an approximation to a fair estimate of the
fancied task of Archimedes ; and would strikingly evince the utter
insignificance of human skill and science when contrasted with
the powers of naiture.

Obaenatums on Friction / on the Bigidity of Cordage f and on the

Strength of Materials,

237. In making calculations or estimates of the effective force
of moving powers applied to machinery, it is always necessaiy to
admit certain deductions on account of the obstacles to fireeaom
of motion arising from friction, the rigidity of cordage, or the im-
perfections of the materials of which machines must be construct-
ed. All these subjects are of the highest importance to practical
mechanicians, and are therefore deserving of the most accurate
attention; but it will be sufficient here to describe briefly the
nature of these obstructing or retarding forces and to notice the
methods usually adopted for lessening or correcting the inconve-
niences they may produce.

How rapidly might the theoretical lever of Archimedes have enabled
hira to move the earth ?

What are the elements of ealeaktion to show the practieal resalt of
such an attempt ?

In what light would the computation place huoiaa skill and aitifioial
powers ? '

On what accounts are deduotions from the theoretical effects of mar
chines rendered necessary ?
- .. - I I — • - - - — - — -

* Recreations in Mathematics and Natural Phiioaopby, edited by Dr.
Charles liattoo, vol ii. p» 1%

. FRICTION. 109

238. It is the well-known consequence of friction that when
one substance moves in contact with another, either at rest or
movin? in an opposite direction, more or less force must be applied
to produce motion in proportion to the roughness or smoothness
of the surfaces of the two bodies. No substance can be perfectly
smooth: npt^ven polished steel or g^hss. Those surfaces that to the
naked eye seem free from i^e slightest inequalities are found,
when examined by a powerAil microscope, to be covered with in-
numerable rising points and hollows, like the face of a file ; and
sometimes to be intersected by abundance of irregular ridges and
furrows. Now when surfaces, such as have been just described,
are made to move in contact, the prominent parts of the one will
pass into the depressions of the other, and thus occasion more or
less difficulty in procuring lateral motion.

239. Though friction, from its effect in retarding motion, lessent
the advanta^ derived from maohineryf and often causes inconve-
nience, yet It is one of those properties of matter which we find tc
be of almost indispensable utility. If all bodies were destitute of
friction it would be very difficult for us to grasp or retain in our
hands any solid substance ; a penknife, a ruler, or a book would
slip through our fingers, if not held tightly ; and in using our
hands for any purpose, such a degree of muscular power must be
^exerted as would be extremely feitiguing and inconvenient. But
without friction it would be still more difficult to use our feet than
our hands ; and no man could walk upright unless he possessed
the skill and activity of a tight-rope dancer, or a performer on the
glack-wire.

240. The consequence of losing the advantage derived from
friction in walking, may be easily conceived, when we reflect on
what takes place when friction is partially destroyed by the streets
and open pavements being covered with ice, as occasionally hap-
pens during the winter season. Arming the soles of the shoes
with list, or with projecting nails, and covering the ice with saw-
dust, ashes, or other loose substances, are among the usual methods
resorted to at such times, to restore friction, and enable people to
walk steadily.

241. Friction is likewise advantageously used as a means of
sharpening or polishin? various substances, by rubbing, grinding,
and other operations of great importance in several arts and manu-
factures. This property of matter may even be applied to the
production of motion ; at least it may be made the medium of com-
munication between one part of a machine and another. Thus

To what is the resistance from friction always proportioned ?
What is the true nature of surfaces comroonly considered. wnoo/A^
By what means is the true character of surfaces to be detected ?
What is supposed to be the real mode of action by which friction op*
poses, i*etard8, or destroys motion ?

Of what advantage is friction in the ordinary purposes of life ?
How is its importance in walking made apparent r
How is friction employed la manufacturing processes ?

Ik

110 BIXCBANIC8.

wheels are sometimes covered on their peripheries with baff-lea«
ther, and one of thehi being set in motion will then turn the other,
by the friction of the rough surfaces of the leather, acting as if the
wheels had been furnished with innumerable series of minute
teeth.

242. Such are the benefits of friction, but in many cases it
proves a very inconvenient proper^ of matter, hindering freedom
of motion, and tending to obstruct it entirely ; and hence in the
construction of machinery various contrivances are adopted to
lessen or destroy the effect of friction. Systematic writers have
distinguished this property of matter into two kinds : namely,
1. That which takes place when two flat surfaces are moved m
contact, so that the same points of one surface are constantly ap-
plied to some part of the other ; and 2. The friction that takes
place when one body rolls over another, so that the points of con-
tact of both surfaces are perpetually changing. The former may
be styled dragging friction, and the latter rolling friction.* It
must be obvious that the retardingr effect of the former must be
vastly greater than that of the latter kind of friction. It is for this
reason that plumbers, masons, and carpenters, when they want to
move a heavy mass of metal, stone, or wood, place beneath it
several cylinders of hard wood, by means of which such a mass
may be dragged forward without coming in contact with the
ground, and the immense friction of the first kind or dra^n^ fric-
tion which must otherwise occur, is changed into rolling friction
or rolling motion. On the oftier hand rolling Motion is converted
into dragging friction, by shoeing or locking the wheel of a car-
riage in going down a steep hill.

243. On the principle just stated depends the utility of those
parts of some complex machines, called friction-wheels. In
wheel-work the chief friction takes place between a wheel and the
axle on which it turns ; and to diminish its effect it is usual to

How may it serve as a means of communicatinc motion ?

Into how many kinds has friction been divided by systematic writers ?

Which of these exists in tlie mechanical devices for moving heavy
masses ?

What kind of friction exists at the axle, and what at the periphery of
a carriage-wheel ?

How is the friction of the axis of carriage-wheels diminished by means
of rollers ?

* The terms rotting" and dragging friction seem more appropriate than
interrupted and continued friction ; as expressive of the mode of action
by which they are respectively produced* A true case of rolling friction
takes place only when the rollers are so situated as to have no necessity
of employing axles ; as when cylinnders or cannon-balls are placed be-
Beath heavy weights. The wheel-carriage is not a case of this kind, for
it only transfers the friction which would take place at the periphery if
the wheel were locked to the axle, which experiences a dragging friction
within the box. — See on this subject Journal of the Franklin Institute,
vol. 5, p. 57.— E».

RIGIDmr OV CORDAGE. ' 111

construct the axle and the box, or central part of the wheel, of
very hard substances, the surfaces of which are not only rendered
as smooth as possible, but also coirered with oil, or some other
unctuous matter, which facilitates the motion of the corresponding
parts. But where it is necessary to obtain the utmost facility of

motion, a method has been adopted for subdi*
yiding friction, by letting the axle of a princi-
pal wheel move on two or more small wheels,
as in the marginal figure. These are named
friction-wheels. *

244. In estimating the eSeci of friction, so
many circumstances must be taken into the
account, that the result, in any given case, may
afford but little assistance in deciding others. It may however
be stated, as the most important deduction from repeated experi-
ments, that friction does not depend on the extent of the surfaces
on which it acts, but chiefly on the degree of pressure to which
they are subjected ; so that, the surfaces continuing in the same
state, increase of pressure will produce increase of friction.

245. When a heavy body is placed on an inclined plane, it will
.have a tendency to slide ; and consequently will remain at rest on
such a plane, only when the retarding effect of friction is greater
than the tendency for motion, caused by the inclination of the
plane. Hence the angle of inclination at which motion on an in-
clined plane commences, has been styled the angle of friction /
and it will of course be different in different cases, according to
the nature of the rubbing surfaces, and the degree of pressure.

246. The rigidity of cordage is another property of solid bodies
which interferes with the freedom of motion in some kinds of ma-
chinery. It must necessarily depend on the peculiar nature of
the materials used, since the more flexible they are, the more rea-
dily will they become adapted to the wheelti or spindles around
which they are coiled ; and the smaller will be the interruption
of regular continued motion. It is principally when very thick
lines are used, such as the cables for heaving anchors for very
large ships, that this rigidity of cordage becomes a serious impe-
diment to motion, requiring the expenditure of great force to over-
come it. Iron chains have been advantageously introduced into
the maritime service, instead of cables ; and are likewise employ-

What is ODe of the most important deductions from experiments on
fnction ?
What is meant by the angle offiiction'P
Is this angle constant or variablll ?

To what does the rigidity of cordage usually offer its resistance ?
What substitutes for large cables have of late been adopted ?

• The railroad cars of Winans, Howard, and several others, employing
friction- wheels, have been invented in the United States, and will be
found described in the early volumes of the Journal of the Fiwiklin In-
stitute. — ^Ed.

1 12 MBCHANIC8.

ed for various purposes in the arts to which ropes alone were for-
merly considered applicable.

247. In the construction of machines much must depend on the
strength and firmness of the materials of which they may be com-
posed. Thus, in the case of one of the most simple machines,
the leyer, suppose a long pole to be applied to raise a considerar
ble weight, much of the effect of any power would be lost in con-
sequence of the bending of such a wooden lever at that part which
rested on the fulcrum ; and therefore an iron bar, nearly inflexible,
of the same length with the pole, would enable a man to move any
given weight or resistance with less exertion.

248. The hardness, tenacity, elasticity, and other properties of
bodies, on which their relative strength must principally depend,
vary so greatly even in different specimens of the same su|>stance,
as wood or metal, that few rules of general application can be
given for computing the degrees of force, which may be applied
with safety to the particular parts of any complex machine. Any
solid substance, as a bar or rod of iron, may be subjected to ten-
sion or pressure in different ways : as, (1), by suspending to it a
great weight, or endeavouring to stretch it longitudinally ; (3),
by weight or pressure applied to crush or compress it ; and (3),
by weight or pressure applied to the centre of a bar or ro4 i<«
extremities alone being supported.

249. It appears from experiment, that in the first case, the
length of a rod remaining the same, its strength will be increased
or diminished in proportion to the area of its transverse section;
thus, as 27 tons weight will tear asunder an iron bar one inch
square, so a bar but half a square inch in the section will be
broken by a weight of 13^ tons ; and so on in any given proper*
tion. Ooncernin? the capacity of bodies for resisting compres-
sion, but little is known with certainty. Much appears to depend
on the form of a body, for a cubic inch of English oak required to
crush it a weight of 3860 pounds ;- but a bar an inch square and
five inches high ?ave way under the weight of 2572 pounds ; and
if longer it would manifestly have broken with a less weight.
Mr. Rennie, one of the architects of London Bridge infers from
calculation that the granite of which the great arch of that bridge
IS constructed would bear a pressure equal to four tons upon every
square inch of its upper suriace.

250. As to the strength of bodies exposed to transverse or late-
ral pressure, one or both enda bein? supported, it depends on the
dimensions of a section of the body in the direction of the pres-

On what circumstnnces must the usefulness of machines chiefly depend ?

Name some of the physical properties of materials which Tary their
usefulness in die mechanic arts.

In how many ways may a solid rod of any material be subjected to me-
chanical action ?

How is direct tendon applied? how crushing pressure? how crass
strain?

What inference has been drawn by Rennie from experiments on era*
aite ?

UOVINO P0WSR8. 118

fmre. Thus a beam having the same length and breadth with
another, but t-wice its dr'pth, will be four times as strong ; and a
beam double the length of another, but with the same breadth
and depth, will have but half as much strength. Hence the
strength of solid bodies is not by any means to be estimated by
their absolute magnitude.

251. Hollow cylinders are much stronger than solid ones of
equal length and weight ; and therefore it appears an admirable
provision of nature that the bones of men and other animals in
those parts requiring facility and power of motion are more or less
of a cylindrical shape, with cavities in the centre, which in birds
are filled only with air, whence partly their capacity for fli^t ;
but in men and beasts the cavities are filled with a light oily nuid,
which congeals after death, forming marrow. The strength or
efficient power of an animal depends chiefly on the accurate con*
struction and adaptation of its several parts.

252. Some very small creatures possess muscular power, in
proportion to their bulk, incomparably greater than that of the
largest and strongest of the brut^ creation. A flea, considered
relatively to its size, is far stronger than an elephant or a lion ; as
will appear from comparing the distance the insect would leap at
one bound with its actual dimensions, with reference to the spring
and dimensions of the quadruped. Some marine animals, as the
whale, are of vast bulk ; nature having provided for their conve-
nience by giving them a medium of great density to inhabit.

Moving Powers,

253. The original forces which produce motion, and which have
been denominated Moving Powers, or Mechanical Agents, are cf
various kinds, depending on the natural properties of bodies. Gra-
vitation or weight is an extensively acting power aflTecting matter
in all its different forms, and affording the means of originating
motion for many useful and important purposes. By the proper
application of weight is excited and maintained the equable motion
of wheel-work, as in a common clock ; and the same power differ-
ently adapted is made to act by percussion, in pile-driving and
numerous other operations. Currents of water owe their velocity
to the weight of the descending liquid, yielding a kind of movin?
power on which depends the effective force of water-wheels ana
other hydraulic engines.

254. Elasticity is another property of matter which gives ener-

What relation exists between tbe dimensions of a beam and the resist-
ance which it is capable of opposing to cross strain }

What advantage does the hollowness in the bones afford to the strength
of animals ?

How are we to judge of^ the relative strength of insects and of large
animals ?

What is one of the most common mechanical forces, and in what dif<«
fenmt modes is its efficacy applied ?

s d

114 BIECHANtCS.

gj to vanous mechanical agents. Elastic metals, as steel, mann*
factored into springs, are used in the construction of watches or
chronometers ; and the contractile force of springs is employed for
many other purposes, as in roasting-jacks and weighing-machines.
Liquids, though compressed with difficulty, display a high degree
of power when thus treated ; and machines of vast energy hare
been invented, the effect of which depends on the expansive or
elastic force of compressed water. The elasticity of air is like-
wise an abundant source of moving power. Steam-engrines, such
as were used in the early part of the last century, were made to
act through atmospheric pressure, arising from the joint influence
of the weight and elasticity of the air ; but since the vast improve-
ments in machines of this description, in consequence of the re-
searches of Watt, and other experimental philosophers, steam or
elastic vapour is employed as the sole moving power, and so
managed as to produce effects far beyond those of the old atmo-
sphenc engines.

,255. Heat must be regarded as a movinjv' power, the efficacy
of which depends on its tendency to dilate different kinds of mat-
ter. It also converts solid bodies to the liquid state, and liquids
under its influence are changed into vapours or gases. Hence
indeed is to be explained the 'operation of the steam-engine, in
which alternating motion is proauced by the expansive force of
steam or water raised, to the state of vapour by means of beat.
Combustion is a chemical process, often excited by heat, an4
during the progress of which heat is always developea ; and from
this source is derived moving power of vast intensity, as occurs
in the discharge of shot or balls from fire-arms, through the ex-
plosion of gunpowder. In this case the moving power arises from
the sudden expansion of gases formed by the combustion of solid
matter ; but engines have recently been constructed the action of
which depends on the formation of a partial vacuum by the in-
flammation of oxygen and hydrogen gases in close vessels, and
the consequent production of water.

256. Machines may be set in motion by means of electricity,
galvanism, or magnetism ; and forces, which have been chiefly
regarded as objects of curiosity may be extensively applied to
useful and important purposes. In a French periodical publica-
tion (Journal de Geneve, 1831]), some account is given of an elec-
trical clock, invented by M. Bianchi of Verona. This timekeeper
has neither weight nor spring, instead of which the constant vibia-

State some of the applications of elasticity to purposes connected with
the arts ?

What is the difference in principle between the atmospheric en^ne of
Newcomen and the modern steam-engines of Watt and Evans ?

Qn what is the efiicac^ of heat as a moving power dependent ?

In what other modes is heat occasionally applied to produce mechanie
ction ?

What other imponderable agent, besides heat, is occatioiuilly employ-
d as a moving force ?

HITMAN BTREKOTH. 115

tion of the pendulum is maintained by the impulse of electricity,
which it receives by moving between two galvanic piles, the ball or
or bob being furnished with a conductor, which in its oscillations,
approaching either pile, alternately, is repelled by the discharge
of the electric fluid ; and the regular action of the whole of we
machinery is kept up.

2^7. lliese cursory observations will afford some general ideas
of the nature and extent of the moving powers originating Irom
the influence of elastic fluids, heat, and electricity ; but the further
discussion of these topics must be referred to the subsequent por-
tion of this work, where the phenomena connected with these
subjects will be distinctly noticed. There are, however, besides
those moving powers, the operation of which depends on the phy-
sical properties of matter in diflferent states of aggregation, other
mechanical agents, the eflfects of which arise from the vital ener-

fy of animated beings ; and concerning these some details may
ere be properly introduced.

258. The application of the natural strength of man must have
preceded the employment of all other moving powers ; and we
know from history, that ever since a very remote period brute ani-
mals have likewise been rendered subservient to the purposes of
art and industry. The employment of oxen and horses in the
labours of the fleld must have originated in the earliest ages ; and
the art of training beasts of different kinds to exert their strength
for the benefit of man has been known and practised among almost
all nations except those in the very rudest state of society.

259. The mechanical effects produced by the muscular exertions
of living beings cannot be subjected to calculation on precisely
the same principles as the moving power of a weighing-machine
or a steam-engine ; nor even can they be estimated with so much

Erecision as the efficient power of a windmill or a water-wheel ;
ut there are modes of obtaining data whence to determine the
value of animal strength as a mechanical agent, which may serve
to indicate the comparative product of labour from that and other
sources, and enable us to discover their relative importance for
any given purpose.

260. The usual method of computing the mechanical value or
efliciency of labour is from the weight it is capable of elevating to
a certain height in a given time, ue product of these three mea-
sures (weight, space, and time) denoting the absolute quantity of
performance. But these measures have obviously a mutual rela^
tion which will affect the result ; for great speed will occasion a

Describe Bianchi'sjnivanie clock.

On what do the efrects of animal efforts depend when emplojed for
mechanical purposes f

What were among the earliest xooUc moving forces emplojed in the
arts ?

Can the power of animals be aceuratelj computed bj their weight and
velocity ?

What three elements enter into the oomputttion of animal power ?

116 MECHANICS.

waste of force, and shorten the period daring which it can be ex-
erted. It was compated by Daniel Bernoulli and Desa^liers
that a man could raise two millions of pounds avoirdupois one
foot in a day. But some writers have calculated that a labourer
will lift ten pounds to a height of ten feet every second, and
continue to work at that rate during ten hours in a day, raising in
that time 3,600,000 lbs. But these estimates are certainly incor-
rect, and appear to have been founded on inferences drawn from
momentary exertions under favourable circumstances. Smeaton
states that six good labourers would raise 21,141 cubic feet of
sea-water to the height of four feet in four hours ; so that they
would raise about 540,000 pounds each to the height of ten feet
in twenty-four hours.

261. Coulomb has famished some of the most exact and varied
observations on the measure of human labour. A man will climb
a staircase from 70 to 100 feet high, at the rate of 45 feet in a mi-
nute; and hence, reckoning the man's weight at 155 pounds, the
animal exertion for one minute would be 6975, and would amount
to 4,185,900, if continued for tftn hours. But such exercise would
be too violent to be thus continued. A person might ascend a
rock 500 feet high by a ladder-stair in twenty minutes, or at the
rate of 25 feet a mmuto : his efforts are thus already impaired,
and the performance reduced to only 3875 in a minute.

262. But with the incumbrance of a load the quantity of action
must be yet more remarkably diminished. A porter weighing *
140 pounds, who could climb a staircase forty feet high two
hundred and sixty-six times in a day, was able to carry up only
sixty-six loads of fire-wood, each weighing 163 pounds. In the
former case, his daily performance was very nearly 1,489,600;
while in the latter it amounted to only 799,920. The quantity
of permanent effect in the latter case* therefore was only about
600,000, or little more than half the labour exerted in mere climbing.
A man, drawing water from a well by means of a double bucket,
may raise 36 pounds one hundred and twenty times a day, firom a
depth of 1 20 feet, the total effect being 5 1 8,400. A skilful labourer
working in the field with a large hoe produced an effect equal to
728,000. When the agency of a winch is employed in turning a
machine, the performance is still greater, amounting to 845,000.

263. The effective force of human exertion differs according to
th6 manner in which it is applied. From some experiments made
by Mr. Robertson Buchanan, it was ascertained that the labour
of a man employed in working a pump, turning a winch, ringing

What arc the sappositions adopted by Bernoulli andDesaguliers in re-
gard to the amount of human effort ?

To what results did Coulomb arrive in respect to the speed of human
movements, and to the continued daily labour of men when working only
to raise their own weight, and when carrying up additional burdens ?

According to what circumstances does the effective force of human ex-
ertion vary r
■

* The uteftd elect in the former ease sras 0; in the latter it was 490,990.

HIJHAK 8TRENOTH. 117

a bell, and rowing a boat, might be represented respectiyely by the
numbers 100, 167, 237, and 248. Hence it appears that the act of
rowing is an advantageous method of applying human strength.

264. A London porter is accustomed to carry a burden of two
hundred pounds at the rate of three miles an hour ; and a couple
of Irish chairmen will walk four mileaanhour, with a load of 300
pounds. But these exertions are by no means equivalent to those
of the sinewy porters in Turkey, the Levant, and other parts bor-
dering on the Mediterranean. At Constantinople, an Albanian
will carry 800 or 900 pounds on his back, stoopmg forward, and
assisting his steps by a short staff. At Marseilles, four porters
commonly carry the immense load of nearly two tons, by means of
soft hods passing over their heads, and resting on their shoulders*
with the ends of the poles from which the goods are suspended.

265. The most extraordinary instances of muscular exertion in
the carriage of burdens are those exhibited by the cargueros or
carriers, a class of men in the mountainous parts of Peru, who
are employed in carrying travellers. Humboldt, in relating the
circumstances of his descent on the western side of the Cordulera
of the Anded, gives some account of the cargueros. It is as usual
in that country for people to talk of going a journey on a man's
back, as it is in other countries to speak of going on horse back*
No humiliating idea is attached to the occupation of a man-carrier ,
and those who engage in it are not Indians, but Mulattoes, and
sometimes whites. The usual load of a carguero is from 160 to
180 pounds weight, and those who are very strong will carry as
much as 210 pounds. Notwithstanding the enormous fatigue to
which these men are exposed, canying such loads for eight or
nine hours a day, over a mountainous country, though their backs
are often as raw as those of beasts of burden, though travellers
have sometimes the cruelty to leave them in the forests when they
&11 sick, and though their scanty earnings during a journey of
fifleen or even thirty days is,not more than from 11 to 12 dollars,
yet the employment of a carguero is eagerly embraced by all the
robust young men who live at the foot of the mountains.*

266; The different races of mankind display much diversity of
muscular strength ; though in all cases much must depend on the
constitution and habits of the individual. M. Peron f has stated
the results of some interesting experiments which he made to

In what kind of exertion did Buchanan find the greatest, and in what
the least advantageous employment of the strength of men ?

What striking examples can you enumerate of the transportation of
heavy loads ?

Who are the cargueros, and what feats of strength are related of them
by Humboldt ?

* See Humboldt's Researches concerning the ancient inhabitants of
America ; with Descriptions of the most striking Scenes in the Cordille-
ras. London, 1814.

t Voyage de Decouvertes aux Tcrres Australes, fait par ordre da gou-
▼eroment pendaut les annees 1800— 4i.

118 MECHANICS.

discover the relative mechanical power of indiridnals of different
nations. For that purpose he used an instrument called a Dyna-
mometer, which, by the application of spiral springs, to a gradu-
ated scale, afforded the means of estimating the forces exerted by
the persons who were tlje subjects of his experiments. He col-
lected by this method a number of facts, which he conceived sui^
ficient to enable him to deduce from them the medium forces or
powers of exertion of the inhabitants of the Island of Timor, of
New Holland, and Van Diemen's Land, and to compare them
with those of the English and the French. The following is the
order of arrangement, commencinjg with the weakest: Manual
force— -Van Diemen's Land, N. Holland, Timor, French, English;
The proportion between the two extremes is nearly as 5 to 7.
Lumbo-dorsal force, I force des rein8'\ — ^the order the same as
before; but the proportion between the extremes, as 5 to 8.

267. The labour of a horse in a day is usually reckoned equal
to that of five men ; but then the horse works only eiffht hours, while
a man can easily* continue his exertions for ten. Horses display
greater power in carrying than in drawing ; yet an active walker
will beat them in a long journey. Their effective force in traction
seldom exceeds 144 pounds, but they are able to carry six times
that weight.* The pack-horses in the West Riding of Yorkshire,
England, are accustomed to convey loads of 420 pounds over a
hilly country ; and in many parts of that country the mill-horses will
carry the burden of even 910 pounds, for a short distance.

268. The most advantageous load for a horse must be that with
which his speed will be greatest in proportion to the weight car-
ried. . Thus, if the greatest speed at which a horse can travel
unloaded be 15 miles an hour, and the greatest weight he could
sustain without moving be supposed to be divided into 225 parts,
then his labour will be most effective when, loaded with 100 of
those parts, he travels at the rate of five miles an hour. The com-
mon estimate of horse-power adopted in calculating the effect of
steam-engines is wholly hypothetical. It is stated hy Watt to be
that which will raise a weight of 33,000 pounds the height of one
foot in a minute of time, equal to raising about 90 pounds four
miles an hour. Another estimate reduces the weight to 22,000
pounds raised one foot in a minute, equivalent to 100 pounds 2^
miles an hour. This mode of calculation seems to have been intro-
daced as a matter of convenience, when the use of horses in mills
and factories was superseded by that of steam-engines ; and must
have been adopted in order to show the superiority of steam-
To what extent did Peron discover thnt different nations vary in the

forces which they can exert in different modes of exertion ?

In what manner do horses exert their strength to greatest advantage ?

What is generally found to be their effective force of traction ?

What will be found the most advantageous load for a horse ?
' What is the estimate of horse-power assigned by Watt in calculating

the effect of steam-engines ?

- - —^

* It does not follow that it is better lo vuepack^honet than wagona.— Ela

ANIMAL BTRXNOTH. 119

engines over horses according to the most exaggerated statement
of the power of the latter.

269. The ass, though far inferior to the horse in strength, is yet
a most seryiceable beast of burden to the poor, as he is easily
maintained at little cost. It has been found that in England, an
ass will carry about 220 pounds twenty miles a day ; but in
warmer climates, where he becomes a larger and finer animal,
he may be made to trot or amble briskly with a load of 150
pounds.

270. Dogs are now frequently used for draught in yarious coun^
tries. The Kamtschatdales, Esquimaux, and some other north-
ern people, employ teams of dogs to draw sledges over the frozen
surface of snow. They are harnessed in a line, sometimes to the
number of eight or ten, and they perform their work with speed,
steadiness, and perseverance. Captain Lyon, when he visited the
Arctic regions, had nine of these dogs, who dragged 1610 pounds
a mile in nine minutes, and worked in this manner during seven
or eight hours in a day* Such dogs will draw a heavy sledge to
a considerable distance, at the rate of 13pr 14 miles an hour; and
they will travel long journeys at half that rate, each of Uiem pul-
ling the weight of 130 pounds.*

271. The elephant was used by the Romans for the purposes
of war, as it is still in India, and other oriental coun^nes. His
strength is reckoned equivalent to that of six horses, but the quan-
tity of food he 'consumes is much greater in proportion. An ele-
phant will carry a load of 3000 or 4000 pounds ; his ordinary pace
is equal to that of a slow-trotting horse ; he travels easily 40 or
50 miles a day; and has been known to go 110 miles in that
time.

272. The camel is a most valuable beast of burden on the sandy
plains on both sides of the Red Sea; for ^traversing which, the
animal might seem to have b'een expressly created. Some camels
are able to carry 10 or 12 hundredweight; others not more than 6
or 7 ; and with such loads they will walk at the rate of 2^ miles
an hour, and travel regularly about 30 miles a day, for many days
together, being able to subsist eight or nine days without water,
and with a very scanty supply of the coarsest provender.

273. The dromedary is a smaller species of camel, chiefly used
for riding, being capable of travelling with greater speed than the
larger camel, but not equally proof against exhaustion. The best
Arabian camel or dromedary, after Uiree whole days' abstinence

For what parposes have dogs often been employed P
At what speea, and with what loads, can the Arctic dogs travel ?
At what speed, and with what load, can the elephant travel?
What circumstances of its constitution adapt the camel for usefulnesa
in the particular climate where it is found to subsist?
For what particular labour is the dromedary adapted ?

* The exhibition called the Hall of Industry, shows the force of dogs
applying their strength on tijlexible inclined pfanej-^En,

120 MECHANICS.

from water, shows manifest symptoms of ^at distress ; though
itmi^ht possibly be able to travel five days without drinking;
which, however, can seldom -or never be required, as it appears
that, in the different routes across the desert of Arabia, there are
wells not more at the utmost than 3^ days' journey from each
other. Exaggerated statements have been given of the speed of
this animal ; the most extraordinary performance of which the
traveller Burkhardt ever obtained authentic information having
been a journey of 115 miles in eleven hours, including two passa-
ges across the Nile in a ferry-boat, requiring twenty minutes each.
The same traveller conjectured that the animal might have travel-
led 200 miles in twenty-four hours. A Bedouin Arab has beea
known to ride express from Cairo to Mecca, 750 miles, upon a
dromedary, in five days. Twelve miles an hour is the utmost
trotling-pace of the smaller carnel ; and though it may gallop 9
miles in half an hour, it cannot continue for a longer time tnat
unnatural pace. It ambles easily at the rate of 5^ miles an hour;
and if fed properly every evening, or even once in two days, it will
continue to travel at that rate five or six days.

274. The lama, or gpianaco, is a kind of dwarf camel, which
is a native of Peru ; and it was the only beast of burden employed
by the ancient inhabitants of that country. It is easily tamed,
feeds on moss, and being admirably adapted for traversing its usual
haunts, the lofVy Andes, it is still employed to carry go^s. The
strongest of these animals will travel, with a load of frQm 150 to
200 pounds, about fifteen miles a d&y over the roughest moun-
tains. There is a smaller animal of a similar nature, called the
Pacos, which is also now used by Ihe Peruvians in transporting
merchandise over the mountains ; but which will 'carry only from
50 to 70 pounds.

275. Oxen have- been, in many countries, employed in the la-
bours of husbandry, instead of horses. They are, however, infe-
rior, not only on account of the softness of their hoofs, which
renders them, if unshod, unfit for any except field work, but
likewise as being comparatively unprofitable. A team of oxen
capable of ploughing as much land as a pair of horses will require
for support the produce of one-fourth more land, after allowing for
the increase of weight and value.

276. In some parts of Europe the goat is made to labour, by
treading a wheel to raise ore or water from a mine. They are, in
England, sometimes harnessed to miniature carriages for children;
and in Holland the children of the rich burghers are thus drawn
by goats, gaily caparisoned, and yoked to light chariots. The

What is the greirtest speed of the camel ?
At what constant rate can it usually travel ?
What is the load and speed of the'lania of South America ?
Why are oxen inferior to horses in the labours of liusbandry r
In what manner has the goat been employed as a mechanical agent ?
In what reg^ion, and for what purpose^ is the rein-deer made subier*
vient to the purposes of man ?

ANIMAL BTRENOTH. 121

rein-deer of Lapland is a most fieiriceable beast of draught in the
frozen regions of the north. Two of these deer, harnessed to a
sledge for one person, will run 50 or 60 miles on the sttetdi; and
they have been known to travel thus 113 miles in the coarse of m
day.

At what qwed can this anioMl triTel f

The foregoing statements and illustrations will, ih general, be
found sufficient for the class of students for whose use this work
is chiefly designed. For the use of teachers and others who may
desire to pursue the subject more into detail, and to fidd rigorous
demonstrations of the principles above laid dewn, we would make
the following references to works which i^ay with more or less
facility be obtained by the American reader.

Cambridge Mechanics, by Prof. Farrar, p. I3---378.
Fischer's Element's of Natural Philosophy, pi 10 — 53.
Playfair's Outlines of Natural Philosophy, p. 19—168.
Boucharlat, translated from the French by Frofessor Courtney
Gregory's Mechanics.

Library of Useful Knowledge, article Mechanics^ three nura*
bers.
Robinson's Mechanics, edited by Dr. Brewster, in 4 vols.
Young's Mechanics.
Lagrange M^canique Analytique.
Biot Traits de Physique.
Journal of the Franklin Institute, passim.

Many more works might be named, but the above it is believed
will constitute a sufficient collection of subsidiaiy works to aid
the teacher in his private investigations under the different heads
embraced in the preceding treatise.— Ed.

HYDROSTATICS.

]« As the science of Mechanics treats of the phenomena depend*
faig on the properties of weight and mobility in solid bodies, so
Hydrostatics relates to the peculiar effects of the weight and mo^
bili^ of liquids. The term hydrostatics properly denotes tiie
stability of water,* or in a moie exteasire aem>tation, the pies-
sure and equilibrium of liquids at rest. The effects produced by
the flowing of water or any other liquid, have sometimes been
regarded as appertaining to a distinct department of natural phi-
losophy, named Hydraulics ;f and occasionally the whole doctrine
of mechanical science as applicable to liquids has been treated
of under the designation of Hydrodynamics,^ which, however,
seems to possess no such peculiar property as to warrant its gene-
ral adoption ; and therefore the term Hydrostatics may be retained
as denoting the science whose object is to explain the phenomena
arising from the influence of gravitation on water and other
liquids whether in the state of rest or in that of motion.

2. Liquids differ in some of their distinguishing properties from
9olids, and in others from gases or aerial fluids; forming an inter-
mediate class of bodies. A solid, by the disintegration of its parts,
9iay be reduced to a state bearing some resemblance to that of a
liquid, thus fine sand or any lighf powder will yield to pressure in
^very direction, almost as readily as water; but the resemblance
is still extremely imperfect Viscous fluids, ds train oil or trea-
cle, approach to the nature of solids ; and indeed the distinction
between such liquid substances and some of the softer solids, as
butter or honey, depends much on their relation to heat, their con-
sistence or relative density varying with the temperature to which
they are exposed.

3. As the effect of temperature on different bodies will consti-
tnte the subject of a separate treatise, it will be sufficient at pre-
sent to state that the peculiar degrees of density and tenacity of
unorganized substances, constituting the respective states of soli-
dly and fluidity, with their various modifications, seem to be
ehiefly influenced by heat and pressure ; so that a particular sub-
stance, as water, may exist under different forms, depending on
the circumstances in which it is placed. Thus a certain degree
of cold will convert water into a hard solid, as ice or hail, which,
when melted by heat, produces a liquid differing in no respect
|rom the water of which it was formed ; and this when exposed

To what it the term hydrostatiet properly applied ?

To what is hydrauUct sometimes appropriated ?

What other term has been used to denote the meohanical properties
and effects of liquids ?

In what manner mar solid substances be made to resemble liquids ?

What class of liquids bear an analogy to solid boflies ? What circam-
•Stanees influenee tlie density and tenacity of unorganized substances ^

^ * From Xfi.^, water, and a-r»ri(, standing. f From T}»p, and mxe;, •
P>ps. t From T^««, and iw^/nit power.

188

or Ltqumi lt|

to a sttttcietitly M^ t0nii>6ntafe, trill e^tporatt o# be^ctee
which may be agfain eonaensed or lestoteA to the liquid itato by
cold. Mercury commonly occurs in the form of a very denaA
liquid ; but it may, like water, be condensed or frozen by exporara
to an extremely low temperature, and be made to boil or erapo*
rate by subjecting it to a great degree of heat. The other melali
differ from mercury only in remaining solid in higher temperatores
than that substance; but they all melt with various degrees of
heat, and become sublimed or eraporated when the heat is gveatly
tais^ above the melting point.

4. Since die same kind of matter may exist under diffiBWol
states or forms, it follows that ]i<]uids must be composed of tlia
same particles as solids, and tbe difference between a liquid and
a solid may be conceived to arise, merely, from peculiar modifier
tions of the cohesive attraction which takes place between thi
constituent molecules or particles of such bodies respectiTely*
The particles of elastic solids must be capable of a sort of yibra-
tory motion, from sudden pressure, but they will always tesumS
the same position as soon as the vibration ceases, unless it be ss
violent as to occasion a permanent separation of the partielssi
when the solid becomes broken or pulverised. Now liquids have
their constituent particles, held together like those of solids, by
eohesive attraction, but they oscillate on the application of ths
slighted impulse ; and Uiere seems to be such a general relstioa
of all the particles to each other, that when the oonnexioa between
any two particles is broken, by shaking or otherwise agtiitiiif
the mass of which they form- a portion, tiiey readily became sl»
tracted by any other particles with which they may happen Is
iSome in contact, new cohesions take place, and when the dis»
furbing fome Is removed, the general equilibrium is re s tored
throu^oat llie liquid mass.

5. The cohesive attraction between the particles of liqaidfe is
demonsttated by the gl<^ular figure which tiiey assume, when no
external force interferes with the aggregation of the mass. Thid
appears in the case of mercury thrown in small portions on aefains
plate, or on any surface which exercises on it no chemical zUn/b^
tion ; when the minute portions into which it will become sepataledi
Will be found to be perfect spherules, the larger ones only behii|
slightly flattened by the pressure occasioned by thdr own wetgt3
<rti the plate. Similar sphemles, consisting of drops of wi&B^

Throttg^h what sucoeMtve changes of state miiy bodie« oceasionallj paur

Give some examplet of these changes ?

Whence arises Ae difference between a liqwd rtnda soMd body?

Of what action are tbe particles of elastic solids necesaarily sSMcpllS
We?
. l)y what force are tbe particlea of liquids held together T

>^hat constitutes the difference between breaking a solid and separat-
ing th^ parts of a liquid ?

Ho# IS tbe cohesive attraetioh between die particles of a liquid deteSa-
atraled?

KM HVDROfTATlCB*

aie formed by dew or rain on tiie broad leares of some' kinds of
regetables, as those of the common cole-wort or cabbage. If^
howerer, Uie drops become laiffe, as when two or three run t(^[e-
ther, they spread out at the edges, sinking down, and becoming
flattened, partly through their own weight, and p«urtly owing to
the attraction between the water and the surface of the leaf.

6. The general appearance or figure which liquids assume when
al rest is me joint effect of the extreme mobility of their constita-
ent particles, of the gravitation of liquid masses, and of their at-
traction for the solids on which they are sustained. Hence when
m liqtdd in any considerable quantitjr is poured into a vessel of
any shape whatever, it adapts itself exactly to the internal surfiuse
of the vessel, the superior or unconfined sur&ce of the liquid form«
ing a horizontal plane, usually raised a little at the sides or
border of the vessel, where the liquid is attracted by the contain*
ing solid with which it comes in contact.

7. When an immense mass of liquid presents a continued sur-
lace, its form will be a portion of a convex sphere ; because the
oollective gravitation of all its particles towards the cejitre of the
earth causes it to partake of the general figure of the terrestrial

globe. This, indeed, will be the case with comparatively small
odies of liquid ; but when it is considered that tne radius of the
sphere, of which any such liquid surfiice formed a part, would be
equal to half the diameter of the earth, it must be manifest that
the difierence between the surface of a small portion of such a
•phere and a honzontal plane would be too inconsiderable to be
distinguished. Vast collections of water, however, as the open
sea, afford decisive indications of super^cial curvature, among the
most striking of which is the fact Uiat when a vessel first comes
in sight its masthead alone is visible, and the lower parts appear
successively as it approaches the observe, rising as it were out
ef the bosom of the aeep.

8. Among the properties in which liquids differ most remarka-
bly firom gases, is the power of sustaimn^ pressure to a consider*
able extent, without undergoing any obvious change of volume.
Common air, steam, and other elastic fluids, as they are termed,
may be compressed by very moderate force, and on its removal
Ihey expand to their original dimensions, as may be ascertained
by squeezing a blown bl^er ; but a leather bsj? or strong bladder
fillea with water, and secured so tiiat none of the liquid can es-
cape, may be burst by forcible compression, but cannot be made
to exhibit any sensible degree of contraction. Such indeed is the

On what three cirenmatanoes does the figure assumed by a liquid at
rest depend ?

What is the external form of a lai^e mass of liqnid ?

Why is this form taken rather than any other ?

What sensible evidence is afforded of the spherical form of the earth?

In what respect do liquids differ essentially from gases ?
. How was water formerly re^rded in respect to compressibilitj ?

On what experiment was this opinion founded ?

vomwvsmwm Itf;

extrabrdmary leiiistanee of water, when sobjeeted to piMMre dn
ail sides, that it was long regarded as absolutely inconiprsstiUe.
This opinion was partly rounded on an experiment made m the stx>
teendi century, by the members of a sdenfific society at Florence
called Academia del Ciraento. Those phiiosopfaers coneeiYadtlM
idea of enclosingr a quantity of water in a hollow gflobe of beateA
gold, and exposing it to the powerful action of a screw prssa*
when it was found that the water was forced through the potot
of the gold bill or case, standing in drops like dew oft its exterior
surface. But this experiment, can by no meaos be eonsidemd as
demonstrating, the entire ineomptesstbiitty of the liqoid; fyr
though it obnously displayed va^t resistance to the eompretoiag
force, it might hate ondetffone the utmost limit of eondensattoa
before the exudation took place ; and the experiment Was ansatift-
facUjTj^ as affording no means whatever for appreciating the actual
volume of the water at the moment when it penetratad the solid
envelope. In fact, notiiing more could be inferred from such an
experiment, but that water is not susceptible of unlimited con*
densation.

9. The fallacy of the formerly generally reoeired notion of tiM
absolute ineompresMbility of water was proved by some ingeik
niously contrived experiments by Mr. Canton, a feUow of tho
Royal Society of London, in 1761. He showed tbat water, ift*
cjuded in a glass tube with a large bulb or hoUoW globe at its ea&*
tremity, expanded and consequently stood higher in the tul>e Whw
placed under an exhausted receiver iik»a wheM suiiiected to ti&^
pressure of the atmosphere, and on the eontraiy was condensed pt^
portionaily, by pressure equal to the weight of two atmospherM*
He made similar experiments on spirit of wine, olive oil, and
mercury, from which it appeared that those liquids tmdeigo coft^
densation, but in difTefent degrees, when subjected to compie»«
sion*. In conducting these experiments proper precautions wem
adopted to prevent any inaccuracy arising from Tariation of tem*
perature ; and the follpwing table exhibits the results obtained
when the barometer stood at 29^ inches and the thermometer a*
50 degrees.

10* Spirit of wine underwent compression amounting to

0.000,066 of Its bulk.
Olive oil, - - - 0.000,046
Rainwater, - - 0.000,046

Sea water, • . - 0.000,040
Mercury, - - - 0.000,003
Hence it appears that mercury is far less compressible than water

Wbftt legttimaie inference ean be drawn from the Florentine txpeti'
ment ?

By whom was the fallacy of the opinion formerly entertained retp^et-
ing the compressibility of water first demonstrated?

btilte the manner in which Canton's experiments were condosted^

What oUier liquids betides water wore proted by Canton to he
pressiUe? ,

L 2

Ittt^ Bmmon'ATiot.

11. More reeenily, experiments on this interesting subject hare
been instituted by M. Oersted, a philosopher who has greatly
distinffnished himself by his scientific researches ; and the resolts
of his mvestigations, which appear to hare been very carefully con-
diieted, correspond nearly with those of Canton, ihe contraction
of water, under pressure equal to the weiefat of an additional
atmosphere, according to the experiments of Oersted, amounting
to 0.000,045.

12. Experiments have been undertaken in England with a yiew
to ascertain the effect produced by subjecting liquids to compres-
sing forces of vast enervy, far beyond those employed in the re-
searches of Canton and Oersted. In 1830,. an account was laid
before the Royal Society of London by Mr. Jacob Perkins, of
some experiments from which he inferred that water had suffered
a compression of about one per cent, of its bnlk by a pressure
equal to 100 atmospheres ; and in other experiments the com-
pressing force was au^ented to 336 atmospheres, which caused
a contraction of the liquid to the amount of nearly 3J per cent.
These results were obtained by including water in tiie cavity of a
oannbn, fixed vertically in the earth, and driving more water into
it with a forcing piimp ; and corresponding experiments were
made by si^iking water inclosed in a proper apparatus to a great
depth beneath £e surface of the sea, and observing the degree of
compression it had undergone.* These operations, however,
oould not be regarded as equally accurate with those previously
described ; thougfi the deductions from them have been corroborat-
ed by 'the result of subseqnent investigation.

13. In 1836 Mr. Perkins made public other experiments on the
eompression of water, of which also Im account appeared in the
Philosophical Transactions. The machine he employed was
eomposed of a cylinder of gun-metal, 34 inches in length, and
having an internal cavity to which was adapted a steel pump, with
a water-tight piston, by means of which water could be injected
into the body of the cylinder. A lever apparatus was properly
snnexed for the purpose of measuring the degree of pressure ; and
80 adjusted that the number of pounds pressing on its piston indi-
cated exactly the number of atmospheres equivalent to the degree
of compression.

14. That part of the toparatus in which the liquid is enclosed^
the condensation of whicn is to be measured, is called by the ex-
perimentalist a piezometer.f It consists of a strong glass tube,
eight inches in length and half an inch in diameter, closed at one

To what results has Oersted been led by his experiments on jrater ?
How many atmospheres of pressure are required to eondense water t6
the amount of one per oeiit or its ordinary bulk ?

• See Philosophieal Transactions, 1890, and Abstraet of Papers in Phi-
•lodtiieal Transaetioos, vol. ii. p. 134.
, t From the Greek ni><«, to preiS| and Mirpor, a meanire.

coMnusnoD. 1ST

extremity and open at the other. This tabe mait be carefUly
filled with water freed from air, and being ioverted wtiile the
water is prevented from eecaping by the application of a thi>
membisne to ita mouth, it mast be inserted in a wider tnbe or
glass, the upper part of which is filled with water,-and the lowar
part with mercnry; the small tube contains a hair|4prin(( presmiw
against its interior surface so as to retain its position when foiMd
upward ; and this spring is in contact with a steel diek marine
fi^elj in the apper tube, and &om its inferior weight sappOTled
by tiie surface of the roeTcnry below. A frame of strong win
retains the small tabe in its situation; and the pieiometei bMOg
thns arranged is introduced into the receiver of the compveMor,
filled with water at a temperature of 50 degrees, when the pnmp
being' screwed into its place, any required degree of presenre maj
be applied. When the experiment was carefully conducted it w»a
found that water, under the inBuence of a force aqnal to SOOO at- .
ospheres, was dimiliished by 1-13 part, as indicated by lk«

i.wiu

By irhat part of 1(9 original bulk i
torve of SOOO atraospherea, or 30,000 i
eiTcn meu of water f

WUt is the eonitniclioD of Perkini

128 HTORO8TATI08.

perhaps be more clearly comprehended from the annexed figure, ia
which R is the metallic cylinder, G the wider glass tube with a
quantity of mercury, M at the bottom; gw the piezometer dipping
into the mercury below and kept steady by the wire cage C near
the top ; €? is the steel disk, and a the hair-spring to be moved up-
ward when the water in g is compressed, and the mercury with
the disk d rises, and to remain and indicate the degree of com •
pression a^er the experiment. The use of the force pump P with
tts two valves, e, 9, and that of the safety valve V , with the lever
and weight W serving to determine the force applied, will be
readily understood. The apparatus of Oersted substitutes a
strong glass receptacle foif the metallic one of Perkins ; and his
piezometer is a nearly capillary tube in which a thread of mercury
rises by the compression and forces before it 1^ water with which
the whole upper part of the tube, (hermetically sealed at top,}
is filled. Oersted employs to compress the liquid instead of the
steel pump, a strong thumb-screw inserted into the top of the
brass cap with which his glass receptacle is closed. He also en*
closes a thermometer, not nermetically sealed, to mark the de^ee
of heat, if any, developed by the effect of mechanical compression.

16. Though it is manifest, from the preceding statements, that
liquids undergo great compression unaer certam circumstances,
jet the degree of compressibility of such liquids, as water, is so
inconsiderable when the compressing force is moderate, liiat no
sensible effect is produced. Hence in all calculations concerning
the action of water, at rest or in motion, in ordinary cases, it may
be regarded as an incompressible fluid.

17. Liquids in general possess the property of elasticity; but
like solids, some of them display that property to a greater extent
than others. When a solid disk, as an oyster-shell or a flat stone,
is made to strike the surface of water at a small angle, as in the
sport which schoolboys call making ducks and drakes, the solid
will rebound from the water with considerable force and frequen-
cy. So a musket-ball impincrin^ obliquely on water will take a
zigzag course, en ricochet, as the French express it. Water dash-
ed against a hard surface, as when it is poured agrainst the side of
a china basin, or let fall on a plate, shows its elastic force, in
flying ofl* in drops in angular directions. Experiments on the
elasticity of drops of water, spirit of wine, or any similar liquid,
may be made in a shallow wooden box, having its bottom and
sides thinly covered with any light insoluble powder ; for the drops
on being impelled against the side of the box, or even against
each other, will reboimd like miniature cricket balls or marbles.

Might this instrument be employed with advantage to measure the
compression suffered by water in deep^ea experiments ? Describe th#
arrangement 6( apparatus employed oy Perkins in the compression of
water. How may water be regarded under the influence of moderate
changes of pressui*e ? What evidence is afforded of the elasticity of
water by the impinging of solitl bodies upon its surface ? How may we
deDioustra'ce the elasticity of drops of water ?

WEIGHT 07 LIQini>8. 129

«

' 18. Mercury is yet more elastic, as might be shown by placing
a small quantity of it in a little case made by bending at rig^t
angles the sides of a common playing-card ; and on inclining it so
as to make the metallic fluid strike one of the raised sides of the
card, the shinin? globules would recede with a velocity propor-
tioned to the violence of the shock. The effects thus exhibited
appear to be exti^mely similar to those observed in the case of
elastic solids. A globule of mercury impinging on a hard snfw
£ice becomes slighuy flattened, but instantaneously resuming its
curved fiffare, it recoils like a bent spring suddenly liberated. In
some hydraulic operations the elasticity of liquids becomes a pro-
perty of considerable importance, variously augmenting or modi-
fying the efficient force of particular kinds pf machinery.

•

Weight and Fruaure of Ltquids,

19. Among the absurd doctrines heretofore generally received,
but which have been exploded by the light of modem philosophy,
must be reckoned that of the non-gravitation of the particles of
liquids on each other. That liquids as well as solids possess
weight was never denied ; since every one must have learnt &om
experience that a cup or a bucket filled with water would require
a greater exertion of force to lift it than when the water was re-
moved. But it was observed that in drawing water from a well,
so long as the bucket remaincfd under water very little effort was
required to raise it, while as soon as it emerged from the surface
of the liquid, the loaded bucket would press downward with a
force proportioned to the quantity of water contained in it. This,
and the general observation that heavy bodies were easily raised
while under water, gave^se to the vague idea that a liquid did not
gravitate in its own element, and that therefore a body surromuM
by any liquid was destitute of weight.

20. The following experiment sufficiently proves that this is
not the case. Let a strong phial, with a stop-cock fitted to it, be
exhausted by means of an air-pump, and the stop-cock beingr turn-
ed let it be suspended from one arm of a balance, so that it may
be entirely immersed in a vessel of water, weights being placed
in the opposite scale of the balance to keep it in equilibrium ; then
if the stop-cock be opened the water will now in and fill the phial,
which will immediately sink, and to restore the equilibrium the
same weight must be added that would counterpoise the water it
contains u weighed alone : thus, if the bottle would hold exactly
four ounces of water, a weight of four ounces would be required
to make the balance stand even as at first.

How are analogous experiments on mercury condueted ?

What appears to be the effect of the impact of a drop of mercury upon
a hard tunace ?

What opinion was formerly entertained respecting the gravitation of
Uquids upon their own mass ? .

From what circumstance did this idea probably take its rise ?

How it its incorrectness conclusively demonstrated \

130 HTDBOffTATICB*

31. The apparent diminntion of the weight of bodies nnder'vra*
ter is owinjr to the particles of the licjuid mass grayitating equail^r
in every direction ; so that the interior portions of any nqaid, or
of solids immersed in liquids, are subjected to the same degree of
pressure on all sides; and therefore a body surrounded by water is
partially supported by it, and consequently may be raised through
the liquid with greater ease than in the air, a fluid, the relatire
density of which is so yery inconsiderable. Liquids are not less
powerfully affected by gravitatiye attraction than solids, but they
exhibit different appearances under its influence, owing to their
being constituted differently, so that their particles moye freely
and almost independently of each other.

22. All the constituent particles of a solid are firmly connected,
and they thus act with combined effect in producing pressure or
impact ; but a liquid yields to force in any direction, and is liable
to be separated into small n^asses, the effect of which is compare*
tively inconsiderable. A basin of water poured from a great
height on a man's head would hardly be felt more than a curreni
of rain ; but if the contents of the basin, supposing it to hold a
quart, were suddenly changed to a solid mass of ice, it mi^ht oc-
casion a fracture of the skull. But though a liquid in Tailing be-
comes almost dissipated through the resistance of the atmospherci
it displays ^eat force when it can be made to act hi a continuous
column. Hence the power of a mill stream in tiirnin^ largO
wheels either by weight or pressure; and the tremendous violence
of a cataract, sweeping away great stones or other ponderous
masses which may present any obstruction to its impetuoui
course.

23. The effect of a liquid mass when its particles are protected
from dispersion, and thus enabled to act in concert, like those ot
a solid body, may. be amusingly illustrated by means of the little
instrument called a water-hammer. It consists of a strong glass
tube, about twelve inches long, and nine or ten lines in diameter,
having three or four inches of water included in it; which bein^
made to boil and form steam by the application of a proper heat,
the tube ihust be hermetically sealed by means of an enameller's
lamp and a blow-pipe, so that when it becomes cool, a vacuum
will be formed above the water by the condensation of the inclur

How Is the apparent loss of weight in bodies immersed in water to t>e

explained ?

To what is the difference attribatable between the phenomena exhibit-
ed h^ li()uids and those observed in solids, when under the infloence of
gravitation ?

How are the constituent parts of each held together ?

What simple experiment would illustrate the influence of a change of
form, in modifying the effect of water ?

What examples may be cited of great energy displayed by a falliof
liqnid }

By what apparatus may the percussion of a falling mass of liquid be
illustrated ? ^

Describe the water-hammer.

ded Bteam. On shaking such a tabe Tertically, ths water, riBiag
a few iachea and sinking suddenly to the bottom of the tube, pro-
duces a sound like thai arising &om the stroke of a imall htmuner
on a hard bodVi whence the name of this instiument, the action of
which depends enuiel; on the exclusion of the air, so that tha
water moves in a dense mass.

S4. He pressure of liquids extending equally in all directions,
S liqoid mass will have all paita of its surface at the same level,
whatever be the farm of the vessel in which it is contained, s«
Igogas diere is a &«e commnnicalion tkioughout.

96. In the preceding figure let A B repieseni a slass vessel
closed except at the two raised estremitios, and fillen with water
to a height above the horizontal line; then suppose four dif-
ferently ^aped tubes C, D, E, F, open at both ends, to be in-
Berled in the ohlong part of the vessel, with their npper extremi-
ties not rising so high aa those standing at the sides; it will he
found that the liquid will pass laterally Into the tube C, ascend
directly in D, and circuiloualy in E, while it both descends
and ascends in F, rising equally in all the tubes, and spouting
out till the water is reduced in the side tubes to the level of the
Is of the internal ones, when the equilibriiim being esta-

blished the liquid will remain at rest. Thus it fallows that any
number of columns of a liquid, freely communicating, whatever
may be their respective diameters and figures will alwajfs have
the same vertical height.

S6. Yet though all the particles of a liquid mass will press
erjually on each other, it must be manifest that the collectire
weight will be proportioned to the depth beneath the surface, so
that the bottom of the containing vessel necessarily sustains the
weight of a column having the greatest vertical height of the
liqud with an area equal to that of the base itself.

27. If the vessels A, B, C, D, and E, have water poured into
them in such quantities that it may stand at the same height id
each, the ptesaures on their bases respectivelj will he as the
several columns marked 1,3,3, 4. Hence the amount of the pres-

WTnl ooineqnenBe retulti from the equal preiiore of liquidi, in regard
to ths height of ill lul-fiw !

What fiiSueMe ha ■ ilie figure snd liae ofihe pirta of conUinlnR <!«••
■ell on the heighi 10 which liquida »i]1 riK wiihin tliem rpipeeiinlj' f

WhU Tnemurei the preHure exeiviaeU b; a oohiain of liquid cu lb*
bottom of iu aoBtaiuing TCUcJ / ■

133

BTllROSTATXCf*

sure of any liquid may be ascertained by multiplying the vertical
height at which it stands by the extent of surface of its base.
Tlius suppose the water in the vessel B to stand at the height of
four inches, and the area of the base to be eight square inches, the
pressure will be eoual to thirty-twp inches of the fluid ; but if the
water should stand at the same height in the vessel C, having a
base only half the area of the former, the pressure will be but half
or only sixteen inches, though the capacity of both vessels may
be exactly the same. The diameter of a vertical column com-
municating with an extended base may be relatively inconsider-
able, as in the vessel £, notwithstanding which it will cause Hie
same degree of pressure as a column of the same height with a
diameter corresponding to the base throughout.

28. This effect of the vertical pressure of liquids may be vari-
ously exhibited, and its results are curious and important. Hence
the principle involving the peculiar mode of pressure of liquid

masses has been termed the Hydro-
static Paradox. It may be illustrated
by the following experiment. Let a
cup or wide-mouthed jar, filled with
water, be poised by han^ng it to tiie
arm of a balance, by loadmg the oppo-
site scale with the requisite weights;

then after marking exactly the height

at which the liquid stands, pour out a jpart of it, and plunge into
the midst of the jat a conical block of wood, supporting it with
the hand or by means of the apparatus represented in the annexed
figure, taking care that the block shall not touch the sides or bot-
tom of the jar. If it be plunged just deep enough to raise the
remaining liquid to the same height as at first, the balance will be
again exactly equipoised ; and the block may be so large as to
leave only a thin film or hollow cylinder of the fluid without at all
disturbing the equilibrium. It is uf no consequence what* is the
weight or shape of the body introduced, for a piece of cork or a
blown bladder held in the jar will produce the same effect, if its
bulk be sufiicient to raise the water to the required height.* .

By what mode of catpulation may we aseertAin that pressure ?

If a vessel representing the frustum of a eone be filled with liquid, and
successively placed on its two opposite bases, what will be the relatioQ
between the pressures exercised in the two cases ?

What is meant by the kydrostaUc paradox ?

What experiment exemplifies the kind of pressure exercised by liquids?

How can we prove that the loss of weight from plunging a body into
water is only appai*ent ?

* An ingenious apparatus for drawing water from a vessel ia which a

_j

HTDR08TA.TIC PKEfliimB. |8|

99. There is ftnodier striking mode of illustrating the efiect of
liquid pressure, by means of a kind of machine called the Hydro-
static Bellows, a figure of which may be seen in the margin. It
is composed of two fiat boards united at the sides
by flexible leather, and havinff a long narrow ver«
tical tube, communicating with the cavity, with a
funnel at the top, for the convenience of pouring in
wat^ or any otiter fluid ; and a ^hort lateral tuba
with a stop-cock may be added to discharge the
water occasionally. If now water be pourS intq
the long tube it will fill the cavity and consequejit-
ly separate the boards, and by adding more water
the instrument ma^ be made to support any given
weight, in proportion to the height of the vertical
column. Suppose the boards to be about 330
inches superficial measure, four ounces of water,
•standing at the height of three feet in the tube, will keep the
i>oards separated when loaded with 416 pounds.

30. Two stout men standing on the upper board, one of them
4>y blowing into the tube may fill the cavity with air instead of
water, so as to raise the board on which they stand, and by stop-
ping the pipe with the finger to prevent the air from escaping,
\hej may keep themselves supported.

31. llie force of water pressing on an extended surface by means
of a small vertical tube may be shown by fixing such a tube in a
<water-tight cask or other close vessel, which, whatever its
strength, might be burst by filling it with liquid, and adding
•more through the tube, till the weight of the column became too
.great to be supported by the sides of the cask. The effect depends
wholly on the height of the tube, its diameter being immaterial.
A hogshead fiUed with water and exposed to the pressure of a
column in a narrow tube, twenty feet high, woald burst with great
violence.

32. Astonishing effects are sometimes produced by the pres-
-sure of water modified in the way already described. As when a
shallow body of water is collected in a close cavity under ground.

Describe the constrcation, and explain the principle, of tlie hydrosta-
tic bellows ?

la what manner might the same principle be applied to maiDtain a re-
^lar blast of air for the blow-pipe ?

In what manner do we demonstrate the importance of height of eo-
lamn to the eflfect of liquid pressure ?

In what manner may the effect of hydrostatic pressure on portions of
the earth's surface be manifested ?

solid has been made to float until the liquid has the same level as at first,
and then weighing -the quantity drawn out against the solid which had
been floating, proves tlie same genei-al position as the arrangement above
described. It moreover shows that the weight lost by the solid, and the
upward piressure of the liquid which is the cause of'tliat loss, are both
equal to the weight of water so displaced. — Ed.

134 HTVSOSTATICS.

if B narrow opening be made froin ahigheTiuriacecommnnlcBting

with ihe cavity, and il should become filled by rain or anow water,
whatever might be the fonn of the aperture, if it was water-tight,
ea HOOD as the communication was effected between the tub&-Hbs
opening and the cavity, preSBore would take place in every di-
recIJon, in a degree proportioned to the vertical height of the open-
ing and the area of ilie oavity ; in consequence of which the
flupeiincumbent mass might be cent from its foundation, aod a large
building or even a mountain might be overthrown, as h; an earth-

33. The principle of hydrostatic pressore was discovered, or at
least first satisfiictority demonstrated, by the celebrated Pascal,
about the middle of the seventeenth century ; and he showed how
an engine might be constructed, acting throu^ the force of a
column of water, by means of which one man pressing on a small

fiston might counterbalance the efforts of one hundred men
roueht to bear on the surface of a large piston. Yet notwith-
standing the distinct description of what the ingenious discoverer
terms "a new machine for multiplying forces to any required
eitent," 'more than a century and a
half elapsed before the idee was fiiUj
developed, and applied to practical
> purposes, by Mr. Bramah, the engi-
neer, in the construcrion of his hydro-
static press.

34. This machine consists of a
solid mass of masonry or strong wood-
work, E F, firmly filed i and con-
nected by uprights with a cross-beam.
B represents a strong table, moTing
vertically in grooves between the up-
rights, and supported beneath by the
piston A, which rises or descends within the hollow cylinder Ij,
and passes thrsugh a collar N, fitting so closely as to be water-
tight. From the cylinder passes a small tuba with a valve open-
ing inwards at I, and D is a lever which works the piston of the
small forcing-pump C H, by which water is drawn from the
reservoir G, and driven into the '- ' t . :■

t

driven into the cylinder L, so as to force up tl_ .

At K is a valve, which being relieved from pressure,

ij turning the screw which confines it, a passage is opened for

the water to flow from the cylinder, through the tube M, into the

reservoir G, allowing the piston to descend.

35. The effective force of such a machine must be immensely

Who Brit dcmonalrated the prineiple of h;droMt]c preanire) accord
lag to the height of column >

WhM *p|ili«itian did PikbI propoie to mike of (he principle of pre»
■qre BcoDi-ding to the Krei of the biBe of Ihe contBininr veisel /

By vbom, ani «I what period, wmi Ihe iden of Paic^ fully realized >

What ia the conalmction of Bramah'a preaa !

' Pascal de I'eqnilibre del liqneara, ediL S, 1664, ch. ii.

PRESSURE OF LKUilDS* 185

great, combining as it doe? the adTantages of solid and liquid
pressure. The amount of the latter is to be estimated by the
relative diameters of the two pistons ; so that if the piston H be
half an inch in diameter and the solid cylinder or piston A one
foot, the pressure of the water on the base of the piston A will be
to the pressure of the piston H on the water below it, as the
square of 1 foot or 12 inches^ 12 X 12 =" 144, to the square of ^
an inch, .5 X .5 »= .25; that is as 144 square inches, to i of a
square inch, or in the ratio of 576 to 1. To this must be added
the advantage afforded by the lever handle of the forcing-pomp,
depending on the relative lengths of its arms ; and supposing the
power to oe thus increased tenfold, the effect of the machine will
be augmented in that proportion, or will become as 5760 to 1.

36. As the hydTostatic press acts with a comparatively trifling
degree of friction, it may be made to produce an infinitely great
amount of pressure ; its efficiency in fact being limited only by
the measure of the strength of materials employed in its construc-
tion. SoiQe idea of the power of this engine may be derived from
the statement that with such a press, only the size of a common
tea-pot, a person may cut through a thick bar of iron with no more
effort than would be required to slice off a piece of pasteboard
with a pair of shears. It has been used in making experiments
on the tenacity and strength of iron and steel, being applied so as
to tear asunder solid rods or bars ;* and in packing bates of cotton
or trusses of hay, it has been employed to compress them to con«
venient dimensions for stowage on board ships.
. 37. Tlie principle of hydrostatic pressure has been ingeniously
applied to a purpose of great practical utility by Dr. Amott, in
the contrivance of a hydrostatic bed for invalids. It is so con-
structed as to keep the body of a person reposinff on it, sustained
by a mattress on a lic[uid surface, yielding freely in every direc-
tion,, and therefore entirely exempted from any irregular pressure:
thus the irksomeness, as well as the serious evils caused by con-
finement to one position for a long time, and the consequent inju-
ries which persons enfeetbled by disease sometimes incur, may be
wholly prevented.

38. The pressure of water or any other Uquid against the bot-
tom of a vessel in which it is contained may be regarded as the
common effect of gravity, which acts in the same manner on solid

What method will enable us to estimate the adyaotage of a Bramah'a
press of known dimensions }

What advantage has the hydrostatic press oyer the screw press and si-
milar machines r

Wliat limits the efficiency of this apparatus ?

What remarkable applications of the hydrostatic press illustrate its
foi*ee and usefulness ?

What is the construction of Amott's invalid bed ?

In what respects does the pressure of a liquid within a containing vessel
differ from that of a solid under the same circumstances ?

* See Bncyclopedia Metropolitana—- Mixed Sciences, yol. i. p. 70,

18ft BtBBMTATieS.

paint often becomes an object of importanoe ; since it will indi*
eate the most efficient means for sustaining a floodgate or any
similar surface ag^ainst the pressure of a body of water. The
position of the centre of pressare must depend on the figure of the
surface and the depth of the head of water. Supposing tlie sur*
h,ce to be a perpendicular parallelogram, the centre of pressure
will be at two-thirds of the distance nrom the level of the water to
the bottom ; and if the figure of the surface be an equilateral tri<»
angle, at three-fourths of the distance from the vertex to the base.

44. On the principle of the lateral pressure of liquids may be
estimated the pressure sustained by solids immersed at any depth
beneath a liquid surface. Thus, if it be required to find the prea*
Sure which a diver sustains when he has descended in water to
the depth of 33 feet, or rather to duch a depth that the centre of

gravity of his body may be exactly 33 feet beneath the surface of
le water-; then as the extent of surface of a human body, at a
medium, may be estimated at lO square feet, the product of that
number multiplied by 33 will give the quantity of water in
6ubic feet, the weight of which must be sustained by tke diver at
the depth just stated. Now as one cubic foot of water weig}i8
1 000 ounces avoirdupois, the weight of 320 cubic feet will be
830,000 ounces or 30,000 pounds.*

45. The equability of itte pressure in every direction renders
fuch an immense weight supportable ; though it occasions con*
siderable inconvenience to persons learning to dive, from the
intense pain caused by the pressure of the water on the drams of
the ears, even at the depth of 18 feet below the surface. It ap»
pears probable that diving in very deep water, at length, has the
efiTect of rupturing the membrane callea the drum of the ear, after
which pain in that organ is no longer felt by the diver ;f but there
must be a limit to the depth to which the most experienced diver
can descend, since at a very great depth the compressing force of
the liquid mass wpuld be so augmented as to expel entirely the
air that had been retained in the cavities of the chest and head^

Oh what circumstance vMI its position depend ?

At what point in a floodgate in the form of a rectangular parnllelo*
gmm miglit a single force on the side Opposite to th^t pressed bj the
water be applied, so as to resist the whole pressure of the liquid within ?

How may we find the amount of pressure ppon the body of a diveri
when at a given distance below the surface } *"

Why is not a person crushed by the weight of liquid above hitn, when
placed many feet below the surface of water ?

^ What peculiar seoflation it felt at fint by persons aneccastomed to deep
diving ?

Wnat is supposed to tiike pUcc when the iueonventeBce at first felt is
found to cease ?
Why may nOt k man descend to any depth below the Surface of water?

Wi^"*^ ' I II ■ ■ I I r n I 1 l i 1 1 I I ■ 11 I f I I I I II I I 1 ■■ ■!■■

}

* The manner of ascertaining the weight of any body relatively to iti
bulk will be described in the next sebtiOn, in treating of specific gravi^.
t See Uardy'i Traveli In the Iiiterier of M^iee. LoBttOD^ l^dOb

THS SPIiaT LBVCt. IM

and contracst fhe bnHc of the whole body in such a manner as to
tender ascent to the surface no longer practicsdile.

47. The uniform pressure of liquids in every direction, and the
consequent equality of action and reaction among the parts of
liquid masses cause them to assume a lerel surface under all cir**
cumstances. This property of liquids has been advanta^onsly
employed in the construction of instruments for ascertaining the
relatire heights of any given points, as in taking levels in survey*
ing, and in various operations in which it is requisite to deter*

mine the accuracy of a horizontal
plane. Such an ipstrument may
consist of a glass tube of consider*
able length, as represented in the
margin, open at both ends, which
must be raised or turned upward to
tiie same height ; and the tube being
filled with water or mercury, when
it is placed in a horizontal position, the liquid will stand at the
same level on both sides. Upon the open surfaces of the liquid
must be placed floats, each carrying upright sights with cross*
wires, wl\^ch standing at right angles to the lengm of the instru-
ment, when it is properly adjusted, the intersections of the wires
will be situated in a horizontal line ; and consequently on look*
ing through the sights at any distant object it can only be seen
exactly opposite the intersections of the wires when it happens to
be in Uie i>anie level.

48. The spirit level, an instruident adapted to the same pni^
poses with the preceding, consists of a glass tube, closed at both
ends, and filled with alcohol, except a very small space occupied
by a bubble ef air, which, in whatever situation the tube may be
placed, must rise to the highest part of it. When, therefore, the
tube is fixed in a horizonUu position, the bubble will stand pre*
cisely in the centre of the tube and in contact with its surface.
Such a level may be used like the waternlevel, above described^
for ascertaining the accuracy of^ a horizontal plane ; or it may be
mounted in a frame with moveable sights adapted to a quadrant^
by means of which the angular distances x)f objects may be deter^
mined with the utmost degree of correctness.

49. The property which liquids possess of preserving an eXaet
level in different tubes or vessels communicating with each other
is of the highest importance, as indicating an obvious mode of
conducting water from one situation to another. Thus from a lake
or reservoir this useful fluid may be conveyed in pipes or tunnels
underneath streets and buildings to any given distance, and sup^
plied to the different quarters of a town or city, at any height not
exceeding that of its source. The whole amount of the daily

WhHt is the general conMmetion of liquid levelling instrument! f
What is the rorm and use of the spirit level f
Qn what prineiple are we eaablcd to oondoet water wdcr {fOUild* sad
ihroagh irregular tubes ?

140 HYDROSTATICS.

sapply of water to the cities of London and Westminster appears
to be nearly 26,000,000 gallons, more than half of which is deriyed
from the Thames ; and as most of it is delivered at heights much
aboye the leyel of the riyer, it is necessarily raised by artificial
pressure by means of steam^ngines.

50. Though water and similar liquids may be transferred to any
imaginable distance through a series of communicating tubes bent
into numerous angles, descending and ascending, and made to
issue freely at a height nearly e(|ttal to the source ; yet it is found
in practice that obstruction, arising from the friction of the liquid
against the sides of the tubes, especially where they form acute
angles, and from the accumulation of bubbles of air in long nar-
row tubes, may cause great inconyenience ;' and hence large pipes
are more adyantageously employed than smaller ones, and aque-
ducts or open conduits are to be preferred in some situations.

51. In the south of Europe may be seen the remains of stupend-
ous aqueducts constructed by the ancient Romans, forming open
canals supported by numerous arches passing across wide yalleys,
and exhibiting eyen in decay striking memorials of the architec-
tural skill and industry of &ose to whom they owe their origin.
From these magnificent works on which such immense labour
must haye been bestowed for the purpose of conducting water on
one descending plane, it has been hastily inferred that the an^
cients were entirely ignorant of the effect of hydrostatic pressure ;
and of the means of making water rise to the height of its source
after passing through a lower leyel. But this notion is utterly
erroneous, j£r in the great work of the celebrated naturalist, Pliny
the elder, it is expressly stated that water will always rise to the
height of its source ; and he adds that tubes of lead must be used
to carry water up an eminence.* Passages to the same effect
might be adduced from other ancient wnters, containing plain
allusions or direct statements relatiye to the consequences of the
pressure and flow of water. Indisputable eyidence that the an-
cients were not ignorant of this principle has been afforded by the
researches made among the ruins of Pompeii, where the remains
of fountains and baths show that the inhabitants of that city,
which was destroyed in the rei^ of the Emperor Titus, were not
unskilled in the means of causing water to ascend through pipes
and conduits. The reason why the Romans did not adopt the
method of conducting water through large tubes was chiefly
because they were unable to construct such tubes as would b«

»

Of what nature are the impedinients to the motion of liquids in oon-
dait pipes ?

In what manner were the aneients accustomed to eondact water from
ft distance into their cities ?

What evidence^have we that the ancient Romans understood the prin-
eiples of hydrostatic pressure as applicable to subterranean conduits ?

• Plinii Hist. Natural, lib. zxxvi. cap. vii. See Leslie's Elem. of Xat
Philos. pp. 411—413.

lUlN. 141

water-^ght -when exposed to the pressure of a considerablo
column of liquid. Their water-pipes were made of lead, eartlien*
-Ware, or wood, and were in many respects inferior to those used
in modern times.

52. Some of the most remarkable phenomena of nature are
owing to the tendency of liquids to form coherent masses, to
become extended over the surfaces of solids, and to flow in any
direction till they find a common level. Water is the most abun**
dant of all liquids, and if we trace its operations under the seve^
ral forms of rain, springs, fountains, running streams, lakes, or
Tivers, communicating with the extended ocean, the peculiar pro-
|)ertie8 which constitute the distinctive character of liquid bodies
-will be recognized in the effects which they produce. Some
notice has already been taken of tiie different states of aggrega-
tion which water assumes when exposed to certain degrees of
temperature, being expanded or converted into vapour hj heat,
and condensed by cold.* It may be considered as making its
£r8t appearance sls a liquid in the form of falling rain, which con-
sists of^ drops of water recently produced by the condensation of
dqueous vapours.

53. ''The drops of rain vary in their size, perhaps from one
twenty«fifth to one-fourth part of an inch in diameter. In parting
irom the clouds, they precipitate their descent till the increasing
resistance opposed by the ahr becomes equal to their weight, when
Ihey continue to fall with a uniform velocity. This velocity i^
therefore, in a certain ratio to the diameter of the drops ; hence
thundet and other showers in whieh the drops are large pour dovrn
fester than a drizzling rain. A drop of the twenty-finh part of
an inch, in falling through the air, would^ when it had arrived at
its uniform velocity, only acquire a eelenty of eleven feet and a
half per second ; while one of one-fourth of an inch vrould acquire
A velocity ff thirty-three feet and a half.**f

54. Experimental inquiries have freqnelitly been instituted ad
to the quantity of rain which had fallen at any particular place
during a certain period. An eslimate of the amount of aqueous
fluid discharged from the atmosphere might be formed from
Observing the quantity of rain-water descending on the roof of a
house or any other building, provided the whole could be collected
and measured before any poi^ion of it had been dissipated by
evaporation, and an exaet measurement could also be obtained of
the superficial area of the surfaOe on which the rain had fiillen.

Why did not the ancients carry all their aqueducts Beneath the surface
of the f^roand ?

What limits the yelocity of water descending in the form of rain ?

To what is the resistance of the air te falling drops of water propor-
tioned ?

How might an estimate he formed of the amount of water descending
annually over the surface of a country ?

' - - - , - . . ■ ., ■

* See Mechanics, 54. t Leslie's Treatise on Heat and Moisture.

142 RYDROBTA.TICS.

There are some situations ia which this plan might be executed
without much difficulty.

55. But a more generally applicable, though perhaps less satis-
factory method of ascertaining the daily, weekly, monthly, or
annual fall of rain, in any situation, is by means of an instrument
called a pluviameter, or rain-gauge. This instrument- has been
variously constructed, and the different forms which have been
recommended may each have their particular advantages ; but the
general object of all of them is the collection of rain falling on an
area of known extent, as a few square inches, aod providing
for its accurate measurement. Below is represented a ram-
page which has at least the merit of simplicity, as showing, on
inspection, the quantity of rain-water which may have fallen on a

certain area during any given time. It con-
sists of a quadrangular-topped funnel. A,
the opening of which may be ten inches
square, terminating below m a reservoir B.
Through the neck, or opening between the
funnel and the reservoir is inserted the gra-
di^ated rod C, to which is adjusted the hall
D, made of cork or light wood, so that it
may float on the surface of the water in the
reservoir, and the upper part of the rod being
marked with divisions into inches and parts
of an inch, will indicate by its ascent the
depth of water in the reservoir. The sto^
eock E serves to let off ihe watei after its quantity has been noted,
or at any stated periods. A more simple instrument consists of a
conical vessel about 8 or 9 inches high and 5 inches in diameter,
placed in a convenient frame and furnished with a rod, graduated
progressively to correspond to the varying size of the cone.

56. According to the observations of Mr. Daniell, ^e average
quantity of rain which falls in the neighbourhood of liondon, m
me course of a year, amounts to 23.1 inches ; the greatest quan-
tity falling generally in the month of July, and the least in Febru-
ary ; and the whole quantity falling durine the first six months
being not much more than half that in the last six months of the
year.*

57. Leslie has remarked that in general twice as much rain
falls on the western as on the eastern side of the island of Great
Britain, and that the average quantity may be reckoned at 30
inches. According to this estimate, the whole discharge from
the clouds in the course of a year, on every square mile of the sur-
face of Great Britain would at a medium be 1,944,633, or nearly

How is the rain-gauge constructed and applied ?

What depth of rain generally falls in London in the coarse of a year ?

* Meteorological Essays and Observations. By J. F. DaoieU, F. R. S<
9dedit

ORIGIN OF SPRINGS. 143

' 2,000,000 tons. This gives about three thousand trais of "water for
each English acre, a quantity equal to 630,000 imperial gallons.*
68. It may be questioned whether the very limited extent of
any observations which can be made by means of rain-guages
afifords ground for perfect confidence in die results they afford ;
and hence wherever experiments can be prosecuted on a larger
scale it is desirable that they should be recorded ; as the conclu-
sions already obtained might thus be either confirmed or cor«
rected.

59. Ther^ is one singular circumstance attending the fall of
rain calculated to throw some doubt on' the absolute accuracy of
the common mode of observation, which is, ** that smaller quan-
tities have been observed to be deposited in high than in low
situations, even though the difference of altitude should be in-
considerable. Similar observations have been made at the sum-
mit, and near the base of hills of no great elevation. Rain-^ages
placed on both sides of a hill at the bottom, always indicate a
greater fall of rain than on the exposed top."*

60. It appears, however, that larger quantities of rain fall on
extended tracts of elevated ground than at the level of the sea ;
but that at stations abruptly elevated above the suriece of the
earth the amount diminishes with the ascent. The mean annual
fall of rain at Geneva, as Calculated from observations during
thirty-two years, amounts to 30.7 inches ; and on the Alps, at the
Convent of the Great St. Bernard, the mean of twelve years is
60.05 inches. According to M. Arago, who has traced a pro-
gressive decrease in the annual amount of rain from the equator to
the poles, not less than 123.5 inches fall in a year on the Malabar
.coast, in latitude 11 ^ deg. N. ; while in latitude 60 deg. the
quantity is reduced to 17 inches.

61. The water that falls from the clouds as well as that derived
fxom melted snow and similar sources, if the surface with which
it comes in contact happens to be loose and porous, will sink into
the bowels of the earth, penetrating in any direction till it meets
with a stratum of clay, or some other dense and almost impervious
substance, which may cause it to accumulate and form subterrane-

IIow ^eftt a weight of water has Leslie supposed to fall on a square
mile of the surface of Great Britain.

What is found to be the relative quantity of rain falling in high and low
stations ?

How are the quantities of rain found to tary on high table lands, and at
the level of the sea ?

What remarkable example of this rariation can be adduced ?

What are the relative quantities of rain falling in the torrid and in tlie
temperate zones respectively ?

Explain the manner in which water reaching the earth from the clouds
is eventually disposed of?

* Leslie on Heat and Moisture ; see, also, Proceedings of the British
Association at Cambridge, 1833, for a report of experiments made at
Yorit.r— Ed.

144 WIMMMrATIOS.

ous lakes or reservoirs, the contents of which eccasiooally are
raised to the surface in various situations by hydrostatic pressnns.
Thus sometimes in digging wells it is necessary to penotnUe to a
great depth before water can be obtained, but at length when the
source is found the water rises with such rapidity in the shaft
that has been opened as sc^arcely to leave time for the well-sinkera
to make their escape from the ascending column.

62. The term Artesian wells has been recently applied, espe-
cially in France, to wells formed in the manner just described,
by the ascent of water through openings made by bcuring down
and introducing tubes which traverse the superitNT strata, and c<»b-
municate with subterraneous springs or reservoirs, from which
the water rises through the tubes by hydrostatic pressure, neariy
or quite to the surface ; constituting in the latter case perpetual
fountains, such as occur on the e^fetem coast of Lincolnshire,
England, where they are called Blow Wells. They are also fre-
quent in Artois, in the Netherlands, and hence they have derived
the appellation of Artesian wells, from Artesium, the ancient
name of that country.*

63. Water collected in subterraneous passages by infiltration
sometimes passes below the bed of the sea, and forms a sort of
Artesian fountains, which flow at intervals depending on the ris*
ing and falling of the tide. A remarkable ebbing and flowing
stream of this kind was discovered in 1811, by boring in the haip-
bour of Bridlington in Yorkshire ;f and submarine fountains have
been met with at the mouth of the Rio los Gartos, in South Ame-
rica, at Xagua, in the Island of Cuba, and elsewhere.^

64. By means of such underground canals formed by nature,
streams of water and even great rivers, after sinking into gulfs
and cavities in the earth, make their appearance again at the sui^
face, in some cases far from the spots where they descended. $
Gulfs of this kind, in which rivers and rivulets lose themselvee,
occur in the Alps of Jura, and other limestone mountains ; and
where the upper surface consisting of a bed of tenacious clay pre*
vents the absorption of the rain-water by the soil, openings into
the more porous strata beneath whether natural or artificiS, may

What evidence haye we of the existence of extensive collectioni of
vater under the surface of the ground ?

To what is the term Artesian wells applied ?

What is the origin of that terra ?

In M'hat remarkable situation has the formation of Artesinn wells been
occasionally prosecuted ?

• See Notice of a Lecture on Geology, by Dr. Buckland, in the Ke-
port of the British Association, vol. i. pp. 100, 101.

t See a Paper by John Storer, M. D. in the Philosophical TraDsaotioni,
for 1815. Abst. of Papers in Phil. Trans, vol. ii. pp. 6, 7. v

I Numerous Artesian wells, both salt and fresh water, have been fbrmed
in the United Stales. ^Ed.

§ See Humboldt's Travels, vol. ii. p. SIS.

be made the me^as of conyeiting a maiahy waate into s ftrtOe
plain,*

65. When rain falls on the summits or elevated sides of hillt
and mountains, if tlie surface be solid rock or clay, the liquid, by
its natural tendency to flow till every part of its exposed surface
has attained a common level, collects in rills, which find or form
for themselves narrow channels, through which the water descends
to th^ plains below ; there, the confluence of springs from vaiioos
sources produce lakes or rivers, which in general ultimately com-
municate with the ocean, or |nrith some great inland sea, like the
Caspian or the Lake of Aral, both which are below the levri of
the Mediterranean ;t and other, lakes which have no outlet mual
be situated in valleys or basin-shaped cavities, either below the
sea-level, or surrounded completely, by walls of rock or compact
earth, which prevent the egress of the liquid mass,

66. Rivers in their passage to the deep sometimes fonn grand
and beautiful cataracts and waterfiUls, where the collective streamt
jafter being coniined in a narrow channel, bursts abruptly over a
precipice with astonishing force, dashing on the lower surface,
snd rising again in clouds of misty sprav. Such are the famous
Falls of Niagara, formed by the water of Lake Erie ; the Gatarael
of Tecquendama, on the Rio Bogota, in South America, described
by Humboldt; the Fall of the Ithine at Schaffhausen; and the
icatairacts of the Nile, at Syene, now Assouan, in Upper Egypt.

67. The currents, which have been thus rushing with impetah
ous force over the same surfaces for successive ages, cannot bqt
have had a considerable effect even on the hardest rocks of which
their beds are formed ; and hence the heights from which these
torrents descend being gradually worn down, alterations take
place, and the cataracts must at leng^ lose much of that foimiii-
able and impressive ^pearance they now exhibh. It is owing
no doubt to such chanffes that the descriptions given by ancieoi
travellers and geographers of some of the most remarkable cata-
racts by no metos correspond with their present state.

68. Rivers formed by natjire are running streams, whose velo-
city depends on the inclination of the surfiice of the coun^
through which they pass. They have in various ages and in dil^

In wbftt instances is water known to have eoUeeted in basins below tl|S
level of the sea ?

What influence are cataraicts known to exercise on the rocks over
which they descend ?

Why are the accounts of ancient travellers not always verified by the
present appearance of cataracts ?

On wliat does 4he velocity of natural streams depend ?

• See an account of the draining of the Plain of Falans, near Mar-
seilles, by sinking «hafl% from the surface into the cavernous strata be-
low, which conveys water through subterraneous channels to the barb«|ar
rf Mion, near Cassis, Ibrraipg spouting qunngs, or Artesian feuAtaini.—
Arcana of Science .for l^S, pp. £35, £d6 ; from Hericart de Thury.

t See the Report of the British Association at York, 9.939.

N

146 HTBR08TATICS.

ferent parts of the world Wen made the means of interconrse by
inland navigation between distant places. For this purpose, how-
ever, they are but imperfectly adapted ; since, besides the obsta-
cles arising from rapids and cataracts, there must always be
difficulty in ascending the stream of a river proportioned to the
rapidity of the descending current. Hence in many countries
navigation for the purpose of interna] communication is in a great
degree confined to the larger rivers and tide- ways, and to the
numerous artificial canals which have been constructed chiefly
since the middle of the last century; and the smaller naturaJ
streams, crossed by wears, mills* and manufactories of various
descriptions, may thus be most eifectively rendered subservient to
the promotion of national industry and wealth.

69. A navignbl^ canal usually consists of several continuous
bodies of water, sometimes of c^nside^able longitudinal extent,
and each one haVing a perfectly level surface, the water being at
rest. In a country intersected by npmerous mountain ridges and
▼alleys, the formation of a lon^ unbroken line of canal must in
general be attende4 with difHcuities, and can seldom be effected
at all except by erecting massive aqueducts supported on arches,
and stretching from one point to another over the lower grounds,
and elsewhere by carrying subterraneous galleries or tunnels
through intervening hills.

70. Canals, however, generally Consist of several longitudinal
Irasins at different levels, and to preserve or rather ocbasionally
to form communications between these, for the passage of vessels,
iocks are constructed wherever a variation in the level takes place,
and thus vessels may be raised or lowered, according to circum-
atances. Locks are nothing more than small basins, with flood*
gates at each ^d, placed across the canal, from side to side, and
3ius includin^a portion of its water between them. To transfer
a vessel from the higher to the lower level, the water in the inter-
vening lock must be raised, by opening sluices at the bottom, to
the height of the upper level, then the floodgates on that side
being opened, the vessel is to be dniwii into the lock, the gates
through which it h^s passed are to 'be shut, and the water in the
lock suffered to sink Uirough sluices to the lev^ of the lower part
of the canal, and the lower floodgates then being opened the
vessel may proceed on its passage till it reaches the next lock,
where the same process must be repeated* The transfer of a ves-
sel from a lower to a higher level is effected by the contrary
operation of raising the water in the lock, instead of sinking it,
while the vessel remains inclosed in it. *

71. The passage of vessels in either direction through a lock
«annot take place without the loss of a considerable quantity of

What eircamstance limits the usefulness of rirers for purposes of Da-*
vigation ?

Of what do artificial canals commonlT consist ?

In what manner is a cnmniunicatioii effected from a reach of canal at
one level to that at another ?

spscnric jxraxtty. 147

"tratar, which mast in each case be allowed to escape from the
higher to the lower level of the canal. Where the supply of water
therefore is not very copious, and more especially when the appli-
cation of artificialmeans is requisite to obtain it, the loss becomet
a serious inconvenience, and eource of expense. This has |ed to
different schemes for the convejrance of canal-boats from one level
to another, without any expenditure of water.

73. One method of effecting this object is by means of a snS'
pension-lock or moveable basin, containing a body of water saf
ficient to float a canal-boat, and capable of being alternately raised
to the higher and depressed to th^ lower level of two correspond*
ing parts of a canal, separated from each other by floodgates, with
a space between them in which the suspended basin might be
raised or lowered, so as to take in and discharge the boat. This
scheme does not appear to have been put in practice, at least not
on an extensive scale; and from the complication of Uie machinery
requisite,, it would probably be found liable to insurmountable ob-
jections. In some situation, the basin terminates at a certain
point, and another basin commcncinff at a lower level, boats are
transferred from one basin to another by inclined planes.

w Speeifie Gravity,

73. The terms Density and Specific Gravity have been le*
peatedly introduced in the preceding pages; and their genml
signification has been in some degree elucidated already. It will
however be necessary now to explain somewhat more fiilly the
signification of those terms, not only as applicable to liquid bodies,
but likewise with reference to soHcfs and gases ; and to describe
the means by which the specific .gravity of any substance may be
ascertained.

74. In deserUyinff the effects of hydrostatic pressure, we have
hitherto considered them as owin^ to the presence of a single
liquid ; the illustrations of the ptinciples of the science now under
review having been chiefly drawn from the phenomena exhibited
by water alone, in several situations and circumstances, as afford*
ing results more simple and uniiorm than those which are observed
when different liquids are placed in contact with each other, and
when their combined pressure on solids as well as their mutual
action must be modified accordingly.

75. It has been sufilciently demonst^ted that a single liquid, ae
water, will always stand at the same height in two or more open

What diiad vantage attends the transfer (^ boats frani one level to an*
other by means of locking ?

What methods have been proposed or employed to obviate thejoiit of
water in the transfer of boats ?

To how many classes of bodies are tlie terms density and specific gra*
vity applicable ?

Whence' resdlts the equality of height at which liquids rise in tiiliea
coramanioatitig with each other?

t48 HYDROSTATICS.

fabes freely coiDmnnicatin^ with each other, whaterer may be thdr
peculiar forms or dimensions ; and this indeed is a necessary con-
sequence of the common tendency of every liquid to act with equal
force in all directions, producing equality of pressure on the solid
body or bodies by which it may be encompassed, and extending*
itself, where unconfined, till every portion of its surfsuse has as-
sumed a common level.

76. When two liquids or any greater number, differing from
each other in specific gravity, are placed in contact, as when in-
cluded in a glass jar or bottle, unless they are capable of uniting
to form a chemical compound, it will be perceiir^sd that each liquid
becomes arranged in a separate and distinct stratum, the heaviest,
Or that which has the greatest speciiid gravity, sinking to the bot-
tom of the jar, and'presentinga level suiikce above, on which rests
the next heaviest liquid ; the others in the same manner taking
flieir places according to their respective degrees of relative or
Specific gravity. Thus mercury, water, olive-oil, and sulphuric
ether, might be poured into the same phial, in which they would
form separate layers, standing one above another, in the order in
which they have been mentioned ; water being much lighter than
mercury, oil lighter than water, and ether yet lighter than oil.

77. Many liquids, differing in specific gravity, may be mixed
by agitation so as to form a compound ; but if the lighter liquid be
poured gently on the^ surface or the heavier, they will for a long
nme remain distinct, but little action taking plac»» 9^eU where the
surfaces meet. E v^ry body knoWs that water may be mixed with

Sort wine or snirits, both which ftre lightet ^hantfam liquid, as may
e shown by ttie following experiments.

Suppose A B to repreSI»nt a double^odled vessel
the only communicatian between the upper and
lower portions of which is through the tube C and
D ; then if the part B be filled with water to the
neck, and A witii port wine, so as to rise above
the tube D, still no mixture or alteration in the
state of the liquids will take place, for the lightest
sV ^ -III ^<^^Py^°? ^® hi^est situation will retain it un-

. M il disturbed. But if the lower part be filled with

port wine, and the upper with water, the former
fluid will ascend through the tube D, and the latter descend through
the tube C, till they have entirely changed places. A vessel
of this construction, having the upper part transparent, and ihe
lower part opaque, would form an amusmg philosophical toy, by
means of which might be exhibited an apparent conversion of water
into wine. An analogous experiment may be made by taking a

What happens, when two liqiirds, incapable of chemical union, and of
different specific mvities, are put into the same vessel ?

With what liquids might this trath be illustrated ?

Is the actual mixture of two liquids capable of combining, a eeuCain
Sonseqoenee of placing the one upon the other ?

Bj what arrangement of apparatus mig^t this be exemplified ?

SPECIFIC ORAVITT.

1^

small bottle, with a lonpr narrow neck, not more than the sixth of
^n inch in diameter, which is to be filled with spirit of wine, tinged
red, by infusing io it raspings of sanders wood, or yellow, bv pttt>
ting into it a small quantity of saffron ; the bottle thus filled with
the coloured spirit is then to be placed at the bottom of a deep glass
jar of water, when the spirit will be seen to ascend like a red or
^Fellow thread through the water, till the whole has reached the
surface.

78. Bodies, differing in specific gravity, «nd incapable of com-
bination, may be shaken together in a phial, and mixed for a time,
but will separate completely on being allowed to remain at rest.
Such ia the effect exhibited in the following mimic representation
of the production of the four elements from chaos. A glass tube,
about an inch in diameter, closed at one end, or a deep phial, being
nearly filled with equal parts in bulk of coarsely powdered glass,
oU of tartar, proof spirit, and ifhphth^, or spirit of turpentine, tho
former spirit tinged blue, and the latter red,* the tube or phial
must be secured with a cork ; and when it is briskly shaken th«
four imaginary elements will form a confused dull-looking mass,
but on setting the phial upright, and suffering it to remain undis*
turbed for some time, an entire separation will take place between
the several portions of the chaotic mixture: the powdered rlase
at the bottom representing earth ; the oil of tartar, floating twoye
it, water; the spirit, with its cerulean tint, occupying the place of
air ; and the glowing naphtha at the top designed as an emblem
of elementary fire.

79. When two liquids, varying in specific gravis
ty, are included in a bent tube, as represented ia
the annexed figure, they will not stand at the same
height on both sides of the tube, like a single liquid ;
but their respective heights will be in the inverse
ratio (if their specific gravities. Thus, as any given
bulk of mercury weighs nearly fourteen times as
much as an equal bulk of water, one inch of mer-
cury, M, would equipoise about fourteen inches of
.' Water, W, on the opposite side of the bent tube.
Neither the form nor the dimensions of the tube are
of any importance to the result of this experiment;
for as in other cases 6f hydrostatic pressure, a small quantity of
water may be made to counterbalance' the larger quantity of the
heavier fluid mercury, provided the column of water stands per-
pendicularly fourteen times as high as the column of mercury.

What happens when two liquids incapable of eombination are diaken
together?

Describe the apparatus known by the name of the foar elements.
. What occurs where the bent part of an inverteil sypon is oeeupied by
mercury, and one of the branches is afterwards filled with water ?
— i ■ II I . ■ ■ ■ -^^i—i—

* The blue tint may be communicated to the proof spirit by adding a
small portion of tinctai*e of litmus ; and the other spirit may be coloare4
with dragon's blood. .

60. On th« piiseiple now stated, a read j method might
be contrived foe aBcertaininr the relative weights or spe-
cific gravities of any two fiquids, as oil and water, or
water and ether, or spirit of wine. For this purpose it
would merely be requisite to procure a glass tube, bent
and graduated as represented In the margin; then on
pouring into the npnght brancbee, equal qaantities by
weight of the respective liquids, their relative weights
woald appear on inspection; being inveisely as -the
heights to which they would rise in the branches of the
tube. The aecoiacy and utility of such an tnetrument
would be angmented by filling the lower portion aFtfie
hibe with mercury, and the graduated branches being of equal
diameter, given weights of anv liquids, which would not ttct che-
mically on the mercury, would show, by their respective heights
An either side, how much greater space an ounce, a dram, or any
ether quantity of one liquid would take an than an equal quantity
Of the other ; and hence it would appear how far tiie specific gra-
vity of the latter exceeded that of the fonner.*

61. As the specific gravi^of aliquid is indicated by the relative
Apace which any given portion by weight occupies, so in the same
tnanner the specific gravity of a solid body may be inferred from
fbe bulk of Water or any liqnid of known specific gravity, which
an ounce, a pound, or any similarly ascertained quantity of the
•olid would displace when plnnged in the liquid. Oa this princi-
ple depend the usual methoas ordetBrmining the specific giavitieB
of'bodies, by means of hydroetatio balances, hydrometers, an-
^meters, and oleometers.f

8Q. The discovery of lius fundamental principle of science has
been generally ascribed to the Syracusan philosopher, Archime-

3, and the circumstances relating to it are thus reported by
Titraviua.^ Hlero, King of Syracuse, having ordered an artist

Hdv might Ihe printiple InTalved in (htt dperiment be applied to de-
termine (he relstiie weight) of difierenttiqnidi?

What is the relilion between Ihe denfitf of a liquid and the apaoe
frhieh B giten weight of it ointt oooupj ?

How ma; Ihe ipeciGo graTilj of a tolid be foond withool reducing it to
taw partieiibr Form or bulk i

What inMrumenta are emplnjed to determine reUlive weighti of bo-

*An inatrumenl in iniich the two open endi of Ihe tube are turned
dawnvanli Hnd dipped into lepkrate eopi of Ifonidi, and la die bent or
upper part of which an eihaniting ■prin); ii applied lo prodaee a l^rtial
vaouum to raise the liquid, irmueb more eonieniont in praotioe. It h«i
Ifing been known in France 11 the " ireometra ■ ponipe." A modifica-
llon hT Dp. Here ii called the lilrameler.

tTlie former of ilieae inilrumenti ii u eslled from tbe Greek rt.p,
♦»ter, and UiTps,, a measure ; and the latter from A(ai>t, light, or hai-
hgeampiniiveleviir.indu,,,,.. (»K)m«ter«le«tbe*aliMaf lamp oil.

t Architector. lib. g. cap. 9.

8P8CIFIC ORAVITT. 151

to make him a golden crown, af%er it was completed foond soma
cause for suspicion that the goldsmith had imposed on him by
mixin? with the gold, with which he had probably been famished
from the royal tre^sary, an inferior kind of metal. The investi-
gation of this matter was referred to Archimedes, who appears to
have been unable for some time to contrive any satisfactory method
of. ascertaining whether the crown consisted of mixed metal or
pure gold. At length, on the occasion of his getting into a batht
ne observed that the water rose on the sides of the marble buin
or reservoir in which he stood, in exact proportion to the bulk of
his body beneath the surface of the fluid. At once the idea flashed
on his mind that every solid plunged under the surface of water
must displace nrecisely an equal bulk of that liquid ; and aft
solids, bulk for hulk, are some lighter than others, the companip
tive or relative gpravity of two or more solids might be ascertained
by immersing equal weights of them in water, and observing the
quantity of liquid displs^ed by each of the solids. Convinced
that he could by this means find out whether Hiero's crown had
been- adulterated, the philosopher is said to have leaped from the
bath, in a fit of scientific Ecstasy, which rendered him insensible
to every thing except the importance of the principle he had dis-
covered, and running naked through the streets, he exclaimed
eloud, '''£vp(»A— -^£tf^MCdu" '^ I have found it out !— I have found
it out !"

83. In order to applv his theory to practice, he procured a mass
of gold and another of silver, each having just the same weight
with the crown : then, plunging the three metallic bodies sno-
cessively into a vessel quite filled with water, and having care-
fully collected and weighed the quantities of the liquid which had
been displaced in each case, he ascertained that the crown was,
bulk for bulk, lighter than gold, and heavier than silver ; and he
therefore concluded that it had been alloyed with the latter metal.

84. In comparing the relative or specific gravities of bodies, it
is necessary that Uiere should be some standard to which the
respective weights may be referred. It might be stated that pla-
tina is as heavy again as silver, and that cast iron is not much
more than half as heavy as mercury ; but it would not be possible
from these data to decide whether silver would sink beneath the
surface of mercury ; for though it is clear that cast iron would
float on mercury, yet unless some further information were ^ven,
no comparison could be made between the relative gravities of
silver and mercurjr. Supposing, however, it be known that mer-
cury is thirteen times and a \mf the weight of water, silver ten

What historical account is given of the discovery of this method of de-
termining specific gravities ?

What process was performed by Archimedes to detect the amount of
alloy in Hiero's crown ?

What standard is it customary to assume in speaking of the relative
weights of bodies ?

What renders any such standard neceasaiy?

152 HYDROSTATICS.

.times and a half, iron seven times and a half, and platina twem^*
one times, it will be obvioas tiiat the last-mentioned metal wonid
sink in mercnry, while silver as well as iron would remain sus-
{>ended on it.

85. Tables of the specific gravities of a great multitade of
bodies have been constructed, showing their relative weights,
expressed in numbers denoting in what ratio they exceed or fall
below that of water. The adoption of this fluid as the standard
of specific gravity is attended with several advantages, which
have induced philosophers in general to consider its density,
under certain conditions of temperature and atmospheric pressure,
as affording a convenient point of comparison to whicn may be
referred the densities of other bodies, whether solids, liquids, or

gases.* The extraordinary power of water to resist compression
y mere mechanic force, except under such circumstances as can
rarely take place, f is one of the advantages it presents ; but in
the prosecution of experiments of a delicate nature, the pressure
of the atmosphere must be taken into the account in order to en-
sure accuracy in t}ie results of our calculations. Alternations of
temperature, as to heat and cold, also affect the bulk of water so
considerably as to render it absolutely necessary that any sub-
stances, wljjpse specific gravity we wish to ascertain by experi-
mental comparison with that of water, should have the same tem-
perature with the standard liquid, or that allowance should be
made for any unavoidable difference of temperature. Purity of
the watery fluid is likewise, as may be supposed, indispensably
requisite ; rain-water carefully distilled, and thus freed from all
foreign impregnation, is therefore to be preferred in the prosecu-
tion of experimental inquiries.

86. In the London Philosophical Transactions for 1798, is a
memoir by Sir George Shuckburgh Evelyn, containing an account
of numerous and important experiments on the specific gravity of
water, which have served as the foundation of subsequent re-
searches. He found that a cubic inch of pure distilled water, ihe
barometer standing at 29.74 inches, and Fahrenheit*s thermome*

What advantages belong to the standard actually adopted, beyond what
are possessed by other substances ?

What relation has temperature to the metliod of determining specific
gravities ?

What is the weight of a cubic inch of water at mean temperature and
pressure ?

•

* The relative density of gases is sometimes estimated by comparison
with that of atmospheric air, as the standard : but the ratio of the specific
gravity of atmospheric air compared with that of water being known, that
of the other gases may be deduced from computation, when their several
relationsinpoint of density to atmospheric air have been ascertained; and
on the contrary the relations of tbe other gases to atmospheric air, as the
standard of specific gravity, may be computed from a table of specific
gravities, including the gases, and referring to water as the common unit
of density. — See Treatise on PneumaUa,

t See 10^15 of this article.

TABLE OF SI^ECmC eiUTlTIES.

163

t^r at 66 degrees, weighed 352,587 grains troy. Now it is a well
ascertained fact that water attains the utmost degree of density
jjQSt before it freezes, its bulk being relatively less at 40 deg. of
Fahrenheit or 8 deg. abpye the freezing poitat, than at any point
either higher or lower in the scale.*

87. The difference of the weight of a cubic inch of distilled
water at 40 deg. and at 60 de?. is somewhat less than half a ffrain
troy, whence it may be made to appear from calculation that a
(subic foot of pure water, at its greatest densilTf weighs almost
exactly 1000 ounces avoirdupois, or 62^ pounds. If, therefore,
^e specific grayity of water be rejtfesented by the number 1000,
each of the numbers in the following table will express the eor-
lesponding weights of a cubic foot of the several bodies included
in It. llius a cubic foot of pure gold would weigh 19,258 ounees
ayoirdupois, and an equal bmk of cork but 240 ounces.

88. Spemfie GravHieg oftarums SoUds^ LAq^ida^ and CrOMeM^OM com*

fared t»(ih Water at 60 Deg.

Platina, laminated

. 22,069

purified •

. 19,500

€K>ld,ca8t •

. 19,258

hammered •

. 19,361

standard, 22 carats 17,486

MeH^ur^, fluid

. 13,568

solid •

. 13,610

Lead, cast

. 1^1,352

SUver, cast

. 10,474

hammered

. 10,510

Bismnth, east

. 9822

Copper, cast

• 8788

Brass, cast •

• 8395

wire •

. 8544

Nickel, cast

. 7807

Iron, cast

. 7207

malleable

. -7788

Steel, soil

. 7833

tempered •

. 7816

Tin, cast

• 7291

Zinc, oast

• 7190

Sulphate of Barytes, or >

Ponderous Spar >

Oriental Ruby
Brazilian Ruby •
Bohemian Garnet •

Oriental Topaz •
Brazilian Topaz
Diamond • •

Natural Magnet •
Fluor Spar

Parian Marble, white .
Carrara Marble, white
Rock Crystal
Flint
Sulphate of Lime, or

Selenite .
Sulphate of Soda, or

Glauber Salt .
Chloride of Sodium,

or Commoif Salt
Phosphorus'

4430

4283
3531
4188
4010
3536
3521
4800
3181
283V
2716
265S
2594

2329

2200

2130
1770

At wbat tempemtare is water at tbematest density >

What is the weight of a eubio foot ofwater at its greatest density f

What Would be tne weight in ounces of a enbie foot of platina ?

Would a block of siWer sink or ^wim in a bath of roeronry ? why ?

Would a piece of steel sink or swim in melted copjicr P

What would be the effect of dropping a bar of lead into a pot of meltci

tin?

How many times more matter in a eubio foot of saltpetre than In a llko

bulk of water ?

« See Treatise on FyronomicM, .

154

HYDROSTATICS.

\

Nitrateof Potash, 01
Saltpetre

Sulphur, native •

Plumbago, or Black Lead

Coal

Sulphuric Acid, or Oil >
of Vitriol . 5

Nitric Acid

highly con-
centrated

Muriatic Acid, liquid,
or Spirit of Salt

Sea-V^ater

Ice . . .

Alcohol

Proof Spirit

Sulphuric Ether

Naphtha

Linseed Oil

0:ive Oil

Oil of Turpentine
Aniseed .
Lavender •
Cloves

Camphor

Yellow Amber •

White Susfar

2000

2033
1860
1270

1840

1271

1583

1194

1030
930
797
923
734
708
940
915
870
986
894

1036

1078
1606

Honey • «
White Wax
Caoutchouc, or Gum

Elastic
Ivory • •

Isinglass

Milk, cow^s ' •
Butter • •

Mahogany •
Lignum Vite
Dutch Box
Ebony •

Heart of Oak, 60 years >

felled . 3

White Fir
Willow

Sassafras Wood .
Poplar •
Cork
Chlorine, formerly called

Oxymuriatic Gas
Carbonic Acid, or fixed

air . « «

Oxy^A Gas
Azotic, or Nitrogen Gas
Hydrogen Gas •
Atmospheric Air •

1469

968

933

1917
1111
1032
942
1063
1333
1328
1177

1170

569
585
482
383
240

3.02

1.64

1.34
0.98
0.08
1.21

89. If the specific gravity of water be represented by 1 instead
df 1000, then that of platina will be 22.069, the last three fiffures
t>eing taken as decimals ; the specific gravity of standard gold will
be 17.486, that of sea-water 1.030, that of olive oil 0.915; and so
on throughout the table, the three right hand figures representing
decimal parts, except those denoting the specific gravities of the
gases, the numbers of which must be thus altered to indicate ths
xelations of their specific gravities to that of water.
Water - - - - 1.
Chlorine ... 0.00302

CafbonicAcid - - 0.00164
Oxygen Gas - • • 0.00134
Nitrogen Gas - - - - 0.00098

Which would sink most rapidly in vater, a piece of flint, or one of na-
tive sulphur?

When alcohol and linseed oil. arc put into the same vessel, which will
vccnpy the higher part ?

^ Determine the snme, with re^rd to water and honey — oil of turpen-
tine and cow's milk — ^proof spirit and naphtha — sulphuric ether and oil
of lavender.

When the specific gravity of water is taken as unity, what must we eon-
aider the last three figures of each number in the table ?

SPECIFIC 6RAVITT OF THE HITMAN BODY. 155

Atmospheric Air - - 0.00131
Hydrogen Gas - - 0.00008

90. From the foregoing table it will appear that almost all bo-
dies will float on the surface of mercury ; gold and platiua, and
their alloys, being the only substances known of higher specific
gravity than that metalic fluid, except one or two recently disco-
vered metals of rare occurrence.* Many bodies will float on the
surfaces of metal while in fusion : and thus earthy and other sub-
stances found in metallic ores rise in the state of scorie to the sur-
face of the melted metal in the process of reduction. The lava
discharged from volcanos is a very dense fluid, partly metallic;
and hence stones of vast bulk and weight are ireqiiently seen
swimming on its surface M^^ile it remains ip the liouid state. "

91. Most kinds of wood will float oi^ Water, and out ftw, as fir,
vrillow, and poplar, on rectified spirit. The solution of a solid in
any liquid increases its density : thns sea-water is heavier, bulk
for bulk, than pure water ; and an ^^ which will sink in the lat*
ter will swim tn brine. Hence it sometimes happens that a heavy
laden vessel, after having sailed in safety across the salt sea, sinks
on entering the mouth of a river ; owing to the inferior specific
gravity of the fresh water.

92. The specific gravity of the human body during life is in
most cases nearly the same with that of river water, and coincides
more exactly with that of sea-water ; so that there are probably
but few persons who would not float very near the surface of the
sen in calm weather. Corpulent people are, bulk for bulk, light-
er than those of sparer habits ; for the adipose membrane or fat of
animals is inferior in specific gravity to water ; whilst lean flesh,
unless the blood and other juices are drained from it, js of higher
specific gravity than that fluid, and bone is proportionally much
heavier than the soft parts of the body. Hence it might be infer-
red that the power of^ floating on water does not depend entirely
on the relative specific gravity of the solids and liquids which en-
ter into the composition of a human body ; and accordingly we

Whieh of the gaseous bodies has the greatest speoifie gravitj ?

How many and which of them are specifically heavier than atmosphe-
ric air ?

Which is the lightest of ^seous substances ?

Why do the impurities of metallic ores rise, when melted, to the sur-
face of the mass ?

What is the nature of lava ejected from volcanos ?

What effect on the' specific gravity of any liquid is produced by dit-
aolving in it a portion of^ any solid ?

To what maritime occurrence is this fact applicable }

What is the relative specific gravity of the human body compared with
fresh and with sAlt water respectively ?

* Iridium, a peculiar meCsllic substance discovered by Mr. Smithson
Tennant, in combination with crude platina, has the specific gravity of
18.6 ; and Tungsten is a rare and difficultly fusible metal, the specifia
gravity of which is stated to be !?.&

156 BYPBOSTATICS.

find that the hody of a person destroyed by drowning, or throws
into water immediately after death, will sink far beneath the suf-
iace ; but after several days have elapsed a body thus treated osa-
ally rises to the level of the water, in consequence of its having
become specifically lighter than that fluid, from the accumulation
of gas within the body, produced by incipient putrefaction. It is
then chiefly owin^to the air included in the cavities of t!he body
durinjgr life, especially that portion contained in the lungs, that a
man is enabled to float on the surface of a pond ox xivei.

93. There are, however, some credible accounts extant of pei«-
sons whose bodies were so much inferior in specific gravity to
water, that they could not descend beneath its surface ; not pos-
sessing that *' alacrity in sinking," which may be literally attri-
buted to most individuals. In 1767,*there was a priest residing
at Naples, named Paulo Moccia, whose extraordinary facility of
flotation attracted much public attention. This ecclesiastic could
Bwim on the sea like a duck ; when he assumed a perpendicular
position, the water stood on a level with the pit of nis stomach ;
and it is stated that when dragged under the water by one or more
persons who had dived fi>r that purpose, as soon as he was re-
leased, his body would rapidly rise to the surface. It appears that
the weight of this gentleman's body was thirty pounds less than
that of an equal biuk of water. This peculiarity of conformation
doubtless depended partly on his being extremely fiit, and having
very small bones ; besides which, probably his lungs were capable
of holding a larger quantity of air than is usual, and there mi^ht
also have been an accumulation of air in the abdomen, arising
from the disease called tympany, or from some other cause.

94. Most very corpulent people, who are at the same time
Btron? and healthy, woald perhaps find on trial that their bodies
would float on water ; and those who do not happen to be endowed
with a superabundance of fat might still in almost all cases, with
a little application, acquire the habit of floating with facility.
The capability of breathing freely aad at regular intervals is es-
sentially requisite to enable a person to support himself on the
surface of water. The head, and the upper and lower extremities
are relatively heavier than the trunk of uie human body ; and the
head especially, from the c^uantity of bone of which it is com-
posed, is the heaviest part of the whole mass, yet unless the face
at least be kept above water respiration cannot be continued. It
is therefore or the highest importance that ^ persons should be

Will a fat or a lean person float with the greater facility in water f

What will generally occur when a haman bod^ is throwo into water ?

Why does the body of a drowned person rise to the surface after being
some days in the water f

What extraordinary instance of specific lightness in the human bodyU
recorded ?

On what circumstances did it probably depend }

What operation is it neoessary to perform while attempting to float ^
the surface ?

Ani OF 0WI1OIINO. 157

'perfectly aware of the precautions necessary for this purpose; so
that any one accidentally falling into the water, and heing unabld
to swim, may be instructed how to escape a watery graye.

95. A person suddenly immersed in water, if not absolutely
deprived of self-possession by fright, should, on coming to the
surface after the first plunge, endeavour to turn on the back, care-
fully keeping the hands down, with the palms extended towards
the bottom of the water, the legs being suffered to sink rather
lower than the trunk ; the only parts alwTe the surfiuse will then
be the &ce and a small portion of the chest: at each inspiration
more of the head and chest will rise above the water, and perhaps
thosQ parts will at first be for a moment covered with the aqueous
fluid at the interval of expiration of the air. Every thing depends
on making no effort to raise or keep out of water any part except
the face, and endeavouring to keep the lungs, and consequently
the chest as much expanded as possible, without using any irre*
^lar exertions in breathing ; and it may be proper to caution per*

. eons thus circumstanced against struggling or screaming, as worse
than useless ; for in case any one who mi^ht yield assistance
should be within call, it would be best to wait till the first alarm
Lad subsided, and then the involuntary bather, conscious of com-
parative security, might use his voice with due effect, and with-
out increasing the hazard of his situation.

96. But an acquaintance with the art of swimming can alone
give a person perfect confidence of safe^ when b^ accident im-
mersed in water. It is to be lamented that this is not a more
general accomplishment ; for it is one which must frequently prove
of great utility ; and it is much to be desired that it should become

'a branch of education at schools for boys, as being of higher im-
portance than the more fashionable arts of dancing, fencing, or even
gymnastics.

97. It may be questioned whether written instructions alone
would enable ?mj one to acquire a fiicility in swimming; and ad-
mitting their utility, it would be inconsistent with the purpose of
this work to afford them more than a cursory notice. In swim-
ming', as in floating, the chief object of attention must be to keep
the race above water, while the limbs are immersed ; but from the
different position required, it must be apparent that in swimming,
not the race alone, but nearly the whole head must be sustained
above the sur&c^. In making a first attempt, the advice of Dr.
Franklin may be followed, where he directs the learner to walk
into water till he reaches a place where it stands as high as
his breast, and dropping into the clear stream an egg ; as soon as
it has reached the bottom, he is to lean forward, resting on the

What measures shoulcf be adopted when one is suddenly immersed in
water ?

What importance ought to be attached to the art of swimming?

What is the first step towai'ds the acquisition of that art ?

How may the learn<ur be made ^osible of the buoyant power of the
water?

O

158 HTDR08TATICS.

water, and endeavour to take up the eg^^, when he will become
sensible of the upward pressure or resistance of the fluid ; and find-
ing that it is not so easy to sink as might have been previously ,
supposed, the young adventurer would acquire confidence in his i
own efforts, the valuable result of experience.

98. Corks or blown bladders fitted by strings passing under the
arms and across the chest, will afford material assistance in sup-
porting the upper part of the body in a proper position ; but they

I perhaps rather tend to retard than facilitate the progress of the i

earner, by leading him to form a false estimate of the resistance of
the water; so that as soon as he makes an experiment without the
corks he finds himself obliged to recommence his task, and study
it on a different plan which might as well have been adopted at
first. If, however, corks or bladders should be used, it is highly
necessary that they should be secured from slipping down to. the
hips, andf thus causing the swimmer to fall with the head vertically
downwards, and incur the most imminent risk' of drowning.

99. As less exertion would be required in the position of float-
ing than in that of swimming, there would perhaps be some ad
vantage in acquiring the power of flotation, as above described,
previously to attempting to swim. This hanng been effected, the
learner might, instead of the common expedient of using corks,
procure a two^nch pine plank, ten or twelve feet long, and placing
It in the water, lay hold of it with one or both hands and push it
before him while learning to strike with his legs. But this or any
other artificial mode of practice, that may be adopted, should be
laid aside as speedily as possible, as the learner cannot too soon
make himself acquainted with the full effect of the pressure of
the fluid in which he is movin?, and with his own strength and
power of action ; and till such knowledge is attained he will make
but slow progress in the art of swimming.

100. The method of communicating buoyancy to solids of greater
specific ^vity than water, and enabling them to float in that
fluid, by inclosing within them air or gas, is susceptible of appli-
cation to a variety of useful purposes. It has accordingly been
adopted in the construction of swimming-girdles, life-preserving
belts, and air-jackets, which like the bladders noticed above, are
merely bags of different shapes contrived so as to be inflated with
air, and worn round the upper part of the body. Life-boats or
safety-boats, as they are sometimes called, are rendered buoyant
by forming in their sides air-tight cells or lockers, of sufficient di-
mensions ta prevent the boat from sinking even when every other
part of it is filled with water. It has recently been proposed to
extend this principle to vessels of any size, and thus to prevent

What objection exists to the use of eork jackets and similar expedients
to increase the buoyancy of the body when learning to swim.

What use may be made of the swimming board while learning the art?

Explain the construction and use of the girdle employed for the same
purpose.

How are life-boats made incapable of sinking ?

CAUSES 07 BUOYAKCT OV SOLIDS. 159

heavy laden meTchant ships or men of war from foundering at
The scheme consists in tne employment of copper tubes of a cy*
lindrical form, hermetically closed at the ends and sufficiently large
and numerous to contain as much atmospheric air as would causa
a ship to swim, when in consequence of haying sprung a leak it
would otherwise sink. It is stated by the inventor of &ese safety
tubes, Mr. Ralph Watson, that an eighty-gun ship, even when im-
mersed from leak, would not require the application of such tubes
to a greater extent of displacement of water than would be suffi*
cient to support 240 tons of its Immense weight.

101. Fishes, in general, are provided by nature with a peculiar
apparatus, which enables them to swim with the utmost facility,
and to ascend close to the surface of the water, or descend to a
considerable depth beneath it, by means of a membranous bag or
bladder containmg air, which they can distend or contract, and
thus alter their specific gravity according to circumstances. The
toa<l fish it is said distends its stomach oy swallowing air, to ae»
sist it in swimming, and becomes puJQfed up like a blown blaJJe^
in the same manner as the globe or balloon fish.

102. An experiment has oeen previously related exhibitiog the
effect of the pressure of water upward in supporting a plate of
metal, in contact with the lower extremity of an open cylinder,
from which it may be infeired that solids of the highest specific
gravity, as gold or platina, may be made to float on water or any
other liquid, provided the floating body be of such a form that its
upper surface may be protected from the pressure of the liquid by
a column of air, the depth of which bears a certain proportion to
the specific gravity of the solid. It is thus that a china tea-cup,
though much heiavier than an equal bulk of water, will yet float
on that liquid if placed in it with its cavity upwards and empty ;
but on pouring water into it, the cup will descend in consequence
of the air within its cavity being displaced by the heavier fluid ;
till at length, when so much water has been poured in as to ren-
der the cup and water together heavier than a quantity of water
equal to the space the cup occupies when immersed to its edge,
it will sink to the bottom.

103. A iraft will float, because it is absolutely lighter than water,
and a life-boat also for the, same reason; but vessels in general,
firom the cock-boat to the largest main of war, owe their buoyancy
to their concave form. Hence ships need not be built of fir or any
light wood, since not only the heaviest woods might be used but

How are Watson** safety, tubes to be applied for the security of veiaels
at sea ?

To what is the power of vertical movement in fishes attributable ?

How may the heaviest of metals be made to float on the lightest of li
qaids ?

What quantitjT of water will it be necessary to pour into a floating ba
sin in orcfer to sink it to the water's edee ?

How is the floating of a raft to be explained ?

}6(> HTDftOSTAtlCS.

eren the heaTiest metals, to constnict floating vessels ; and indeed
steam boats made of sheet iron have recently been tried, and found
to possess the requisite properties for ploughing the waves with
perfect facility and safe^.

104. Floating bodies maj be employed to raise heavy snbstan-
ces from the bottom of a nver, pond, or basin of water.' Thus a
sufficient number of air-tight casks might be attached by ropes or
chains to a large block of granite at the bottom of a river near its
entrance into the sea, and the ropes being adjusted to such a length
as to keep them strained tightly by the buoyancy of the casks at
the lowest ebb of the tide, the block would be raised by the up*
ward pressure of the casks at high water. Perhaps this method
of raising or lowering ponderous masses of stone might be advan-
tageously applied to practice in building bridges or piers within
the tide-way of a river.

105. The common method of regulating the supply of water
conveyed by pipes into a cistern by means of what is called a ball-
cock, depends on the action of a hollow globe of such dimensions
relatively to the thickness of the metal as to keep it always float-
ing on the top of the water in the cistern. A long wire is con-
nected with the ball at one end, and at the other with a valve or
stop-cock, on which it aicts as a lever, opening it when the long
arm of the lever is allowed to descend by the sinking of the bafi
attached to that end, when the water flills in the cistern, and on
the contrary closing the valve, when, by the rising of the ball with
the water, the cistern becomes full, and the lever presses on the
valve or cock and keeps it shut, so that the cistern can never be
filled beyond the proper height.

106. The power of floating bodies may also be applied in a dif-
ferent manner to the purpose of rendermg buoyant other bodies
attached to them ; and amonff the various applications of this prin-
ciple may be noticed the ingenious invention calted the water-camel,
used in Holland and also in Russia and at Venice, to enable large
and heavy laden ships to pass shoals or sand-banks. The method
of effecting this object consists of thd application of two long
narrow vessels adapted to the sides of the ship, and being hollow
and water-tight they are filled with water, and then let down, and
firmly secured on each side of the ship, aher which the water is
to be pumped out of them, and the whole mass, consisting of the
ship and camel is thus rendered specifically lighter than before,
ana drawing less water than the ship alone did previously,
the shoal or sand bank may be passed without danger of ground-
ing.

How does it differ fi-om that of an ipoA steamboat ?

To what usefal purpose may the prfnciple of floatation be applied ia
eonnexion with submarine operations ?

In what manner is the same principle applied to re|^late the aeeess off
water to a cistern ?

Explain the construction and use of the water-tamd ?

CENTRE 0¥ BUOYANCY. 101

107. The tendency of a floating body to assume a partiealar
position when partly immersed in a liquid, and to retain or lose
that position according to circumstances, may be ^ucidated by
reference to the doctrine of the centre of gravity, as explained
with relation to solids.* When a solid body, specifically lighter
than water, is placed on its surface, it will sink to a certain depth
at which the absolute weight of the body is exactly counter-
balanced by the upward pressure of the water. The point at
which the entire weight of a body acts with neatest effect must
be its centre of gravity ; and that point at which the sustaining
efforts of the liquid are most effective may be termed the centre
of buoyancy, which must evidently coincide with the centre of
gravity of the portion of water displaced by the floating body ; and
if the body be of uniforn} structure with the centre of gravity of
that part of it which is under water. A floating body cannot
maintain itself in a state of equilibrium, unless its centre of gr»-
vity be situated in a vertical line over its centre of buoyancy, or
immediately under that point. In the former case it will be in
the state of instable equilibrium, and in the latter in that of stable
equilibrium.f .

108. Hence the necessity of placing iron bars, stones, or other
heavy substances in the hold of a ship by way of ballast when it is
not freighted, or is laden with very light merchandize, in order that
its centre <^f gravity may not be elevated too much above its centra
of buoyancy. It is not requisite that the centre of gravity should
be reduced below the centre of buoyancy, for though such a dispo-
sition would contribute to the stability of the vessel, the resist-
ance to its passage through the waves would be so ffreat as to
make it sail heavily. In determining the proper situation of those
points regard must be had to the shape and dimensions of a ves*
sel as well as to the nature of the cargo or lading, and the manner
of stowing it ; and on a due attention to these circumstances its
security and rate of sailing must in a great measure depend.

109. The methods adopted for ascertaining the specific gravities
of bodies are founded on the relation between bulK or dimension,
and weight, which may be determined by various operations,
according to the nature of the several substances, whether solid,
liquid, or gaseous, to which they are applied. The relative den-

^^at takes place in regard to the centre of gravity of a floating body ?

How deep vill such a body when specifically lighter than water always
sink in (he liquid }

What name is given to the point at which the whole buoyancy of the li*
quid ipay be conceived to be concentrated ?

What will be the relative position of the centre of gravitjr and of the
centre of buoyancy of a body floating at rest on the surface ot water ?

Why are heavy articles stowed in the hold rather than on the deck of a
. vessel ?

— '

* See JHechamct, Nos. 125—133. f Ibid. 137—141. .

o 3

Idi HYDROSTATICS.

sity of different solids may be discovered by simply weighing a
cubic inch of each ; but unless the process of measurement and
that of weig^ng are both executed with scrupulous accuracy the
result must be uncertain, and the former of these operations at
least, must, in many cases, be difficult, and in some impracticable.
Hence the method adopted by Archimedes is to be preferred, and
it may be improved by merely weighing the subject of the experi-
ment first in air and then in water, and noting the loss of weight
that takes place in the latter case, as that must be equal to thd
Weight of the water displaced by the substance under examin»*
tion.

110. On this principle is constructed the hydrostatic balance^
which may be used to dcjtermine the specific gravity of liquids,
as well as that of solids. For this puxpose a globular or egg^
shaped mass of glass or crystal must be suspended by a hair or
fine silk thread from a hook at the bottom df one of the scales of
an accurate balance, and its weight is then to be ascertained first
in the air, next in distilled water, and lastly in the fluid whose
specific gravity is required ; then by deducting the loss of weight
of the fflass in water from the loss observed -WDBii it was weighed
in the liquid, the specific ^vity of the latter, with reference to
tiiat of water, will be obtained. By using a glass globe of such
dimensions as to lose 1000 grains in water, its loss of weight in
any liquid would at once indicate the specific .gravity of that
liquid.

111. Insoluble solids denser than water are easily subjected to
experiment; but any insoluble solid body, which is specifically
lighter than water, requires, in order that its specific gravity
should be ascertained, the addition of some heavier substance, so
that the joint mass may be made to sink in water ; then its welgrht
in air and in that liquid respectively being determined, the specific
gravity of the lighter solid will be the difference between the
weight of the heavier body in water alone, and that of the joint
mass, deducted from the difference of their weight in air. Solid
substances, soluble in water, such as salts, may have their spe-
cific gravity ascertained by weighing them in alcohol, or some
other liquid which will not dissolve mem, and their specific gra-
vity, water, being the standard, may be found by computation ;
or they may be weighed in water aiter being defended from its
action by coating them thinly with melted bees-wax.

What are some of the methods of determining the speeific gravity of
bodies }

What is the eonstmction of the hydrostatie balance, and how is it ap*
pUed t6 this pnrpose ?

What method is it neeessary to adopt inaaeertaining the speeifiegrayity
of solids lighter than water ?

How can we take the specifie gravity of solid bodies whieh are sohible
m water?

CAPILLARY ATTRACTIOX.

163

112. The most usual and coiiTeiiieiit method of
ascertaining the specific gravittes of liquids is hj
means of a hydrometer. This instrument, as re-
presented in the margin, consists of a hollow glass
hall B, with a smaller ball of metal C, appended
to it, and which, from its superior weight, serves
to keep the instrument in a vertical position, to
whatever depth it may be immersed in a liquid*
From the lai^ ball rises a cylindrical stem A D,
on which are marked divisions into eoual parts ;
and the depth to .which the stem will sink in water,
or any other liquid fixed on as the standard of spe-
cific ^vity being known, the depth to which it
sinks m a liquid whose specific gravity is required
will indicate, by the scale, how mucn greater or
less it is than that of the standard liquid.

CapiUary Mradion,

113. Liquids ije distinguished by the property of preserving
a level siunace when at rest, and rising to tne same height in any
number or variety of communicating tubes ; an effect resulting
firom the joint action of the cohesion of their particles and the in-
fluence of universal ^vitation. But there are certain circum-
stances in which liquids may be placed, in consequence of which
the phenomena wiU be remarkably modified, and a portion of a
liquid mass may rise far above the common level, and preserve its
elevation, as if exempt from the power of gravity. Water may
be made to rise perpendicularly to a great height in an exhausted
tube ; and even mercury, one of the heaviest of fluids, may be
seen to be elevated in the same manner in a barometer tube 39
or 30 inches above the level of the liquid in the basin, into which
the open end of the tube is plunged. But in these cases, as we
shall subsequently show, the innuence of mvitation is distinctly
perceptible, and the liquids rise in exhausted tabes, in consequence
of pneumatic pressure.*

114. There is, however, another case in which liquids rise above
&eir common surface level, not being inclosed in exhausted tubes,
but in tubes open at both ends, or between solid plates nearly in
contact. This phenomenon is styled Capillarity^ and is said to be
caused by Capillary Attraction.f Instances of the operation of

How are the fpeeifie gravities of liquid subatanees eommonly ascer*
tained ?

Explain the eonstmction of the hydrometer ?

In what manner may water be made to rise above the general level of
its mass f

Is the exhanstjon of tubes in all. eases neeessary to prodoee that effeet ?

To what phenomenon is the term eapillarity applied ?

—~*~~~— ~*™^^™*— »"i^^-^^

* See an aeoount of the Barometer, in Treatise on Pneumatics,

t From cabiUutf a hair, or capUlam^ hair-like, in rcfiereiice to the
tmall bore or tubes which produoe these elfeeta.

IM

RTOROffTATICS.

this principle are constantly taking place aroand us, and thongrb
highly interesting, they are overlooked by common observers.
If a slice of stale bread an inch square, and three or four in
length, be held perpendicularly with one end immersed in a small
quantity of water or milk, the liquid will ascend through the pores
of the bread till it is entirely absorbed* and if there is a sufficient
quantity of it, the bread will become saturated with the moisture.
In the same manner water or any aqueous fluid will ascend and
spread through a lump of sugar or a heap of sand, if the base of
either be immersed in the liquid.

115. Tubes of glass having a very small bore, and therefore
called capillary or hair-like, if dipped a little way beneath the sur*
fece of water, will cause the liquid to ascend to a height bearing
a certain relation to the diameter of ^e tube. If that diameter
be 1-50 of an inch, water will rise to 2^ inches ; if it be but 1-100
of an inch, it will rise 5 inches ; and so on in the inverse ratio
of the diameter of the tube. Similar effects may be exhibited by
means of two plates of glass, placed as represented in the margin
in a shallow vessel of water, so that their edges on one side, A O,

may be in contact, and the other^
B D and' E F, somewhat separated.
The liquid will then rise between
the plates, standing highest on
that side where they most nearly
approach, and gradually declining
towards the sides that are separated,
the upper surface of the elevated
portion of the fluid forming the
curve F G H, the height of the li-
quid at any point, as H, being great-
er in proportion, as it is nearer to the side of the plates A C.

116. It is in consequence of capillary attraction that a sponge
imbibes water, blotting paper absorbs ink, or that oil arises amidst
the fibres of the cotton- wick of a lamp. These effects are mani-
festly owin? to a common cause, and we learn from experiment
that it is only under certain conditions that they take place. Thus,
all liquids will not rise to the same height in the same tube, for
water will rise higher in a capillary ^lass tube than alcohol, and
neither of these liquids will rise at all in the finest metallic pipe,
nor in a glass tube, if the inside of it be greasy. Mercury, on the
contrary, will not rise in a clean glass tube, especially if it be
wetted ; while it becomes elevated, when the inside is lined, with
a very thin film of bees-wax or tallow.

117. Some remarks have been elsewhere introduced, relative to

From what exhibition of the principle is its name derived ?

How may the progressive increase of capillary attraction be experi-
mentally exhibited ?

Is the same amoont of capillary attraction exhibited by a solid towards
all sorts of liquids?

CAPILLARY ATTRACTION. 165

the effect of cohesive attraction on the particles of liquids, cans-
ingr them to assume a ^lohnlar figure, and on the modifications
produced hy the attraction of solids with which the liquids may
oome in contact.* It is on the joint operation of these causes
under particular circumstances that the phenomena of capillarity
appear to depend. It is found from observation that when fiuida
rise in capillary tubes, the surfaces are concave or depressed in
the centre ; and on the contrary, when the fluids do not rise, they
have convex surfaces, or stand highest in the middle. These ef«
feets are mamfestly owing, in the first case, to the superiority of
the attraction between the liquid and the tube over that between
the particles of the former ; and in the second case, to the in-
feriority of the former attraction compared with the latter. Hence
also if water be poured into a glass tumbler it will rise somewhat
a(t the edges, while mercury poured into the same vessel would
be depressed at the edges.f

On what causes do the phenomena of eapillary attraction depend ?
What surfaces do liqoidt in tubes ordinarily present ?
What causes the diversatjr in this case ?

* See No. 5 of this treatise.

f See Joomal of the Franklin Institute, yoL xit. p. 147, for some inge-
bIOUs experiments on capillary attraction, by Mr. J. W. Draper.— Eb.

The following, among other treatises, may be profitably eoiH
suited in regard to this branch of philosophy, and will generally,
perhaps, be attainable without much difficulty by the Americaa
teacher.

Cambridge Mechanics, by Prof. Farrar, p. 289 — 368.
Fischer's Elements, p. 83 — 111.

Playfair's Outlines of Natural Philosophy, vol. i. p. 168—* 193.
Gregory's Mechanics for Practical Men, Philad. edit. p. 284^
301.
Library of Useful Knowledge — ^Treatise on Hydrostatics*
Robinson's Mech^ical Philosophy, vol. ii.
Edinburgh Encyclopedia, article Uydrodynamica,
Hydrodynamique, Bossut.
Hydrodynamique, Prony.
Traits de Physique, par Biot, vol. i. chap. 22.
Mecanique Celeste, translated by Bowditch, book 10.

HYDRAULICS.

I When the ecjailibrium arising from the weight and conse-
Gueni pressure of hcjuids is disturbed, motion will take place ; and
the laws by which it is regulated are the same with those which
govern the motion of solid bodies. The velocity of flowing water,
uke that of falling bodies, depends on gravitative attraction, and
is to be estimated on the same pinciples ; and the phenomena
exhibited by jets of water, or other spouting liquids, are analo-
gous to those displayed by solids projected through the air, the
effects in both cases depending on the operation of similar causes.

2. Among the circumstances which influence the motions of
liquids, one of the most important is the weight of the air, pro-
ducing atmospheric pressure ; and to this force the most powerM
and useful machines for raising water chiefly owe their efficiency.
Such are the various kinds ot pumps, flre-engines, and siphons,
which are rather to be considered as pneumatic than as hydraulic
machines, resembling in their mode of action the barometer and
the common syringe ; their construction and effects may ^ere-
fere be most advantageously investigated and explained in treat-
ing of pneumatics, indeed that branch of hydrostatical science,
which relates to the motion of liquids, is so intimately connected
with the theory of motion, as applicable to all fluids, whether
liquid or gaseous, that in a systematic treatise the subjects could
not with propriety be separated.

3. At present, we shall confine our attention to the effects of
the motion of liquids on different parts of connected masses, or
on solids with' which they may come in contact ; and afterwaids
briefly notice the construction and mode of action of those ma-
chines whose power depends on the weight or pressure of flowing
liquids, or on the pressure or impact of liquids on solid bodies.

• 4. In consequence of the imperfect cohesion of their constituent
particles, liquids present some peculiar appearances, when they
fall through the inilnence of gravitation. A continuous solid mass
will always remain at rest while its centre of gravity is supported ;
thus it may be sustained by net-work, or suspended by a line, as
securely and steadily as if it were inclosed on all sides ; but an
unconnected mass, as a heap of sand, can have no common centre
of gravity, and therefore to preserve its stability everv separate
gram must be supported. Water, or any similar liquid, in order
to keep it in the state of equilibrium, requires support even to a
greater extent than a disintegrated solid, or powder ; for such is
Sie peculiar attraction existing between the particles of a liquid,

Whftt laws regulate the motions of liquids ?
On what does the velocity of flowing water depend ?
What circumstance modifies the motions of liquids?
Under what two general divisions may liquid motions be examined ?
What peculiarity is presented by liquids when falling in obedience to
gravitation ?

166

FORCE 07 FLOWING WATER. 167

that unless the whole mass be supported laterally as ve.l as at
the base, it will spread on that side where the pressure is with-
drawn till every part has attained a common level. This property,
and its effects in producing pressure in liquids at rest, have been
already noticed, and those which are exhibited by flowing liquids
axe how to be "developed.

5. When water contained in a deep vessel is suffered to escape
from an aperture in the bottom, it flows in a continued stream,
formed by the pressure of the liquid aqting against that point from
which the support has been withdrawn. The combined effect of
the hydrostatic pressure, and the cohesion of the particles of the
watery fluid causes various movements in the flowing stream,
which may be accurately observed by using a glass jar, and mix-
ing with ^e water some very small pieces of amber, or sealing-
wax, the specific gravity of which exceeding that of water but in
a trifling degree, they will be carried down with the current, and
exhibit its internal motions.

6. The annexed figure will serve to show the man-
ner in which the liquid descends^at first in horizontal
strata, and afterwards, when a portion has escaped^
the surface becomes depressed in the centre, till
at length, when it approaches the bottom, it as-
sumes the form of a funnel, or hollow inverted
cone, which it retains till the vessel is nearly
emptied. If the aperture be made in the side of
the vessel, and close to the bottom, the same ap-
pearances may be observed, with the exception of
the hollow cone, which in this case does not occur, the liquid re-
maining level at the surface till it sinks down to the orifice. As
the common direction of the particles of the descending liquid is
towards a central point, indicated by the course which the floating
fragments of sealingwax take towards the aperture, the stream must
become compressed, and consequently somewhat contracted at that
point. Its situation depends much on the size of the aperture ;
and when that is very small, and the side of the vessel in which
it is pierced extremely thin, the greatest contraction of the jet
will take place at the distance of about half the diameter of the
orifice beyond it ; and at that point tlie diameter of the liquid vein
will be to the diameter of the orifice nearly in the proportion of 5
to 8, whatever be the height of the liquid in the vessel from which
it flows. This contraction of the liquid vein may be equally ob-
served when the discharge takes place from an aperture in the
side of a vessel, and likewise when the liquid is projected verti-
cally upwards, as in jets^-iPeau,

' What force projects and maintaiDs the continued stream of vater flow*
ine from a deep vessel P

How may the interior motions in such a vessel be rendered apparent ?

What appearance on the exterior of an orifice results from thie inter-
ference of^the particles of liquid seeking the outlet?

Within what limits does the corUractea vein approach the diameter o£
the orifice ?

168 HlfDSAULICa.

7. The point of greatest contraction in a stream of flowing
water, or of any other liquid, must manifestly be also the point
where it has the greatest velocity, as it is there that the hydro-
static pressure acts with greatest effect. In estimating the velo-
city Ota liquid issuing from an aperture in the side or the bottom
of any vessel, it will he found to depend on the vertical height of
the water within the vessel ; and in every case it will be equal to
the velocity that a body would acquire in falling through a space
equal to that height. Hence it cannot be uniform unless the
water is supplied as fast as it is discharged, and thus kept ^ways
at the same level. ,

8. Suppose two vessels, one of which is 5 inches in height, and
the other 20 inches, to be filled with water, each having a circu-
lar orifice at the bottoml-5 of an inch in diameter, if both be open-
ed, atid the vessel kept constantly full by a supply of water above,
the taller vessel will discharge about 21 ounces of water in a
quarter of a minute, and the shorter vessel about 11 ounces in
uie same space of time. Thus, estimating the relative velocity of
the stream in the two vessels by the quantities discharged by each
in a given time, that of the stream from the taller vessel will be
to that from the shorter, as 2 to 1, nearly ; and the velocities would
be exactly in that ratio, but for the effect of Motion between the
particles of the liquid and the sides of the vessel, and the resist-
ance of the air, which proportionally diminish the discharge fi'om
the taller vessel somewhat more than that from the shorter one.
Now taking the velocities as 2 to 1, the height of the taller vessel
being to that of the shorter as 4 to 1, it will appear that the velo-
city m either case is as the square root of the height of ^the column
of liquid in the respective vessels ; for 1 x 1 = 1> and 2x2 »= 4.

9. It may, therefore, be generally stated, that independently of
the irregularities occasioned by friction and other causes, the
maximum velocity with which a liquid flows from an aperture in
the side or bottom of a vessel will be as the square root of the
depth of the vertical column within the. vessel. Hence the velo-
city of a flowing liquid depending, like that of a falling body, on
.gravitation, it follows that a stream issuing four feet below the
surface of a liquid mass will have double the velocity of one issu-
ing at 1 foot below the surface ; at the depth of nine feet the velo"
citjr will be treble, at 16 feet fourfold, at 25 feet fivefold, and so
on in proportion to the depth of the aperture below the sur&ce.
It must be recollected that these comparative estimates are to he
Tegarded as results deduced from the influence of gravitation
alone, therefore in practice allowance must be made for the effect
of friction and atmospheric resistance, and the dimensions and
form of the aperture must likewise be attended to in making expe-
riments and calculations.

In What part of a jet will the greatest velocity necessarily be found ?
On what cireumstanee does the velocity of issuing curi'ents depend ?
Why does not the rapidity of flowing liquids correspond exactly with
Ike aqoare roots of the heights, or heads or pressure ?

10, From the experimeDts ot fioMilt it appenn tint dw »etiml
qoftntitT of water discharveil &om oriGces of the same dimeosiop*,
under different dejreeB ofnrasHnrB, ia hr less than might be in-
ferred from calculation. TTie following table* of theoretical and
practical dischargfea through circular onficea one inch in diameter
will elearly exemplify this principle.

Height oftjie liquid Conipatcd diMhapge per Aetoil diielurp Per
■tore the orifice. miaate. In eubio indiel. per mlnale. el.

1 foot - - 4437 - - 3819 - 63^3

6 feet - - 10123 - - 6977 - 63,0
Iflfeel - - 14317 - - 8860 - 81,8
IS feet • - 17533 - - 10831 • 61,1

11. The phenomena exhibited hj apouting liqnida iriien the
current is directed rertically upwards, are equall^ wifti tiuMD of
descending currents under the influence of gnritation; and ••
bodies projected perpendicularly in the air rise to aheight eqnal to
thai from which they mast hare descended, to acquire the Telocity
with which Uiey were propelled,! *" Kqnids apouting from a short
pipe directed upwards, rise to a height equal to that of the liquid
column by the pressure of which they were ejected. .In the nui>
ginal figure let A repreaent a cistern filled with water at the con-

slant height BC, then if four bent p^Vpt
"" D, E, F, G, be inaerled at different di>-
tanoea below the surfiice, the lets will
dl rise to neatly the same level, that of
the line B C The resistance of the
atmosphere and the mutual friction be-
tween the particles of the Bsoendisg
current, both, however, counteraci its
force, so that it is only when the orifices
of the pipes are extremely small that
the eletalion of the jets becomes cod-
•iderable relaUrely to the hydrostatic pressure. Yet water may
be made to rise in spouting streams even aboTe the level of the
reservoir from which it issues, by introducing a current of ur in
such a manner Ihat it may be mingled with the stream, and tne
fluid thus becoming speciBcally liehter than the water in the le-
aervoir, the la,lter is more powerfully acted on by the incumbent
weight.

^Vhl^ do the riprrimenli of Bo«™t prove la regard to tha dltehsr^e'

Doe» the (llfftmiee between llie tiieoretleal md the aelnal dinhtrta
incretae or diminish by *n incretu of head >

Whil reUiion eiiiCB betveen the head of pKRore and th« belsnt (0
lihieh 1 liquid will be projeeied upwahdi ? , . , ,. .-.

In what minner ma; a Hqnld be made to rin In a jet above the lerd

dimen!

U J 1^,
^ ( Much

170 . KWDKAXJttCB.

12. The ooncouTse of the aerial and aqueous fluids produces
musical sounds, somewhat resembling those from the harmonica,
but not so soft. That the sounds are caused by the particles of
the air striking against those of the water is evident, because,
when the ilax of the water is stopped, and the air suffered to issue
alone, nothing is heard but a hissmg noise very different from the
preceding.*

13. It has been ascertained from experi-
I I I B I ™^"^ ^^^ ^ greater quaotity of water will be

I "^ I I discharged in a ffiyen time ^om the side or

■— j |— J L-i r- * bottomof a yessel, through a.short projecting

tube, than from a simple aperture of the same
dimensions. The tube, however, must be
entirely without the vessel, as in fig. B, for
continued inside, as at' A, the discharge
be lessened instead of being augmented,
also depends on the figure of the tube
and that of the bottom of the vessel, since
more water will flow in the same time through a conical or bell-
shaped tube than through a cylindrical one, and a further ad van*
^ge will be gained by giving a correspondmg shape to the bottom
of the vessel, as at D. These effects depend on the interruption
to the conflux of the aqueous particles by the sides of the rising
tube in the vessel A, and the greater facilities afforded for their
escape in different degrees by the forms of the apertures in the
vessels B, C, and D ; and the last of these, coinciding most ex«
actly with the figure of the flowing stream, is best adapted to pro-
mote the discharge of the liquid.

14. When pipes, or tubes, of considerable length are used to
conduct water from a fountain, the effects will be modified by va-
rious circumstances, the quantity discharged depending on the
length and dimensions of the pipes, their direction m inclination,
ana the number and abruptness of the angular bondings which
take place in their course.

15. When a stream of water is propelled through a cistern or
basin containing water at rest, it will have such an effect on the
entire mass as to set it in motion, and cause a great part of it to
mix with the current, and make its escape. Owing to this pro-

{»erty of flowing liquids, it is possible to drain a lake or mareh by
eading a stream descending from a higher level to the border of
the lake, when it will sweep through the stagnant water, and

Whiit phenomenon aeoonpanies a jet of mixed air and water isiuing
from a pipe ?

On wnat cireomitanees do the effects of short tubes of adjittage depend ?

What additional causes of resistance are to be considered in long tubes ?

' What occurs when a stream of water is directed along the surface of
« basin of the same liquid ?

To what is this effect attributed ?

• y. Beudant Traite Elemeutuirc de Physique, 1829, pp. STl, 272.

FORMATION Of WAtES. 171

mdually drawing it into its vortex, carry it off over the opposite
bank. Venturi, an Italian philosopher and engrineer, made use a£
this inethod to drain a marsh near Modena, by coDdacttng throag^
it a rapid descending stream.* This effect is prodacM by fne-
tion between the particles of the liquid, and thus the water in
motion communicates its impnlse laterally, till the whole mass is
affected, and gradually entering the current is carried off.

16. The friction which takes place between the particles of
water and those of the air is productive of some curious and in-
teresting phenomena. To this cause is owing the current of air
caused by the fall of water from an eminence, of which a remark-
able instance is adduced by Venturi, in a cataract which rushes
from the glacier of Roche Melon, on the rock of La NoTalese,
near Mount Cenis.

17. The agitation of tiie sea by the wind, and the transforma-
tion of its surface into a mass of foaming waves and mountain
billows during a storm, is another important and striking effect
of the friction of air and water. That the formation of waves
depends on this cause is convincingly proved hj the experiments
of Dr. Franklin, who ascertained that by pounng oil on the sur-
face of a pond to the windward, in stormy weather, the ripples
with which it was covered might be made to subside ; and it
^appears that this method of calming the waves by pouring oil on
their surface has in some instances been found advantageous at
sea. From its inferior specific gravity the oil forms a floating
film, which defends the surface of the water from contact with the
currents of air, and tiie friction between the wind and waves is
vastly diminished, in the same manner as that which takes place
between solids is by the application of unctuous matter.

18. The effect of the pressure or impact of flowing liquids on
.solids immersed in them, is, as in other instances of hydraulic

pressure, ^aUy influenced by circumstances, and therefore the
general pnnciples arising from theory must be adopted with con-
siderable limitations when applied to practice. It must be mani-
fest that when a flat solid surface is moved perpendicularly
against a liquid, the resistance will always be in a certain pro-
portion to the extent of the solid surface ; and when such a plane
surface is exposed to the action of a flowing liquid, the effect must
be greater or less according to the degree or the velocity of the
stream. Hence may be deduced the general rule, that the effect
produced by the pressure of flowing water, aetingr perpendicu-
iariy on a flat surface plunged beneath it, is in Uie compound

f

To what useful purpose has this experiment been eonverted ?

How is the elevation of waves to be explained ?

What experiment is eonoeived to demonstrate the correetness of this
explanation ?

In what proportions are solids resisted when moving through liquids.'
■ ■ I ■ I ■ « ' •

• See Leslie's Elements of Nat. Philosophy, vol. i. pp. 397, SOI | and
Bflcbodson's Journal, 4to. 1798.

rt% IITINUITLI9S.

ratio of the Bqaate qf tiie Telocity of the iBtream and that of tite
laolid sttifaee. If the^urfaee be pieaented obliquely to the dliee-
tion of the stream, ^he effect must be less than when it is pelrpen-
dioular to the suT&ce of tthe current; and the diminution of pres-
sure arising from such a cause will be proportioned to the mcli*
nation of £e solid surface. Its amount in any given case may
be calculated on the same principlei^ as the enects of inclined
iplanes in mechanics.

19. When a liquid acts by impact on a solid plane, causing it
to turn round an axis, in the manner of the float-boards of a water-
wheel, there will be a certain point in. that plane, where, if the
whole force of the stream could be concentrated, it would produce
the same effect as when that force is distributed over the whole
surface of the plane. The point thus indicated is the centre qf
. percussion, some notices of which have been introduced else-
whercrf*

BydrauUfi Mo/ehines*

30. The object of hydraulic machinery is chiefly that of raising
water from a lower to a higher level, which effect n^ay be pro-
duced by hydrostatic pressure or impact, on liquids and solids,
either alone, or in conjunction with atmospheric pressure. The
construction of those machines whose operation depends on the
latter cause must be referred to the treatise on Pneumatics ; but
there are other machines which mxy be properly noticed at pre-
sent as their modes of action admit of satisfactory explanation
on the principles of hydrostatic science.

21. These may be distinguished into three classes : namely,
machines for raising water by mechanical means only; those
which act by the weight, pressure, or impact of water, on solids ;
and those in which the efiect is produced by the reactive force or
intermitting action of flowing water.

2d. A common draw-well, from which the water is lifted by
means of a bucket and windlass, affords an example of a machine
of the first class. JBut the comparatively small quantity of water
that can be raised at once by the use of a single bucket confines
its employment to domestic or occasional purposes.

23. The chain-pump is a much more efficient engine, though
very similar in its mode of action to the preceding. The figure

What advantage is possessed by the obliquity of Uie sorfaee against
ivhieh the resistaDee is.applied ?

^ At what point Id a float-board may the whole action or reaction of a
liquid be conceived to be applied ?

On how many different principles are machines for raising water con-
structed ?

Into how many classes are those machines divided, which depend for
^eir efficiency entirely on hydrostatic laws ?

* See Msehamc9t No. 183. See Col. Beaufoy's experiments on Hy«
.draulie action, in which a vast variety of forms, veloeities, and modes of
action are detailed.-^£D.

TB« rHAiK-rtm?.

173

in the margin represents it u consistinff
of a number of plates or flat disks of
wood, D D D D, attached horiiontallj
to an endless chain, and passing round
two wheels, E and F, by turning which
the chain and plates are carried through
a water-tight cylinder, the lower end of
which is plunged beneath the surface of
water, and its internal dimensions are ex-
actly adapted to receive 'the plates, which
successivelv entering the tube when
drawn up by the revolving chain, form
so many buckets filled with water, which
they carry up and discharge into a cistern
above, or when used as they commonly
are en ship-board, into a pipe that may
— — " discharge it again into the sea. The mik

chine may be set in motion by a winch, or other means applied
to turn the upper wheel. The chain-pump will act with greater
effect when the cylinder can be placed obliquely than when its
direction is exactly vertical.

24. The rope pump is a less efficient modification of the chain-
pump or bucket-engine. It is composed of wheels, one under
water and the other above, having on their peripheries several
grooves, through which pass endless ropes ot very loosely spun
wool or horse hair; and the upper wheel being made to revolve
with great velocity, the water which adhere»to the coarse ropss
may be raised and discharged above by presfure. The water is
here attached to th6 rope by simple cohesive or capillary attrac-
tion.

25. The Persian wheel, which la used to raise water not only
in Persia but also in Egypt and other eastern countries, consists
of a large wheel, to the nave of which are suspended a number
ot buckets, in such a manner that in the revolutions of the wheel
they successively dip into a pond or stream of water over which
the wheel moves, and the buckets thus being filled ascend with
their load till each in turn reaches the summit of the circuit,
where there is a contrivance for tilting each bucket, so that it may
discharge its contents into a cistern or reservoir, and it then de-
scends with the revolving wheel to be filled again. Such a wheel
may be put in motion by any mechanical means ; or if it be em-
ployed to raise water from a running stream, floatpboards may be
added to make it revolve like an under-shot wheel.

Explain the netion of the ebuin-pump.

In what position will the ehain-pump aet to moat adTantage ?

By what species of mechanieal action is water raited on a rope pampr

Explain the coDstruction of the Persian wheeL

.174 HTDOAnLIGR.

Q6. The cochlioD 01 screw
of ArchiroedsB, derives its

designatioD from & preva-
lent opinion tbat it was Ihe
invention of the Sjiacasan
tage. But it is not men-
, tioned by Vitravius amone
the discoveries of Archi-
medes, and there is aome
ground for believing that it
was, before bis dme, need
in Egypt to raise and cany off the Bnperfliioas water letl in the
low grounds ai)er the inundaUons of Uie Nile; sothat the ques-
tion as to its ori^ remains ondecided. Its farm, as represented
in the margin, is that of a helix (as the name partly implies,)
consistiag of a flexible tube like a hollow corkscrew wound roond
a solid cylinder, which may be made to revolve by turning a
winch, or by attached wheel-work. When it is placed in an ob-
lique position, with the lower opening of the screw immersed in
a cistern, or any other body of water, the liqnid will enter below,
as the orifice dips beneaft it in each revolution, and be carried up
■nd discharged above ; the peculiar form of the machine facilitat'
'ing the elevation of the water.

S7. The most important machines belonging to the second class
are different modifications of water-wheels, l^ey are respectively
termed undershot wheels, overshot wheels, and breast wheets.

llie undershot wheel is said to
be of earlier origin than the others ;

tfnd it is likewise the mos"

:s shown i
id figure, of a wheel o
phery of which are fixed a nnmbu'
of flat boards at equal distances, and
set at right angles to the plane of-the
wheel. They are called float-boardai
, and the wheel being so placed as for
' its lowest point to be immersed in
flowing water, it is set in motion by
tiie impact of the water on the boards as they successively dip
. into it As a wheel of thia kind will revolve in any stream which
furnishes a current of sufficient power, it may be used where tha
descent of the water is bv far too trifling to turn a bieaat wheel,
mnch less an overshot wlieel.
3S. If all tha float-boards are vertical to the centre of the wheel.

To whom !■ the inrentian of the coshlion eonimonl* aKribed t
Into hair man; bUiki ire nrtical wiler-vheeli diTided >
WI)M narae ia (irm to that part of an nodtnhot wheel whiah reoeiiM
die inintet of the water I
la what dmadimi ii the pe«ulia»adTanla|« of thia Und o( wtwd la to

WATEB-VREKU. - ITS

n the figure, the wheel will work eqnillT well in eilher direc-

flowing or the ebbing lide. But in any other situation ■ wheel ii
to be preferred in which the float-botirds incline towards the cof-
rent, and thus the effect of the stroke is increased ; but it a^ars
from experiment that the best position is when the inclination of
the float-boards is bat inconsiderable.

S9. The overshot wheel diflen
. from the foregoing in the manner in
which itis acted on bj water, receiv-
ing- its impulse not from the impact
onlj, hut from the weight of water.
This kind of wheel, as maj be con-
ceived from the figure in the margin,
can only be used where a considcT-
able fan of water can be obtained.
On its peripberr are fixed a namber
of cavities caUed buckets, being
closed on both sides, but having
Qpeninga, so that the water, conducted by ale vel trough of the same
breadth with the wheel, ma; fill each bucket in succession, as it
reaches that point in the circuit of the wheel at which the weight
of the water ,can begin to act on its circumference. From the pe*
culiar form of the buckets they retain the water partially till they
have descended to near the lowest point of the circuit, and having
discharged their contents into the tail-stream, thej ascend on tha
opposite side tobe filled as befpre. As the overshot wheel requires
the greatest fall of water to make it act, so is it likewise the most
powerful with reference to the effect produced, by the momentum
of flowing water.

36. The breast wheel is a sort
of machine having an inlermediata
character compared with the un-
dershot and overshot wheel. It
has float-boards like the former,
but they are converted into buckets
somewhat af^r the manner of
those in the chain pump, as thej
move in" a cavity adapted to the
circumference of the wheel, is
shown in the raar^n. The water
passes through thia eavi^, enter-
How are the Soati of an undenhot wheel to be Kt with retpect to dm

Deacribc the eonitrDsliDn ind action of nn
What reUlion hti the power of the ovei
vhecli niing the ume qDKn1il]r and fall of water I
Wlwtii the cDnatruDtion of tiie breail wheel .'
InwhitptinudoMitTCMmble the other two foTinief wibn^wheebf

176

BYDRATTLIC8.

inor it nearly on a level with the axis of the wheel. In this case
the liquid acts chieHy by its weight; and the ipachine, though
less efficient than the overshot wheel, is more so than the other.
It is, therefore, only used where the fell of water happens to be
peculiarly adapted for the purpose.

31. Among the hydraulic machines belonging to the third class,
which derive their power from the reaction of flowing water, is
one called Barker's Mill, as having been invented by Dr. Barker,
towards the close of the seventeenth century. This engine, as

represented in the annexed figure, consists of a
-|P — HL hollow cylindrical metal pipe, A B, of consider-
3?^ able height, and terminating above in a funnel-
shaped cavity. The pipe is supported in a ver-
tical position, by resting below on a pointed
steel pivot, turning freely in a brass box, adapt-
ed to receive it ; and the upper part has a cy-
lindrical steel axis,C D, passing trough a board,
supported by uprights at the sides. The hollow
tube, A B, communicates with across tube, E F,
closed at the extremities, but having adjusti-
ble orifices at the opposite sides, near each end
of the cross tube. A pipe, G, above, communi-
cates with a supply of water, which it dis-
charges into the funnel at the top of the vertical
pipe B ; and the supply must be so regulated that the pipe may
oe kept constantly filled with water without running over ; while
the orifices in the cross-pipe at E and F will deliver the water
with a force proportioned to the height of the column in the tube
A B, and the apertures being iii opposite directions, the spouting
currents will communicate a rotary motion to the vertical tube and
its axis C D, to which may be attached a toothed wheel connected
with any other machinery.

32. The action of this machine does not, as sometimes stated,
depend on the resistance of the atmosphere to the jets from the
cross-pipe ; but is wholly owing to the hydrostatic pressure of the
column of water in the vertical tube, which exerting great force
on the interior of the horizontal tube, and that force being removed
from the points whence the water issues, the pressure or reac-
tion on the corresponding points on the opposite parts of the in-
terior of the tube tends to make it revolve, the action of both lets
producing motion in the same direction. Hence it is often called
the reaction wheel. The theoretical investigation of its peculiar
properties and mode of action, has engaged the attention of the
celebrated mathematicians, Leonard Eiuer and John Bernoulli^

By what meehanical property does the water produce its effect on this
wheel ?
■On what principle is Barker's mill constructed ? ,

Is the presence of the air necessary to the action of this roachiq^e ?
Ou what part of the revohing arms is the moving force really exerted ?

.both of whiim xapreeent it aa ezhibi^g'.a UBlhod of emplojing
-the force of walor aa a moving power, BupeiicH- to any other.

' 33. Among machineB whose effects depend on the fotee of flow-
ing water may be included the Hjdmulic Ram, inveiited, oi '*

chiefly froin the momentum of a current of water, suddenly stop-
ped in its coarse, and made to act in another direction ; and as it
produces a kind of intermiltipg motion, owing to the alternate re-
treat and access of the stream, accompanied with a noise arisinir
&om the shock, iU> action has been compared to theiiutting of
lams ; and hence the name of the machine.

34. Several historical facts, in regard to the employmant of the
percussive forae of liquids lo elevate portions of their own iiaUt
are cited hy .writers on this subject, prior to the invention tX
MontgoLGera Seiier hydnaaUque. The filing of pipes to conrtj
water from one level to (mother, could scarcety fail to render ap-
parent the immense power momentarily uerted when a colamn
of water descending with. considerable velocity is suddenly aneit-
ed. A raost slnliing example of this was euiibited (Dec. 1834)
At the Philadelphia water worfcs, in which, by a little deranvemept
in .the action of the valves of the force pump, the column of water

Jaom itiifi basin JOO feet high, was suddenly met by the machipa

-yiritii a force which burst the air vessel with onexplosion tike that
:ef artilleix, tearing asunder the cast iron at a part where the dn
•meter of the vessel w^ three feet, and ^e thickness of tha

.jgaetal full an inch and a half of perfectly boimu) caatiog. Sereia]
inch bolts of .wiought iron which had «onfin^ the upper part of

,4ie T46a«l were likewise lam KWfj.

35. The essential parts of the hydranlis ram, as exhibited br
Montgnlfier, are repreBented in the marginal G^re. A, is a heu
of water, connected with the tube or tunnel B, closed at the extre-
mity C, but having an aperture at D, to which is adapted a nlva
formed by a ball of porcelain or copper, hollow, so aa to be not

e percUMion of a liquid GOtamnbavo
been obKrved !
Explain the Hver*! parti of the machine ioTenled ii MontgoltlsT.

178 HT0RATTLIC8.

more than as heavy aprain as an equal Yolume of water, and sup-
ported near the orifice by a sort of muzzle or cage. F, is a reser-
voir of air, with an opening from the tunnei B, and a valve E fit-
ted to it, but lifting upward, and prevented from displacement by
a muzzle over it. From near the bottom of the air-vessel F pro-
ceeds a pipe G, which may be continued to any griven height to
which it is requisite that the water should be raised. The tube
B, is called the body of the ram ; the tube G, the tube of ascen-
sion; D the stoppage valve, and £ the ascension valve.

36. Now the former valve being open and the latter shut when
the water begins to run, it at first escapes through the stoppage
valve D, but soon acquiring a momentum, from the accelerating
velocity of its fall, it drives the ball D against the opening and
stops the passage in that direction; the reflected stream then
strikes up the valve E, and water enters into the air-vessel F,
through the ascension valVe : the ball D, as soon as it is relieved
from pressure, falls into its muzzle, and makes way for the water
again to escape through the stoppage valve, while the other valve
closes through its weight and the reaction of the compressed air
in the reservoir. The renewed momentum of the stream presently
shuts the stoppage valve, and lifting the ascension, valve, more
water enters the air vessel, and as soon as the orifice of the pipe
G becomes covered, the pressure of the air drives the water up-
ward ; for that which has been admitted through the ascension
valve cannot return, and more being added at each stroke of the
engfine, it may be gpradually raised to an indefinite height.

37. The absolute effect produced must, in any given case, de-
pend on the fkll of water to supply the engine, and the diameter
and lengths of the tubes. M ontgolfier erected a water ram in his
garden, with an artificial fall of water of 7^ feet, by which water
was raised to the height of 50 feet, in tubes two inches in diame-
ter : the water expended in four minutes was 654 pints, that ele-
vated 52 pints. Comparing the power expended^ (554 X 7.5=3
4155,) with the eftect obtained in this case, (52x50=3700,) we
get the result 2700-r 4 155=^5, or the effect is sixty-five per cent,
of the power, while with the best forms of overshot wheels the
effect sometimes exceeds 85 per cent. In another machine, with
a fall of about 34 feet, water was raised seven times that height,
and the stoppage valve closed one hundred and four times in a
minute. Improvements were made on the original construction
of the hydraulic ram by the son of the inventor, who obtained, in
England, a patent for his construction.

Wliy doci not the stoppage valve remain permanently in oontact with
its seat when once elevated by the force of the current ?

On what does the absolute effect of the hydraulic ram depend ?

What proportion did Montsolfier find between the power eocpendea
and the effect produced in the elevation of water ?

WORKS oir HTSiuirLxcs. 179

The subject of hydraulics embraces two different objects.—
The first, a theoretical riew of the nature of the forces exerted by
'water in motion, and the peculiar phenomena accompanying its
movement, whether in open channels, closed pipes, or the organs
intended to receive and employ its mechanical efficiency; and the
second regards it as a branch of engineering. Teachers will find
the two departments often blended together, and the topics be-
longing to both promiscuously treated. But in some recent pub-
lications they have been very properly distinguished, and the
science of the matter, with its various theoretical developements,
arranged under appropriate heads. In this manual, the object
of which is to treat chiefly of the' sciences, the former class of
treatises deserves particular mention.

Theoretical calculations are to be found in Cambridge Mecha-
nics, pp. 369 — 417.

Gregory's Mathematics for Practical Men, pp. 302—329.

Treatise of Mechanics, by the same author, 3 vols. 8vo. 1896.

Venturi's Experimental Inquiry, translated by Nicholson.

Lectures on Natural Philosophy, by Dr. Young.

Belidor's Architecture Hydraulique.

Prony's Nouvelle Architecture Hydraulique.

Dubuat's Principes d'Hydraulique.

Traite El^mentaire d'Hydrodynamiqne, par Bossnt.

The volume of the transactions of the British Association at
Cambridge, contains an able report by Mr. Rennie, on hydrau-
lics, as a branch of engineering, which has been republished in
the Journal of the Franklin Institute for January and February,
1835.

For an account of experiments on water power the reader may
consult Smeaton's Reports, Evan's Millwright's Guide, Banks
on Mills, and Journal of the Franklin Institute, (report of commit-
tee on water-wheels.)

V

PNEUMATICS.

1. Thk obJe($t of that branch of physical dcieiice \rhich has
bfeen denominated Pneumatics,* or Aerology,! is to Explain and
illustrate those phenomena which aris^ from the weight, pr^ssure^
or motion of common air or other fluids possessing the same gene-
ral properties. The distinction between liquids and those more
elastic fluids called air, gas, vapour, or steam, depends in a great
degree on occasional causes, espedally dn tefmperature and pres-
sure. Those effects which are' to be attributed to the operation
of heat and cold, or diversity of temperature, are oh sereral ac-
counts of sufficient importance to be made the subject of detached
investigation, comprehending a review of the relations of heat to
all natural bodies, whether solids, liquids, or ^es ; and tracing
the general influence of temperature in the production of those pe-
culiar forms of matter. Therefore, though it will be impossible
to explain the phenomena of atmospheric pressure, and its effects
on solids and liquids, without adverting to the influence of tempe-
rature, a more extended survey of that important subject must be
referred to the subsequent treatise on that branch of science which
has been termed, Pyronomics, or the laws of heat.

2. There are two kinds of aeriform bodies; namely, those which
are always in the gaseous state, under common circumstances of
temperature and pressure, thence named permanent gases or airs ;
and those which become gases chiefly at high temperature, and
Which therefore may be styled non-permanent ^es or vapours.
Common air, or atmospheric gas, affords an obvious specimen of
a permanent elastic fluid, ana steam or vapour of water of a non-
permanent elastic fluid.

3. These different specW of gases possess many properties in
common ; and there is reason to believe that those gases which,
have till recently been regarded as capable of existing only in the
form of permanently elastic fluids, might be reduced to the liquid
state by subjecting them to extremely low temperature and very
powerful pressure.

4. Mr. Faraday has effected the condensation to the state of a
liquid of the gas called carbonic acid or fixed air, as well as
several other gases previously considered as permanently elastic

What 18 the object of the science of pneumatics ?

On what rests the distinction between liquids and |;aseoas bodies ?

What imponderable agent is necessarily involved in the phenomena of
atmospheric action ?

How many kinds of aeriform bodies are found in nature ?

What constitute their distinguishing properties ?

What has Faraday proved in regard to toe state of carbonio acid and
other gaseous bodies ?

* From the Greek Uviv/tm, breath, or air ; or nviv^mrws* atrial.
tFrom *Anf, air{ and A*re(, a discourse, or treatiie.

180

GENERAL FROFERTIXS OT ME. 181

.fluids, by the combined operation of pressure and low tempera-
ture.* And Mr. Perkins, whose experiments on the compres-
sibility of water have been already described, extended his opera-
tions to gaseous bodies, and from his statements it appears that
he succeeded in reducing atmospheric air to the state of a limpid
liquid, by a pressure equS to the weight of twelve hundred atmos-
pheres.f Should the observations oi those gentlemen be confirm-
ed and extended to all those now called permanent gases, it will
be evident that their existence in the liquid or gaseous form de-
pends entirely on their relations to temperature and pressure, the
various airs and vapours being all susceptible of conaensation un-
der different circumstances.

5. Airs and vapours, or permanent and non-permanent elastic
fluids, however, though they may be considerea as forming but
one class of bodies, yet from the vast diversity of their relations
to heat, admit of bemg applied to very different purposes ; and
hence, in treating of their physical properties, the distinction
between them must be carefully kept in view. It will, therefore,
be conducive to perspicuity to notice in this treatise the properties
of the permanent gases, such as atmospheric air ; leaving the cir-
cumstances which constitute the discriminating characteristics of
the non-permanent gases, and especially of steam or the vapour
of water, to be more fully investigated in the division of this
work, appropriated to the aoctrine of Heat.

General Properties of Mr,

6. Common or atmospheric air is an invisible or perfectly trans-
parent fluid, the ultimate particles of which appear to be destitute
of cohesion ; and hence air has a disposition not only to sink
down, and spread out laterally like liquids, when unconfined, but
it is also equally capable of expansion upwards ; so that any por-
tion of this fluid will speedily become dissipated and lost, unless
It be inclosed within a solid air-tight vessel or other receptacle,
such as a bladder, or retained in an open vessel by the pressure
of a liquid on its surface.

7. That air is porous in a very high degpree appears from its
readily yielding to pressure ; but like all material bodies it pos-
sesses the property of impenetrability, for though a considerable
bulk of this fluid may be forced into a comparatively small space,
there must be a limit beyond which the utmost pressure will cease
to have any effect. The resistance of air to pressure may be

Wliat effect did Perkins obtain by subjecting air to the pressure of 1200
atmospheres ?

To what conclusions are we conducted by these experiments on gaseous
bodies f

What are the most striking sensible properties of atmospheric air }

What appears to be the mutual relation of its particles to each other ?

* See Abstracts of Papers in Philos. Trans., vol. ii p. 192.
t idem, p. 290.

Q

demonstrated by meanu of a syrin^ of any kind wiih a Bolid pis-
ton ; for if the pipe or lower opening be nrm)y closed after the
piBlon has been drawn up so as to nil the barrel wilb air, it irill
be found impossible to thrust down the piston again completelj
while the pipe remains obstructed.

8. Let a tall narrow-mouthed glass jar or bottle b«
^ half filled with water A, and a funnel C, with a ioag
tube, be inserted in the mouth of the bottle, as repre-
sented in the margin, and firmlv secured at D, by luting
or by passing' it through a cork, in such a manner that
the included air at fi cannot escape between the fun-
nel and the mouth of the bottle, "nien water bein'g'
I poured into the fnnnel, little or none of it would pass
Into the bottle ; for if the funnel had a tube several
feet, or eien yards in length, so as to give the sdmin-
tage of strong hydrostatic pressure, though in diat cas«
the air at B would be compressed into somewhat smal lei
space, yet no imaginable force would fill the bottle,
which of course would burst under a certain degree of pressure.

9. Another property of air is compressibility, in which it dif-
fers most esseniially from liquids. It has been elsewhere stat^
that water undergoes no apparent diminution of bulk from pres-
sure unless vast force is applied to it; and other liquids in dif-
ferent degrees resist compression, though readily dilated by heat
and contracted by cold. But airs and gases, though, as we hare
just shown, manifestly endowedwith impenetrabilily, yet display
a facility of contraction and expansion under the influence of pres-
Bore, which is completely independent of temperature. They are,
however, most powerfully affected by changes of tempemture also ;
their bulk increasing oi diminishJDg with the degree of heat to
which they are esposed.

10. That the particles of air can be compteBsed, or driven by
external force closer to each other than they were before that force
wa3 applied, must be apparent from the eiperiments adduced to
prove the impenetrabili^ of air; for whno those experimeote

t.i!iw of ai

show that the particles of the compressed fluid cannot be destroy-
ed, but will, when exposed to the utmost force, aUll occupy a cer-
tain space, yet itappears that contraction always takes place under
the iimuence of pressure to a certain extent; and hence may be
inferred anotherpropertyof air already noticed, namely its porosity.
11. The compressibility of aii may be experimentally illus-
trated by means of a strong glass tube closed at one end, like a
barometer tube, and having fitted to it a piston, consisting of a strong
iron wire or rod, with moistened leather fixed to one end, so that
it may move up and down in the tube quite air-tight. Then, the

Hov ii lir proved Id pOBieti the properly of Impenetnbililj ?

In ivhal miDDcr it impenetrability mBnifested in filling ■ bottle wi&

Hon' do gue» differ from liqcidi in regard to eompreMibility f
How niay the sompreMibililj' of air be eiperimentally llluitrated i

WEIOBT.OF AXE. 169*

tube being full of air, the piston is to be adapted to the open endy
and if it bn oantiously premised down, the air may be reouced to
about one-half of its origfinal bulk, without using much force, and
by stronger pressure the fluid may be yet further condensed, but
at length the resistance will be such as to preclude the possibility
of any greater compression.

1 2. TTie most re.narkable among the properties of air is elas-
ticity, depending on its expansive power, in consequence of which,
when its dimensions have been reduced by pressure, it immediate*
ly recovers its bulk on the cessation of the compressing force.
Thus, if the piston of a common syringre is pushed down while
the air is prevented from escapinj^ by the pipe, as soon as the pres-
sure is withdrawn the piston will be raised by the expansion of
the included air. To this property of air or gas is owing the force
with which a pellet of wet paper is driven from a scnool-boy's
popgun ; and this insignificant little engine acts on the same prin-
ciple with the air-gun and other philosophical instruments, which
will be subsequently noticed.

IS. Gravity or weight is another very important property of air,
which it possesses in common with solids and liquids. Common
air, as being comparatively lighter than water, will when set free
below the surface of that liquid, rise through it, in the fbrm of
transparent bubbles. This is an effect of hydrostatic pressure, in
consequence of which bodies of inferior specific gravity to water
when immersed in it are pressed towards its surface ; and thns it
happens that a cork, a drop of olive oil, or a bubble of air or eas
will fkxit on the surface of water, and when forcibly pressed be-
neath it, rise s^gain to the top as soon as the force tnat kept it
dpwn ceases to. acL

14. The weight of air may he ascertained in the same manner
as that of liquids or solids, by the common operation of weighinff
it with a balance. But in consequence of its extreme expansi^
bility, some peculiar precautions are necessary in performing this
operation, even when no ^reat nicety is reouirea. These, however,
will be subsequently noticed; and it wilt be sofficient at present
to state that bv means of a large bottle with a stop-cock and a
syringe adapted to it, the weight of a given quantity of air may
he discovered. For suppose ue stop-cock to be left open and the
bottle weighed in that state, when of course it will be full of air,
then the weight of the bottle and the included air having been
noted, the air must be drawn out, as completely as possible, by
screwing the syringe on to the stop-cock, and wbrkinff the piston ;
the stop-cock is then to be turned so as to close the bottle, which
on being weighed again, after being unscrewed from the syringe,
will be found to have lost a portion of its weight equal to that of
the quantity of air which it would hold.

Wlmt eflTect, resulting^ from elnsticity, follows the compression of ail ?
Wlwt familiar facts illustrate this position ?
What causes the rise of bubbles ot air through a mass of liquid i
How is the vrnffht of air demonstrated ?

184 PKETTMATICS.

15. A cubic foot of air weiffhs about 523 gprains ; and conse-
quently a cubic inch will wei^ somewhat more than .3 of a grain,
therefore if the bottle would hold three pints, its capacity, solid
measure, would be rather more than 100 cubic inches, so that if it
could be perfectly exhausted, it ought to weigh .3 X 100=30 grains
more when weighed with the stop-cock open, than it does after
the air has been extracted from it. By using an air-pump instead
of a syringe, a bottle with a stop-cock may be so nearly exhausted
of air, as to leave behind no quantity sufficient to interfere in the
slightest degree with the result of this experiment.

Different Kinds of Airs or Gases.

16. Common air, which forms the atmosphere surrounding on
all sides the earth which we inhabit, was long supposed not only
to be a simple elementary body, but even after its mechanical pro-
perties had been investigated, and great progress had been made
m the study of the laws of nature, very erroneous ideas were re-
tained concerning the composition of air, and it was imagined
that sdl elastic fluids were essentially the same. It is now known
that atmospheric air is a compound, consisting of two different
species of air or gas, one of which, called oxygen cas, and
sometimes vital air, is necessary to the support of animal life ; and
the other, named nitrogen or azotic gas, when inspired alone, is
injurious to animals. Both these gases are capable of entering
into combination with many other bodies of very different kinds,
and producing compounds, some of which are usually in the solid
or liquid state, and others in the form of permanent gases. There
are likewise other gaseous bodies besides oxygen and nitrogen
which have never been decomposed, and are therefore considered
as simple forms of matter ; and these, together with the various
compound gases, constitute a very numerous -class of bodies,
which possess different degrees of elastici^ and weight, and by
their consequent pressure on solids and liquids, produce equili-
brium or motion ; and hence they are capable of being applied to
various importint purposes.

17. The pecaliar nature and effects of the combinations of the
gases with each other and with solid and liquid substances cab
only be ascertained by the application of the principles of chemi-
cal science ; but the action of the gases or airs, so for as it depends

What is the weight of a cubic foot of air ?

About how many g^raini less will a three-pint bottle weigh when ex-
hausted than when filled with air ?

Wiiat opinion prevailed among the early philosophers in regard to the
nature of air ?

How were all gaseous bodies regarded ?

or what materials is atmospheric air composed ?

What analogy have oxygen and nitro^n with other gaseous bodies ?

What differences exist in the mechanical properties of the gases?

How far does tlie examination of gaseous bodies belong to the science
of pneumatics ?

OASES AKD TAPOURS. |80

on their mechanical properties, forms the appropriate sabject of
Pneumatics.

18. Though atmospheric or common air, as bein^ by far the
most abundant and generally diffused of all elastic fluids, is ther^
fore usually employed as the medium of pneumatic pressure, yet

' since the recent researches of men of science have made us ao*
quainted with the variety of those fluids and their several proper-

. Ues, it appears that some of them may be adapted to the purposes
of art with greater advantage than others, and atmospheric air is
no lon^r the only kind of gas made use of as a moving power.

19. ^The discovery of elastic fluids much lighter than the atmo-
sphere has given origin to the art of Aerostation, or soaring through
the air in an inflated balloon ; the explosion of ^unpf^wder, and the
projectile force of balls, shells, and other missiles dischargred from
artillery, depends on the elasticity of a peculiar kind of air formed
by the deflagration of nitre, sulphur and charcoal, composing gun-
powder; and the combustion or burning togrether in close vessels
of oxygen gaa with another kind of gas called hydrogen forms
water, which, being a liauid, nearly the whole space taken up by
the gases previously to their combustion becomes a vacuum, and
thus pressure may be produced, and a moving power obtained.

20. The application of the vapour of water to cause motion by
the alternate expansion and condensation of steam affords an ex-
ample of the advantageous adaptation of a non-permanent gras to
the most important purposes; and if convenient means can be
discovered for the liquefaction of common air and other gases by
pressure and reduced temperature, as appears probable from the re-
searches of Mr. Faraday and others, it may be expected that ma
chines will be invented as far superior in some respects to the
steam-en^nes now used, as they are to those which were con-
structed m the early part of the last century.

21. As the mechanical effects of the different grases when they
act by pressure must depend on their relative specific ^vities, it
is of importance that those should be accurately ascertained. The
following table will show the respective weights of equal quanti-
ties by measure of several elastic fluids, including those which
are of the greatest importance, on account of their nrequent occur-
rence and we valuable purposes to which they have been applied.

Weiglit of 100 eabie inches. Speciiie gravity.
Atmospheric air - - 30.5 grains - 1.

Oxygen Gas - - - 33.8 - 1.111

Nitrogen Gas ... 29.25 0.973

Nitrous Oxide - - r 46.5 - - 1.527

To what mechanical purposes h^ve the gases other than common air
been applied ?

By wfwt species of force is motion impressed on projectiles?

How does the alternate formation and condensation of non-permaneot
vapoor afford a mechanical a^nt ?

ilow much do 100 cubic mches of eopsroon air weigh ? How mufih
|b(e ftme hnljc ^ ^syrgep? of ni^cogen ? ^ nitrpas./mi|e,? byd^qgeni cai^

186 PNEUMATICS.

Hydrog;en Gas - - - 2.12 - *- - 0.069

Carbonic Acid - - 46.5 - - ' - 1.529

Chlorine Gas - - - 76.3 ... 2.500

Subcarburetted Hydrogen Gas* 16.9 - - - 0.555

Carburetted Hydrogen Gas* 29.6 - . - 0.972

Steam - - - - 18.8 - - - 0.519

22. From this table it may be perceived that gaseous bodies
differ greatly from each other in specific gravity ; chlorine being
2i times the weight of common air, and hydrogen only about
7-100 the weight of that fluid, so that common air is nearly 15 times
the weight of hydrogen. Steam has but little more than half the
weight of atmospheric air, and hence it rises through the air, in
the same manner that a piece of deiil or cork rises in water.

Elaatidty of Mr,

23. The most obvious and effective property of air is its elastic
city, to which, with its gravity or weight, are to be attributed the
phenomena of equilibrium, or motion in bodies under the influence
of pneumatic pressure. In addition, therefore, to what has been
already stated concerning these properties, a more detailed inves-
tigation of their nature and action will be requisite in order to the
fmler elucidation of that branch of science now under our notice.

24. The elasticity of air appears from its resistance to pressure.
The application of a heavy weight, or any external force to a
woolpack or a bag filled with twisted horsehair would cause the
pack or bag to give wav, and become more or less contracted, bat
on the removal of the force it would expand to nearly its original
dimensions.! What thus takes place is manifestly owing to the

bonic acid ? chlorine ? subcarburetted hydrogen ? carburetted hydrogen ?
steam ?

What substance is generally assumed as a standard of comparison in
stating the specific gravities of'^the gases ?

What are the several specific gravities of the gases compared with that
substance as unity ?

How many times heavier is common air than hydrogen gas ?

How much lighter is common steam than atmospheric air ?
' What property of air is most important in reference to its mechanical
agency ?

* The gases procured from the distillation of coal and from oil eon-
sist principally of subcarburetted hydrogen, or li^ht inflammable air,
and cnrburetted hydrogen, or heavy inflammable air. "As these gases,
which are now i^eneraily used for lighting streets and shops, are lre»

Jiuently mixed with other gases, the specific gravity must vary,, in dif-
erent specimens, with the degree of purity. Coal-gas, after it has been
purified is foand varying in specific gravity, from .450 to .700 ; while oil*
ns, which contains a larger proportion of'^carburetted hydrogen, is much
neavier, and therefore yields more light in proportion to its bulk.

t The manner in which elastic bodies act is strikingly illustrated by the
novel application of spiral springs of iron wire in the Gon8ti*ttction of
ckitic ehairs and beds. Pr. Paris, who notices this inveotion in his

ELASTIC FORCE OF AIR.

187

form and texture of the incladed substances ; the particles of which
are separated by numerous interstices, and therefore readily yield
to the force applied at the surface, which drives them nearer to*
gether without destroying their elasticity, or disposition to regain
their previous situation, and hence they recede from each other«
when the force which made them approach is withdrawn. A
bladder filled with air may thus be compressed by saueezinff it
with the hands, and it will swell out agaiD as soon as it is reliev-
ed from the pressure, owing to its particles being endowed with
a power of repulsion ; for in proportion as they are left at liberty
they exhibit a tendency to expand in every direction, so that their
absolute dispersion through boundless space can only be prevented
by the influence of pressure.

25, The elasticity of the- air is most convincingly
demonstrated by the operation of the machine called
an air-pump, the construction of which is similar in
principle to that of the syringe.. By adapting two
stop-cocks to a common syringe, and forming by
means of them a communication with a vessel of
convenient shape and dimensions, a rude and im«
perfect kind of air-pump might be contrived, by
means of which air included in the vessel might
be considerably rarefied or condensed. The efirect
thus produced will appear from the annexed figure,
in which A B represents a syringe with a solid pis-
ton, C a cock, which when open, leaves a commu-
nication between the barrel of the syringe and the
glass globe E ; and D another cock which opens
a communication with the external air. If now
we suppose the piston to be at the bottom of the
barrel, and the cock D shut, then on drawing the piston up
to A, a part of the air in the globe will rush into the barrel,
and the whole mass of the included air will become expanded ;
the cock C is then to be closed and the cock D opened, when
the piston being pressed to the bottom of the banel, the air it con-
tained will be expelled through the open cock ; this is next to be
shut, and the cock C opened, and on drawing up the piston again,
the air in the globe will become further rarefied ; and these opera-
tions, the alternate opening and shutting of the cocks, and raising
and depressing the piston, may be continued till a high degree of
rarefication is produced. This apparatus is called an exhausting
syringe.

Whence does this property beeome apparent ?

What familiar illustration shows the nature of this action ?

How is the dispersion of air through the regions of space prevented ?

What machine demonstrates most satisfactorily the elasticity of air ?

Explain the simplest form of this machine. •

^ Philosophy in Sport made Science in Earnest,*' says, '* Down itself can*
not be more ^ntle nor springy $ and such beds never inquire to be abakcii
or moilff." .

188 fsimutTicB.

86. The same apparatus may be employed to effeet the con-
densation or Ihe innfosed air, by drawing up the piston with th«
cock C shut, and D open, and thrusting dofrn with C open, and
D shutj far by continuin);r this process, air would be made to enter
by the cock D, and he afterwarts forced into the globe E. The
apparatus now lakes tlie name of a condensing' syringe; though
Talves which open and shot by the mere pressure and expaii-
eion of the included air are usunlly suhatituted for the stop-
cocks. Valves are more convenient than stop-cocks, as re-
quiring less labour and attention on the part of the operator ; but
a mncli higher degree of exhaustion can he etTected by means of
a syringe furnished with the latter tlian by using the common ex-
hausting syringe with valves ; yet these are generally adopted in
the construction of exhausting and condensiug syringes and air-
pumps, as being much less expensive than slop-cocks, and mors
easily kept in proper order.

37. The air-pump, asm
be inferred from its b|
; tion, is a machine i
tracing air out of a cloa«
vessel, and thus producing
within it a degree of raie-
&ction nearly approaching to
a vacuum; it being impos-
sible, as we ahalT siriise-
quendy show, to fbna a per-
fect vacnum, by this or any
■ other apparatus ; though the
I exhaustion may be carried
' 80 far that the remaining air
will not at all interfere with the results of our ekperiraents.

28. The Sgure in the margin exhibits a ^^tion of an air-
pnmp, from which it may be perceived that it essentially consists
of two exhausting syringes, so arranged that they can be vorked
.alternately. The syringes are marked A A, and their pistons ate
moved mi and down within the barrels, by the racks or toothed
rods B B, adapted to corresponding teeth on the periphery of the
wheel C, having a winch ot handle M, by which it may be tumel
-60 as to raise and depress either piston successively. Each of
the pistons is furnished with a valve by which the air escapes aa
the piston descends, and Uiere are other valves D D, at the bottom
of each barrel, which become closed by either piston in its de<
scent, but when it is drawn up, open a passage into the tube E £,
communicating with the caviar cf the glass bell F, «aUed a re-

Wbat purpOK ii Krved by the alop-oocfci or tbItci of an eibanitiaB '
syringe?

IiuigBii^pamp.adtqoalelo produce perfect exh>unipn«)lhin.a oo^
<|luniiwTew«l?

Id vhai manner it motion nnitllT oommiuiiciled to the JfUt/tVSf-iV*

THE AIR-FUIIP. . 180

ceirer. From the tabe E passes off another tube 6, the extremity
of which opens into the bell-shaped tube K, within which is a small
basin H, containing mercury, and the small tube I, closed at the
upper end only, has its lower end plungned beneath the surface of
the mercury. At L is a stop-cock, which when closed cuts off
the communication between the receiver and the syringes, and
which must therefore be opened while the machine is put in ac-
tion. Another stop-cock, not shown in the figure, closes a pass*
age through which the external air may be admitted under the re-
ceiver, when the result of an experiment has been ascertained.

39. There is so little difference in the mode of action of the
air-pump and the exhftusting syringe before described that the ef-
fect of the former will be readily understood. Either syringe in
turn, by the elevation of its piston, and the consequent closure of
the piston-valve and opening of the valve D, draws a portion of
air from the receiver F, through the tube E E ; and the alternate
depression of each piston, by the elasticity of the air inclosed in
the barrel, shuts the valve D, and prevents the air from returning
into the receiver, at the same time that it opens the vaive of the
descending piston, and finds a passage into the upper part of the
barrel^ whence it is expelled by the piston in its next ascent.
Thus, the reciprocal action of the syringes, by means of the rack-
'work, may be continued, till the requisite degree of rarefaction
be produced in the air within the receiver. The only part of the
apparatus requiring further explanation is the air-gau^, consisting
of the tubes IC ana I, and the basin of mercury H, with which the
latter tube is connected.

30. The air within the tube K, by its pressure on the surface
of the mercury in the basin, will keep that portion of the same
fluid in the tube I raised to a height exactly proportioned to the
density of the included air, which must be the same with that in
the receiver, in consequence of the communication by the tube G ;
and thus the height at which the mercury stands in the small tube
I will serve as a gauge or measure of the elasticity and weight of
the included air, being always in the inverse ratio of the rarefac-
tion which has taken place.

31. It may be proper to add that the edge of the receiver must
be ground perfectly smooth and level throughout its circumference
that it may fit closely to the brass plate of the air-pump on which
it rests ; and that it may prevent the entrance of the air more ef-
fectually, it must be smeared with grease, or, as is more usual, set
on a collar of oiled leather, and thus the junction of the receiver
with the surface below it will be rendered impervious to the air.

What is die purpose of tbe mercurial apparatus connected with the
air-pump ?

What elopes the lower valve of the pump on the descent of the piston
within the barrel ?

What ratio is preserved between the height of mercury in the gaug«
and the degree of rarefaction in the receiver f

What practical precaations are usually necessary to preserve the rare-
factioa obtained by the action of tlie air-pump ?

. 32. A multitnde of es^rimentS) serving to diemonstrate tiie
eiasticity as v/ell as the weight of air, may be satisfactorily per-
fprmed by means of this machine, which was originally invented
by Otho Guericke, a German philosopher, in the latter part of
the seventeenth century, and having been rendered more effective
by the skill and science of Boyle and Hooke, it subsequently un-
derwent numerous improvements, some of the most important of
which we owe to the ingenuity of Smeaton, the celebrated en^-
neer, of Dr. Prince of Salem, in Massachusetts, and Dr. Hare
of Philadelphia.* But the principle and general plan of this phi-
losophical mstrunient, under the various forms, in which it has
been constructed, correspond with the descriptive statement al-
ready laid before the reader.

33. The elastic force df atmospheric air may be rendered obvi-
ous by placing under the receiver of an air-pump a biadder, which
l>as been about half filled with air and firmly tied at the neck so
as to prevent it from, escaping; for on. exhausting the receiver gTa<
dually, the bladder will be, seen. to. swell, from the expansion, of
Uie air within it ; and if the exhaustion be continued long[ enough,
the bladder will bursty from the elastic force of the air it con-

V tained, no longer counterbalanced; by pressure on. the extemol
surface.

34. A SjC^uare or flajt glass phial, filled with air,^ well corked and.
fk^ened. with wire, if pl^cea uuidieX' the receiver, will crac^ from
t)ie expansion of the air within it« a$. soon a/sl^e pressure is with.-
drawn from its sui&ce by the- exhaustioa of the receiver. A phial
of the usual shape would resist force applied internally or exter-
nally, much, better than o^e:Virith flat ^idess in consequence of its
arched figure ; hence t)ie globuls^v or hemispherical shape oC the^
receiver, renders it best adapted for its purpose.

35. Shrivelled apples, prunes, or ir^isiiis, with the^r skins un-
broken, when placea under a receiver, on the ^ir beipg exhausted*
will become plump from the elasticity of the air included in those,
fhiits ; and thus a bunch of dried raisins may be made to assume
the appearance of a fine cluster of grapes, and 9 similar apparent
renovation may be efiected on the apples and prunes ; but on rea4-
n^itting the air into the receiver thcf fruity would all resume the
wrinkles which betray their age.

36. If a large glass globe with an open n^puth have a piece of

By whom and at vhat period vas the air-pump invented ?

Who have contributed towards its improvement ?

How is the elasticity of the air proved by the experiment of the fiaccid
bladder ^

What will occur when a thin flat or square, phial is placed under a r^
eeiver, and the air exhausted while the phial remains corked ?

How may shrivelled fruit be temporarily restored to a plump appeas-
ance ?

Explain the experiment of the ^la99 ghbcand bladder,

* $e^ Jourofkl of the Franklin InitUute, vol. xiL p. SOS.

SXFERmENTS Ol9 ttfB fiLisnCITT OF AIB. 191

bladder tied over it, so securely that the air within it cannot es-
cape while the bladder remains whole, and it be set under a re-
ceiver, while the air is being withdrawn from it, that within thtf
globe will expand by its elastic force, and raise the bladder to a
convex shape, distending it more and more as the exhaustion in-
creases, till at length the bladder will be ruptured, and the air in
the globe Will expand itself through the receiver.

37. Let a small syringe, having a weight fastened to the han-
dle of the piston be closed with a cork at the end, tied down with
a piece of bladder, so that on pulling up the piston the air could
not enter; then let it be suspended in an inverted position with
the weight downward, under the receiver of an air-pump ; upon
extracting part of the air from the receiver, the weight at the han-
dle will drftw down the piston, and on readmitting the air the
piston will rise again. In this case the partial exhaustion of the
receiver lessens the elasticity of the included air so considerably
that it is unable to support the weight ; and on letting in the air
again, it will recover its elastic force and raise the piston with the
attached weight, in the same manner as it would be raised by the
I pressure of £e external af^.

38. A very amusing exhibition of the effect pro-
duced by the elasticity of the air may be made by
means of the apparatus represented in the margin.
Hollow glass figures, about an inch and a half in
lengfth, resembling men or women, must be procured,
each having a hole in one foot, and the glass must be
of such thickness that the figures will float near the
surfiiice of water when they are filled with common
air. They are then to be immersed in a tall glads
jar nearly filled with water, and covered on the top
with a strong bladder, fastened air-tight. * If the blad-
der now be pressed inwards with the finger, the water
being almost incompressible, and the air quite the re-
verse, that contained in the little images will yield to
the compressing force, and becoming contracted, wa-
ter will enter, and the images thus becoming speci-
fically heavier than they were at first will descend
towards the bottom of the jar ; on the pressure above being re-
moved, the air in each image recovenng its elastic force, will
expel the water, and the images will rise as before. By forcing
a little water into one or two of the figures before they are placed
in the jar, they may easily be made to float at different heights ;
and thus their motions may be greatly varied, by regulating the
pressure on the bladder. These diminutive images have been

Kdw may the tyrmgre and-weighi be made to exhibit the alternate ex-
pansion and contraction of air }

Describe the pneamatie tor called the bottle of imps.

In what manner may the imps be made to rise from the bottom ol
water^ and how is the experiment to be explained '

192 PNEUKATJC^S.

whimsical iy called bottl&>imp8; and their agility must appear
wonderful enough to those who are ignorant of the cause of lU*

30, The elasticity of the air may be further illustrated by
placing an open jar containing a single glass figure, and filled
with water, under the receiver of an air-pump, only the figure
must be just heavy enough to sink to the bottom of the jar under
the usual pressure of the atmosphere. Then on exhausting the
receiver, and thus diminishing the elasticity and consequent pres-
sure of the included air, the density of the water remaining the
same, the figure will gradually rise, as the air becomes more rari-
fied, till it reaches the surface of the water, where it will float,
till the air is again admitted into the receiver, on which it will
descend to the bottom of the jar.

40. Abundant proof of the compressibility and elasticity of the
air may be drawn from the consideration of the mode of actioA
of the common domestic utensil, a pair of bellows. This will at
onc^ appear on attending to the effect of the valve or leathern flap.
This valve rises when the boards are separated, and the air enters
through the hole in the lower board, which on pressing together
/the boards again becomes closed by the falling of the valve, and
the air having no other vent, makes its exit through the pipe in a
dense current.

41. The double bellows, used by blacksmiths and other artisans,
differs from the machine just described in having an intermediate
board, which is fixed, while the others are moveable, so that it
consists of two air-chambers instead of one ; and a hole in the
middle board, with a valve, suffers the air which has been drawn
into the lower chamber through the hole below to pass into the
upper chamber, where it becomes condensed by the pressure of a
weight fixed to the upper board, and is discharged in a continued
stream through a pipe connected with the upper chamber. The
lower board is moveable, and when it sinks by its own weight,
the valve opens, and shutting again when the board is raised by
means of a lever or some other contrivance, the air is prevented
from escaping by the valve-hole, and is therefore forced into the
upper chamber.

42. A kind of bellows or blowing machine, constructed entire-
ly of wood, was^nvented at-Bamberg in Bohemia about 162.0, and
was thence called the Bamberg bellows. It consists of two boxes,
in the form of cylindrical sectors ; one fitting into the other so as
not to prevent it from being moved up and down, but without siif-
«

A How is the common hand bellQws constructed ? ;<

.?, How is a Constant stream of air maintained by the double bellows 7
Of what does the Bamberg bellows consist ?

* French writers on natural philosophy usually exhibit a siurie figure
in describing th^ counterpart of this experiment. To this little ena-
melled figure, petite figure d^email, thery give the name of Ludion. — ^V.
Sigaud de la Fond £lem. de Pbys., vol. lii. p. 162. Beudaut Traite Eleou
de Phys., p. 305.

BEito's FOi7inr4nf. IM

fering the air to escape between the sides isi the boxes. It is
n^dfess to describe it more fully, as the mannei in which it acts
may be easily conceived from what has been stated aboYC. Va-
rious modifications of this machine have been adopted in esta^
blishments for smelting metals, and other purposes connected with
the arts and manufactures.

43. The effect of air acting by its elastic force on the surface
of water may W variously euiibited in the formation of Jet$ (Peau^
or spouting fountains. Let a strong decanter be filled to about half
its deight with water, and a glass tube of small bore be 4>as8ed
into it nearly to the bottom, and fixed air-Ufl^t, going through a
holo drilled in a cork, with a piece ai bladOer tied over it and
round the tube. This bottle is then to be placed under a tall re-
ceiver, on tliTe i)late of an air-pump ; and on the receiver being
exhausted, the air within the bottle will expand, and pressing on
the surface of the water, cause it to issue from the top of the tube
*n a jet, the height of which will be proportioned to the degree
of rarefaction of the air under the receiver.

44. Compressed air may be made to pro-
duce a similar effect, which may be thus dis-
played : a strong bottle somewhat more than
halt filled with water, as represented in the
marginal figure, by the line D E, must have
a tube A C fitting into its neck, and capable
of being opened or closed at pleasure, by:
tnrning the stop-cock B. A condensing sy«
ringe* being adapted to the tube at A, and
the stop-cock opened, air is to be forced into
the bottle, which rising through the water,
will by its density press strongly on the^sur-
face of that liquid ; then afler turning the
|top-CQck the syringe is to be removed, and a small jet-pipe being
fitted to the tube A, the stop-cock is to be opened, and the elasticity
of the condensed air in the bottle will drive up the liquid in a jet»
the height of which will gradually diminish, as the mcluded air,
by its expansion, approaches nearer and nearer to the density of
the external air.

45. A small phial, with a w;ell fitted cork, having a little tube
or a stem of a tobacco-pipe passed through it, and reaching near-
ly to the bottom of the phial, partly filled with water, will, on
blowing strongly into the bottle through the pipe, exhibit effects
precisely ansdogous to those of the apparatus just described.

46. The machine called Hero's Fountain, resembles in princi-
ple those noticed above, difieriog from them only in ^e manner
m which the compression and consequently increased elastioi^
of ti^e air is produced. This is effected by means of a. column.

Explain the eonstroction of the foantain in vaeao.

How is the force of air applied in. the eofkpreMed tdr fiuntain f

•In what reapect doc* HoriVfbttataia differ from the preeedingf ?'

194

PNEUMATICS.

of water, as will appear from inspection of the
annexed ficrare, representing one of the numer-
ous forms in which such spouting fountains have
heen constructed. It consists of an open vessel
A, from which a tube passes downward to the
vessel B, from the opposite side of which another
tube forms a communication with a close cavity
over the basin C, having a jet*pipe extended al-
most to the bottom, and open above to the air.
Water having been introduced into the basin C,
more water is to be poured into the vessel A, till
it runs down the tube, and fills the lower part of
the vessel B, and compressing the air in it, and
in the other tube and cavity above it, the water
in the basin C, will, by the elastic force of the
condensed air,^be driven in a jet from the aperture
at D ; and by adding water to that in the vessel A, the enclosed
air may be so compressed as to expel nearly all tlie water from
the basin C. The principle on which Hero's Fountain acts has
been heretofore adopted in Germany, in forming machines to raise
water from mines ; but they have been laid aside since the pro-
gress of science has led to the construction of far more powerful
and efficient engines adapted to the same purpose.

47. A familiar example of the elastic pressure of the air occurs
in the frothing of bottled ale, porter, cider, and the sparkling or
creaming of champaigne wine, when uncorked and poured into an
open vessel ; the air which those liquors contain, on being released
from its confinement in the bottle, escaping in numerous bubbles
covering th6 surface cf the liquor. Ginger beer contains a quanti-
ty of air or gas, formed by a chemical process, and such is its
elastic force, that if the ginger beer has been properly prepared,
the included air will drive out the cork with a loud report, as
soon as the string with which it is tied down is cut through.
Hence also the bursting of bottles filled with cider, perry, and
other liquors considerably impregnated with air, when' well cork-
ed and secured with wire.

48. What is called soda water is manufactured by compressing
carbonic acid into water by inechanical means ; and it therefore
can scarcely be preserved except in strong bottles of a peculiar
form, frrm which it spouts with violence through the elasticity of
the condensed gas, as soon as the cork is removed. Air readily
combines with water, though not to any great extent, under the
usual pressure of the atmosphere. This will be evident on plac-
ing a glass of water under the receiver of an air-pump ; for on
exhausting the receiver the air will issue in a multitude of small
bubbles from the surface of the water.

To wliRt purposM has this fountain been applied in raining^ operationa?
In what tRmiliai* operations is the elastic loroe of gaseous matter es-
caping^ from a liquid made eonspicoous ?
What is the nature of the preparation called toda vHOfrf

RAREFACTION. 190

49. It has been already stated that a perfect yacnaru cannot be
obtained, even by means of an air-pump of the best construction.
The impossibility of completely exhausting the receiver of an air-
pump ; so far as it is not owing to the imperfection of the machinery,
depends on the identical property of the aerial fluid which causes
the air-pump to act : for the elasticity of air is always in the di-
rect ratio of its density ; so that when half the air is extracted
from any vessel, the remaining half will expand to fill the whole
space, its density and elasticity being diminished in the same pro
portion. Thus if half the air could be exhausted frrm a receivei
by the first stroke of the piston, and one-half of what was 'eftbj
the next stroke, the quantity removed by every subsequent strrk«
must manifestly be but one-half of that removed by the str ke irr
mediately preceding it : in fact, there must always be a remaindr .,
however trifling it must at length become. It will be evident t!iat
though an indefinitely small quantity of air must thus remain
after working an air-pump for any imaginable period of time, yet
that quantity would soon oecofne so extremely inconsiderable as
to have nearly the effect of a complete vacuum.

50. Suppose the proportion of capacity between the barrel of
an air-pump and the receiver ,to be such, that one-fourth part of
the air would pass from the latter into the barrel at each stroke
of its piston, then the quantity remaining in the receiver after the
fifth stroke would be less than one-fourth of the original quantity ;
and as the decrease would go on in a geometrical progression,
thirty strokes of the piston would leave in the receiver only 1-3096
of the quantity it contained at first. Hence it will appear that
If the receiver be not less than the barrel, the smaller the differ-
ence between the size of the receiver and that of the barrel, the
more rapidly must the rarefaction of the included air take place;
and though with a small receiver the air may be highly rarefied
In a short time, it cannot be entirely withdrawn.

51 . It must also be observed that the extent to which the rarefaction
can be effected will be limited by the pperati'^n of the rarefied air
on the valves at the bottma of the barrels ; for as the elastifity of
the air " remaining in the receiver is the cause of the cpenrng of
those valves, they will at length cease to act, when the exhiustion
has been carried so far that the expanded air has net elastic force
enough to overcome the very small deg^ree of resistance caused
by the weight and friction of the valves.

52. Another obstacle to the rarefaction beyond a certain limit
will arise from the resistance to the opening of the piston-valves

Why cannot a perfect vactiant be AbtnineH by menna of ibe »if-piimp r

What would be the rate of exhaustion if the barrtl had one half of ih<i
cafmcity of the feeeU-er '

State }-onne other relation between the bulks of the cvlituTer and re-
ceiver, and compute the degree of exhaustion after a certain number of
•perations.

What influeoee has the nature of the lower valve on the extent of rare-
laciion ?

When must tlie rarefaction neeessarilj cease ?

during the descending stroke, owing to the #ant of soffielent elas-
deity in the highly rarefied air to ovetcoine the pressure of th6
atmosphere on those vadves. Various imptovetoents have been
made in flie constraction of air-pomps, which have considerably
lessened the imperfections in these machines now stated,* and
thongh from the essential properties of air the formation of an
absolute vacuum in the manner described must bei impracticable,
yet the ingenuity of modem artists has ehabled us to produce
within a receiver any degree of exhaustion requisite for the most
delicate and interesting experiments.

Weight of the Air,

53. The phenomena depending on the influence of gravitation
on air, and its consequent gravity or weight, are of equal im*
portance with those ansmg from its elasticily ; and the subject
therefore demands a more extended investigation than has been
already afforded to it.

54. Direct evidence of the weight or ponderosity of air may
be easily obtained by means of an air-pump. For by ascertain-
ing the weight of a bottle»of known capacity before and after it
has been exhausted of the air contained in it, the loss of weight
after exhaustion would show the gravitating force of the air which
had been extracted from it, and if the experiment be accurately
performed it would appear that a cubic foot of air would weigh
533 grains. The same quantity by measure of water would weigh
1000 ounces avoirdupois ; hence it must follow that water has
about 840 times the weight of air, bulk for bulk ; and this result
corresponds sufficiently with the estimate of the specific gravitf
of atmospheric air compared with that of watet, as stated in the
table of specific gravities, in Ihe treatise on Hydrostatics.f

55. As air then has a determinate weight like all other ponder-
able kinds of matter, it must produce pressure in the same man-
ner as other heavybodies, and that in proportion to its mass and
specific gravity. The weight of 1000 cubic inches of atmospheric
air must, from what is stated above, be greater than that of a
single cubic inch of water, and consequently if the pressure of
such a mass of air could be made to act on a small Surface, it
would produce a greater effect than the pressure of a cubic inch of

What other obstacle to rareraction exists in the construotioh of the
iiir-pump ?

In what manner may the air-pump be employed to ascertain the weight
of the air ?

By what number of tim^s does the weight of water exceed that of air^

* Such is the object of Dr. Prince's improvement, who makes Use of
a iiibsidiar]r piston to take off the pressure above jtbe main pUtons, after
the exhaustion is nearly oompleted.-^ED.

t See ffydroitaticM, No. 88.

ATMOSPHERICAL PRESSURE. 197

wiLtet. Now the most direct mode of causing the pressVire of a
given bulk of air to act by its gravity on a sunace of a certain ex-
tent would be, by forming a cylindrical or square column of air,
the base of which should be exactly of the extent required. This
could not be conveniently effected by artificial means, except in
columns the height of which was but inconsiderable; but in the
atmosphere around us nature presents a mass of air of great alti«
tude, the vertical pressure of which on any given space may be
ascertained b^ direct .Experiment.

56. A receiver, or any other air-tight vessel, placed on the plate
of an air-pump, would become fixed to it by the exhaustion of the
included air, in consequence of the atmospheric pressure on its
surface. Some idea of the amount of this compressing force may
be obtained by placing the palm of the hand over Uie top of a
glass cylinder open at both ends, the lower opening resting on the
plate of an air-pump, and the upper opening being covered by the
hand so closely as to prevent the air from entf ring in that direo*
tion, the cylinder beings partially exhausted the weight of the
atmosphere pressing on the back of the hand would not only be
sensibly felt, but would also be found to be so considerable befoo
complete exhaustion had been effected, as soon to occasion pain
and inconvenience. ReckQ)[iing the weight of the atmosphere
upon every square inch of surface, to be nfteen pounds, the pres-
sure on the hand placed over an exhausted receiver, the top of which
it would just cover, would be equal to about sixty pounds.

57. A more exact estimate of the weight of the
atmosphere may be formed by attending to the result
of an ex|Jeriment to show its eSect on the surface of
two hollow hemispheres, from which the air has heern
extracted by moans of an air-pump or exhausting
syringe. Thf se hemispheres, constructed of brass,
should be furnished with handles, or hooks, by means
of which they may be suspended ; one of which
may be fixed, but the other should be removable.
In the tubular neck to which this handle is screwed
is a stop-cock, which being opened, and the handle
removed, the hemisphere is to be screwed on the
pump-plate, or on to an exhausting syringe ; and the
other hemisphere having been fitted to it, a vacuum is to be form-
ed in the interior by working the pump. The stop-cock must
then be turned so as to prevent the re-entrance of air, and on un-
screwing the brass globe, and refixing the handle, it will hp found
that the hemispheres composing it are firmly united by the pres-
sure of the external air. Suppose the diameter of the globe to

How might we conceive the pressure of a given balk of «ir acting by
ks weiglit alone to be exercised f

With what illustration of this subject does nature furnlsti us?

What causes the adhesion of an exhausted receiver to the plate of aa
air-puiiip .' ^

UoW 18 the correctneM of this explnnation made apparent?

b8

198 pmtVMATfcs.

be 6 inches, the surface of a section through the centre would btf
about 28 inches scjuare ; and hence the pressure of the air upon
one square inch bem^ known, the force requisite to separate the^
hemispheres, supposing the exhaustion to be nearly complete,
might easily be computed.

58. This is usually termed the Magdeburg experiment, it bay
ing been originally contrived by Otho Guericke, of Magdeburg
the inventor of the air-puinp ; and it appears to have led him to
that important discovery. For the manner in which he originally
conducted the experiment was by filling the space included be
tween the hemispheres, when pressed together, with water to ex
pel the air, and then pumping out the water, while the air was
prevented from re-entering by turning a 8t(^>-cock. Having thus
ascertained the fact of the existence of atmospheric pressure to a
great decree, he preceded to the invention of the air-pump, by
means of which the exhaustion of the joined hemispheres could
be much more readily and conveniently effected than by the operose
process he had at first adopted. This ingenious philosopher
operated with two copper hemispheres, nearly a Magdeburg ell*
in diameter ; and ttie amount of pressure on such an extent of 8ur«
face was so great, that when the interior cavity had been exhaust*
ed, the separation of the hemispheres could not be effected by the
strength of twenty-four horses, twelve being harnessed together
on each side, and dragging in opposite directions.

59. That the weight of the atmosphere is always pro^
portioned to the vertical' height of the column of air

Sressing on any extent of surrace, may be demonstrated
J means of a glass tube bent as represented in the mar-
gin, and open at both ends. The diameter of the tube
being the same throughout, if mercury be poured into it,
it will rise to the same height, D C, in either part of the
tube. Then let the extremity, A, be closed by placing
over it a piece of moistened bladder, firmly secured by
melted resin or sealingwax ; and the mercury pressed on by the
iiir above it, of the common density of the atmosphere, would
always remain at the same height, D ; but the column of mercury
in the other part of the tube havin? its surface exposed, would
ns6 or sink with the variation in density of the atmosphere. Thus
if such a tube were carried to the top of any high tower or moun-
tain, the column of air would be shortened by a space equal to
the height of the situation and the mercury^ in some degree re-
Describe the manner of proTihg the pressure of air acting by its weight
bn the Magdeburg hemispneres.
How did Guericke at first produee the ▼aeuum in his hemispheres ?
What aeoount is given of the size and effieaoy of his apparatus ?
In what manner can we prove what pi^ssore the air exeroiset On tbi
exterior torfaees of bodies }

• See WioUer'i BlemenU of Kat Pbiloiopby, fing. Tr.| ITSTt ^^1^

THi BAROMKTBR. IM

Ilered from pressure, would rise in the space C 6. On the con-
trary, if the tuhe coold be removed into a deep mine, the mercary
on the open side would sink below C, being pressed by a loflier
column of air than when at the surface, where the heigrht of the
mercury was first noted.

60. Such an instrument as that just described would be a
species of barometer,* since it would indicate the varying weight
of the atmosphere. But the instrument to which the appellation
of barometer has been given is differently constructed, and better
adapted to afford a correct estimate of the amount of atmospherie
pressure at different times, or under varying circui^stances.

61. The invention of this valuable instrument appears to be justly
attributed to Torricelli, professor of mathematics at Florence, io
the earlier part of the seventeenth century. He was the pupil of
the celebrated Galileo, who seems to have been the first among
modern philosophers who had any idea that air possessed the pro-
perty of weight; though he was not aware of the mode ot its
operation in producing atmospheric })ressure, and the numerous
pnenomena constantly resulting from it.

62. It had been accidentally observed that in raising water by
means of a pump, the height to which it could be drawn in what
is called the suction-pipe never much exceeded 33 feet; since
when the piston of a pump was elevated more than about that
height above thd surface of the water in the pump-well, the liquid
no longer followed the piston. The drawing up of the piston of
course torms a vacuum in the pipe below it, and the consequent
rising of the water into the void space was accounted for, or ra-
ther attempted to be explained, by the philosophers of the six-
teenth century, by the hypothetical principle that ** Nature abhors
a vacuum," and therefore causes the water to ascend in order to
prevent the vacuum from taking place.

63. The dogma of Nature's abhorrence of a vacuum is a com-
plete absurdity ; and the phrase was invented, like many others,
some of which are still current, to conceal the ignorance of those
who pretended to universal knowled^. It was, however, gene-
rally adopted at the period just mentioned ; and till it was disco-
vered that, when a vacuum was actually formed in the suction-
pipe of a pump, water would not rise to fill it much above 33 feet,
no one seems to have thought of questioning the propriety of the
current opinion on the subject.

VVhftt is the nature and purpose of the baromefer f
Who was the inventor of that instrument ?
Who first conceived tlie idea that air possesses weight ?
By what expression did ancient phUosopbers explain the rising of water
in a common pump ?
In what lierhtare we to view this esTiression ?

* This term signifies a measure of weight, from the Greek B^f a^, a
weight, and m/t«ov, a measure. The instrument described in the text
would bear k nearer relation to AlIte'siytDpiezomfctier than Ip the com*
SMNi faerometBr.— £0i

200 PNEUMATICS.

64. Some engineers at Florence finding themselves foiled in an
littempt to raise water by a pump from a well of greater depth
than usual, applied to Galileo for advice as to the means of raising'
water to a greater height than 33 feet, or at least for an explana-
tion of the cause of a phenomenon which they could not reconcile
with the generally received hypothesis concerning the ascent of
water in the suction-pipe of a pump. Galileo is said to have told
the inquirers that " Nature's abhorrence of a vacuum did not ex-
tend to distances greater than 33 feet, and therefore that at that
point her efforts ceased." It has been questioned whether the
philosopher really expressed himself in this manner, ](hough the
story has often been repeated ; and it may at all events be con-
cluded that if he gave such an answer to those who applied to
him, he could hardly have considered it as a satisfactory solution
of the difficulty which had been started. Accordingly in his
writings he attempts to account for the phenomenon on other
principles, but the real cause of it seems to have eluded his pene-
tration.

65. Probably the discussions to which this circumstance gave
rise suggested to Torricelli the idea that the ascent of water Itk
the suction-pipe of a pump was caused solely by the pressure of
the atmosphere on the water in the well, and that as the weight
of the atmosphere at the earth's surface could never vary to any
great extent, it therefore never greatly exceeded what would be
sufficient to raise a column of water in a vacuum to the height of
S3 feet. The happy thought occurred to him of verifying his con-
jecture by making an experiment with a fluid much heavier than
water, as he perceived that if the ascent of water depended on at-
mospheric pressure, the same pressure on the heavier fluid would
raise a column of it to a prbportionally inferior height. With
this view he fixed on mercury, as the heaviest fluid known, at
common temperatures ; and having procured a glass tube, open at
one end, he filled it with mercury, the specific gravity of which
compared with that of water was as 13^ to 1 ;^then immersing the
open end of the tube in a small jar of mercury, and suffering a
communication to tak6 place between the mercury in the tube and
that in the j^ar, he observed that the fluid sunk till it stood in the
tube just 30 inches above the level of that in the jar below. This
experiment was so far completely satisfactory ; for as 13^ : 1 : : 33
feet : 2^ feet = 30 inches ; thus the weight of the atmosphere
pressing on any given surface was found to be equal to that of a co-
lumn of water of the same extent at the base and 33 feet in height,
or of a similar column of mercury only 30 inches in height.

Wliat is said to have be^n the Veply of Galileo to inquiries on this sub*
ject.'

Did that philosopher understand the true cause of the rise of fluidi into
an exh:iiisted tube ?

To whom do we owe the true explanation of this subject ?

In what manner did Torricelli demonstrate the correctness of bis views
Fn regard to the pressure of the air ?

THB BAROMSTXR. 201

66. The batotbeter now in general use as a weMier-g1ad8 iA
nothing more than a tube of proper len^h filled with mercury, and
either dipped at the open end in a smsul cup of the same fluid, or
else having the opien end curved upwards, 60 that the mercury
may be exposed to the preissure of the atmosphere : a scale of
inches also is adapted ' to the upper part of the tube, extending
from 27 to 32 inches, that it may appear by inspection at what
height the mercury stands under the pressure of the atmosphere
at any particular time.. This useful instrument was at first called
the TorTicellian tube, from the name of the discoverer of the prin-
ciple on which its action depends ; but it subseauently received
the designation of a barometer, noW universally aaopted.

67. After the effect of atmospheric weight and consequentpres*
sure had been ascertained by the decisive experiments of Torri-
celli, the subject was further investigated in France, chiefly by
the distinguished philosopher Pascal, and by "Father Mersenne,
in 1647. The former reflectin|f on the effects of atmospheric pres-
sure, it occurred to him that the weight of the column of air, de-
pending on its vertical height, must be greatest in low situations,
and decrease in ascending an eminence. To try this principle by
the test of experiment, he requested a friend who resided m Aa-
vergne, to ascertain the relative heights of a barometrical column
&t the bottom, and afterwards at the top of the Puy de Dome, a
high mountain, situated in that province of France. The effect
took place as Pascal had anticipated ; and he himself subse-
quently made corresponding observations on a barometer, removed
mm the level of the street in Paris to the ^uinmit of a lofty
church-tower.

68. As a philosophical instmmeht, the baromb'ter is highly ns^
ful, hot only for the purpose of astcertaining the niaiiy and hourly
variations which are taking place in the atmosphere in any HveB
situation, arising from causes connected with the science of m^
teorology, and for other purposes of a similar nature ; but likewise
'2A affording means for accurately estimating the heights of moun-
tains, or in fact of any places whatever above the level of the sea.
For either of these purposes, however, it is necessary that a baro-
meter shotild be very carefully and accurately constructed ; and
in making observations by means of it, especially in the measure-
ment of heights, various precautions are required, and the effect
of temperature in particular must be taken into the account in
making any calculations.

69. It must hence be obvious, that as a weather glass, the uti
lity of such instruments as are commonly used must be extremely

»

Deseribe the manner ef constructing^ the bArometer. ■
By what name was this instrument iormerly known ?
What application of the barometer was made by Pascal /
In what manner was his principle verified ?

To what particular purposes is the barometer applied in meteorologT«
What particularly requires attention itk the measurement of heights b^
the barometer ?

20t mtKuxjfnch,

limited ; for as Ae height of the mercoTT at atkj Ume must denend
partly on the elevation of the place of obaervatioo above the level
of the sea, no correct judgment can be fonneil relative to the den-
sity of the atmosphere, as afTecting the slate of the weather with-
out reference to the situation of the instrument at the time of
making the observation ; and a series of observations at any given
place would be required in order to enable a person to form a pro-
bable opinion of the change of weather to be expected after the
rising or tailing of the mercury.

70. One source of imperfection in the instrument, which ren-
ders it diMcult Ut determine the extent of those slight variations
in the height of the mercurial column which are yet interesting to-
the meteorological observer, has led to some peculiarides of con-
struction, by means of which the scale of observation might be
enlarged, and minute changes in the height of the mercury be ren-
dered obvious. One method of effecting this purpose is )>y means
of what is called a wheel barometer, the external ap-
pearance of which few persons can be unacqu^nted
with, as such instruments are generally preferred for
domestic rise.

70. The construction of the wheel-barometer may

be thus described, with reference ta the Rgure in the

I margin. It consists of a lube, ABC, hermetically

sealed at A, and open at C ; and of such a length that

the distance from C to A may be about 33 inches. The

tube must be entirely filled with mercury, which on

placing it in a vertical position will subside in the part

A B, till the difference of the levels E and F will bo

equal to the height of a column of mercury which will balance

the weight of the atmosphere, so that any change of pressure will

have an equal effect on the mercury at B and F, and thus tlirough

whatever space the fluid may rise at E, it will be depressed to

the same extent at F. Upon the surface of the mercury at F floats

a small ball of iron, suspended by a strong thread over a pulley

P, and to tlie other end of the thread is attached the weight W,

not so heavy as the floating ball. The axis of the pulley passes

through the centre of a large graduated circle G, and carries an

index H. which revolvea as the pulley turns round. The weight

W being just heavy enough to counterbalance the iron ball and

overcome tiie friction of the pulley, the iron ball rises and falls

freely, aa the surfiice of the mercury on which it floats is elevated

or depressed by the weigljt of the air. Now if the circumference

of the wheel P be S, inches, then one entire revolution will corre-

WbV IB the bnromelei-, aa commDnlT eonitructed, ill Bdipleil lo the
pui'tinsr:!! nf iniliotiii); tlie «Rli^ of wfiliier f

Huw liaa it li«en found piiiiitieable to enlarge the Male of bsromelrio
tsi'lniiona, so us to rem! ilighl diRcreiieei?

Given desci'iptiori of llie wheel-barometer.

Wljat is tlie purpDie of the weight luipvudeil on the exterior end ol

THE WATSBrBAAOKETER. 203

spond to an alteration of level amounting to 2 inches at F, and
therefore to an alteration of 4 inches in the height of the barome-
tric column. And as the graduated circle may convenieotly be 40
inches in circumference, 10 inches of that circle will correspond
to I inch of the column, and 1 inch of the circle to 1-10 of an inch
of the column ; so that variations, amounting to much less than
the tenth of an inch will be distinctly perceptible.

73. As already stated, the utility of the barometer as a weather-
glass must depend on certain circumstances, with reference to the
situation of the observer ; and not the least attention ought to be
paid to the words "rain," "fair," "changeable," &c., freaaently
engraved on the plate of a barometer, as they will be founa to a&
ford no certain indications of the correspondence between the
heights marked and the state of the weather.

73. General rules for calculating changes in the weather from
the barometer can seldom be adapted to all situations ; and there-
fore- those who may be desirous of obtaining the means for form-
ing a correct judgment, as to subsequent alterations in the state
of the atmosphere, from the indications of the degree of atmosphe-
ric pressure at any time afforded by the barometer, must devote
much attention to the subject; without which, written rules would
only mislead 'the observer, and long application to the practical
study of the instrument would render rules unnecessaiy. One
circumstance, however, may be worth mentioning, which is, that
changes of weather are indicated not so much by the actual height
of the barometrical column, as by its variation of height, and the
manner in which that variation takes place.

74. Among the methods which have been adopted to obtain the
most accurate estimates of the effect of atmospheric pressure may
be noticed the compound barometer, in which water is added to
the mercury in the tube, and the mean height of the barometrical
column being thus augmented, the variations which arise from
the varying weight of the air will be more considerable than in a
common barometer, and therefore may be more distinctly observ-
ed. Such instruments, however, are liable to certain defects and
disadvantages, which render them inferior upon the whole to
those of the usual construction.

75. A barometrical column, composed of water alone, from its
extreme sensibility to changes of atmospheric pressure, must af-
ford much greater facility for noticing the more minute alterations
which are found to be constantly occurring than the mercurial ba-'
rometer. But a water barometer must necessarily be a most un-
wieldy machine, and consequently could be adopted in but few
situations, even where the expense and inconvenience attending
its construction might be of little importance.

What reliance is to be placed on the prognostics of weather sometimea
found on the scales of barometers ?

What circamstance, in regard to changes of weather, deserves parti-
cular attention ? What adTaotage is possessed by the eolamn composed
of water and mercury f

304 nikviUTici*

76. M. Pascal, whose philosophical researches have heen al-
ready noticed, made some interesting experiments at Rouen, la
Normandy, in which he, hy means of glass tubes, 40 feet in
length, ascertained the effect of the pressure of the atmosphere on
water, and also on wine ; and he Iband that when mercury ^tood
in the common barometer at the height of 2 French feet 3^ inches,
water was raised in one of his tubes to the height of about 31 1-9
feet, and wine to that of 31^, Thus, though the difference of
specific gravity in these two liquids must have been but inconsi-
derable, yet it occasioned a sensible diference in the manner in
which they were affected by atniospheric pressure. The experi-
ment on water was repeated by Roger Cotes, professor of philo-
sophy at Cambridge, £ngla)id, in £e beginning of the last cen-
tury ; but in both cases the object was merely that of a temporary
exhibition for the purpose of distinctly demonstrating the operatien
of aerial gravity and pressure.

77. A permanent water-barometer, however, has heen erected
by order of the Royal Society of London, in the -hall of entrance
to their apartments, the tube extending upwards in the well of a
winding staircase. It consists of a fflass tube 40 feet in length,
and I inch in diameter at the lower end, nearly cylindrical through-
put, beingonly a little narrower at the upper extremity than it ia
below. This instrument is well adaptea to show the various pe-
riodical alterations, or as they have been termed, oscillations of
the atmospherical column, and some observations, with tables form-
ed from them, have already been laid before the Royal Society,
by Mr. Daniell. It has been noticed, that the rise and fall of the
column of water in this barometer precedes, by one hour, the cor-
responding changes in a mercurial barometer; and it is stated
that in windy weather the water is in perpetual motion, its fluc-
tuations in the tube having been compared to the breathing of an
>niroal.*

78. It has been already observed, that the elasticity of the air
is always in the direct ratio of its density ; or in other words, that
the greater the densitv of any portion of air, the greater will be
the degree of elastic force which it is capable of exerting. -The
best modern air-pumps are so constructed as to serve for the
condensation as well as the rarefaction of air; but for the former
purpose, a condensing syringe may likewise be employed ; and
either method may be adopted in making experiments on the elas-
ticity of compressed air.

79. Among the most interesting applications of the force of air

What relfttion did Paseal find between the heights of columns of wine
and of water eauivalent to the weight of the atmosphere }

In what peculiar manner is the water barometer found to be affected ?

What advantage does it possess over the mercurial barometer^ \a regard
to the indication of diurnal fluctaations ?

In what ways may the artifiicial condensation of air he effected I

■^r«* «^ •««•< »« #^- ««• -*-^' •« F« « ^ ■ «^ ^%.» •

• Areana of Soienee, 1833, pp. fi8^fl64

TKE AtR-OVN. ft05

in a state of hig^h condensation is that of projecting by such xneann
bullets or other missiles from an air gan. It is somewhat re-
markable, that this instrument appears to haire been in use before
the discovery of the air-pump or the barometer ; for it is mentioned
in a work entitled " Elemens d'Artillerie," written by David Ri-
vaut, who was preceptor to Louis XIII. of France, and he as-
cribes the invention to Marin of Lisieux, in Normandy, who pre-
sented an air-gpn to Henry IV. It is also stated, that an air-gun
was preserved in the armory of Schmettau, on which was the date
1 474. But these instruments were far inferior to modem air-ffuns,
from which they must have differed considerably in the mode of
constraction.

80. The air-gun, like the common pm or musket, consists partly
of a long metal tube adapted to receive a ball, but the breech end
of the tube or barrel has an openipg to admit condensed air be-
hind the ball, which, acting by its elastic force, propels it with a
velocity, proportioned to me degree of condensation of the air.
Though the effect is produced in the manner just described in all
air guns, yet the mechanism or arrangement by which the admis-
sion of the air is regulated varies in different instruments. Some
of tK^m are furnished with a syringe for compressing the air, in*
eluded within the butt of the gun, and there is an exterior tube
surrounding the barrel, so that the air is forced into the space be-
tween the tubes, and the ball having been introduced into the bar-
rel which it fits closely, a valve is opened by pressing on a knob
or trigger, and the air rushes from the cavity formed by the outer
tube into the chamber behind the ball, which it expels from the
barrel, continuing to act upon it by its expansive force till the ball
has passed from the mouth of uie air-gun. Other instruments
have but one tube,' for the reception of the ball; and the air is
compressed by a condensing sjrringe into a strong brass or copper
globe, which when filled, can be detached from the syringe, and
screwed to the butt of l^e gun, and by a contrivance similar to
that already described, a bullet can be discharged, by drawing a
trigger. Irie butt may be made to hold a magazine of balls,
which can be admitted one at a time into the chamber, and a por^
tion of the condensed air escaping on opening the valve, several
balls may be projected from the air-gun in succession, but in this
case, as each discharge will diminish the density and elasticity
of the remaining air, the velocity and effective force of the balls
will also progressively decrease.

81. From what has been stated relative to the density and elas-
ticity of air, it will follow, that all bodies on the surface of the

What are some of the iroportant app1ieation« of condensation ?

What are the essential part's of the air-gun ?

To what is the Telocity of the ball proportioned f

What two different aiTang^ements of parts are occasionally applied for
retaining the condensed air ?

Can this instrument propel, with equal velocities, seVeral balls io iao-
cessiOD, without renewing the cfaam of ftir '

200 f9fBUMATIC8.

earth, SQstain a pressure from the superincambent atmosphere
etj^ual to the weight of a column of water, about 34 feet in height,
with a base corresponding in extent to that of the body or bodies
pressed upon. This pressure may be estimated at from 14 to 15
pounds on every square inch of surface, being the weight of a co-
lumn of mercury 30 inches high, and 1 inch square at the base.

82. Hence it must be evident that every human being constant-
ly has pressing on the body in every direction a weight equal to
15 times as many pounds as there are square inches on the sur-
^e of that body. Suppose then the sur&ce of a man's bod}r to
measure 2000 square inches, the force of the atmosphere pressing
on that surface would be equal to 30,000 pounds. It may appear
unaccountable that so vast a pressure should be perpetually in
operation, without our being sensible of the weight or experienc-
ing any inconvenience from it. This however is owing to the
uniform manner in which the 'force acts in all directions, so that
the body is supported by the pressure on one side against the
equal pressure on that which is opposite ; and it is only when the
equilibrium is destroyed by removing the force in one ditection,
that its effects become perceptible, as is shown by an experiment
previously described, in which the hand is exposed to atmosphe-
ric pressure by placing it over a partially exhausted receiver. All
the cavities of the body also are either filled with air or with
denser fluids, so that they resist compression from the external air
as perfectly as the firmest solids.

83. Some idea of the weight of the whole atmosphere, encom-
passing the earth on every side, may be formed from a calculation
which has been made to determine what must be the diameter of
a sphere of lead, the weight of which would be equal to that of
the entire atmosphere ; and from which it appears that the sphere
must have a diameter nearly 60 miles in length ; which would
eonrespond in weight with a mass of water sufficient to cover the
whole surface of £e earth to the height of 34 feet.

84. It is an interesting matter of speculation to what height the
atmosphere extends from the surface of the earth. If the density
of the atmospheric column were imiform, its vertical height mi^ht
be readily calculated ; for as water is nearly 850 times heavier
than air of the common density, and a column of water 34 feet
high is equivalent to an atmospherical column having a base of the
same extent, it is evident that the height of such a column of air
of uniform density must be 850 times 34 feet, or 850 X 34=28,900

What amount of atmospheric pressure is sustaiDed by all bodies on the
surface of the earth ?

What pressure is applied to the bodjr of a person of ordinary size f

How is the body enabled to sustain this pressure without inconvenience ?

What would be the diameter of a sphere of lead equal in weight to the
whole atmosphere of our g^lobe ?

To how thick a stratum of water over the whole globe would this bd
equivalent ?

Ifow may we calculate the height of an atmosphere of uniform density,
equal in weight to that of the earth ?

ALTITUDE OF THE ATMOSPHEBE. 307

B=5 miles 833 yards and 1 foot, or nearly 5^ miles. But the den-
sity of tlie air varies at different distances from the sorface of the
earth, iu consequence of its elasticity.

85. Air may be conceived to consist of innumerable strata or
layers, forming a concentric shell, surrounding the solid eiobe we
inhabit ; and the lowest stratum being compressed by the whole
weight of the superincumbent mass« must necessanly be more
dense than the next above it, and the density decreasing in pro-
portion to the increase of height or distance i&om the earth's sur-
face, no definite limit can be assigned to the extent of the atmo-
sphere.

86. Cotes, in his Hydrostatical Lectures, has stated the relative
density of the atmosphere at different heights, as deduced from a
comparison of the specific gravity of air at the common level of
the earth's surface with that of air at a certain elevation as ascer-
tained by means of the barometer. Thus the rarity of the air being
four times greater at the altitude of seven miles than at the sur-
face, and the rarity of the air augmenting in a geometrical pro-
gression, while the altitude increases in an arithmetical progres-
sion, it will follow that at the height of 14 miles the atmospnere
would be 16 times rarer than at tb^ surface, at 21 miles 64 times
rarer, at 28 miles 256 times, at 35 miles 1024 times, at 70 miles
about a million of times, at 210 miles a million of millions of mil-
lions of times, supposing air to be capable of indefinite expansion.
Hence, also, at the altitude of 500 miles, if the air could continue
to expand at the same rate, a cubic inch of the common density
would be dilated througli a greater space than a sphere equal in
diameter to the orbit of Saturn.* This, however, is a purely hy-
pothetical estimate, for it is founded on the presumed infinite
kvisibility of matter.

87. The observations of Dr. WoUaston, " On the Finite Extent
of the Atmosphere,"! tend to prove that air consists of ultimate
indivisible particles ; and the expansion of a medium composed
of such particles must cease at a certain point where the force of
gravity acting downwards, upon a single particle, would be equal
to the resistance arising from the elastic or repulsive force of the
medium. At such an altitude, therefore, the elasticity of the
atmosphere would be completely" extinguished, and thus a physi-

How mvij we conceive the atmoBphere to be arranged upon the lurfaee
of the globe ?

What limit can be assigned to the height of the atmosphere ?

State the calculated progressive rarefication of air as dependeot on
elevation.

On what supposed propertj of air is this calculation founded ?

What views did Dr. Wollaston advance on this subject ?

What equality of forces would limit the expansion of air ?

, * See Cotes's Hydrostatical and Pneumatical Lectures. Sec. edit
Cambridge, 1747, p. 1S4.
t AbstracU of Papers in the Philoi. Trans., toI. ii. pp. 160— 16S.

908 PNEUMATICS.

cal limit might be assigned beyond which it eoold not possibly
extend.

88. In making calculations relative to the density of the air at
different heights, or forming rules for the determination of the cor-
respondence between atmospheric altitude and pressure, for prac-
tical purposes, such as the measuring of eminences by means of
the barometer, several circumstances must be taken into the ac-
count. Thus it is not only necessary that the exact heiffht of the
mercurial column at different levels should be ascertained, but due
regard must also be had to the influence of temperature, the effect
of vapour suspended in the air, and the latitude of the eminence
whose height is to be determined. The indefatigable spirit of
research of modern experimental philosophers and mathematicians
has triumphed over these ditficulties so far as to have furnished
us with general principles and formulae, by the application of
which to the results of carefully conducted experiments, the per-
pendicular heights of the principal mountains in every part of the
world have been discovered.

89. From calculations founded on the barometical formula con-
trived by the celebrated mathematician, Laplace, and adapted to
the estimation of heights, it appears that at the elevation of
52,986 metres, French measure, or 173,796 English feet,* the rarity
of the air as equal to the utmost degree of rarefaction which can
be obtained in the exhausted receiver of an air-pump. This mani-
festly connot be the extreme altitude of the atmospheric column,
nor is it possible to decide that point with certainty. But it ap-
pears from the observations of astronomers on the duration of
twilight and the magnitude of the shadow of the earth by which
the moon is eclipsed, that the rays of light from the sun are
affected by the medium through which they pass at the distancS
of from 40 to 50 miles from the earth's surface ;f and therefore it
may be reasonably inferred that the atmosphere extends to the
altitude of at least 45 miles above the level of the sea.

90. The pressure of the air arising from the joint effect of elas-
ticity and weight is the cause of a ffreat number of phenomena
constantly taking place around us, and immediately depending on
the operations of nature or art. It is thus that the effect of the
instrument called a sucker, used by schoolboys, is to be explain-
ed, It consists of a disk of moistened leather, with a string
by which it may be suspended with any weight attached to it

What three circumstances are to be taken into account in measuring
heights by barometric observations ?

At what height has Laplace calculated that air will have as great a
rarity as it is possible to produce by the air-pump }

What height of our atmosphere is deduced from the observations of
astronomers on the duration of twilight, and the magnitude of shadows
in 'eclipses of the moon ?

* A French metre is 39.37 inches English measure, or 3.28 feet,
t S^e Treatise oti,0ptic9.

EFFECTS OF ATMOSPHSIUC PRE8SU1IS. S09

and as Its smooth moist surface may be pressed so closely against
the flat side of a stone or other body, that the air cannot enter be-
tween them, the weight of the atmosphere, pressing on the upper
surface of the leather, makes it adhere so strongly that a stone
of weight proportioned to the extent of the disk of leather may be
raised by lifting the string. If the sucker could act with full
effect, a disk an inch square would support the weight of 14
pounds; but the practical effect of the instrument must be variabley
eren supposing that it was accurately constructed.

91. Wnenever surfaces are brought into such close contact that
the air qannot insinuate itself between them, they will be pressed
together with a force corresponding to the extent of the surface
of contact. Hence glass-grinders and polishers of marble find
that the substances on which they are operating by friction, when
reduced to a state of extreme smoothness, become united by atmo-
speric pressure so firmly that great exertion is required to sepa-
rate them, and the circumstance is the cause of considerable m«
eonyenience.

92. The adhesion of snails, periwinkles, limpets, and some
other crustaceouB animals, to rocKs and stones, is effected on the
same principle. The sur^ces of their shells at the opening are
capable of being exactly fitted to any plane surface ; these ani-
mals have the power of producing a vacuum within their shells

, when dius fixed, and the atmosphere consequently presses on
them with a force proportioned to the extent of exterior surface.
It is thus, too, that a common house-fly is enabled to run with
great facility up a perpendicular pane of glass, or on the under
side of a horizontal plane, as the ceiling <n a room. The feet of
the insect are provided with cavities, the sides of which being
adapted to the surface of glass, &c., by some internal mechanism
the cavities are exhausted, and the pressure of the atmosphere on
the minute surface of the feet supports the insect against the
power of gravitation. That such small animals may be thus sus-
tsdned will probably appear less extraordinary than that a similar
power of running up a perpendicular plane should be possessed by
a much larger creature.

93. But Sir Everard Home, who, by means of microscopical
observations explained the structure of the fly's foot, as connected
with its mode of progression on walls and windows, also investi-
gated the anatomy of the foot of the lacerta gecko, a kind of
lizard, found in the island of Java, which walks up and down the
smoothly-polished walls of the Javanese houses, pursuing the
flies on which it feeds, and it runs \ipwards to its retreat in the

How is atmospheric pressure illustrated in the stone-sucker ?

What facts do marble masons experience connected with the same
principle ?

What facts in natural history prove the application of atmospheric
pressure to the position "and locomotion of animals ? .

What enables flies and other insects to walk upon upright turfacei and
ceilings ?

How are the feet of the gecko formed^

82

010 FNBUMATI08.

root's of houses, though its weight is sometimes 5} ounces. It
has on each foot five toes, and on the under side of each of these
are sixteen transverse slits, with serrated edges, and pouches be-
tween them, by means of which the animal is enabled to form a
vacuum within the ' cavities, produced by the application of the
loose membranes, surrounding the under surface of the toes, tc a
wall or any other smooth plane.* Nature has provided animal?
of far superior bulk to this lizard with a similar organization, and
for the same purpose.

94. Sir E. Home, from an examination of a specimen of the
amphibious marine animal, called by naturalists the walrus, from
the Arctic regions, discovered that there is an analogy in structure
between the hind foot of the walrus and the foot of the fly ; so that
this large clumsily-shaped animal is enabled to proceed upon the
smooth surface of ice against gravity, by the adhesion of the feet,
owing to atmospheric pressuTe.|

95. Those who are but slightly acquainted with natural history
can hardly be ignorant of the faculty belonging to the fish called
remora, which fixes itself firmly to' the side of a ship or to that
of a larger fish, as a shark ; and thus it travels without the exer-
tion of swimming from one part of the ocean to another. It has a
sort of sucker on its head, by the application of which it becomes
attached, the pressure of the surrounding water having the samie
effect in this case as that of the air in those previously noticed.

96. Among the experiments which have been devised to demon-
strate the elastic pressure and weight of the atmosphere, the fol-
lowing are well aulapted to the purpose.

I.

97. Take a quart bottle and drill several holes in the bottom,
then set it in a wide-mouthed ju^, and having filled it quite full
of water, cork it securely. On lifling it from the jug it will be
found to hol(i water notwithstanding the perforations, the pressure
of the atmosphere preventing its escape ; as will appear on taking
out the cork, when the water, being equally pressed above and
below, will run out through the holes till the bottle is emptied.

II.

95. A wine-glass filled with water maybe held with the mouth
downwards without spilling a drop. The means by which thi»
seemingly marvellous effect is 'produced are extremely simple. It

What advanta^ does the walrus enjoy in conseqaenee of the peculiar
structure of its feet ?

By what apparatus is the remora enabled to adhere to the sides of a
vessel or those of a larger fish ?

In what manner may a vessel, the bottom of which is perforated, be
itJU made to hold water ?

• See AbstracU of Papers in Philos. Tr^os. from 1800 to 1830, vol. ii.
p. 38.
t Idem^ p. S1&

EZFERniENTS ON ATM08PBSSIC PRESSURE. Sll

i3 merely necessary to place a piece of paper on the surface of the
water with which it must be every where io contact, and also
with the rim of the glass, which is then to be inverted ; and as
the air cannot get in to act on the liquid above, its pressure is ex-
erted against the under surface alone.

III.

99. A tumbler or goblet may be filled with water, and the
surface being covered as before with paper, which may be held
up with the palm of the hand, while it is suddenly inverted, then

5 lacing it on the surface of a smooth table, the paper is to be with«
rawn, and the water will remain suspended in the glass; which
will adhere closely to the table from the pressure of the atmo«
sphere. Any one may now be safely challenged to lift the glass
yertically without spilling every drop of the water ; for it would
require some exertion to move the glass at all upwards, and as
soon as it was elevated on one side, the included water would
sink down and escape.

100. It is in consequence of the unrestrained pressure of the at-
mosphere that liquor will not flow from a cask after it has been
tapped or pierced, unless another opening be made as a vent-hole
\n the upper part of the cask : for till this is done the force of the
air pressing on the mouth of the tap, having nothing to counter-
balance it, would support a column of liquor, if the cask was air-
tight, the height of which would be proportioned to the specific
gravity of the liquor. When, however, the air is enabled to act
through the vent-hole above, the pressure below is counterba-
lanced, and the liquor descends and runs through the tap by the
eflfect of its own weight. The operation of the same principle
may be observed in using a tea-pot ; for there is always a small
hole in the lid through whidh the air enters, and without which
the liquid would not now from the spout, if the lid fitted close, aa
it ought.

101. Many circumstances of frequent occurrence may be traced
to the influence of atmospheric pressure acting irregularly. The
stoppage of a supply of water from wells and foimtains during a
frost is sometimes owing to this cause ; for the frost does not ex-
tend far beneath the sunace of the earth, but it consolidates it so
as to prevent the access of air to the channels of water from which
fountains and wells are fed, and thus the atmosphere pressing
only on the open well prevents the water from entering it as usual,
till a thaw takes place, and the ground again beconung pervious
to the air, it acts on the feeding springs, and the water rises in
the well.

How is the paradox of the inverted glass of water to be explained ?

How ipay a full goblet be inverted upon a table withoat spilling its con-
tents ?

How is atmospheric pressure concerned in the ^ischai^e of liquid from
a cask, ura, or other close vessel ?

In what manner can you aseoaDl for Uie oceational fikiknre of spriogt
la severely cold weather I

212 PNEUMATICS.

109. The instniment, called in French TAte-liqnenry
or Chanteplenr, and that used in filling essence-bottles,
act on the principle of atmospheric pressure. Their con-
struction and eflfectwill be readily apprehended from in-
spection of the annexed figfure, which represents a small
conical tube, A B, open at botii ends ; and when in this
state the lower orifice is plunged beneath the surface of
any liquid, a portion of it will enter at A, and fill that
part of the tube A C, which is immersed in the liquid ;
if then the upper extremity B be closed air-tiffht by
placing lY^Q thumb over it, the tube maybe lifted out of the liquid,
and the pressure of the air below will prevent it from escaping,
till the thumb is removed, and the air tnus allowed to act on the
surface of the liquid at C. The length of the tube to raise water
in this manner might obviously be extended to more than 30 feet ;
as the height of the liquid column which the atmosphere would
keep suspended would be greater or less according to its specific
gravity. Such instruments of a moderate length are conveniently
applicable to the purpose of withdrawing a small quantity of any
liquor through the bung-hole of a cask.

103. The Siphon* sSfords another^illustration of the principle
under discussion. It is employed for the purpose of decanting or
drawin? off liquors, and is variously constnicted. If an open tube
of small diameter, bent into the shape of the letter U, be filled with'
water, and the curved side turned upward, the liquid will be sus-
pended b^ the pressure of the air oq the open extremities, while
the tube is held in such a position that the columns of liquid in
both legs shall be exactly of the same height ; but if the tube be
inclined at one side more than the other, so as to destroy the equi-
librium, the water will run down and escape through that end
which is at the lowest level. So if a common siphon, or bent
tube with one side longer than the other be filled with water, and
inverted or held with the open ends downward, the atmospheric
pressure acting equally on ooth sides, and the liquid columns be-
ing unequal, the water will escape through the longest leg, falling
in virtue of its own specific gravity. But if, when such a siphon
is filled, its shortest les be plunged beneath the surface of water,
not only will the liquid all run out of the longest leg, but it will
also rise in the shorter, and be discharged from the other in a con-
tinued stream, till it sinks below the open end of the shorter leg.

104. If the siphon be used without previously filling it with

Hov may we l^pplj the pressure of air to the purpose of raising small
quantities of liquor from a cask ?

To what length might a tube for this purpose be extended ?

What is the constiniction and use of the siphon }

What principle besides that of atmospheric pressure is eoncemed in
prodacing the continued action of the siphon ?

1 Why will not the siphon act without previously filling both legs of the
tube ?

"•.i^

* from the Greek s«f «», a tube.

THE SIPHON.

21S

tbe liquid to be decanted, thoaffh the liquid will rise in the shorter
leg, it will not ascend beyond its own level, so as to pass oyer
tlie bend of the tabe, and escape, unless the air be drawn out of
the longer leg. Hence the utility of that kind of siphon repre*
sented m the margin, the peculiarity of which entirely consists in

the addition of the tube C, open at the up-
per end, and communicating below with
the longer leg of the siphon S. The shorU
er leg A then being plunged into the bung-
hole of a cask, or into any other yessel
containing liquor, the opening B is to be
stopped with the hand, or otherwise, and
by suction at C, the liquor may be made
to pass over the bend and fill the leg B,
when being suffered to escape, it continues
to flow, as long as the extremity A is im
mersed in it. Large siphons of this sort,
made of copper, iand furnished with a stop-cock, just aboye the
opening B, may oflen be seen in action ; being used by the dis-
tillers and liquor-merchants to draw off spirits.

105. The Wirtember^ Siphon, shown in the following figure,
when once filled with liquid, will remain so, and hence may be
hung up in that state ready for use. One leg A being plunged
intr a yessel of the liquid to be drawn off, it will escape through
the open extremity B, in consequence or the addi-
tional pressure of the liquid in the yessel at A ; thus
it will appear that this siphon acts somewhat diffe-
rently from those of the common construction, though
it is applicable to similar purposes.
' 106. Tantalus's Cup, or the Magical Goblet, is an
\9 amusing philosophical toy, which consists of a cup
with a cayity at the sides or bottom, or both, with
which the longer leg of a siphon communicates ; so that when
water is poured into the cup high enough to oyercome the pres-
sure of the air on the end opening into the cayity, the liquid will
sink in the cup, and run into the cayity; and thus it can neyer
rise so high as the mouth of the figure within which the siphon is
concealed ; and the classical fable of Tantalus is realized. There
must be an aperture near the rim of the cup to admit air into the
cavity, or rather to suffer it to escape, and by closing it with the
finger, the cup may be filled to the brim ; but as soon as it is un-
closed the water will sink as before. If a hollow handle, com-
municating with the lateral cavity, be fitted to the cup, the hole
may be so placed at the inner side of the handle as to escape no-
tice ; and the effect will appear astonishing to those unacquainted
with the theory of atmospheric pressure.

A

To what practieftl purpose is the siphon frequently applied ?
What advantage is possessed by the Wirtemberg siphon ?
lo vbat manner it the cup of Tantalus eonstruoted r

214 PNEUltlATICS.

107. Intermitting fountains, or periodical springrs, are found in
some places, and from the capricious and apparently unaccounta-
ble irre^larity of such streams they haye lyeen regarded as mira-
culous, in dark ages, and have ^ven rise to abundance of su|>eT-
stitions among the common people. There is a remarkable spring
of this kind called Lay well, near Torbay, in Devonshire, England,
and the peculiarity of this and other intermitting fountains, may
be satisiactorily shown to arise from the operation of siphons
formed by nature, communicating with subterraneous reser-
voirs.*

108. The siphon may be made available for the purpose of con-
veying water over the side of a pond or reservoir into another,
provided the latter is on the same or a lower level than the form-
er. It was thus very ingeniously applied by a French engineer,
M. Garipuy, in 1776, to discharge the surplus quantity of water
from the canal of Languedoc, when it had been raised above the
proper level by the influx of water at the mouth of the river €la-
ronne during a storm.

109. Whenever water is conveyed by pipes from a higher to a
lower level, over an intervening eminence, the principle on which
the siphon acts must be adopted ; and thus water may be made to

Sass over any height not much exceeding 30 feet. It is thus con-
ucted from Lochend to Leith, near Edinburgh, through pipes,
the intermediate ground being 8 or 10 feet above the fountain head.
It is necessary tlmt the water should be driven in the first instance
beyond the most elevated part of the pipe by a forcing-pump, and
it then continues to flow by the influence of atmospheric pressure.
But as air, always loosely combined with flowing water, will
be gradually extricated from it at the bend or highest part of the
pipe, it will at lens;th there accumulate so as to stop the flux of
the water. When this happens, the forcing-pump must be worked
to renew the current.

110. From what has been stated with regard to the siphon, it
follows that it can only be used for transferring liquids from higher
to lower levels ; therefore when water or any other liquid is to be
raised by means of atmospheric pressure, some kind of pump
must be employed. Pumps are variously constructed. The mar-
ginal figure below, (III9) represents the common suction pump,
which is nothing more than a sjrringe so contrived that the water

With what natural phenomena ai*e the principles of the siphon con-
nected ?
From what source do intermitting springs derive their supply of water ?
For what hydraulic operations may ^he siphon be employed ?
How high may water oe made to pass over a barrier r
In what manner is the siphon tnink for such purposes usually filled ?

• For an account of LAywell, see Philos. Trans., No. 424; see alsoNos.
119, 189, 192, and 884. There is an interesting paper on the noted inter-
mitting spring at Giggleswick, in Yorkshire, by Mr. Gough, of Kendal, io
Nicholson's Journalof Natural Philosophy, 8vo.

THK aucnoN-FCMP. SIS

drawn into it puses througb the piston b; meani of ■ valre, and
is diacharged above it, instead of being again forced ont below
The iDTentian of this instniment is attributed b^ VitniTlus to
Ctesibiue or Cteaebes, an Athenian eagineer, who lired at Alexan-
dria, in Egypt, about the middle of the second century before the
Christian era; and the construction of syringes, fir^^nginea, and
other machines acting on similar principles is described by his
scholar Hero, in a treatise on. Pneumatic a, still extant.

111. The suction-pump consists of

below in a perforated ball, throng
which the water in the well enters
freely into the suction-pipe; and at
its other extremity is a valve D, open-
ing upwards, and affording a com<
■nonication, when open, with the ap-
perpipe A. In thia pipe, consrituting
I the barrel or body of the pump, the
piston B moves air-tight vertically,
and by its valve C opening upwards,
it permits the water to pasa above it
and be discharged at the spout. Now
suppose the piston to be at the bot-
tom of the barrel in contact with the
valve D, on lifUng the fonnei by dn-
preasing the lever handle of the pump, connected with the piston-
Tod at ¥, the valve C will be closed by the pressure of the air
above, and a vacuum being thus fanned in the barrel, the same
pressure on the surface of the water in the well, will drive it up
the suction-pipe, and raising the valve D, the water will enter the
exhausted barrel, whence by depressing the piston, the valve D
wilt be abut, and that at B rising, the water will pass upwards and
he discharged through the apout. . The first effect of working
such a pump must be to form a partial vacuumin the barrel of the
pomp, and the upper part of the pipe E, and it wLH be only after
Uie whole of the included air has been expelled through tlie pis-
ton-vaive, and replaced by water in the pipes, that the liquid be-
gins to fltiw, the atmospheric pressure below taking full effect,
while the equivalent pressure above is counteracted by the manual
force applied to the handle of the pump.

lis. The suction-pump cannot raise water beyond the extent of
action of atmospheric pressure, the ntniost limit of which will be
about 34 feet; eo tiiat the height of the valve D above the level
Dewribe the GonitraeiiDn and operation of ihe common pnmp.

By whom were ijringet RnJ Ere-enginei Sral delcribed >
Wbil clotei Ibe upper rain of the luelion pomp berore it bai bewne
immerted In water !
What ii IhE firu t^eraUon which takes place within the barrel of the

els nfEVKATlCS.

of tho waEei in the well must never exceed that distance ; and
unless the pump be accurately constructed, so that the piston in
its descent fits close to the bottom of the bHirel, so as to fonn a
perfect vacuum in its ascent, the water will not rise to the ex-
treme height in the suction-pump. It must appear somewhat
paradoxical, that though this will be the effect when the pump is
in the best working order, the valves and pipes being air-tight,
yet a pump, the suction-pipe of which has been damaged, so that
a small quantity of air can enter, will raise water uearij as high
again as a good pump.

113. A tinman of Seville, in Spain, ignorant of the principles of
science, undertook to construct a suction-pump to raise water from
a well 60 feet deep : nhen the machine was finished, he was coa-
founded at discovering that it hod no power to raise water at all,
and enraged at his disappointment, while some one was working
the pump he struck the suction-pipe with a hammer or aie, so
forcibly as to crack it, when to his surprise and delight the waler
almost immediately began to flow, and he found that he had thus
attained his purpose. This happened about L7G6, when M. Le-
cat, a celebrated surgeon, then at Rouen, in Normandy, being in-
formed of it, made a similar experiment on a pump in hie-garden,
by boring a small hole in the suction-pipe, 10 feet above the level
of the water in the cistern, and having adapted to it a stop-cock,
he found that when it was open the water could be discharged at
the height of 65 feet. Instead of from 30 to 34 as before.* The
circumstance admits of an obvious explanation, ihe effect being
analogous to that exhibited by Jelt-d'eaii, when air i$ mingled
with the acending column of liquid.f 'Hius in the case of the
pump, the air presses in through the slit oi aperture in the suc-
tion-pipe, and becomin'T mixed with the water m its ascent, forma
B compound fluid, far lighter than water alone, and therefore acted
upon by atmospheric pressure more
readily, and thus it produces the phe-
nomenon described. However, as
there are other and more efficacious
methods of raising water to great
heights, the contrivance just noticed
: is not to he recommended.
I 114. The Lifting-pump, as repre-
i sented in the margin, acts in much
I the same manner as the preceding,
but the machinery is reversed. It
consists of a hollow cylinder or barrel
A B, in which Is fixed the valve G, a
little below the level of the water in
' the well or reservoir. A piston F
1 with a valve opening upwards, fits
i into the lower part of the barrel, in
i. pp. 238, S39.

THE FORCINCkPCIIP.

S17

which it is moyea yertically by means of the frame B C D E, eon*
nected with the mston-rod 1. Now when the piston is at the bot-
tom of the barrel, the pressure of tlie atmospnere on the surftca
of the water in the well will open the piston valve ; and the wa^
ter will rise to the same height within the barrel as without; and
on lifting the piston, its valve F will close, and the water a^ove
it will be driven by the openinff of the valve G, into the upper
part of the barrel : then tne piston being depressed again, the
valve F will open to admit more water into the lower part of the
barrel, while that above will be prevented iirom returning by the
closing of the valve 6 ; and tiius by continued working of the
piston, the water will rise in the barrel till it escapes by the spout,
115. In both the suction-pump and the lifting-pump, the water
will be discharged by jets, uidess a kind of reservoir is made b^
the enlargement of the barrel above the spout, in which case it
may be made to flow in a continuous stream.

116. The forcing-pump is another form of this use-
ful machine, combining m a ffreat degree the proper-
ties of those already described. It is composed of a
hollow cylinder, the lower end of which dips into the
water in the well ; just above the valve, in the upper
part. of this cylinder, a lateral pipe branches off, nav-
mg at a short distance from its origin another valve,
both valves opening upwards ; and in the upper part
of Uie cylinder or barrel is a solid piston or plonger,
moving air-tight vertically. Now if the piston be
depressed io the lower vaJve, and then raised, tliat
will open, while the valve in the lateral pipe remains
closed, and the pressure of the atmospnere on the
water in the well will cause it to rise a little and ex-
pel a part of the air.through the firet valve ; the pis-
ton then being lowered mat valve will close, and
the air above it be expelled through the other valve ; thus every
elev^Uon of the piston will make the water rise higher in the cy-
Hiiiiei ifM it has expelled all the air, and it will consequently, at
the next uiting of the piston, pass above the first valve, and the
piston oeing again lowered, as the liquid cannot descend, the
valve being closed, it will be forced into the lateral pipe, through
its valve, and as it is prevented from returning again by that valve,

What is the greatest hei^t to idiich water may be drawu up by a well
eon struct ed pump of this form ?

In what manner was it disooTered that a mixture of air and water may,
by the action of a pump, be raised higher than 34 feet f

How is this action explained ?

Of what does the lifting^-pump consist ?

Is the liftine-purop limited to any particular height to which it nrx
raise the liquid column ?

In what manner is a constant stream maintained either in the lifting or
^e suction-pump ?

Describe the form and action of the forcing-pump.

Which of the preceding pumps constitutes a part of thit ?

216 AEROSTATICS.

it will continue to ascend with eyery down-stroke of the pistoni
and may thas be raised to any height required

J[17. In a pump of this kind, the stream will be intermitting,
tiidess there be a cistern above the spout, to fonn a head of water
which may act by hydrostatic pressure ; or the same object may
-be more effectually attained by closing the force^ipe, so that a
portion of condensed air may press on the surface of the wa'^er
after it has passed the yalve, and an open tube, fitting air-tight,
entering the chamber, and having its lower extremity, plunged be-
neatii the svrface of the water, uiat liquid will be driven up it by
the pressure of the included air, and form 2.jet-d*eau^ or flow in a
regular stream, according to the disposition of tiie spout or moutii*
piece.

118. The fire-engine is a modification of tiie forcing'-pump, con-
sisting essentially of two working barrels, like an air-pump, but
fitted witii solid pistons, and valves corresponding with those of
the forcing-pump ; and thus water is drawn from any reservoir or
other source of supply, and propelled into a strong air-chamber,
from the upper part of which passes a tube, having its inferior ex-
tremity dipped under the surface of the water, which is thus
driven through it by the pressure of the condensed air. The tube
just mentioned may be connected with the part that enters the
air-chamber by a universal joint, and thns its extremity may be
convenientiy turned to throw water in any direction; or as more
usual, it may have fitted to it a flexible leathern pipe or hose, by
means of which the stream may be conducted to any spot where
it may be made to act with the greatest effect.

AEROSTATICS.

119. The laws which regulate the ascent and descent of floating
bodies have been generally elucidated in treating of specific gra-
vity, as connected witii the science of Hydrostatics. It was there
demonstrated that liquids differing in density when placed in con-
tact would assume an arrangement depending on their relative
weights or densities, the heaviest always sinking to the bottom of
the containing vessel, and the others floating at heights corres-
ponding to those weights.* Solids, immersed in liquids, in the
same manner either sink or float according as they may be heavier
or lighter than the medium in which they are placed. Thus if a
vessel were partly filled with mercury, and water standing above

What device maintains a eonitant efllax In the forcing-pump f

What are the essential parts of the fire-engine ?
*' What device enables the fireman to direct the stream in aty directioi^
•eeording to circumstances ?

* See S^drostiain^ No. 76.

OE VEGI7NDATI0N. Si9

It, th^n on dropping into it a piece of tfon, the sdid metel wvold
be seen to fall tniou&rh the upper stratum of the liquid maM, and
stop at the surface of the lower stratum, as consisting of a metallie
fluid more dense than the solid metal.

120. An analogous effect might be exhibited with gases of dif-
ferent densities. If a quantity of carbonic acid or fixed air wsra
to be poured into a large glass jar, so as to fill the lower half of
it, the upper part of the jar would be occupied by atmospheric air,
as the lighter of the two fluids ; and any bodies of specific gravity
intennediate to these gases, as soap bubbles, being let loose oyer
the jar would fUl through the upper stratum of gas, and be ar«
rested by the lower, on me surface of which they would float, josl
as a cork would float on water.

121. A great number of substances of yarious kinds are sas-
pended in the atmosphere within a moderate distance from the
sur^e of the earth ; some of them, in consequence of their ex-
treme minuteness, belonginff to the class so nicturesquely &»»
scribed by Shakapeare as ^^ Sie motes, that pe<^le the sanb^un.**
These floating coiq[>uscttles appear to be numerous in proportion to
the heat of the air ; and hence they are much less frequent in win-
ter than in summer.

122. ** We are ignorant of the precise nature of this fine pow-
der. Perhaps it may be a mixture of inert matter extremely
divided, with the exauisitely small germs of various species of
organized bodies, as tne eggs of insects, the seeds of plants, and
likewise the fecundating powder from the stam^us of flowert. It
is in fact known from the observations of naturalists, Uiat under
many circumstances, animalcules and minute vegetables of dif-
ferent species become developed, though it is impossible to pei^
eeive the germs from which tney are derived. It is certdn, also,
that flowers famished with pistils only, (as tho«ie of the date
palm,) are fecundated, and bear fruit, though the plants furnished
with stamens are found at considerable distances, and even sepa*
rated from the others by vast tracts of sea. All these observations
tend to confirm the hypothesis of the transmission of germs and
fecundating jpowders by means of the atmosphere. Indeed we
take aature m the fact, as it were, under many circumstances ;
thus plumose or tufted seeds are frequently observed flying in the
air, as those of the lettuce, the dandelion, and others, with which
children sometimes amuse themselves. And it mav be perceived
that the seeds of many species of vegetables are Uirntsiied with
delicate membranes or wings ; as, ror instance, those of the fir

What analogy exists between tbe pt^enomena of liquids and those of
gases, when different kinds are poured Into die same vessel ?

fxive examples of diat analog.

In what manner is the floating dost of die atmosphere seen in warm
snnny weadier to be aeeounted for ?

Is the ascension of those substances trom the earth rendered probable
oy any known facts in natural history ?

What seems to foe the desig^n of the thin membranes and delicate
tamer wiih «rhi«h the seeds of certain plants are famished?

tSO ASBO0TATICS.

the elm, te., wlkieh seem formed expressly in order that the #i]ui
may raise them, so that they may be transported in all directions,
and thns contribate to the propagation of the species to which they
belong.

• 123. "Relatively to the fecundating powders, it may be re-
remarked, that in n^rests of pines and firs, at the period of flower
ing, the ground is covered for several days with an extremely fine
li^t powder, which becomes raised in the air by the winds in
prodigious quantities, and conyeyed to distant places, where the
descending clouds have, been often mistaken for showers of sul-
' phur. Also during the season of the flowering of wheat, the fe-
cundating dust, or pollen, may be seen floating over the fields
like a thick mist."*

124. The modern art of aerostation, or as it has been more cor-
rectly styled aeronautics, depends on the application of the prin-
ciple of specific gravity to the action of gases on solid bodies, and
the consequent motion of the latter through the atmosphere.
After the invention of the air-pump, when the mechanical proper-
ties of the air had been experimentally demonstrated, the feasi-
bility of contriving a machine for the purpose of navigaling the
atmospheric regions became a favourite subject of speculation
among men of science.

125. Bishop Wilkins, a distingnished mathematician, and one
of the earliest members of the Royal Society of London, was so
&r convinced that a method of travelling through the air might
be discovered, that he hazarded the opimon that the time would
come when a man about to take a journey would call for his wings
as familiarly as he mi^ht now for his boots. But the idea of taking
advantage of the principle of specific gravity to form a flying-engine,
that should rise m consequence of its bein^ lighter than an equal
bulk of air, appears to haye been first publish^, if not conceived,
by Francis Lana, an ingenious Jesuit. The scheme he proposed
was that of attaching to a car four hollow globes of copper, which
were to be exhausted by means of an air-pump ; and which he
imagined would have sufficient ascending power to elevate the
ear and the aeronautic adventurer. It seems to have been merely
a theoretical project, which must have failed in the attempt to ex-
ecute 'it; for neither globes of copper nor any other substance
known could be manufactured in such a manner as to be at once
buoyant, from the thinness of the sides, and strong enough to re
sist atmospheric pressure.

What remarkable appearance is often exhibited by the turfaee of t
earth in the flowering season of pines, firs, &c.?

^ How earl^, and by what oocarrenoes, were men iaduced to atterop' aS-
rial navigation ?

What appears to hare been the earliest coneeption of thii tubjcet, and
jiow did it differ from the idea of Lana ?

WI17 was the project of the latter impracticable ?

* Beudant Traite Elem. de Physique, y; . 3C , 535.

THB Ant-BAXXOOK. ttl

136. Nearly a century had elapsed after the pabliealioii ot the
abortive plan just noticed, when the discoTCiy of hydrogen gas,
or inflammable air, by Cav^idish, about 1766, and of its remanui-
ble inferiority of density compared with common air, reviTed the
epecuiations of philosophers on the subject of aeronautics. Dr.
Black, of Edinburgh, soon after ascertained, by experiment, thai
a thin bladder filled with hydrogen gas would rise to the ceilinff
of a lo^ room, and remain suspended till it was taken down ; ani
several years subsequently tiie subject was further investigated
by Cavallo, a Portuguese jgentlenian, residing in England, who
was a fellow of the Royal oociety.

Id7. It was, however, in France that the invMition of the aii>»
balloon took place. Two brothers, Joseph and Stephen Montgol*
fier, paper-makers, at Annonay, constructed a large aqnaie baff of
fine silk, and caused it to ascend in an inclosed diamber, and a&
terwards in the open air, by heating the air within it by means of
burning paper. After several preliminary enerimeots, a ballooa
was constructed at Paris, consisting of an elliptical bag, 74 Ihet
in leng&, and 48 in breadth, with an aperture below, near which
was suspended an iron grate for burning wood and straw, and A
boat or car attached for the reception of aerial travellers ; and in
this machine the first ascent was made, in October, 1783, by Pil^
tre de Rosier, superintendent of the Royal Museum. Other ex-
periments of the same kind followed, with balloons rendered buoy*
ant by the admission of heated air.

128. But this method of aerostation was liable to inconveni-
ences and imperfections, which rendered it- less eligible than diaft
of employing balloons inflated with hydrogen gas, the chief ob»
jection to which arose from the expense attending it. This, how*
ever, was obviated by means of a public subscription ; and D^
cember 1, 1783, M. Charles, professor of natural philosophy, aft
Paris, and his companion M. Robert, ascended flrom the gardens
of the Tuillcries, by means of a balloon filled with hydrogen or ii^
flammable air. The success of this undertaking demonstrated
the superiority of this mode of construction ; and it was consc^
quently adopted by many other experimentalists, both in France
and elsewhere. Lunardi, an Italian, was the first aeronaut who
exhibited in England ; and amon^ those who distinguished them-
selves by their enterprising spirit, or philosophical researcheSt
Fmidst die fields of air, may be noticed the names of Blanchard,
Gamerin, Zambeccari, Dr. Jeffries, W. Windham Sadler, and
Oay-Lussac, the last-mentioned of whom, in 1804, ascended from

How k>og vat Lana's scheme pablished before the discovery of hydro-
gen gns ?

What experiment by Dr. Black is probably the earliest form of ballboii
ascension ?

In what manper did the Montgolfiers effect the elevation of their dlkl^ig?

What is related of the form and size of the first balloon vith which %p
aeronautic expedition was made by Rozier ?

Why was net hydrogen adopted by the eariiesl aeronaatt }

Who were among that nomber ? . . . . ^

T 3

S23 AEBOSTATICfl.

Pttis, ftmuhed ^ith instninients for making meteorological ob-
servations ; and from the descent of the mercury in his barometer^
he inferred that he had, when at his utmost elevation, attained the
height of about 23,000 feet above the level of Paris ; and this ap-
pears to be the greatest distance from the sur^ice of the earth to
ivhich any person has hitherto risen by means of an air-balloon.

129. Several accidents have occurred to aeronauts in the prose-
eutions of their adventures, and some have lost their lives ; as
Pilatre de Rozier, who, after repeated successful ascents, was
killed, together with M. Remain, in consequence of the balloon tak-
ing fire in which they had attempted to pass from France to Eng-
land, in June, 1785 ; MadameiBlanchara, the wife of the aeronaut,
mentioned above ; and W. W. Sadler, who, after having made
thirty atmospheric voyages, in one of which he crossed the Irish
Channel, was precipitated from his balloon, owing to the car
striking against a chimney, at the height of about fort^ yards
ftom the earth. Notwithstanding these and other fatal disasters,
aeronautic expeditions have been so frequently undertaken, that
most persons must have had opportunities for witnessing them ;
but though several useful purposes to which air-balloons might be
applied have been 8ugj|nested, the difficulty of managing them has
himerto prevented their adoption except as objects of display.

130. The air-balloon consists of a light bag of thin silk, of a
globular or elliptic shape, and rendered air-tight by a coating of
varnish, made by dissolving gum-elastic in spirits of turpentine.
When thus prepared, it must be distended witii some elastic fluid,
lighter than common air ; and it will thence acquire an ascending
power equal to the difference between its weight, including the
attached car and its contents, and that of tiie bulk of atmospheric
air which it displaces. Suppose the diameter of the silk globe
to be 20 feet, its circumference will be about 63 feet, its superfi-
cial measure about 1257 square feet, and its contents, solid mea-
sure, 4190 cubic feet; then if it be filled with ^ having only i
of the specific gravity of common air, and admitting that a cubic
foot of the latter would wei^h 1| oz., and that 1 square foot of
taffeta or thin silk would weigh 1 oz.:^

The weightof atmospheric air displaced will be 6285 oz.

The weight of gas in the balloon ... I571i
The weight of the taffeta .... 1257

2828i

3456}
To vhAt height did Gav-Uinae awend ?
To what purpoie have baUooni been hitherto applied ? •
or what doei the air-balloon eonsitt ?
What would be the aieensional foree of an unloeded balloon of silk 90

feet in diameter filled with hvdrogen of a apeeifie gi^vity i that of
mon air? Cakulats m lioiiar priasiplca the ions of a b»lloon dO

eom-

^^ ^ ^ feet

■n QiamBterr

TBS PARACHUTE. 229

131. Hence the inflated balloon wonld wei^h 3456 os., or 316
pounds less than an eqaal bnlk of common air ; and therefore snch
a balloon, with a car and its contents attached, weighing 200
pounds, wonld hare an ascending force equal to 16 pounds. But
if it were filled with pure hydrogen gas, having a specific grari^
but 1-13 that of common air, its power of ascension would mam«
fektly be augmented in a hiffh degree.

139. Aeronauts in general were accustomed to use inflammable
air, procured by dissolving pieces of iron or zinc in sulphuric acid
diluted with water; a tedious, troublesome, and inconvenient
operation, which was never found to produce gas approaching to
the specific gravity just mentioned. Hence Mr. Green, who has
distinguished himself by the number of his aerial expeditionSy
amounting to about one hundred, determined to make a trial of coal
gas. From some preliminary experiments he ascertuned that the
ascending force of a balloon three feet in diameter, when inflated
with gas from coal, was equal to 11 oz.; and that when filled
with hydrogen gas procured in the usual way, its force was not
more than 15 oz. He therefore, in his ascents in the neighbour*
hood of London, availed himself of the convenience of procuring
gas from the coal-gas companies, which he found to be sufliciently
adapted for his purpose.

133. The accidents which occurred to some of the earlier aero-
nauts suggested the idea of contriving a method of descending
independent of the balloon, if circumstances should render it desir-
able. The first experiments for this purpose were made by Le
Normand, in 1783 ; and Blanchard subsequently constructed a
machine resembling a large expanded umbrella, called a para«
chute, which he let fall from a height of 6000 feet above the earth,
with a dog in a basket suspended from it. A whirlwind arrested
its descent and swept it above the clouds ; but it soon approached
the balloon again, when the dog recognized his master, showing
nis uneasiness and alarm by barking ; another current of air then
carried him out of sight, and he ultimately landed in safety, though
not till after the descent of the balloon. Gamerin, who used a
parachute 25 feet in diameter, with a basket attached to it, descend-
ed from the air by this means, several times, both in France and
in England ; and on one occasion from the perpendicular eleva-
tion of 8000 feet.

134. On the principle of the parachute depends the buoyancy
of numerous light bodies presenting an extended surface to the
air ; and thus a little canopy made by attaching four strings to the
angles of a sheet of paper with a light weight in the place of a car,
if dropped from an eminence will descend but slowly to the ground.

What has recently been mbstituted for hydrogen in the inflation of
balloons ?

What reUtiTe aseensional forces will be gWeo to balloons by ooal-gat
and hydrogen respectively ?
' What is the form and what the ol;ject of the parachate ?

What ae«oants are given of the use of this apparatoa '

224 AEROSTATICS.

Some experiments founded on the observation of such facts, made
in Germany, may here be noticed. Zacharia of Roaleben, conceiT-
ingr the possibility of forming a flying boat, constructed, by way
of trial, a case of liffht wood coyered with linen, in the shape
of a flat obtuse-angled keel, 5^ feet in diameter, and i a foot deep,
weighing 14^ pounds. On the 17th of September, 1823, this ma-
chine was launched from a scafibld on the race-course of Wen-
delstein, the scaflbld being 27 feet hi^h, and standing on a rock
100 feet above the surrounding plain; so that the perpen-
dicular height was 127 feet; and the boat flew to the distance
of 153 feet. A second flying boat 7i feet in diameter, i a foot
deep, and 25 pounds in weight, which was launched from the
scaffold on the same day, took a somewhat more elevated path, and
landed after a flight of 158 feet. These experiments appear to
have been expensive, and the result was not sufficiently flattering
to induce the projector to repeat them.*

135. Attempts have been made at different periods to construct
wings for active fliarht through the air ; but tney have all proved
abortive. The celebrated historian, William of M almesbury, in
his account of the conquest of England by the Normans, men-
tions an alleged prediction of that event, by Elmer, or Oliver, a
Benedictine monk of M almesbury, in consequence of the appear-
ance of a comet, in 1060. This monk appears to have been a
learned and ingenious man, who was skilled in mathematics.
But his claim to notice at present is grounded on his being the
earliest English aeronaut on record ; though his speculation was
not only unsuccessful but unfortunate. For the historian informs
us that Elmer, having affixed wings to his hands and his feet, as-
cended a lofty tower, whence he took his flight, and was borne
u^on the air for the space of a furlong ; but owing to the violence
ot the wind or his own mismanagement through night, he fell to
the ground, and broke both his legs.f

136. The famous Roger Bacon, who died towards the end of
the thirteenth cenjtury, in his treatise on the Secret Works of Nsr
ture and Art, expressly asserts the possibility of constructing
machines in which a man sitting might move through the air, by
means of wings, like a bird flying.:^: In the fifteenth century,

Whnt success h&s attended the various attempts which have been madie
to employ aerial boats ?

How early do attempts of this kind appear to have engaged the serious
attention of speculative men ?

What success attended the flights of Elmer, Dante, and Degen ?

• Elements of Natural Philosophy. By Prof. Vieth, of Anhalt-De§-
aau, (German.) Leipsic, 1823. p. 208.

+ Gal. Malmesbur. de Gestis Regum Anglorura, lib. ii. cap. 13.

^ " Possunt etiam fieri instrumenta volandi ut homo, sedens in medio
rnstrumenti, revolvens aliquod iiigenium, per quod ale artificialiter comr

eositSB aerem verberent, ad modum avis volantis." — ^Epistola Fratris B.
^aconis de Secretis Operibus Nature et Artis. Hamburg. l$7fL p. 3^.

.*i

THE SKY-ROCKET. 825

John Baptist Dante, a mathematician of Perngria in Italy, excited
the astonishment of his contemporaries hj his aeronautic exploits.
But his career i^as unfortunate ; for we are told that after he had
repeatedly crossed the lake of Thrasymene through the ab, he
took his flight from an eminence in his native city, when his ma-
chinery becoming' deranged, he fell on the roof of a church, and
fractured his thi^h. The Journal des S^avans, December 13, 1678,
contains a description of a flying-engine contrived by a locksmiUi
of Sabl^, in the county of Maine, in France, by means of which
the inventor descended from a second floor window, and proposed
to fly from a height over a river. Professor Vieth says, that the
late^ experiments on the art of flying were made by a watch-
maker at Vienna, named Degen ; but they seem to have led to no
practical results of importance.*

137. The ascent of sky-rockets affords an interesting object of
philosophical speculation, and the phenomenon has been variously
accounted for by men tf science. The rodket consists of a cy-
lindrical case or cartouch of thick paper filled with a composition
of gunpowder, charcoal, steel filings, and other inflammable mat-
ter ; with a head technically styled ** the pot," at the upper ex-
tremity ; and a light stick, to which the rocket is affixed laterally.
Its flight, like that of other projectiles, depends on the sudden expan-
sion of compressed air, formed by combustion. The cause oi the
ascent of the rocket is, that whereas it would, if it were not for the
aperture below, be equally pressed on all sides within by the ex*
panding ^s, and would remain at rest, but this pressure, like that
' of steam m a boiler, will often on a small portion of its inner sur-
face greatly exceed the weight of the containing vessel. In suck
cases, the opening of an aperture sufiUciently large, will project the
container in the direction opposite to that in which the opening
takes place. It will be perceived that from this account of the effect,
the operation would be the same in vacuo as in the open air. In
fact the effect is no more due to the impindlnff of the escaping gas
against the air below, as Dr. Hutton ana ouiers have supposed »
than the effect of eflluent water in Barker's mill is to be attributed
to the same cause. Several steam-boilers which have exploded
in the United States have gone off through the air like rockets,
having first formed a rent in such a part as to allow the issuing ^
steam to urge the enormous mass forward by its elastic action.
One occurrence of this kind at Pittsburg was, at the time, de
scribed as having been accompanied by a train of light ; as if the
issuing stream had been an inflammable mixture.^ A revolving

Of what does the sky-rocket consist ?

On what does its flight depend ?

What causes the rocket to ascend when the contents are inflamed ?

What analogous effects on a larger scale have sometiines been wit-
nessed ?

By what method is the rapid developement of gases obtained in the
rocket ?

• Elem. of Nat Philos., p. 209.

S26 AEROSTATICS.

apparatus, likQ a Barker's miU, only adapted to the action of air
instead of water, may be set in motion by condensed air ; but will
reTolre with rather more velooity if placed in the receirer
of an air-pump, and, afVer exhaustion, set in motion by allowing
the external air to find an entrance through the reyolving arms*
Dr. Hutton justly remarks that the rocket would not rise unless
the elastic fluid were produced in abundance; and hence the
necessity for piercing in the centre of the rocket a conical hole,
and thus the composition when inflamed bums in concentric strata,
of much greater extent than the circular disk to which the com-
bustion must otherwise be confined, and the expansive gas is
formed in quantities sufficient to' produce the required effect.

138. Among the amusements of schoolboys there are few more
susceptible of application to useful or curious purposes than that
of flying paper-kites. By means of such a machine, which he
eonstructea by stretching a silk handkerchief over a wooden frame,
Dr. Franklin demonstrated the identity of liflrhtning with tlM
electric fluid ;* the paper-kite has been employea to convey a line
to the shore from a vessel wrecked on a rocky coast ;f and a few
years ago, a Mr. Pocock, of London, made repeated experiments,
by means of which he ascertained the possibility of travelling in
a carriage drawn by two paper-kites, supported at a moderate ele-
vation, and impelled by the wind. The elevation of the paper-
kite in the usual manner, with a line attached to a loop on the
under-side of the machine, is satisfactorily elucidated by Dr. Pv
ris, who has shown that the ascent of the kite affords an example
of the composition of forces, the mode of aotion of which is esoii*
bited in the following diagram.

139. The kite is here represented rising from the ground, the
line W denoting the direction and force of the wind, wnich filling
on an oblique surface, will be resolved into two forces, namely.

To what naefol parpoaes has the kite been occaalonall/ converted ?
On what principle is its ascent to be explained ?

* See Treatise on Electricity,

t See Transactions of the Uondon Society for the Encouragtment of
Arts, Manufactures, and Commeroe, v«l.xii.

THE BITINO-BtLL. 227

6ne parallel with it, and anoliher peTpendieolar to that snrftce, and
the latter only, repTesented by the line Y, will produce an effect,
impelling the kite in the direction O A ; and the tension of the
6tnng, at the same time, in the direction P T S, will canse the
machine to ascend in the diagonal O B of ihe parallelogram O A
B T.* The ascent of the paper-kite not only depends, as may
be thus perceiyed, on the same principles as those which goyem the
morement of bodies on inclined planes ; but it may also be fairly
affirmed that the path of the kite in rising is an actual inclined
plane, up which it is drawn, by tiie tension and weight of the
string.

140. A well constructed kite maybe made to ascend when
'liiere is little or no wind stirring ; for, by running with it held by
the string and inclined obliquely, the air on its interior sur&ce will
be compressed, just as it i^ould be by running with an expanded
Tunbrella held out; and by yeering out the string and running at
the same time, the kite is drawn up an inclined plane which it
forms for itself by the ^dual compression of the successiye pop>
tions of air oyer which it moyes.

7^ Diving-bell.

141. As sdr produces peculiar effects when its density is inferior
to that of the lower atmosphere, so likewise are certain effects
produced by air, the density of which has been augmented by
<eompression or otherwise. Condensed air, if not contaminated
•with deleterious gases, may be breathed with impunity by ani-
mals for a considerable time ; though its effects are yarious on dif*
ferent individuals, and some experience considerable temporary
inconyenlence frdm inspiring it. Mr. Bills, of New York, has
founded on this property of compressed air an improved method
of bottling sparkling liquids, such as ale, cider, and perry. His
plan consists in conducting, the whole operation of drawing off,
bottling, corking, and secunng the liquors in question, within an
air-tight chamber, into which such a quantity of air may be com-
'pressed by a condensing pump or engine, that it may alwajrs
afford a degree of pressure on liie sur&ces of the liquors sufficient
to prevent the escape of the gas to which they owe their sparkling
quality.

142. But the most interesting and important purpose to whidi
the respirability of compressed air has been applied, is that of en-
abling persons to descend to a certain depth beneath the surface
of the sea, by means of the machine called a diving-bell. The

What path does it aetually detoribe in rising?
How may the kite be made to rise in a calm?
How is the ascent in this ease produced f

What effect on the respiration of animals is produced hj air above the
eomraen density ? ^

What application of suoi air has been made to purposes in the arts ?

, • Philosophjin Sport made Science in Earnest. New edit 18d3,p S36»

228 . AXKOBTATICS.

compressibtlity and impenetrability of atmospheric air may be
both at once demonstrated by the simple experiment of holding
by the foot an inverted beer-dass, and plunging it perpendicularly
in a jar or basin of water, when the portion of air within the beer-
^lass will be compressed and diminished in bulk, in propor-
tion to the depth to which the glass was pressed beneath the sur-
face of the water : but a limit would occur beyond which manual
force woiUd not drive it. If a small bit of lighted wax-taper,
attached to a cork, were placed on the water and included under
the inverted glass, it would bum in the compressed air longer
than in an equal bulk of air at its usual density ; but the air would
be consumed hj the combustion of the taper till it became reduced
to about one-third, and the residue would be found unfit for respi-
ration and the support of animal life.

143. A diving-4)ell is merely a large conical or pyramidal ves-
sel, made of cast iron, or of wood, the latter loaded with weights
to make it sink. It is usually furnished with shelves and seats
on the sides for the convenience of those who descend in it ; and
several strong |rlass lenses are fitted into the upper part for the
admission of light. There is likewise a stop-cock, by opening
which the air, rendered impure by respiration, may from time to
time be discharged and rise in bubbles to the surface of the wa-
ter ; and provision must be made for the regular supply of £resh
air, which may be sent down through pipes from one or more
large condensing syringes, worked on the deck of a vessel above.
The bell must be properly suspended from a crane, or cross-beam,
furnished with tackles of pulleys, that it may be lowered, raised,
or otherwise moved, according to circumstances.

144. Some have supposed that the- ancients were acquainted
with the use of the diving-bell, and apparent allusions to it occor
in the works of Aristotle. But the earliest direct notice of such
a machine is probably to be found in a tract *' De Motu Celerri-
mo,'* by Jbhn Taisnier, who held an office in the household of the
emperor Charles V. He states that some experiments were mado
in ^e presence of that prince, at Toledo, in 1538, by two Gredcs,
who descended under water several times in a brazen caldron,
without wetting their clothes, or extinguishing lights which they
carried in their hands.* Since the middle of the seventeenth een
tury, diving-bells have been often used for the purpose of recover
ing valuable property which had been shipwrecked.

145. In recent times, the expense attending the construction of
a diving-bell, and the difficulty of managing so unwieldy a ma-
How are the eoropressibilitj and the impeneti'abilltj of air demon-
strated ?

How is the power of compressed air to support combustion proved ?
What is the description of the diving-bell ?

How are tlie operators in a diving-bell supplied with air during their
continuance beneath the surface ?

What historical account is given of the invention of the diving-bell ?

" ... - .,-... ■ . ■ -■ ■

* y. Schotti Techoica Curiosa, lib. vi. cap. 9.

DIVING HABITS. tt9

dhine, have led to the inyention of less operose and moTB eoiiT»>
nient methods of makings submarine inyestigationa ; bat there iS
one instance of the successful employment of diving-bells for the
recovery of treasure from the sea, which occurred in 1831, and
that attracted attention on account of the skill and enterprise
displayed in the conduct of the undertaking. In December,
\ 1830, a British frigate having sailed from Rio Janeiro for Eng-
land, with 810,000 dollars on board, struck on rocks, and was
sunk at Gape Frio. Captain Thomas Dickenson, an officer on
that station, obtained permission to attempt the recovery of the
treasure; and not being able to procure 'a diving-bell at Rio, he
adapted to the purpose Uie ship's iron water-tanks, and constructed
a huge crane 158 reet in length, and 50 feet above the level of the
sea, from which to suspend the bells. Though the bells were re-
peatedly lost, the undertaking was prosecuted by Captain Dick
enson and other officers, till ultimately 750,000 dollars were xe>
covered, besides a (quantity of marine stores and other articles.

146. Diving habits, or jackets, adapted for descending under
water, have been variously contrived ; and among such machines
are the Scaphandre, invented by the Abbe de la Chapelle ;* and
Klingert's smachine for walking under water ;f but these, dioogh
ingenious, are probably inferior to the apparatus recently employ-
ed at Portsmouth, England, by Mr. Deane. The essential pait
of his machinery consists of a capacious metal helmet, covering
the head and neck, restin? on the shoulders, and attached to the
body bv straps. In the front are three oval windows of strong
plate-giass ; from the lower part of the helmet passes a bent tube
for the discharge of air which has been breathed ; and from the
upper part proceeds another tube connected with a flexible pipe,
through which fresh air is forced from above. Armed with tnis
head-piece, and a waterproof dress, the adventurer descends from
the side of a ship by a ladder to the bottom of the sea, which he
can explore at his leisure, and walk to any distance within the
length of his air-pipe. To counteriialance the upward pressure of
the water at any considerable depth, it is requisite that leaden
weights should be attached to the body, in addition to the weight
of the helmet, and thick leaden soles &r the shoes.^

147. Some curious inventions, for the purpose of submarine na-

What objection exists to the general use of diving-bells for submarine
explorations ?

What instance can you cite of the suceessful employment of these m»-
ehines fbr the recovery of lost treasure ?

How is Deane*s diving apparatus constructed ?

What limits the extent to which the diver can extend his examinations
when usine this apparatus ?

How is me body prevented from rising from deep water in the excur-
sions taAien with 'diving dresses f

* y. Sigaud de la Fond Elem. de Fhjrs., vol. ii. p. 249.
t See Tilloch's Philosophical Magazine, vol. iii. p. 172.
i Nautical Magazine.

U

280 AEROSTATIOS.

yigatioft, have been invented in the United States. Robert Ful-
ton, the successful inventor of the steamboat, contrived a machine
of this kind, called a Torpedo ;* and David Bushnell invented a
sabmarine vessel in which a man might pass a considerable dis-
tance under water ; and by means of this, and its accompanying
magazine of artillery, an attempt was made to blow up a Bri-
tish vessel in the harbour of New York, during the late war with
England, f This project appears to have failed mereljr from the
difficulty or rather rnkpossibility of attaching the magazine to the
bottom of the ship, which was attempted by means of a sharp iron
screw, which passed out from the top of the diving-machine, and
communicated with the inside by a water-joint, being provided
with a crank at its lower end, by which the en^neer was to drive
it into the ship's bottom. The machine affording no fixed point
to act from, the power applied to the screw could make no impres-
sion on the ship ; and thus this bold adventure was disconcerted.^

Describe the method of Bashnell for blowing up an enemy's ships.
Why did this plan prove unsoccessfal ?

* y. Montucla Hist des Mathemat., t iii. p. 78.
' t For a description of this curious engine, see a paper on ** Submarine
NaTigation,** by Charles Griswold, in Silliman's American Journal of
Science, vol. ii. p. 94.

f For a report on Norcross's diving apparatus, see Journal of the
Franklin Institute for January, 1835, vol. xv. p. 25.

The following scientific treatises may be advantageously con*
suited in reference to the department of Pneumatics :^

Playfair's Outlines of Natural Philosophy, vol.i. pp* 242-^262.

Library of Useful Knowledge, treatise on Pneumatics.

Gregory's Mathematics for Practical Men, pp. 346—^52.

Ferguson's Lectures on Select Subjects, pp. 195—227.
. Cambridge Mechanics, p. 377, where the motion of gases is
treated to some extent, and p. 403, theory of the air pump and
other machines depending on the atmosphere.

De Luc Recherches sur les Modifications de 1' Atmosphere.

Philosophical Transactions, vol. Ixvii. pp. 513. 653.

Cavallo's Philosophy, vol. ii.

Playfair on the Causes which affect Barometric MeasurementSi
in the Edinburgh Philosophical Transactions, vol. i. p. 87. ~

ACOUSTICS.

1. The science which has heen designated by the terms Acou-
stics* and PhonicSff treats of the causes and effects of Sound, and
the manner in which it is perceived by the organ of hearin?. The
idea of sound is excited in the mind when the motions which take
place in any of the bodies around us are such as can be commum«
cated to the auditory nerve and thence to the brain. This effect
is produced by means of the organization of the ear, the tremulous
motions c^ vibrations of the air being propagated to the tympanum
or drum, a thin membrane which closes the aperture of the ear;
behind the drum is a cavity in the bone which forms the side of
the head, separated by another membrane from an inner cavi^,
from which branch' off variously-formed tubes or canals, which,
as well as the inner cavity called the labyrinth, are filled with a
limpid fluid ; and an expansion to the auditory nerve, or delicate
layer of nervous fibres bein? distributed over the internal surface
of the labyrinth and canals, it thus becomes the medium of sensa^
lion with regard to sound.

2. There is a passage called the Eustachian tube, which ex-
tends from the back part of the mouth to the cavity immediately
behind the membranous drum, through which air passes, and
therefore the drum vibrates freely when acted on by the sonorous
undulations of the external air, which are communicated from the
membrane of the drum by a chain of very minute bones and mus-
cles passing from it to the membrane over the entrance to the la-
byrinth, ana corresponding undulations being* produced in the con-
tained fluid, impressions are propagpated to the nervous lining of
the labyrinth, and thence to the brain.

3. Hence it must be apparent, that the sense of hearing, de-
pending as it does on the perfect operation of so complicated an
organ as the ear, may be impaired oy various causes, or entirely
destroyed when the essential parts of the organ are originally
wanting, or so greatly injured by disease as to be incapable of
performing their functions. Thus some persons are born deafi
the organization of their ears being so defective that they are ut-

What is (he object of the science of acoustics ?
Under what circamstances is the idea of sound excited in tlie mind ?
How is the effect produced ?
What is the tympanum of the ear ?
What is the inner cavity of the ear designated ?
How is its internal surface lined ?

What appears to be the immediate instrument of sensation in regard to
aound ?
What is the position of the Eustachian tube ?
What is the natural consequence, in regard to language, of an original

want or an early destruction of the organs of hearing ?

■II II .- .. «

* From the Greek Axev*, to hear,
t From «»v>i, a voice, or sound.

231

232 ACOUSTICS.

terly incapable of perceiring sounds, and tiierefore can neyer ae-
qtiire the faculty of speech by imitating vocal languafi[e. Such
unfortunate individuals, incapable of obtaining knowledge by the
jDSual channels, may, however, be qualified ror hi^h degrees of
mental cultivation by the modes of instruction contnved, or rather
greatly improved, by L'Epee, Sicard, Braidwood, and others, who
have most meritoriously devoted their talent^ to ^e instruction of
tlie deaf and dumb.

4. Though the functions of the orffan of hearing are clearly as-
certained, as to the general principle of action, yet the peculiar
purposes of the several parts are by no iptieans equally obvious ;
nor is it certain that any of them, except the auditory nerve, are
absolutely essential to the perception of sound. Some persons na-
turally have an aperture in the membranous drum of the ear, and
in others a similar defect id produced by disease ; but in either
case, though the faculty of hearing is commonly somewhat im-
paired, it 18 not destroyed, not even when, owing to abscess in
the ear, the chain of bones* between the membrane of the drum
and that covering the entrance to the labyrinth has been disu-
nited. In that case, probably, the vibrations of the air impinging
on the inner membrane cause the requisite undulations in ue nuia
within the labyrinth.

5. There are persons who occasionally amuse themselves and
their companions by drawing a quantity of tobacco-smoke into the
mouth, and then expelling it tnrough one or both ears; a feat
which can be performed only by those who have a natural or arti-
ficial perforation of the membranous drum of the ear ; and thus
they can force the smoke through the Eustachian tube, into the
cavity of the drum, and discharge it through the perforation just
mentioned.

. 6. In practising the art of diving, it appears that those engaged
in it on first going into deep water become subject to most intense

Sains in the ears, which continue till they have reached certain
epths, when the sensation of something bursting within the ear
with a loud report terminates the pain, and they can then descend
as low as may be necessary without any further inconvenience.
There can be no doubt that all this is occasioned by the vast pres-
sure of the water on the drum of the ear, and its consequent nq>-
ture ; and probably it would be found on investigation, mat pearl-
divers, and others accustomed to deep diving, nave the auditory
faculty more or less impaired.

What effect on the faculty of hearing has a rupture of the tympanum ?
N "What experiment proves Uie existeoce of a passage between the mouth
•nd the external ear?

What sensation precedes the relief obtaiued by dirers when they fir»'
go into deep water ?

* This chain consists of three distinct bones, called, from their re*
pective forms, the hammer^ die atvoU, and the stirrup bonet, — maUeut,
mczM,'and stapes*

80N0BOU8 VIBRHTION. 29$

7. Though air is ihe usual medium of sound, it is not essential
to the formation or the propagation of sonorous vibrations. Some
substance however, either solid, liquid, or aerial, must form a
continuous connexion between the sounding body and the ear ; for
sound cannot be conveyed through a vacuum. If a small bell be
suspended under the receiver of an aii^pump, in such a manner
that it can be struck with a hammer without admitting air to it,
when partial exhaustion has taken place, the sound will be weak-
ened, and after the rarefaction has been carried as far as possible,
oo sound will be heard on striking the bell. If the experiment
be made by inclosing the bell in a smaU receiver full of air, and
placing that under another receiver from which the air can be
withdrawn, though the bell when struck must then produce sound
as usual, yet it will be quite inaudible, if the outer receiver be
well exhausted, and care be taken to prevent the sonorous vibrap
tions from being propagated through any solid part of the appaia*
tus.

8. As sounds become weak when tiie air surrounding the sono*
reus body is rarefied, so on the contrary, any sound, as that of a
bell, will be perceived to be much louder when the bell is struck
m a vessel filled with highly compressed air, than when struck
-with the same force in a vessel of air of the common density.
Ileoce, too, it happens that when a pistol is fired on the top of a
high mountain, where the air is comparatively rare, the report is
not so loud as when it is fired at the base.

9. That liquids conduct sound with no less facility than air
may be ascertained by ringing a bell under water, when it will
be heard as distinctly as when rung above the water. And a
person diving under water would plainly hear the sound of a bell
struck in the air at a moderate distance; If both the hearer and
the sounding body be immersed in the same mass of water, the
sound will appear much louder than when passing through an
equal extent of air.

10. The propagation of sonorous vibrations through liquids may
be rendered visible ; for, on rubbing gently with a wet finger the
edge of a drinking-glass, half filled with water, sound will be pro-
duced, and the surrace of the water will be covered with minute
undulations. The intensity or loudness of sound in fluids appears

What function does the air perform in regard to the sonoroas body and *
to the ear ?

What experiment proves the necessity of a medium for the transmis-
uon of sound ?

What is the effect of highly condensed air on the loudnesi of sounds
produced within it ? . ^

What other evidence is afforded of the effect of pressure on the inten-
sity of sound f

How can we prove that liquids conduct sound ?

Docs it^ppear from experiment that Uquids aro better or wovse oon-
ductors of sound than air r ^ >

How is the pronagatton of sonoroas vibrations in Uqiiids rendered vi-
sible ?

u3

834 Acovsncs.

to be at^mented in proportion to the increase of their specific gra^
▼ity. Taas water, being so mach denser a fluid than air, sounds
produce a stronger effect in the former medium than in the latter;
and theiefore it may be regarded as a wise provision of the Au-
thor of Nature, that the organs of hearing in fish are much less
perfectly developed, and consequentlj less sensible to the impres-
sions of sound than tiiose of terrestrial animals.

11. Solids, when they possess elasticity, convey sounds to the
ear more readily and effectively than gases or liquids. If a per-
son, hard of hearing, places one end of an iron rod between his
teeth, while the other end rests on the edge of an open kettle, he
will understand what is said by another directing his voice into
the kettie, more distinctiy than if the voice of the speaker passed
through the air, so that he might converse in this manner with
any one at a distance at which he would not hear under common
circumstances. When a stick is held between the teeth at one
extremity, and the other is placed in contact with a table, the
scratch of a pin on the table may be heard though both ears be
stopped. When sounds are propagated in this manner, the sono-
rous vibrations must be conveyed through the mouth and along
the Eustachian tube to the interior part of the organ of hearing.
. 13. Among the evidences of the transmission of sound through
solid bodies, may be mentioned, the common experiment of tying^
a ribbon or a strip of linen, cotton, or flannel, to the upper part of
a large poker, so that it may be supported vertically by holding
the two ends of the ribbon; which are to be brought in contact
with the ears, and pressed against them, so as to close them, then
on swinging the poker so that it may strike as gentiy as possible
against a bar of the fire-grate, or any other metallic substance, a
deep sound will be distinctiy heard like the tolling of a lam bell;
and yet if the ribbon be removed from the ears, and the poker sus-
pended by it, and struck in the same manner, the sound will be
nardly perceptible. Some experiments will subsequently be no*
ticed, which show that sound not only passes much more readily
through elastic solids than through air, but also that it traverses
the former with abundantly greater velocity.

13. That peculiar kind of motion in bodies which gives rise to
the sensation of sound has been characterized by the term vibra-
tion, because a striking analogy may be traced between the tremu-
lous agitation which takes place amon? the particles of a sound-
ing body, and the oscillations of a pendulum. The nature of so-
norous vibrations may be illustrated by attending to the visible

According to what circumftanee does their eonducting power appear
to be aogmented ?

What conducting power for sound is possessed 'by elastic solids com*
pared with that of other classes of bodies ?

In what manner may a person partially deaf be enabled to carry on a
conversation ?

What easy experiment Illustrates the transmisnon of sounds by solids ?

What iiarae is given to the moUoo by which sound is produced /

:»

NATVIUB OF 80N0R0TO VIBRATIONS. t8i

X

motiont which ocenr on strikhii^ or
twitching a tightly-extended cord or
wire. Suppose such a cord repre-
sented hy the central line in the mar-
ginal figure to be forcibly drawn out
to A, and let go, it womd immedi*
ately recover its original position b j
virtue of its elasticity, but when it reached the central point it
would have acquired so much momentum as would cause it to
pass onward to a, thence it would vibrate back in the same man-
lier to B, and back affain to 6, the extent of its vibration beinc[
gradually diminished oy the resistance of the air, so that it would
at leuffth return to the state of rest. The string of a violin or a
karp drawn aside thus, and suffered to vibrate freely, would pro«
pagate its vibrations to the body of either instrument and to the
surrounding air, and thus a tone or musical note would be pro*
duced and rendered pereeptible to the ear.

14. The air usually encompassiuff sounding bodies on every
tide conveys the sensation of sound in all directions ; therefore
the aerial vibrations, or, as they have been termed pulses, most be
communicated successively and generally throughout the whole
space within the limits of whieh Siey are capable of affecting the
ear. We may conceive ^his to happen in consequence of minute
expansions and contractions of the particles of air, which, thus
pressing on the contiguous particles around them, excite corre-
sponding motions, extending every way from a common centre.

15. 'Aese soniferous undulations of the air have been compared
to the waves spreading in concentric circles over a smooth pond
of water when a stone is thrown into it. And thus as the liquid
waves are propagated not only directly forward from the centre,
but also if they encounter any obstruction, as from a floating bo-
dy, they will bend their course round the sides of the obstacle and
spread out obliquely beyond it, — so the undulations of air, if in-
terrupted in their progress by a high wall, or any similar impedi-
mmit, will be continued over its summit, and propagated on the
opposite side of it. From this description of the nature of sono-
rous vibration it will be perceived to consist of the alternate dila-
tion and compression of certain portions of air or other bodies act-
ing mechuiically, so as to cause corresponding effects throughout
a given space ; and the motion thus originated, produces no per-
manent change of place among the particles of Uie sonorous mass,
but simply an agitation or tremor, so that each particle, like a

What figures are successiTely assumed by a string or wire Uurown into
a state of vibration ?

What purpose is served by the body of a stringed instrument f

How are aerial vibrations or pultet eommunicated ?

How mav this communicatioa be accounted for withont sapposiog the
particles oi air to move out of their respective places }

To what have soniferous undulations been compared ?

What analogous effects favour the sapposidon of their nmiUritj ?

280 AC0TT8TICS.

pendnlam that has been made to oscillate, recorers at length its
original position. * Hence sound is communicated through the at-
mosphere by the propa^tion of minute vibrations of its particles
from one part of the fluid mass to another without any translation
in motion of the fluid itself.

16. ** Perhaps we may most distinctly coneeire the kind of &ct
here spoken of, by comparing it to the motion produced by the
wind in a field of standing corn : grassy waves travel visibly over
the field in the direction in which the wind blows, but this appear-
ance of an object moving is delusive. The only real motion is
that of the ears of grain, of which each goes and returns as the
stalk stoops and recovers itself. This motion affects successively
a line of ears in the direction of the wind, and affects simultane-
ously all the ears of which the elevation or depression forms one
visible wave. The elevations and depressions are propagated in
a constant direction, while the parts with which the space is fill-
ed only vibrate to and fro. Of exactly such a nature is the pro-
pagation of sound through the air. 'rhe particles of air go and
return through very minute spaces, and this vibratory motion runs
through the atmosphere from the sounding body to the ear. Waves,
not of elevation and depression, but of condensation and rarefac-
tion, are transmitted ; and the soited thus becomes an object of
sense to the orpan."* *

17. That vibration of the particles of bodies which Has been
indicated as the cause of sound must have a certain degree of ve-
locity in order to produce the required effect. An extended cord
may be so slack that when made to vibrate it will yield no sound,
its motion being too slow and weak to propagate sonorous undu-
lations through the surrounding air. In order that sound may be
procured the tension of the cord must be increased; and it will
then be found, that the length remaining unaltered, the number of
vibrations in a given time will be augrmented in proportion to the
additional tension of the cord.

18. It has been^ ascertained by experiment that a vibrating cord
will not produce a sound distinctly appreciable by the most deli-
cate ear, when it makes less than about 32 vibrations in a second.f
But the susceptibility of the organs of hearing to grave or acute
sounds appears to be different in different individuals, lliere are
some curious observations on this subject in a paper published in

To what natural appearance may we compare the soniferoQs waves ?
Of what real nature are the waves of air r

Will every deg^ree of tension in a cord enable it to produce audible
sounds ?
What has experiment proved in regard to this matter ?
Are all ears equally susceptible to the same classes of sound }

• Wheweirs Astronomy and General Physics considered with Refe-
rence to Katural Theology, b. i. ch. xiv. pp. 117, 118.

t Savart asserts that he has proved by experiment, that a perceptible
sound is produced by a eord giving eiglu single vibrations in a secODd.—
Go. .

LIMITS OF A|n>IBI.S FJBRCEPTION. SS7

the Philoeophical Transactions, by Dr. WoUaston, ** On Sonndt
Inaadible by certain Ears/' The attention of this ingeniom
philosopher was attracted by the circumstance of finding a person
insensible to the sound of a small organ-pipe, which, withrespeet
to acuteness, was far within the limits of his own hearing. Ho
was hence led to try the effect of different modes of wedcening
the sense of hearing in himself; and he found that by closing the
nose and mouth, and expanding the chest, the membrane of the
drum of the ear, being subjected to extraordinary tension by ex*
temal pressure, made the ear insensible to grare tones, witiioat
affecting the perception of sharper sounds.

19. This fact affords some evidence in favour of the opinion
that the membranous drum of the ear, by means of its apparatus
of bones and muscles cohnecting it with the internal membrane
over the labyrinth, is capable of tension and relaxation so as to
adapt itself to receive ana transmit aerial undulations haying d^
fisrent degrees of velocity ; and hence it may be concluded diat
the power of perception of low or high tones depends on the state
of the membrane of the drum and parte united to it.

20. The range of human hearing includes more than nine oo*
taves, the whole of which are distinct to most ears, though the
vibrations of a note at the higher extreme are six or seven hundred
times more, frequent than those which constitute the gravest audi*
ble sound ; and as vibrations incomparably more, nequent may
exist, we may imagine, that animals like the Grylli (cricketo or
grrasshoppers), whose powers appear to commence nearly where
ours terminate, may hear still sharper sounds, which we do not
know to exist ; and that there may be inseeto hearing nothing in
common with us, but endued with a power of exciting^ and a sense
that perceives the same vibrations which constitute our ordinary
sounas, but so remote that the animal who perceives them may bo
said to possess another sense, agreeing with our own solely in the
medium by which it is excited, and possibly wholly unaffected
by those slower vibrations of which we are sensible.***

21. Though sound may be propagated through an infinite mass
of air to veij considerable distances, yet ite intensity or loudness
diminishes m proportion as the sonorous vibrations extend firom
the spot where they arie produced. The rate of diminution of in*

What facta did Dr. WolUston observe on this nibjeet ?

In what manner may the senie of hearing for gimve tones be volants-
rily weakened ?

On what is the power of peroeiving sounds of different degrees of
acuteness probably dependent ?

How extenrive is the ransre of human bearing ?

What difference in the degree of frequency must exist between the
extremes of the audible scale ?

What are probably the endowments of insects in regard to sound ?

How is the intensity or loudness of sounds affected by the distance
from the sonorous body ?

• Abstracta of Pap. in Philos. Trans., toL ii. p. ISd.

238 ACOUSTICS.

tensity may be inferred from mathematical calculation as well as
ascertained by experiment; and the results, which confirm each
other, show that other circumstances being alike, the intensity of
sound will be the inverse ratio of the square of the distance of
the place of observation from the sounding body. The distance
to which sound can be transmitted through the atmosphere, de-
pends in some degree on the direction of the wind and local cir-
cumstances. Most persons residing within a few miles of a very
large bell must have observed that the sound of it will be audible
or otherwise, in certain situations, according to the quarter from
which the wind blows. Under favourable circumstances sounds
may be conveyed to great distances. Instances are recorded of
the report of a cannon having been heard thirty leagues from the
place where it was fired.*

22. The absolute velocity with which sound is propagated
must depend on the nature of the medium by which it is conveyed.
Atmospheric sit bein^^ the general medium of sound, the investi-
gation of its conductmg power has at different periods occupied
the attention of men of science. Cassini, Picard, and Roemer,
members of the French Academy of Sciences, in the latter part
of the seventeenth century, Vnade experiments from which they
inferred that sound travels 1 172 feet in a second of time ; Dr. Halley,
and Flamstead, the astronomer royalf^ho pursued the inquiry m
England, were led to the conclusion that the common velocity of
sound was 1142 feet in a second ; and this deduction was confirm-
ed by the varied and extensive researches of Dr. Derham, in con-
8e(]uence of which it has been generallT adopted by subsequent
writers on this branch of science. This statement, however, is
now considered as requiring some correction on account of the
influence of temperature; and from a comparison of the experi-
ments of Derham made in the day-time, with some more recent
nocturnal observations of French academicians, it appears that the
actual velocity of sound, at the zero of temperature of the centi-
grade thermometer (32 deg. of Fahrenheit) is about 1130 feet in
a second ; which likewise agrees with other accurate experiments
of professor Pictet of Geneva.

23. By adopting either of the numbere last stated sufiiciently
correct calculations may be made df the distances of objects as
inferred from the relative velocities of light and sound ; the for-

According to what law'doey it vary ?
On what does the absolate velocity of sound depend ?
: What is the absolate veloeitj of sound.in air at S2° Fahrenheit ?
How is a knowledge of that velocity applicable to the measure of dis-
tances ?

• When the explosion of the volcano of Cotopaxi, in Pent, took place,
in January 1803, the noise it occasioned was heard day and night, like
continued discharges of artillery, at the port of Guayaquil, 52 leagues
distant, by the travellers Humboldt and Bonpland. — ^Edinburgh Review
for Xov. 1814, vol. xxiv. p. 142*; from Humboldt's Researches.

YELQGITT 07 SOUND. 9Stf

mer from its extreme celerity bdnff regarded as appearing instan-
taneously* on its production, at distances not exceeding a few
miles. Thus supposing a flash of lightning to be perceived, and
on counting the seconds that elapse before the thunder is heard,
we find them to amount to 3^ ; then if we reckon the velocity of
sound at the rate of 1130 feet in a second, it will follow that the
thunder-cloud must be distant .1 130 X 3^=:3955 feet. In the same
manner may be discovered the distance of a ship at sea, if we
see the flash of a sun fired firom it, and ascertain the number of
seconds that elapse oefore the report becomes audible. In defect
of a stop-watch a rough estimate of time may be made by any
person, by counting the pulsations of the artery at his wrist, which
m most young people in health will amount to about 70 in a
minute.

24. Sounds are propagated with greater or less velocity through
gases according to their density ; and thus a sharper tone will be
produced by a sonorous body in hydrogen gas than in atmos-
pheric air, and a graver tone by the same body in carbonic acid or
fixed air. Vapours of water, spirit of wine, or ether, are capable
of conveying sounds with .degrees of facility proportioned to their
respective densities, as appears from experiments made at Arcueil,
near Paris, by Blot, BerthoUet, and Laplace, the first-mentioned
of whom published an account of their investigations in 1807.
The vapour of ether conveys sound almost as readily as atmos-
pheric air; for a beil, the , sound of which in air could be heard
at the distance of 158.5 yards, was heard in the vapour of ether
at that of 143.7 yards. f

25. Expenments on the conduction of sound by water were
made a few years ago, by Messrs. Colladon and Sturm, in the lake
of Geneva. The method of operation was to sink a large bell seve-
ral feet below the surface of the water, strike it a smart blow wiUi
a hammer, the handle of which at the same instant brought a blaz-
ing port-fire in contact with half a pound of gunpowder to produce
a signal. The sound was heard nine miles by means of a spe-
cies of ear trumpet, sunk in the water, and having a broad spade-
like surface facing the direction in which the sound came. The
times were accurately noted, and the distances having been care-
fully determined by triangulation, the velocity, per second, was
found to be 4709 feet.:!:

To what expedient m&y one resort when not famished with a time-
keeper to note the time elapsed between the perception of light and of
sound in any given explosion ?

How are the different gases related to each other in regard to the
transmission of sound ?

With what proportionate velocities do the vapours of different liquids
conduct sound ?

In what manner has the conducting power of water been determined ?

* See Treatise on Optics,

t See Nicholson's Philosoph. Journal, 1812, 8to. vol.zxz. pp. 169. 173,

I See Annales de Chym. et de Phys. yol. xxx?i

240 ACOUSTICS.

96. Examples haye been already adduced of the facility witn
which solid bodies transmit sounds. To these it may be added,
that the North American Indians avail themselves of this pro-
perty of solid matter, applying their ears close to the prround in
order to discover the noise made by approaching enemies, when
the distance is too great for the sounds to be conveyed throueh
the air. Upon the same principle is founded the utility of the
stethoscope,* an instrument invented some years since by Dr.
Laennec, a French physician, to ascertain the state of the cavities
of the body, especially the chest, as to health or disease. It con-
sists of a wooden cylinder, one end of which being placed in con-
tact with the surface of the body to be examined, and the other
resting against the ear of the observer, then by ^ntly striking
the body with the knuckles or otherwise, sounds will be perceived
indicative of the existence of abscess, schirrus, or any other altera*
tion of structure which may have taken place.

27. Dr. Chladni, a German philosopher, who distinguished
himself by his investigations relative to acoustics, estimated the
velocity of transmission of sounds by the tone produced by vibr^
tion, or in other words, by the musical note emitted by a rod or
bar of any substance when struck. By thus comparing the sound
of a rod made to vibrate longitudinally with that of a column of
air vibrating in a tube of the same length, he found that the velo-
city of sound in air being represented by 1, the velocity of
sound transmitted by tin would be - - - • - 7J
By silver ---------^

By copper -- - - - - - - -12

By iron -------- -17

By different kinds of wood - . - from 11 to 17

Iron and glass appear to be among the best conductors of sound,
which they transmit at the rate of 17,500 feet, or more than 3 miles
in a second.

28. Some very interesting experiments on the capacity of solids
to conduct sounds were made by M . Biot, at Paris, in which the
research was prosecuted by more direct means than those last
stated, and different results were obtained, whence the velocity
of the transmission of sound through cast iron appears to be in-
ferior to the preceding estimate. - M. Biot took advantage of the
circumstance of laying down trains of cast-iron pipes in the French
metropolis to form an aqueduct 3120 feet in length. At one^ex-

What peculiar use do the American Indians make of the conducting
power of solids }

What purpose does it serve in the practice of medicine ? "

What IS the construction and use of the stethoscope ?

What is the relation of the metals to each other m regard to the cob-
duction of sound ?

Wliat solids appear to he among the best conductors of sound ?

In what manner did Biot determine the relative conducting power of
iron and of air ?

* From the Greek Zriido;, the hreast, or chest, and zxoo^fw, to examine.

TRANSMISSION OF SOmUD THROVOH SOLIDS. Ml

tremity of the tabes was fitted a riiiff of metal of the same <
as the orifice, in the centre of which were fixed a clock-bell and a
hammer which could be made to strike at pleasure, in such a man*
ner that the hammer would fall on the bell and on the ring of metal
just mentioned at the same instant : thus the sound of the latter
being transmitted through the solid metal or tube itself, and that
of the former through the aerial canal or cavity of the tube, the
perceptible difference of the time of transmission by the respeo*
tive mediums might be determined. It was found that by placing
the ear against the other extremity^ of the pipe two sounas were
distinctly heard, and the time being very accurately noted, by
means of a seconds watch, it appeared from a mean of many ex-
periments that sound is transmitted with 10^ times greater Telocity
through cost iron than through air, trarelling through the former
at the rate of 11,865 feet in a second.

29. It is a commonly-receiTed opinion that acute and grave
sounds are transmitted in all directions with equal velocity ; and
an experiment made by M. Biot on the same train of pipes that
served for those just recorded tends to confirm it. He caused ft
man at one extremity of the train to play various airs on the fiute,
placing himself at the other end to observe the effect. Now a
piece of music consisting of a series of notes varying from acute
to grave and the contrary, and forming a peculiar melody, adapted
to a certain measure, which regulates strictly the intervals of^tha
successive tones, it must follow that if at the distance of '3180
feet any difference had been perceived in the velocity of the dif-
ferent notes, the music would havfe become confused and imperfect
at the distance just stated. This, however, was by no means the
case, the melody being as perfect when thus listened to as in the
immediate focus of the sounds.

30. There can be no doubt that acute and grave sounds are trans*
mitted through spaces of no very considerable extent without any
perceptible difference of velocity ; for otherwise there could be no

• such thing as harmony, or the concord of sounds varying in tone
or pitch, except in the immediate vicinity of the source of sound.
But that all sounds pass with equal celeritjr through the same me-
dium to any imaginable distance seems improbable; and more
numerous and precise experiments than have nitherto been made
would be requisite in order to Enable us to decide the point in
question.

^31. Sounds certainly in some respects Interfere with each other.
Thus one stfnorous body being made to vibrate, all others near it

Whftt was the result of his experiments ?

What is the rate of tnosmission of grave compared with that of aeiila
toancls ? *

How did Biot eondiMt his experiments on this sobjeet ?

Why are weftHowed to strnposean equally rapid transmission for sonadt
of nil degrees of aeotcncss r

What occur* when of scveHil toneMrat bodies near each o(hcr» and
toned to accord* one is thrown into a state of vibimUoo /

X

S4S Acoirmcs.

iMipable of producing the same tone will yibrate also ; and there*
fore when one body is made to produce a certain note, probably
its soniferous yibrations would be checked or interrupted by the
emission of a more powerful or discordant sound from another
body near it. Hence weak sounds generally are drowned by loud
ones ; and on the contrary, during the silence of ni^t, many gen-
tle sounds become perceptible which, amidst the din arising from
daily labour, business, and pleasure, especially in a crowded city^
are comjf^letely stifled ere they can reach the ear.

7%eory of Muateai Sounds,

33. Most persons, in whom the sense of hearing is perfect, pos-
sess the faculty of distinguishing certain relations between sounds
differing in tone, that is, being more or less grave or acute one
than another ; and such persons are said to have a mimcal ear, or
en ear for music,* because pieces of music consist of combinations
of such tones or sounds as those just mentioned. The manner in
which musical sounds are formed by different instruments, and
the peculiar circumstances on which their mutual relations depend,
will now be the subject of inyestigation.

Hov ara weak Bounds affected by the.OGeurreoee of more povcrfttl
ones in their Ticinity i
What ia meant hj**a tnuncdl ear?*'

* There are persons who, though endowed with the sense of hearing
in perfection, yet appear to be utterly destitute of an ear for masie.
Ther seem to haye no perception whatever of the pleasure ^nerally ex-
eited b^ successions or melodious sounds, and therefore (if their own
professions are to be believed) they cannot properly distingfuish one tone
from another, or discriminate between the noise of an itinerant nusie-
grinder and the performance of a musician possessing exquisite skill and
taste. It would be unreasonable and unjust to attribute the alleged indi^
ference of such persons to caprice, and to doubt their veracity ; for it
would be difficult to point out any motive which could induce a person to>
eoonterfeit an insensibility to the ** Concord of sweet sounds." The
writer of this note hdard a clergyman of his acquaintance, after haTing
witnessed the singing of Catalani, declare that he was utterbr unable to
ascertain in what respect her performance excelled that of'^a common
ballad-singer, gravely averring th|it he thought the melody of the one
just as agreeable as that of the other. It is deserving of notice that indi-
viduals distinguished for poetical talent have been destitute of an ear tar
mnsic. This was the ease with the celebrated poet Pope, one of tlie
Jnost exquisitely skilful masters of the melody of terse that ever existed,
who was unable to perceive any difference between the compositions of
Handel and the vilest attempts of a wandering fiddler. It appears, like-
wise, that a highly distinguished ^t of the present age. Sir Walter
ocott, though not incapable of enjoying music when performed by others,
was utterly unable to acquire a practical knowledge of music ; and that
when young, bating been placed under the tnitioa of an eminent teaeher
bf nnsio at Edinburgh, the attempt to instruct him was relinquished,
after a short time, on the groimd that he was totally deficient in that io-
ditpensaUe requisite for aeqairiay the arfr-«« masical ear.— See Annual

iography, toK xt ii. p. IT9.

33. 1^ ha3 been already stated that the chazactea^ «C a found as
to gniyityoT acuteness, is determined by the number of yibrations
in a ^ven time made by the sounding body, and thence propa^ted
ihrough the air, or some other medium, to the ear. A sonorooA
body, as for instance, a bell, the dimensions and general form' of
which remain unaltered, will, when struck, always emit the same
sound ; for though its sonorous vibrations may be more or lest
powerful according to the manner in which it is struck, th^y will
always be isochronous, or equal in equal times. Suppose then a
series of three bells to differ relatively in size, so that the largest
should vibrate when struck 256 times in a second, the next 513
times, and the smallest 1024 times, it would be found that the first '
bell would yield the sound or tone called middle C of the piano-
forte or harpsichord, or that note produced by pressing down the
central key of the instrument ; the second bell would yield a tone
an octave above the former; and the tiiird bell one an octave
higher still ; for the larger bell would yield the graver sound.

34. The number of vibrations which take place in a soundinff
body, and the consequent tone which it yields, depend on severu
circumstances connected vdth the peculiar form and consisteaoe
of the body ; and hence the variety of musical instruments, the
distinguishing properties of which. 'depend on the diversity of
modes ia whi^ harmonious sounds can ee formed and propagated.
*I1he manner in which the times are emitted by an extended string
or wire will afford an example of the modifications of sound pro*
duoed by alterations of the state and condition of the string as to
its dimensions and tension.

35. U When two strings of equal diameter are eaually streicb-
ed, the relatiyei numbers 3i their vibrations, and of tne consequent
tones t&ey yield, will be in the inverse ratio of thetr lengths : thns
if two strings A and B have the sam^ size and tension, and if A
have double the length of B, the former will vibrate only half as
many times in a second as tl^ latter, a:nd vKll yield a note an oo-
tave below the latter.

2. When strings have the same lengtii and tension, the num-
bers of their vibrations and respective tones will be in the in
verse ratio of their diameters.

3. When strings have the same diameter and the same length,
die Aundiers of £dir vibrations and relative tones will be in the

What remiirluible examples may be eited cf per«otit waDdng thif fit-
•wllT?

What rdation alwaya •ohtiftfl bietweea die vabration^ of a tonereos
body of Ifivarialale dimenqioot ? '

What nfuml>erfl of vibration m^st ^r^ bell* roak« in order that their
tones d»ouid be an octave apart, and the lowest one «piTespood with te
Diddle C dr the piano \

On wfiat eareumstanees does the nfunber of vibrations In a sounding
body depend ?

What relation subsists between die numb^t of vibmtioDf eon^axetf
If ith tlie lengiha of strings ?
' What, compared with their diamctenJf

iu

direct ratio of tlis sqnaTe roots of ihe weights by which they ara
■tretched.

36. It will easily be eonceived that a string may be extended
so Blackly, that when made to vibrate, no aadible sound will be

Sroduced ; and from experimental obsetrations it may be inferred
lat a string ribraiing less than eight * times in a second will not
yield a perceptible sound.

37. Columns of air included within tubes, when thrown into
the state of eoiioioua vibration, yield tones bearing certain relations
to their lengths ; and other circumstances remaining unaltered, a
tube of any given len|[th capable of yielding a musical tone, will,
when reduced to half that len^h, yield a tone an octave higher
than before. The Following scale will show the relative lengths
of open tubes requisite to produce a succession of octaves, com-
mencing from Ihe lowest audible sound, and with the numbers of
the vibrations taking place during the emission of each sound.

' Scale of Oetava atrreiponding toith certain laigtkt of optn Organ-

-, 38> Those who have any acquaintance widi mnsicBl notation
will, on inspection of the preceding table, perceive that the tbinl

What, compared nith Ihe •Irelehing veighti or lautmt ?

yfhnt it (lie relation, in point nrneulcneii, belwcEa the loaetof a |^pe
or ■ giiien lenglh bikI of one but hair (hat length !

Through bow iimnv ocUvei in muiie ma* pipei riia b» dimlniahinc
their length rrom SS feet lo 1 j inebea .'

* Ai proved bf S*v*rl.— Revue EDejclopediqne. JuiUet, 1S3I, aod
Ann. de Chim. voL iiivK^Eiil

irosicAL woxnxwk Mi

Mid fourth tftftTes maxked with the treble end bias ^lels, with the
singrle line between them, include three octaves, while the linet
above the treble clef and those below die base def piay all be
considered as so may ledger lines. Peraoss who have no know-
ledge of music may be informed, that this scale of nine octaves
not only includes the utmost range of musical tones ever employed
in practice^ but also that the notes at either extremity of the scale
are rarely introduced, but few instruments being adapted for the
production of such tones.

30, Tlie Aumbers at the bottom of the jfofegoiog scale denote
Ihe half vibrations performed respectively i^ a see<»d by the seva*
ral cMumns of air whose lengtns are stated above. Souorout
vibrations, like those df a pendulum, extend on either side of the
point occupied by the vibratinr body when in the state of rsst»
Suppose A B, in the marginal figure, to be an ex<*
tended string or wire ; if it be drawn aside to C, aaA
aufieied. to vibrate, its oscillations will carry it alter-
nately on either side of the central point E ; and ita
passase from C to £ may be termed a semi-vibiatioiiy
but when it has arrived at E, its momentum will canao
it to proceed toD, and thue a complete ^l^ratidn must
include a certain space on either side of the central
point or line of rest, to which the string will graduallT
return as its motion progressively declines throogk
the resistaace of the air.
40* If we consider the manner in which sound is propagated.
It will be manifest that it can only aflfect our ears by meana of
pemi-vibrattons, foe the soncvous undulatimis of air or anv other
conducting medium consist of contractions and dilations through
indefinitely minute spaces ; and the impression of any particle of
air on the drum of the ear must be made in its seBai-vibratiQn to*
wards-the ear, while the corresponding semi-vibration will act ia
the opposite direction.

41. Hence, in, estimating the relations between die tones of a
aonorous body, as the strinj^ of a harp or pianoforte, and the nuni'
ber of its isochronal vibtations, it is usual to reckon the complete
vibrations.; and therefore the jiumber of efiective or perfect vibrio
tions answering to each of the notes in the preceding scale will
be just half the number stated at the bottom of .the scale ; and
these numbers will correspond with those of the sonorous vibra-
tions of bells mentioned above.

42. Musical instruments yield Aot only octaves, but also a

How exteDMve if theaetnal range ordhiarily employed in maiical eonw
position ?

"^ftt reaembhtnee exists between the oieillstioiit of a pendulum and
the yibrations of sonoroas bodies ?

How is it eastomary to reokoo the number of vibrations of a sonorous
body ?

What is the differenee between one tone and another on a musical ia-
stroment, commonly called ?

X2

346 ACOTTSTICS.

ysriety of intermediate tones, which have certain relations to each
other ; and the difierence between one tone and another, is termed
an interval. When two tones or notes sounded together produce
an agreeable effect on the ear, the combination is called a musical
concord ; and when the effect is disagreeable, it is called a dis*
cord. It appears from experiment, that any two notes will form
a consonance or concord, more or less perfect in proportion as the
relation between the numbers of their vibrations is more or less
simple. Thus if one note is the result of a number of vibrations
double that of those belonging to another note, the former will be
an octave to die latter, ana their vibrations will be relatively as
9 to 1.

43. It has been already shown, that any series of vibrations
successively duplicates of those preceding them will form so
many octaves, all denoted in the gamut or musical alphabet by
tbe same letter. Indeed the agrreement between notes produced by
a series of vibrations, when those corresponding with the higher or
acuter note are exactly double, quadruple, ei^t times, &c., those
•orresponding with the lower note, is so perfect, that in musical
emnposition, octaves are considered as having the same effect with
notes whose vibrations are-«qual, and which are therefore said to
be ah unison.

44. The common musical scale or gamut imcludes seven inter-
vals, between one octave and that next above or below it, and
consequently it consists of eight notes takitig in the two octaves.
These notes have been distinguished by certain names, each form*
ed of a single syllable ; but it is more usual for teachers of music,
in this country at least, to designate the notes by the first seven
letters of the alphabet, and thus the octaves are always denomi-
nated by die same letter a^ that from which the scale begins.

- ^ 45. In any series of notes or tones the number of corresponding
vibrations will always increase in a certain ratio to the increasea
acuteness of tone ; and on the other hand, if the notes be pro-
duced by a string of a given diameter and tension, its length must
decrease in proportion to the increase of sonorous vibrations and
acuteness of tones* The relations between the numbers of sono-
rous vibrations and the lengths of strings required for the produc-
tion of the notes forming a single octave will appear from the fol-
lowing table of the notes of the gamut, or diatonie scale :

What 18 meant by the terms concord Bn6 diocord?

How is the relation of the numbers of vibrations required for two
notes, connected with their respective effects on tlie ear ?

What f elation has the numher of Tibralionsin a string producing a
l^iven tone to that of another sounding an octave below f

How are octaves regarded in musical composition ?

How many intervals hat the «ommon musieal scale or gattint ?

How are the notes designated ?
, How will the number of vibnitiont in any series of notes always be
compared to the acuteness of tone }

, When a string of given diameter and tension is eoDsSdersd, how will
the watoiest of notes vary f

MUSICAL tNTBUVALS.

Uf

Kan.e.ofNote. "tf*^^,^™^

Belatite Leogte
of Strings.

c

ut . •

1

1

D

- rei - -

9-8

8-9

E

ml - -

6-4

4-5

F

. fa - .

4-3

3-4

6

sol -

3-3

2-3

A

. la - .

6-3

3-5

B

•

- SI - -

15-8

8-15

C

ut - -

3

1-2

46. Sueh is the musical scale that appears to be foondedl on th«
relations between sonorous Tibrations and the peiceptive poweia
of man; for it has been generally adopted with slij^t modifica-
tions by the inhabitants of all countries with whose mnsie'we
hare any acquaintance. A comparison of this table with the scale
of octaves in a preceding page will show how the gamut may be
applied to successive octaves, the notes in every octave being
divided by similar intervals from each other.

47. The eight or rather seven notes of the gamut (the last
being an octave of the first,) are not however separated by equal
intervals. . On. observing the relations between the different nunk>
bers of vibrations, we shall find that the relation or interval be-
tween C and D is as 8 to 9 ; that between D and E, as 9 to 10 ;
between £ and F, as 15 to 16 ; .between F and G, aff 8 to 9 ; be-
tween 6 and A, as 9 to 10; between A snd B, as 8 to 9; and
that between B and C, as 15 to 16. Thus it appears that the
intervals S, ^, g9 a9 ^^ a* ^^ nearly equal ; and they are there-
fore regarded ss whole tones; bat the intervals $ and Jl^are but
htdi more than half either of the others, and hence they are
named semi-tones. In transposing pieces of music from one key
to another, attention must be paid to the places of the semi-tones,
and hence the principal use of the marks called flats and sharps ;
the effect of which cannot be understood without some practical
acquaintance with music.

48. Bat though this gamut or musical scale may be considered as
the groundwork of all existing music, it must be admitted that it
does not appear to have been always known or adopted in its pre-
sent state, but to have formerly consisted of those notes only
which are separated by complete intervals or whole tones ; for,
in the old Scotch and Irish tunes, the semi-tones are wanting,
and hence the peculiar effect of the national music of those
nations. And it has been stated that the oldest national airs

' What ia the relative nnroher of ▼ibrationt required to prodeee 6 of
the diatonic scale, when the C below it ia produced bj a number taken as
unity .'

What will be the relatiTe lengths of string in the two cases ?

On what is the generally received masicai scale apparently founded ?

Are all the interrals of the gnmnt equal ?

State the actual intervals between the several letters.

Of what (lid the gamut formerly consist f *

What netci of the asale are waotiog in lbs awns of teveral aationi?

248 ACOUSTICS.

of the Orientals, the people of the North of Europe, and erm
those of the Italians, exhihit the same characteristic oniission of
the notes F and B, thus increasing the intervals now occupied hj
the semi-tones in the received scale, so as to make Uiem exceed
whole tones,

49. The combination of notes into a successive series, in which
one musical tone or sound is heard at a time, constitutes melody
or air in music ; while the synchronous production of sounds, of
the union of two or more successions of musical tones is requisite
to form harmony or music in parts. There is thus a radical dis-
tinction to be made between melody and haimony, sometimes im«
properly confounded, the former consisting of music simple and
-unaccompanied, and the latter of musie in a more complex and
artificial form.

50. The construction of harmony or composition of accompani-
ments for musical airs requires an acquaintance with the concords
and disccnrds of the scale of notes ; in order that the composer
may know ho^ to introduce them in such a manner as to gratify
the ear and produce the liighest effect. Next to the octave, the
most perfect consonance of tones is that produced when the nom-
-bers of the vibrations of two notes are in the ratio of 3 to 2, or
when the lower note is formed by a string or other sonorous body
which makes but 2 vibrations, while the string fotmias the hig)^
-er note makes 3 vibrations. . Such a concord is oallea a fiflth, as
S in the preceding table ; C, the lower note, beinff formed by a
string which may be 1 foot in lenrthf and G, the mth note above
it, by a similar string only f of a foot in length*

51. If the ratio of the vibrations be as 5 to 4f that is, if the low-
er note makes 4 vibrations in the same time that the hi^^ier makes
5, the concord called a third will be produced, as |. %Vhen the'
ratio of the vibrations is as 5 to 3, the lower note making 3 vi-
brations while the higher makes 5, the concord called a sixth wiH
be produced, as J. And if the ratio of the vibmtions be as 4 to 3,
the lower note making 3 vibrations while the higher makes 4, die
interval will be a fourth, as ^, which is sometimes reckoned a con-
cord, as the effect in harmony is not unpleasing. The same ob-
aervation will Spply to the minor third, in which the ratio is that
of 5 to 6, as g; and the minor sixth, in which the ratio is as 5 to 8,
as ^, the lower note B making 5 vibrations, while the higher G
makes 8.

52. The discords are the second and seventh, the former of

What constitutes melody ?

To what art is the knowledge of musieal ooneordi and diseordt requi-
site ?

Which concurrence of notes gives next to the octave the most agreea*
ble impression ?

What are the relative numbers of vibrations produced by striags
yielding^ the concord ofjift/u?

How is the third produced ? tixth P fourth?

How are the minSr ^^rif and the minor tixfh respectivelv produced ?

Wliioh ^wo setjB pf aptes sounding together pro$lttoe^thp.4tc«rdsJ?

TIBRATIOHS* OR IfATSfl. 849

which pfodneed by two notes soundiiig together, die intertBl be*
tween which is only a tone or a semi-tone, is particularly disap
greeable. The major seventh is the discord produced by notes
whose yibrations are in the ratio of 9 to I61, as { ; and the minor
serenth is also a discord, arising from notes whose Tibrations are
in the ratio of 8 to 15, ad § : Doth these are sometimes intro-
duced.

53. The absolute number of yibrations necessary to constitute
any giren tone or musical note can hardly be determined with per-
fect accuracy ; for the tone of an instrument which naight be pr^
sumed to be permanent, as a bell or an organ^pipe, can hardly be
supposed to be unaffected by the state of the air ; besides whieht
there may be other circumstances which may cause occasional
yariation m the number of the sonorous yibrations eyen of a belU
Nor is it probable that the yibrations of a string or wire, under the
sanMT circumstances of length, diameter, and tension, would yield
exactly the same number of sonorous yibrations, in difieient states
of the atmosphere, and under different degrees of temperature.

54. Hence considerable difficulties woidd attend any attempt to
ascertain by experiment the relations between sounds or tones, and
^e yibrations of the sounding bodies. It appears, howeyer, from
a paper in the Memoirs of the Royal Academy of Sciences at Ber*
lin, 1893, th^t some results have been obtained, as the fruit of
experimental researches, which agree as nearly as eould be ez«
pect^ with theoretical estimates preyiously made, and whieh
may therefore serye as the basis of future calculations of the num*
bers of sonorous yibrations conesDondinsf with the di^erent tones
and semi-tones of the musical sealei

55^ The tone or note wfatose conesponding yibrations haye been
made the particular object of inyestigatien is that mariced A, oe-
Cupyinflf the' second space from the bottom in the> stave distin*
guished by the treble clef, being the note produced by the third
string of the yiolin, and a sixth aboye middle C of the pianoforte.
The following are the numbers of the yibrations or wavee in a
second connected with the note in question, as deduced from ob-
seryations made in*different orchestras:

Theatre at Berlin - - - 437.33
Italian Opera at Paris • - 434.17

• ' 'French Opera ... 431.34

Comic Opera - - - - 437.61
56. The difference between these numbers, serves to c(Mrrobo-
rate the remarks alrsady made on the difficulty of deciding by ex-
periment the absolute number of yibrations which may take place

• How are the miyor and (he minor §evenih seycrallj produoed ?

Is it certain that the same stringy or other sonorous body always yields
under apparently similar eireamstanees the same number of vibrations f
What musioal note has been the object of particular attention in expo*
rimeiits on this subject ?

• Hov near an agreement wjts found in respect to that note in the- loot
oreheitras at whieh it was czamioed ?

MO ACOOSTICB*

wkea the jMioeptitm. of any gp:Teii tone or maneal BOidid is pto-
dttced. Still the resulte obtained are TaLuable, as, by eompannff
thma with calcttlatioiis made on different grounds, measures ra
the ratios of sonorous yibrations may be dedaeed which seem do-
servinffof confidence.

57. The number 436| ia nearly a mean between those deriyed
from the observations made in the Parisian orchestras ; aiid by
adopting it as that of the number of sonorous vibrations corre-
sponding to the note A fonned by the third string of the violin
when open, the number 256 will be obtained as that representing
the vibrations connected with middle C, or the sixth below A.
For since the vibrations of A are to those of C-as 5 to 3, those of
the former being 436| in a second, those of the latter must be
356 ; because, as 5 : 436} : : 3 : 356. Now this last number being
taken to represent the vibration^ eorxespondin^ to the note C,
mariced in music by the tenor clef, the octaves m the desoending
or ascending scale will be denoted by numbers which are so ma^jr
duplicate multiples of unity.*

58. When an extended string is madf to vibrate by striking it
or drawing across it a violin-bow, it will yield a tone depending
on its dimensions and tension; but besides this, which may be
called the fundamental tone, the string will, when the nbhition ia
oaused by striking it, emit not only its fundamental or proper note,
but also other relative tones, espedally the third and the 6&k
above the proper note. The co*existence of these relative tonea
with the princi|>al one depends on the excitement i^ vibration^
corresponding with the division» of the string which would fom
*he principal concords to the fundamental note. When the string
is made to vibrate by means of a violin-bow tiie sound is simple
and distinct, arising firom the fundamental tone only«

• 59. If a single string of a harp or pianaforte be struck* oChei
strings of the same instrument tuned in fifths and thirds to the
former will be thrown into the state of sonorous vibration, and aa
the original tone becomes weaker the relative tones or symp«tih«tie
conc<^s will be more distinctly perceived. The effect produced
on strings by the vibration, of other strings near th^on, tuned so aa
to form concords, may-be visibly demonstrated by placing small
bits of paper bent in4he form of the letter Y inverted thus a on
one or more strings, so tuned as to yield tonea an octave, a mlh,

What open 8triii| of the violin ewreipoiids to tike note ia ^ettioD I

What namber m vibntiont nay we assume for its rate of vibntieft
per second ?

What will be the number for mtdcflSs C of the piano ?

How manj times can we divide this number and its successive quotients
by 2, before we an'ive at 8, die lowest numbef of vibratioM wliieh 8a*
vart found to produce andible sounds }

Can a string by a single stroke be made to yield more than a aiogis
tone?

Illustrate this position.

• Sec Scale of OctoveS, of thtji artiolc. No. 87.

TiBiULTXiro n;A.TX8. m

or a t^ii'rd above a (larticalar string' ; and, <m earning the latter te
Vibrate strongly, the oilier strings will suffer corresponding vibrar
tions, as will appear from the bits of paper falling off. Hence sii^
galar effects are sometimes produced by the sympathetic inftaenoe
of s^aoroQs vibrations.

60. An account of some remarkable experiments illustrative of
the subject under disenssion is given oy J. B. du Hamel, a
French philosopher of the seventeenth century ; which are the
more deserving of notice, as they an cireomstanttally recorded.
Afler observing that a glass cup or goblet ma^r be broken by a
man's voice, the wriiter adds, ^ First of all it is aeoessaiy that
the tone which the glass is adapted to yidd should be ascertained
liy ringing it, as may be done by giving it a slight fillip with the
linger ; then, the voice being accommoaated to Siat tone and gm-
dually augmented in loudness and raised to the octave above the
original tone, the imperceptible minute particles of the glass
sh&en by reiterated concussions will be agitated with tremnlons
undulations, which^ increasing by the continued operation of the
concussions, will at length attain such force that the glass will
fiy in pieces. Some caution is necessary in the choice m a glasay
whi6h should be quite clean, free from any lines or flaws on the
surface, and capable of yielding such a tone when struck, as the
Toice of tHe inaividual mridng the trial can easily reach."

61. Another experiment eniibited at the same thne or place is
also thus describea : ^ Two ghtss goblets are to be procured, into
which water is to be pourea to the depth of ^o or three inches,
and they must then, by the addition of more water to one or Uie
other as may be requisite, be made to yield the same tone when
struck. This having been effected, if a small portion of bent wire
be placed across the edge of one glass, then on rubbing the edge
of the other lightly with a wet finger, the senorous vibrations thus
excited will be communioated to the fflass with the wire on ite
edge, and whil& sound is produced the light fragment of 4he wire
will be seen dancing as it were to the musie of the glasses."*

63. The sonorous vibrations of plates or disks formed of elastic
solids, as glass or metal, mav be traced and rendered visible, by
methods pointed out by Dr. Chladni, whose researches concerning
the doctrine of Acoustics have been referred to elsewhere. He
ascertained that sounds might be elicited from plates of glass

Off what does the effeet probably depend ?

What oeeurs when a nngle ttriiig of^an iMtrmnent if ttmek }

How may this be made visible ?

What remarkable effects of sympathetic vibration were obtained by Da
Hamel ?

In what manner does the experiment succeed with the greatest eer*
tainty ?

In what manner did CMadni operate to produce mnsieal vibratioDS in
<ilastie plates ?

* J. B.tfai Hamel Operom Philosqph.» t li. Nerimb. 1681. 4to. pp.
4I1» 565.

252 . . AcouiTics.

ground smoolli on Hm edges, by drawing the bow of a violin over
any part of the edee of sach a plate ; and that when sand had been
previonsly strewed over the surface of the plate, it would become
arranged in certain lines according to the manner in which the
plate was supported. M . Oersted, who repeated, with various
modifications, the experiments of Chladni, ascribes the production
of iines in sand, or any other light powder, as the dust of lycopo-
dium, strewed on vibrating plates, to electricity.*

63. Mr. Faraday has recently proposed a different explanation
of these phenomena, which attributes them to the formation of
currents in the air surrounding the vibrating plate which, pro-
ceeding from the more fixed to the agitated parts of the plate, pass
upwards and involve in* their vortex any light particles of matter
which they encounter. He showed that the current of air could
foe interrupted by walls of card, when the light particles took
different directions. He observed that particles of heavy sub-
stances, as sand, went to the lines of rest because the current of
air was too weak to carry them in its course ; but that light bodies,
as powder of lyeopodium, being easily affected by the air in its
motion, passed m a contrary direction.

64. In confirmation of this view of the subject Mr. Faraday
stated that when plates are made to vibrate i^ water instead of
air the effect is different, particles of sand being then carried from
the quiescent to the agitated parts of the plate, as the lighter par-
ticles were in air ; and also, that when plates are made to vibrate
in a vacuum, even the lightest particles pass to the lines of rest,
there being no current of air to sweep them in the opposite direo-
tion.f

65. These peculiar figures formed on vibrating plates, though
apparently resulting from simple causes, present sometimes singu-
lar appearances. The arrangement of the lines of sand, or other
substances, depends on the manner in which the vibrating plate
is supported, and the point at which the violin-boMT is struck against
its border ; as also on the form of the plate, and other circumstan-
ces already noticed. Some idea of the nature of these figures may
be derived from the annexed repnesentations ; the first figure being
produced by holding a square plate of glass with a pair of toogs
in the centre, and passing the bow over the middle of the edge at

To what did Oersted attribute the formatioa of nocb/ Unet in Chladni'i
experiments ?

now did Mr. Faraday explaia them ?

What occurs when plates vibrate in water ?

What in the meuttm of an air-pump ?

On what does the peculiar arrangement of sand on the Tibrating plate
appear to depend f

What arrangement of lines will be given by a square plate held by the
eentre and rubbed with the Tiolio-bow in the middle 'of one edge ?

* See Nicholson's Jourital of Natural Philosophy. 8vo. toI. x. p. 258.
t Arcana of Science, U30, p*77i ft'om Jooroal. edited at the Royal
Institution.

MUSICAJk QftfimiENTS. 9§9

either side; uid the other anangemeats depend on the diape of

the plate and the mode of striking it.

6j6. It is in consequence of the resonanees or sjrmpathetie proj^a-
gation q{ sounds, that in a large apartment^ tones are sometimes
emitted from the walls, floor, ceiling, or farnitore ; owing to the
eiiccitement occasioned by the tone of an instrument or a man's
yoice acting on some object adapted to yield a tone in concord
with the original tone. It may even be observed that one part of
a floor or any other surface will be thrown into the state or sonor-
ous vibration by one sou^d, and another by a different sound ; and
the tremulous motions thus produced in various objects may be
perceived by the sense of touch.

67. The ancient Romans were well acquainted with the doctrine
of resonances, and availed themselves of their knowledge in orde?
to facilitate the propagation of sound through their theatres. The
method they adopted was to inclose in the walls of those build-
ings hollow globular vessels, so fixed as to be excited into sonor-
ous vibration by the voices of the actors, and thus add considera-
bly to their effect.

98. Musics^ instruments, how much soever they may differ one
from another as to the mechanical modes by means of which they
are made to produce soniferous vibrations, have one common pro-
perty, namely, that they all yield the same tone relatively to the
numbers of their vibrations. Hence the term concert pitch, or the
isound qf a fundamental note corresponding to a certain number of
vibrations performed in a given Ume by the sonorous parts of
several instruments which are to be used in conjunction. Differ-
ent methods are adopted by musicians for obtaining an invariable
tone, from which they may compare and regulate any number of
instruments to be used in concert; and the tone of this note being
decided, they proceed to adjust the strings of violins, violoncellos^
and other such-like instruments, so that they may all correspond
with ^ach other, as well as with those instruments which by their
construction are fitted to yield permanent tones. This operation
is called tuning, or putting instruments in tune.

69. Sometimes the fundamental tone is ascertained by means

MThat oceations the emisuon of tones from the walla, floors, and fat •
niture of an apartment ?
' What advantage was taken of this prinetple by the aneient Romans,?

What dominon property have all musical instruments I

What is meant by Uie term coocart pitch ?

What three methods are employed oy anasMiaiis to ^t the invariabU
tonen or cQRfmrt pitsh ?

264 ^ A001T8TIC9.

of a piteh-pipe, which consisto of a tube capable of being length-
ened or shortened at pleasure by the introduction of a moveable
plug ; so that by blowing into it at the mouth-piece, either of the
notes of the gamut may be produced. Another instrument for
obtaining fixed and determined tones is the monochord,* which
is merely a string or wire of given length and diameter, the ten-
sion of which may be regulated by certain weights hanging from
one end, while the body of the string passes over two bridges or
other solid supports, and the other end is firmly secured.

70. But the most usual instrument employed by musicians as
the index of a 6indamental tone is that styled the tuning-fork. It
is a steel rod curved nearly into the figure of a sugar-tongs, but
having a short handle fixed to the convex si^e of the curved part,
and terminating in a knob : it may be made to yield sonorous
vibrations, if it be held by the handle so as to leave the prongs
free, and, after striking one of the prongs smartly against the edge
of a table or any other solid body, setting the knob a^nst the
table. The sound or tone emitted must depend on the dimensions
of the rod or its prongs : those that are used for tuning pianofortes
or harpsichords yielding the tone called middle C ; and other tun-
ing-forks giving the sixth above it, or A, the note which ought to
be produced by the third open string of the violin, whence the
other strings of that instrument are aSjusted.f

71. Instruments of music may be arranged in classes, according
to their forms or modes of action. It will be sufficient here to
distinguish them into stringed instruments ; pulsatory instruments
including bells, drums, &c, ; those in which sound depends on the
vibrations of elastic rods, hemispheres, or plates ; and wind in-
struments. The varieties of the first and the last of these classes
are extremely numerous ; and many of them were invented at a very
early period. It has been questioned which of the two may be
justly reckoned the most ancient. A recent ingenious writer
seems inclined to decide in fayour of stringed instruments. He
says, *^ The lyre or harp is surely as ancient as any instrument on
record. The mythologist ascribes the idea of producing sound by
the vibration of a string to Apollo ; which is said by Censorinus
to have suggested itiself to him on his hearing the twang of the
bow of his sister Diana.*'^

72. Among the principal varieties of stringed instruments are
the violin, tenor, violonc^lo, and double bass, in all which the

What is the coDStruction of the monochord ?
Into^ what four classes are musical instruments distinguishable ?
Which classes present the greatest variety ?

On what circumstance in the action of strineed instruments does the
performer chiefly rely for the extension of their range of notes ?

* From the Greek M&ve;, one ; and XepiTii, a chord, or string.

t The tuning-forks of diflferent nations give different tones for the
aame letter. A London and a Vienna Afoik. have sometimes been found
about one-third of a note apart— Ed.

f Philosophy in Sport made Science in Earnest, edit 188S, p. SOO.

PUL8ATQRT UifTAITIIENTfl.

relative grayity or acutenesB of the tones ihey emit depends peitlj
on the tension and diameter of the strings, and partly on their
lengths, which are regulated by stopping them in certain parts
snccessively by the application of the nngers, principally near
the neck of the instrument, while the stopp^ or open string, as
may happen, is made to vibrate by drawing across it a bow
armed with horsehair. As more than one string may be put into
the state of sonorous vibration at one time, harmony or musie
in parts, as well as melody, taxy be elicited from the violin and
similar instruments.

73. In the hands of skilful performers the violin exhibits nnri*
vailed powers ; as those who have witnessed the magical execu-
tion of Paganini, will in general be readily disposed to admit.
Those who have never hea^ him may acquire some fiunt idea of
his extraordinary skill, from the circumstance of his being able to
produce abundance of excellent music from his instrument, after
naviog made a monochord of it, by taking away all the strings
except one. •

74. The guitar somewhat resembles the violin in figure and
construction, but it is played on usually by twitching the strings
with the fin^rs, and a variety of notes may be produced by stop-
ping the string with the left hand, so as to regulate the numbers
of Sieir vibrations and consequent tones. The perfcmner gener-
ally uses the guitar to furnish an accompaniment to the voice : its
power alone l^ing inconsiderable. The harp is likewise played
on with the fingers, but ita strings are numerous and all open*
The pianoforte and the harpsichord have also distinct strings for
each tone and semi-tone ; and like the hsjp they are adapted for
the performance of music in parts ; so that they may serve either
for playing symphonies or other pieces of music wholly instm-
mental, or for accompaniments to the voice.

75. Pulsatory instruments of music display considerable varie-
ties of form, comprising the double drum, the opposite ends of
which yield different tones when struck, for the parchment cover-
ing one extremity is, by regulating its relative degree of tension,
made to yield a sound which is a fifth in tone different from that
of the other extremity; kettle-drums consisting of copper hemis-
pheres, the open ends of which are covered with parchment, and
two such drums properly tuned being used, they may be intro-
duced instead of a double drum, but will be distinguished by a pe-
culiarity of intonation, though yielding the same notes ; the tam-
bourine, a well-known instrument, resembling in principle the
preceding ; besides some others of a similar nature.

What remarkable faet proves the power of the violin ?

In what chief circumstance does the g^iitar differ from the violin ?

For what purpose is it generally employed ?

Are the strings of the harp, pianotorte, and harpsichord capable of
being varied in tone by alterations of length at the pleasure of the per-
tormer ?

Enumerate some of the chief pulsatory instruments.

▲CKytMTiOi.

76. Bells, gon^ &e., are open hemispheres, er conical instni-
ments made of some sonorous metals : the latter of which, used
in China, are large and very powerful instniments. Among t&-
cently-invented musical instruments is one called the Harmonicon,
Constructed hy ranging in one or mcNre lines a numher of small
oblonff disks of glass, each adapted, by its vibrations when struck,
Co yield one of the notes of the gamut or common musical scale,
inchidinff two or more octaves according to the size of the instru-
ment: the disks are fixed securely 'at one end only, so that they
vibrate freely on striking them with a hammer much like the ham-
iiieTS of a pianoforte.

77. Glass hemispheres or bell-shaped goblets, fixed in a frame,
{knd tuned to the gamut, by pourin? in more or less water, form
an agreeable instrument of music, played on by striking the edges
wiA a violin-bow, or by being thrown into the State of sonorous
vibration by ^ntly touching them with wet fingers. There are
several varieties of these instruments, which, as well as the pre*
ceding, have received the names of harmonica* and harmonio
glasses.

78. Wind instruments display no less Variety in their construe'
tion and mode of action than stringed instruments ; and in the
opinion of some antiquaries they were invented at a more remote
period than the latter. The general principle thev involve is
that of the production of sounds by the vibrations of columns of
air, usually contained in tubes, whose relative lengths and those
of the included columns determine the numbers of the synchronous
waves or vibrations to which the tones or musical sounds emitted
owe their character as to gravity or acuteness.

79. Instruments of this class have been distributed into three
kinds: (1.) those in which the contained column of air is made
to vibrate by blowing forcibly into one end of an Open lube ; (|S.)
those in which the vibration of .the air is caused by hlo^ng
through a solid month-piece, at one end, which merely limits the
size and figure of the aperture, and thus 'adds to the force with
which the air is introduced through it; (3.) wind instruments
played on with a reed or very elastic mouth-piece, the primanr vi-
brations of which highly augment the sonorous vibrations of the
column of air.

80. 'There is likewise a distinction to be made between tubes
open at both ends, without any lateral apertures, and those whidi
bave several such apertures, the obvious effect of which must be
to lengthen or shorten the tube, or rather the column of air in it,

To what nation is the gong chiefly confined ?

How are harmonica ccAistnlcted t

In what manner may the tones of musical glasses be varied so as to tone
an instrument constructed of them }

What circumstance determines the gravity or acuteness of tones giren
by tubes in wtml instruments?

/Into how many and what classes are instruments of this nature distin-
guishable ?

WIND IM8TR171CENT8. M7

OR tbe dimenmons of which the sonorous yibiaUont and eoneomi-
tant sounds must depend.

81. Ampng the first mentioned species of wind instruments
must be included the inimpet, the bugle-hom, the French^oniy
Pan's pipes, and some others, which however they may differ in
form, or in the effect of the tones they yield, are all made to sound
by blowinpr through a circular aperture ; and from these the Ger^
man flute is distinguished merely by having the aperture through
which air is admitted in the side of the tube, while the end
is closed. . To the second species of instruments belongs the
flagelet, which is played on by means of an ivory mouth-pieccy
having an aperture of invariable dimensions. The third species
of wind instruments comprehends several varieties, some having
mouth-pieces possessing a slight degree of flexibility, as the clsk
nonet; others are played on with a reed, forming a highly flexi-
ble mouth-pijBce, as the hautboy and the bassoon. The diversity
of sounds produced by different sets of organ-pipes, answering to
the respective stops of the instrument, depend on the pecmiar
forms oi the pipes, and especially on the manner in which the
air is admitted into them.

82. The Jew's-harp, an instrument too generally known to need
description, and commonly despised as utterlv insignificant and
inharmonious, is however deserving of particular notice, not only
as being a wind instrument affording sounds on somewhat diffe-
rent principles from those above described, but li^iewise becausCf
in the hands of more than one performer, it ha^ been found cap^
ble of producing considerable effect, and exciting the admiration
of musical amateurs. As the Jew's-harp has no cavity it is al-
most inaudible when struck, till it is placed between the lips and
teeth of the performer, and thus the sonorous vibrations on which
its tones depend are formed in the moath, the tongue or bent wire
belonging to the instrument acting the part of a reed.

83. Three tones or notes only can be produced by means of a
single harp ; the lowest of which may be termed its fundamental
note, and the others are its principal concords the third and fiflh.
From a scale so limited it would be impossible to derive melody,
much less harmony; and therefore the instrument was neglected
by regular musicians, though commonly used among the peasantry
in many parts of Europe, and particularly in the Netherlands and
in the Tyrol. Some kind of improvement was effected by the
Tyrolese, by uniting two Jew's-harps, or using two at once ; aod
this method was adopted by a Prussian soldier, mentioned in the
Memoirs of Madame de Gemis, as having acquired the art of play-

Whftt 18 the purpose of the holet usually seen in instruments of this
sort ?

Give examples of each of the three classes of wind instraments.

On what do the different tones of organ-stops depend ?

What is neeessary to the production of sound by the Jew's-harp ?
. What is its range of scale, and what the notes it can actually yield?

By whom has it been extended and improved ?

t 2

26d AcxMmrtcs.

Sng on this instrament with nb nmeh skill and tast^ lliat he inras
heard with pleasure and surprise by the king, Frederic the Great,
who possessed considerable knowledge of music, and was him-
self a good performer on the German mitek

84. But to the more recent labours <^ M. Eulenstein we are in-
debted for the complete developement of the powers of this little
instrument. He deroted ten years to the study of its capabilities,
and the means of removing its defects ; and having ascertained
the compass of tones belonging to it, as stated above, he conceii^
ed the idea of extending its power, and snpplying Uie intervals
wanting, so as to complete the gamut through several octaves, bjr
joining sixteen Jew's^iarps, atra ihen tuning them by fisttng morb
or less sealing-wax at the extremity of the tongue. By means of
this construction he effected his object, ar by rapidly changinir
from one harp to another, he could elicit any iseries of tonee, and
perform pieces of music, in a manner ^hich delighted and iisto-
Idshed those who heard him.

85. It appears bo^ from theory and experiment, ihat in the fun-
damental sound of a tube open at both ends, the portions of thb
included column of air on the opposite sides of ^e centre of the
tube move in directions contrary to each other. This principle is
ingeniously confirmed and illustrated by Mr. Wheatstone, in a
papier read before the Royal Institution of London, Mareh 16,
1832 ; when he exhibited the phenomenon in question, by meanft
t)f aii apparatus consisting of a leaden tube about an inch in dia-
meter and thirteen inches long, bent nearly into a circle, so that
its two extremities might be opposite to each other, vrith a small
Space between them. Within this space, equidistant from each
end of the tube, was held the vibrating part of a square plate Of

flass thrown into a state of vibration, either by means of a violin-
ow, or a hammer, so as to produce its lowest sound, or that de-
noted by Chladni's first figure. By this arrangement, the plate
advancing in its vibration towards one end of the tube, and re-
ceding at the same instant from the other, the effects neutralist
each other, and no resonance, or augmentation of the original
sound takes place. Ill the middle of me tube was a joint, \niich
allowed either half to move independently round the axis of th^
tube ; and thus the two ends could be brought -to the opposite
sides of portions of the plate which were vibrating at the same
moment on contrary sides of the neutral plane : in this case the
impulses were made at the same instant towards each end of the
tube, and the augmentation of sound was considerable.* Hence

What remarkable tttentioa baa been bestowed on the developement of
Ms poweri }

How did Ealenatein tune his instrument ?

What appears to be the kind of motion whiefa takes plaee in the co«
lamn of air within a tube c^ien at both ends f

Desoribe Wheatstone's method of exhibiting this prineiple.

"•- ■•■

• Report of British Assotiation^ p. 5S6.

VIBRATION OF COLUMNS OF AIR. S59

it appears that in a tube or pipe, open at botK ends; tiie ribralln^
eolamn will be double, and therefore only half the length of that
in a similar tube closed at one end ; so that the latter would yield
ft tone an octave lower than the former*

86. Mr. Wheatstone investigated the modes of vibration of co^
lumns of air in conical tubes, and ascertained diat the air in t tube
of this form, excited into vibration at its closed end, or at the sum-
mit of the cone, yielded the same fundamental sound, and th^
tome series of harmonics as a cylindrical tube open at both ends.
Thus he showed that the trumpet, French-horn, and hautboy pipes
of the organ, all being conical pipes, produced the same sounds
•s the Cremona pipe, a cylindrical tiibe^ excited in the same man-
ner, and only half their length. He likewise compared the haut-
boy, a conical tube, with the clarionet, a cylindrical tube of the
i|ame leuffth,* and demtmstrated that in the former the fundamen^
tal sounds were the same, absolutely and relatively, as in the
flute, a tube of the same length, open at both eads ; and that in
^e latter the fundamental sounds were relatively as those of a
tube of similar length closed at one end.

67. A tube or pipe, the upper aperture or mou&-piece of whKbK
Is placed dose to the lips, as in the ease of the trumpet, Ffench-
hom, or clarionet, is to be considered as open at the lower end
only ; and thus its tones are relatively deeper and more powerful
than ^ose of the German flute or flagelet, tubes open at both ends ;
for the aperture through which the flute is blown or made to
sound, is not covered by the lips of the performer; and though
the mouth-piece of the flagelet is covered in playing on that in-
strument, it is reduced to the state of a tube open at ho&i ends,
in consequence of its having a lateral aperture near the upper ex-
tremity.

88» The theory of musical sounds may be elucidated from the
consideration of the manner in which tones are produced from ^e
French-horn. As the harmonics or concords of a fundamental
bote may be obtained by the division of a vibrating string into
certain proportions, so the same series of tones may be formed by
tiie spontaneous division and subdivisi(^ of the column of air con^-
itained in the French-horn. When this instrument is used ib con^
cert, it must always be adjusted to a certain length, by increasing;
or diminishing the number of the cranks, or circalar tubes of which
it is compost ; so that the gravest tone it will yield may corre-
spond with the key-note or fundamental tone of the piece of musio

To what result did his inyestigfttion lead ?

What relation has the tone of a conical to that of a cylindrical tube of
die same length ?

How is a tube or pipe to be regarded when the mouth-piece fits close
h> the lips }

How 18 the French-horn adjusted to a particular concert-pitch ?

Which of its notes ought to be adjusted to the key-note ?

* The bell-shaped part of the clarionet has no effcict on its tone.

260 AC0VSTIC8<

to be perfonned. Sappose this tone to be C, if then the horOf
properly adjusted, is blown gently, this note will be heaid ; a
stronger blast will double the number of the sonorous vibrations,
and produce an octave above the first note ; by increasing the force
of the blast may be obtained in an ascending series a fifth, then
an octave above the second C ; then a third, a fifth, and an octave
above the third C ; then a fourth octave, including nearly all the
tones of the common musical scale.

89. Thus in the French-horn, the common bugle-horn, and
other instruments formed on the same plan, the different tones are
produced by varying the impulse given to the included column of
air in blowing them ; while in such instruments as the Grerman
flute, the same effect is more perfectly obtained by altering the
length of the vibrating column, which is done with the requisite
ease and rapidity by the apertures alternately opened and closed
by means of the fingers or keys.

90. The iEolian harp, in point of construction, is a stringed
instrument, but its sonorous vibrations are caused by the impulse
of the air ; and its tones may be characterized as the music of
nature improved by art. It usually consists of an oblong deal
box, four or five idches hi^h, and aaapted to the aperture formed
by nearly closing a sash window, so that the current of air passing
throuffh the opening may sweep over wires or harp-etrings^ ex-
tend^ lengthwise upon the top of the box, in which there must
be sound-holes like those of a violin, and the wires are to be
supported and stretched by a bridge at either end. Four strings
or wires may be tuned so that the third may be an octavo above
the first, the second a fifth above the first, and the fourth a fiAh
above the second. But various arrangements may be adopted, in
consequence of any of which, alternately increasing or diminish-
ing strains of wild harmony will be elicited from me instrument
by the fluctuating impulse of the wind.

91. A colossal imitation of the instrument just described was
invented at Milan in 1786, by the Abbate Gattoni. He stretched
seven strong iron wires, tuned to the notes of the gamut, from
the top of a tower fifty feet high to the house of a Signor Mos-
cati, who was interested in the success of the experiment ; and
this apparatus, called the Giant's Harp, in blowing weather,

Jieldea lengthened peals of harmonious music, now swelling in
oud chorus, and seeming to fill the atmosphere, then dying away
on the breeze like the soft tremulous murmurs of a common iEo-
lian harp. In a storm this aerial music was sometimes heard at
the distance of several miles.

What will enable the perfonner to increase the aenteness of the tone
to an octave ?.

In what manner does the mode of rarying the acuteness df sound in
the common bugle differ from that in the German flute ?

What is the common construction and mode of applying the iEolian
harp ?

What aceount is given of a remarkable instrument of-this conitnie-
tipn?

VIBRATIONS OF 1N8SCT8. 861

93. The music of nature exhibits boundless Tariety as to the
combinations of tones and ^ die several modes in which they are
produced. But besides the warbling of the feathered choir and
abundance of other vocal sounds with which we are familiarly
ac(iuainted, there are some which are constantly emitted under
cntain circumstances, yet, though curious and mterestingr, they
seldom attract our notice.

93. Bees, gnats, and many other winged insects in their pas-
sage through air excite sonorous vibrations by the viewless
flutterings of their wings or other membranous parts of their
structure. The intermitting note of the grasshopper is probably
the result of a similar mechanism ; but some insects of this tribe
seem to be furnished with a peculiar organization for the pr(^
ductioa of their music.

94. Dr. Hildreth states that the American Cicade, or locusts^
are furnished with bagpipes on which they play a variety of notes.
<* When any one passes they make a great noise and screaming
with their air4>ladd« or bagpipes. These baes are placed undet
and rather behind the wings* in the axilla, and something in tho
manner of using the bagpipes, with the bags under the aims* I
could compare them to nothing else; and indeed I suspect the
ilrst inventor of the insteument borrowed his ideas from some in-
sect of this kind. They play a varbty of notes and sounds, ona
of which nearly imitates the scream of the tree-toad."

95. Some birds yield musical tones through the percussion of
fhe air by their wings in flijj^ht This circumstance, which pel>*
ha^ has escaped the attention of naturalists, is particularly ob*
senrable in the lapwing, or as it is sometimes called from its crft
the pewit. This bird is an inhabitant of the furze-clad downs oi
Wiltshire; and when it stoops near the ground, in its circling
couise through the air, as it approaches the observer, a sound may
be heard resembling the distant tone of a French-horn, entireljr
distinct from the dissyllabic scream from which it derives its
provincial name; and which is formed like the cries of other ani-
mals in the throat or larjrnx. The peculiar clanging tone iiist
m^itioned seemed, as far as could be guessed from repeated ob*
servations, to be nearly the same note with the middle C of tha
harpsichord. It is manifestly caused bv the reverberation of the
air against the hollow sides of the broad wings of the bird in its
rapid wheeling flight; and it is heard only when it happens to
come very near the observer.*

Qow are winged insects generally found to produce sound t
How is the American locust farni bed with musical instruments ?
By what means other than die Toiee are birds sometimes found to give
musical tones ?

* The common night-hawk affords a familiar illustration of the effbct
of rapid stooping through the air, producing the ** boo-oo** often heard on
a warm summer evening. — ^En.

263 ACOUSTICS.

The Human Voice,

96. Among the most curious works of nature, must be reck-
oned the organization on which depend the tones of the human
voice. The most ancient physiologists regarded the trachea or
windpipe as the immediate organ of sound, comparing it to a flute,
and ascribing the Yoice to the impulse of air against its sides in
its passage into the lungs. But Galen controverted this erro-
neous opinion, by showing that the voice is formed during the
expiration of air, or its passage from the lungs, and in its es-
cape from the larynx, at the back of the mouth. Besides the
lungs, which propel air in the same manner as it is propelled by
a bellows into the pipes of an organ, ^e parts ess*ential to the
production of voeai sotmda are the trachea or windpipe,^ the larynx,
and its appendages.

97. The windpipe, as the term implies, is merely a cartilaflrl-
nous canal through which the air issues from the lungs; ttie
larynx is an enlarged continuation of the windpipe, formed, like
it, of cartilage or gristle, membrane and muscle ; but it is more
complicated, •terminating above in two lateral membranes, which
approach near together, leaving only an oblong narrow opening,
called the glottis. The cartilages of the larynx admitting of some
degree of motion by means of their attached muscles, tiie mem-
branes of the glottis, which are connected with them, may be
extended or slackened, and thus the vibrations of the air passing
through the glottis are regulated, and sounds are modified as to
tone. Tendmous cords or ligaments are also extended within
the larynx, which are supposed by some physiologists to co-
operate with the membranes of the glottis in producing sonoroas
vibrations.

98. The glottis, or rather the membranes which compose it,
thus appears to form the immediate organ of sound ; which has
been aptly enough compared to the reed of a hautboy, since * c is
composed of thin vibrating plates, with a narrow variable opening
between them. But the surpassing delicacy of the orgranization
in the construction of the glottis abundantly demonstrates the
superiority of the works of nature over the most elaborate efforts
of art. Dodart, a French physician, who, in the beginning of the
last century, investigated the structure of the vocal orgrans, made
a calculation whence he inferred that the intervals of sound capa-
ble of being perceived by the ear correspond to contractions of
the glottis less than 1-9632 part of its diameter* It is probable,
however, that the diversity of tones is caused not merely by aUe-

What part of the oreans of speech did the anoientg regard as the im-
mediate cause of sound ?

Who controverted this opinion, and on what ground ?

What, besides the windpipe, is essential to the production of Tocal n^
terance ?

What is meant by the larynx? what, by the glottis?

What office does the glottis appear to perforin ?

ORGANS OF SPEECH. . 263

rations in the dimensions of the glottis, hut is partly dependant on
the lengthening or shortening of the entire tuhe of the trachea, in-
cluding the larynx, and hy corresponding alterations in the form
and size of the cavity of the month. Yet the power of varying
the tones of the voice in singing must depend cniefly on the sus-
ceptibility of the membranes of the glottis, the firmness and elas-
ticity of the cartilages of the larynx, and the strensrth of the mus-
cles by which they are moved, and that of the muscles of the chest
concerned in respiration.

99. That the sound of the voice wholly arises from the pas-
sage of air from the lungs through the glottis, is proved by the
fact that when the windpipe is wounded below the glottis so that
the air .in expiration passes through the wound, the power of
forming sounds is destroyed ; while a wound in the throat which
leaves the glottis and parts below it uninjured, produces but little
efiect on the voice ; and if a piece be cut out of the windpipe of a
man or any animal similarly constituted, the power of uttering
sounds of which he is thus deprived will be restored by carefully
closing the artificial opening in the windpipe^ so that the air no
longer escaping by it, may pass through the glottis as usual.
Hence those unfortunate persons who cut their own throats so as
to wound the windpipe but not the large blood-vessels, imme-
diately breathe through the wound and hecome silent, hut as soon
as the wound is dressed and the air no longer passes through it,
the power of speaking is restored.

100. The tracheal canal, including the larynx, may even be
entirely detached from the animal to which it belongs with-
out losing its property as a vocal instrument. The celebrated
naturalist Cuvier, having cut off the head of a screaming bird so
as to leave the glottis and parts below it entire, the creature stilt
uttered cries for some time after its decapitation, the organ of
voice remaining uninjured. An animal recently dead may be
made to utter sounds as when living, as appears from experi-
ments made by M. Ferrein, in 1741, and repeated by M. Piot, a
few years since. The latter gentleman employed in his researches
the larynx of a pig, with the trachea attached to it, and to the
opening of the latter, he fitted the bellows of an organ, and by
varying pressure on the larynx with his hand he could increase
or dimmish the aperture of the glottis while forcing the air through
it, so as to imitate exactly the grunting of the pig. The same
philosopher subsequently constructed an artificial glottis, the
lamina, or membranous plates forming the opening being made

On what operations besides the enlargement and eontraotion of the
glottis is variety of tone supposed to depend ?

What diract proof have we that the voice is formed at the glottis ?

What experiment did the celebrated naturalist Cuvier institute on this
subject f

u what manoer did Biot imitate thf voice of the living animal ?

364 ACOUSTICS.

of gram elastic ; and having adapted it to the pipe of a pair of bel*
lows, he was thus enabled to produce vocal sounds.*

101. The aperture of the glottis is naturally more contracted in
females and in males before the age of puberty, than in adult
males ; and therefore, women and children have shriller voices
than men, the difference of tone commonly amounting to about
an octave. The entire compass of voice in female singers is usu-
ally more extensive than in men ; for though their scfie of musi-
cal sounds commences at a relatively high tone, it ascends yet
higher in proportion.

102. The human voice may be so modulated as to form a vast
variety of musical sounds or tones with rapidity and precision fai
beyond the effect of any instrument formed by art ; for V9cal mu-
sic, on account of its superiority over instrumental music, in point
of expression, must always be regarded as the highest excellence
of the art. But the vocal organs not only afford tones or sounds
distinguished by relative gravity or acuteness, but also modificar
tions of sound, forming the basis of language ; and to the posses-
sion of the faculty of speech, and the interchange of vocal and
audible signs, man is greatly indebted for his superiority over the
brute creation.

103. It is during the transmission of the sonorous vibrations
through the mouth that the peculiar effect is produced which com-
municates to the ear the Sounds of letters and words, constituting
language. The most simple of these articulate sounds are those
corresponding with the vowels, the differences between which
depend on emarging^or contracting the cavity of the mouth while
they are uttered. The consonants, which, it hardly need be ob-
served, cannot be enunciated without the addition of a vowel,
require more complicated motions of the parts of the mouth ; and
hence some of them are called gutturals, as being formed in the
back part of the month ; some dentals, as requiring the application
of the tongue to the teeth ; and others labials, because they cannot
be distinctly pronounced without jnoving the lips.

104. The success of the researches of men of science concern-
ing the theory of vocal intonation has occasioned different attempts
to produce speaking machines, the operation of which should de-
pend wholly on mechanism. In 1779, a prize was offered by the
Academy of Sciences at St. Petersburg, for the best dissertation
on the theory of voi^el sounds, illustrated by actual experiments ;
and it was awarded to 6. R. Kratzenstein, an account of whose

How is the opening of the glottis in females compared with tfiat in
males ? To what amount do they generally differ ?

How is the human voice, in point of variety and rapidity of executiqp,
eompared with musical instruments ?

On what condition of (he vocal organs do the different vowel sounds
depend ? On what three parts of the mouth are the consonant sounds of
Utiguage mainly dependent ?

* v. Sigaud de la Food Eleo). de Phys., vol. iii. p. $51; Teystedcs
filem. de Phys., p. ^14.

FORMATION OF VOCAL 80UN2>S BT MECHANUM • 9U

researches was published in the Transactions of Ihe Academy.
This igenious philosopher showed that the sounds of the four
vowels, A, E, 0, and U, might be obtained by blowing throng
a reed into several tubes, the forms of which are represented in
the annexed figures 1, 2, 3, and 4 ; and that the sound of I, as

Sronounced by the French and other continental nations was pio-
uced by blowing at a, into the pipe No, 5, without using a reed.
Kratzenstein continued his investigations, but probably he did not
obtain results of greater importance, as he never published any
further account of the progress of his inquiries.

105. M. Kempelen, of Vienna, who distinguished himself by
the construction of an automaton chess-player, which has excited
much attention, also devoted his ingenuity to the contrivance of a
apeaking machinis. He succeeded so ^ as to produce an instru-
ment capable of uttering certain words and short phrases in French
and Latin. The sounds appear to have been produced by means
of a single cavity, the form and dimensions of which might be
modified at pleasure. It has been described as consisting of a
box, about tiiree feet long, placed on a table and covered with a
cloth, under which the operator in exhibiting its powers introduced
both his hands, one of whicb probably was employed in pressing
on keys which might communicate with pipes aner the manner
of those of an organ. This machine was only shown to the pri-
vate friends of the inventor, and it does not appear that it was ever
completed.*

106. A gentleman of Cambridge, England, has more recently
prosecuted experiments on the formation of articulate sounds ; and
having adopted the method of Kempelen, in using a single cavity,
he found that by blowing through a reed into a conical cavity the
vowel sounds could be produced by altering the dimensions of the
aperture for the passage of air from the cavity by means of a
sliding board. He also found that when cylindrical tubes, the
length of which could be varied by sliding joints, were adapted
to the reed, the series of vowels could be produced by gradually
lengthening the tube; and when its length was augmented in a

Give some account of Kratzenstein's vocal pipes.
To what extent did Kempelen, the inventor of Maelzel's automaton
chess-player, carry the imitation of human speech ?
Of what did his spealcing machine consist }

* See Nicholson's British Encyclopedia, art Airaaomss $ Brcwstor'i
Edinburgh EneydopSBdia,. art AuroiwiToa.

z

"^0^ ACOTTSTICS.

'Certain proportion the same vowels were repeated but in inverted
order; and bj a further augmentation of the length of the pipe, a
'Second repetition of the vowels took place in direct order?

107* The de^pree of accuracy with which some birds, and espe-
'cially parrots imitate the human voice, depends more on their
"propensity to mimic the sounds they hear than on any peculiar
-qualifications they possess for giving utterance to articulate tones;
'and therefore many other animals, beasts as well as birds,
'might be taught to speak by any one who was disposed to bestow
Bumcient time and labour on such a task. Pliny the Elder men-
tions certain nightingales, which ^oke Latin and Greek ;* but, as
the authority of that ancient writer may be considered as some-
what dubious, it will be more to the purpose to observe, that
Father Pardies, a learned Jesuit, gravely asserts that dogs had
been taught music, and that one of them was so apt a scholar,
that he could sing a duet with his master. The celebrated phi-
losopher Leibnitz has given an account of a dog which he saw
and heard speak, after it had been under tuition three years. This
animal could pronounce thirty words, such as iea^ coffee^ chocolate ;
'and he merely repeated them after hearing them from his master*!

Reflection of Sound,

108. When sonorous vibrations are propagated through a mass
of matter of great ex^nt, and of uniform density and elasticity,
'as when they are continued uninterruptedly through the open
air, the sounds will be heard alike in every direction, and become
dissipated or lost in the surrounding space. But if they impinge
on some obstacle which interrupts their progress, they will be
. driven back or reflected ; and thus a wall, the face of a rocky cliff,
the surface of water, or even a dense body of vapour, may cause
the reverberation of sound.

109. The reflection of sound takes place ac-
cording to the same laws that govern the re-
flection of perfectly elastic solids. Hence the
sonorous vibrations being propagated in right
lines, the ang^e of reflection is always equal
to the angle of incidence. Thus suppose a
sound to /be emitted from A in the annexed
figure, and to impinge on a dense plane, £ B
F at B, it will return in ^e same line B A, and
the reflected sound will be heard at A after the

are nventioned
'reflection

ioned of this power in quadrapedsr What is meant by the
of sound ? According to what laws does it take plaee ?

* Plinii Histor. Natur., lib.x. cap. 42.

t Histoire Critiqae de PAme des Betes. Amsterd. 1749. t.ii. p. 50.
^3e la CoDDoissance des Betes, p. 129. Histoire de I'Academie dea Sei-
coees. An. 1715. p. 3.

£CH0S8. %n

original soaDdfOonstitatioffwhatis tennedanecho.* If^howeTerf
the sound be emitted from jD, bo tliat the line of its direction may
forqi an oblique angle with the plane E F, it will be reflected from
B in the line B C, forming a similar oblique angle with the j^ane
E F. The velocity of the reflected sound is precisely the same
with that of the direct sound; therefore the sound will De returnecl
from B to C in the same time that it passes horn D to B, Hence
the sound uttered at D will be heard by a person stationed at C at
the end of a period double that which it takes to pass from D to
B. So that as sound travels at the rate of 1130 feet in a second*
if the distance from D to B should be 282 } feet, the echo will be
heard at C in half a second, for in that time the sound would be
conveyed 282^ x 2 = 565 feet.

110. It is requisite that the reflecting body should be situate^
at such a distance from the source of sound that the interval be-
tween the perception of the original and the reflected sounds may
be sufficient to prevent them from being blended together. When
they become thus combined the effect is termed a resonance, an4
not an echo. The shortest interval sufficient to render sounda
distinctly appreciable by the ear is aJl)out 1-10 of a second;
therefore when sounds follow at shorter intervals they will form a
xesonance instead of an echo. So that no reflecting surface will
produce a distinct echo unless its distance from the spot whence
the sound proceeds should be at least 56} feet, as the sound will
in its progress forward and return through double that distance^
1X3 feet, take up 1-10 of a second. Resonances, or combinationa
of direct and reflected sounds are heazxl more or less in all inclosed
places of moderate extent ; and as they occasion some degree of
confusion in the perception of sound, inconvenience arises fron^
this source in rooms appropriated to the purposes of oratory, the
voice of a speaker being heard but indistinctly, especially in some
situations ; hut in a concert-room such resonances are rather ad-
vantageousi at least they would add to the effect of instrumental
music,

111. Some echoes will repeat but one syllable or distinct sounds
while others will repeat several in succession. Hence the dis-

- tinction of monosyllabic and polysyllabic echoes. As it would.

Constnict and explain the diagram relating to this subji;et ?

What is meant by the angle of ineidenee of sound f

What by the angle of reflection ?

What is the r<4ation of those angles to eaeh other }

What are the aomfUirattTe velocities of original and of reflecCed.sound f

What name is given to the iBtermin(g^tng of original and refleetedi
aound ?

How far must a reflecting surface be placed from the source of sound,
in order that an echo should be dirtinctly heard ?

What disadvantage t« occasioned bj resonance^

in what oases may it be found beneficial ?

What distinction has been made between echoes ?

—

* ^om the Greek *H;c»» a reflected sound.

26^ ACOUSTICS.

be impossible topronounoB more than ten syllables in a second intel-
ligibly, it must follow, that if the reflecting body^ausing an echo
should be so near the speaker as to return his voice in 1-10 of a se-
cond, the last syllable only of a word uttered would be distinctly re-
echoed, for all the preceding would be confounded together. There-
fore, if the distance of the reflecting object were but 56} feet, the
sound in going and returning through twice that space would take
up but I-IO of a second, and the echo would consequently be mono-
syllabic. If the distance of the reflecting object were 1 13 feet two
syllables mightbe returned ; and in general there would be as many
syllables repeated as the multiples of 56} in the number of feet
between the source of sound and the reflector.

1 13. It will be obvious that two persons may be so situated that
one may hear the echo of the voice of the other without perceiv-
ing the original sound ; for the voice impjnging obliquely on a ro-
flecting surface may be conveyed uninterruptedly to a person
placed in the line of reflection, while some intervening obstacle
may prevent the direct passage of the sound. Thus two persons
standing one on each side of a mirror might see the reflected
figures of each other in the glass, though an opaque body might
entirely conceal from either uie real figure of the other.

113. Such echoes as have been now described would be produced
by a single reflecting body, which would necessarily return or repeat
but once each original sound. The most remarkable echoes, how-
ever, are those which have been termed polyphonous, because
they multiply sounds, or yield several repetitions^ of a single ori-
ginal sound ; the echo arising from successive reflections from a
number of different surfaces. Numerous instances of extraordinary
echoes are related by travellers and other writers. Dr. Plot, in his
History of Oxfordshire, England, gives an account of an echo in the
park at Woodstock, that would repeat seventeen syllables in the
day-time, and twenty at night : the air being more elastic duringr
the day than in the colder season of the night, the sound would
travel faster, and be returned more speedily by the diurnal thaa
by the nocturnal echo. On the north side of the parish church of
Shipley, in Sussex, there is an echo which will repeat twenty-one
syllables.

114. Among the echoes which repeat the same sound many
times one of the most noted is that mentioned by Father Kircber
and Gasper Schott in the seventeenth century, and subsequently
by Misson and Addison, as existing at the Marquis of Simonetta's
TiUa, near Milan, in Italy. It is produced by two parallel walls
constituting the wings of the building, and the echo, which is
best heard from a window between them, will repeat a single

How is the number of syllables echoed necessarily limited ? /

In what cases mav echoes be beard to the exclusion of direct aounda f

What are meant by poljphonous echoes ?

From vhat do the^ proceed ?

What examples ot poiTSTllabic echoes have been recorded f

What cases of polyphonous echoes have beep noticed ?

REFLECTION OP BOUND VROU CURVED SURFACES. 5^

word more than forty times, and the report of a pistol nearly six^
times ; but not with perfect distinctness except early in the morn*
ing or late in the evening, in calm weather. There is an anaIo->
gous echo at Verdun, produced by two towers distant from each
other about 165 feet ; and a single word uttered loudly by a persoi^
standing between them, wUl be heard repeated a dozen times.

115. On the Rhine, near Lnrley, is a remarkable echo, described
by Dr. Granville, caused by the reverberations of sound from the
rocky banks of the river, which may be heard to the greatest ad-
vantage firom a boat in the middle of the stream . This echo repeats
lAusical sounds, gradually fading on the ear till they die away i
and U resembles the polyphonotus echoes, which are heard on the
Lakes of Killamey, in Ireland. A wonderful echo is mentioned
by Father Gassendi in his remarks on Diogenes Laertius, since he
states that Boissard heard the first verse of the iEneis,

Arma viramqae cano TroJ« qui pricniia ab oris,

repeated eight times at the tomb of Metellus, an ancient monument,
near Rome. At Genetay, near Rouen, in Normandy, a curious
echo is said to exist, the effect of which is such, that a person
singing will hear only his own voice, while others at a distance
^ear the echo and not the original sound ; sometimes the reflected
sound seems to approach, and at other times to recede, and the
sounds are heard m different directions, according to the situation
of the hearer.

116. The reflection of sound, ii^
stead of producing an echo, may
have the ef^t of concentrating
sonorous vibrations so as to render
sounds audible veith the utmost
distinctness at considerable dis-
tanced from the places where they
are emitted. This may happen in consequence of repeated refleo-
tions from ^ curved or polygonal surface, so that the sound beine
uttered in the focus of one reflecting sur^e it will be conveyea
to the ear placed in the focus of another, reflecting surface. Thu9
a sound too weak to be heard in the direct line A B, in the margi-
nal figure, may be augmented by reflection from B to C, and thence
to A, and also by a number of intermediate reflections from a, A,
<^9 ^9 ^9 /) 2ind various other points, all tending to A ; so that al
whisper or the scratch of a pin, which could not be conveyed di-
rectly from B to A, would be heard plainly by accumulated reflec-*
tion from different points in the circular surface B C A.

117. The most trifling sound may thus be heard from the oppo-

What stngfttlarity exists in the echo of Gene^y .'
What effect, other than eehOes and resonances, may be pl^tMcd by
the reflection of sound ?
In what manner may this result be obtained ?
niusti'ate this by diagram.

z2

2^0 ACOUSTICS.

site side of the circular gallery at the base of the dome of St.
Panics cathedra], London, hence called the whispering gallery.
There is also a whispering gallery in Gloucester cathedral, where
a narrow passage seyenty-fiye feet in length extends across the
west end of the choir; and the wall forming fiye sides of an octa*
gon, the Toice of a person whispering gently at one end of the
gallery is carried by reflection to the ear of a person on the other
side of the choir.

118. In a very similar manner sound is concentrated by reflec-
tion, from the focus of one reflecting surface to that of another in
an elliptical vault. The cupola of the Baptistery of Pisa is thus
constructed ; so that a person placed in one focus may distinctly
hear a whisper uttered in the other focus, though it would be in-
audible in the intermediate space. The cathedral of Girgentl, in
Sicily, affords an example of a similar construction; in conse-
auence of which the gentlest whisper may be, plainly heard from
tne cornice behind the high altar by a person at the great western
door, a distance of*two hundred and fifty feet. The ecclesiastics,
i^rnorant of this circumstance, had unluckily placed the confes-
sional in the focus of one of the reflecting surtaces, and persons
who happened to have found out.the place whither sounds were
tenveyed, amused themselves for some time by resorting thither
to hear secrets intended only for a confessor : at length one of
these indiscreet listeners was punished by hearing his own wife
confess her frailty; and the affair becoming public, 3ie confessional
was removed to a more secure spot.

119. The concentration of sounds sometimes produces very
singular effects, of which an instance is thus related by Dr. Ar-
nott : «« It happened one da^r on board a ship sailing along the
coast of Brazil, far out of sight of land, that the persons walk-
ing on deck, when passing a particular spot, heard very distinctly
during an hour or two, the sound of bells, varying as in human
rejoicings. All on board came to listen, and were convinced ;
but the phenomenon was most mysterious. Months aflerwards it
was ascertained, that at the time of observation the bells of the
city of St. Salvador, on (he Brazilian coast, had been ringing on
the occasion of a festival : and their sound, therefore, favoured by
a gentle wind, had travelled over perhaps 100 miles of smooth
water, and had been brought to a focus by the concave sail in
the particular situation on the deck where it was listened to."*

ISO. There are some echoes for which it is more difficult to ac-
count. Such for example, as that observed by M. Biot in the
aqueducts of Paris, where on speaking at the extremity of a tube
9^1 metres in length, the voice was repeated six times. The in-

In what celebrated strnctares is the coneentration of toond exemplified^
What is related of the cathedral of Girgenti in Sioily ?
How have mariners occauonally experienced the effects of coneen .
tnted sounds ?

* £l«ineDti of Ptyrsiof, vol. i. p. 538.

THE INVISIBLE LADY. 271

tervals of these echoes were eoo^l, each heing aboat half a second ;
the last Tetarning to the ear after three seconds, that is, after the
time requisite for the sound to pass through the space of 951 me^
tres. Similar echoes have been noticed in the long galleries of
mines. M. Beudant observes that probably, in the experiment
of Biot, the tubes were not laid exactly in a straight line, nor
throughout of the same diameter ; and that in the galleries of
mines it may be imagined that the walls or sides were not parallel.*

121. The conveyance of articulate sounds by means of tubes
through considerable distances from one part of a building to
another is now commonly practised. By tkis method a message
or inquiry can be communicated from a person in his study or
office, in the upper part of a high building, to clerks or workmen
in the lower part, without loss of time or inconvenience.

132. The facility with which the voice thus circulates through
tubes was probably known to the ancients, and certainly to the
cultivators of philosophy in the middle ages. Pope Sylvester II.9
whose proper name was Gerbert, was almost tne only man of
science living in the tenth century, and not now forgotten. By
his contemporaries he was regarded as a magician, because among
the wondenul machines he constructed was a speaking head of
brass. Albertus Ma^us, and Roger Bacon, in the thirteenth
century, incurred similar odium, in consequence of their having
formed speaking figures. There can be no doubt that each of
these ingenious men adopted the method now described of con-
veying sound from a distance, so that it might appear to proceed
from an inanimate bust,

123. By far the cleverest deception of this kind was an exhibi-
tion which took place at Paris several years since and afterwards
in London, appropriately styled the Invisible Lady, since the ap-
paratus was so contrived that the voice of a female at a distance
was heard as if it originated >from a hollow globe not more than a

. foot in diameter, suspended freely from wooden framework, by
slender ribbons.

124. A perspective vfew of the machinery, and a plan of tht
globe and adjoining parts as constructed by the inventor, M.
Charles, are given below. It consisted of a wooden frame, much
resembling a tent bedstead, formed by four pillars A, A, A, A,
connected by upper cross-rails, B, B, and similar rails below,
while it terminated above in four bent wires, C, C, proceeding
from the angles of the frame, and meeting in a central point. The
hollow copper ball, M, with four trumpets, T, T, issuing from it

How may polyphonous eehoea in pipes ftnd mines be explained ?

To vhftt useful purpose may the conduetion of soand bjf long tubes b«
applied ?

What instances of ingenious deception have been formed on this mode
of transmitting soand ?

To what imputation did the^ sobieet their authors ?

How was M. Charles's invisible lady constructed ?

* Beudant Elem. do Phys., pp. 367, 368.

at right angles, hang in the centre of the frame, bdn^ connected
with the wirea alone by four oariow ribbons, D, D. Anjqaestioii
or obserTation uttered in a low Toice eloae to tiie open mouth of
one of the tiumpeta eliciled a reply which might be heard from
all of Ihem, the sound being perfectly distinct, but weali, as if it
was emitted by a very diroinutiTe being.

135. The real speaker was a female concealed in an adjoining
apartment, and the means by which her roice was made to tsaua
from the globe in the manner stated were at once Tery simple and
ingenious. Two of the trumpet months, T', T", as represented
in the plan, were exactly opposite apertures .leading to tubes in
two of the cross-rails, which meeUng at the angle A, opened into
another tube descending throngh tlie pillar, and. which was con-
tinued under the floor into an adjoining apartment, where a person
sitting might hear wbat was whispered into either of the Hum-
pets, and return an appropriate answer by the same chaimel.
This machinery differs from the common speaking-lubes, previ-
ously noticed, merely in (he addition of the hollowhall and trum-
pets, by means of which the voice is reflected Irom the cavity of
the globe through the trumpets T, T, into the tube of commu-
nication; and thus the effect produced is rendered abundantly
mysterious to those unacquainted with the principles of Acon-

136. The Speaking-trumpet, used by captains of ships, is aa
instrument adapted for increasing the intensity or loudness of
sounds, by multiplied reflection. It consists of a conical tube, as

Ip what manner did his inao>iinery differ from onHnarj ipeaking-tnbea ?

▼ENTRILOQinSM. 279

represented above, open at both ends, about tbree feet in length,
and formed of copper or tin-plate. On speaking slowly and dis-
tinctly with the mouth applied to the opening' A, the sound after
being reflected from the sides of the tuoe in the direction of the
cross-lines, will escape from the wide extremity B, and be con-
Teyed to a distance proportioned to the leneth of the tube and the
diameter of its bell-shaped aperture. Kircher, in his ** Phonurgia
Nova," published in 1673, claims the invention of the speaking*
trumpet, referringr to his *' Musurgia,*' 1650, for an account of the
instrument; but nis contriyance seems to have been constructed
on the principle of the tubes of communication mentioned above,
rather than on that of the speakinff-trumpet, which was more pro-
bably invented by Sir Samuel Morland, who gave a description
of it in a work which was printed in London, 1671, under the title
of ** Tuba Stentorophompa." It is by no means necessary that
the speaking-trumpet should be composed of a metallic substance,
for the sonorous vibrations will be propagated in the same manner
and with equal effect through a similar tube lined with cloth.

137. A Hearing-trumpet is nothing more than a funnel-shaped
tube, like a reversed speaking-trumpet ; which, when the small
end is held to the opening of the ear, conveys to the person using
it articulate sounds from without, increased in intensity by re-
peated reflections from the inside of the tube. M . Lecat invented
a double acoustic tube, by which concentrated sounds might be
conveyed to both ears at the same time ; and which may be used
with great advantage by those whose auditory faculty is im-
paired.

138. The art of ventriloquism appears to depend in some de-
gree on the reflection of sound within the mouth. Professor Du-
gald Stewart attributed the talent of exciting the perception of
articulate sounds, in such a manner as to {pve them the effect of
emission from various distances and directions, wholly to decep-
tion and the power of imitation. Those who have witnessed the
curious monopolylogues, as he appropriately styles them, of Mr.
Mathews, in his theatrical exhibitions, will readily admit that
feigned dialogues and other vocal illusions, in which the sounds
apparently issue from an object very near the performer, may be
solely the effect of exauisite mimicry.

139. Tlie celebratea Peter Pindar, or rath«r Dr. Waleot, pos-
sessed considerable talent as a mimic, and he sometimes amused
his friends by the display of his skill. He would c^uit his apart-
ment on the first floor, as if to speak to a person waiting for him.

By whom it its inveDtion claimed ?

Is a metalUo sabstaaee neoessary to produce the effect of this instm-
ment?

What iroproTeroent on the hearingwtrumpet was ioTented by^^cat ?

In what peculiar art is the reflection of sound ap|>lied ?

In what manner are we to account for the decepuon of the ear by mU
mies wlio personate several characters at the same time }

What anecdote illastrates this view of the subjeot ?

974 . ACOUSTICS*

and shotting the door on those within, he would stand at the stair
head, and hold a fancied conversation with his laundress, the bard
speaking alternately in his own natural tone of Toice, and in the
shriller Key resembling the Toice of a female. He would repre-
sent the visiter as demanding payment of her account, and, ia
spite of the excuses and expostulations of the bard, raising hex
Toice in reply to each apology, and becoming gradually more and
more abusive, till at length, when the company, who heard all,
might suppose that he hsS lost all patience at the woman's per^
nacity and insolence, the dialogue would be suddenly terminated
by a noise which seemed to indicate that he had Idt^ked the laun«
dress down stairs. Such an exhibition, whimsical enough to
those not in the secret, would obviously reauire much le§s skill
and address than the scenes whece the perrormer stands in pre-
sence of the audience.

130. The deceptions of the ventriloqM^t are produced in a dif**
ferent manner, requiring not only the faculty of disffuisin^ the
voice BO as to imitate other sounds, but also the art of determining,
the apparent source of sound. Among the numberless examples
of the feats on record relative to the professors of this art is the
following, related as an ear-witness, by Van Dale, the author of a
treatise on Onicles. There was, ia 1685, in the hospital for the
aged at Amsterdam, a woman, seventy-three years, old, whose
name was Barbara Jacob!, and who was visited by a vast multl-*
tude of persons on account of her talents as a venmloquist. She
lay on the side of a small bed» the curtains being undrawn, and
turning as if to talk to a man near her whom she called Joachim^
she returned answers to her own questions or remarks in a feigned
voice. She would accuse her supposed companion of gallantries
with other females, make replies for him, sometimes as if he was
laughing, sometimes crying, now uttering groans, and then sing*
ing songs, and all with such address ana enect, Uiat the illusion
was complete; and the bynstanders occasionally would not be
convinced that she had no associate till they had satisfied them-
selves by searching the bed.*

131. This woman, who was famous for her skill, is also mei^
tioned by Balthasar Bekker, in his curious treatise entitled *' The
Enchanted World;" and another Dutch writer, John Conrad Am?
man, in his Dissertation on Speech,, states, that he had heard her
singular dialogues, and that the feigned voice she uttered seemed
to issue from a spot at least two paces from her. The Abb^ de la
Chapelle, a learned Frenchman, who was a Fellow of the Royal
Society of London, wrote a distinct work on ventriloquism,f in

How does Tentriloauism differ from mimicry ?
' What instance of their combination is recorded by Van Dale }
What account of the effect and its cause is given by Amman ?

* Antonii Van Dale Polyatri Harlemensia DUsertatiooea de Origine
ao Progressu Idolatriae et Superstitionum, 4lo. p. 65S.

t Le Ventriloque ou PEngastrimylbe. Par M. de la Chapelle, Cenaevr
Boyai a Paris, &o. Deux parties. A Londrea, 1773. ISmo.

▼simtnjOQmsH* 276

which he has collected a great number of notices of peraons
skilled in this art, and of their singular exploits.

132. One distinguished ventriloquist to whom his work relateSy
was the Baron Ton Mengen, a nobleman of Vienna, who in a let-
ter addressed to M. de la Chapelle, states, that he had acquired a
facility in speaking as a yentriloquist, by mere practice com-
menced when a boy, but he professes himself unable to commu-
nicate the art to another. Amman in the passage referred to
above, expressly asserts that the effect is produced by speaking
during the act of inspiration, or drawing air into the langs, in«
stead of speaking while the air is passing out as usual. The
Abb^ de la Chapelle attempts to controvert this notion, which,
however, seems to be founded on correct observation, as articulate
tones can be thus formed differing in intensity from those emitted
in the customary mode. Yet some further modification of the
speech appears to be requisite to produce all the effects described;
and one of^the most essential peculiarities will consist in the art of
enunciating all, or nearly all, the letters of the alphabet without
moving the lips.

133. It is a remarkable circumstance that the art of ventrilo-
quism is j^ractised among the Esquimaux, by individuals who
have acquired among their countrymen the reputation of beinflr
wizards. The late Captain Lyon, one of the officers who visited
the Arctic regions a few years since, had opportunities of wit
nessing the exhibitions- of one of the most skilful of the Esqui-
maux ventriloquists, and he published an interesting account of
the observations he made on those occasions. One of the most
important circumstances which he noticed was that the ventrilo-
quist, after having uttered a protracted sound, which lasted while
Uaptain L. had made two inspirations after holding his breath as
long as possible, emitted a "powerful yell, without a previous
stop or inspiration of air."* Doubtless the previous lengthened
sound was produced during inspiration, and therefore the succeed-
ing yell relieved the distended lungs. This observation thus

. agrees with those which have been made on other ventriloquists.
But there seems to be little probability that this subject will be
perfectly understood till some skilful ventriloquist may choose to
investigate the manner in which the vocal organs are employed in
forming such anomalous sounds, and communicate the result of
his researches to the public.

Among what barbarous natioot has the art of veotriloquitm been prac-
tised ?

In what manner does Captain Lyon account for the prolonged utter-
ance given by the Esquimaux ?

• Private Journal of Capt G. F. Lyon. 1824. pp. 358, 361, 366.

276 ACOUSTICS.

Book» on the tubjut of AeouaticB,

Playfidr's Outlines of Natural Philosophy, vol. i. pp. 281-^91.
Robinson's Mechanical Philosophy, 8vo. vol. iv. pp. 376 — 537.
Library of Useful Knowledge, treatise on Pneumatics, pp.
29—31.

Chladni Traits d'Acoustique, Paris.

Rush's (Dr. James) Philosophy of the Human Voice, Phila.

1827.

£diuburgrh Encyclopedia, article Aeoustiea.

Cassini on Sound, Mem. of the French Academy ibr 1738,
p. 128.

Savart Recherches sur les Usages de la Membrane du Tym-
pan. Ann. de Chim. vol. xxvi. p. 5.

Nollet on the Hearing of Fishes, Mem. of the Fr. Acad. 1743.

Euler on ^e Propagation of Pulses, Berlin Memoirs for 1765.
p. 335.

Dulong on the Velocity of Sound in different Gases, Ann.
de Chim. vol. xli. p. 113.

Works in the department of acoustics are very numerous ; but
the number of those which treat the subject with perfect clear-
ness, combined with that exactness which the case seems to require,'
is perhaps smaller than in most branches which have for an equal
len^ of time been cultivated by the curious and the learned.
This arises, in part, from the intrinsic difficulties of the subject; in
part, from the fact that many persons, as stated in a preceding page,
are insensible to the nicer shades and distinctions of sound, and
in no small degree from the discouragements long felt in attempts
to make the deductions of theoretical investigation c6rrespond with
experimental results. The far-reaching mind of Newton himself,
was not able to grasp all the causes which modify the transmis-
sion of sound. Indeed the law of La Place, that the theoretical
result of Newton must be corrected by multiplying it into the
square root of the ratio of the specific heat of air under a constant
pressure, to its specific heat under a constant volume, could not pos-
sibly have been applied in Newton's time; for the whole doctrine
of specific heats was then unknown. Again, the means of ex-
hibiting, in all their variety, the numerous phenomena of vibration,
have only for the last few yeara been supplied by the laboura of
Chladni, Savart, Biot, Colladon, Faraday and others. The art of
composition in music, and skill in its perrormance, have sometimes
been mistaken for the science from which they derive their princi-
ples. The art of rhetoric has in like manner been conceived to
constitute ike science of vocal utterance, until Dr. Rush pushed
his analytical researches to the extent of dissecting the very ele-
ments of speech, and displaying those operations of the organs
which combine to produce or to modify every element.-*>ED.

PYRONOMICS.

1. Among (hose branches of natural philosophy which have at-
tained the rank and importance of distinct sciences, in conse-
auence of the researches of our contemporaries, must be included
laty the object of which is, to explore the properties and opera-
tions of heat. To designate this department of human knowledge,
Leslie has adopted the term Pyrdnomics, which signifies the laws
of heat.* The effects of heat, or rather of relative temperature,
6ave 80 striking an infiueiice on all bodies around us, and it pro-
duces such varied and singular consequences, that whether con-
sidered as arising from the modifications of matter with which we
are most intimately ac<}uainted, or as depending on the presence
of some peculiar subtile agent, it must be desirable that they
should be closed and arranged so as to form a systematic series
of facts and observations; and to subh av system the appellation
Pyronomics may be appropriated, as expressive of the objects, to
which it relates.

2\ The cause of heat has formed a 'subject of dontroversy amongf
modern philosophers; some of them ascribing the phenomena in
which it is concerned to intestine motions of the minute particles of
l>odies, analagous to those which give rise to sound; while others
ha,ve endeavoured to prove that neat arises from the presence
of a peculiar fiuid, or ethereal kind of matter, such as that which
&as been regarded as the cause of light. If such a fluid exists, it'
must be destitute of weight, for it has been ascertained from ex-
periment that the addition of heat to dny substanc(9 produces no
alteration whatever in its gravitative fofce.

3. Dr. George Fordyce instituted researches conceining the
effect of temperature on the weight of bodies, whience it was con-
cluded that the abstraction of heat from water occasioned an in*
crease of weight. He inclosed 1700 grains of water in a glass
globe, and having sealed it hermetically, after ascertaining the
weight of the globe and its contents with the utmost accuracy, he
immersed it in a freesing mixture, and on weighing it again after
the liquid had become entirely congealed, he found it had gained
3-16 of a grain.f Guy ton de Morveaii, and Chaussier, made cor-
responding experiments in Friance, on water and sulphuric acid,
from the results of which they drew the same inferences.

To what IB the term Ffronomics properly trpplied ?

To what cause have diflTereot phiioaopbert attributed tiie ph«ndiiteiui
of heat }

To what result were Fordyce, Morveau, and ChauHie^ eondueted itt
regard to the effect of temperature ori the weight of bodies ^

* From the €k<eek iiDp,4ire, and Ne^^, a law.
t See PhllotOphiMa Transactions vol.lxxy.

2 A 277

278 FTR0NOMIC8.

4. Oth^r philosophers, however, made comparative trials of the
weight of water and sulphuric acid in the liquid state, and when
reduced by cooling to the solid form, without being able to detect
any difference of weight appreciable by the most delicate balances.
The apparent effect of the variation of temperature on the weight
of booies as noticed in the above experiments, may be attributed
to the influence of the frozen mass on the density of the surround-
ing air; therefore, if we admit the observations to be perfectly
correct, it m^ still be contended that they are not conclusive.

5. Count Kumford, having suspended a bottle containing' wa-
ter, and another containing spirits of wine, to the arms of a balance,
and adjusted them so as to be exactly in equilibrium, he found
that it remained undisturbed when the water was completely fro-
zen, though the heat the watetr had lost must have been more than
sufficient to have made an equal weight of gold red-hot. If, there-
fore, with Lavoisier and his associates, (the founders of what has
been styled the Antiphlogistic System of Chemistry,) we suppose
heat to be matter rather than motion, it must be allowed to be an
imponderable fluid, and also, as denominated by some philoso-
phers, a:n incoercible fluid, for though it passes through some bo-
dies with more difficulty than through others, there is no body or
or kind of matter which can completely arrest its progress.

6. The terms Caloric* and Matter of Heat have been adopted
to designate the hypothetical causes of those phenomena which
are referred to the science of Pyronomics ; and without admitting
the separate existence of such an agent as the caloric of the French
chemists, the term may sometimes be advantageously employed
to denote the amount of effect produced by relative changes of
temperature in the same or in different bodies.

7. Various phrases occur in our own and in other languages
expressive of the effects resulting from alterations of temperature*,
both as regards the impression on our senses, and the changes of
form or structure produced in the bodies around us. When we
touch a substance more heated than the hand applied to it, the
sensation arising from it is styled warmth ; and that caused by a
substance less heated is named chilliness or cold. But warmth
and chilliness, or heat and cold, thus used, are merely relative
terms ; for a substance which would excite in one person the sen-
sation of heat might at the same time seem cold to another ; and
if a man, after holding one hand near a fire for a few minutes, and

Hovr has the weight of bodies, as affected bj heat, been explained ?

In what manner did Ramford seek to determine the qaestion of the
ponderability of heat ?

What other distinfaishing property, besides that of imponderable^ be-
lones to the nature of heat ?

To what is the term calorie applied^'

How may it be shown that hot and cold, warmth and chilliness, are re-
ktive terms ?

* From the hktiu odor, heat

EFFECTS OF CHAMOB €9 TEMPERATtRI. STO

laying the other on a cold stone or maible slab, wero to dip both
in a basin of Inkewarm water, the liquid would warm the cold
hand and cool the warm one.

8. '^ There is, perhaps, no subject," says Hutton, '' in which the
language and ideas of men are more vague, or more distant from
true science, than in those of heat and cold. The reason of this
will not be difficult to assign. Heat is a term which is applied
in different cases ; for it is both a principle of action in external
things, and a principle of passion in our sensitive mind. But
this is only a part of that intricacy with which this apparent-
ly simple subject is necessarily InTOlved ; for when heat is a
principle of action in external things, there are two different effects
which occasionally follow this principle as a cause. First, bodies
are by heat distended or 'expanded in their volume : here is one
distinct effect. Secondly, without being thus distended by heat,
bodies, in receiving that distending cause, are made to lose their
hardness, or concretion, and become fluid in their substance : here,
then, is an effect distinctly different from the other ; and both of
these are perfectly different from the feeling of heat and cold
which is the immediate information of the sense."*

9. Among the terms indicating various effects or operations of
heat, or the appearances which we are accustomed to ascribe to
its action on the several substances around us, are some which
imply a connexion between heat and light : thus a body like led-
hot iron, or burning coals, is said to glow or be glowing; a lumi-
nous vapour Issuing from a burning body is called flame, and
such a body may be said to flame or blaze; when any substance
exhibiting these appearances becomes dissipated or dispersed
through Sie air, either wholly or partially, the operation is styled
burning or combustion ; and when a heated body emits light with*
out combustion, it is said to be incandescent, or in a state of incan-
desence. The appearance of heat and liffht in conjunction is oflen
designated fire, a term used by ancient philosophers as character-
istic of the matter of heat, and regarded as one of the four ele-
ments.

10. The effects of heat on our senses are too variable and falla-
cious to afford any important assistance in the investigation of its
nature and properties ; and the principles which form the foundsp
tion of the science of Pyronomics must therefore be derived from
the consideration of those phenomena which always appear under
certain circumstances, so that we can produce them at pleasure

Whence has ariBCD the va^enest of langoage employed on this tub-
jeot?

What two circumstances have particularly contributed to increase the
misconception of terms in relation to it ?

What is meant by gUrmng P burning and incandetcence ?

What character was by the ancient philosophers attributed to fire ?

On what must we rely for obtaining the principlet of pyronomics ?

^ ■■ ■ ■ ■ P 11 ■ I I ■! ■■ —111 I l^« .l^^l I , ■ ■ ■■ — ^— ^ ■ ■ ■ ■' ■

* A Dissertation on the Philosophy of Light, Heat, and Fire, by J,
Button.

%86 moKoncfl.

hj proper arruigBments, and estimate with precision the leenlts
of combinatLons by means of whieli heat is accumulated or die*
persed, and its operation on bodies induced, modified, <»r termi-
nated.

11. The distioguishinff properties or effects of heat, or those
phenomena which arise nom its addition to material bodies, or
more simply from the au^entation of their temperatore, are of
three kiods. 1. Mere dilatation, or increase of Tolume, which
.occurs in solids, liquids, or gases, without any change of fi^rm.
2. Transformation of a solid to the liquid state, as in dissolving
ice or melting lead ; and of a liquid to the gaseous state, as in
fbrminff steam from water, or yapour from any liquid, by boiling
or distillation. 3. Destruction of the texture of bodies by com-
bustion, in consequence of which new combinations of the con-
^tituent particles of bodies are formed, the inyestigation of which
faTls within the province of the chemist rather than of the natoral
phUosopher.

Sources of HeaL

\% Before proceeding to notice in detail the prc^rties and ph&>
pomena of heat, some attention may be advantageously direeteid to
the sources or efficient causes of heat. Hiese are radiation finona
the sun together with light ; certain mechanical operations, as
friction, percussion, and coQipression ; and a variety of.ehemieal
operations, especially combustion* Heat is also evolved from
living animals and vegetables, either by the immediate influence
of the vital powers of organized nature, or in consequence of
chemical processes, modified by the peculiar properties of organie
matter.

13. That the sun is the grand fountain of heat, or the prin-
cipal cause of elevation of temperature by its action on bodies
exposed to its rays, is a fact too obvious to require demonstration.
As to the manner in which the effect is produced, different opin-
ions have been entertained. The generally received popular notion
concerning the nature and constitution of the sun, as the .source
of light and heat, is that of an inextinguishable mass of matter
in a state of intense conflagration; or, in other words, a globe of
perpetual fire, the idea of which has manifestiy been derived from
comparison of the property of giving heat and lig^t common to
the sun, and to flames arising from combustible substances.

14. Sir William Herschel entertained a very different opinion
relative to the nature of the solar orb, which from numerous te-
lescopic observations, he was led to imagine to bo an ppaqne

Of how many Vinds are the phenomena arising froip the a^gment^Uoa
of temperature ? *

' How many and what are the known aourcea of heat ?

Whence has the idea pf the ignfimu nature of the son been derived ?

What view did Sir W. Herachel entertain on this subject ?

HEAT OF THS SUN. 281

globular masSf encompassed by an atmosphere consBttnflr of trans-
parent elastic fluids, from the decomposition of which heat and
iiffht were continually proceeding; and he conceived that the body
of the sun, far from being the seat or fountain of perpetual fire,
might with probability be regarded as a world furnished and in-
habited like the earth to which we are confined. Interesting,
however, as such speculations may be, they are not necessarily
connected with the subject before us, and must therefore be dis-
missed with this short notice.

. 15. In reference to the sun as a principal source of heat, the
question wiU recur, whether heat is a peculiar kind of ethereal
matter, which may be emitted from the sun, or rather according
to HerschePs hypothesis from the solar atmosnhere, and radiating
through space together with light, be absorbed by,Tarious bodies
on the surfaces of the earth and other planets, producing the e&
fects of sensible warmth, expansion of solids and fluids, melting
of solids, and boiling or evapdration of liquids, and in some cases,
destruction of substance accompanying chemical combinations ;-—
or whether heat and its concomitant phenomena just mentioned
may not arise from the propagation of motion through space ; the
sun or its atmosphere being the grand exciting cause of >heat, and
producing it in a manner analogous to that in which sound is pro-
duced by a bel).

16. According to the former mode of theorizing we must admit
the existence of a peculiar penetrating fluid emanating from the
region of the sun, and entering with ffreater or less facility into
all terrestrial bodies, on which it produces its characteristic e€>
fects: and on the latter supposition, it .must be admitted that
there exists in the space between the sun and the planets an ethe-
real medium of inconceivable tenuity, through wJiich vibrations
can be propa^ted with velocity immensely beyond those to which
are to be attnbuted the phenomena of sound.

17. Heat, as depending on the influence of the sun, is unequally
distributed over the surface of ihe earth. Owing to the oblique
position of the earth's axis, the solar rays fall more directly on
certain parts of its surface at one season pf the year than at ano-
ther ; and the inequality of effect arising from this cause, is in
some degree compensated by the greater length of time during
which the sun shines continuously on those parts of the earth
where the direction of the solar rays is least favourable. Thus
under the equator the length of the days never exceeds twelve
hours, while they increase in extent on proceeding towards the
poles, and within the arctic and antarctic circles, the sun shines
six months together, and then for six months remains invisible.

What part is acted by the bud on the sapposition that heat is mate-
rial ?

What on that of its being the vibratioD of an ethereal fluid ?

In what manner is the heat of the son distributed over the earth ?

How does the obliquity of the sun*s annual path to the plane of the
equator afiect the heat of difib-ent parts of the globe f

2a2

282 PTitoNOKici*

18. The relative inflneiice of tiie san as the cause of lieat, at
different parts of the earth's snrfiice, attracted the attention of
Dr. Halley, who laid the result of his observations before the Royal
Society, in a ** Discourse conoeminff the Proportional Heat of the
Sun in a^ Latitudes, with the meuiod of collecting tiie same.**
He notices, the influence of local circumstances on the tempera^ue
of different regions, reierringr to the effect of the neigrhbourhood of
kigrh mountains, and wide> sandy deserts. But his ?raad object
was to ascertain the relation between temperatare ana latitude, as
depending on the direction of the sun's rays, and the respective
intervals during which they operate.

19. Since the beginninff of the present century, a vast number
9f facts have been accumulated relative to this interestiiig subject ;
and the observations on temperatare made in different parts of the
world, and especially in high northern latitudes, have enabled
Qien of science to form much more aoouiate conclusions relative
to the- temperature of the eardi as influenced by its positicm at di^
ferent seasona of the year, than those deduced from mere theoreti-
eal cidculatiotts.

80.* From a comparison of the researches of Humboldt m the
equatorial regiotis of the earth, with those of Captains SccMresby
and Parry in the colder climates of the north, it has been eon*
eluded that there must exist two poles of maximum cold, one in
America, and the other in Asia; and that the utmost depression
of temperature takes place, not at the. north and south poles of the
earth, out at those imaginary points. Temperatare and climate
must chiefly depend on the figure of the gr^t continents of the
world ; besides which there are a variety of circumstances which
must tend to modify the influence of solar heat as connected with
ihe situation of places on the suriace of the earth.

31 . Mr. Atkinson, in a paper in the *< Memoirs of the Astronomi*
cal Society of London," (vol. ii.) estimates the mean temperature of
the equator at 860.55 ; andtfaat of the pole at — 10o.5S. Mr^ Forbes
regards the former as decid<idly too- greats and says ** it is prob»-
ble that the mean temperature of the equator does not exceed 81<*.5
or 820.'* i^e mean temperature at the pole cau only be inferred
from, observations at very high latitudes ;. and hence Uie following
table of mean temperature becomes interesting and important.

Lat Mean Temp. Obserrer.

Melville Island - - - 74P - ij© - - - Parry.

Fort Bowen - - - - 73 J ----|-4 --- The same*

What poiDt did Hallej seek to detqmiiiie in regiird.to solar beat?

Is the temperature of different part* of the earth's surfa^ dependcaii
solely on latitude ?

What important eonelnsion has been dedaeed from the resc^rohes- of
travellers in the polar and c»qoatorial regions ?

What is Mr. Forbes*s estiiqate of the mean temperatare of. the eqjoa
tor?

Hov ean we infer that of the pole ?

What is the mean teiqperatare of the year at Melville IfUpd f

What is its distance in degrees from the north pole ?

CENTRAL BEAT OF THE EARTH. 983

LM. Mean Temp^ Obterrcr.

Igloolik 69i----f7 --- Parry.

Winter Island - - - 66| - - 4- 9 J - - * The same*
Fort Enterprise • «> - 64i « ^ •• + 15^ .<• - - Franklin.

82. ** M . Aiagfa had concluded from the results of Scoresby^
Parry, and Franklin^ that the mean temperatme of the pole is 13^
Fah« This^ howerer, is upon the idea that the cold is at a maxi-
ttom at the pole^ which is not probable : it cannot, howerer, be
aaiich short of that intense degree."*

23. The relative decrease oi heat in ascending above die smw
face of the earth is a subject highly deserringf of investigation*
Mi. Forbes says, «*Tlie true law of decrease <n temperatnie, such
as it would be if the earth was removed^ must be souJi^ lor probft*
bly by snccessiTs stages of balloon obsenration, commencing at s
oonsidmble height above the surface." The best obserrations
on the leladve diminution of heat at mcieesin^ heights, are those
of Humboldt, derived from ezperiments made m the equatorial re*
gions of America. The general result oi his researches gives ISl
toises of ascent for a diminution of 1 deg, of Reaumur's thermo<*
meter. Comparative observations at Geneva and on Mount St*
Bernard afford a coincident resul4hig^y remarkable, the diferenee
of mean temperatuxe of the two stations being 8 deg. 64 miB.t
Reaumur, for 1069 toises, which gives 123^ for 1 deff. Reaumur,
er 352 feet for 1 ideg* Fahrenheit , and this is probably the most
correct mean result which can at present be attained 4

24. An inquiry has been started whether the climate of a par*
ticular place, or that of the g^obe in general, has materially altered
during the period of historic record ; and some writers have been
inclined to decide in the affirmative, aUeging the statements of
historians that the vine was cultivated extensively for the pro-
duction of wine in England formerly, though the summers in
that country are now too cold to bring the fruit to &e requisite
degree of maturity. Circumstances of this nature, however, can*
not be considered as decisive; and it may be generally con-
cluded that we have no authority for assuming sucn a change.

25. Yet though the tempemtnre of the earth may be regarded
as permanent, so fer as it depends on the heat d^ived from the
sun, there seems to be great reason for believing that the whole

What is Arago'ii oonelauon respecting polar meso temperature ?

How is the decrease of temperature above the earthS surface to be
known ?

Whftt appears to be abdut the mean rate of diminution in temperature,
according to increase in height ?

From what observations mis this result been obtained ?

What may we infer respecting the present compared with former states
of terrestrial climates ?

Ill . I IM I PI I I I I I I Mill ■ ■ ■ ■ I I M il— — ^^—

*^ Report upon the Recent Progress and Present State of Meteorolo|7.
By James D. Forbes, F. R. S., in Rep. of the British Asaoeiatioii am
1888, p. 816.

t Idem, p. 219*

384 PYBOMOMICS.

mass of the terrestrial globe is undergoing a gradual process of.
cooling, from an originally ver^ intense temperature. Baron
Fourier, who has distmguished hmiself by his investigations re-
lating to this curious topic, has proved that the heat may be very i
intense at a short distance from the surface, and yet from the ex- j
tremely bad conducting power of the crust or exterior strata of the
earth, that it may exert no sensible influence on the climate : he
actually computes it as not amounting to 1-30 of a degree of the
centigrade thermometer. Towards the centre of the earth the
heat may be of the most extreme intensity, and the phenomena of
earthquakes and volcanoes may be imputed to its influence.

26. The process of cooling, which at first must have been com-
paratively rapid, may be considered as having reached such a rate
as to be imperceptible. From experiments made at Paris, in the
caves under the Observatory, it is probablB that the influence of
the solar rays does not extend more than about 100 feet beneadi
the surface. Therefore the heat there will be nearly invariable ;
and throughout the superior strata a constant influx and efllux of
heat must be going on. As the influence of the sun does not ex-
tend beyond a certain depth beneath the surface of the earth, it
might be expected that beyond such depth the temperature would
continue the same all the year round in every inferior stratum re-
latively to its position; and that this is the case has been ascer-
tained by M. Cordier, from a collation of numerous facts observed
in Cornwall, Saxony, Brittany, Switzerland, AAierica, and other
parts of the world. It also appears that in all the inferior strata
the temperature increases as we descend, without any exception ;
ft circumstance decisively proving, that there must be a source of
heat in the centre of the earth.*

27. Hence it may be concluded, in conformity with the most
rational geological speculations, that the planet which we inhabit
was at a very remote period in the state of fusion, and like other
semi-fluid masses revolving rapidly under the influence of central
forces, it has assumed its peculiar form, a flattened spheroid.
During how many ages thetenestrial globe continued to emit heat
from its surface before compact strata were formed, such as the
various modifications of primitive rocks which we behold, and
how much longer time elapsed before those rocks became the

On what other cause than solar heat does climate depend ?

What conclusions has Fourier derived from his inTestigationa of this
subject f

To what cause may earthquakes and volcanoes be imputed ?

To what depth beneath the surface of the ground does the influence of
solar hciat probably extend ? ^

What is constantly takingplaee at levels above the irwarifible stratum?

How is the temperature ot the strata, below the invariable one, found
to be at difierent seasons ?

What relation has it to the depth of the several lower beds?

What does this prove in regara to the temperature of the central parts «
of the earth ?

* Report of British Association, p. 221.

PRODUCTION OF HXAT BT FBICTIOK. 9BB

iiasis of this world of land and water, nnmeroiidT peopled wiUi
living beings as at present, it would be aiterly aseless to attempt
to conjecture. It is sufficient for us to be able to state, as the re-
sult of the most accurate and eztensire observations, that the in-
ternal heat of the earth no longer affects in a sensible degree the
temperature of its surface, or of the strata immediately MMEwath
the surface ; and therefore the varieties of climate, and Ihe alter-
nations of heat and cold in the different seasons of the year, as
weH as the changes attributable to incidental causes, are chiefly
owing to the in&ence of the snn, as the general ^cientsonrce
of heat.*

38. Among the mechanical means of producing, or rather xtt
exciting heat, friction is. perhaps the most usual and effective. In
sawing wood, or borinff metal, it may be observed that the sub*
stances thus exposea to friction soon become sensibly warm*
The wheels of carriages sometimes take fiie, from friction against
the axles when in rapid motion. In some rude oountries, as isi
Patagonia, the inhabitants avail themselves of this mode of pro-
curing fire. They either rub tosether two pieces of hard dry
wood till flame arises, or more artifleiaUv insert the blunt-pointed
extremity of a rod of hard wood in a small cavity in a thick plank^

What besrioff have the obserred ftett in regard to iobtemaeaa heat

upon geoloncal theories ?

What efl»et has tbe internal heat of the earth on the t^iBperatore of
\X$ sarfaee f

By what mechanieal means raaj heat be excited f

What remarkable faett prove the efficaej of frietieB In produdog

■•^^•■■r"

* Some very remarkable ioilanees have been reeorded of extreme
heat, as DOtioed by several obserrera. In Winkler's Elemeota of Natu-
ral Philosophy, toI. i. pp, 17^—182, are two tables of very high and very
low temperatures obsenred at different times, and in Tarious ^tnationSy
eolleeted by Professor Heinsins. The highest atmospherie temperatore
whieh he reeords was obsenred at Senegal, on the eoast of Afriea, in lat.
16* N. when the beat was 86^* of Deliue's thermometer, eorresponding
to 1084° Fahrenheit. This temperature was considerably below that oIh
senreo at Bagdad,, in August, 1819, as stated in the Journal of Science,
Literature, and .the Arts, edited at ^he Royal InstitntioB, ISflO, vol. ix.

6, 493, <* On tbe 86tb of August, last year (1819}, the thermometer at
agdad rose in the shade to 190^ Fahrenheit, and at midnight was 108°:
many persons died, and tbe priests propagated a report that the day of
judgment was at hand." The p;reatest heat accurately^ obsenred in Eng^
Tana, of which we hare authentic accounts, took place in 1808 and 18S5 ;
July 13, 1808, the thenuometer, according to the Royal Society's Regis*
ter, rose to 93°.5 ; and Mr. H. CaTendish's thermometer at Clapham, to
96°. Dr. Heberden observed the heat in July, 1825, and found that on the
18th of that month the thermometer stood at 96°, and on the following
day at 95°.~p49ee Philos. Trans. 1896, part ii. p. 69.

In different parts of the United States the thermometer is freqiieBtly
known to rise, for a few hours at a time, to 95 or 100 degrees. On the
8th and 9th, S6th and 27th of July, 1834, the thermometer In Philadel«
phia stood at from 94 to 98 degrees, according to different exposures.-*
Ed.

2S6 PYRONOMICS.

and tarningr it with great velocity between their hands thus obtain
sparks and flame.

29. Count. Rumford instituted some important experiments on
the effect of friction in producing heat.* Having observed that
great heat was excited during the operation of boring cannon, he
procured an unbored cannon, with the large projecting piece two
feet beyond its surface, which is usually cast with the cannon to
ensure its solidity ; this projecting piece, was boared and reduced
to the form of a hollow cylinder, attached to the cannon by a
small neck ; the apparatus being wrapped in flannel to prevent
the escape of heat, it was made to revolve on its axes by the
power of horses, while a steel borer pressed against the bottom
of the cylinder. The temperature of the metal at the commence-
ment of the operation was 60 de^. and the cylinder, having made
960 revolutions in half an hour, it was stopped, and the tempera-
ture was found ndsed to 130 deg. In another experiment a borer
was made to revolve in a cylinder of brass, partly bored, thirty-
two times in a minute ; the cylinder was inclosed in a box contain-
ing 18 pounds of water, the temperature of which was at first 60
deg., but rose in an hour to 107« and in two hours and a half the
water boiled.

Stockenschileider, an ingenious mechanic of Nieuburg on the
Weser, invented a machine, by means of which great heat might
be produced, and water boiled by friction.

30. Air does not appear to be necessary to the production of
heat by the attrition of solid bodies. Boyle procured sensible
heat by making two pieces of brass m()ve rapidly in contact under
an exhausted receiver. Pictet, of Geneva, repeated the experi-
ment with success, and found that the introduction of a soft sub-
stance between the rubbing surfaces, such as cotton, occasioned
an increase of heat. Sir H. Davy insulated an apparatus fbr re-
citing heat by friction, by placing it on ice, in the vacuum of an
air-pump, under which circumstances heat was produced. He
likewise ascertained that two pieces of ice might be melted by.
rubbing against each other, either in the air of a room below the
freezing point, or under an exhausted receiver.

What practice amone nide nations is founded on this principle ?
In what manner did Rumford investigate the relatioB of heat to firietiont
To what result do his experiments conduct us?
What applications have been made of this principle ? ^
What extraoadinary results were obtained by Davy in his experiments
OR friction ?

" ' V

*The experiments of Rumford seem to prove the incorrectness of that
tl\eory which ascribes a material character to caloric 9 and as he ascer«
tamed that the borings taken from his cannon had not undergone any di«
minution of capacity for heat, it is difficult to ascribe the vast amount of
beat developed to any other cause than the vibration of the metal pro-
duced by the mechanical operations of rubbinr snd abrading.— £b.

t A plan has been started in New England of heating manufactories
and other buildings by the friction of metallic wheels, actuated by tlw
lame moving power which drives die machinery.— En.

COMBUSTION. 287

31 * Compression produces heat either in solids, liquids, or g^ases.
An iron bar may be hammered till it is red hot; and water strongrly
compressed ^ves out heat, as appears from the experiments of
Dessai^es, as well as from the interestingr researches of Mr.
Perkins on the compressibility of liquids, which hare been noticed
elsewhere.* Solids also give out heat when violently extended,
as may be ascertained by stretching quickly^a piece of Indian
^bber, and immediately applying it to the lip» when a sensible
'degree of warmth will be felt. Mr. Barlow, in his acoount of
some experiments on the cohesion of malleable iron, states it as a
curious fact, and deserving the attention of philosophers, that fr^
quentty at the moment of rupture the bai acquired such a degree
of heat in the fractured part as scarcely to suffer a person to hold
the bar grasped in his hand, without a slight painful sensation of
buming.f

32. %ut the effect of compression is exhibited in a more striking
manner in the production of heat from gaseous fluids, as common
air. When air is forcibly compressed by driving down the piston
of a syringe, nearly closed at the end, great heat is produced;
and syringes have been construced for the express purpose of
procuring fire, the heat evolved by the compression of air in this
manner being sufficient to kindle ary tinder or touchpaper.
■ 33. The chemical operations in the progress of which heat is
produced are numerous, and among the .most remarkable causes
of the evolution of heat from bodies becoming united, so as to
form chemical compounds, are those arising from combustion. All
substances are not capable of undergoing combustjion, and hence
the division of bodies into two classes, namely, combustibles, or
inflammable bodies, and incombustibles, or non-inflammable bo-
dies. Among the former are vegetable substances in general, as
wood,cbarcoi3, and oils ; most animal substances, as hair, wool,
horn, and fat ; and all metallic bodies.

34. The class of non-combustibleS> includes stones, glass, and
salts. The latter, when exposed to high degrees of heat, under
such circumstances that they cannot undergo chemical decompo-
sition, may be made to display the usual appearance of fire, or the
combination of light and heat, variously designated by the terms
flowing, red heat, or white heat, denoting different degrees of
incandescence, and when no alteration has been produced by the
high temperature to which they may have been exposed. But

combustible bodies are very differently affected by heat. ^ Some

I

What calorifict action attends mechanieal compression }
" Which class of natural, bodies illustrates most strikingly the influence
of condensation ?

Wliat division of bodies Has been formed in reference to the property
of undergoing combustion ?

What natural substances belong to each of these classes ?

Describe the effects of beat on these different classes.

^■^— ^^»^^-^-^— ■ I I H ill ^— — ^-^— ^.^_— — »^..j»»»^i^.^— ^—^i— >

♦ See Treatise on Hydrostatics^ Nos. 13 — 15.

t Enoyolopsed. Metropol.— Mixed Scienoei, p. 70.

2S8 FSTRONOmCS.

of them at comparatively low temperatures become combined with
the oxygen gas contained in the atmosphere around them^ and
they all undergo similar transformations at certain temperatures^
and during 9uch processes heat in the form of fire is frequently
exhibited.

* 35. Among the simple instances of the effect of cheoueal e(M&-
bination in causing the appearance of heat may be noticed the
increase of temperature that takes place when water is mixed
with alcohol, and which may be readily perceived on applying the
hand to a phial containing the two fiuids just after they have been
introduced into it. But me mixture ef water with sulphmie acid,
or, as it is commonly called, oil of vitriol, causes a much greater
augmentation of temperature than the preceding; for if an ouDee
of sulphuric acid be poured into a bottle^ containing ei^fat ounces
of water, the glass will be so much heated as to render it impossi-
ble to hold it ; and a more violent heat may be produced by increas-
ing the proportion of the acid..

Charaetenstie Effects or Properties of Heat,

I. DILITATION OR EXPANSION OF BODIBS.

36. The most obvious and direct effect of heater exaltation of
temperature is to add to the bulk of the heated body, or to increase
its dimensions^ generallj^in all directions. This takes place in
solids, liquids, and gases, without altering their essential pr<yp^r^

ties. The expansive effect of heat on solids,
may be exhibited by means of a cylindrical
bar of iron, as represented in the marginal
figure. When cold, it will be found that the
cylinder A B will exactly fit into the space
C, in the brass gauge annexed ; and it will
also pass throng^ the aperture D ; but when^
heated by pinning it for some time into boil-
ing water, it will be so much expanded tiiat
it will no longer fit into the space, or enter
the aperture. If the bar be coded* either slowly by exposure to
the air, or suddenly by covering it with ice or snow, it will again-
be contracted, and pass into the cavities as before. The more
highly the bar is heated the greater will be the amount of its ex-
pansion ; and on the contrary, when cooled, its contraction Will be
in proportion to the reduction of its temperature.

37. The effect of heat in expanding solid bodies, and especially
metals, has been advantageously applied to practical purposes.

What examples illustrate the effect of chemical combination ob the de*
velopement or heat ?

What immediate alteration follows the increiise of teroperatixre in all
foi*m8 of matter ?

In what manner may it be easily exhibited in the case of solid matter ?

What useful applications are made of expansion in the common arts
of life ?

^ DILATATION or flOLti>8 BY HKAT. t6t

ThvLB coopers, in fixing iron boops on a cask, make them pMyiousTy
tery hot, and being adapted in that state to the periphery of thtf
Cask, tbeir contraction in cooling binds together the staves of tlie
cask. Wheelwrights also nail on the iron tire or band, while it Is
nearly red hot, to the wooden wheel of a carriage,' and as the mstal
Contracts in Cooling, it clasps the parts firmly together.

38. llie expaAsibility and contractibilitt of iron as an efiaet of
temperature demands the particular attention of arehitects and en-
gineers, now that metal is so fVequentiy substitated for wood and
stone in the construction of ioofs of buildings, pillars, arohss, and
^'r other jmrposes. Due illowaiice should always be made for
tAj alteration of dimensions in metallic beaiAs or supporters, ds^*
j^i^i^g on alternations of heat and cold at different seasons of
the Voar, or arising from other cau^s. in the iron af^hes of Sotilh*
wark Bridge, erected by the late Mr. John Rennie, over theThamesi
the extreme vsCriations of atmospheric temperature, occasion a dif-
ference of height at different times, amounting to about one inch.

39. A curious example of the influence of heat on the dimeii'*
sions of solidGf was exhibited some years Since at Paris, in tb0
method adopted for restoring to a perpendicular direction the de-
clining walls of a gallerv in the Abbey of St. Martin, now the Con*
i^ervatory of Arts and iSades. The weight of the roof had pvea^
•ed outwards the side walls of the structure, and ^xeifed aopro-
heuslons for its safety, when M. Molard contriTod to render it
secui^ by the fo]\owii\g process : Several holes were made in the
walls opposite to each ouier, througli Which were introduced itoA
bars stretching across the gallery, with their extremities extending
beyond the walls; aud to these projecting parts were setewed
strong iroii plates, or rather large broad nuts. Each alternate bat
i^as then heated by means of powerful lamps, and tiieir lengtili
being thus increased, the nuts wnich had become advanced beyond
the walls were screwed up close to it, and the bars suffered t*
Oool. The powerful contraction of ttie bars drew closer the walla
of the buildmg ; and the same process being applied to the inter-
mediate bars, and repeated several times, the walls were gradually
and steadily restored to the upright position, and the danger appr»-
hendied from their declension was averted.

40* Mussichenbroek ascertained that heat not only expands me-
tals, but also different kinds of stones, chalk, burnt brick, and
glass. Such substances, however, must be perfectly freed from
moisture, otherwise increase of tcfmperature will occasion contiao*
tion of volume, by dissipating the moisture. Thus wood some*
times acquires an increase of specific gravity by drying. From

To what elasses of utisant is ihh brAieh of the lubjeot paitkUktrly
ImiKirtant f Why f

What ezeroplification of this is seen in bridge boilding?

Describe the &ikctii6d of MolaVd to rcMore walls to their vertieal pbsi«
tion.

In what ease may an apparent eontraabon toHew the api^UcaUon of
heat?

9B

200 PTRONOMICS.

some experiments to determine the weight of different kinds of
wood at various degrees of dryness, recorded by Mr. Barlow, it
appears, that in some cases there is a considerable augmentation
of specific gravity. A piece of Riga fir, 1 1 inches thicK, lost i of
an inch in seasoning, and the weight of a cubic foot was increased
from 546 ounces to 562 ; a piece of American pitch pine, 7^ inches
in tiiiickness lost i of an inch, and the increase of weight of a cubic
foot was as 518 to 524 ; a block of Halifax spruce spar, 5| inches in
diameter, was reduceil to 5|, and the difference of specific gravity
was as 541 to 544 ; and a block of Canada spruce spar, 4^ inches
in diameter, lost i of an inch, and the difference of specific gpravity,
or weight of a cubic foot, was in the ratio of 485 to 513 ; but in
most other cases, tihe loss of weight was greater in proportion than
the diminution of bulk, so that the specific gravity was dimi>
nished.* «

41. The effect of temperature in the expansion and contraction
of glass is an object of common observation, and becomes the
cause of frequent accidents. Though the degree of expansion
which takes place in glass at any given temperature is proportion-
ally much less considerable than that produced in metals, platina
excepted, yet from the irregularity of the effect, glass is easily
broken by the sudden application of heat. Glass ffoblets and
tumblers are very liable to fracture, when water heated almost to
the boiling point is poured into them ; and the dan^r will increase
in proportion to the thickness of the glass ; for this substance, ad-
mitting heat to pass through it but slowly, the inner surface be-
comes heated and distended bj the hot water before the outer sur-
face, and the irregular expansion causes the vessel to break. In
this manner, the glass chimneys now in seneral use for oil-lamps
and gas-burners are often destroyed. M. Cadet de Yaux state»
that uie danger of fracture may be prevented by making a minute
notch in the bottom of the tube witn a diamond ; and in an esta-
blishment where six lamps were lighted every day, this precaution
being adopted, not a single ^lass was broken byneat in the course
of nine years. A bottle of wme placed near the fire in cold weather,
will sometimes fly, as it is termed, especially if a draught of air
falls on one side while the other is receiving heat; and the glass
cylinder or plate of an electrical machine may be cracked and
spoiled by incautiously placing it in a similar situation.

42. On the relative expansibUUy of different metals by heat de-
pends the operation of compentotiDg pendulums, used for time-
keepers and astronomical ciocks.f

Is the increase of density a nnifonn result of the process of drying
wood?

State the experiments on this subject

What peculiar quality in glass renders the effect of expansion in that
substance conspicuous ?

How does Cadet de Vaox prevent the fracture of lamp glasses by heat ?

* See Encjdop. Metropol. — ^Mixed Sciences, ?oL i. p. 186»
t See Treatise on Mkchanica, No. S24.

DILATATION OF LIQUIDS AND OA8S8 BT BSAT. 291

40d The tnilixence of heat, as a dilatingr or expandingr powers
applied to liquids, is ffreater than in the case of solids. But the
degree of action which it exerts is different with respect to diffe-
rent liquids ; so that ether is more readily expanded than rectified
spirit, the latter is more expansible than water, and water more so
than mercury. This might be experimentally demonstrated by
fillin&r Uie bulb of a large thermometer tube with each of these
liquids in succession, and then presenting the bulb to a lamp at
precisely the same distance, and obserying the height to which
the liquid would rise in each case in a given time.

44. As different liquids undergo different degrees of expansion
at the same temperature^ so the expansibility of one liquid will
be found to increase or diminish under variations of temperature
in a different ratio from those which regulate the expansion of
other liquids. This irregular effect of heat is chiefly observable
in liquids which boil at a comparatively low temperature, as is
the»ca8e with water; while mercury, which requires a great de-
gree of heat to make it boil, or become evaporated, undergoes
nearly the same amount of expansion by the audition of any given
quantity of heat, whether at a low or high temperature; and
hence its utility in the construction of instruments for «measuring
heat.

45. The influence of temperature on the bulk or dimensions of
ateriform bodies, whether permanent or non-permanent, is more
strikingly exhibited liian in the case of Uie liquids or solids. This
may be ascertained by taking a bladder half filled with air, and
Irin^ it so that none can escape, when, if it be held near the fire,
me included air will expand till the bladder is fully distended ;
and if while in that state it be plunged into cold water, the air
will contract in bulk, and the bladder become flaccid. Such a
bladder if very thin, would form an air-balloon, which would as-
cend, when heated, to the ceiling of a lofty room, and fall down
as soon as by the gradual cooling of the aix within it the specific
gravity of the mass was reduced below that of an equal body of
the sunoundingr air. The expansibility of air by heat may also
be demonstrated by means of Uie apparatus represented below.

46. It consists of along glass tube A B, with
a bulb at one end, and open at the other, which
is plunged into the jar of water C ; then on
heating the bulb by means of the lamp D, the
air wiSiin the tube will become expanded, and
issuing in large bubbles from the aperture B,
it will rise rapidly through the water in the jar;
On removing the lamp aner a considerable por-
tion of the air has been expelled, the water will
rise in the tube to supply its place, as the tube

How are liquid and solid bodies eomparatiTelj aifeeted by inerease of
iepiperatm-e ?
What relation ha?e the different liquids to each other in this respeot?

Qools. The reapplication of the lamp to the hulh ^ a greater ihh
iance than hefore will again dilate the inelnded air and depieat
^e i^rater in the tube ; and the liquid n),ay he made thus to liae
fuid fall alternately by cooling and heating the hplh.

47*. It is of importance to observe that airs, gaees, and Taponrs,
are all alike affected by giyen quantities of heat; that is, they not
only all expand in the san^ proportion at certain degrees of tem-
perature, but their rate of expansion under any increase of tern*
perature is likewise uniform. In this respect it will be perceiyed
that gaseous bodies differ from solids and liquids; for while both
^e latter kinds of matter display the utmost dissimilanty in their
relations to heat as an e^smdiog power, the former always under-
go expansion iu exact proportiou to the degree of temperature to
which they are exposed.

hittrumenU far Measuring Hea$.

48. The universal influence of heat on the dfinensions of ma-
terial substances affords a convenient method of estimating th^
Relative quantity of heat which will produce any given effect ; for
since it appears that a certain increase of temperature will always
be accompanied by a certain degree of expansion of bulk, it fol-
lows, that if we ci|n estimate correctly the degree of expayisibq in
any given case, we may thence infer the amount.of temperature.
tJpon this principle depends the utility of those philosophica}
instruments called Thermometer^* ^nd |^yrometer8.f

4 J. Among the former of these instruments is that which ire*
quently accompanies the barometer, indipatipg b^ the expausiot
of mercury, or some other fl^id, the relative temperature of thf
atmosphere, at different times or plape^. The mercurial thermomev
ier consists essentially of a glass tube with a bulb at one extremity,
and which having been filled -y^ith mercury at a certain tempera-
ture, introduced through the open encl, has been hermetically seal*
ed while full, so that no air c^ afterwards enter it. As tlie tube
and mercury in it gradually become cooled, the inclosed fluid con-
tracts and consequently smks, leaving aboye it a vacant spaoe of

•

* In what decree will the san^e liquid be fonod expanded by e^oal quai^-
tities of heat when applied to it at different teinperatares ?

In what class of liquids is this prinoipie most strikingly verified }

Hpw is the effect of temperalnre on aeriform bodies exhibited ?

How may the ascent of a mass of heated air be Visibly illustrated ?

Explain the manner in which the expansioa of air is proved by hating
a glftu bulb.

rlow are the different kind* of air and vapour relatively-expanded |j
Jieat ? '

Of what use to science and arts is the principle of expansion ?

Of what does the mercurial thermometer consist ?

What two points are usually established on the tube before graduating
the scale of a thermometer ?

* From the Gi*eek e(pAi«$, hot, or e«p/iii, heat, and Marfov, a meaiare.
i From xsuft fire, and ji«irf«t

INVEMTIOK OF THE THEIUIOMETER. 203

v^eaum, througli which it may again expand on Ihe application'
of heat. To such a tube it is necessary to add a scale snowing at
what height the mercuTy will stand at the temperature of freezing
water, and what will be the rate of expansion at any other poin^
as that of boilin? water, together with the amount of expansion
at regular intetviSs between those two points.

50. In what is called the centi^de thermometer, now used in
France, the freezing point of water is marked on the scale zero ^0^^ ;
and theboiling point 100^, the intermediate space -being accordingly
divided into one hundred equal parts, regularly marked and num-
bered ; and as the scale may be continued to any required extent,
above or below zero, any degree of temperature may be thus as-
certained, at least between the freezing and the boiling points of
mercury ; and as this metallic fluid requires a far more intense
cold than water does to make it freeze, so it will take a much
greater de^ee of heat to make it boil ; and the scale may ttius be
extended m both directions. Mercury freezes at 40 deg., or 40
centigrade degrees below zero, or tiie freezing point of water; and
it boil3 or becomes sublimed, in vacuo, at -f- 350 deg., that is, it
takes a hi^er temperature by 250 centigrade degrees to make it
boil than is required to make water boil.

51. Any fluid mi^ht be employed to mark, by its relative ex-
pansion and contraction, the temperature to which it might be ex-
posed; and thus sulphuric acid, water, alcohol, oil, and air, have
been variously adopted in the construction of thermometers for
different purposes. The invention of this usefhl instrument ap-
pears to have occurred in the early part of the seventeenth century ;
and the mode of measuring heat first employed was by observing
the expansion of air confined in a ^lass tube. It is rather uncer-
tain with whom thi^ idea originated ; but among those who have
laid claim to it may be mentioned Cornelius Drebbel, of Alkmar,
in Holland, and Santo Santorio, professor of medicine at Padua,
in Italy ; and it is not improbable that this method of discovering
the irelative effect of high or low temperature may have been in-
dependently adopted by both those ingenious men. Drebbel, who
passed some part of his life in England, in the reign of Charles I.,
certainly introduced tiie knowledge of the thermometer into that
country.*

5^ The original thermometer was a very imperfect instrument.
It consisted of a glass tube with a bulb turned upward, and the
lower portion of the stem partly filled with a coloured liquid, and
inverted in a globular bottle partially filled with the same liquid ;
80 that the portion of air included in the bulb and upper part of
the tube was exposed to atmospheric pressure, and tnerefore the

How is the freezing point marked on the eentigrade thermometer?
How the boijing point ? "

At what temperature on that scale does mercury freeze ? and at what
point does it boil ?

By whom and at what period was the thermometer inyentcd ?
What was the ooostruotion of tlie oi'igiiud thermometer ?

2b2

994 PTRONOMICS.

cflbot of h^ on it coold not be accurately appreciated* It was
ipdeed merely adapted to afford a general estimate of the influence
of temperature on the bulk of air ; much in the same manner as
i( is eihibited by the apparatus preyiously desciibed.* This kind
of thermometer was improved by the French philosopher Amon-
tons ; and Leslie, Wollaston, and others, have adopted several
modifications of the air-thermometer, as a delicate instrument for
indicatiuff trifling variations of temperature; but the extreme sen-
sibility of air to the impression of heat must ever confine its utility
to suchpurposes as those just mentioned* ' -

53. The greatest defect in the early thennometers arose from
the want of a regular scale of temperature, with fixed points to
form a medium of comparison between the ejects of heat as ex-
hibited under different ciici^nistances, or in its operation on difife-
Tent bodies, fioyle proposed the congealing point of oU of aniseed
•for this purpose ; but Dr. Hooke witfai greater ])ropriety recom-
mended me nreezing point of distilled water ; and Sir Isaac Newton,
adopting this as the comraencemeqt of his scale, or the point zero
(0^), he ascertained that 34^ would mark the boilingr point of war
tcr, as indicated by the relative expansion of linseed oil, the fluid
which he used to nil his thermometer.f

54. The discovery of two fixed points for the thermometrieal
scale contributed vastly to the improvement of the instrument;
but ^hat of Newton was rendered imperfect by the nature of #ie
inclosed fluid, which did not move freely within the tube^ and by
the inconvenient length of the degrees of the scale. Hence other
men of science employed theniselves in contriving b^ various
methods to augment the utility and accuracy of this instrunient.
Beaumur, in France, invented a thermometer filled with tinged
spirit of wine, with a scale divided into 80 degrrees between the
freezing and the boiling points of water. Bu( as spirit of wine boils
at a lower temperature ttian water, and as it bould afford no indication
of any degree of heat beyond its own boiling point, on this ao-
Qount, the spirituous fluid was exchanged fof mercury ; and such
^ n^rcurial thermometer, wi|h Reaumur's scale, was in general
use in France till the period of the revolution, when it was super-
f^eded by the centigrade thermometer, already noticed.

55. The emplo]pient of mercury as the most suitable ffluid for
the thermometer is usually attributed to Fahrenheit, a native of
Pantzic, who settled at Amsterdam as a philosophical instrument
maker ; and his instruments having the merit of great accuracy

Who arc amon|[ the Inpravers of this iMtrameDt ?•

What defect existed io the origiiial ^thermometers f-

What limits the useful application of the air thermometer ?

What peculiar disadvantage had the thermometer of NcwUm?

In what manner did Reaumur divide his scale ?

^iVhat scale has sc^perseded that of Reaumur io Fraoce f

* See above, No.4Sw

f See Cotet*s HydsSilst Ust. Apptndis, No. H.

DIFFEBSNT KWM O? tlHEIUIOlIETERfl* fig9

and i^ciatiieas id executioii, became muck soagfat «il«r, and hjui
name has been permanently connected with that form of tha
thermometer, bow generally u^ed in Ki^land, Holland, and the
tlnited States. It appe^rsy however, from the statement oi Bobf-
haave, that the improvement of the thermometer, so jfar as relates
to .fiUiflff it with mercury, and fixing on the peculiar scale denomi-
nated i&sr Fahrenheit, ought rather to be ascribed to Olaus Roe*
mer, a Danish philosopher, to whom we owe the discovery of the
velocity of light.* The peculiarity of this scale is, that it does
not commence at the freezing point of watec, but descends mnch
beldif it ^

^. It is usually stated ^at Fahrenheit obtained the point,
whence he commenced the graduation of his therraometei9, by
producing artificial cold from the nfiixture of common salt and
snow ; but from the aathority jvat dted, it appears that zero of
Fahrenheit's, or rather Roen^er'a scale, was d^ved from tho
lowest degree of temperature, or greatest cold which had been
observed in Iceland, whicfi wap ^ed on from an erroneous sup-
position that this was the extreme of Iqw temperatose which was
ever likely to become the object of philosophical investigation.

57. Among the numerous modifications of the thermomet^pro-

Sosed by> in^nious men, as adapted to the general purpose of in-
icatin^ vanation of ^mperature, the only one besides &e preced-
ing which requires to be here noticed, is that of J. Delisle, member
of ths Academy of Sciences, at St. Petersburg. It differs prin-
cipally from other instruments in having a scue, the graduation
of which commences at the boiling point of water, and is reckoned
downwards : the distance to the freezing point being divided into
150 de^ees. Its use is nearly confinea to the Russian ^empire,
where' it is eenerally adopted by men of science.

58. As the centigrade diermometer, originally invented by
Olaus Celsius, of Upsal, in Sweden, ai^d that of Fahrenheit, are
at present commonly used in registering observations on tempera-
ture, in France and Great Britain, while those of Reaumur and
Delisle have been employed by several eminent philosophers in
making and recording their peculiar observations, it becomes re-
quisite that the means should be afforded for ascertaining ^e
relativer value of degrees of teipperature, according to either of
these scales, and of oonvertii^g any given nun^ber of degrees be-
longing to one scale into degrees belonging to that with which we
are moist ^miliar. Fahrenheit's scale, commencing at zero (0^),
ascends to 3d^ the freezing point of water, and thence to 213^, ths

In what conntriei n tjie scftle of Fahrenheit chiefly used ?
To whom belonrs the applieatlen of mereury, and the original nse of
the scale adopted oy Fahrenheit f
How did Roemer actually obtaio the zero pf his instrameot ?
Where did Delisle commence the |;raduatioo of his thermometer ?
Where and by whom was the ceQtigrade thermometer inveoted ?

1 9Pfirbayii £kaatata GhMiitt, t V p. rail

296

PTR0N0M1C8.

boiling point; so that there are 180 degrees, in the scale, between
these fixed poiifts.

59. The following table exhibits conresponding numbers of the
several scales of Fahrenheit, Reaumur, belisle and Celsius, or
that of the centigrade thermometer, from a temperature 13 degrees
above the boiling point, Fahrenheit, to 96 degrees below zero.

Fahr.

Reaumar

Delisle

Centigrade

•

224

85 3-9

10

106 6-9

212

80

100

Boiling point of Water.

192

71 1-9

16 4-6

88 8-9

V

160

56 8-9

43 2-6

71 1-9

128

42 6-9

70

53 3-9

96

28 4-9

96 4-6

35 5-9

Blood heat

64

14 2-9

123 2-6

17 7-9

38

150

Freezing point of water.

14 2-9

176 4-6

17 7-9

>

32

28 4-9

203 2-6

35 5-9

39

31 5-9

209 1-6

39 4-9'

Freezing point of mercury.

64

42 6-9

230

53 3-9

90

54 2-9

251 4-6

'67 7-9

J Greatest known degree
I of cold.

96

56 8-9

256 4-6

71 1-9

60. Hence it will appear, that 1° of Fahrenheit's scale is equal
to 4-9° of Reaumur's^ 5-6° of DeUsle's^ and 5-9° of the centigrade
scale. Therefore in order to convert any number of degrees of
Reaumur into corresponding degrees of Fahrenheit^ the-give'n num-
ber must be multiplied by 9 and divided by 4, and if it be a number
above zero, 32 must be added to the product, and the amount will
be the degree required ; but if the number be below zero of Reau*
mur^ and above zero of Fahrenheit^ that is any number less than
14 2-9, the product must be subtracted from 32; and if it be a
number below 14 2-9, 32 must be subtracted from the product,
to obtain the degree required^ In the same nianner the correspon-
dence between degrees of the centigrade scale and Uiat oi Fahrenheit
may be ascertained, only the gi^en number of centigrade degrees
must be multiplied by 9 and divided by 5. To convert decrees of
Delisle into degrees of Fahrenheit^ the given number must oe mul-
tiplied by 6 and divided by 5, and tl^ product subtracted firom
212 will be the number required ; but if the number be below
zero of Fahrenheit^ or 176 4-6 Delisle, 212 must be subtracted
from the product; and if the number required be degrees of Delisle

Name the boiling points on the four therraometric scales.

What are the freezing points on them respectively ? congealing point
of mercury .' greatest knovrn degree of cold ?

What rules can he given for converting degrees of Reaumur, Cel8iu%
mod Delisle respeotively into those of Boemer or Fahrenheit ?

tibayp 9ero, fil9 must be «dded U> the pT^dnet to obtiin ts» iiiimbei
jequired, deootiiiig Uij9 cojrraspoDding degree of FahrcnheU,

(SI. The ooeprcuriai thenno^ietpr i9 the meet coj^Teoient ioetni*
inei^t ipr mi&s^vaing any dep^e of temperature hetween 644 deg^
Fahrenheit, at which the lu|uid hoil9, and 39 deg. below zero, ai
iwhich it fieezes. For the n^iisur^tioii of more intense de^rrees
of cold, 9 thermpI^^teir may be employed ^led with alcohol, tmged
red by means oif alkaaetproo^; f^r that fluid, when otherwise per^*
fectly pure, wil) remain uncongpaled at a temperatjare much lower
ihan th«t at which mercury fireeaes.

6)3. M t^ere is no known lic^iaid that continues unevaporated at
a higher temperature than mercury, the relative effect of very hig)|
^legrees pf heat is usu|il|[y est^i^ated by t^e alteration of bulk that
iakes placQ in eolid bodies. Heat gpnerally expands substances
of sJl kinds ; but ]y[r. We^glV^ood c^bserv^ that fine porcelain clay
becomfcs cqiitr9cted by exposure to gr^at )ie9t ; and he found, 09
investigatiop, that pieces of pure elfiy c^urefolly dried, and then
^po^ed to a )red heat ip a furnace, exhibited ^ sensible degree of
pontr^oD, which Temataed ytk^n they «g?in became cool ; and a»
ft further appear^ that the pontraetioa proceeded with the aug-
mentation of heat, till yitrific^tion took place in the clay, he con-
^^yed the i^ ef fi»ii|ui)g ^ pyrometer, 01 measurer of heat, cod-
insting q{ a wmb^r pf tes^pieces pf prepared clay, in the shape
pf ^m^l datte^ed eylipdeis, ^ad |l scale composed of brass lodf
i imsb s^u^j^i apd 9 ieet lopg, i^jsd tp 9 him^ piate, obli<|uely
inclining mwards, so as to be som^iyhi^t i^j^^ toffether ^t oi|e
end thaii at the other, and marked witii a scale of equal parts,
poijciq^pnping at t)^p yndpt extremi^.

63. A^ the contr^ict^oh pf the claj pieces is permanent, a fresh
pne ipu^t be ?S^ for i^ch trial, which is to be jgaade b^ exposing
opp or mor^ te^t-piepes to tne heat, the intepity' of which is to ba
aiscertained, and when thus heated and aarain cooled, the contrac-
tion that h^ occurred i^ to be measured by sliding the piece be-
tween the brass rod^ so far as it will gOy and observing the dimi-
nution of bulk as indicated, by the scale, all the pieces being
adapte4 exactly to $t the widest pi^rt of the scale before their ex-
posurp to the heat^ ^e estimation of iiirhich is the object of experi-
ment. The seeipingly anomalous .effect of heat on which the
proper^ of tins instrument depends may be accounted for, as the
result of the concentration of tne particles of tiie clay by the more
intipdpte union of the alliaceous and silipeous earths of which
it is composed.

64. £aoh degiee of Wedgvood's scale is equivalent to 130 de

Within whfX limits mty'tlie mersuritl thermometer be employed ?

In wliat manner bfts- it )>een vsaal to eptimate very high degreci of
heat ? On vhat observation did Wedgwood found the constmetion of
his prrometer ? Can Wedgwood's standard pieces be repeated^ VMed
for the same purpose f

In what manner is the eootraetion of thp poreelgin pieecs to he it*
•ounCedforf t f

208 mtoNOMiCfl.

grees of that of FahTenheit; and the former commences at 1077^^
of the latter scale. The mode adopted for institutingr a compari-
son between the two scales was by observing the expansion of a
P3rrometric piece of silyer and of a test^piece of clay, as relatively
exhibited at 50 de^. and 212 deg. Fahrenheit, and at higher tem-
peratures as indicated by the brass scale. Having thus obtained
a common measure of high temperature, the inventor of the py-
rometer proceeded to make various researches concerning the
melting points of metals, and other subjects ; and it may be stated
as the result of his inquiries, that the greatest heat he ever pro-
cured was from an air-furnace, amounting to 160 deg. Wedgwood,
equal to 21,877 deg. Fahrenheit.

65. Doubts have been started whether the contraction of clay
affords a uniform measure ef temperature ; and the more recent
investigations of M. Guy ton Morveau, and Mr. Daniell, render it
very probable that Wedgwood formed his comparison of the py-
Tometric and the thermometric Scales on an erroneous assnmption
relative to the melting point of silver. Hence the calculations
grounded on experiments made with his pyrometer are not to be
absolutely depended on ; though the instrument is well adapted to
the exigencies of the i>otter, as affording the means of ascertaining
the heat of furnaces with sufficient exa^ness for many purposes.

66. A great many pyrometers have been invented by varioos
experimentalists, exhimting different methods for measuring, with
more or less accuracy, the relative expansion of bars or wires of
iron, or of some other metal.*

Several of these are con-
structed on the principle of
that represented in the mar-
sin, in which a bar of metal,
A, may be so placed, that
when expanded by the heat
of a lamp B, one extremity
will press against a lever
and cause an index, C, to
move along the graduated
arc D; and by means of such a pyrometer, the effect of heat, ap-

Slied in the same manner, for a ffiven length of time, to bars of
ifferent metals having the same length and diameter, may be as-
certained.

67. Mr. Daniell contrived a pyrometer adapted to measure the

How did Wedgwood unite the indieations of bis scale to those of the
eommon thermometer ? What relianee is to be plaeed upon it as an ab-
solute melisore of temperatare ? To what practical purpose may it be
usefully applied ? On what principle have pyrometers generally been
constructed ?

* For descriptions and figures of a number of pyrometers, invented
by ingenious British and foreign philosophers, see a Treatise on the
Thermometer and Pyrometer, published hj the Societjr for the ProoM*
tioa of Useful Knowledge.

DtDieU.

Fahrenheit

. 63°

441«

66

463

• 87

609

93

644

. 94

658

140

980t

. 163

1141

267

1869

319

2333

364

2548

370

2590

497

3479

XETALIJO FTROKETERS. 399

expansion of a rod of plandna, made to move an index OTer a
liial-plate divided into 360 degrees, each equal to 7 degrees of
Fahrenheit. He published an account of experiments made oy
means of his pyrometer, the result of which may be subjoined, as
bein^ probably the most exact yet published relative to the effects
of high temperatures.*

Melting point of tin .

bismuth .

lead •

Boiling point of mercury •
Melting point of zinc . •

Red heat visible in full daylight
Heat of a common parlour fire •
Melting point of brass •

silver .

cooper

gold

^^— — — cast bon .

68. Pyrometers, or rather metallic thermometers, suited for mea*
Buring with great accuracy small variations of temperature, have
been constructed by contemporary artists, among whom may be spe'
cified Breguet and Frederic Honriet, of Paris, and Holzmann, oi
Vienna^

In the prosecution of delicate experiments on the influence of
temperature, those thermometers may be most advantageously em-
ployed in which the effect of heat is exhibited by the expansion
of air, included in a small tube with a scale annexed.

69 Among the more recent and useful forms of such instmmentSy
the more important is that called the Differential Thermometer,
invented by Leslie, and described in his ^* Experimental Inquiry
into the Nature of Heai."

It consists of two bulbs or glass spherules A
and B, 'connected by the tul^ C D E F, bent
twice at ri^bt angles, and supported by a wooden
stand G. Within the tube is a small quantity
of coloured sulphuric acid ; and when a heated
substance is brought near to the bulb A, the in«
closed liquid recedes, and rises on the opposite
side, where its relative height, as indicated by
the scale attached to the tube E F, will show
the degree of expansion of the air in the tube'
and bulb A C D. One of the principal advan-

* Subsequent to the publicatioo of this table, Mr. Daniell published
others, difrering very considerably from these. Besides which, Mr.
Prinsep of Benares, in the East Indies, has published the resalts of some
experiments with an air pyrometer, and the editor of this work has made
numerous experiments with his steam pyrometerj^ described in the Amer.
Jour, of Science and Arts, voLzxii. p. 96.* -En.

t Probably too Iow.^Ed.

fag«8 attending the nM 6t this inst^tnent is its not h ^ing liable tor
error f^om changes in th^ temperature of the atmosphere ; for the
heat of the surrounding air mus^ a6t on both bulbs in the same
tnanner, therefore when a heated object is applied to ohehnlb only^
the whole effect produced by k will appear fttAd the AWeteat
amount of expansion of the enclosed air ; or if a cold object be
applied the effect will bd equally obvious from the different co>
traction which takes place ; and nence the instrument is named i
differential thermometer.

' 70. The actual amount of expansion that takes pX^te tn dtffbreiit
bodies raised to the same temperature is, as already obe^rved, by
no means equal. According to rec^t experiments of Herbert oi
the expansion of solids by heat, it appears fliat rods of gl^s and seye-
ral metals, of the same length at I3ie freetilng point of water, were
variously extended at Ibe Soiling point. Tlius the longitudinal
dimensions of each being supposed* divisible into 100,000 parts, at
33 degr. Fahrenheit, each sabstance, at 212 Aeg, wad au^6nfed
in the following proportions :

Platina • , * • • 85 parts*

Glasa 86

Gold 94

Iron 107

Copper • • • • 156

Br&ss 172

Silver 189

Tin 212

Lead , • • • • 262*

71. Liquids also expand unequally at different temperatoiesi
and different liquids are variously affected, by the same tempera*
ture. The irregular expansion o^ liquids interferes with the re*
suits of experiments made by means of common thermometers;
but mercury as exhibiting more uniformity in its rate of expalision

What substance did Daniel! adoftt for the measure o^ high tempera-
tures?

What temperafure did he assign for that of redness in dkyliglit ?

What did he obtain fbr th<6 melting point of silver ?

What for that of cast iron f

What species of experimeols may be adyantagidosly proiecuted with
the air thermometer ?

What liquid is employed in the. differential thermometer ?

What is one of the chief advantisges of this instrument ?

Are all bodies equally adapted tt> the formation of instruments to mea*
wire heal by expansion r Why ?

By how many miUIonths of its leneih, taken at the freezing point, will
a bar of platina be found expanded wnen raised to the boiling point oi
water ? a bar of iron ? of silver ? of leftd ?

How ar^ liquids affected by equal aagmehtatlonk t>t temperature in dif*
f^rebt parti of the scald ?

• Vieth's Elem. of Nat Philos. (Otriii.), p. 514

^

RATE OF EXPANtlOU OF LIQUIDS. 801

than other fluids, as water or alcohol, is better adapted than they
are for thermometrical investiffations. Indeed the more readily
liquids evaporate under the influence of heat, the grreater will be
their dilatation, when it takes place without change of form ; and
therefore ether and alcohol expand more in proportion at relatively
high than at low temperatures, and mercury, which requires a
great heat to make it boil, increases its rate of expansion mors
slowly.

73. The following table of the expansions of liouids is derived
•from the researches of Mr. Dalton, who ascertained that an eleva-
tion of temperature from the freezing to the boiling point of water
would canse an increase of volume m the ensuing proportions.

Mercury as 1 to .

. 00900

Water . . .

0-0466

. 0*0500
0*0600

Sulphuric acid . . •

Muriatic acid • • •

• 0*0600

Oil of turpentine

0*0700

Ether

• 0*0700

Fixed oils • • •

0*0600

Alcohol ., • , •

. 0*1100

Nitric acid •

•

0*1100

73. Aeriform fluids, as before stated, all dilate alike, and undergo
uniform degrees of expansion at different temperatures. This pro-
perty of gases and vapours depends on their being destitute or co-
nesian, so that the influence of heat operates on them simply and
independently, its effect noltbeinj^ modified by any opposing power,
as in the case of solids and liquids. From the experiments of
Gay Lussac in France, and those of Dalton in Endand, it appears
that all elastic fluids, whether airs or vapours, wnen raised from
the temperature of 33 deg. Fahrenheit to 212 deg., become ex-
panded nearly in the ratio of 100 to 137.5 ; or 100 cubic inches of
gas at the freezing point of water, if heated to the boiling point,
would be augmentea in bulk to 137i inches. Hence the expan-
sion of volume for each degree of the centigrade thermometer
would be 0.375, or reckoning the bulk at zero as 1 (unity), the
augmentation for each degree would be 0.00375. Dalton estimates
the increase of bulk for every degree at 0.00372, which would be
nearly equivalent to 0.00208 for each degree of Fahrenheit's tiber-
mometer.

How is the rate of dilatation related to the bnilinr point of liqiuds }

What example! prove the truth of this prinej{>le 7

How much IS mercury expanded by the addition of 180 degrees Fah-
renheit to its temperature at the freezing point ?

On what property of gases is their uniform rate of expansion supposed
to depend ?

How roach is the bulk of a gas enlarged by heating it from the freez-
ing to the boiling point ?

What will be the rate for one degree f

2C

WZ PYKONOMICS,

74. Tlias it appears that the density of sabstanceB generally
bears a certain relation to their temperature, being augmented by
cold and diminished by heat, or in other words, contracted by ex-
p'osure to a low temperanire and expanded at a high cempera-
ture. So far as we can judge from experiment, the maximum
density of solid bodies must be at the lowest temperature which can
be produced. And the same may be stated with respect to liquids
which are not susceptible of being solidified by cold, or frozen.
But thisjdoes not always hold good with regard to freezing or
congealing liquids ; and water is especially remarkable for its prof
perty of expanding in the act of congelation, whence, as is generally
known, vessels are liable to be burst, in winter by the freezing of
aqueous liquors contained in them ; and loose ice is always seen to
float on water, in consequence of its inferior sp^ific gravity.

75. From the researches of Deluc and others, it appears that
pure ^ater acquires its maximum density at the temperature of
40 deg. Fahrenheit, whence, if the cold be increased, it expands
till it reaches the freezing point ,32 deg.; so that ice at 32 deg. has
the same specific gravity as . water at 48 deg. But for this pro-
perty of water, large ponds aiid lakes exposed to intense cold
would not merely be frpzen over, as is usual in the winter season,
but they would become entire masses of solid ice. For ice once
formed, if heavier bulk for bulk than the water beneath it, would
sink to the bottom of the pond or lake, and rem^n there to be
augmented by fresh descending portions, as long as a frost lasted ;
but its relative levity causes it to continue on the surface of the
liquid which it protects in some degree from the cold atmosphere,
and congelation consequently proceeds more slowly.

76. This remarkable property of liquids near the point of con-
gelation is certainly not, as generally stated, peculiar to water, for
other aqueous fluids are afiected in the same manner ; and there
is reason to believe that metallic and oth^r substances, which have
been melted by exposure to great heat, contract in cooling only to
a certain point, and then expand, like water, so that the density
of a mass of metal just become solid will be inferior to that of
the same metal a few degrees above that at which it takes the
solid form.

77. Reaumui; states that iron, bismuth, and antimoay, are more
condensed just before they beco^me solid than aflerwarids; and he
observes that hence figures Qast in iron are correctly marked, from
the expansion of the metal in cooling, which causes it to press
into the most minute indentations of uie mould. Sulphur exhi-
bits the same appearances, when used for taking impressions of

What must be the maximum density of solid bodies ?

Why are closely corked bottles burst when their liquid contents freeze f

Why does ice not sink to the bottom of cold water ?

Is the property of expanding near the freezing point confined to a no-
gle liquid ?

What causes the accuracy with which iron and other metals fill Um
moulds, and thus yield *< tharp caatin^i?"

LATBNT BSATk

medals ; and it is probable that all bodies capable of fosioii by heat,
would, under similar circumstances, be found to have less density
at the point of solidification than jast before the commencement
of that process. As to the cause of this phenomenon, the most
probable conjecture is that of De Mairan, who, in his Treatise on
Ice, ascribes the expansion of freezing water to the new arrange*
ment of i(s particles consequent to crystallization, so that die
minute and still invisible intervals between the molecules of the
mass are larger or more numerous in ice than in water 8 deg. above
the freezing point. But this interesting topic demands farther
investigation.

Latent Heat^ and its In/hterue on the Forms of jBodiea,

78. No indication is afforded by the thermometer of the abso-
lute quantity of heat which any substance mav contain, but merely
of the amount of free or sensible heat capable of producing a cer-
tain degree of expansion in a column of mercury. If a quantity
of ice at 32*^ Fahrenheit be placed in a jar set ii? a basin of water
considerably heated, the ice'wiil gradually melt, absorbing heat
from the water through the sides of the jar ; but though it must
thus receive successive portions of heat, they would produce no
effect on a thermometer within the jar, the mercury in wliich would
remain at the ff-eezing point till all the ice became dissolved. So
that any quantity of beat thus absorbed by ice in the act of thaw-
ing would becomie combined With the liquid, constituting what is
termed latent htat, as not being appreciable by the thermometer.

79. Different bodies require different quantities of heat to raise
them to the same thermometrical temperature, or at least they are
differently affected by exposure to the same temperature. Thus,
if a quart of water and a quart of olive oil be removed, from a
toom in which the heat of the air is but 40^ Fahrenheit, to another
room heated to 80°, both liquids would gradually acquire the lat-
ter degree of heat, as might be ascertained by placing a thermome-
ter in either liquid. But the oil would be perceived to have become
raised to the temperature of 80° much sooner than the water ; and
hence it has been inferred that a smaller quantity of heat is re-
quired to produce an augmentation of 40° of temperature in the
former liquid than in the latter. As oil becomes heated more
speedily, under the same circumstances, than water, so likewise
it cools faster than water ; as would appear on reversing the pre-
ceding experiment.

To what does De Mairan attribute the diminution of density in bodiei
at their points of congelation f

To what is the indication of a thermonieter limited f

In what change of a solid bodj is teimble converted into latent beat ?

When equal quantities of different bodies are exposed to a change of
temperature, what difference may we expect to find among them while
undergoing that change ?

Exemplify this in the case of two liquids.

804 FYSOHOIIICS.

60. When ec|aal quantitiet of the same body at different tem-
perataies are mixed, the temperatare of the mixture will be at the
medium between those of the two portions : thns a pint of water
3^9 added to another pint at 98^ would produce a quart of water
at 65^ ; half the difference between the temperature of the hot
water and the cold (33^) having been taken from the former and
added to the latter* But the result is very different when dissimi-
lar bodies at different temperatures are mingled : for if one pound
of water at 156^ be mixed with one pound of mercury at 40^, the
common temperature will be 153^, instead of 98^, the medium
temperature, which would have been that of' the mixture if water
had been used instead of mercury.

81. From this experiment, it appears that the water gires up
4° of its heat to raise the mercury 113^ ; whence it has been
concluded that water has a greater capacity for heat than the me-
tallic fluid, in the ratio of 113 to 4, or 38 to 1. If the experi-
ment be reversed, by mingling one pound of mercury, at 166^
with one pound of water at 40^, the common temperature will be
44^ ; the mercury having been deprived of 113° of its heat, while
the water has acquired but 4°. A pound of gold at the tempera-
ture of 150° added to a pound of water at 50^ will raise the tem-
perature of the liquid but 5°, while it will lose 95°, the commoQ
temperature being 55°. Hence the relative eapadiy for hoot of
gold and water would be as 5 to 95°; so that the capacity of
water for heat must be 19 times greater than tiiat of gold. Bat
the results of different experiments on specific heat, vary con-
siderably from each other. Thus Lavoisier and Laplace make
the specific heat of mercury .039, water being 1.000; Petit and
Dttlong make it .033 ; ICirwan .033, and Dalton .0357; these dif-
ferences are, probabl^r, to be attributed to the different methoda of
conducting the experiments.

83. Several attempts have been made to ascertain with greater
precision the quantities of heat given out by different substances
under various circumstances. Lavoisier and Laplace constructed
for this purpose an instrument called a calorimeter, adapted to
measure the quantity of ice melted by different bodies, in the pro-
cess of cooling from any given temperature to 33° Fahrenheit.
Various precautions were employed to prevent the access of exter-
nal heat, while the-cooling process was going on, and forestimat*
log with exactness the quantity of water produced by the fusion
of the ice within the body of the instrument

83. Afler having determined from experiments with the calori-
meter, that the heat absorbed by one pound of ice in. melting would

What will be found to take place on mixing equal quantities of the
same body at difTerent teraperatures }

What two liquids afford a striking illa8h*ation of this point }

What term is applied to signify the relatWe power which different bo*
dies possess of absorbing heat }

What method was adopted by Lavoisier and Laplace to measure the
heat gireo out in cooling ?

SPECIFIC BEAT OF BODIES. 805

be sufficient to raise ah eqaal weight of water from 33^ to 157^9
or 135 degrees,* the French philosophers proceeded to ascertain
the relative quantities of heat evolved by different bodies, in cool«
ing, through a certain number of thermometrioal degrees, as also
in other processes. But the results obtained by means of this in-
strument are liable to inaccuracy from various causes, which render
it difficult, if not impossible, to collect the* whole of the water
produced from the melting ice ; for it has been rendered probable
Ihat a part of the water thus formed may sometimes be congealed
again in its passage through the lower part of the calorimeter, so
that the quantity obtained will affi)rd no certain measure of the
effect of the evoluti<m of heat from the body under investigation.

84. Other experimentalists have therefore had recourse to di^
fer^it methods of appreciating the specific heat of Various sub-
stances. Count Rumford invented a calorimeter, for estimating
the quantity of heat given out, in the cooling of heated bodies or
other processes, by observing the increase of temperature in a
body of water, adapted to receive the heat evolved n'om the sub-
jects of his experiments. On a similar principle is founded the
method of ascertaining the capacity for heai^ or as it is also term-
ed the apeeifie heat of gaseous fluids, employed by MM. Delaioche
•End Berard.

85. Another mode of conducting researches of this nature, con-
sists in jioticing the time required to cool any substance through
a certain range of temperature, as indicated by the thermometei,
End comparing its rate of cooling with those of other substances.
The experiments of Leslie, and those of Dalton, on the specific
heat of different bodies were thus conducted ; and a similar plan
was puraued by MM. Dulong and Petit in their experiments on
metals.

86. All these methods of operating are more or less liable to
objection ; and the results thus obtained can only be reg[arded as
Wording some approximate estimates concerning the relative influx
ence of temperature on different bodies. Two other methods of
determining specific heat, have recently been put in practice. The
first, is that of Weber, who measured the heat given out by
stretching a bar of metal, and observing how much Uie elasticity
had been diminished by the loss of heat. The second, is the me-
thod of evapotation, employed by the editor of this work, and

What quantity of heat did they find to become latent by the melting of

iee?
To what objection is the calorimeter exposed ^ ,

How did Rumford attempt to determine specific heats } ^

To what purpose did DielaTtMehe and Berard apply this method ^ ^
What method was employed by Dalton, Leslie, Dulong, and Petit fof

thedeterminittg^ of specific beats r

What other modes of arriving at the same object have been adopted }

* Dr. Black estimated th^ heat required to melt a eiven quantity of
tee as equal -to that which -would raise the temperature of the same weight
of water from 33 to 162 or l40dcigrees.

2c 2

806 PTSONcmics*

desoribed in the American Journal of Science,* together with
formule for calculating the specific heats. ,

87. As the general effect of heat is to cause an increase in the
volume of bodies exposed to its action, producing expansion com-
monly in all directions, but in different degrees according to the
nature of the substance on which it operates, an estimate of iib»
quantity of heat thus operatinj^^, or rather of the amount of the
effect thus produced, may in this manner be obtained ; and the in-
struments adapted for measuring heat on this principle have be^i
described. But, as already stated, important changes may be
caused in bodies by the additicm or abstraction of l^t wi&out
affecting the thermometer in the usual manner; thus solids by ex^
posure to heat may be converted into liquids, and the latter, when
heated, boil or beocmie evaporated, or altered firom the liquid to ibm
gaseous or aeriform state. It was by observing the dissimilar effect
of heat on given portions of ice and water, both at the temperature of
32°, being placed in equally advantageous circumstances for receive
ing heat, that Dr. Blaclc was led to form his theory of latent heat, as
the cause of the lique&ction and vaporization of different bodies.

88. It may be stated as a genenu principle, deduced from nur
onerous experiments, that when any substance becomes liquefied
or melted by heat, a quantity of heat appears to be absorbed by.
that substance in the process of fusion, which cannot be s^>pre-
•ciated by the thermometer ; though the depression of temperatuia
in bodies placed in contact with the melting substance is found to
be very considerable. Thus water may l;^ frozen by placing a
small bottle partly filled with that liquid in a basin containing
pounded ice or snow mixed with the salt called muriate of lime;
and supposing the temperature of the water to be 52^, or 30^
above the freezing point, it will gradually give out heat till con?

gelation takes place, and the quantitjr of heat which thus escapes
om it Will be absorbed by the frigorific mixture of snow and salt,
which will progressively melt or become liquefied, but will retain
the same thermometrical temperature so long as any part of the
mass continues undissolved* ^

89. On this principle depend the artificial modes of reducinff
liquids to the solid state, oy means of freezing mixtures, which
usually consist of minend acids or powdered neutral salts, mixed
with snow. Analogous effects will be observed when fusion
takes place at a high temperature. Thus spermaceti melts at

What effect, besides expansion, takes place in bodies by additiooa of
heat?

What first led to the formation of Black's theory of latent heat ?

What p;eneral principle is applicable to the heat of bodies undergoing
iiquefaction ?

Explain this principle in the process of freezuif water by frigorifif
mixtures.

At what point would spennaceti remain stationarjr when exposed in iti
lolid state to the effect of heat ?

^^' • Vol. liii. p. 279.

ABSORPTION OF HEAT DURING THE MELTING OF 80LID8. Wf

the heat of 113^, which temperature it will retain as long^ as any
portion remains solid ; so that a person might dip a finger into the
melting mass while fraffments continued undissolred, but when
the fusion was completed, any addition would raise the thermome*
ter aboye the melting point. Tin becomes fused at 443^, and lead
at about 602^, and at those temperatures respeetiyely, the metala
would remain during the proeess of fusion ; out after it was com-
pleted, they might be raised to a red heat. And lead, melted and
tiien made red hot, in a crucible, would immediately be cooled
down to its melting point by throwing into it a piece of solid lead.*

90. As absorption of heat or diminution <xf temperature in suiw
rounding bodies always takes place when a sohd substance is
melted or changed to the liquid state, so^ieat isgiyen out when the
contrary change occurs of a liquid into the solid state. If a strong
solution of Gmuber salt (sidphate of 8oda\ made with hot water
be poured into a phial, and corked up' while warm, nrovided it be
left quite undisturbed, the salt will remain dissolved when below
the temperature at which it would otherwise crystalliae; then on
suddenly opening the bottle a mass of crystals will be immedi*
ately formed, and their production will be accompanied with an
elevation of temperature easily perceptible by the hand applied to
the. outside of the bottle. When water is poured on quicklime it
loses its liquid form, and, entering into comoinattons with the cal*
eareous earth, constitutes the pulverulent solid called slaked lime,
giving out the same time abundance of heat, a great part of which
IS carried off by a portion of the water rising in the form of misty
Tapour.

91. When liquids are exposed to heat they become conrerted
more or less readily into aeriform fluids ; thus water is changed
into steam, and ether and alc(Aiol into inflammable vapours ; and
generally all liquids, heated without being decomposied, assume
Uie caseous form at certain temperatures, and are condensed to
the liquid state again }>y exposure to cold. Different liquids re-
quire different degrees of temperature in order to their conversion
into the form of vapour. Water boils or becomes evaporated at
21S deg., while alcohol enters into ebullition at 173} deg., and

How long would it retain this teinpeniture ?
Whut is Uie melting point of tin ? what, that of lead f
What pheoomenon presents itself when lic^uids are converted into solids^
What causea the heat which is observed in the process of slaking lime r
What effect of heat follows the exposure of liquids to its continued in-
fluence ?

At what temperatares does boiliog or vaporization take place in water ?
in alcohol ? in ether ?

* An important investigation of the latent heats of tin and lead, and
of yarions alloys of those and other metals, has been made by M. Rud-
berg, and will be found in the Annates de Chjm. et de Phys., vol. zlviii.

IK 353, in which he has shown that alloys have two stationary points, an-
ess mixed in certain proportions, probably those in which they form
oomplete chemical eompounds, and teave no ezoeu of tither ingredient.
—Ed.

308 nrftowoicicB.

ether at 100 deg. But similar changes take pl^ce to a certain'
extent at almost any temperature; for all kinds of aqueous liquids
slowly evaporate when exposed in shallow masses at the coldest
season of the year ; and spirituous or ethereal liquids cannot be
preserved Ion? in that stale' at ordinary temperatures except in
closely-stopped vessels.

92. Oily and saccharine liquids do not very readily evaporate
in cold weather, hut they also beccmie dissipated through the air
after longer exposure than those of a more volatile kind. This is
a wise provision of nature, for if water ohstinately retained its
liauid form at all temperatures helow 312 deg., the moisture that
fell to the earth in the state of rain would never be evaporated
during the hottest summers ; and abundant inconvenience would
arise from the presence of liquids which are now removed more
or. less speedily at all seasons, through the agitation of the sdr.

93. Evaporation at low temperatures takes place from the 8u^
faces of solids as well as from those of liquids. • Even ice and
snow may be observed to diminish in quantity from evaporation
during the continuance of a frost. Some interesting experiments
on this subject were made in the vtrinter of 1814-15. On the
eastern coast of Lake Winnepie, latitude 52 deg. N. November
S8, 1814, Mr. Holdsworth hung up a disk of ice, SI inches tbic^,
weighing QO tt>s.; on the 14th of February it had lost 17 oz., the
hi^est temperature in the interval being 23 deg. Fahrenheit. The
experiment was Continued till the 31st of Maroh, when the
entire loss of weight of the ice by evaporatioiuamounted to 41bs.;
and though the temperature on the 26Ui and 28th of February had
been as high as 36 deg. during more than two hours each day, no
dropping took place from the ice, nor was any moisture percepti-
ble on its surface. Februai^ 16tlM815, snow was suspended in
a crape bag or handkerchief, which, together with>the snow,
weighed 30 oz. In ten days it had lost 2 oz.; and in nine days
more an additional 2 oz. ; on the 14th of March, the total loss was
6 oz., or one-fifth of its weight in twenty-six days; the crape
continued dry during the whole period.* Hence it appears that
a very considerable amount of evaporation takes place, from solid
ice, when the temperature of th« atmosphere is below that of
freezing water.

94. Among those circumstances which materiallly affect the
evaporation or liquids, one of the most important is atmospheric

Will evaporaU(M always require the same temperature as vaporiztUhn?
What would be the consequence, were the law of nature different from
what it actually is in this particular ?
What bodies other than liquids evaporate at low'tempe'ratures ?
What experiments are related in cdtlncxion with this subject ?
What interesting general conclusion rnay be drawn from them ?
Wliat circumstance materially affects the rate of eTaporation }

. * Jouroftl of Science, &c., edit, at Royal iDstitution. vol. he. pp. 458}
4S4.

ABSORPTION OF BBAT DUUNO EVAPORATION. M9

pressare. All lianids readily become eynpoiated in a higUy nure*
lied medium. Mercury is sublimed with a small degree of heat
in the vacuum formed in the upper part of a barometer tube ; and
water may be made to boil in an exhausted receiver of an air-pump
at a temperature much inferi<» to that at which ebullition takes

Elace under common circumstances. In the same manner the
oiling point of water becomes lowered, in proportion to the rare-
faction of the air, in ascending high mountains ; and in general the
boiling points of all liquids will vary in some degree according to
the pressure of the atmosphere, as indicated by me barometer.

95. Ether, when placed under an ex-
hausted receiver, rapidlv evaporates. It
may dins be made to boil while water

£ laced in contact with it freeses. To ex*
ibit this phenomenon, a small flask, B,
mnst be procnied, made of thin fflass,
and nearly fitting into a bell-shaped arink*
inff-glass, C, as represented in the figure.
Euer must be poured into the g^ass, and
water into the flask, so that both liquids
may stand at the height of the dotted
line, A D, and the apparatus thus arrange
ed is to be placed under &e receiver of
an air-pump, on workiug which the eUier will boil or be converted
into vapour ; and as it requires heat for this purpose, it will absorb
it from the containing vessels and the water which it sorronnds,
and the latter liauid uius deprived of its heat will be reduced to
a temperature b^ow the fireezing point and beeome ice.

96. Tlie diminuticm of temperature produced by the evaporation
of ether is so considerable, tnat by means of it mercury may be
reduced to the form of a solid. This may be effected by inclosing
a small quantity of mercury in a flattened spheroid of thin glass,
and covering it with thin muslin on which ether is to be dropped
as fast as it evaporates, and the heat will Uius be so rapidly ah*
stracted from the mercury that it will soon be frozen to a solid
mass.

97. Water alone will boil speedily under the exhausted receiver
of an air-pump, at the temperature of about 100 degrees of Fah«
renheit; but in this case the ebullition soon ceases, in consequence
of die pressure of the steam or aqueous vapour,. which occupies
the space from which die air has been withdrawn.

98. The manner in which a liquid may be made to boil by dimU

In what manner may water be made to boil below filS??

What two contrary effeets may be exhibitejl by withdrawing atmoa-
pheric prettare from the sarface of two liquids ?

Explain the manner in whieh the experiment is to be eondueted.

What eife«t may be produeed by dropping ether on a eapsale filled
with mercury ?

At what temperature will water boil in the ezhaotted receiTer of an
air-pump ?

810

niBhing thi pressure of the Btmoaphere on its sarfoce mvj b«
amuEmglj exhibiled by means of the following' experiment:

Let a Blop-cock be fitted into the neck of a Flo-

^ rence flaak, containing a amall quantity of \n.Wt,

and after holdinglhe flaakovettheflameof aspirit-

lamp till the water boils and partly escupea in lbs

form of Bt«Hm through tbe stop-cock, let it be end-

denly removed from the flame, at the eame time

shutting the stop-cock ; the ebullition -will soon

ceaae, and the flask is to be plunged into a jar of

cold water, sa represented in the margin. The

water in the flask will instantly begin Co boil again,

inconsequence of the condensation of the inciud^ steam, andtlu

vacuum thus farmed in the upper part of the flask. If it be kept

long immersed in the jar of water, the ebullition will cease front

the gradual cooling of the water within the flask; but if it be

taken out of the jar and held near tbe hre, fresh et«ana will be

formed, and the ebullition may be renewed by plunging the that

afresh into tbe cold water.

S9. The mode of making liquids boil at a comparativety low
lemperatuie by the diminution of surface pressure, has been-ad-
vsntageously adopted in Bome manufacturing processes. Thus it
has been applied to practice in the art of refining sugar, the saccba-
rioe Bymp being concentrated by this means to the pcdnt at which
it crystallizes or granulates, without any hazard of its burning,
or becoming decomposed by excess of heat. In the preparaiiDn
of vegetable extracts for medioal purposes, similar processes have
been adopted ; and also in making jellies or other kinds of cod-
fectionfu^.

100. Distillation is an operation conducted on similar principles
with those just described ; but the object is different, for the TBpanr
or steam' which, inrefining sugar, -or making eilracta, is diBsipated
and suffered to escape, as nBeless, is on the contrary, in disdlte-
tion carefully preserved, and reduced again to the liquid form by
condensation. The method of distilling at a low temperature,
by removing the pressure of the atmi!i^phere, haa been profitably
employed in eases where it was a principal object to obtain pr*
ducts as free as possible from an empyreumatic flavour, or peculiar
disaereeable taste. Mr. Henry Tritlon has invented an appers'
tus MT distilling spirits, by means of which Ibe vapour is railed
in a vessel not exposed na nsnal to the fire, but surrounded with
hot water; and the pipe proceedingfrom tbe upper part of it, after

'.r mo]' ebullition at low tempenture be exhibited f

■ ■ ' ne eold ?

g under diminiihed

in differ from (he mere eoncenmiion OT iiquMi'
i> diiiillaiion at low temperature) chlefij Mgoi^

nty of Trittoo') diitilline Rpparatusf

FAPIN*8 BIGB8TER. 811

passing in thei asnal way through a large body of cold water, ter-
minates in a spacious cavity or close vesse), from which the atr
can be extracted by an air-pump or exhausting sy rin^. A similar
process has been used with advantage in the distillation of vinegar*

7 101. As liquids boil readily at comparatively low temperatures
when the pressure of air or elastic vapour on their surfaces is in-
considerable, so they remain unevaporated at relatively high tem-
peratures, if exposed to extraordinary compression, as when con-

f fined in a strong close vessel. Such an engine is that galled Papin's
Digester, from the name of the inventor. It consists of a metallki
vessel 6f a cvllDdrical form, with very thick sides, having a cover
fitting air-tight, and confined by a cross-bar fastened with screws.
When, such a vessel, partly filled with water, is exposed to the
heat of a fire, a quantity of vapour wiU be formed within it, which
being prevented from escaping will press powerfully on the sur-
face of the liquid, and prevent ebullition, though the heat of the
water be raised far above the boiling point, while the quantity and
elasticity of the included vapour or steam will also be augmented.
The digester ought to be furnished with a safety-valve, which may
9pen when the steam acquires a certain degree of force, below the
estimated pressure which the sides of the vessel would be capap-
ble of withstanding, and thus the risk of its bursting if over-heated
wou^ be obviated. Such machines are employed for extracting
the gelatinous matter from bones to make portable soup, and for
other purposes.

102. The temperature of steam is always the same with that of
the liquid from which it is formed, while it remains in contact
with that liquid > and as the elastic force of the vapour is in-
creased in proportion to its degree of heat, the amount of pressure
which it exerts will depend on the temperature at which it is
formed. Hence the distinction between high and low pressure
steam and steam-engines.

103. When steam begins to be produced, as in the process o[
mskking water boil, and the heat overcomes the atmospheric pres-
sure on the surface, small bubbles are formed adheriog slightly
to the sides of the vessel, as may be conveniently observed by
using a Florence fiask or any other thin glass vessel* These bab-
bles are formed most rapidly at those points against which the
fiame is most strongly directed ; and if any particular portion of
the surface of a common boiler be more intensely heated than the
surrounding parts, and the metal become uncovered by the liquid,
'^hen the water again comes in contact with it, very elastic steam

Under what circumstances may liquids be made to undergo a high tem-
perature without evaporating f

Describe Papin's digebter.

What relation exists between the temperature of vapour and that of
the liquid from which it rises ?

What dhtinction arises from the diiferenee of temperatures at which
it is produced ?

In what manner is the production of steam first manifested ?

la what parts of a boiler will its developevenft be most eoDSpicaoas f

S13

vill be aaddenly fonned, which may cause the baiter to bnrat
Such appears to be the most probable mode of accounting for the
DumeiooB accidents reaultiag ftoiu the employment of steam m a.
moTiDg power.

104. Mr. Perkins bas inrenled an impioTed steam-boiler, in
which a constant circulation of the water ia kept up, through a tube
or open cylinder in the centre of the boiler, which sweeps off the
bubbles trom the heated surface of the vessel as fast as they are
produced ; and thus ^e formation of Hteam goes on with uniform
regularly. He had obserred that two Teasels being filled with
water, and one placed within the other, when heat ib applied so
that it can only reach the inner ressal through the liquid oontained
in the outer one, no steam-bubbles will rise in the former, while
they will be rapidly formed in the latter. The fluid in the ex-
terior Teasel, consiating of water mixed with air-bnbblea, and ^1
in the interior Teasel of water only, the contents of the two Tea-
's at the same temperaluie will differ in specific ^ruTity, those
' ' <■.-..• .^ . K therefore the

of the outer Tesael of courae being the lightest, tf

bottom of the inner Tessel be remoTed, aa that it will ci

an open cylinder with its upper edge a little below Ibe surface of

the water in the larger Tessel, and supported in that position, at

showD in the annexed figure, the unequal density of the fluids in

the eiterior and interior parts of the

boiler, when expoaed to the action of fire,

will cause the formation of a circulating

cnrrent.

lOS. The bubbles contained in the
water of the outer Tesael, adjoining &s
■fire, will riae continually to the su^ce ;
when at a low. temperature with a power
somewhat exceeding the difference be-
tween the specific graTities of air and
water, butif the heat be augmented, and
the bubbles formed more rapidly, the
difference of specifio graTJty of the re-
spectiTe ftuida will be increased, and
alao the Telocity and force of the current.
For the fluid in the inner Teasel or cy-
linder, being free from bubbles, will, in consequence of its aupe-
lior apecific grafity, descend and arrange itself beneath the risiog

;t Teasel, and tbua continue the circulation.

106. If the fire be urged so as to produce an extremely intense

'heat around a boiler of uiis construction, ao rapid and forcible will

be the ascending current, that it will draw into its Tortex and

carry upwards sand, graTel, or stones, or almost any kind of

la vbat manner hai Perkiua uught to render the aelion of the tartite

EipUin the mtaner in which the olrenUiioa li maintained by (lie pr»
dBDiioa and ampe ftf tnponr ?

TBS STBAK-XiroiKE. 818

hstvy sabalance of moderate sise which may happen to be in
the boiler, sweeping off, in its ascent, all the steam-Dubbles which
form on the interior surface of the boiler, and keeping it dear from
the vapour which might otherwise interrupt the free passage of
the heat which it receives into the water ; and by the uniform and
constant agitation of the whole mass of the liquid, a regular and
abundant absorption of heat takes places, and steam is evolved
with astonishing rapidity.

1Q7. The steam-engine, a machine of the highest import^nee,
the effective power of which depends on the expansive fbfce of
steam or vapour, its general construction and mode of action may
here be described. The vapour of water occupies a space aboat
1700 times larger than the bulk of the water from which it is
formed ; iind hence it may be conceived that in consequence of
|ts expansibility it must strongly resist compression, and that the
impetus thus obtained may be variously directed or modified so as
. to produce motion.

108. At what period steam was first employed as amoving
power is uncertain* However the mode of thus applying it, was
known as early as the middle of the seventeenth centnry, since
the Marquis of Worcester in his ^ Century of Inventions," pub-

^ lished in the reign of Charles II., describes a machine for pro*
ducing motion by the force of steam ; but though the idea of using
stecim as a moving power seems to have occurred to several persons
about the same period, the invention of the steam-engine properly
so called may be fairly ascribed to an ingenious man named New-
comen, who was a locksmith in the West of England; and a
patent for such a machine, for raising water from mines, was taken
out by Newcomen, in Coniunction with Captain Savery, an engi-
neer, who probably contributed to the improvement of the inven-
tion.

109. The mode in which steam is made to act is by causing it to
raise a solid piston working in a cylinder, like that of a forcing-pump
OF fire-engine ; and the piston-roa rising by the impulse of expand-
ing steam admitted into the cylinder below it, must communicate
motion to a beam or lever connected with it. When the piston is
thus raised, if the steam be suddenly condensed or withdrawn
from under it, a vacuum will be formed, and the pressure of the
atmosphere on the piston above will drive it down. It may then
be raised afresh by the admission of more steam, to be condensed
in its turn, and in this manner the alternate motion may be con-

What striking effects are said to be prodaoed by the eurrents between
the two cylinders of Perkins's boiler P

What is the relation between the bulk of steam and that of the water
from which it is produced f

How early was the force of steam, as a meehanieal agent, probably ap-
plied ?

Who invented the steam-engine ?

tn what manner is the force of steam applied in that machine f

In what way did the atmospheric engine of Neweomen become efliw^
Ive aAer the piston had been raised hv the steam ?

9D

814 FTRONDMICS.

tinned indefinitely. Newcomen's claim to be considered as an
inventor depends on his having been apparently the fiist peisoo
who conceived the idea of condensing the steam the moment it
had effected its object, by throwing into the cylinder a jet of cold
water.

110. Two very important improTements on the original engme
were made by the celebrated James Watt. He observed that the
cooling of the cylinder by the water admitted into it lessened the
expansibility of the steam it received, and that thus much power
was dissipated : to prevent which, he contrived a method of with-
drawing the steam from the principal cylinder into another, io
which the condensation takes place, and from which the wat» it
yields is returned to the boiler to form fresh steam. The other
miprovement consisted in employing the expansive force of steam
to depress the piston as well as to raise it. In the original, or
atmos{^eric engine, the piston, as above stated, was driven down
by the more impulse of atmospheric pressure ; but in the improved,
or double-action engine, steam is admitted into the cylinder abore
the raised piston at the same moment that it is removed below it;
and thus the expansive force of steam is exerted in the returning
aa well as the ascending stroke, and a much greater impetus is
given to the machinery than by the old method.*

On what does hii elaim to the inyention of the steam-engine rest?
In what did the two principal improyements of Watt eonsitt ?
How does bis doable-aeting engine differ from Newcomen's, in regard
to the effective mover ?

* The following notices concerning the invention and improvemeDt cf

removing the fii*e continually from one par
Captain Saverj exhibited a model of a steam-engine, Jane 16, 1699,
which is described in the Philosophical Transactions. The date of St-
▼ery and Newcomen's patent for a steam-engine is in the year 1705 ; asd
the latter ** introduced the piston.'* Among the improvers of this vils-
able machine. Dr. Young mentions the names of De Moura, SmeaUm,
Beighton, Francois, who contrived *' an engine without a piston, workinf
the cocks by a tumbler ;*' Droz, Cartwright, Hornblower, Woolf, and
Edelcrantz, besides Watt.— iectorea m J^tuhtral PMUwphy, 1807, tol
ii. pp. 257, S58.

BnCUFnON OF THB VflUi-BMeim.

111. Tha prBcediny figni
net idea of the principal pi

e reader to form « cor-
e princTpaTpBTts of a Bleam-eagine, and of ita mode
ot action. A B denotes the principal cylinder ; P its piston, act-
ing bj its rod Y on the extremity of the beam G H, the other ex-
tremity of which is connected with the llv-wheel j C, a tube or
passage by which steam formed in the boiler is conveyed to the
conducting pipes T U, to be admitted on either side of the piston
P alternately; 0, flie fly-wheel, which by the rods R S, moving
eccentrically, acts upon the rectengnlar lever V, which by means
of the valve Z regulates the admission of steam by the conduel-

Delinate aeittnilelj (he uTenl parti of WaCli'i doable-fteting lov
preonre eneine, snd cxplmin their url
HkkE ■ ibrnvinf oT the whota o^ni, umI ihow It

$l6 mioiemacB.

ing pipes; M, the condenser; X, a tabe by which the steam
passes from the cylinder into the condenser ; N, a tabe to contey
the water after condensation to the pump L ; F, the piston of the
pump L, worked by its rod E attached to the beam 6 H ; K the pis-
ton-rod of a pump to inject water into the condenser.

112. From this description, the mode of action of the engine
may be readily understood. Suppose the piston P to be at the top
of the cylinder A B, the lower part being filled with steam, then
by means of the lever V, the steam-valve Z will be drawn down
so as to admit steam by the upper branch of the conducting pipe U,
into the cylinder above the piston : and at the same ihne a pas-
sage will be opened to let the steam below escape into the con-
denser. Thus the piston will be driven to the bottom. of the cy-
linder, when the steam-valve again opens to admit steam by the
lower branch of the conducting pipe T, into the cylinder below
the piston, while the other passage also opens to permit the steam
above the piston to escape throu^ the tube X into the condenser.
Thus the mannei' in which the piston alternately rises and falls
is shown, and by the connexion of its rod with the lever G H, it
works the pumps, and turns the fly-wheel, whence the moving
power may be propagated through trains of machinery for any
purpose requirea. The fly-wheef may be moved in the manner
represented in the figure, by a crank connected with a rod descend-
ing from the arm H of the great lever ; or the toothed wheels
called the sun and planet wheel may be applied, in the mode that
has been explained elsewhere.*

113. Various other arrangements are adopted for modifying or
regulating the motions of different parts oi the machine. 'Hius
the piston rising vertically is connected by a system of jointed
rods with the extremity of the arm G of the great lever ; and as
that lever turns on a pivot, the end of the arm must form an arc of a
circle, but by means of the rods the motion is so modified that the
piston is allowed to rise in a curve of double curvature of so large
radii at the point described, as not to differ sensibly from a right
line.f Another contrivance, regulating the velocity of a steam-en-
gine, is that called the governor, previously described, which by the
rising of the revolving balls closes, and by their descent opens (he
passage from the boiler to the cylinder ;:(: and there are yarions
others adapted to particular purposes. '

114. It has been mentioned that the degree of the elastic force
of steam depends on the amount of pressure sustained by the sides
of the vessel in which it is formed. In the common low-pressure

By what two methods has the alternating motion of the piston rod been
ooiivei'ted into ooittinued rotary motion ?

For whnt purpose is the system of jointed rodi invented by Watt ap-
plied to the steam-etigine ?

What is the elasticity of steam used in engines acting on the principle
•f Watt ?

• See Tieaiise on Mechanics, No. 219 f Ibid. No. 890.

t Ibid. No. 228.

BioH-FRvannu snoinb. 817

engine tbe steam need is generally fonned under a yi e esm e not
exceeding twenty pounds on a square inch, and therefore when the
expansive force of the steam exceeds it, die valve opene, and the
force of the steam is consequently reduced. This pressure is only
five pounds more than that of the atmosphere, and the boiler is fur-
nished with a safety-valve, loaded with that weight to each square
inch of its surface ; but in the double-action engine, the pressoie
of die atmosphere being excluded, the whole pressure of twenty
pounds is by the aid of the condenser made available ; and thus
such an engine, if its piston be of equal size, will have the same
power as a high-pressure engine worKinff witii steam of the force
of thirty-five pounds on the square inch, because fifteen pounds
are here employed in overcomine the resistance of the atmosphere*
into which the steam is finally thrown.

115. The hiffh-pressure engine, being simpler in eonstruetion,
as well as smsiller, than the double-action low-pressure engrine,
is more advantageously used than the latter, where it is requi*
site to employ considerable power within a confined space;
and therefore it has been adopted in steam carriages.* In these
engines, the steam is not condensed, but is suffered to escape*
after it has acted on the piston ; and as it is formed under extra-
ordinary procure, varying from fifty or sixty to two hundred and
sixty pounds on the square inch,* its expansive force is relatively
▼ery great The attention of diose who have been engaged in
the construction and improvement of steam-carria^ has there-
fore been chiefly directed to the contrivance of boilers in which
high-pressure steam may be formed with the least possible risk
of explosion; and Mr. Goldsworthy Gumey and others appear to
have so fiair succeeded as to have produced carriages worked by
steam in which persons may travel at least as safety as in coaches
drawn by horses, and witn a degree of velocity incompurahly
greater.

How maeh of the force of high tteam is lost by the reiistinee of the
air to its final expulsion from the cylinder ?

What renders the high-pressure engine pceoliarly adapted to loeomo-
tive carriages ?

* See Gordon's Treatise upon Elemental Looomotioo, 1858, pp. 09*
10, 96, and lOa

9d9

OUptr EvamU Bttcm En(^.

OLIVER EYAlfS*8 8TBA|| ENOIKB. 819;

116. Thehigh-pressnie steam engine invented by Oliver Evans
of Philadelphia, in 1784,* and for ^ich he obtained a patent from
the state of Maryland in 1787) (the confederated states noVhaving
adopted a general system of patents,) is the original of all those
powerful machines which for the last few years have astonished
the world with their wonderful performances.

117. The accompanying figure is a pretty accorate representi^
tion of a model of Evans's engine, now in the collection of the
Franklin Institute. By a comparison with the machine of Watt,
it will be seen how greatly the ffenius of its inventor had simpli-
fied the structure which has c^led forth such lofty encomiums
from some of Watt's biographers. It must be remembered that
the inventor was not insensiole to the peculiar adaptation of his
machine to the purposes of locomotion on land, but that this was
one of the express objects of his patent. The parts of the high*
pressure engine will be understood by a reference to the figpure.

118. A is the working cylinder, to which the steam, equal to
several atmospheres in pressure, is admitted by the pipe o and
the rotary valve v.

B is the boiler of a cylindrical form, with a return fine placed
below the centre of the outer shell, so as to be constantly covered
with water when it stands about at the level of the line B.

S is the smoke pipe springing from the interior flue, after the
latter leaves the head or the boiler.

119. G is the fire grate or furnace, from which the flame passes
in the direction indicated by the arrow.

P is the force pump, which draws water from a reservoir of hot
water, R, placed above its own level. This water is kept hot by
the steam which escapes from the cylinder A after it has perform*
ed its office there.

p is the pump rod connected with the moving beam above.

V is the safety valve connected with the boiler and furnished
with a CTaduated lever and weight to regulate the pressure.

120. I is the working beam connectea to an upright support by
the two rods A; A;, to the oscillating triangle T, the pump roap^ the
otston rod /, and the shadde^bar 6, which last gives motion to the
ny-wheel W.

^ is a toothed wheel, geared to another of the same diameteri
which being connected with the two equal bevel wheels at «, com*
municate motion to the rotary valve v,

8 is the escape pipe, by which the steam is conducted to the
tank or reservoir R.

By what means is the flow of iteam into the ojlinder in Gvans's eogine
allowed to take place ?

How is the interior flue arran^d in reference to the water line ?

How is the hot water force pamp arranged with relation to the reier>
voir from which it is supplied ?

What is the purpose of the two small hevel wheels teen in the figure ?

- ■ -

* JBvans began to btdld steam-engines on his plan in 1801, but in 1794

he had sent drawings and specifications to England, where they r e m a i n ed

AD the deadi of Mi. Saiqpaon, hy whoot they were carried out— En.

FmONOKICS.

Propagation of Heai,

121. When any considerable mass of matter, whether conBist-
ing of a single substance, as a body of water or atmospheric air,
or of seveial substances mingled together, exhibits a uniformity
of temperature, if another substance, either more or less heated
than the general mass be added to it, the equilibrium of tempera-
ture will be partially disturbed, for a time, and then restored ; the
whole mass taking heat from the substance added to it, if the
latter be comparatively hotter than the iHass, and giving out heat
to it, if it be relatively cooler. Heat is thus propagated, or com-
municated from one body to others, having a tendency to become
generally diffused among bodies, and cause them all to exhibit
Sie same degree of thermometrical temperature. There are two
modes in which the propa^tion of heat may take place ; namely,
by conduction, and by radiation.

122. Propagation of heat, by conduction, always takes place
when any substance is brought into contact with another which is
relatively colder. Hence it is that the temperature of the air in
deep cellars' and caves seems to be higher in winter than in summer.
The degree of heat in such places is at both seasons nearly the
same ; but the surface of the body in winter being colder than the
air of a subterraneous cave, will attract the heat firom it, and in the
summer, on the contrary, the air will rob the body of its superior
heat. It appears, from the experiments of MM. Bertholet, Pictet,
and Biot, that heat is communicated more readily by a stroke or
blow from a heated body than by simple contact.*

123. The laws of the propagation of heat through bodies by
conduction, may be deduced irom the following experiment : sup-
pose a bar of metal, two or three yards in length, to be placed m
communication with a constant source of heat, and let ten holes
be bored in it at equal distances from each other, from one end to
the other, and filled with mercury, thermometers being plunged
into the fluid metal in all the holes ; then deducting the diflTerence
ofvthe temperature of the air from that of the several thermome-
ters, we obtain the temperature of the bar at so many relative dis-
tances from the source of heat. These distances must necessarily
constitute an arithmetical progression of numbers, and it will be

What effects result when a nass of matter at one temperature is mixed
with another mass at a different degree of heat ?

In how many modes does the propa|;ation of heat take plaee ?

What is meant by the term conduction ?

What effect has percussion produced by a hot body different from that of
simple contact ?

In what manner may the laws of the {Htipagation of heat through bodies
be estimated ?

What prog^ression will the diminutions of temperature follow wb^n the
points of the solid under examination are at distances from the souroe
of heat forming an arithmetical series ?

t I.I I .1 I I I ■ I ■ ■ . I ■ .1 i

* Memoirea d*Aroueil, t ii. p. 447.

CONDVGTOni OV HEAT. 8ftl

found that the iie^rease of temperature will take place in a geo*
metrical progression, forming a rapidly diminishing scale of num-
bers. The rate of diminution of heat is indeed so rapid, that it
would be impossible to raise the temperature of one end of a bar
of iron two yards and a half in length, a single de^ee, by any
heat applied to the other extremity ; ror the heat requisite for that
purpose would be greater than what was sufficient to melt the iron,
as might be shown by calculation.

124. Though heat has a tendency to spread by conduction
through all bodies, yet some receive and give it out with much
greater facility than others. Among solid substances the power
of conducting heat varies very considerably. Metals in general
conduct it more readily than wood, and the power of conduction
is different in different metals. Hence the handle of a metal tea-
pot or coifee-pot is commonly made of wdod ; since if it was of
metal, it would become too hot to be grasped with the hand, soon
after the vessel was filled with boiling water.

125. Dr. Ingenhousz ascertained the difference of conducting
power among several metals, by dipping into melted bees-wat
cylindrical rods of various metals of the same dimensions, and
when the equal coating of wax on all the rods was become solid
by eooling, ne plunged them to the same depth into heated oil,
and from the difference of time required to melt the wax, in each
case, he inferred the conducting power of the respective metals.
It thus appeared that silver was the best conductor of heat, then
gold, tin, copper, platina, steel, iron, and lead. So that the power
of conduction in metals seems to be independent of their density,
tenacity, or fusibility ; for the specifie gravity of silver is inferior
to that of gold or platina, yet its conducting power is srrenter^
while it has less tenacity than either of those metals; audit is not
80 readily fusible as tin or lead.

126. Next to metals, precious stones, as the diamond, the topaz,
and other dense earthy compounds, appear to be the readiest con-
ductors of heat : then stony bodies, porcelain, and glass, and po«
reus earthy compounds, such as brick and pottery. Wood con-
ducts heat very imperfectly, whether in its usual state or in that
of charcoal ; either of which may be held by the fingers very near
the part which is burning and red hot.

137. Animal and vegetable substances of a loose texture, as
fhr, wool, and cotton, are extremely indifferent conductors of heat*
Hence their utility, either as natural or artificial clothing, in pre-
serving the warmth of the body, in consequence of the obstruo-
tton they present to the passage of heat through ^em. It is pro*

How far might a bar of iron be heated by exposing one end only to the
most intense heat ?

Which class of solids comprises the best oondaetors of heat ?

To what practical purpose is the low conducting power of wood appli-
cable?

MThat order did Ingenhousz find among the metals in regard to oonduet-
ine power ?

What class of bodies hold the leeeiid place in eonductioD ?

SSS PSSONOKICB.

bable. howBTer, that in such cases the effect pa^f depends on
the quantities of air contained in the interaUceB of such loose sub-
stances ; since air is one of the very worst conductars of heat

128. Liquids conduct heat verr elowl; and imperfectly^. If
mercurj be poured into a jar, and boiling water poured over it, lh«
metallic fluid will receive heat but slowlj from Ae water abots
it. A themometer let down a few feet below the surface of a
pond or of the eea, would, on being dr&WD up, indicate a low«c
temperature than thai of the sur^e water; for the latter, healed
by the rajs of the sun, would communicate by conduction little
or no heat to the water below. Indeed it has been questioned
whetlier water has the power of conveying heat at all bjr conduction.

139. In the marginal figure, let A represent a ct-
' ' ' ' of water, with an air tbermomeler, C,
it, and having its bulb very near the
surface; B is a small copper basin floatmir on the
water jnst above the bulb, and separated from ll
only by a thin stratum of the aqneoua fluid ; yst,

sj die boiling point, the temperature just below vill
" scarcely be sufficient to produce any effect on the
thermometer: so that it may be concluded that water does not
transmit heat dowDWsirde by conduction.

130. It may be reasonably inquired how it happens that walerb
readily made to boil by the appnca^on of heaU A little conaiiS*
eiation will show that the effect in a ereat measure depends on
the manner in which the liquid la heated, by placing it above the
source of heat. Thus, the lower stratum of the liquid, being expand-
ed by the heat communicated to it throngb the bottom of the con-
taining vessel, rises to the top in consequence of its inferiority of
!.p_ -raviiy, and the water above einks down to supply its

specinc graTily, and the water above einks down to au
place and be heated in the same manner, till the whole m

ires the same tempeiatore. The mode in which ebullition k
militated by the formatioD of air-bubbles, and the ensuing circn-
lation of the fluid in ascending and descending currents, baa been
already described.

13t. Air, like water, appears to have no observable effect on the
propagation of heat by conduction ; and it may be concluded that
gaseous fluids conduct heat, if at all, with degrees of difficulty in-
creasing in proportion to their rarefaction. It is owing to the ei-
treme rarefaction of the atmosphere at great distances from lh«
level of the earth's surface, as upon high mountains, and

On whmt Dircamitincc da puroui, ftniraal and vegetable aubitkODCS pn^
bablv depend for Iheir low oaniiuoting power .'

What fasta and eiptrimeiita ahov thulow condiielin^poverof liqajdi)

Explain the ipparetua bj whicb Ihii principle ia illnatrated.

Haw doea it appear thai the rapid oDinmuniiwtlaa of heat to nler ii
eoniiilenl with iti low conducting power ?

In what muuer do gaaeout bodiei ooniainuicate heat f

THE CALORIC ENeiNE. 829

its increased capacity for lieat, that the excessive cold obsenrable
in such situations is to be attributed. Yet though the atmosphere
is so bad a conductor of beat, substances may be warmed or cooled
by the relative temperature of the air; for the expansion of air by
heat, and necessary production of aerial currents, causes a rapid
transmission of heat through the air, and thus the temperature of
any body in contact with it may be raised or lowered according to
circumstances. Air also readily conveys heat by radiation, as will
be subsequently explained.

7%6 Calorie Engine.

132. The principle of commuidcating heat by eirculatian^ is ap-
plied in connexion with the rapid absorption and subsequent com-
munication of heat by\ metallic bodies, in the construction of a
machine which has recently been invented b^ Mr. Ericson, of
London. It is called the Calonc Engine, and is actuated by the
successive dilatation and contraction of a quantity of compressed
and partially heated atmospheric air, or other permanent gas;

133. This air or gas being made to circulate in opposite car-
rents through a series of small metallic tubes, causes a constant
transfer of heat from one part of the machine to another, whereby
an alternate dilatation ana contraction of the impelling medium is
effected and kept up.

134. Thus the Cfaloric Engine possesses a novel and itnnortant
feature when compared with the steam-engine, viz., that oi beinff
actuated over and over again by the same heat, or nearly so ; ana
it may be added, that it presents to natural philosophy an illustrar>
tion of the fact that heat does not lose its energy in producing me-
chanical force, but remains in undiminished quantity after having
caused the dilatation which produces that force. At the same time
this new invention presents to mechanical science a wide field for
improvement, since (unlike the steam-engine) its principle is such
that the quanti^ of force it produces has no other relation or pro-
portion to the niel it requires than that established by a more or
Jess perfect machine and transferring apparatus.

135. The action of the Caloric Engine and the transfer of the heat
will be ready understood by referring to the annexed diagram*

On what prineiple is Ericson'i «a1orlo engine oonstraeted ^
What esseoUal feature does it possess different from that of the steam-
engine ?

136. The engine conBiHta of two cjUnders, A and B, ofuWTUSi
diameters, the 1a^ one beioo always kept at a high temperature,
and the small one always cool, Thesecjlinders are provideii with
piatons and ralreB, similar to those of a high-pressure Eteam-eD-
gitie, and their piston-rods are connected so that the one pistoa
cannot move withoat the other. The two cylinders commonicale
with each o^er by means of a number of small tubes, C, pasaipf
thraugh a vessel, D, called the regenerator, and all termtnallogH
chambers or caps, E and F, attached to the ends of the recfnereloi,
•nd saarraoged that the hot air, after having performed its duly in

Id wbit ititc !■ the larger cylinder of Briown krpi .'

Exolaiu (ha NTeral parti of thii niBhiue, and Hata (heir Mierd d»-

THE CALORIC ENGINE. 325

the laree or 'forking cylinder A, passes tiirough the pipe G, into
the both/ of the regenerator D, for the purpose of giving out its heat
to the small tuhesC, in its passage towards the small cylinder B,
and thereby becomes cooled and reduced in volume.

137. In order to effect this more completely, a number of parti-
tions, H, having segments cut out alternately from their tops and
bottoms, are introduced into the body of the regenerator, giving a
very circuitous motion to the hot air in its passage from the pipe
G to the pipe K.

The cold air, forced, by the action of the piston of the small
cylinder in a contrary direction, through the small tubes O (these
being also provided with small partitions for changing or intermitt-
ing the particles of air), will, during its passage from the cap F
to. the cap £, on its way to the hot cylinder, take up the heat im-
parted to those tubes by the contrary hot current which passes
through the body of the regenerator, and thereby become neated
and enlarged in volume.

13d.. It will be evident that if compressed air be admitted into
;both cvlinders od one side only of their pistons, the greater sur-
iface of the one will be acted upon with greater force than the less
surface of the other ; hence motion must ensue : and by reversing
the position or the ^^ slide valves" at the termination of each stroke,
it will be continued. It need hardly be stated that the difierence
of the volttmes of air contained in the two cylinders will cause
neither deficiency nor accumulation during the action ; because in the
Jarge cylinder the air is in a heated state, and in the small one cold.

139. Some loss, of heat will of course be unavoidable in the
transferring process, and this is compensated by passing the air,
previous to its entering the hot cylinder, through a series of small
tubes, L, communicating with the cap £, and induction-pipe Q,
and exposed to fire, contained in a stove, M, the combustion being
supported by ordinary draught, and the waste heat made to pass
round the regenerator, and carried off at N into a common chim-
ney. At the same time the air which has passed through the body
of the regenerator still retains a small quantity of heat when en-
tering the pipe K ; it is therefore passed through tubes O, immersed
in cold water, or exposed to some other cooling medium, previous
to entering the small cylinder.

140. The marked difference, then, between the caloric engine
and the steam-engine consists in this : that the heat, which is re-
quired to five motion to the caloric engine at the commencement^
is returned by the transferring process, and thereby made to work
the engine over and over again, requiring but a small addition of
heat to compensate for losses caused by radiation, &c. ; while on

What is the purpose of the small perforated partitions in the regene-
nitor, and in its inoluded tubes >

To what cause of loss is the air in this engine exposed ?

How is that loss supplied I

How is the quantity of disposable foree in this engine proportioned to
the size of the two cylinders respectively ? _

2£

826 FYRONOMICS.

the other hand, in the steam-en^ne the heat is constantly lost by
being thrown either into a cold condenser, or into the atmosphere,
like so much waste fuel.

141. From what has been stated it must be inferred that those
bodies which absorb heat most freely also part with it most rapidly ;
that is, they are sooner heated and more speedily become cooled
than other bodies. Metals, which are generally the best conduc-
tors, and therefore communicate heat soonest, cannot be handled
when raised to a temperature of more than 120 de^.; water be-
comes scalding hot at 150 deg> ; but air applied to uie skin occa-
sions no very painful sensation when its heat is far beyond that
of boiling water.

142. Some curious experiments on the power of the human body
to withstand the influence of heated air were made by Sir Joseph
Banks, Sir Charles Blaed^n, Dr. Solander, and Dr. George For-
dyce ; and an account of them was published in the Philosophical
Transactions for 1775. These gentlemen found that they could
remain for some time without inconyenience in a room where the
heat was 52 deg. above the boiling point. But though they could
thus bear the contact of the heated air, they could not bear to
touch any metallic substance, as their watch-chains or money.
Eggs placed on a tin frame in the heated room were roasted hard
in twenty minutes ; and a beef-steak was overdone in thirty-three
minutes. Similar experiments have been often repeated, especially
by persons who have made public exhibitions of their power of
sustaining heat, in which, however, there is nothing extraordinary,
or which may not be explained as the result of habitual practice.

143. Mr. Chantrey, the celebrated sculptor, made some obser-
vations analogous to those just noticed, by means of a stove or
oven which he uses for drying plaster casts and moulds. A ther-
mometer suspended in this heated cell, usually stands at 300 deg.
yet the workmen enter and remain in it occasionally some minutes,
without difficulty. Persons unused to such a temperature found
that they could easily support the heat for a short time ; but one
gentleman inadvertently entering the oven with a pair of silver
mounted spectacles on, had his mce burnt where the metal came io
contact vnth the skin ; thus experimentally ascertaining the dif*
ferent effect of air and silver at the same temperature.

144. On the strong attraction of metals for heat, and the conse-
quent facility with which they abstract it from other bodies, de-
pends, in a great measure, the effect of Sir Humphry Davy's
safety-lamp, to be used in mines, or other places infested with that
kind of inflammable gas called fire-damp. Flame is gas, or air

What relation exists between the power of bodies to absorb and ti
eomniunicate heat ?

How is the difference of varioas suhstancaa in this particular ttriking^j
exhibited ?

Describe the experiments of Banks, Blagden, and others.

What curious observations were made by Chantrey ?

On what principle is the usefulness of Davy's safety-lamp dependent?

RADIATION OF HEAt. 327

in the state of combustion, and all gases leqaire a rery high tem-
perature to make them bum ; so that the name of gas becomes
extinguished by lowering its temperature. This may be experi-
mentally demonstrated by approaching to a weak flame a laree
mass of iron, as by gradually lowering a thick iron ring over the
flame of a small cotton thread dipped in oil ; which, being deprived
of its heat by the metal, would go ouL

145. In a similar manner, the temperature of any inflammable
vapour may be reduced below what may be termed the burning
point, by passing through fine wire-gauze. Thus, if a small por-
tion of camphor be placed in the centre of a piece of wire-gauze
about a foot square, and a lighted candle applied to the under sur-
face, the vapour of the camphor will be kindled and bum below
the gauze, without passing through to inflame the camphor upon
it. Hence may readily be understood the effect of the safety-lamp,
which i^ kind of lantern of fine wire-gauze, within which a candle
or wick, fed with oil, will bum in security amidst an atmosphere
of fire-damp ; for though the vapour may enter and become inflamed
within the lantern, the flame cannot pass throu^ the close tissue
of the wire-gauze to occasion an external explosion.

146. Heat is not only communicated from one body to another
by conduction, or by means of circulating currents, but it is also
conveyed to considerable distances, through any elastic fluid, as
air, by radiation . This mode of the transmission of heat resembles
that in which light is propagated ; and, as light and heat are fre-
quently transmitted together by radiation, uie effects of radiant
heat were generally attributed to the light by which it is observed
to be accompanied. The ancient Greeks and Romans were ac-
quainted with some of the extraordinary effects of radiant heat
produced by burning-glasses ; and thus Archimedes is said to have
consumed the ships of the Romans by such instruments, during
the siege of Syracuse ; and several centuries later the philosopher
Proclus in the same manner destroyed the fleet of Vitalianus,
before Constantinople.

147. Many experiments have been made in modern times on the
effect of the transmission of radiant heat through convex lenses,
and of its reflection from concave mirrors, which show that br
these means its power may be vastly augmented, and which tendf,
upon the whole, to corroborate the statements of ancient writers
relative to the action of burning-glasses.

148. The following results are said to have been obtained from
tke exposure of different substances to the rays of the sun, col-

What i« the true natare of flame ?

How can we prove that flame is prevented from traversing wire-gauze
by the cooling of the burning gas ?

What other substances besides permanent inflammable gases, may be
kept below the burning point, by wire-gauze.

Explain the difference between radiation and conduction.

What evidence have we that the ancients knew the effect of radiation f

What results have been obtained by modem ezperimeots on burning
lenses?

928 PTRCWOMICS.

lected by means of a lens two feet in diameter, with a focal dis-
tance of three ells, in experiments made at Leipsic in 1691.

149. Pieces of lead and tin were instantly melted ; a plate of
iron was soon rendered red hot, and afterwards fused ; a burnt

, brick was conyerted into yellow glass; and amianthus, one of the
most refractory bodies, was in a short time reduced to the state of
black ^lass.* Analogous experiments were subsequently per-
formed in France, with a more powerful lens, constructed by order
of M. Trudaine de Montigny; and in England, with Parker's
bumine lens, which was presented to the Emperor of China, when
I^ord Macartney was sent on an embassy to the court of Pekin.
This last instrument was a double convex lens, three feet in di-
ameter, three inches thick in the centre, and weighing 213 pounds.
Its aperture, when set, was 32^ inches ; its focal distance G feet
8 inches : but the focal length was generally shortened by a smaller
lens. The most refractory substance fused was a cornelian, which
required 75 seconds for its fusion ; a chrystal pebble was fused in
6 seconds ; and a piece of white agate in 30 seconds.t

150. Important experiments have likewise been made with con-
cave mirrors and with combinations of plane mirrors, which,
though relatively less power&l than lenses, may more conveniently
be rendered efficient at greater distances. M. Dufay used both
parabolical and spherical mirrors made of plaster of Paris, gilt
and burnished ; and with one of the latter, 20 inches in diameter,
he set fire to tinder at the distance of 50 feet. The Abb^ Nollet
made corresponding experiments with concave mirrors constructed
of pasteboard, covered with silver or fold leaf and bumished4
fiut the most remarkable experiments ot this nature were those of
Buffon, who had a machine composed of one bund red and sixty-
eight small plane mirrors, so arranged that they all reflected ra-
diant heat to the same focus. By means of this combination of
reflecting surfaces he was able to set wood on fire at the distance
of 209 feet, to melt lead at 100 feet, and silver at 50 feet.$

151. The heat of the sun may be concentrated by means of a
concave mirror, or by being transmitted through a convex lens ;
but the heat of burning bodies in general, though readily reflected
by a concave mirror of metal, produces little or no effect by means

What remarkable effects were obtained by the Leipsic experimenters?
What refractory materials were fused by Parker's lens ?
What kind of reflectors were used by D«fay for burning mirrors f
What apparatus was employed by Nollet and Buffon for the same pur-
pose ?

How may the heat of the sun be concentrated ?

How is common culinary heat affected by transparent solid lenses?

* Sigaad de la Fond Elem. de Physique, vol. it. pp. 172, 173.

1 Dr. Young, in Lect. on Nat. Philos., vol. ii. p. 407, from Cj»vallo.

^ y. Histoire de PAcademie Roy. des Sciences, An. 1726, p. l65. NoI>
let Lecons de Physique, t. v. p. 218.

§ y. Hist de l*Aeademie Kov. des Scien., Aq3,1747, p. 82; and 1749,
p. 305.

KAMATIOM OP Snt&T. n9

of B lena. In a paper pnbliabed in the Memoirs of the Academy
of Sciences of Paris, in 1683, bj M. Mariotle.he stated that ndiant
heat from a common fire, concentrated b; a concave mirror, baa
ita effect destroyed bj the interposition of a plate of glass ; aod
the Swedish philosopher, Scheele, from numerous eiperimentSf
inferred that glass intercepts entirely the radiant heat of a fire ;
and that a glass miiroi reflects the li^t, but preTenta the paaeage
of the heat, while a metallic mirror reflects both heat end light.
It hae however been since diacovered, that though the hnat of bum<
ing bodies commonly exhibits different effects, in passing through
glass, from those which are perceived in the passage of solar heat,
Uiejr may probably de|iend on the fer inferior iotenciity of the heat
arising from combustion compared with Uiat of the aun. For
when a very intense artificial heat with light is produced, as that
of charcoal ignited by a voltaic battery, if a small lens be placed
before the brilliant star of fire thus obtained, and its focus be cast
on the ball of a delicate air thermometer, some eleralion of tem-
perature ma J be perceived.

153. That heat radialee fhun bodies in right lines, and that it
Inay be reflected to a focal point by a mirror, like light, may he de-
monstrated by the apparatus represented above. It consists of
two concave mirrors, A and B, of planished tio or plated copper,
about one foot in diameter, and placed exactly opposite each other,
at the distance of about ten feet. In the foco»of one mirror, at C,
must be placed a heated bod^, as a ball of iron ; and in the focus
of the other mirror, at D, a differential thermometer. The rays of
heat, then, impiogingon the mirror A, are reflected through the air to
the mirror B, whence they converge to its focus at D, and produce
an effect on the thermometer propottloned to the degree of heat
of the iron ball or other heated body.

What renill vii lUcred to have been obtiined in rtprd to this nb-
j«i bj M. Muriotle and tay Schiiele f

What is now found to be the Tad in regard to the ptuagc of tbe beat,
produced bj- MDibuition, throogh glaia >

WliBI eiplRnRtian it to be giTen of the difference Iwlween th»t and Mlar
heat ? Demribe the apparatui eslled Picia't cenjunctroe tmrrtrt.

How doet it appsar (hat (he eSeot on the tfaennoiDCter ia not tha eoo-

teqaenee of direct roMxttion/

9iS

990 PTROlVOHICS.

153. That this etfeet is not produced by the mere dispersion at
the heat through the air may be evinced by holding a pasteboard
scteen between the mirror B and the thermometer, when the latter,
though as near the source of heat as before, would be hardlj, or
not at all, affected by it. And if the ball be moved out of the locus
of the mirror A, towards the thermometer, though thus brought
nearer to it, the effect will be greatly diminished.

154. The flame of a candle, or a flask of boiling water, being
substituted for the heated ball, die same effect will be produced.
If a body yielding a stronger heat, as burning charcoal, or a red
hot ball of iron, be placed in the focus of one mirror, and a piece
of phosphorus in that of the other, the phosphorus will be instantly
infiamea ; and in the^ame manner may be effected the detonation
of fulminating silver, or the deflagration of gunpowder. For the
exhibition of the latter experiments may be sSopted by Sir H. Da-
vy's arrangement of the mirrors, vertically opposite to each other.

155. An extraordinary and somewhat problematical phenome-
non which may be exhibited by such mirrors, is the apparent
radiation of cold. For if a ball of ice or snow be substituted for
the heated iron in the focus of the mirfor A, the thermometer will
show a reduction of temperature. It has been hence inferred by
some, that cold is a peculiar kind of subtile fluid, capable of being
propagated by radiation ; but the effect h^ been more generally
attributed to the abstraction of heat from the thermometer by the
frozen mass opposite to it.

156. Those bodies which reflect heat most powerfully, like the
polished mirrors above described, do not acquire heat from the
rays impingin? on tlieir surfaces ; so that such a mirror might be
held a long time opposite to a fire without becoming perceptibly
warmer. But if the surface of the metal be made rough by
scratching it with sand-paper, or covered with paste mixed with
chalk or lamp-black, it will rapidly absorb the rays of heat, instead
of reflecting them. Hence it appears that the effect of radiant
heat greatly depends on the state of the surfaces of bodies.

157. " Leslie discovered, by experiments made in 1802, that the
heat emitted by radiation was affected by the nature of the sur-
face exposed. The action of a blackened surface of tin beinff
100, that of a steel plate was 15, of clean tin 13, of tin scra^eo
bright 16, when scraped with the edge of a fine file in one direction
36, when scraped again across about 13, a surface of clean lead 19,
covered with a gray crust 45^ a thin coat of isinglass 80, resin 96,
writing-paper 98, ice 85. Heat as well as light is so projected
from a surface, as to be equally dense in all directions, conse-

Whftt eaaes of incipient eombastion may be produced by reflected heat ?

How is the apparent rftdiation of cold exhibited ? and how explained ?

What relation appeavs to subsist between the reflecting and the absorb-
ing power of bodies }

What is the effect of roughening or colouring the surface of a reflee-
tor?

What discovery did I^estie make io regard to the radiating power oi
different surfaces?

R&DliTlNO fetrltf ACES. Sdl

quently from each point, in a quantity which is as Ae sine of the
angle of inclination. The radiation is not affected by the quality
of the ffas in contact with the surface, but it is not transmitted by
water."*

158. As polished metals ahsorb heat yery slowly, so heat ia
but slowly emitted from the surfaces of such metals ; and thus
boiling water would continue at a high temperature much longer
in a Sliver tea-pot than in one of black earthenware ; so that ves-
sels of polished metal are best adapted for preparing tea or other
tegetable infusions.

159. Substances of a li?ht and very brilliant colour reflect heat
readily, but do not absorb it ; while black or very dark coloured
bodies absorb the heat that falls on them, reflecting little or none
of it If pieces of white cloth and other pieces of black cloth
be laid, in similar circumstances, on the surrace of snow, it would
soon become melted beneath the black cloth, but remain perfectly
solid under the white. In some of the mountainous parts of Eu-
rope, th^ farmers are accustomed to spread black earth or soot
over the snow, in the spring, to hasten its dissolution, and enable
them to anticipate the ipenod of tillage.

160. It may be generally assumed that all bodies of unequal
temperature tend to become of equal temperature ; if in contact,
by conduction ; if at sensible distances, by radiation of the excess
of heat; and in the latter case whether the radiation reach the
cooler body directly, or by an intervening reflection, f

What class of surfaces emit or radicUe heat most readily ?

What influence has colour on the absorbings and reflecting powers of
bodies respectively ?

What advantage is taken in Europe of Franklin's discovery respecting
the melting of snow beneath black surfaces?

In what two modes do all bodies of unequal temperatures tend to an
equality in this respect ?
" ■ ' ' ' ^^^■^^^^— — ^

* Dr. Young's Lectures on Natural Philosophy, vol. ii. p. 407.

t See Report on the present State of our Knowledge of the Science
of Radiant Heat, presented to the British Association, by Professor Pow-
ell, in 1832,

33!^ PVHONOMICS.

Works of reference on the subject of Fynmomies,

The following^, among other works od the science of heat, ma^
be consulted in fhrther prosecuting the study of this department.

Thompson's Trealise on Heat and Electricity.

Library of Useful Knowledge, treatise on Heat.

** ** " " ^ on the Construction of

Thermometers and Pyrometers. Two numbers.

Webster's edition of Brando's Manual. Boston.

Turner's Chemistry, by Dr. Bache. Phila. edition.

Ure's Dictionary of Chemistry. Article ddorie,

Leslie on Heat and Moisture.

Crawford on Animal HeaL

Dalton's New System of Chemical Philosophy.

Leslie's Experimental Inquiry into the Nature of Heat.

Walker on Cold.

Many articles in Silliman's Journal, the Journal of the Franklii
Institute, Annales de Chimie, and other contemporary periodicals.

OPTICS.

1. Among the grand sources of our l^nowledge of the works of
nature is tiie faculty or sense of sight or yision, to which we owe
the perception of light and colours, and the means of judging
concerning the forms and appearances of the numerous bckiies
around us. The highly curious, interesting, and important phe-
nomena with which we thus become acquainted constitute the
subjects of the science of Optics,* or the theory of light and vision.

2. This department of natural philosophy may be considered as
furnishing topics for investigation under different points of view :
1. As relating to the gener;u properties of light, and its effect on
the organ of vision ; 2. With reference to the reflection of li^ht
from the surfaces of bodies; 3. With reference to the refraction
of light, or the alteration it undergoes in passing through transpa*
rent bodies ; 4. As regards the phenomena of colours ; 5. As re-
spects certain modifications of reflected and refracted light, which
have been characterized as resulting from the polarization of li^ht.

3. Aijaong the multitudes of bodies which we can perceive,
some sure visible by their own light, and these are styled luminous
bodies; while others have no such illuminating property, and can
be seen only by means of the light afforded by the former. Lu«
minous bodies consist of those which are original and permanent
sources of light, as the sun, fixed stars, and probably comets ; and
those which exhibit light only under certain circumstances, es«
pecially while undergoing combustion, as in the case of minuter
fragments of steel struck off by the collision of flint with steel, or
in the common process of borning a candle, oil in a lamp, or coal«
gas.

4. Any bodies which do not interrupt the passage of light, of
which admit of other bodies being seen through them, are called
transparent bodies ;f those which prevent entiiely the passage of
light are termed opaque bodies ; and those which allow other bodies
to be seen through them obscurely and imperfectly are named semi-
transparent substances. Transparency and opacity, however, de-
pend much on the relative tiiickness or thinness of substances ; for
even air, which affords less intermption to the passage of light

What classes of phenomena are embraced in the science of opticsP
Under how many and wbat different views may this science be regarded?
On what is the distinction of lundnotu and noti'luminoua bodies founded }
How are the terms tranaparent, tetm-iran^Mrent, and opatpte respee-

tively applied ?
Are transparency and opacity to be regarded as absolute or relative

properties of matter ?

• — - .._i. _ . I . ■_ _i__i_mi^»- ■ ^^ • —

* From the Greek O9rT0A<a«, to see.

t The words tranaparerU and diaphanout are synonymous ; the former
being derived fi'om the lAtin, trantt through or beyond, and pareru, ap-
parent; and the latter from the Greek, Ai«^«yi|(, shining throngh, or
tnmduceot.

333

334 OPTICS.

thao any other kind of matter, is not perfectly diaphanous ; nor
will the densest metal completely preyent the influence of light.

5. It has heen calculated that tne atmosphere, when the rays
of the sun pass perpendicularly through it, interrupts from one 1-5
to i of their light; but when the sun is near the horizon, and the
mass of air through which the solar rays pass is consequently
Tastly increased in thickness, only 1-212 part of their light can
reach the surface of the earth. '' By a peculiar application of my
photometer,*' says Sir John Leslie, ^^ I have found that half of the
mcident light, which might pass through a field of air of the ordi-
nary density, and 15^ miles extent, would penetrate only to the
depth of 15 feet in the clearest sea^water, which is therefore about
5400 times less diaphanous than the ordinary atmospheric medium.
But water of shallow lakes, although not apparently turbid, be-
trays a greater opacity, insomuch that the perpendicular li^ht is
reduced one-half in descending only through the space of six, or
even two feet.

6. The same measure of absorption would take place in the
passage of light through the thickness of two or three inches of
the finest glass, which is consequently 500,000 times more opaque
than an equal bulk of air, or three hundred times more opaque
ihan an equal weight or ma>88 of this fiuid. But even gold is dia-
phanous. If a leaf of that metal, either pure or with only 1-80
part of alloy, and therefore of a fine yellow lustre, but scarcely
exceeding 1-300,000 of an inch in thickness, and inclosed between
two thin plates of mica, be held immediately before the eye, and
opposite to a window, it will transmit a soft green light, like the
colour of the water of the sea, or of a clear lake of inoderate
depth. This glaucous tint is easily distinguished from the mere
white light which passes through any visible holes or torn parts
of the leaf. It is indeed the very colour which gold itself assumes,
when poured liquid from the meltinff-pot.

7. A leaf of pale gold, or gold alloyjdd with about 1-80 part of
silver, transmits an azure colour ; from which we may, with great
probabilit;^ infer, that if silver could be reduced to a sufficient de-
gree of thinness, it would discharge a purple light. These noble
metals, therefore, act upon white light exactly like air or water,
absorbing the red and orange rays which enter into its composition,
but suffering the conjoined green and blue rays to eflfect their paa-

What portion of the sun's light, when vertiosl, is supposed to reach the
earth ?

How great a portion is interrupted when the sun is near the horizon }

What calculation has Leslie instituted between the transparency of air
and that of water }

What is found to be the degree of opacity in shallow lakes compared
with sea- water ?

How much does the opacity of the best glass, weight for weight, exceed
that of common air ? What examples of transparency are found in bodies
commonly reckoned opaque f

How is the eoloar of light found to vary when transmitted through ho>
dies which are imperfectly transparent I

801TRCB6 OF LIGHT.

sage. If the yellow leaf were to transmit only 1-10 part of the
whole iiv;ident ligrht, we should only conclude, that pure gold is
850,000 times less diaphanous than pellucid glass.

8. The inferior ductility of the otner metals will not allow that
extreme lamination, which would be requisite, in ordinary caseg,
to show the transmission of light. But their diaphanous quality
may be inferred, from the peculiar lints with whicn they affect the
transmitted rays when thev form the alloy of gold. Other sub
stances which are commonly reckoned opaque, yet permit in yari
ous proportions the passage of light. The window of a small
apartment being closed by a deal board, if a person within shut
his eyes for a few minutes, to render them more sensible, he will,
on opening them again, easily discern a faint glimmer through the
window. If this board be planed thinner, moire light will succes-
sively penetrate, till the furniture of the room becomes visible,
and perhaps a large print may be distinctly read.

9. Writing-paper transmits about the third part of the whole ia-
cident light, and when oiled it oflen supplies the place of glass ia
the common work-shops. The addition of oil does not, however,
materially augment the diaphanous quality of the paper, bat ren-
ders its internal structure more regular, and more assimilated to
that of a liquid. The rays of light travel without much obstruction
across several folds of paper, and even escape copiously through
paste-boa?d."*

10. The chief sources ef liffht, as already observed, are perma-
r;.ently luminous bodies, or celestial fire, especially the sun ; and
terrestrial fire, or that given out during combustion, or incandes-
cence. There are, however, some oases in which ligrht is exhibited
under circumstances apparently unconnected with the influence of
the solar rays, or of terrestrial fire. The exhibition of light ao-
companies many electrical phenomena ; as lightning, the luminous
traits produced by brushing with the hand a cat's back in the dark,
and many others which will be more particularly noticed in the
subsequent part of this volume.

11. Phosphorescence is another kind of luminous exhibition,
where light is emitted without sensible heat, and the effect seems
to be but remotely, if at all, dependent on either of the grand
sources of luminosity which have been pointed out. Common

What relation may be computed to subsist between the transparency
of gold and that of glass ?

What would be the comparative transparency of gold and common air ?

How may the pai'tial transparency of wood be demonstrated }

What is the effect of oiling paper on its power of transmitting light?

What are the chief sources of light ?

From what sources, independent of these, are the phenomena of light
oecasionally produced ?

What is the distinction between phosphorescenee and the luminont-
ness of burning bodies ?

* Leslie's Elements of Natural Philosophy, vol. i. pp. 90 — SS.

886 o^ncfl.

phosphoras* is a highly comhustihle hody, burning fiercely ata
eertain temperature, with intense liffht and heat: it also gives ont
light at a very low temperature, without apparent heat, but this is
the effect of slow combustion : it was, however, ascertained by
Dr. Van Marum, a Dutch philosopher, that phosphorus covered
with dry loose cotton, or sprinkled with resin, would shine under
the exhausted receiver of an air-pump,f a situation in which it
seems impossible that any combustion can take place, on account
of the deficiency of atmospheric air. But there are phosphores*
cent bodies which yield light under circumstances which have no
connexion whatever with the process of combustion.

12. Decayed wood, and sometimes peat or tivf, have been ob-
served to shine in the dark ; and some kinds of fish, as soles,
whiting, tench, and carp become luminous when tainted, but
before they grow putrid ; lobsters and crabs often display phospho-
rescence in similar circumstances ; and also batchers' meat, occa*
sionally.

13. There are many animals of the lower orders that emit light
in greater or less abundance while living. Among insects, the
^low-worm {Lampyrtu Splendidula) is the most generally noted for
Its illuminating powers, in European countries, and the common fire-
fly of the United States ; there are, however, other insects in some
degree possessing similar properties, as the common centipede,
found under tiles or flowerpots in gardens, which when irritated
gives out bright flashes of light. But the most remarkable shining
insects are natives of the \Vest Indies and South America; and of
these the Elater Noetilueus^ a coleopterous^ insect, affords a splen-
did specimen. ** It is an inch *lon^, and about one-third of an inch
Broad, giices out its principal light from two eye-like tubercles
placed upon the thorax ; and the light emitted from them is so
considerable that the smallest print may be read by moving one
of these insects along the lines."$

14. The sifrface of the sea is observed by the mariners to be
occasionally illuminated ; and the light generally, if not always,
is produced by certain marine phosphorescent animals. There are
some peculiarities in these luminous appearances, which have been
described as exhibiting five varieties ; *' the first shows itself in

How did Van Marum exhibit the phenomena of phosphoreecence ?

What examples of a purely phosphorescent appearance can be men-
tioned }

How docs it appear that neither combustion nor putrescence is neces-
sary to the production of phosphorescence in animal substances ?

Among what tribes of animals are the phosphorescent classes chiefly
found ?

• From c»f, light, and «oji,., to bear.

t See Arcana of Science, for 1832, p. 130 ; and Brewster's Edinburgh
Journal of Science, N. S.

\ So called from the character of the wings.

% Introduction to Entomology. By Rey. W. Kirby and W. Spence.
Syo. Yol.ii. p. 41 3.

PHOSPHORESCENCE OF THE SEA. 88t

scattered sparkles in the spray of the sea, and in the foam ereated
by the way of the ship, when the water is slightly affitated by
the winds or currents ; the second is a flash of pale light, of m<H
mentary duration, but often intense enough to illuminate the water
to an extent of several feet ; the third, of rare oocuirencet and
peculiar to ffulfs, bays, and shallows, in warin climates, is a di^
fused pale phosphorescence, resembling sdmetimes a sea of milk,
or of some metal in a state of igneous lique&ction ; the fourth
presents itself to ^e astonished yoyager under the •sf>pearance of
^ thick bars of metal of about half a foot in length, igmted to white-
ness, scattered over the surface of the ocean, some rising np and
continuing luminous as long as they remain in yiew, while others
decline and disappear; and the fifth variety is in distinct snots oil
the surface, of great beauty and brilliancy. The light of the first
variety b more 'brilliant and condensed than that of any of the
others, and very much resembles every way the r^ gold and silver
rain of the pyrotechnist. This together with the third kind are
produced by myriads of various minute crustaoeous animals, the
smaller Medusm and Moikuea^ and perhaps some JnnelideM s the
second appears to proceed from the gelatinous Medusa, of a laifor
size ; the pyrosoms are the cause of the fourth kind, which may
be often witnessed by vessels bound to India, or the eastward of
the Cape of Good Hope, occurring in the calm latitudes near
the line.

15. **The Sapphirina Indieaior,Ka insect piomewhat resemblioi^
in appearance the woodlouse ((mictM), and about one-third of an
inch in length, emits the last vaiiety enumerated, which appears
to be limited to the seas sitifated to the norUi and \yest of a line
dravm frogi the Cape of Good Hope to the southern extremity of
the Island of Ceylon."*

16. Some flowers have been remarked to emit flashes of li|[ht
while growing on the plants to which they belong. These miua-
ture lightnings sometimes are perceived of a summer evening, in
warm close weather, issuing from the petals of the African and
the common marygold, the nasturtium, and the tuberose.

17. Many mineral bodies give out light under particular circum-
stances. This is the case with some diamonds, and varieties of
rock crystal, which become luminous on being removed into a

.dark room after exposure to the jays of the sun. What is called
the Bolognian phosphorus, is artificially prepared by mixing into
a paste, with gum tragacanth, powdered sulphate of barytes, or

How many and what varieties of luminousDCU are exhibited at aea?

What causei the fint variety ?

How 18 the fourth kind to he explained ?

In what part of the oeean is the pyrosoma found ?

Where the sapphirina indicator r

What plants are known to emit luminous flashes ?

Under what eireamstanoes mav certain minerals appear to emit lidit ?

What artificial imitations of these substances have been preparedr

• Thompson's Zookf ical Besmirches and llloitntioiii. lUSL 8vo»

8P

jpdiidefcms spar, anid 4iWding^ tbe mass into thin cakes, wMeh are
to be carefauy calcined in an open fire and suffered to cool slowly:
Aey then sbine in the dark after heingr exposed to the sun.

19. Canton's phosphorus, iH^hieh consists of sulphnret of lime;
tad Baldwin's phospnoras, which is nitrate of lime, hare analo-
gous properties ; and ore^teP'Shells, calcilied by putting* them into
e oOu Or charcoal fire, tor about an hour, and when cold taking off
tl thin Seale ftdm the inside, will be found to be become pfaospho-
leseent. There are minerals which are rendered luminous in the
idiurk by exposare to a temperature of red heat, as phosphate of
Hme, from cstramadnre, and some kinds of fionr or Derby^hird
ispar, fbtid eaibonalie oi lime or swinestokie, quartz, a&d ponderous
^ar,

19. Light may be elicited firom riolent friction or collision of
fncombustible bodies, just as fire is from filrnt and steel. Thus
tnight sparks may be produced by striking one piece of common
'flint or rock crystal agEiinst another; rubbing together two pieces
of bonnet-cane will cause the emission of light, in eonseqnence
of the epidermU or sealy coating of the cane being composed of
ailiceous earth; and loaf sugar yields a pale light, from the col-
lision of two lumps in the dark, the effect being mei^ly the exhi-
bition of phosphorescence, for though sug^r is an inflammable sub-
'Matice, the Inminoos appearance is unaccompanied by combustion.

30. Light, considerea as the cause of vision, or the medium by
trhich objects become perceptible to sight, exhibiting a yarfety
of tints to the eye, has generally, since the publidation of Sir
Isaac Newton's theory of light and colours, been ascribed to the
emissioa of a pecuHar ethereal fiui<l from the sun and all other
luminoQS bodies. This subtile fluid or ether was supposed to be
perpetually streaming in all directions from the sun and fixed stars,
travelling with a Telocity 900,(W0 times that of sound through
the idr, and yet consisting of particles so extremely minute as to
pass through the densest substances without at sJfl altering their
'fltru<!ture, or interfering during their progress in the slightest de-
gree with each other. Descartes, who died in 1650, had advanced
% different hypothesis to account for the action of light, founded
on the admission of the existence of an ethereal fluid, not subject to
amotion of translatioh, or passage firom one part of space to another,
hut capable of being thrown into the state of undulation by the
impulse of luminous bodies, and the undulating motion being in-
idefinitely extended, would obviously propagate the influence of
light through any given space.

21. The theory of undulation, as it is termed, was adopted and
improved by Huygens, the oontemporary of Newton, whose sys-

By what meehanical means mar eombaitible bodies be made t» emi
light?

Is WKf real oonbiMtkni piiidueed in these oases ?

"What theories did Newton advanoe to ataount for the pbeaomieiia ol
light and ▼ision }

MA #faisi rate did he fiad it neeessny to soppopo lighl to tmv^l

CAUSE ANB MODE OF > PBOVAOATION OF LIGHT. WSt

lemof emanatio* or omisttoB ef Uyht, psop#8ed Vy tke latter» j/nn^
ci{|a^ly through tke authority of his great name, prevailed vlmoit
universally till about the middle of Uie last ceotory, when it was
attacked by Leonard Euler, who, in hia " Letters on different sub-
jects of Natural Philosophy, addressed to i^ German Princess,'*
has pointed out the difficulties whi(^ occur in attempting to ex-
plain ^e phenomena of light according to Newton*s doctrine o^t
theory of. emanation, and has advanced man^ striking arguments
in favour of the theory of uvdalation, showigg the analogies be-
^een the modes of propagation of light and sound, and denonr
strating the ^neral agreement of the hypothesis with those factsr
which constitute the basis of ojptical science. JSuIer, however,
gained but few converts among his scientitc contemporaries, and
the opposite theorv was generally admitted as correct till about
the beginning of the present century, when Dr. Thomas Young,
in the Bakerian Lecture, read before the Royal Society, in 1801,
entered into an elaborate disquisition concerning the Aeoiy of light
and colours ; and deduced from the principled &id down by New-
ton himself, the three following hypotheses:

23. 1. That a lumimfbrous ether,, rare and elastic in a high de-
gree, j^rvades the whole universe, 2. That iindulations are ex-
cited in this ether whenever a body becomes luminotts. 3. That
the fieasation of different colours depends on the frequency of
vibraUons excited by lighX in the retii^ To these he added a
fourth hypothesis, assuming that all material bodies have an at-
traction for the ethereal n^ediilm, by means of wldch it is accumu-
lated widiin thek substance, and for a smalf distance around them.
In a state of greater density but not of greater elasticity.*

23* Subsequent discoveries, have tended to confirm the theoijT
of undulation, which affords a more unobjectton^le mode of ex-
plaining the phenomena of polarized light, and otl\er appearances,
than is furnished hy the theory of emanation; and the fonaer han
been embraced by the most distinguished phiiosojphers now livinj^
or recently deceased ; as Fresnel, Arago, Sir Joiiii Herschel, Sfr
D. Brewster, and others, who by their own discoveries have coor
tributed to extend the boundaries of science.

24. The propagation of light always takes place in right lines,
projecting on every side from luminous bodies. Such radiating
tines or rays, diverge from each other in their passage, forminjr
what is called a pencil of light, as exhibited in die margual figure.

What substitute for the li^eory of emission was adopijted hj peseartefl,
fioygens, and Eoler f

\» hat three positions were ettablUlied by pr. Yoaii^ on this siAJcet?

What relation did he assame to subsist between the uiminiferous ether
and comnion matter f

WiucH of the two theories appears at present, to posABss the gr«fUer
iMinber of advocates among the cultivators of this scieapei

In what lines does the propagation of lighjt take pUc<e /

, * See Philos. Transact for 1802 { «nd A)>a|tf.^c|t of Papers in th« l^l*
*1^£aiia^ voL i. pp. 64, 65.

840

OPTICS.

A tiikiilar effect maybeprodaced'by^admittingriiito a darkened

room, through *a minute aperture in a win-
^^^. dow<«hutter, the light of the sun which would
^'*^=^' he perceived proceeding in a diverging bun-
dle or pencil of rays ; and on presenting to
it a flat board, a luminotis image would be
formed, increasing in diameter with the in-
crease of distance from the aperture at which
the plane was held, and which, by variously
inclining the plane, might be made to assume elliptical or other
curved ng^res*

5^. Images df variously-shaped bodies seen -by light thus ad-
mitted tiirough a small opening are always in a reversed position,
in consequence of the obuquity or divergence of the rays of light.

That this effect iQust take place will be readily perceived from
the preceding figure, which shows that the rays in passing through
the opening must cross each other, and thus raysj^omin^ from the
superior parts of objects, impinge on the relatively inferior portion
of the plane, and those from the higher parts strike on that portion
of the plane below the other rays : the spectra or images produced
must consequentlv be inverted.

36. It is stated above that the dimensions of the images thus
formed decrease in proportion to the distance from the opening at
which they are ^ituatea. Thus if the plane on which tne image
of an object is received, be placed at exactly the same distance
before the aperture, as the object stands behind it, the size of the
imaffe will coincide with that of the object, for the pencils of rays
on either side would be alike. If, however, the plane be removed
nearer the aperture than before by one-half, the image will be but
one-fourth of the size of the former ; at one-third the distance, its
size would be one-ninth ; at one-fourth the distance, one-sixteenth ;
the diminution taking place in the ratio of the squares of the dis-
tances of the plane from the aperture. The intensity of light d^
minishes in tne same proportion : thus suppose a candle to be

Bv what experiments ean this be proved ?
How ' ^*^ '•' " '

formed id

How is the rectilinear toarse of fays proved by the images
s dark room by ligbt admitted tbrongh a simple aperture f

What ratio will the size of sueh an image nave to the distance of the
iereen from the apertare ?

H6w does the intensity of light vary with the distanee of the nuSoaf
of light?

RELATIVE IKTKNSITT OF LIGHT. 941

placed at iAte distaiiee of one yard from the laee of » dial pr ItM*
|»iece, the ligrht thrown on it may be lepresented by the Aunbei
1 ; if then it be remoied back to tviro yards, the light will be b«|
i as much as before; at 3 yards l-9« at 4 yards 1-16, at 5 yanl4
1-25, at S5 yards 1-635.

27. This reduction of lig^t^ in proportion to the distance of ih»
luminous body, is the necessary effect of the divergence and co»*
sequent dispersion of the radiant pencil ; and hence it may readily
be conceived, that an inconsiderable light can only be visible at «
comparatively trifling disltaaioe, and .that its inflaence in nodering
Bon-lumiaous objects visible, must ^ limited to a much aborts
distance than the ^j^ame point at w^ch its li|[ht m411 be pe»
ceptible.

S^. The apparent sise of aU visible obJe<pts is. to be explained
fm the same principles with those that govern tihe Ibrmation of
images by liffht transmitted through an aperture, as ^ust desciibed^
When we take a view of an illuminated body, its un&ge becomef
traced in shadow, exhibiting,, however, its proper colours on tht
ietina, a nervous membrane that lines the interior ^surface of tii«
eye. A more particular description of the structure and apparenl
uses of the difmrent paits of the organ of vision will be introduced
after the nature and causes of the refraction of light ^ave been exr
plained; but the relation between visil^e images qf ciijects and
ahe angular distances ef the objecta theq^selves may here be gkA^
pisriy poioted out^

3d. From what has been previously stated concerning the diitii^
Aution. of the light of a pencil of rays iii proportion to the distaneea
of the point whence they diverge, it must be evident that the
nearer to the eye any object may be placed, so mueh more numsf
fotts will be the rays of light passing from it which can act upon
the eye so as to form the image on the retina. The number of th*
•rays indeed will increase or decrease as the. squsM^s of tlie diff
lances of objects, afWr the manner already described. This, hoWr
ever, is to be understood as the law that regulates the propagatioi
pf light simply and independently of the medium it traverses $ fyf
air, the most transparent of bodies, interrupts in some degree tbf
passage of light through it, as elsewhete ob&iervedj and thereforf
the apparent dimensions of objects must be considerably infl«»
enced by the nature of the medium through which they are be-
held.*

30. The angle formed by the crossing of the rays of light pas^
ing from the opposite. extremities of a visible object is ^led Uio

What causes this Tatiation ?

flow is the apparent size of olgects dependent on their distance f
How mu^t the apparent brightness of an Object be affected bj its n^ai^
taess to the eye ?
What is ineaDt by the an^e of vision?

• See the subsequent part of this treatise, relating to the Refraction of
Light.

2f2

m OPTICS*

angle of vision. Now tkat an|rle will be relatirely v^ry eontnct-
od when the radiating lines afe emitted from an object extremely
minnte, or from one placed at a great distance. Thus there are
insects too small to be visible to the naked eye even when brought
as near to it as possible ; and some objects of immense size, as
^e fixed stars, each df which probably is many thoasand times
larger than the earth, appear as mere points from the remote situ-
ations they occupy \ while there are doubtless multitudes of other
atars yet more remote, and therefore quite invisible, even when
tite heavens are surveyed through the best telescopes. Unless
tbe angle of vision be niore than one second of a degree, the object
whence the rays proceed will not be visible,^ without it be yeiy
strongly illuminated.

'31. The velocity of light is so great that it was long supposed
to pass instantaneously through any given space ; and though it
ASS been ascertained that it occupies a certain time in its passage
proportioned to the distances traversed, yet so rapid is its appa-
rent motion, that in observing the effect of light at places a few
miles distant from each other the time need not be taken into the
account. The rate at which light is propagated was discovered
by Olaus Roemer, in making observations on the eclipses of the
satellites of Jupiter.

32. If the transmission of light were instantaneous, it must be
obvious that the reflected light of the* sun would take up no mora
time in passing from anv one of the planetary bodies to the eartbi
when they are farthest mm us, than it does when they are lieaiw
est; and as the situation of the earth with respect to the other pla>
nets is different in different parts oi her orbit, the satellites of Jti«
piter, on emerging from the shadow of that planet, would be seen
as quickly when the earth was in one part of her orbit as in ano>
ther. But this is by no means the case ; and the effect of the
transmission of light is such, that when the earth is between Ju-
piter and the sun, the satellites, after being eclipsed, are perceived
rather more than eight minutes sooner tiian they ought to appear
according to the time as calculated by the most accurate tables ;
and when the earth is in the opposite part of her orbit, so that the
sun is between this planet and Jupiter, the satellites emerge about
eight minutes later than the calcmated or mean time.

Bjr what two cireumttances may that angle be dimioished till the ob-
ject beeomes iropeixeptible ?
, Give some iUottrations of both caiei.

What is generally the least anele under which an objeet can be seen ?

What was formerly thought or the motion of light ?

Why do we make no account of the time occupied by light in trayera-
ing distances on the earth's surface ?

How would the instantaneous transmission of light through space ent"
ble ut to see the heavenly bodies in different parts of the earth's orbit ?

What is found to be the &ct in regard to Jupiter's satellites ?

TXEOCITY 07 LIGHT.

84S

33. In tiie annexed diamm, let S re-
present the sun, A and B Sie earth in dif-
ferent parts of her orbit, J, Jnpitfer, D, his
nearest satellite entering the shadow of
that planet, and C, ^he same satellite,
emergring from the shadow. Now the time
of the Commencement or termination of an
eclipse of the satellite, as stated from cal-
culation in tables, is the instant at which
the satellite would appear to enter or
emerge from the shadow, if it could be
seen by an obserrer from the sun : and it
is found from repeated observation, that
the eclinse takes place about 8 minutes
earlier than tiie calculated period, when
the earth is in the nearest part of her orbit,
as at A, and 8 minutes later when she is
in the opposite part of her orbit, as at B.
Hence it will be apparent that light takes
up 8 minutes. in passing throush a space
equal to half the diameter of the earth's orbit, or the distance be-
tween the earth and the sun, which is ninetj-fiTe tnillions of
miles; so that it moves at the rate of 95,000,000 -~^ 8 X 60a
197,916, nearly 200,000 miles in one second.

34. The aberration of the fixed stars also shows with what
speed light is propagated ; Dr. Bradley having ascertained that
this phenomenon depends 4>n the motion of the esnrth in her orbit,
in eonnezion with*the velocity of light. The effect thus produced
cm the apparent places of the fixed. stars at different times, termed '
aberration, is familiarly explained by Professor Robison. He ob-
serves, that if hailstones were falling perpendicularly, they would
pass freely through a tube held steadily in a vertical position ; but
if the tube were moved round in a circle while the hailstones were
falling they would impinge against its side, unless the tube were
inclined forward, at an angle of 45 deg., supposing the velocity
with which the tube was moved was equal to that of the falling
hail. **In the very same manner, if the earth be at rest, and we
would view a star near the pole of the ecliptic, the telescope mast
be pointed directly at the star. But if the earth be in motion
round the sun, the telescope must be pointed a little forward, that
the light may come along the axis of the^tabe.

.*•
What is the true time of commeDeemeot or termination of the eclips^
of one of those tatdlitea P

Cnnstrnet and explain the dian^ro, showinr the velocity of light.
How long does it require to traverse a semi-diameier of the earth's orbil?
How far will it travel in a second ?
On what does the aberration of the fixed stars depend ?
By what supposed arrangement of apparatus may the abetration of
light be illastrated ?

What mast the absolnte direetion of a telesoope be in regard to tha
poiitioii of the body viewed ?

4644 opTiGfl*

35. *'The proportion of &e ireloeity of light to the 8appt>sed
Jocity of the earth in her orbit is nearly that of 10,000 to 1 : there-
fore the telescope must lean about 20 see. forward. Half a year
ftfler this, let the same star be viewed again. The telescope must
again be pointed fifi sec. ahead of the true position of the star :
but this is in the opposite direction to the iomet deriation of the
telescope; because the earth, beilkg now in the opposite part of
her orbit, is moving the other way. Therefore the position of the
star must appear.to have changed 40 see. in the si^ months. It
is easy to snow- that the consequence of this is, that ^very star
must appear to hare 40 sec. more longitude when it is oti oar me-
ridian at midnight than when it is on tiie mendian at mid-day.
The effect of this composition of motionA, which is most suscepti-
ble of accurate examination, is the following. Let the declinal^on
of some star near the pole of the ecliptic w obserred at the tihie
of the equinoxes. Jt will be found to have 40 sec. more declina-
tion in the autumnal than in the yemal equinox, if the obsenrer
be in the latitude of 66 deg. 30 min. ; and not much less if he be
in the latitude of London. Also CTery star in tiie heayene should
appear to describe a littie ellipse, whose longer axis is 40 sec"*

36. As the total absence or privation of light produces darkness,
eo the' partial defect of light occasions shade; and when any
opaque body interrupts the passage of the rays of light, the figure
of that body or its outiine surrounding a dark area will be fonned
<on any plane surface beyond the opaqve body, constituting its
shadow. The depth or darkness of the shadow is always in di-
rect proportion to the intensity of the light ; and if an opaque body
he illuminated bjr several lights at ones, diffisientiy situated, as
many shadows will be formed as there are lights present; as
may be observed with respect to any object inr a room where two
or more candles are burning. If the luminous body be laiger
than the opaque body that intercepts its rays, the shadow will be
in the figure of a pyramid the hsae of which will be equal to the
surface of the opaque body, and its extent will depend on tiie dis-
tance at which the luminous body is situated firom that which in-
tercepts its light

What is the proportion between the Telocity of light and that of the
Mirth's progressive motion ?

How much mast a telescope be inelined forwaifd of its real object in
order to see a remote luminary ?

. Why will it appear to be again in adranee of iu true poiiUea st the
end of half a year ?

On what is the intensity of 4iadow8 dependent 1

What will be the form of shadows when ^ radiant i« larger than the
Intercepting surface p

* £lements of Mechanical Philosophy, IQO^ ?ol, i pp. $^ jgfijf«

CAUSE <n SHADOWS.

d49

37. Thus, in the preceding diagram, eappose S to lepresent the
snn, y the planet Yenns, and £ the earth; then if the two planets
were of equal size, Venus being nearer than the earth to the sim
would cast a shorter conical shsuiow than our planet.

38. If a shadow be formed by an opaque body of exactly the
same dimensions with the luminous body whose rays it intermpts,
the shadow will be a cylinder of an area equal to that of the two
bodies, and extending infinitely in length. But if the luminous
body be smaller than the opaque body Sie shadow will be a trun-
cate pyramid, the larger base of which must be at an infinite
distance. In the former case the rays of light will proceed in pa-
rallel lines till they are intercepted, and the consequent shadow
will preserve the same dimensions throughout its indefinite extent;
and in the latter case the rays will be divergent, and the shadow
formed will increase in dimensions ih proportion to the distance
between the two bodies.

39. This will further appear from considering tile relative di«
mensio'ts of the bhme figure whe.n the shadow is ^rown on a

eane surface at differept distances from the source of light Thus
e A b« an object illummated by the ii^ht of a candle, and B, C,
D, E, be a succession of screens, B being the nearest, and £ the
most distant; the former, therefor, will have the 8o>alle8t shadow,
and the latter the largest.

40^ The proper 'shadow or dark outline of an opaque body,
formed when it intercepts the rays of light from a luminous body,
is always encompassed or bordered by a kind of demi-shadow, or

as it is termed, penumbra.* Thus,
in the annexed diagram, let S !».
present the sun, or anjfother source
of light, and A B any opaque body,
the true or proper shadow of which,
on the plane M N, will- be termi-
nated by the tangential line C A F,
and the space B A F will be entirely
shaded ; but the penumbra will ex-
tend firem F to I, iilclading the space

How il thit illastrated in th^ soltr system ?
Exhibit And explain the dias^m on this subject
What form will the darkened space possess when the opaque body and
the radiant are of the same size ?

* From the Latin pene, almost, and umhra, a shadow

F AI,'w)iiehwUl bep*iliaU7en]i|:falea(id, the ihsdeendtullfdi-
IBinishing ta It recedes from F towiTds I, wken it wilUie lenniut-
•d bj ibe second tannnt SAL

41. An ecIipM of the ma being occsncwed bf the interpondoB
of the opaque bodj of the moon betweea the earth and tfaeton,
Ibe proper shadow will be a frustum of a cone coUTergtiv basa
the moon towajda the earth, and tiieie will be a fainter Amaowot
feanubra (urrounding the foriMr. Thia is ahown in the amieied

are, wh«e S repreaenla die auiii M, Ilia moop, and L, the eanb)

Ihe dark apace is denoted by the inner conical ahadow, bejonj

_i;_.. :.i .-j^ — lends the penumbra.

moral regarded aa beino' perfectly black,

J they cBrtainly must be where the light of a laminoiu body i
ompletely excluded by an opaque body pla^ before it, fii
tiadowa, as they comqwal j appeaj, are found to Taiyin colour i

well a.

E reduced by the sun at diSerent hours of the da3f, and thme cauwd
y_ different aorta of lights, if attentively examined, will be pet-
eeived to consist of green, blue, violet, or red ^ta laore or Jul
Rallied by black.

43. The figure of the enligfateDed part of an opaqne hodj:, sen
%y meana of a fixed light, dependa on tk^ rel^ye position of di«

Let L, (n the mamnal

fgure, be the place tf ibe

luminouB point, the rit

nation of the observer;

then, aa opaque aphen

placed at A would noi aa-

pear at all enlightenEo;

at B, the enlightejMd pa^

tion of the sphere vodd

asaume'the form of aeres-

cent; at C it wouid bes

aemi-cirele ; at D it would approach to a circle ; and at E the circle

wonld be oomplete. The ^enameDa would be repeated, but in

What will be ila form, if the opsqoe body be Uraer tfian the lanUKiu

'"' -it by.

lydiagn
« form of the taitrihai pcopev diadov ia f«lri{i>eaafdM

How arc the nicBeaaiie iippearancei ofthe mooa tobe explaiaed?
Drav and detcribs (he dUgrtm for iu jiAata,

BSFLECTIOlf Qt LIGHT. 94T.

ia^m OTder, in pioceedtng tbroo^ the oppottte posUioa«« from,
S to A. iThe extent of the enligkiened part visible from the point
O, ifl determined by the tanmiis drawn from the small sphere to
the points L and O : thus a o marks, the enlightened part which is
visible of the sphere D.

44. This diagraAi and deseription will serve to explain tih«b
j^Msea of the mooD, or her various ajppearanees, as enlightened
vf l^e sun, and viewed from the earth in different parts oi her or->
biL 'PhsLt saielUte being invisible at the changet or new moon, as.
at A, and afterwards exhibiting more and move of her surface tlU it
^eoolnee 4 eompkte eileulai disk, or frill moo&«,aa at E; afrei
which the waning moon txavelft oft to Ay whenee afresh saeoesaion
of changes tskes place.

CATOPTRICS.

45. BcsiDtts the g^nerd effect of light in tendering .visible ail
objects within the influence of tiie rays extending from luminous
bodies to those around them, there are peculiar phenomena which
take place when the rays are thrown on substances presenting very
bright or smoothhr pohshed surfbces. For in such cases, if the
sunaces are very highly poliehed, the^ are no longor visible wh«i
Aus illuminated, but exhibit perfect pictures of any objects placed
in front of them, that is between the light and the polished surfiice*
llie effect just desmbed is one with which all adult persons in
civilised countries Are so* &mi]iarly acquainted, in consequence of
the general use of mlrrois or looking-glasses, ihat they cease te
excite wonder or observation ; but brute animals, ignorant savases,
and doubtless very young children, when they b^old for the first
time an image in a mirror, must suppose it to be a real object.

46* The effect of such an exhibition on a game-cock has been
often noticed, and that brutes and duldren may be thus deceived
may easily be admitted ; but it might be 8»plehended that a savage
could hardly be ignorant of the effect of light on the smooth sur*
face of clear water, and that he would therefore view without sur-
prise his own image in a looking^ass. This, however, is not
always tiie case ; and a modem traveller relates an amusing story
of a savage, who, on being shown his face in a pocket glass, be-
came excessivdy alarmed, and could by no means be induced to

At what p6itit does the moon become iDTiaable from imraenioii in the
■m'sniya?

How may an opaque anrface be resdered invaaible ? .

What familiar lUoatration^ the ease ia afi^rded by the uae of mirrora ?

What evidenee have we that only the image formed by the mirror is
really visible ?

What confirmation do tmveUen affordof the eorreetDaasof this sappo*
aition in regard to mSrrorft )

948 OPTIC8.

approach again eitW the ^aaa or its owner, coneeinng that Uie
inaividual who could take & UkenisB^ by means of his mystehoos
machine, might perhaps appropriate his proper person, and keep
him and sell nim for a slave. ^

47. The common term reflection has been adopted to denote the
direct effect of light in foroducing imaoes of bodies in their proper
places, and also the inairect effect of li^t in forming, by means
of polished surfaces, images of bodies m some place or places
different from that where they appear by direct light. The latter
kind of reflection alone, or that caused by variously-formed mirrors, ^
or more or less pe^ectly polished surfaced, constitutes the sobject
of that branch of science called Catoptrics.* Reflecting'sar&ces
may form images, the apparent situation of which will be mora
distant from the observer than the surface or mirror itself, as hap-
pens in the case of common looking-glasses, or convex mirrors;
or the images may be formed between the eye of the observer and
the reflecting surface, and may therefore appear in the air, as
will be perceived in some cases where concave mirrors are used.
Other singular effects may be exhibited by xAeans of cylindrical
or conical mirrorsy or by various arrangements of mirrors, and by
eombinations of them with other optical glasses.

JReJUetion from Plane Surfaces,

48. RavB of light are reflected acc(»dinff to the same laws that
regulate the motions of perfectly elastia solids $ for a ray impinge
ing on a reflecting surface will be returned or reflected m such a
manner that the angle of incidence ehall be exactly equal to the
anffle of reflection.f Hence if a rav of light fails hoiizontiJly on
a plane mirr6r held vertically, it will be reflected in the same right
line; but if it falls obliquely, it will be reflected with the same
deg[ree of obliquity ; that is, the returning line and tiie line of
incidence will form similar angles with a perpendicular drawn
between them.

49. Thi9 may be shown by admitting
^p through a small aperture, into a dark cham-
/ her, a ray of light, and receiving it on a me-
/ tallic mirfor A B ; if it fall obliquely in the
direction C D, it will be reflected in the line
D £, and will form a )ueid spot at £ on a
plane properly placed to meet it. Since all
^^ rays falling on a reflecting sur^e r^atively
preserve, after .reflection, directions cone-
To what two teta of phenomena it the term refleetioD applied ?
What i« meant by eatoptrics f

In how many positions, with reference to their distance from the cy^
may imares be formed by refleeting sorfaeet ?
According to what laws are rays of light reflected!
How may this be experimentally demonstrated ?

• From the Greek KiiT«rrp«», a mirror,
t Sec Treatise ouMeehamci^ No. $L

EFFECT OF PLANS MIRRORS.

a49

sponding jnrith those they had previoasly, therefore a ray falling on
a mirror perpendicularly would be reflected on itself, and cons^
quently could produce no lucid image.

50. The relative position of the ima^e of
an object as seen in a reflecting plane will be
such that every part of the image will appear
as far behind the plane as the object itself is
before it. Let A jB represent a plane mirror,
and £ F any object, as an arrow ; then draw,
from the pomts E and F, the perpendiculars
E 6 and r H to the surface of the mirror,
and produce those lines to e and /, so that
E G shall be equal to e 6, and F H to /H,
and ef will be the position of the image
which will be exactly equal to the object, as

the quadrilateral fij^ure G ef K will be equal to the quadrangle
G E F H. From inspection of this figure it will be perceived,
that the rays of liffht proceeding from that pah of the object nearest
to the surface of tiie mirror wiU be reflected so as to form the part
of the image nearest to the plane of the mirror in the opposite di-
rection. Hence when trees or buildings, or any other objects, are
reflected from an horizontal*plane, as Sie surface of a pond or a
smooth stream of water, they will appear inverted ; for their lower
parts being nearest to the reflecting surfiace are deen immediately
within it, while their tops seem to hang downwards, or to extend
deeper beyond the surface.

51. When a mirror, C, in the annexed
:^^ figure, is inclined forward at an angle of 45

b deg. an object A B, if placed in a vertical
posiUon, will form an horizontal image a b;
and if the position of the lobject be horizon-
tal, that of the image will be vertical.

52. A person standing before a plane mir-
ror placed vertically opposite to him, will not

' perceive the image of his whole person, if

the length of the mirror be less than half his
height. But if the upper part of the mirror be inclined forward,
more of the image will become visible, in proportion to the dimen-
sions of the mirror, than when it is placed vertically ; and hence
a person may view himself from head to foot in a looking-glass

Explain the diagram relatin|^ lb' the incident and reflected rayt.

Into what line will a ray falling perpendicular to a reflecting surface b*
reflected ?

Explain the relative poutioo of parts of an image formed by a reflect-
ing plane*

Why do trees, bailding«,dr other objects seen by reflection from a mr-
face of water, appear inverted ?

How may a vertical object be made to produce a horizontal image ?

How long most a vertical plane mirror be, in order that the whole
person may be seen by an eye immediatelv in front of it ?

What expedient enables us to see the whole person in a small mirror ?

350- opncs.

of a moderate size, by giving it a due degree of incliiftition, bat
then the image as well as tne mirror wul appear in an oblique
position.

53. Any one looking into a fixed mirror, and at the same time
stepping backwards or advancing forwards, would perceive his
image Siso to recede or approach, but with double the velocity of
the actual motion. This will be understood by recurring to what
has been stated relative to the angle under which the image is
perceived.

54. A number of images may be formed and peculiar effects
produced by means of two mirrors, either inclined or parallel, and
opposite to each other, for the image of an object which is delinea-
tea behind one mirror may thus serve as an object to be reflected
(rom the surface of another mirror.

65. If any object, as M, N, be
placed between two plane mirrors in-
clined towards each other at an an-
gle A C B, several images will be
perceived, all situated in the circum-
ference of a circle. This may be de-
monstrated by drawing the image in
its place behind each mirror, and con-
sidering each image as forming an
object in its turn, the image of which
is also to be drawn. Thus it will be
perceived that the imag^ of M N in
the mirror A C is m n, while its
image in B C is M' N' ; and in the same manner the ima^ formed
by the reflection of the first image mn'inbC will be M" N", while
the image of M' N' in a C will be m' n'. It will further appear
that to" n" is the image of both M" N" in the mirror 6' C, and of
m' n' in the mirror a' C, one of the images covering .the other, if
the angle A C B be 60 degrees, or the sixth part of a circle, as
in the diagram ; but, if the angle be any greater or less, the image
to" n" wUl be two-fold; that is, the two images will not ex-
actly coincide. On this principle is formed the Kaleidoscope,*
invented by Sir D. Brewster, and by means of which the reflected
images, viewed from a particular point, exhibit symmetrical figures
under an infinite variety of arrangements of beautiful forms and
colours.

56. If the two mirrors are placed opposite and parallel to each
other^ an indefinite multitude of images will be perceived, becoming
more and more indistinct by repeated reflection, till at last they

How may the image be made to advance or retire ?

How may the image of one reflection be made the object of anodier?

Explain this by the diagram.

Wnat optical toy it constructed on the principle of multiplied reflectioDi

What appearance results from reflections between parallel mirrors \

* From the Greek ]u\ef , beautiful, s«{o(, a form, and Xs9ri», to Tiev.

ATMOSPHERIC RVLECTION. 8U.

yanish in obscurity. This effect maj be advantageously obserred
in an apartment where two mirrors are fixed on opposite sides of
it, with a lustre, or some such object between them. One of the
rooms at Fonthill Abbey, built by the eccentric Alderman Beckford,
was wainscoted, as it were, with mirrors of plate-glass ; and
thus it presented to the spectator an interminable yista on every
side, filled with a seemingly infinite multiplicity of objects.

57. Common mirrors are formed of. glass, to the back of which
is attached an amal^m or mixture of tin and quicksilyer, which
adhering to the surtace of the glass forms a smooth polished plane,
capable of reflecting the rays of light which impinse on it more
abundantly than almost any other kind qf mirror. The principal
reflecting surface in this case is that where the metallic covenng
joins the back part of the glass ; and the image there' formed under
ordinary circumstances is so bright and distinct as to prevent any
other from bein^ perceived. If however, a lighted candle be held
before a glass mirror, so that its rays may fall on the glass obliquely,
several images may be perceived ; as a feint one at the outer sur-
face, another mach more intense just behind the former, and seve-
ral others gradually receding, and becoming fainter and fainter, till
they vanish in the distance. The first faint image «is formed by
reflection from the outer surface of the glass, the second, or prin-
cipal image, at the surface of the amalgam, and the others by
reflection within the glass. These interfering secondary images,
though of no importance in a cofbmon mirror, would produce con-
fusion in more delicate optical instruments, such as the reflecting
telescope, the mirrors of which therefore are constructed enturely
of polished metal, which, presenting only one reflecting surrace,
affords a single image.

58. Among the natural phenomena produced by the reflection
of light, by far the most important is that of atmospheric reflec-
tion, for without it ^few objects would be visible excepting those
on which the rays of the sun might fall in a direct line between
^at luminary and t6e eye. But &e rays of light falling on the
particles which compose the atmosphere are thence reflected in
every direction, and thus daylight is produced even when the
whole visible hemisphere is covered with clouds, and th6 face of
the sun is hidden from our view. But for reflection, all opaque
bodies would cast perfectly dark shadows ; and on turning our
backs to the sun the objects before us would be involved in the
deepest obscurity.

59. Some of the less usual phenomena depending on atmos*
pheric reflection are extremely curious, as that called the Mirage,

Where has this experiment been exhibited on a grand scale ?

How are common mirrors constracted ?

What part does the glass act in the formation of images ?

How are the numerous images, visible in an oblique direction to the faiw
face of the mirror, produced ?

Why are not glass mirrors employed for reflecting teleteopes ?

What would be the effect of opaque bodies, if the atmosphere were dea-
(itute of reflecting power i

OPTICS.

and a rariety of Atrial spectra of an analogous kind. Tbe milage li
l^nerally perceived on sandy plains in hot climates, as in Egypt and
in South America ; and it has been often described by trayellers.

60. In the middle of the day, when the sun shines on ^e level
surface of the sand, the appearance of a sheet of water is obser-
ved at the seeming distance of about a quarter of a mile ; the de-
ception being so complete, that any person unacquainted wiUi
its pause would inevitably suppose he was approaching a lake or
river. Like real water, the spectral lake reflects objects around, so
that houses, trees, and animals, are perceived with the utmost dis-
tinctness in this sinffulai mirror. As the observer advances, the
visionary stream recedes, still keeping at the same apparent dis-
tance, but with changes of scene, by the disappearance of images
first beheld, and the formation of new ones^ from other objects,
as they successively become liable to reflection.

61. The French philosophef Monge, who witnessed this phe-
nomenon in Egypt, published a satisfactory explanation of it in
the first volume of the Decade Egypfienne; and about the seme
time a similar exposition of the cause of it was given by Dr.
WoUaston, in {he London Philosophical Transactions. The latter
also produced an artificial mirage m the heated air over a majBs of
red-hot iron ; and he observed the same appearance in bodies seen
across the surfaces of two differentiy refracting fluids placed one
above the other in a transparent ^ssel.

62. He thus accounts for th% phenomenoi^: in the middle of
the day, the sandy soil becoming very hot, the stratum of the
air in contact with it acauires a veiT elevated temperature, and
hence, being dilated, its aenaity ift mnnd inferior to Uiat of the
strata immediately above it, and the luminous rays which fell
on this dilated stratum, at an angle comprised within a cer-
tain limit of 90 degrees, are reflected at its surface as from a mir-
ror; and they convey to. the eye of the observer the reversed
image of the lower parts of the sky, which are then seen on the
prolongation of the rays received, and consequently ^pear below
the real horizon. In this case, if nothin?^ corrects the erroTi
the limits of the horizon will appear lower and nearer than they
really are.

63. If any objects, as villages, trees, or the like, render it evident
to the observer that the limits of the horizon ar^ more remote, and
that the sky is not so low as it seems, the reflected image of the sky
wtU appear to form a reflecting plane of water. The villages and
the trees will emit rays which will be reflected just as rays

How is the mirctge formed ?

Give some account of that appeannce.

How may the effect be imitated ?

What explanation did WoUastpn give of that phenomenon ?

How 18 the imagination led to the supposition that the reverted image
of the lower part of the sky is nearer and lower than the true visible ho-
riEon ?

How does it appear that trees, buildiogs, ke., ought to appear it i uie d
in tbe inverted image of the aky }

AERIAL nreCTRA. 8ft

would have been if coming from the part of the sky intercepted by
them. And these rays will prodace a reversed image below the
objects seen by direct rays. The limit at which the luminous
rays begin to be reflected being constant, and the rays that fbrm
the largest angle with the horizon appearing to come from the
point nearest to the spot where the phenomenon commences, this
point must be at a constant distance from the observer : hence if
ne advances, the border of the lake will seem to recede, as actually
happens.

64. MM. Jurine and Soret, in September,' 1818, observed, on
the lake of Geneva, a phenomenon analogous to the mirage, but
which, instead of being caused by horizontal reflection, was pro-
duced laterally by the heating of vertical strata of air on the sides
of mountains, which border on the lake where the phenomenon
occurred.

65. Those meteoi^logical phenomena, called paraseline* and
parhelion,'!' appear to be produced by reflection. \Vhen the moon
rises after mid-day, and consequently at a time favourable to the
appearance of tl^ inirage, if the light of the sun and the clearness
of the atmosphere ajilow the moon to be seen just as she gets above
the horizon, two images of that satellite ma^ be perceived. The
parhelion is sometimes observed at sea, but it is a much more rare
phenomenon than the preceding, depending, however, on a simi-
lar cause. Among the instances Recorded of the appearance of
these mock suns may be mentioned the occurrence of four solai
images observed at Rome in March, 1629, que being much tinted
with the colours of the rainbow, and.the others faintly coloured*
Parhelia were seen by Cassini, in 1689 ; and they have also been
noticed in England, Scotland, and America.^

66. On similar principles to those which serve to explain the
mirage depend those appearances called looming, or the elevation
of objects seen in the distant horizon above their usual level ; such
as the Fata Morgana, observed in the Straits of Messina ; and the
singular apparitions of ships and other objects in the air, some-
times in a oirect position, but more frequently inverted.

67. The following figures are given from drawings of aerial
spectra, observed at Dover, England, in May, 1833. When the
real ship is visible, a double image may be formed, consisting of
an inverted figure immediately over the ship itself, and another

How is the eonstant retreat of the iquigiiiary lake or river to be ex-
plained.

May any other than horizontal strata of heated air prodace the effect of
mirage ?

What other phenomenon witnessed at sea is to be explained on the

principles of atmospheric reflection ?

» — I III ^

* From the Greek preposition n«pM, with, and £f xqvti, the moon.

t From n>^s, with, and hxio;, the sun : i. e., appearing together with*
or accompanying the sun.

% For an account of the frequent appearanoe of parhelia, see Party's
looount of his stay at Meltille Island. — ^Ed»

8 69

S54 OPTICS.

Cffon in «n «Teet pocition, aboTB the preceding. If then it i
•mele figure only, it will usoftlly be inTerted with respect toAe
reu ship below it. S<HDetimeft a doable image, or an erect figon
with one below it inverted, will appear when the Tssael thui re-
(tecled is wholly invisible, or perhaps its topmasts be seen, ivtii]«
the ramaintng parts are liidden bj the convexitj of the euth'i

6S. TTie manner in which these and similar phenomena maj
bn caased by reflection may be comprehended by reference to tM
uialogouB effect of spherical mirrors, subsequently noticed. But
it is probable that, where double images of objects appear, th«
effect depends chiefly on the refraction of light, owing to the ra-
rying density of the atmosphere; and the circamertances mider
wbi^ such a slate of the air may be produced have been poinltd
oat and illustrated by Dr. Wollaston.* The refraction betng
grealast where the change of denstty is the most rapid, and leM
on each side of this point, the whole effect must be similar to that

69. In reference to the Fata Morgana, Dr. T. Young says, "II
may fm<]aently happen in a mediiim gradually rarying, dial a
nomberofdifierent rays of light may be inflected into an^es equal
to the angles of iucidence, and in this respect the effect resembles
reflection rather mote than refraction^"j'

Ikjketian fn?m Comex Sarfaea.

70. The effect of light rejected from a conveK mirror is to pro-
""' ituie picture of any objects placed opposite toil; Uio

formed appealing, to die eye of the otwerrer in frcnt,

d appesimg,

;d bj chinge of

,l»inedoiilhimL„__

What i* the efr«t of rcBvclion froiB > convex mfiror ?

Hov irill the nnp«*rsnceof apeitre >hipa be tfTected liT the ni
remoteneii of the rail «ips •hir'- ■■ '

How ii nrniction ■tTected bj

Whit cirranuUnee nf the si
to be refleeled (o i focus .'

Whst tppesmnce it explained on ihii mppotit

• See ■ ptper "On Double Iniagei eauaed by Atmoipherie BetiM-
don," in nMleaophieal Tmnaaeliontfar 1S00.
t Lecturet OD Nitontl Ptulofopl^, nL il. 303.

EFFECT OF CONVEX MIRRORS. 355

lo be fiitaated within or behind the mirror. Thus the globular
Ibottles filled with coloured liquids in a chemist's shop-window
present in pleasing variety the moving scenery of the street with-
out; the upper hemisphere of each bottle exhibitinfir all the images
inverted, while the lowear displays a duplicate of them in an erect
position. Hollow spheres ot ^lass, covered on their interior su-
perfices with an amalgpm similar to that used for silvering look*
ing-glasses, are sometimes suspended in apartments, where they
present panoramic pictures of surrounding objects ; and convex
mirrors are common articles of ornamental furniture exhibiting
analogous phenomena.

71. The images formed by reflection from a convex mirror must
jalways be smaller than the objects by which they are produced,
because the rays which form them become convergent in their
passage to the eye of the observer. In the annexed figure let -A B
represent a convex mirror, the segment of a sphere, whose radios

is the line 6 C ; and there-
fore the point O will be the
centre of^the sphere, and the
focus of the mirror.

72. Ifan object be placed
at E, atn great distance be*
fore the mirror, its i^iage
will appear behind the mir«
ror at a point near D, which
will become the virtual fo-
cus, and will be situate^d
at half the length of the radius of the sphere, or at the middle
point between Sie imaginaiy focus and the surface of the mirror ;
and the magnitude of the image will be to that of the object in
the ratio of the line C D to C E ; that is, it will be as much
smaller than the object as the line C D is shorter than the line
CE.

73. If, therefore, the object be brought nearer to the surface of
the mirror, the image aldo will approach to meet it, and become
proportiopally enlarged; so that it a part of au]^ object be brought
into contact with the convex surface of tliie mirror, the imaee of
that part wiU appear of precisely the same size as in the -^ject
itself: but unless the object be extremely small, or the mirror be
a segment of a very large sphere, it must be obvious that only a
smsdl portion of an object, can be made to touch the mirrez, and
hence the entire image must ever be to some extent ii^erior in size
to llie object by whioh it is predueed. Not only will the lays

How may bottles of liquid and polished spheres produce double images?

What relation has the size of objeots to that of their images in convex
mirrors ? Explain the diagram.

How far from the centre of curvature wHl an image formed by parallel
rays of light falting on a spherical convex m^irror appear to be situated ?

What will be the effect of bringing the object nearer^ so that rays may
hJldherffeatf

856 OPTICS.

falling directly on the mirror, as E C, be reflected so as to form
an ims^e at D, but so likewise will any incident ray whaterer,
as B M, which will be reflected in the direction M N, so that the
angle B M N will be equal to the angle C M E ;* and when the
eye is at N, receiving the reflected ray M N, it will see the object
E, according to that direction, and tike image will appear in the
mirror at D.

74. A convex mirror by reflection converts parallel rays into
divergent rays, those that Ml on it in diverging lines are rendered
still more divergent when reflected, and coAvergent rays are re-
flected either parallel or less convergent. Suppose then an object

of some assignable mag-
nitude A B, as repre-
sented in the margrin, to
be placed before a con-
vex mirror M N, the
rays of light proceeding
from each part of it wifl
^ir be reflected as if from a

single point, and an ima^e will be formed as before, in the line
drawn from 6ach extremity of the obj^l to the imaginary focus
of the mirror F ; and in the same manner from other points, so as
to form a complete image of the object. And this image must
necessarily appear less than the object itself; for the rays which
proceed from the extremity A will be reflected to the eye as if
they proceeded from the point a, and those reflected from B as if
from the point 5, while the rays from the parts between A and B
will be reflected from intermediate points ; and therefore the image
must appear smaller than the object, by which it is produced.

75. An object reflected from a convex mirror will not only form
an image diminished in proportion but also defective in the outline;
for the virtual focus of reflection will vary for different parts of
the same figure ; therefore unless the object be relatively very
small, or the curvature of the mirror very considerable, the central
portion alone of the object will yield a correct image. Such at
least will be the effect ardess the curvature of the mirror be accu-
rately formed, and expressly adapted to the purpose. The human
eyeball constitutes an admirable convex mirror, reflecting minia-

How majr we conceive the incident rays on a convex mirror to be a£>
fected by an imaginary tang;ent plane at (lie point of incidence ?

How will the rays reflected foy a convex mirror be foand in the three
eases where they are respectively partUlel, dvoergerU and confoergeni be-
fore incidence?

Ilxplain this by a diagram.

♦ Every ray falling obliquely on the surface of a convex mirror may
be regarded as impinging on a point which forms part of a plane tangen-
tial to the curved surface of the mirror; and a line drawn perpendicular
to such tangential plane will bisect the angle formed by Uie incident and
Hie reflocted ray, as is shown by the doited lines in the preceding di»* ^
gram. ... -

SINGULAR EFFECT OF A CHINESE HIHROR. 857

iure images, the^delicacv and beaaty of ^wliich have repeatedly
furnished topics of poetical allusion and metaphor. Here we
perceive a striking instance of the vast superionty of the works
of nature over those of art.

76. A convex reflecting surface of variable curvature may afibrd
many ludicrous caricatures of the human figure, or o( that of any
other animal, especially if the object be brought very near the
mirror. That part of tifie surface which is most protuberant wUl
exhibit a comparatively diminished image, and the effect will be
heightened by alternately advancing and wiUidrawing different
parts of the person, and thus the disproportion between the head
and the body or lower limbs may be rendered more remarkable.
For if the head and trunk be thrown backward, while standiner
near the mirror, the image will display a . diminutive head and
body supported by preposterously swelled and gouty legs ; and
on the contrary, if standing more backward, the body be bent with
the head stretched out towards the mirror, it will* present a mon-
strous bloated figure with a dropsicftl head and body perched op
spindle shanks.

77. Sir David Brewster has published, in the Philosophical
Magazine, an account of a curious convex metallic mirror, recently
brought from China to Calcut^, the general appearance and effect
of which is thus described : '* This mirror has a circular form,
and is about 5 inches in diameter. It has a knob in (he centre of
the back, by which it can be held, and on the rest of the back
are stamped, in relief, certain circles with a kind of Grecian bor-
der. Its polished face has that degree of convexity which gives
an image of the &ce ha)f its natural size ; and its remarkable pro-
per^ is, diat when you reJUd the ram of the sun from the poUshed
surface J the image of the omameniai border andeireleSf stamped upon
the ftocX;, is seen distinctly reflected upon the waU,^^ Mr. Swinton,
the gentleman who transmitted from the East Indies the preceding
statement of this apparent reflection of figures through an opaque
substance, proposed a conjectural explanation of the strange phe-
nomenon, as depending oh the difference of density in different paits
of the mejLal, occasioned by the stamping of the figures on the
Iback, the light being reflected ihore or less strongly from parts
that have been more or less compressed.

78. But Sir D: Brewster, judging from the description, which
alone had been transmitted to him, infers that ** the spectrum in
the luminous area is not an image of the figures on the back ; but
that the figures are a' copy of the picture which the artist has
drawn on the surface of the mirror, and so concealed by polishin?,
that it is invisible in ordinary lights, and can be brought out on^
in the sun's rays.'' He had observed radiated lines and concen-

Wkiat defect hai an image reflected by a convex mirror }
What natural convex mirror surpasses those produced by art f
What effect is obtained hj convex surfaces of variable curvature f
How may the image of the person be caricatured by such an aj^paratus^
What des c ri p tio n and explanation are given of the Chinese mirror with
figured back ?

S58 OPTICS.

trie ciftles to be similarly reflected by the light of the snn from po-
lished steel buttons, which having been finished in a taming-laue,
the lines and rings had been formed on their surfaces by the ac-
tion of the polishing powder or some similar cause, but too faintly
to be visible except m the strongest light. Thus the figures on the
back of the Chinese mirror were doubtless placed there merely to
mislead the observer into a belief that he beheld them reflected
through the metal, while he actually viewed the reflection of a
duplicate of those figures lightly traced and concealed by the
polish on the front surface.

BeJUeiion from*Coneave Surfaces,

79. Concave mirrors exhibit a variety of phenomena dependiog
on the situation of the object with respect to the mirror and to the
observer, some of which are highly curious and interesting. ** The
concave mirror," says Sir David Brewster, '' is the staple instni-
ment of the magician's cabinet, and must always perform a prin-
%ipal part in all optical combinations."* Some of the most ex-
traordmary optical effects in Mature are also produced by reflection
from concave surfaces, the properties of which therefore demand
investigation.

80. The manner in which li^t is reflected from concave minron
may be thus explained : let A C B, in the marginal figure, repre-
sent a mirror forming a

^M part of a sphere whose

yJ L s centre is G, and G C, a

radius; and suppose £

Q I ^v^ fl- to be an obj ect far distant

"^ ' from the mirror, then its

image will appear in
^V front of the mirror at D,

the central point of the
radial line C G. For any ray of light whatever, as £ M, from
t^e object E, falling on the surface of the mirror at the point M,
will be reflected thence in such a manner as to pass through the
point D ; and when the eye is placed at N, the object will be seen
at or near D ; but this Image willbe to the object in the ratio of
C D to M E, and consequently less than the object. If the ob-
ject be made to approach nearer to the mirror, the image will re-
cede from D towards G ; and jf it be placed there, the object and
image will coiiipide ; and the object still advancing from 6, the

In what manner does Brewster suppose the effect to have been pro-
duced in that instmment ?

How had Swinton pi*evioasly explained it }

On what does the variety of appearances exhibited by the eoneate mir-
ror depend }

Draw and explain a diagram by showing the manner in whieh light ii
reflected by a concave mirror.

* Letters on Natural Magic, p. 61.

EFFECTS OF CONCAVE MIRRORS. 359

image will retreat beyond it, till the object arrives at D, when the
image will appear infinitely beyond E, But if the object be placed
yet further forward, between D and C, the image will fall behind
the mirror, and it will look larger than the object.

81. Thus it appears that when parallel rays fall on the surface
of a concave mirror forming a portion of a sphere, they will be
reflected and meet in a point at half the distance between the sur-

^face and the centre of concavity of the mirror. If the rays fall
convergent on a concave mirror, they will be brought to a focus
sooner than parallel rays ; and the focus will be nearer to the sur-
face of the mirror than to the centre of concavity. When the
rays fall in divergent lines, the focus to which they will be re
fleeted will be more distant than that formed by parallel rays.

82. There are three cases to be considered with regard to the
effects of concave mirrors : - '

1. When the object is placed between the minor and the prin-
cipal focus.

2. When it is situated between its centre of concavity and that
focus.

3. When it is more remote than the centre of concavity.

83. 1. In the first case, the rays of light diverging after reflec-
tion, but in a less degree than before such reflection took place,
the image will be larger than the object, and appear at a ^eater
or smaller distance from the surface of the mirror, and behind it.
The image in this case will be erect.

84. 2. When the object is between the principal focus and the
centre of the mirror, the apparent image will be behind the object,
appearing very distant when the object is at or just beyond the
focus, and advancing towards it as it recedes towards the centre
of concavitv, where, as already stated, the image and the object
will coincide. During this retreat of the object, the ima^e will
[|till be icrect, because the rays belonging to each visible pomt will
not intersect before they reach the eye. But in this case, the
image becomes less and less distinct, at the same time that the
visual angle is increasing ; so that at the centre, or rather a little
before, the image becomes confused and imperfect ; owing to the
small parts of Uie object subtending angles too large for distinct

What results in this case from the gradaal approach of the objects to-
wards the surface of the miiTor f .

What relation exists between tite focal distance of parallel rays ftom a
concave mirror and that of its centre of concavity ?

Will convergent rays meet nearer to or further from the cooeave mirror
than imrallel ones ?

How will the comparative distances of the fooi of parallel add diver-
gent rays be found ?

State the three cases of parallel and divergent rays.

What will be the relative size and distance of the object and the
image in tlie first case ?

Will the image be erect or inverted ?
. What will be the distance, positive and relative size of the image, in
die second ase ?

360 OPTICS.

vision, just as happens when objects are vieVred too near with the
naked eye.

85. 3. In the cases just considered, the images will appear
erect ; but in the case where the object is further from the mirror
than its centre of concavity, the image will be inyerted ; and the
more distant the object is from the centre, the less will be its
image, and the further from the said centre, or the nearer the focus,
and the converse ; the image and object coinciding when the latter
is stationed exactly at the centre, as noticed in the preceding case.

86. If an observer view his ownvimage at a considerable dis-
tance beyond the centre of a concave mirror, the iamge will ap-
pear small, faint, and somewhat confused. This is owing to the
smallness of the number bf rays that can enter the eye ; henc^
the apparent distance is augmented or rendered uncertain, so that
the image is conceived to he beyond or within the mirror, and this
misconception increases the conAision. As the observer advsmces
towards the mirror, his image will OTadually appear larger and
brighter, and likewise draw nearer tp him ; but if he do not view it
between himself and the mirror, it will continue still indistinct. At
length he will arrive at the station whence the ima^ assumes a de-
terminate and correct figure, at)pearing perfectly distinct. After a
few trials, the true place for viewing the image may be ascertained
with tolerable accuracy ; and it will continue distinctly perceptible
when the observer moves a short distance backwards or forwards
from the proper position : but advanctug beyond it, the image will
soon begin to appear indistinct, and tlus indistinctness will in-
crease till he arrives so near the mirror as its centre of concavity,
where the ima^ will be lost in confusion. If he still advances,
another image in an upright position gradually becomes visible,
as explained in the preceding case.

87. The most singular and curious effects of concave mirrors
are those resulting from the position .of objects at a greater disr
tance from the mirror ths^n its centre of concavity, as in the third
case above described, when a diminished and inverted image will
be formed in the air between the object and the mirror. In order
that this may be seen to the utmost advantage, particular situa-
tions must be assigned both to the object and. the observer, which
will be regulated by the concavity of the mirror and its consequent
focal distance. For the exhibition of such phenomena, however,
spherical concave mirrors are not so well adapted as those of an
ellipticaf figure, for the latter having double foci, any object placed
m one focus of an elliptic concaive mirror will form an accurate
image in the other focus.

How will these three things be found related to each other id the third
case ?

Why 18 the image of a distant observer seen indistinctly in a concave
mirror ?

Which case of reflection by concave mirrors produces the most inter-
esting phenomena ^

- What Cbrm of concavity ought the mirrors to possess for the exhihiUoD
of these phenomena ?

OPTICAL DSCfiPTICNS.

361

88. The marginal
figure exhibits a con-
yenient mode of ar-
Tan?ement for pro-
ducing optical images
in the air by means
of a single mirror.
Suppose C D to be
one side of a room, or
a screen dividing one
part of the room from
another, and having
in it a square aperture £ F, the centre of which may be about five
feet above the floor. This opening may be surrounded with a
black border, or a gilt moulding, so as to resemble a picture-frame.
A large concave (elliptical) mirror, M N, is then to be placed in
en adjoining apartment, so that when any object is placed at A,
in one focus of the mirror, a distinct image of it may be formed in
tiie other focus at B, or in the centre of the p^erture E F. This
image will be inverted with respect to thd ;>osition of the object ;
therefore if a small statue, bust, or plaster cast of any object be

E laced upside down at A, an observer in the apartment at will
ehold an erect image of the object at B. In order to grive the
greater effect to this exhibition, the object should be white, or at
least of a very bright colour, and should be strongly illuminated
by a powerful lamp, the rays of which must be prevented from
reaching the opening E F.

89. In this case, the image being formed, not in the single fo-
cus of a spherical concave mirror, but in one of the foci of an ellip-
tical mirror, it will not be confused or reduced; but will be rather
.arger than the object. When the image appears in the air, as
here described, it will be distinctly visible only from the point
O, and a person placed at a little distance, on either side, will see
nothing of it. - It, however, the opening E F be filled with smoke,
rising from burning frankincense or other perfumes, the cloudy
vapour will serve as a siureen to receive the reflected image, which
may thus be rendered generally visible to persons within the
room 0.

90. Among the natural phenomena which appear to be caused
by reflection from concave surfaces may be mentioned whaf^is
called in Germany the ''Spectre of the Brocken," a gigantic figure
sometimes seen at a distance upon the highest peak of the Harz
Mountains, in the kingdom of Hanover. It has been ascertained,
from careful observation, that the figure is a reflected spectrum oi

Describe the arrangement ofapparmtus for exhibiting atrial imagei.

Will the images in this case be direct or inverted >

What will be the size and position of the image, with regard to thoie
of the object f

How are refleetiont from eoncave rarfaoet applied to explain th« ipM*
tre of the Broeken '

9H

363 OFTICfl.

the obserrerf suck as might be produced w certain sitnations by
i)Bean8 of a concaye mirror. A singalar instaince of atmospheric
ifeflection, as observed io Sictly, from Moaat Etna, has been no-
ticed by a modem traveller. He says, *^ At the extremity of the
vast shadow which Etna projects across the island, appeared a
perfect and distinct image of the mountain itself, elevated sAotc
the horizon, and diminished, as if viewed itt a concave mirror."*
91. Vajrious forms may be g^iven to mirrors besides those already
described, and thus various modifications of the reflected images
inay be produced. Cylindrical, conical, pyramidical and prismatic
mirrors are sometimes constructed, but ^y merely serve the pur-
pose of creating amusement^ by the singrularity of the efleeis
#hich may be exhibited by meana of muii inslrttmeiito. A eon-
mon method of displaying these optical phenoBMlnaconsista in tlis
Rectification of distorted figures (drawn ibr the purpose,) by re-
flection from certain mirrors. These exhibiCioBS aie Iwmed Ana-
morphoses ;f and the rules for delineatiBg defemed fig^uies to snk
the different kinds of mirrors, with direoti<AS for tbe»r pioper af-
I'angement, may be found in several works relating to opmal wh
strumentsand phenomena.^

MOPTRICS.

93. Rats of light in passing to any distance througlk a medimn
of uniform density will proceeo in ri^t lines ; but if a ray or pen-
cil of rays be made to pass from one transparent medium to ano-
ther, as irom air into wafer or glass, its dii'ection will be changed
at the surface of the new medium, and it will afterwards proceed
in a line varjring more or (ess from that in which it passed through
the air. Hence a ray is said to be refracted ei bent, iu conse-

2uence of its transit firom one medicon to another ; the effect pro-
uced is termed refriaction of light; and the laws by which the

To what uw have «ylih(MeBl lairron been cftiefly Applied ?

By what oame are the changes of figure produced by curved mirrors
deaigiiatedF f

iSYhat difference exists between the course of a ray of light while tra*
versing a unifirm, and that which occurs while pasting througft mvmriabk
medium ?

What is meant by refraction Of right?

To viktkt dtvi^on of optics doM this effect give risie ?

■ ' ■'•

* Travels in Sicily, Greece, and Albania. By the Rev. T. 8. Hughes^
1830. ^

t From tfce Greek preposition Ava, and Mo^ ^«r*«, an appearaiMe : i e,
a reversed exhibition.

t Y. Sohocd Mii|;iti UoiVersttliS, 0. i. Xih, 1 ; P: ]|>uWuil I^erapcatiss
Pratique, t iii. Trait. 5„ 6, 7 ; Wiegleb*s Natural Ma£M»(GermaD\&aMi
Hmtsa^i-IfteeKMIieAfrm Witt Phil. toLiti. '

IffiFRACnOH or LIGHT. Sft

phenoikieiia «n mgalated constitiite iSie science, oi bianch of sci-
enee, called Dioptrics.*

93. Tke «ffeet juat described may be easily subjected lo obser-
yatioii, by laying a piece of money near the centre of the bottom
of a china bovl, or basin, placed on a table or on the door, and
then retxeatingr backward till the money is no lonjsfer visible, being
hidden from the eye by the aide of Ae bowl : if then water be
poared into the vessel, the piece of money will become visible, just
as if the bottom of the basm was raised abore its real level. As
this experiment may be readily repeated, and affords a eonytncing
proof of the posilton aboye stated, it may be proper to observe that
the money, or any other flat object which wilt equally weD answer
the purpose, should be fastened to the bottom of the basin wilh
sealinffwax, liiat it may not be moved from its place when the
water Is poured on it, and that the vessel must be filled to a cer-
tain beijght before the object can be seen.

94. Tiie refiraotion of Ught may be exhibited moss simply by
plunging a straight cane or long ruler obliquely into a pond or
a bnofcet of water, whea it will appear bent at the surfice of the
watery that pait of tke eaoe held by tbe hand in the air appeariaf
to be joined At 9n obtuse angle to the part under water.

9B, There Is, however* one ease ia which rays of light, in their
paasage from one mcdinm to another of diffi&rent density, will pio-
oeed withoui ebanffinr their direction; and that is when their di«
rection is perpendteiuar to the connecting surfaces of the two
mediamsp Thus, if the eve be placed vertically abov« a piece of
money in a basin, it will be seen in the same vertical line whetbei
t^e basin ba empty or filled with water; and for the same reason
a atnsight stick held perpendicularly in water will not assume the
bent fiffUBS whieh may be remarked when it is held obliquely.

96. If, from the p^unt where a ray of li^t passes from one me*
4ium through tha suriace of another medium, we conceive a line
to be drawn perpendicular to that surface, and prolonged indefi-
nitaly beymaa it, the ray afVer refiraction will either approach the
perpendicular more than before refraction, or recede further from
It t£an before. If the medium which the ray enters be more dense
than that which it quits, it will approach the perpendicular; but
if the second medium be rarer than the first, the contrary effect
wiU take place, and the ray will roeede from the perpendicular*

What simple experiment illaBtrates the effect of refraQtion ?

What precaution is required to insure its success f
• Why does a stick appear bent when plunged obliquely into die water ?

Under what condition does change of density in the roedium produce
no change of direction f

What course will the incident and the refracted rays remeetWely take
with reference to a perpendicular to the refracting surface r

Distinguish the case where the ray enters a rarer from that io which it
enters a denser medium.

• From the Greek A<e)rre/(»i, to see through { or Ai«irTfi»,a mathemati*
eal instrameDt for mcasiirkig Iwights.

' »

\ p

\^ K ^

J r

8M OPTICS.

97. These effects may be illnstmted
by means of the mar^nal fiffare. Sup*
pose O- to be the point at which the lu-
minous ray passes from one medium to
another, and that the two are separated
|p by the line B D, representing any sur^
face either plane, concave, or conve:t;
suppose the mediam aboTe B D to be
more rare than that below it, and let
H O represent the incident ray, and O C
the refracted ray, and draw throug^h the
point O, I F perpendicular to the plane B D ; then if the ray H O
nad preserved its direction after passing the plane, the angles
HOI and F O C must have been equal ; but the latter is more
acute than the former, because the line of refraction O C ap-

? roaches more to the perpendicular I F than the line of incidence
\ 0. On the contrary, if the medium below B D had been rarer
than that above it, the ray would have been less refracted than
before, and would consequently have diverged further firom the
perpendicular I F than* the ray H O does, and would therefore
have formed an angle F O A more obtuse than HOI. From the
point as a centre describe the circle I D F B, cutting the direc-
tions of the incident and the refracted ray in the points H and C ;
from those points draw the lines H N and C R perpendicular to
I F. w^iob linM will be the sines of the angles H Q I and POC.

98. It has been ascertained from numerous observations that
these lines are always in the same ratio, whatever be the angle of
incidence at which the ray falls, provided the mediums through
which it passes remain the same; for though there is no fixed re«
lation between the angle of incidence and the angle of refraction,
there is always a certain proportion between the sines of those
angles. H N is called the sine of the angle of incidence, and
C R the sine of the angle of refraction.

99. When a ray passes from air into glass, the sine of its angle
of incidence will be to that of the angle of refraction, in the ratio
of 3 to 2 ; if it passes from air into water, the ratio of the sines
will be as 4 to 3 ; but these ratios will be inverted when light
passes from ^lass or water into air ; for in the former case the
ratio of the smes will be as 2 to 3, and in the latter as 3 to 4.
These ratios, as just noticed, are constant, whatever be the angle
of incidence, for the respective mediums. But they differ con-
siderably for different substances ; and the refractive powers of ^

Draw nnd explain the diagram relating to refraetion.

To what trigoiioiDetrical lines are the refractive powers of bodies com-
pflntble ?

What line on yoar diagram is the wne of the angle of incidence }

Which is the sine of the angle of refraction }

What relation will exist between these two sines when light passes from
air into glass ? from air into water ?

What will be their ratio when U»ht oassct from glass and from water
respectively into air }

ATMOSPHERte ftfttRACTION. Mtf

ddtmiAerftble tiHAiW of hbiien lia^ heMk ttMA&kiAd by eip^iri-
meAt.*

100. No g^tteml principk hn» h^et disodr^ed wKi<$h eonneeti
the feflraetiyo pow«T of bodies Hfiik their other phyeioa] propeitteB;
though it is QSttdly highest ifi the densOst transparent substances,
and m ^itch as are of Sn inflsmtnable aafnreb Sir Isaac Newton
haYiii^ obsenrod th^t seteral inflammable bodies possessed high
refhtcftmig powers, and noticing a SimiMr property m the diamond,
ingonloasly oonjeetnred thflLt getn to b^ ^ inflamthablo sabstanee,
long before its composition was known; and analysis has Verified
his idea, and shown it to Consist of ciystallized carbon.

lOI* As Ihe eriffidCt of any transparent modinm, in the refraction
of light, generally increases with increase of density, so air and
ftpimti Wh^n dense display greater power of refraction than when
domparatiTely rare ; ana heiice some cnrious and important pho^
nomena depend on atmospheric refraction.

103. Ligrht, on entering the atmosphere of the earth, encounters
a medium less rare than me more ethereal space beyond it, and atf
the loWSr portion of the atmosphere is relatiyely the densest, ray*
passing Ihroflgh the air from objects fiir above us must be con-i
siderably refrdOted. From this cause the sun and other celestid
bodies are never seen in their true situations^ unless they happen
to be vertical ; and the nearer they are to the horizon, the greater
will be the influence of refraction in altering the apparent plae«
of any of ^ose luminaries.

What rielationi have dn refraeting powers to the other physical propei^
ties of bodies }

What effect on refraeting power has the increase of density ?

What attnos^heric phenomena depend on this refractive inflaenee ?

l^xplain by diagram the effect of refraction on the apparent place of the
heatenly bodies.

* The quotient found by dividing the sine of incidence b^ the sine of
refraction is called, by optical writers, the index of reaction i and, as
stated in the text, different bodies having different refractive powers will
present different indices. The following are a few of the substances of
which these indices have been experimentally determined :

Piamond • . 8.439
Melted sulphur . 2.148
Glass, 8 lead, 1 flint, l.SdO
OniofCattia . 1.641
Quarts . IMS

Amber

1.547

Water

Oil of Turpehtlne

1.475

Tee

Olive Oil . .

1.470

Ether

AlQih

1.457^

Air

Alcohol

1.S/S

9h9

1.336
1.309
1,057
1.000^
[Btf.

969 OPTICS. .

. 103. Thas a spectator at A, in the annexed figare, would see
the sun rise at C, when its real situation was at S ; and so its ap-
parent place woald be relatively altered till it arrived at the zoDith
vertically above the point A ; bat it can be so situated only with
respect to observers under the equator, or at least in the torrid zone.
In consequence of this atmospheric refraction the sun sheds his
ligrht on us earlier in the morning and later in the evening than we
should uiherwise perceive it. And when the sun is actually be-
low the horizon, those rays which would else be dissipated throuffh
space are refracted by the atmosphere to^inurds the surface of the
earth, causing twilight. The greater the density of the air, the
higher is its refractive power, and consequently the longer the da-
ration of twiliffht.

104. In cold climates, as near the poles, where the year is
naturally divided into seasons of light and darkness, each lasting
six months, the twilisht of the circumpolar atmosphere diminishes
the winter-night of uiose gloomy regions by a period equal to
several days. Hence also terrestrial objects, viewed at a great
distance, are afiected by atmospheric refraction ; and they there-
fore appear more elevated and nearer to the obser^r than they
would if seen through a medium of uniform density.

105. Those optical phenomena depending on refraction, with
which we are most familiarly acquainted, are such as are produced
by the passage of rays of light from any medium, as air or water,
into another more or less dense, and their entering again the Hot'
mer medium after they have traversed the more or less refracting
medium. Thus objects seen through a common reading-glass or
a pair of spectacles, if observed at certain distances, wUl be in
some degree magnified ; and glasses used by short-sighted per-
sons have the e^ect of reducing the size of objects seen through
them. And when any transparent substance is held between the
eye and any object, the rays which render that object visible will
be refracted in their passage from the air through the transparent
substance, into the air agam, before they reach the eye ; and the
effect produced will depend on the refractive power of that sub-
stance, and the figure of its surfaces.

106. The most simple case of this na-
ture is when the denser or more refract-
ing substance is terminated by plane sur-
faces parallel to each other. Suppose
A B to be a section of a plate of polished

flass, terminated by parallel surfaces
t, on which falls obliquely the ray
D C at the point C, it will be refract-
ed on entering the glass, and its direc-

How is the duration of twilig^ht affected by the density of the air ?
What benefit do the polar regions derive from the refractive power of
air ?

. What eflfect on the apparent position of objectSy on the sur&ee of the
mrthp it produced by reinfition/

EFTBCT8 OF TRANSPARENT PLATE8. 8d7

tion will be changed so as to approach nearer to a perfen-
dicular to the plane of the ^lass, passing through it in the line C e ;
but on emerging at e it will be again refracted in the contrary
direction, and will proceed in the line e dj parallel to D C d'»
llius rays being restored to their former direction after being re«
fracted through plates of glass, or other transparent bodies with
parallel surfaces, the effect is not perceptible ; and hence the forms
and situations of objects are not affected by viewing them through
the panes of a glass window.

107. When 3ie plane surfaces of a transparent substance are
not paurallel to each other, different effects will be produced. Let

X represent a section of a medium denser than that
surrounding it, and terminated by inclined planes,
across which pass rays of light, from the point O.
Then the ray O b will be refracted in the direction
b 6^, and after emerging, it will pass in the line b'o;
another ray O a, from the same luminous point O,
will in the same manner be refracted from a to a',
and meet the former ray in the point o. If an eye
be supposed to be placed at o, the luminous point O
will be doubled ; one image being formed by rays
O passing through the surface 6, and another by those

passing through the surface a,

108. If, instead of a and b only, there were three, four, or any
greater number of plane surfaces, the eye at o would perceive a
light or other object at O, multiplied as many times as the num-
ber 6f facets into which the sides a b were thus divided. Hence
also when glass is furrowed into a multiplicity of minute surfaces
by grinding, the rays of light in passing through it are icefiracted
as uom innumerable small facets, and therefore objects are not per-
ceived at all through ,it ; for, if the images of them were formed
in proper directions, they would be too diminutive to be visible.
Such glass, forming a transparent screen, is sometimes used in
the windows of offices and counting-houses, as the light passing
through them is more genersilly diffused, and the shadows are very
faint ; and for these reasons, circular screens of ground glass are
adapted to lamps, hence called sinumbral* lamps.

109. Glass and transparent crystals, but chiefly the former, are
the substances generally employed in the construction of optical
instruments for exhibiting the phenomena depending on the refrac-
tion of light ; and having noticed the effects produced by transpa^

State some of the familiar optical phenomena depending on refraction
How are rays of light affected on entering obliquely andpassingthroogb
a plate of glass with parallel surfaces?
Explain ihis by a diagram.

How will the effect be varied where the surfaces are not parallel ?
What effect would result from multiplying the surfaces of incidence ?
Why does a fuiTOwed or ground surface not give distinct images?
Of what utility is the indistinctness produced by roughened glass?

* From the Latin mM| without* and Un^niya ihadovw

$68 OPTICS.

rent iSodies with plane surfaces, we shall now proceed to investi-
gate the properties of glasses with carved surfaces. There are
numerous varieties of such glasses, usually termed optical lenses ;
hut they mav all he arranged in two classes : (1.) convex lenses,
or those which are thicker in the Centre than towards their horders ;
(2.) concave lenses, or glasses thinnest in the centre.

110. Among convex lenses are the douhle convex, A, to which
the appellation, lens, was originally applied, from its resemblance
to a lentil-seed {lens, in Latm), being bounded by two convex
spherical sur&ces, whose centres are on opposite sides of the lens;
the plano-convex, B, having one side bounded by a plane surface,
and the other by a convex surface ; and the meniscus, or concavo-
convex, C, bounded on one side by a concave, and oit ih6 other
by a convex surface ; the former being a portion of a larger circle
than the latter, and ^erefore the surfaces meet, when produced.

111. There are also three principal varieties of concave glasses;
as the double concave, D, bounded by two concave surfaces, form-
ing portions of spheres whose centres are on opposite sides of the
lens; the plano-concave, E, bounded on one side by a plane, and
on the other, by a concave surface ; and the convexo-concave, F,
bounded by a convex surface on one side, and by a concave one
on the other, but these surfaces when produced do not meet.

112. The varieties of both classes oi lenses admit of numerous
modifications depending on the relative curvature of their several
sur&ces. The radius of a lens will be the radius of the sphere
of which its surfaces form a part, if both surfaces have the
same curvature ; but otherwise each side will have a different ra-
dius. In all the various kinds of lenses there must be a point
where the opposite surfaces are parallel ; this point is termed the

^optical centre of the lens, and a line passing through it perpendi-
cularly to the surface will be its axis. On this line will be situ-
ated &e geometrical centres of the two surfaces of the lens, or
rather of the spheres of which they form portions. A lens is said
to be truly or exactly centred when its (^tical centre is situated

Into how many eUsKSinay lenses be divided ?
How are they distingaished ?

What names are given to the different varieties of eonve^c lenses f
What to the three forms of ooncave lenses ?

According to what circumstances id tiiieir eonstmction do these fonsi
vary in diiKsrant glasses ?
What is meaiit by the optical centre of a lena f
What Une forms the axt« of a lens }
When i»« ISBveeiiaidered c«Mtiy oeotied }

XrrBCTS QF CONTXX LENSES. 369

at a point on the axis equally distant from corresponding parts of
the sarface in every direction $ as then objects seen through the
lens will not appear altered in position when it is turned round
perpendicularly to its axis.

113. The general effect of those glasses which ^je styled con«
▼ex lenses, or which are thickest in the centre, is to render rays
which pass through them more convergent; and that of concave
lenses, on the contrary, to render rays more divergent. The man-
ner in which light is infracted by a convex lens may be illustrated
by means of the annexed figure.

114. Suppose A B to
be a double convex lens,
the axis of which is D'C
G^, and C its optical cen-
tre, then the parallel rays
D A, D'' B, will be so re-
fracted at the two sur-
faces as to meet at G',
which point is termed
the " principal focus" of
the lens. And the pa^
rallel rays £ A, E' C, and
E"B, and also F, A, FC,
and F" B, falling obliquely on the lens, will in a similar manner
be refracted, and have their foci at G and G'', at the same dis-
tance behind the lens.

115. It may be observed that the rays E' C G", D' C G', F' C G,
passing through the centre of the lens, do not alter their direction.
C G' is termed the ■* focal distance" of the lens; and in a d' i.hle
convex lens, formed of equal spherical surfaces, its length will be
that of the radius of the sphere of which those surfaces form por-
tions. In a plano-convex lens the focal distance will be equal to
double the length of the radius of its curved surface. If the lens
be unequally convex, the focal distance may be found by multi*
plying togeUier the radii of its two surfaces, and dividing the pro-
duct by the sum of the two radii, the quotient being the focal dis-
tance required.

116. When converging rays, or those proceeding towards one
point, as D A G, E C G, and F B G, fail on the sunace of a con«
vex lens A B, the principal focus of which is at O, they will be-

"What test may be adopted of the aeearacy of such eentring ?
What is the effect of convex and concave lenses respectively ?
Illustrate the manner that parallel rays are refracted by a convex lent ?
What is meant by the principal focus of such a lens ?
What is meant by thejbcal aUtance?

How may this distance be knowa in spherical lenses of unequal coi>>
Texitv ? ^ .
What will it be in a plano-convex lens ?

How can it be found in lenses having carves unequally convex ?
How does a convex lens affect converging rays ?

370

opncB.

come more coBTergent, u4
will therefore be refneted to
a focas at H, nearer the lena
than the point O. 'fhe more
a distant may be the point a, at
which the rays would meet if
umintermpted, the ferllier will
the point H reoede from the
surftice of the lens towards O,
beyond which point it never ^ocs; and the nearer flie p«nt m to
the lens, the nearer will the point H advance towards it.

117. The points G and Hare named "conjugate foci," because
the place of one depends on that of the other, and thou^ every
lens has only one principal focus, it may have an indefinite num-
ber of conjugate foci, as rays may fall on it converging at innu-
merable angles. The conjugate focal distance, C M, may be
found by multiplyingr the principal focal distance, C, by a C,
the distance of the point of convergence, and dividing that product
by the sum of the same numbers, when the ^iiotiefii will gtv^ tbe
distance required C H.

118. When diverging rays, or those issuing from one point, «•'
E A, and E B, fall on a convex lens A B, the principal focua of
which is at O, the refractive power of the leas will make them
oonver^ to a focus at 6, beyond O. Aa the poiat whence the
rays diverge recedes from the lens, the focas 6 will advance to-
wards it, and when tiie point of divergence £ is inftnitely diatant,
the point G will coincide with the principal focua O, for rays
lasuing from a point at an infinite distance must be virtuaUy pa-
rallel rays. If E approaches to O', the focus G will recede from
O, and when E coincides with OS G will be infinitely distant, or

What are meant by the " canJugtOe fid** of a lens f

How is the conjugate focal distance for convergtng rays fonnd ?

In what position, with respect to the pr/neipal focas^ wilt the ooojn^ate
focus of diverging^ rays be situated ?

What effect will the indefinite distance of the point of dlTergenee pro-
dnce on the position of their focus ?

With what point will it then coincide ?

How may rays af^er refraction by a convex lens become paralld f

Where must the point of diyergeuce be situated in order that they
should be divergent after Kfractlon f

EFFECTS OF COKTEX LEN8EB.

671

the lays will becoise parallel afler refraction. And when F is be-
tween O' and C, as at H« the refracted rays will become diver-
gent, as A L, B K, as if they had proceeded from a focus I, be-
yond 0' and in front of the lens. The points £ and G are termed
the eonjugate foei, as before; and the conjagate focal distance
may be found by imtltiplyinff the principal focal distance by E G,
the distance of the point of cuyergence from the lens, and dmding
the product by the difference of those numbers, and Ikt quotient
will be the required distance C G.

119. Rays of light passing
through concave lenses will, in
most cases, be rendered more
divergent by refraction, whaS^
ever be their previous direction.
Suppose A B to be a double
concave lens, whose axis is E C
e, and C, its optical centre ; thea
the parallel rays D A, F B,
falling on it, will be refracted into the lines A <^ B /, as if they
diverged from a point 0, before the lens, which is its principal fo-
cus. The principal focal distance is relatively the same as in a con-
vex lens, and may be ascertained in the same manner, whether
the sides be of equal or unequal curvature.

130. Whencon-
veiqnng rays D A,
F B, proceeding to
a point G, beyond
the principal focus
O of a concave lens,
fall on it, they will
be refracted into tfa*
diverging lines A <4
and B/, as if they
issued from a focns H in front of the lens beyond 0\ When G,
the point of convergence, coincides with .0, the rays will he pa-
rallel after refraction ; and when the point G falls within the noint
0, the refracted rays will converge to a focus on the same siae of
the lens with G, but on the other side O. G and H are styled
conjugate foci, and the situation of one of them, when the other is
known, may be found by the rule given in the case of converging
rays falling on convex lenses.

What directions will rays generally follow after refraetion by a eon-
cave lens ?

Where is the prineipal focus of such a lens eooeeived to be situated ?

In what manner will the principal focal distance be ascertained?

How will converging rays be refracted, which, before refraetion, aoD-
Verge to a point beyonn the principal focus >

In what manner will tliey be refracted if converging directly towarda
the priocipal foaos ?

S72

omcs.

121. When diverging nj9
D A, F B, from any point F
beyond the focus O' rail on a
concave lens A B, they will
diverge in the directions A d,
B/, as if proceeding from a
point H, between 0{ and C ;
and as F advances towards C,
BO will H likewise: that is,
the more divergent the rays are
^ before refraction the more will
they diverge afterwards. When
the distance F C or H C is given, the other point may be found
by the rule for diverging rays falling on convex lenses.

122. Meniscus, or concavo-convex lenses, have the same effect
on rays of light as convex lenses corresponding with them in focal
distance. Convexo-concave lenses have the same effect as con-
cave lenses agreeing: with them in focal distance.

123. The manner in which images are formed by means of op-
tical lenses may be readily conceived from the preceding figures

and descriptions ; and the ef*
feet of convex glasses, in mag-
nifying the images of objects,
may be further elucidated by
reference to the annexed dia-
grram. Let A B represent a
convex lens, of which C d'ls
the optical axis ; and let £ F
be any object to be examined,
placed between the principal
focus and the surface of the lens ; then a ray E g falling on the
lens parallel to its axis will be refhicted in the direction g C, and
another ray E 6 H, from the same point, falling obli<juely on the
lens and passing through' its optical axis, will be continued in the
same direction withouit being affected by refraction, and the two
rays will become more divergent after passing through the lens ;
whence it follows that if the rav E G H were prolonged beyond
E, it would cut the line g ein the point e, and an eye placed be-
hind the lens would see the extremity E of the object at e; and
rays proceeding: from every other part of the object beings refracted
in a corresponding manner, the image of the object E F will ap-
pear as at e/, and therefore be larger than the object.

What will be the directions after refmetion of rajs diverging from a
point bevond. the principal focus of a eoncaye lens ?

Can divenping rays ever become either parallel or convergent by (be
refraction of such a lens ? Why ?

What two rules apply for finding the focal distances of Meniscus lenses?

Draw and explain a diagram to illustrate the magnifying effect of con-
vex lenses.

What effect is produced by refraction on rays pauiog through the op«
ttcal axis of a eonvex lens ?

DESCRIPnON 07 THE EYE. 373

1S4. Btit if the object be placed at the focns of the lens, thd
rays refracted being chiefly such as were paraUel to its axis before
refraction, the eye will not perceive a distinct image of the object.
If we suppose the object £ F to be placed bejond the focal dis-
tance, the rays £ ^, E 6, from the same pomt E will become
convei?ent after having traversed the lens, and will intersect each
other below the axis, E 6 as passing through the centre of the
lens not having its direction altered by refraction ; all the rays
from different points of the object will take analosons directions,
and thus there will be formed on the opposite side of the lens a
reversed image of the object. And if the lens be fixed in an aper*
ture in a window-shutter, and all light but what passes through
it be excluded, the image may be rendered visible, by placing a
sheet of white paper opposite the aperture to receive it. A room
thus fitted up would be literally a camera obBcura, a darkened
chamber.

The Orgeats of Ftsion,

125. The eyes of animals bear a certain analogy to the optical
instniment called a camera obscura, just mentioned ; for the ima-
ges of external objects, within the sphere of vision, are actually
formed or traced within the eye, in the manner that wUl be sub-
sequently described.

126. In man and other animals destined to inhabit the surface
of the earth, the eyeball is a mass nearly spherical, but soooewhat
flattened in front. Those animals that dwell in the water have
eyes very much flattened, the eyeball in most fishes forming but
half a sphere, and in the ray species, it is but one quarter of the
thickness of a sphere. In those birds that soar to the higher re*

fions of the atmosphere, the anterior part of the eye is sometimes
at, and sometimes in the figure of a truncated cone : the upper part
forming a short cylinder, surmounted by a very convex eminence.

127. The eyes of spiders, scorpions, &c., are merely very mi-
nute points, which it would be difficult to recognise as organs of
vision, if their functions had not been demonstrated by precise
experiments. Millepedes, flies, &c., have eyes oflen very large
in proportion to the bulk of the insect, and composed of a multi-
tude of small facets, or plano-convex lenses united into a hemi-
spherical form, with their axes directed to a common focus.
Many injects have, at the same time, simple and compound eyes :
this IS the case with wasps, grasshoppers, and some others, liiere

Why does the eye not perceive a distinct image of an object at the
principal focus of a convex lens ?

In what position will the image appear when the object is beyond the
principal focus ? Explain the cause of this on the diagram.

In what manner may this position of the image be verified ?

What is a camera obacura ?

To what is the construction of the eye analogous f

What relative sphericity have the eyes of land and of aquatic animals?

What peculiarity is found in the eyes of birds that soar to great
helgbts ? What, in those of qiiders and scorpions ?

874 OPTICS.

exist likewisa maltitndes of animals, id which bo otgan of Tiabn csa

bediscarered ; but it appears, that in such the sense of feeling iaex-
tcemel J delicate, and therefore supplies the defect of the other sensM.
188. In the following deacripUre notices of the organs of vision,
and the phenomena depending on them, our attention will l>e reatrio-
led to the struct4iTe and functions of the human eye. But the ejee
of some quadrupeds, as (he ox or the she«i, «o ht resemble thoaa
of man, that sufficiently accurate ideas of die essential parts of
the eye niaj be obtained bj dissecting and examining an eye of
either of those animals, and comparing its mechaoiBm with the
ensuing description.

199. The annexed fieure exhibits a front

view of the eye, or the anterior portion

* of the eyeball. The white part Hnrroand-

- ing the centre is called the sclerotic* coat

(lunica lelerotiea), a a, and it is coutinaed

' within the orbit, round the hack part of

the eyeball, being formed of a dense m~~~

brane, which includes, t
other parts of the eye. , ,

;, end Aerefore is not cootinned over the front of the eye.

, as in a bag, thi
other parts of the eye. It is perfectly

frayish-b

vpaqne, and therefore is not cootinned over the front of the eye,
tint joins the transparent cornea, f b, which diSers from it chiefly
in being completely pervious to light, and therefore serves like a
window to admit it to the interior of the eye for the formation of
(mages. Within or behind the cornea may be perceived the iris,j:
e. a sort of eoloored fringe, nsually either of a dark brown or a
h-blne tint ; and hen,ce the distinction between black, and
. ... ir gray eyes: but there are persons with extremely light
complexions and white hair (Albinos), who have red eyes, the
iris being red, as in the eyes of a white labit In the centre of
the eye, surrounded by the iris, is a dark circular space of variable
dimensioDB, called the pupil, d, through which the rays of tight
pass into the chambers of the eye.

^ 130. An horizontal section of the

eye is represented in the mai^nal
figure, in which the parts already
described are shown, ae well a»
those of the interior. It will be

terceived Ihat the eye is enveloped
I four membranes or tnnics, the
sclerotic coat, AAA; the cornea,
or homy coat, B B, connected with
the fonner, in the front of the eye;
the choroid coat,$ T T, which
forms a lining to the sclerotic coat,
and on its opposite surface is co-

■ From IhF Greek E<i.4,g„ bird, firm < or, £.i.i,>(i(, hardneu.

t From tho laiin eomtta, bnrnv, or like horn.

i 8o tilled from it> being like the rainbnw {irit), nrion^ coloured.

4 From Ita reieaiblBDBe to anolher mtmbrane called, id Greek, Xi^m

THEORY 07 tiSION. 975

▼eVBd by a black pigment fpismentum nigrum), on which lies the
interior coat of the eye, called the retina,* RR, a delicate retica*
lar membrane, expanded over the posterior chamber of the eyfe,
and proceeding from the optic nerve, O) by which sensations are
conveyed to the brain.

131. The interior of the eye, or Ihe cavity surronnded by the
coats jnst described, is filled by three substances called humours :
the first, or the aqneous humour, D, is afimd situated immediately
behind the transparent cornea, and chiefly in front of the iris ; the
second in situation is the crystalline humour, 0, directly behLnd
^e iris, being a solid, transparent lens, more convex behind than
before ; and the third is termed the vitreous humour, V, a kind of
viscous soHd mass, of a medium consistence compared with the
other two, occupying the posterior chamber of the eye, supporting
the other parts, and contributing chiefly to preserve the globular
fi^re of the eye. Between C and Dis the pupil or openiog in the
ins, 1 1, through which light' is admitted into uie eye ; and behind
the iris the crystalline humour or lens is suspended in a transpar
rent capsule, by the ciliary processes, L L, which proceed from
the iris.

132. The eyes are situated in basin-shaped cavities in the skulU
called the orbits, and there are various muscles attached to the
ball of the eye and to different parts of each oibit, which by their
contraction give a certain degree of lateral or rolling motion to
the eye, ana thus assist in directing the sight towards particular
objects. Eyelids, also moved by muscles, and fringed by the
eyelashes, serve to guard the eyes from dust, and screen or shut
fiiem altogether from the access of too intense a light; and there
are glands for the secretion of fluid to moisten the cornea, and by
the motion of the eyelids keep its surface clear, and in a state
adapted to yield perfect vision,

133. As already observed, the eye may be compared to a ca-
mera obscura, the rays of light from any object entering the pupil.

What .appears to supply the place of the eye io animals which are wki^
Out that organ ?

From what quadrupeds may we obtain specimens, as substitutes, to de«
monstrate the structure of the human eye r

Where is the tunica tclet^Hca situated ?

Where the cornea? the iris? \\vbputil?

What are the names and positions of^the four ooats containing the ho*
mours of the eye ?

Between which two membranes is the black pigment found ?

What substance occupies the front cavity of the ey« ?

What is the nature, form, and name of the body which separates this
from the posterior cavity }

With what is the latter cavity filled ?

Draw and describe a maernified section of the eye.

What is the purpose of the dUary processes ?

lyhat name is given to the bony cavity in which the eye is placed ?

* From the Liatin rete a net, in reference to its resemblance to imU
«rork.

876 oPTici.

and forming an iraagre on the retina, which prodace8*the percep-
tion of a visible object conveyed through the optic nerve to ^e
brain. That a perceptible image is really formed in this manner
on the retina, may be experimentally demonstrated by paring away
the back part of the sclerotic coat of the eye of an ox, with a
sharp knife, till it becomes so thin as to be transparent : it will
thus be converted into a miniature camera obscura, and objects
held before the cornea, will then be seen behind, delineate on
the retina.

134. It may be imagined, that if a luminous point, or illumina-
ted obiect be placed too near the eye, the rays proceeding from it
will form an image beyond the retina, or rather the ^mage they
form on it will be confused and imperfect ; so on the contrary, i
the luminous point be too distant the image will be confused ia
consequence of the rays converging to a point before they reach
the retina.

135. In order therefore that the image may always be formed
distinctly on the retina, provision must be made for increasing or
diminishing the refraction of rays within the eye, in proportion to
the distance of the objects to be viewed. This seems to be effec-
ted by means of the crystalline humour or lens, which is com-
posed of concentric laminae of transparent fibres, by the action
of which its form may be modified so as to adapt the eye to the
distance of different objects. And in various animals the figure
of the crystalline, and its situation with regard to the retina are
varied so as to accommodate the powers of vision in each animal to
Its peculiar circumstances and mode of life.

136. The vision of objects at different distances may possibly
also be further facilitated by the variable pcessure of the muscles
on the ball of the eye ; though it must be concluded, from the
experiments of Dr. T. Young, that their action cannot produce
any alteration in the shape of uie cornea. In viewing near objects,
the pupil of the eye is contracted, fewer rays enter the eye, and
such objects are thus distinctly perceived ; while in viewing dis-
tant objects, the pupil dilates to admit more rays to fall on the
retina. In obscnnty, the pupil of the eye becomes dilated to ad-
mit as many rays as possible ; and in a strong light its dimensions
are much contracted, as may be observed by holdin? a candle
near the eye of another person. Sadden exposure of the eyes to
much light produces an uneasy sensation, from the quantity of
rays admitted through the dilated pupil; and, on passing from

How can we prove that a perceptible image is formed on the retina at
the back of the eye ?

In what cases will images be indistinct ?

What provision is necessary to render objects distinct at different dis-
tances ?

By what part of the eye does the adjustment to distances appear to be
effected ?

What apparent effect on the exterior appearance is produced by effbrti
to distinguish very distant objects ?

Whence arises the pain from sudden exposure to a glare of light ?

ANGLE OF T18I0N. 877

Open daylight into an obscure apartment, objects are not seen till
the contracted pupil becomes enough dilated to take in a sufficient
number of rays to render them visible.

137. An object may be seen distinctly and singly, though sepa-
rate images of it be formed on the retina of each eye. This de-
pends on those images occupying corresponding pomts on either
retina, and thus the directions of the optical axes of the two eyes
intersect each other, and a distinct image is perceived at that
point. If, however, while a person looks steadfastly at any near
object with both eyes open, ne tries to direct his view to some
rather more distant object, without suffering the first to escape at-
tention, a double image will be perceived, one somewhat above
the other ; and, on ceasin? the effort to look beyond the object,
the images will coalesce mto one. Similar effects may be pro-
duced by pressing with the finger on the ball of one eye, so as to
displace its optical axis. Double vision is also in the same man-
ner occasioned by intoxication or by frenzy. Many animals never
see objects with more than one eye at a time ; as most kinds of
birds, lizards, and fishes ; while there are some species of fish
that can only see objects situated above them.

138. Though the perception of visible objects is certainly pro-
duced by means of their images formed on the retina, yet the
panner in which the sensation is conveyed by the optic nerves to
the brain is a mystery which we are utterly unable to penetrate.
There are also some peculiar relations between the images of ob-
jects, and the manner in which they are perceived, which have
given rise to various conjectures, and have never yet been clearly
^plained. Thus, it is certain that the image formed on the retina
is always inverted with regard to the position of the object pro-
ducing it ; just like the images formed by a single lens in a ca-
mera obscura, as may indeed oe ascertained by repeating the ex-
periment on an ox's eye, previously mentioned.*

139. Some writers on optics content themselves with asserting
that we really see all objects inverted, but that the judgment cor-
rects the erroneous perception, a process of the occurrence of
which no evidence can be produced* Others more philosophi-
cally attempt to explain this phenomenon by alleging the format
tion of a secondary image within the eye, reflected uom that re-
ceived on the retina to another plane, by means of which the

When «re distinct inmges formed by both eyes ,prodiiein^ a tingle Im-
pression on the mind ?

On what does this dlstinotnesr depend-?

How may the two eyes be made to see different images of the same
object ?

By what' means other than'TOlnntaryefibrt xatij domble vision be pro-
duced ?

WhaCpeculiarities of sight belong to birds, reptiles, and fishes ?

In what position is the ifnage of an object formed on the retina ?

• See No. 133.
3l2

ng OPTICS.

positioii of the imaspe is corrected.* But further inyestigation is
requisite to enable us to explain the relation between the yisible
direction of objects, and tne position of the images formed by
them within the eye.

140. There are, however, some cases in which the judgment,
with the aid of the other senses,. enables us to correct erroneous
perceptions produced by vision ; and it is thus, by means of the
sense of feeling and by habitual observation, that we ascertain
the figures and relative distances of visible objects. It has been
remanded, that persons born blind from the existence of cataracts f
in the eyes, on being restored to perfect vision by a surgical ope-
ration, after arriving at years of discretion, believe at first that
the objects they see are in immediate contact with their eyes,
every thing appearing to them as if painted on a plain surface ;
and they are unable to recognize objects by sight alone, gradually
acquiring that power by comparing their new sensations with the
real objects by feeling them.

141. A person born blind and just restored to sight by the ope-
ration for the cataract, would not be able to distinguish a die oi
any other cube from a marble or a billiard-ball, without touching
them ; neither would he know the persons with whom he was
most familiarly acquainted, or discriminate his father from his
mother, or his brother from his sister, without examining their
persons and dresses by the sense of feeling, or hearing their
voices. Individuals thus situated acquire the correct sense of
vision only by degrees, like infants ; and it is by experience that
they learn to walk about among the objects around them, without
the continual apprehension of striking themselves against every
thing they behold.

142. The processes by which we judge at all times concerning
the dimensions and distances of visible objects are, in an analo-
gous manner, the result of reasoning on visual phenomena ; and
3ius experience modifies considerably the ideas we form of the
size of any object and its position in space, according to the visual
angle. For instance, in judging by the visual angle, a man would
appear to us much smaller at three hundred paces distance than
at one hundred ; notwithstanding which we are able to form as
exact a judgment of a man^s height at one distance as at the
other, provided there be other objects at hand which may serve as
scales of comparison. Thus, we rectify the image formed under

Whnt means have we of correcting the error of. early impressiooB re-
ceived through the eye ?

What happens when persons of mature age are for the first time ena-
bled to see f

How do such persons acquire the correct sense of vision f

Huw do we obtain accurate ideas of the dimensions and distances of
remote objects ?

* See a New Theory of Vision. By Andrew Horn. Also Eocyolo-
pKdia Meti*opolitana — Mixed Sciences, p. 459.
t From the Greek x»t«p»»tii(, a cataract.

PIST4NT VISION. 870

Ihe Yisual angle by our preconceived idea of' the common height
of a man, comparing it m imagination with the door of a house,
the trunk of a tree, or any other object in view> with the size of
which we are acquainted. Hence, if we see a man three hun*
dred paces off, upon a naked plain, as on a wide sandy level by
the sea-side, he will look very small, and may be mistaken for a'
little child, as we can judge of him only by the yisual angle, and
have no other object near to rectify the erroneous perception.

143. Dr. Amott has adduced an interesting example of the op-
tical effect just illustrated. He says he ** once sailed through
the Canary Islands, and passed in view of the far-famed Peak of
Teneriffe. It had been in sight during the afternoon of the pre-
ceding day, at a distance of more than 100 miles, disappointing

general expectation by appearing then onl}r as an ordinary distant
ill rising out of the ocean ; but next morning, when the ship had
arrived within about twenty miles of it, and while another ship
of the fleet, holding her course six miles nearer to the land, served
as a measure, it stood displayed as one of the most stupendous
single objects which, on earth, and at. one view, human vision
can command. The ship in question, whose side, showing it«
tiers of cannon, equalled in extent the fronts of ten large houses
in a street, and whose masts shot up like lofty steeples, still ap-
peared but as a speck rising from the sea, when compared with
the huge prominence beyond it, towering sublimely to heaven,
and around which the masses of cloud, although as lofty as those
which sail over the fields of Britain, seemed £till to be hanging
low on its sides. Teneriffe alone, of very high mountains, rises
directly and steeply out of the bosom of the ocean, to an eleva-
tion of 13,000 feet, and as an object of contemplation, therefore,
is more impressive than even the still loftier summits of Chim-
borazo or the Himalayas, which rise from elevated plains and in
the midst of surrounding hills."*

144. Various optical deceptions are produced when we are ob-
liged to judge of the sizes and distances of objects merely by the
yisual angle. Thus, any person placed at one extremity of a lonff
avenue, a gallery, or a rectilineal canal, will perceive the trees of
the avenue diminishing in height as they are more distant, the
two ranges of trees seeming to converge towards each other, and
come to a point if the avenue is very long ; and the two sides of
a canal, and the floor and lateral walls of a gallery, in the same
manner, become convergent, and meet in a point when greatly
extended. These optical effects may be imitated by constructing

How are we liable to err in estinutting the size of a person on a wide
level surface ? '

In what case may the grandeur of an object be heightened by contrast ?

What gives the Peak of Teneriffe its peculiar sublimity ?

Why are the Andes and Himalayas less impressive than that peak }

In what cases may the imaginaliou be deceived by an undue reliance on
the eye ?

• Elements of Physics, vol. ii. pp. 264, 865.

^80

OVTIOI.

ike Btdes of a canal or alleys of trees in convergring line^, thb
more distant trees grradually diminishing in height ; and thus the
avenue or canal would appear longer than the reality.

e 145. The annexed diagram
may serve to illustrate the
apparent diminution of ob-
jects Under different visual
angles. Suppose a & to be any
object, as a tree, to an eye n-
tuated at O, it will appear
f under the visual angle aO by
and the dimensions of the image on the retina will have a certain
)>roportion; then, if another tree, e rf, of the same height with
the first, be placed as far ag^n from it, the visual angle will be
eO d, and the apparent hei^t of the latter tree will be to that of
the former, as </ a' to a 6 ; and if a third tree be situated at a furr
ther distance, «/, its apparent height will be to that of the first, as
e'f to a b; that is, the spectator will see three trees really equal
in height, as if they were three times at the same distance of the
relative heights, o 4, </ rf', and e' f.

146. When objects are extremely distant it is impossible to
judge correctly concerning their particular situations ; and hence
an irregular line appears to be an aro of a circle, because we sup
pose all the points to be equally^ distant from us ; and thus when
stationed in the midst of a plain, remote objects seem to form a
circle around us. It is for the same reason that the heavens pre-
sent the appearance of a concave hemisphere sprinkled with stars^
ibr at first view the stars seem to be all equidistant from the ob-
server. A small curved or polygonal line seen afar off appears
to be a small right line ; a poiyedron cut in facets, or an irregular
mass, at a distance, will look like a sphere, and yet further off
will exhibit the contour of a flattened disk. This happens with
respect to the sun aild moon, which we see as circular disks.

147. Optical illusions take place in consequence of the figures
of bodies in motion. If a sphere revolving on its axis be placed
at a distance, it will be impossible to perceive the movement, un-
less there are on its surface spots or visible irregularities, the al-
ternate appearance and disappearance of which may be observed ;
and it is thus only that astronomers haye been enabled to ascer-

Illastrate by diagrara the apparent diminution of objecls by the effect
of different visual angles.

How would a row of trees, of e^ual heights, and situated equidistant
from each other, extending nearly in front of the spectator, be made to
appear by the effect of perspective ?

How eould they be imitated in a picture ?

In what instances may irregular forms be mistaken for those which are
regular ?

What appearance has a spheroidal figure when extremely remote from
the observer ?

What will enable us to judge whether such a body haw a rotary mo-
tion or not? ''-

THB THAUMATROFE. 881

tain the rotation of the sun and the planets, by obsemng spots on
their surfaces.

148. A lighted candle or torch whirled in a circle, the plane of
which passes through the eye, at a great distance, merely ap*
pears to come and go, in a line, from one extremity to the other
of the diameter of the circle. The visible paths of the planets
through the heavens, in their revolutions round the sun, thus have
the appearance of right lines, from one extremity to the other of
which each luminary seems, to a spectator on the earth, alter-
nately to advance and return.

149. The impression of light on the eye is not merely instanta^

neous, but continues during a certain time after the luminous or }-

illuminated object has been withdrawn. From the experiments
of D'Arcy, it has been ascertained that the effect of light on the
retina remains about 1-7 or 1-8 of a second after the li^ht has ac-
tually been removed.* To this cause is to be ascribed the circle
of light formed by whirling round a burning stick, a phenomenon
with which every one must be acquainted. And on the same
principle is constructed the amusing toy called the Thaumatrope,!
contrived by Dr. Paris.^

150. It consists of a number of circular cards, having silk
strings attached to their opposite edges,,as represented in the pre-
ceding figures. By these strings, one of the cards being twirled
round with a certain velocity, both sides of it will be visible at
the same time, and any objects traced on them, as a dog on one
side and a monkey on the other, may be perceived simultaneously.
Hence the parts of the picture being united, when it is whirled
round, the monkey will be seen seated on the back of the dog.
In this case the revolving card becomes virtually transparent, so
that the objects on opposite sides of it may be viewed togetiier.

How nifty a circle be mistaken for a straight line }

With what example of this does astronomy famish as f

Does the image of an object, vanish from the eye the moment the ob-
ject is withdrawn ?

For what length of time has D*Arcy found impressions to remain on
the visual organ ?

What famuiar and amasing experiments owe their interest to the du-
rability of visible impressions ?

Describe the thauroatrope.
■ ' ' '' ■

• See a Paper " On the Duration of the Sensation of Sight,*' in Me-
moires de PAeademie des Sciences, a Paris. 1765. p. 439.

+ From the Greek Bctv/t», a wonder, and Tpia-w, to turn.

f Philosophy in Sport made Science in Earnest.

tS9 emcB.

nearly us they would be if painted on the two snrfaces of a plate
of glass.

151. An hnpTOTement of the thanmatrope, as already described,
has been made by the inyentor, which consists in altering the
axis of rotation, while the card is in the act of revolving, in order
that the images on its opposite sides may be brought in different
positions with respect to each other. This is Ingeniously effected
by affixing two strings to one or both sides of the card, which are
80 connected as to act on different points of the border, according
• to the degree of tension applied to them. The appearances ex-
hibited are thus diversified, and rendered more amusing. A caud,
with a horse on one side and a iockey on the other, may by twirl-
ing it be made to show the rider in his saddle, then by merely
tightening the string, while the card continues revolving, the
jockey may be seen as if making a summereet over the head of
his steed : on relaxing the eitring he will again appear in the sad-
dle, and by various degress of tension other postures may be dis-
played.

153. Many singular effects may be produced by modifications
of the machinery, all depending on the continuance of the im-
pression of visible objects on the retina during the ^pace of about
one-eighth of a second, so that the figures on either side of the
card, when it ia made to revolve, are renewed before the preceding
impression Has ceased its action ; and conse<^uently the figures on
both sides of the card are seen at the same time«

153. Another curious machine has been recently invented,
called ' the Phantasmascope,* the effect of which further illus-
trates the phenomenon of the perception of viubie impreanians
during a certain period after the objects producing them are with-
drawn.

154. In this apparatus as modified by Mr. Faraday-, figures aie
seen through revolving wheels, or circular disks of pasteboard,
with deep nanow notches at their edges. If a transparent star,
highly illuminated, be placed behind a disk of pasteboard or
blackened tin plate, with a single narrow opening extending from
the circumference to the centre, it will necessarily hide the whole
of the star except that part immediately opposite the opening;
but if the disk be made to revolve rapidly, the whole star wul
become visible ; as may easily be conceived from what has been
stated relative to the duration of impressions of light on the re-
tina.

By what device has its action been varied ?

With what, as the least veloci^, mast this apparatus revolve, in order
Cb exhibit its true character ?

What other toy has been founded on the durability of impressions ?

Wliat is the arrangement by which figures are viewed in the pbantas-
masGope ?

How did Faraday contrive to render the whole of an iUumioated ob-
ject visible thi*ough a single line of opening ?

* From the Greek ««»rrf«-/M«, a speotaole, and axor«»,-to view.

155. In the phanlasmaBcppe the paHtebpard disks are painted
Trith a variety of figurea, in different poaitiMis, and the borders
of the disks being cutinto cogs orteethitearine- openings between
them, when made to revolve on a spindle, on lookins at the oh-
Jects as exhibited in a minor, through the opening, they wit) dia-
play the moat direr^ified and groteaqne attitudes.

156. Thus the figures given in the preceding cut, when pro*
^lerly viewed, wonld all appear to be piccuetting like so many
opera-dancers. By different Hrrangements of the openings, and

varied deaigna, ma^ be exhibited, in a aimilar manner, yawning
figures, jumping frogs, creeping serpents, and ■ mnltiplicity ot
otliRT atrange combinations. ,

157. One of the moat curious facta relating to the faculty of
vision is the absolute inaensibilit; to the impression of light at a
certain point of the retina, so that the image of any object fallings
on that point would be invisible. When we look with the right
eye this point will he about 15 deg. to the right of the object ob-
served, or to the right uf the axis of the eye, or the point of most

For what purpoie ii the mirror Introdueed in the Exhibition of th*

Are ill the urti of the retina equallT icnuble to tbe impmnoM of
ficbtf

884 oiPTlcs.

distinct yision. When looking with the left eye the point will be
88 far to the left. The point in auestion is the hasis of the optic
nerve; and its insensibility to light was first observed by the
French philosopher Mariotte.

1 58. This remarkable phenomenon may be experimentally proved
by placing, on a sheet of writing paper, at the distance of three
inches apart, two coloured wafers, then on looking at the left-hand
wafer with the right eye at the distance of about a foot, keeping
the eye straight ^ove the wafer, and both eyes parallel with the
line which joins the wafers, the left eye beinff closed, the right-
hand wafer will become invisible ; and a similar effect will take
place if we close the ri^ht eye, and look with the left. -Accord-
ing to Daniel Bernoulli, this insensible spot is about 1-7 part of
the diameter of the eye.

Chromatics, or the Theory of Colours*

159. Among the properties of light one of the most striking and
curious is that of communicating colours to bodies. Popular lan-
gruaffe ascribes the existence of colours to some inherent qualities
of the substance on whose surfaces we perceive them ; and thus,
in using the phrases a red brick or a green wafer, an uninformed
person would conceive the redness of the brick, or the green tint
of the wafer, to be as much peculiar properties of those bodies as -
the quadrangular shape of the one and the circular figure of the
other. But we find, from experiment, that though colour partly
depends on^the texture of substances, and the nature of their sur-
faces, the essential efficient cause of colour is light, since not
only are bodies destitute of colour in the absence of light, but, as
will be subsequently shown, their colours may be altered by sub-
jecting to certain modifications the light by which they are ren-
dered visible. Hence it happens, that many coloured objects, the
peculiar tints of which are discriminated without difficulty in
broad day-light, appear to wear the same hue in the dusk of the
evening, or by candle-light. It may therefore be properly stated,
that the colour of a subtttance ia the effect of light on a surface adapted
to reflect its peculiar colour.

160. The influence of light in the production of colour is re-
markably modified by refraction. This effect of light is most con-
veniently exhibited by means of a triangular prism of glass. If
such a prism be held with one of its angular edges opposite to

What fact, in regnrd to thi« sabjeet, was discovered by Mariotte P
How is the existence of a point of insensibility experimentally proved ?
What is the size of the insensible spot ?

What is the usual belief and the common language of mankind in re-
gard to the existence of colours ?

What is the efficient cause of colours f

How is this proved ?

What is the true definition of colour ?

What relation has colour to refraction ?

How is the influence of refraction best exhibited ?

CAOHB W CO(,0CR». ^a^l

Ibe aye, tke xibjecti awa tbiouglt U will not be ioaUcd, aa vboii
tIsw^ directly throagh one of tlu flnt utiee or the gUiw, but
they will be more or Ipsa elonntad, according to the angle at
which the prisiu U held, uid will also b« clothed with all ue eo-
loun of the mnbow.

161. The dimeotiom of a ny ofaafau If gfatiato 4i
bjr lefiraotioD, may be more tecanttly titplayed bj adtniuing «
ray through aa aperture in B window-ehatieTiMoa dukened oIhhh
ber, and caaaing it to Ml oa a diaphaoooa priam JL fi C, ^ !••
presented to the preeeding figure. A rsy D thoa ertteriv, wri
■nfTered to pass anobttruoted, would form on a plana anmiea S

the ray may eniM and quii Tt at equal angles, it will be
In snch a manner, as to *
an oblong image called
tally iotn Beven coloured

In ench a manner, as to form on a. Bcreeo M N, properly
divided h

seven coloured apacea, or iMada of on
ing '
green, witf, indigo, vielel.

a oblong image called the aotar apeeinu
" '"I In seven coloured apacea, or baada at oneaaal
T each other in the order repreeented : nd, a

!. These bands are not separated by diatfuat lines, aothatitil
difficult to determine where one ends and BBOlher commenoee, tha
fcveral tints at tbea borders being blended, and each alnuM ia>
perceptibly united with tiioee next it ; the whole speatrum eiduUl*

Ing the aeven pripcipal cotourB, with iatermediate shades m
turea. Indeed some writers enumerate butaix colours in Ihaape^
trum, omitting the indigo; and Dr. Wollastonobsetred, thatwkea
■ very smell lay was submitted to the prisn, there were onlv fea
colours, oamely, red, yeHmiiili'grttit, blot, and vialtl. Qauda qf

Deiei-ibe the eETeotoTa prim of any (raotparcnt nibRanGC vbeo pUscd

HoH it ihe •FpiraliMi of ohite light into id eonitilDenl soloDrfd r|y^

■ooM idmiMgDinulj ditplH^d !

What ii the image of a bnm of liglit retracted bj a prim afwlly d«^

Lito how many an' wh^t apatiei ii the leUr imlnun diridvd t
WKrl are the tva cjiln*u> ft Ibc ipMlriiip r

What Jul In Ruid-todw-Duml^ of .aelaor* trai tfutmd BT tti
WoUaitoaf

3K

dSO OPTICS.

eolonrs more precisely terminated may, however, be obtained by
receiving the ray on a lens before it is allowed to fall on the prism;
and ihe ima^e thus formed will be more extended in len^h and
very narrow.

163. Similar phenomena may be produced by means of other
kinds of light as well as that of the sun ; and all transparent sub-
stances, in masses not terminated by parallel surfaces, have in
some degree the same effect as the glass prism. Hence the dia-
mond, sappbire, topaz, and other precious stones cut in ^ets,
display the prismatic colours ; as also do angular crystals of quartz,
Iceland spar, and many valine and stony substances ; the cut-glass
ornaments of lustres, &c., exhibit the same flittering tints, by
lamp-light ; and the refraction of the sun*s rays m passing through
drops of wateiT' produces a like effect in the rainbow.

164. From the preceding and many other experiments of a simi-
lar nature. Sir Isaac Newton was led to the constraction of a
theory relating to the cause of light and colours, which was, during
a long period, almost universally received among men of science.
Hie production of coloured spectra by the refraction or reflection
of light had been observed long before Newton commenced his
researches, and some imperfect attempts had been made to explain
the phenomena; but he not only showed that these conjectures
were wholly unsatisfactory, but also proposed a highly in^nious
hypothesis, founded on the doctrine of the emanation of light, or
that system which refers the i>henomena of vision, light, and
colours, to the presence and motion of an ethereal fluid, constantly
issuinff from the sun and other luminous bodies. *'The sun>
direct light," says Professor Maclaurin, '^ is not uniform in respect
of colour, not beinff disposed in every part of it to excite the idea
pf whiteness which the whole raises ; but, on the contrary, is a
composition of different kinds of rays, one sort of which, if alone,
would give the sense of red, another of orange, a third of yellow,
a fourth of ^een, a fifth of light blue, a sixth of indigro, and a
seventh of violet ; that all these rays together, by the mixture of
their sensations, impress upon the organ of sight the sense of
whiteness, though each ray always imprints there its own colour;
and all the difference between the colours of bodies when viewed
in open daylight arises from this, that coloured bodies do not re-
flect all sorts of rays fiUling upon them in equal plenty ; the body
appearing of that colour of which the light coming from it is most
composed."*

165. Hence, according to the theory of emanation, white light

By what means may distinctneM between the eoloured bands be ob-
tained ?

Is the presence of solar light necessary to the prodnction of colours bj
irefraction ?

How did Xewton exi)lain the phenomena of the spectram ?

How did the Newtonian philosophers conceire light to be constituted?

Of what did they suppose white light to be composed ?

fc.* I t. I • I ,,. ■- I — . 1 .1 ■ II

* Afaclaoria's Philosophy of Newton, 4to. b.iii. p. 318.

CHROMATIC BSFRACTION. 387

k an assemblage of molecales of Tarioos eoloniSi which maj be
separated from each other by the action of a prism; and bodies,
when exposed to the rays o£ the sun, display any g^iTen coloar
becanse they are so constituted as to absorb all the molecules ex*
cent those of the rays of their own peculiar colour : thus perfectly
wnite substances absorb none of tne molecules, but reflect thie
white or compound light unaltered ; black substances absorb all
the rays, and therefore yield no colour ; and red, yellow, and blue
substances respectively reflect those rays alone by which they are
distinguished.

< 166. The least that can be said in favour of this system is that
it accoimts for all the phenomena ; but it must be admitted that
the notion of material particles perpetually traversing space in all
directions, with almost infinite velocity, is absolutely gratuitous,
and hardly consistent with the simplici^ and economy generally
observable in the works of nature. The appearances likewise
may be explained by having recourse to a different theory, advan-
ced by Huygens, and advocated by Euler, which refers them to
the excitement and propagation of uiidulations through an eth^
real fluid pervading all space, whieh, by inconceivably rapid vi-
brations conveys white or coloured light to the eye, in a^maniter
analogc aS to that in which musical and other sounds are brought
by slower vibrations of the air to the ear.

167. ^* Every simple colour,'' according' to this system, *Me-
pends on a certain number of vibrations which are performed in a
certain time ; so that this number of vibrations mfiae in a second,
determines the red colour, another the yellow, another the men,
«nother tiie blue, and another the violet, which are the simple co-
lours represented to' us in the rainbow. If, then, the particles of
the surface of certain bodies are disposed in such a manner, that
being agitated, they make in a second as many vibrations as are
necessary to produce, for example, the red colour, I call such a
body red : and rays which make such a number of vibrations in
a second may, with e^ual propriety, be denominated red rays ; and,
finally, when the optic nerve is affected by these same rays, and
receives from them a number of impulsions, sensibly equal, in a
second, we receive the sensation of th§ red ^colour.

168. The parallel between sound and light-is so perfect that it
holds even in the minutest circumstanoes. When 1 produced the
phenomenon of a musical chord, which may be excited into vibra-
tion, by the resonance only of certain sounds, you will please to

"What account did thev give of bodies possessing a white eolour ? what
of those which are black r

To what objections is the Xewtoaian theory of emanatitu liable ?

What more simple view of the subject was entertained bj HiijgeM,
Euler, and others I

On what, according to that flieory, do the several colours depend }

In what manner does this theory conceive coloured bodies to oe adapted
to produce the sensations belonging to their several tints ?

What analogy has been traced between the seven colours and the serea
notes of the nusieal gamut ?

4ni6iti6n 18 tkd moflrt f»foper 16' Khikd it, ftiid tli%t other mtan^ 8^
A)et ft only in pt Opontott as th«r «nte l«i e6ft»€M^ho«f i^th it.* Anfl
in is exactly tlie siody^ «s to light tnd ftolotifs; fbr th^i dffleren
eolonts fof ttid ftolaf fepectttatti} oonrMponfd to tile d^thfenf tiiiisietl

'•bnitd8."t

It^. It ofrKht to b6 obi8ehr«d tiiAt Whild ^e pb6iioift«iia of ligltt

'«nd doloimi may be a<3Conftted for aeoordinf to the Ayst^m ^Ema-
nation bt that of nndblation, it is not abft6lat«l:f necossary to adopt
either in reasoningr concemin|r them ; and it will be Mficient hei^
to stste that rays of light, whetb^ they be fcrahis of material par-
ticles, issuing from luminoas bodies, or c^haans of nndnlations
taking place in an etb^real medium, are ahrays propagate in
riglrt lines till they enter or impingo on a r^f)rabdng or re^^c^g
mediom, when they ife either b^nt At csrtafn angles, or returned
in the same lines in which tiiey advanced ; aiS thus^ ifHien a
hmiinous raj is reacted or refected it siill piroceeds in a right
line, tikongh that line may not be pandlel^with its original direct
lion.

170. If th^ oolovred im^ge obtained by mettns of t gla«9 prisih hk
cortended longitadinally, by making it first poto tiirongh st convet
Itoa, it will be foand tAat the rays of diilbrent ooloni^ pbds^s de-
ferent degrees of refrangibility, Or Aie some relatively mere lefittcted
than others. The red tint which forms tlie hifeiior band of ihe
spectrvm prodnoed by tiie apparatus abote described^ e0nftiete ef
iays which have undergone a smallfo ^t'^gvee of refiAietion Uiaii tteee
which eonstitnte the orange tint^ the latter are faometi^h^Brore !#>
fracted than the yellow rays, ilie rsi^action ib greater in the other
rays suocessttely, and most cdnsiderable in Hie Vielel rays^ whidi
therefore form the colonr of tiie superior band 6f tiie speetirdift.

171 . If after the solar spectrum has been rendered mote distinct^
by subjecting a beain of light to a lens befoi^ it is telHcted by
the prism, an aperture be made in the screen^ on which it is re-
eeired, opposite to any one of the coloured bands, a snudl j>encil of
light similar to that of the band will pass tbroagh, and if it be
refracted by a second prism, and even again by a thirds and then
receiyed on another screen, U will not be decomposed any fnrtiier,
but will produce a circular imure of a uniform colour correspond-
ing with that of the band of Which it originally formed aportion.

178. By a similar method Khe various refimngibility of the dU^

In what eircomitanoes in regard to direction do rays of light resemble

iMrrenti of Krand ?

What directions do rays always pursue after refraetsMi >

How are the diflfereatly IM>loiired tuft found 16 be eonstitoted in teteeet

»lo refrangibility ?

Which colour is the leaat refrangible ? which the n^tf

What effect is prodiieed on arty onfe of the cdodred bandk of the Mfeo-

Itatimj by an attempt to decotnpose it by S second refraction f

* Se^ Treatise 00 wicmMAic*, No. &8, 69.

t £uler*s Letters to a German Princesfc Ed. 1888. vd. i |k. tH

CBBOHATXG BXFVIUCTION.

ferently oolduied rays maj be farth^ demoiurtrated. For this
purpose It is only necessary to make the first prism revolre slowly
on Its axis, and thus each part of the spectrum will transmit its
rays successively through the aperture in the first screen, and
form a succession of little circular images on the second, traced
at different heights, the violet colour appearing strongest, the red
weakest, and Uie intermediate tints varying as they are nearer to
one or the other. Thus may be produced an ascending or descend*
ing procession of images on the screen, by turning the first prism
ifi one direction or the other.

173. Rays of light, when reflected, exhibit properties analo-
gous with those w^ich they show when refracted ; those coloured
rays which are most refrangible being also the mostreflexible ; so
that the violet is reflected more readily than the indigo, the latter
more than the blue, and the others in succession becoming less
and less reflexible, and the red least of all. This order of reflex-
ibility explains in some measure the azure tint of the heavens ;
for the atmosphere being a reflecting medium, those rays most
subject to reflection, namely, those which are violet,- indigOf
and blue, are reflected most abundantly, and hence the appearance
of the unclouded celestial vault. To a similar cause is to be attri*
buted the bluish tints of distant mountains, often imitated with sreat
effect by landscape painters, in the dfsplay of aerial perspective*

174. As the decomposition of solar light into variously coloured
rays may be exhibited by the means already stated, so there are
methods by which white light may be composed by uniting
the different colours of the solar spectrum into one image. This
may be most perfectly effected by causing the 8pectn;im to pass
through a convex lens, and receiving the imajge on a card or screen
placed in the focus of the lens, where a circular disk of white
fi^ht will be formed. A mixture of seven powders tinted as the
pnsmatic colours, and in the proportions of the breadths of the
several corresponding bands will produce a whitish compound ;
and seven coloured wafers fixed at proper distances .on the border
of a circular piece of pasteboard woi^d, when it ^as whirled
round, on a pm passing through its centre, display a wheel or
i^ircle of a hue more or less approaching to white.

175. InsteM of reuniting^all the rays of the prismatic spectrum
by means of a lens, eertaiiT parts may be united, a screen being
placed to intercept the others, when tiie image product will not
consist of white light, but of particular tints resembling some
of the simple colours of the spectrum. Thus the yellow and the
blue will form a green, the yellow and red an orange, or the red

How may the degree of refrangibiliCy belongingXo each colour be veri-
fied by a seoond refraction ?

What order do the colours follow in the degree of their reflexihility ?

How is this principle applied to explain atmospheric and other coloura f

How may a beam of white light be formed oat of the constituent ele-
ments of different colours ?

How may the eompound ooloars be formed from the parts of the spe^
trum?

9x2

W9 0VTK8;

tnd Mae a violet tbi ; bht Ih^^e >urtificial eohmn bsn diiMuigiliebaf^
Ue from llie original cdoiirs eorreepoBdisgr with ihem Iby ihstlt
■useeptibility of decompositioD into their constitaent parts wbea
tansmitted anew through a {iriem. However, though eertain
colours thus reunited £nn coloured mirtines, there are other
colours, which when united by a second tiansmiesion through a
conrex lens reproduce white light ; and these are termed complex
ttentary colours.

176. As the colourad rays that coaaipose white light have difie-
lent degrees of refrangibility, it follows that ^ pencil of rays df

liflhty as £ F, in the marginal figure,
JL fiming on a leiks A B, the riolet rays

A b^ig &e most refrangible will bo

2> * C M ^ brought to a fobus sooner than the

I L 5 other rays, as at O, and the red rays

^^ ^ whibh are the least refrangible wil,

^ • meet in a Ibens as at D, lite inter

mediate rays meeting at relative dis^
tanees between those two points. Hence the images which ins
ftnrmed at the focus of a lens will be surrounded by raiious colours
eoB^tituting what is termed ah iris. It is likewise obvious that
those raTS which enter the letis near its border will be more dig*

ried than those which pass tiirough near its axis. Time when
object is viewed through a single ffkss lens, the pftrts of it
most distant frmhi ^ centre will be deformeid, and as just ob»
Serred surrotinded by a coloured irndiation.

177. It has been i^own tiiat rayis of light in passii^ throosh
diffbrent tmntparent mediums are more or less refracted br^f»
▼erted from their original directioh ; and that the degree of refrac-
Ison which IL^ht undergoes Tsries with the medium trough which
It passes. The mamier also in which light is aflTected by being
made to trarerse a refracting substance the sides of Wluch are not
parallel to each other, as a prism of glass or crystal, has beea
staled ; and the phenomenon of the production of coloured light,
iirom the disrpersion or anaSysts of the rays of white light has been
generally illustrated.

178. Sir Isaac Niewton, in making his original experiments on
solar light with n glass prism, ascertalnra that ^e relative
|»ieadths of the colouied spaces in tHie prismatic spectrum, sup*
posing itis whole extent to .be dirided Into 360 parts, would be.
fbr Ike red stripe, 45 parts; ?ortiie orange, 37; fer the yeUow, 48

%iVe sdme example^ df thh tnoSe of prodocing Sotettaediflte eOlour •

Whsi we nesBt by eomplelaenlaiy. colotirt ?

What effect has the difference oT refrangibility io the rays of Ugbt mr
6a the doltitir of an Untige Ibrim^ behind a lent f

With what do such images sppear to be sorrojanded ?

What«ireura8tafice «stises varistaims ia the 4sa;ree of rrfnctjoa Mif*
fered by light ?

WhatfMtsdid Newton 'd&sM^er ia rer •« t» the tdbtive braiM lof
loiar Inads in the ^eetrom?

DigpxBsivc PDVBU or DirnRENT Bosnfl. Sn

£)r the cfMii, 60; tot the blue, M; for A* indigo, 40; aid fer tM
violet, BO. These niimbera, bswever, are by no meuis oonnant,
depending- oa the peculiu natiire of tite refiactiiig bodf emplc^ed
to dissect B ray of wfaite light; and it ia a Twf BJngtilaT ciieciBt-
slsiic«4 that though Newton made ass of pnioui ctnnpoMd of-dif-
feient •ubatances in ths prOBeeudon of his leieuchei, VM bo n^
fleeted to obseire that the dispersion t» divergence of ibe diffb-
lently coloured rays ia gieater when produced by one refisetii^
medium than by Eiaolher: he therefore ernmeoHBly coDeleded that
the refractive and dispersiTe powers nf bodie« always coireapon^
ed; but that is far from bein^ Uie case.

179. The effects of a coDSiderable number of transpsMOt aolidft
and Suids on rays of aqlar light tiaBsmitted through thera have
been ascertained ,by Sir D. Brewater and other scientifte iMnirera^
and tables of theii diBpersive powers have been oonatracted, fioA
which it appears that (h* dis|Mr«i*e and the refractive poweM ol
various bodies held do definiie proponioB to mtA other.

leo. The effect of prisms compeaed of different refracting anb-
atances on a ray trf white li^ may b« explained by reference to
the following figures. Oi) fflFcesaia and sulphur*

Suppose A B to repreaeBt a ssectram, aach as would be ibimed

by IranaiBittiiig a k4« t«j Ihrou^ a prism having its sides of

glass, and filled wi' " ' * "*

A' B', a similai spectTom produced by a
prism illed with sulphuric acid. Then
It will appear tbat the least refrangible
colours, red, orange, yellcte, aumbere*
1, 3, 3, will be more contracted in th«
epeetrmn A B, or that fenned by die <^
of cassia, than in A' B', or the epeetram
bnned by sulphuric acid; and that thb
moat refrangible edoura, blue, indigo,
f ioldt, &, t, 7, will be more expanded
in the forBMir spectnnn than in the laV-
ter: Ibe centre sfoae spectram tying
Just within the blue space in the Hit*
C C and ia the other that Hoe dividing
the green apace, but less unequally.
Hence the coloured spaces bear not the
same proportion to each odier as the lengths of the spectra; and

Do (he namberi dbtained hj bim ifipar t» be Mmitant for all rcfraol-
ini bodiei I

Into what error «ai he led ia ropeM to (he diiperiiw pow«r of re-
fnetiug media I
' What general ttitemeni i< true in rewd to Ihe refraetiK aad the dit>
perurc poiren of dilierent nibuaiiee* T

Conitraet ind eiplain ihe diigram iRaMrati(i|; the diflercnee between
the dtaMnlfe power oTapriimoTaulphurie Hid and

In WiMi enlBarli Ibe

r h'Snoi

SiAe Mectnifh fanned ^ die acid /
that taratti b; the Uir

m^ omcfl.

thit want of eorraipondence mi tenned the irrationality of disper*
sion, or irrationality of the coioared spaces in the spectrum.

181. On this inequality of dispersiye power among transparent
bodies depends the construction of those optical instruments called
achromatic,* or aplanatic, t telescopes ; by means of which tbe
coloured fringes and other defects in images formed by a single
lens are removed, or prevented from interfering with the distinct
observation of the object. Hie principle on which this is affected
is by combining together refracting substances possessing diffe-
rent dispersive powers, in such a manner that the aberration caused
by one shall be counteracted or neutralixed by the opposite effect
of the other.

182. Suppose a compound prism A B C, as represented in the
diaffram below, to be formed by joining two prisms A C D, and
D C B, composed of the same substance ; then if a ray of white
light be made to fall on i% an image of the spectrum will be ob-
tained, the colours of wiiich will be less distinct than they would

have been if the prism A C D alone had been
3 employed, because the light decomposed by

the first prism will be partially recom-
posed in passing through the second ; and
It would be entirely so, if B C were pa-
rallel to A B; because then the second
prism having the same dispersive power
as the first, would be so placed as to coun-
N\ p teract its effect. If, instead of employing
^ two prisms of the same kind, one of the
prisms D B C, be composed of a substance the dispersive power
of which is much stronger than that of the other, ADO, the rays
dispersed in passing through the former prism will be re-collected
by the second, and an achromatic prism will thus be formed* If
the refringent power, likewise, of the prism D B O be the same
with that of the prism ADO, the refraction will not be corrected;
but if the refringent power of D B O is the greatest the refrac-
tion will be in some degree corrected.

183. It is practically impossible to form a perfectly achromatic
prism, because the dispersive power of different transparent sub-
stances differs with respect to the differently coloured rays. But

What is meant by the irraUonaBty of dispertitm?

What advantage is deriTed from the knowledge of dispersive powers
in the oonstruction of optical Instruments ?

Explain the terms aenromatie and aplanatic.

How may the dispersive power of a prism be counteracted }

Does it necessarily follow that the refractive effect most be overeome
by a second refractor which restores tbe white colour of a previously re-
fracted beam ?

Why can we not form perfectly achromatic prisms ?

* From the Greek •yfirivcUiix, and Xfaft** colour : without colour.
t From », privative and nx»vii, error or deception : without error.

ACHROMATIC ftENSES. 9M

^e effeet ibay H nrodfie^ With vegwA to Any l#o eokvn ; asd
it is therefore usual to talte tlto ext^mes of tlM apeetnifti, namely
the ped and riolet, or the ted and bine.

164. If, as Sir Isftac Nelrton too hastily eonelnded, the dispel
site property and tiie refrin^pem poiirei had iat all substanees in*"
ereased and diminished in the mme ratio, fhp refraetiott, as well
as the dispefaion prodneed by cHEie substance would be counter*
acted by another : the rays emerging after th^ doable refraelion
would thud beeoBte parallel. It would henee hare been impossi*
ble to achtomatiae a lena) for if a compound lens were formed by
vnitinr a eOnyex with a eoncaye fflaas in such a manner Uiat the
rays should become depriTed of cMOUr aAei: issainf from it, the^
lirould resutoo their original parallel direction: that is the colon-
vatien and the refrac^a proauced by the -ceayex slass would ba
destroyed both together by tbe. coneave one, and the rays eoald
not^ therefore, be united in h leeus.

186. Thfe e^ect of single convex lenses in itroduoing an India-
linet image with a 6oloafed fRB|e» termed Ooromatic aberration,
|»reTiottsly noticed, will be sueh £at, **f)r paralldl rays, the circle
ef least chtomatic aberrationi or of least cdoar, will have the
stfme absolute magnitude^ whatever be the focal len|fth of .the lens,
provided the aperture remains the same. Now since in a tel&-
acope (with a giyen eye-glaas) the image is nAgnified in prop<»w
lion to the ibcsd leagih of the ol^eot-^lass, it fcmows, that by in-
. ereasin^ the focal Tonglh the magiHtude ef the image mereasest
-whilf^ Siat^f the coloured bofder remains the same 2 by eoa-
tinuittgt therefore, to lAoredse the focal length we^get an image so
laaeb magnified that ^ Mour bears an insansble prcj>ortion to
it. Hence, as lon^ as aimple l^aaes only wore aseai in order to
ootrect the aberrations and aeoure a due quantity ef light, it waa
necessary to have telescopes of very unmanageable length. Soma
of those ceastnicted by Huygheas were of one hundrra and eten
ene hundred and fifty feet fowl Iraigth."*

186. Newton, after numerous attempts to render refracting tele-
Bcopes more portable and effeotive, found himielf foiled, in conse-
<|uehce ef the incorrect opinion he had formed coneertiing the
e<Nrreapoadence of the dispersive with the refringent powere of
bodies, and therefore infenring that *^the improvemOnt of the re*
fracting telescope was desperate,''! he devoted his future lattention

WhM woidd hkte bteen ttie ittbeess of sufih an attempt bad Kewton't
tHppoBiiion been trot }

What is the effect of a ahirle eon^x lent in prodoeing an Hnaee ?

In what proportion to tha focal length of the objeet-gli^w will be the
magnifying power of a teleaoope ?

What ineonTenience arose from the use Of simple lensw in the eon*
rtnictitm of nefraodng telescopes .'

* Short T11eraentat7 Tfektise on Experimental and Mathemhtleal dp*
tieS6 iBy the Rev. Baden Powell, M. A., P. R. B. Oxford, itel p. 101
t Optica. liond. IZQl) 4to. Part I. Prop, vii. Theor. 6.

304

OPTICS.

\

to the c4ni«traet}on of telescopes of a different kind, in which die
images of objects are formed by reflection.*

187. But that which was despaired of by this great man has
fortunately been accomplished by others. The merit of having
first discovered the method of forming an achromatic refracting
telescope appears to be due to Mr. Chester More Hall, a gentle-
man residing in Essex, England, who, abont 1733, had completed
sereral achromatic obiect^^glasses, by the combination of lenses
of different kinds of glass, having different degrees of dispersive
power. In 1747, Leonard Enler published a paper in the Me
moirs of the Academy of Sciences at Berlin, on the improrement
of refracting telescopes. Tliis attracted the attention of seve
ral philosophers to the subject, amone whom was John Dollond,
an eminent optician in London, who having fully ascertained the
diversity of dispersive power in different substances, found that
an achromatic lens mi?ht be formed by joining together crown
glass, or that kind with which windows are glazed, with flint
glass, or that of which cut-glass vessels aind ornaments are made.
168. Peter 0ollond, the son of the optician just mentioned,
made a further improvement by forming an object-glass of three
instead of two lenses, including a concave lens of mnt fflass be-
tween two convex lenses of crown glass. It is now, however,
most usual to employ only two lenses, one, partly concave, of flint
glass ; and the other, double convex, of crown glass. These
glasses have different curvatures, and are formed in such a man-
ner that, after refraction, the red rays and the violet rays will be-
come reunited at the same point : the intermediate rays will also
bo reunited at nearly the same point, it being impossible to reunifia
all the rays in precisely one pomt (as above stated), though they
may be made to approach it sufficiently to prevent the nsnal eflRset
of chromatic aberration.

189. In the marginal figure, represent-
ing the section of a compound lens, the
ray E F felling on the centre of the lens
A B, will pass through it without any al-
teration ; but another ray G H falling <hi
one side of the centre will be divided, and
the violet ray, as being the most refrangi-
ble, will pass through the convex lens in
the linO H I, but the red ray in the line
H K. The concave lens of flint glass
will make both these rays diverge from the
axis E F, the red taking the direction K L,
and the violet the direction IN; and in
passing again through the air to the focns,
the red ray will take the direction L F, and

Who first sueeeeded in overeoroing this difiicultj } What two mate-
rials did Dollond employ for the formation of achromatie lenses?

* See Mibsequent part of this Treatise, ivlatiTe to Optical IhstmmenU,

CAU8B OF TU BAINBOW. 985

tfaa Tiolet the direction N F : both consequently will be remiited
at the point F.

190. Achromatic lenses thas constnicted are still subject to im-
perfection, and hence subsequent attempts have been made to im-
prdve them, which hare been attended with a considerable degree
of saccess. Dr. Robert Blair, of Edinburgh, between 1787 and
1790, having conceived the idea of employing transparent fluids
in the construction of compound lenses, at length succeeded, by
inclosing muriatic acid, properly prepared, between glass lenses*
in forramg an object-glass by which the differently coloured rays
were all li^nt from their rectilineal course with the same equality
and regularity as by reflection. Other experimental philosophers
have occupied themselves in analogous researches ; and especially
Mr. Barlow, of Woolwich, who £is been very successful.* To
the instruments thus constructed it has been proposed to apply the
term aplanatic^ or free from error, as possessing the utmost d^;ree
of accuracy.

191. Amonff the most interesting natural phenomena is that of
the rainbow, &e production of which wholly depends on the re-
fraction and reflection of the sun's rays by clouds or drops of rain,
and the consequent formation of prismatic colours ; and the sub-
ject may therefore here be properly noticed. The bow in the
heavens, as the French correctly term it ^rat>o*«i«-c»e/), is seen
when the sun darts his lays on a cloud dissolving in rain, and the ob-
server places himself opposite to it, with his back turned to the sun*
Sometimes one bow only is perceived, but more usually there are
two bows, the interior or lower one exhibiting brighter colours
than the other, the tints of which are comparatively pale. Both
present the colours of the prismatic spectrum ; but in the interior
DOW the tints gradually ascend from the violet to the red, while in
the exterior bow the violet is most elevated. Some writers remark
that a third bow has been observed, but yery rarely ; and accor-
ding to theory many bows must be formed, though all beyond the
cecond must, in general, be utterly imperceptible.

192. The colours of the rainbow are the result of the decom-
petition of white light, in its passage through the fflobular drops
of water forming a shower of rain. Each coloured ray produced

In whnt manner is it customary, at present, to^irrange the parts of an
aehromatio lens ?

Of what material is the double conrex lens composed ?

Which two rajs are brought together at the focus of an achromatle
lens ?

Trace in the diagram the respective courses of the red and the violet
rays.

' what method was adopted by Blair and Barlow to form their aplaoatie
iostniments ?

On what principle is the rainbow formed ?

In what order are the colours arranged hi the double rainbow ?

Whence do the colours of the celestial bow proceed ?

* See Philosophical Transactions for 1828 $ or Abstract of Papers in
PhiloSiTffanfitVoLil. pp.dM»Mi. ..

bj ^m dacompathiflii twraiatti tiie jg^bbid0« Mid U reteled m
part at the opposite concave surface ; it then traverses the globuls
a^in in a new divection^ and fireaieats itself to escape towards
the observer. A part only, iioMrevier, actually passes -out, and th«
other part is again refle<sted and carried back into the iBierior of
the globule. In this manner a multitude x>f successive reflections
may be cansed, at each o£ which some portion of the light will
tacape, but its intennty becomes more and otore feeble with the
increase of the number of reflections.

193. It is from those rays that thus first issne from the drop on
the side towards which the observer is looking liiat the effect is
produced. Tlie lays wluoh pass oat £tQBi a^obule after havinjf
snffiBred one or mofe lefleetions Ibcm a certain an^e with their
primitive direction. This angle is constant for aa rajrs of the
same nature that pesetiate the globule at the same ineidence, and
which undergo within it the sapie number €£ reflections ; but it
varies for those rays the incidences of which are difiesent, and
which undergo a greater or smaller number of reflections.

194. It wul appear ftom calculation that in a series of parallel
mys of the same nature, which fall on a .globule, and which un-
dergo but one reflection within it, that the angle will be sncce*'
sivSy augmented, irom the nonifial.or diseet ray, at which there
will be no angle,* to a certain limit, beyond which it will decrease
till the ray becomes a tangent to the sphere or globule. Hence
within those limits, the parallel rays entertng the globule venr
ttear together, and undergoing deviation not very diseinular, wiU
vemain sensibly parallel at their escape: a^d therefore an eye
plaoed in the direction of such a bundle of rays will be affected
with a sufficiently vivid sensation of colours ; while elaewhero
encountering onl^ isolated rays, the sensation produced will be
extremely inconsiderable. The rays which thus issue from a fflo-
•bule so as to form a small bundle capable of making s sensible
tmpiession, are termed sj f k aei oua fay«.

195. It is the same wiih regard to rays which undergo t^o le-
4sctioa8, or a greater number, within the globule. There wiU
always be certain limits within which severm parallel rays, nte
together, issuing from the globule and remaining sensibly pwallel,
will produce a distinct sensation on the eye. These limits are
'not me same fbr all kinds of coloured rays, but vary with their
refrangibility. Thus, wUh respect to the red J»y8, which are the
least refrangible, when the ray issues after one reflection, it makes
with the incident ray an angle of 42® 2' ; this angle is succes-

How many reflections occur before the light leaves a drop of water ts
^f9^ to the eye of the obtprver in oofiatituiipgUie priiaftry taiabov f

What general fact can be stated in regard to the angle formed by n^
passing out of a drop of water jifW uiMM:gQi4ig ooS or more r«flectioAs ?
What U nicant by tba mrmal ray in a oi^m of light }
Which of the rays reflf oted from rain<.drops-Ar« termed tjficadmmf
What causes the di fferen c e in t h e p o siti on of the asvenl sva of {^Eob*

What angle with the incident ray,

IRIDISGKNT eiRCLES. fiT

fiirMy snudler fot the otiier cblonred rays, to tfa^ violet, wluteh If
the most refrangible, and for which it is 40° IV, When <he emer-
^nt ray uadergoes two reflections ia the interior of the globiui^
3ie limit in the case of the red rays will be at an angle of $0® 57't
and in the case of the violet rays, at an aiiffle of 54^ 7'.

196. The formation of the coloured bands of the rainbow mmf
be thtts explained : the sun, considered as a aiq^ple luminous point
at an infinite distance^, transoits to the shower a handle of ray^i
of which each gloibule of ws^er receives sooie. Hence on each
of the globitles fall s<»iie efficacious rays, which pass off* to d^*
servers at different points. Thus, the first coloured niy which
ean come to an observer, alter a single reflection in the globnilo,
will be that which makes the smallest angle with its. ojriginal dire<^
tion ; and will therefore be the violet ray, of which the angle
is 40° 17'.

197. All Ihe globules sitnated in the same circle, to the centre
of which the axis of the bundle is incident, will produce the same
sensation, and consequently fonii the first coloured tine. The effi-
cacious red rays which fqim with their original disectioo an ^ngle
of 43^ 2', will produce the last or highest line of the first how ;
and between these extremes there will he the five other colours of
the prismatic spectrum, in the order of their refrangibility. Such
is the mode pf formation of the first or principal bow : its dimen-
iftions will be the difference between 40^ 17' and i2^ 2', and thei«-
fore equal to 1^ 45'.

196. Beyond the red rays the observer will only perceive those
which have undergone two reflections, and their intensity will con-
sequently be more feeble. The first rays in this case will now
be the red^ which make the smallest anrle, equal to 50° 57' ;
forming the commencement of a secondary oow, at a dista^nce firom
Hhe first, .corresponding with the difiTerence between 42P S' and
60^ 57', and therefore equal to S^ 56'. The last or highest line
of the second bow will be the violet, of which the rays will make
with their original direction, an angle of 54° 7', and between thes0
extremes will he found the other colours! The dimensions of .th^
second bow will be 54° 7'-^0° 57'«=3° 10'.

199. It may readily be understood from the preceding ohservfr-
tions hew three or more bows may be formed by successive re-

What angle is made by the ejficaciotu violet with the incident my, after
a tingle reflection ?

What are the angles req>ectively for these two rays after nndergoiii|f
two reflections ?

How is the formation of coloured bands in the rainti^w explained f

What' is the necessary limit \o the breadth of the interior or pnmaiy
Jjtow ?

What rays will come to the eye of the spectator beyond the red ray of
that bow f

What order of colours will be observed in the exterior bow ?

What is the breadth of band between the two bows ?

What is the breadth in degrees of the secondary bow ?

3 L

898 OPTICS.

flections ; and why also they must be too faint to be perceptible.*
We have supposed the son reduced to a luminous spot, and thus
a circular line only of each colour would be produced ; but as the
sun has a sensible diameter, it follows that each band of the bows
must have certain dimensions dependingr on the apparent diame-
ter of the sun.

200. Lunar rainbows occasionally occur, but in most cases they
are faint or colourless, from the inferior intensity of the moon's
reflected light* Coloured halos are also sometimes seen, but they
are amongr the more unusual meteorological phenomena* Clouds
of rare clours, as green, have been noticed by some observers ;f
and the effect ma^ be traced to the same causes with the more
frequent and beautiful rainbow.

Why are not lunar bows generally distinguishable ?
What other phenomenoa anaJogoas to the rainbow is occasionally ob-
served in the atmosphere ?

* Under peculiar eireumstanees, more than two rainbows, or rather
iridtscent circles, may be formed by the refraction of the sun's rays, so
as to be distinctly risible. A remarkable instance of such a phenomenon
is related by Professor Winkler, from ^'L'Histoire General des Voy-
ages," as occurring to the French and Spanish philosophers who were
employed, in the last century, in measuring a degree of the meridian, in
Peru. ** As Don Antonio dfe Ulloa was with the French Academicians
on the high and desert mountain of Pambamarca, in the kingdom of
Quito, eaim of them saw his own image over against the side on which
the sun rose, as in a mirror, and the head of each image encompassed
with three rainbows, having all one and the same cei>tre. The last or
outmost colours of the one rainbow touched the first of the following.
And externally, round all the three circles, but at some distance from
them, a fourth bow appeared, which showed white only. When one of
the spectators moved from one side to the other, the whole appearance
followed him, in the like form and order. And though the observers
were six in number, and stood quite close together, yet each could see
only his own image, and not those of the others. As the figures of their
bndies were portrayed in the middle space of the encompassing rainbow,
the vapours of this space must have been in the state for the incident and
reflected rays to form equal angles." — Elem. of Mxi. Philoi. 3eUn., vol.
ii. pp. 63, 64.

^ t The following instance of the occurrence of this phenomenon is de-
rived from a diary kept by the person who witnessed it : December S4,
1812. ^ Just before sunset, I observed a line of clouds, situated above the
sun, tinged of a most beautiful pea-si*een colour. Above and below the
gi*een clouds were situated clouds ofa dusky purple hue, intermixed with
narrow stripes of orange. As the son was sinking below the horizon the
green belt of clouds became gradually lighter ; and when the orb of dsy
ceased to be seen, the green tinge also vanished. This appearance coq-
tinued about a quarter of an hour."

Mosschenbroek notices the occurrence of green clouds, and observe*
that ** such were seen by Frezier, and are described in his ' Voyage to the
West Indies.' "--JS/bn. o/JVo*. Fhiios., tninslated from the Latin, bv
Colson. 1744. voLii. p.241.

COLOURS OF TBXN PLATES.

Colours of thin Plates,

201. The phenomena of coloured rings observed in the simple
f^xperiment of blowing* bubbles of soap and water ; in thin films
of oil of turpentine or other essential oils floating on water; on
the surface of polished steel when heated ; and in general in thin
jilates of transparent substances, as quartz, Iceland spar, and mica,
are extremely curious, as exhibiting a peculiar mode of the de-
composition' of white light. It may be most conveniently studied
by examining what takes place when a very thin stratum of air
or any other fluid is confined between two plates of glass ; and
the experiment may be advantageously made by placing a convex
lens of small curvature upon a concave lens of a radius somewhat
greater, and on pressing them together the colours will appear
arranged in the form of rings round a central spot, which if the
pressure be sufficiently powerful will be perfectly black, when
viewed by reflected light, but when examined by transmitted
light, as by looking at the sky through the glasses, instead of pla-
cing the eye between the light and the reflecting surface, the cen-
tral spot will be white, and be surrounded by rings, the colours of
which will be complementary to those seen by reflection. Jlence
it appears that the colours seen by reflection and by transmission
of white light through thin plates, are those which form the great-
est contrasts with each other.

202. The coloured rings are seen in the following order, pro-
ceeding from the centre to the circumference, forming different
series of tints.

Colours of thin plates viewed by reflection.

Ist series. Black, blttej white^ yellow, orange^ red,
2d series. Ftolet, blue, green, yellow, red.
3d series. Purple, blue, green, yellow, red,
4th series. Bluish-green, red.

Colours of thin plates viewed through the glasses.

1st series. White, YeUowish^^, black, violet, blue,
2d series. White, yellotv, red, violet, blue,
3d series. Green, yellow, red, bluishrgreen,
4th series. Bed, bluish-green.

Under what different circumstances are transparent bodies capable of
exhibiting coloured rings ? \

How ts the appearance most conTenientlv studied .'

In what different positions must the eye be placed with reference to tiM
glasses to observe the two diffei*ent classes of phenomena ?

Of what colour is the central point in two conjoined lenses when it is
viewed by transmitted light ?

What difference do reflected and transmitted light respectively produce
on every ring of the coloured surface ?

What is the order in each of the four series of colours when viewed by
reflected li|;ht ?

What is It when viewed by transmitted light ?

400 6Pt1x:$*

303. These tints are none of them identical with the simple
prismatic colours. Sir J. Hersehel observes, respecting the re-
flected cc^ours, that ** the green of the third order (or series]) is
the only one which is a pare full colour, that of the second being
hardly perceptible, and of the foiurth comparatively dull, and ver-
ging to apple-green ; the yellow of the second and third orders
are both sood colours, but that of the second is especially ricb
ind splendid ; that of the first being a fiery tint passing into
orange. The blue of the first order is so faint as to be scarce sen-
sible, diat of the second is rich and full, but that of the third
much inferior : the red of the first order hardly deserves the name,
it is a dull brick cdour ; that of the second is rich and full, as is
also that of the third ; but they all verge to crimson, nor does
any pure scarlet or prismatic red occur in the whole series.*'*

204. The breadths of the rings are unequal, becoming narrower
and more crowded, as they recede from the centre ; and the extent
of the rings or eircles denends on the curvatures of the irlasses
between which they are lormed* In order to make experiments
with accuracy a proper apparatus is requisite. That used by Sir
Isaac Newton, m experiments on this subject, consisted of a
plano-convex lens, the radius of the convex surface of which was
twenty-eight feet, and a double convex lens, the radius of whose
surfaces was fi^y feet ; and the latter bein? placed on the convey
surfiaiee of the former, they were held togemer, with any required
degree of j)res8ure, by three pairs of screwsi fixed at equal inter-
vals on their borders.

205. The colours may be shown by reflected light, by pressing
toother with the fingers a concave and a convex ^ass slightly di?
fering in oarvature) that of the former having the largrest radius ;
but it is impossible by this means to maintain equable presBOre,
and the figures become distorted from circles into irregular ovals,
or angular lines. He^ce it will be obvious that no correct obser-
vations can be made on the dimensions of the coloured rings, unless
the glasses can be subjected to uniform pressure. It is also ne-
cessary that the eye of the observer should always be similarly pla-
ced, or at the Same angle of obliquity; for if the obliquity be changed,

How do the colours correspond with those of the pHsmstie spectrum ?

Which of the reflected colours is i>erieet In its Ithid f

What relations have the breadths of the several rings to their distance '
f^om d)€ eomiaon centre ?

On what do their actual breadAis depend ?

What were the forms and curvatures of tlie lenses used by Newton in
cxperimcnd On coloured rings ?

In what simple manner may the rings be exhibited bj reflected li|j^t ?

How must the apparatus be arranged and secured, m order to a aor-
rect appreciation ot the nature of the rings f

What preeautioo is aecessatr in regard to the ere of the tpeotator ?
Why ?

* Encyclopted. Metl«|^lir-*-MtKed Sciences, voL ii. p.4€&

COLOTJBSD BIN08. 401

hr eleratlAgr or de^iressingr the eye or the glasses, the diametois
(but not the col oars) of the rings will change.

206. It is of importance to the explanation of this phenomenon
to ascertain the thicknesses at which the respective tints, or the
several points of greatest brightness and greatest-obscurity occur.
This may be done by means of Newton's apparatus ; for the co-
loured rings being perfectly regular, by exactly measuring their
diameter, may be found the thickness of the plate of air corre-
sponding to each of them ; for the interval between^ a plane and
a spherical surface, the centres of which are brought into contact,
will increase in the ratio of the squares of the distances from
that point of contact.

207. Hence Newton found that in the most brilliant parts of
the circles the thicknesses followed the progression 1, 3, 5, 7, 9;
while in the darkest parts, commencing with the centre, tiiey
followed the progression 0, 2, 4, 6, 8. The same philosopher
ascertained that when water is substituted for air between the
glasses, the proportions of the diameters of the rings will be the
same, but they will be relatively smaller; whence it follows that
the plates of water reflecting any ffiven colours must be more at*
tenuated than those of air. It iuriher appears that glass plates,
to reflect the same colours, must be thinner than those of water ;
and it may be generally concluded that the thinness of the plates
increases in proportion to the density of the bodies of which tiiey
are composed, or rather in proportion to their refracting powers.

208. Air in a plate but half a millionth of %n inch in thickness
ceases to reflect light ; and the same is the case with water at
three-eighths of the millionth of an inch, and with glass at one-third
of the millionth of an inch. A plate of air two millionths of an inch
in thickness, exhibits what Sir I. Newton terms ** the beginninff
of black." A plate nine-millionths of an inch reflects the red of
the first circle or series; one nineteen-millionths of an inch the
red or scarlet of thie second series ; and tables have been calcu-
lated of the thicknesses of plates of air, water, and glass respec-
tively, for each colour of each of the four series given above, and
also for those of three more series, which may be observed ex*
tending in narrower circles beyond the preceding. Air seventy-
How may we aaeertain tlie thickness of those plates of air which pro-
dace the different rings ?

In what ratio do the distances between a plane and k sphere to which it
touches, increase from the tangent point ?

What did Newton ascertain to be the relative thicknesses of the strata
at the brightest parts of the rings }

What prbgression of numbers represents the thicknesses of the darkest
riofrs?

What law did Newton find to prevail when the spaee between his len^*
•es was filled with water instead of tdr?

What result was given by ffkus ?

What general conclusion was derived from a trial of various sab-
stances ?

How maeh must the plates of each substanee be dinuDished before thqr
Xote the power of refleetlng Ught ?

2l2

wreB-oiilUoiiUiB of tn iucli m thiekneiB neiieets ;i nddl»h«wliit9
colour, fonnin^ the boattdaiy of tlie ser^nth or outer circle; and
Veyond thttt thickness it reflects auite white or undecomposed light.

509. As to the csnse of the coiovred rings fonned by thin plates,
NewtoB proposed an exj^anatien fonnd^ on the doctrine <^ the
emanation or light as consisting of molecules traTersing* spacer
and his hypothesis is deserTiB|r of notice as being in some degree
applicable to the undulatory dieoTy, which represents tight as
sensing fipsm the TibratiQins of aiA ethereal medlam.

510. Having ascertained from experiment thait Uie different r(sy%
beccfme reflected at the snocessite thicknesses 1, 3^ 5, 7, &e., am
transmitted on the contrary, at the intermedial thicknesses 0, 2,4,
6, ^., he regarded these laws as resnttiiig ttmn a parttcoler dis-
position of lihe ivminons mdecnles, whi^ he denominaded *^ flti
ef easy reflection," and ««flts of easy transmissien." Tlins he
Concluded iJiat any ray would ^be thrown into a fit of easy reflec-
tion <on fidlingen a plate the thickness of whieh was one of &e terms
of the series 1, 3, S, 7, 9, dec. ; 1 being the first cor least thldniest
M which it became susoeeiible of bmg reflected ;^ and on the
eHiier hand, a ray wonld be m a fit of easy transmissioti when the
tkidcness of the phto wais <»ie ctf* the terms c^ the Miies S, 4, 4^

^l i. Thus fu Newton^s hypofliesis is little more ihan an enun*
eiatiain of facts, but he also conjectuved that Hre fits ef easy ie>
flection and transmission mi^ht depennd on a sort ef magnetic po»
larity beloaging te the particles of light ; to which supposition,
howeirer, it does not appear that the illuMious aulhor htmself
attached any mat imponan6e.

S18. According to the undulatoiy tbeonr both of the enrlhces
t>f ^e ^ihi lamina are concerned in &e proauOtion of thec^^oors ;
and the interference of the ligfht reflected from tlie second surface
with the light reflected from the first iolerrnpts or fhcilitates the
passage of the ray at pertain intervals of tiiickness of tibe j^late.* ^

813. **The Cflleurs cf natural bodies in general are the coloars
bf thin plates, produced by the same tfause Whicfh produces them
in thin laniinaBof air, iflass, &c. ; iris., tiie 'interval between the
anterior and posterior surfaces of the atoms, which, when -im odd
multiple of half the leng&of a -fit of easy refleotion and tzans-
mission for any coloured ray moving within the medium, obstraets
its penetration ef ihe second sorfitoe, and wlien an even, ensures
it. The thichneas^ therefore, of tiie atoms of a medium, land rf

What WRi Kewton's^explanatiaB Afthe MQae of ioolopred rings ?
^ Explain by an example what he meant to express by **^ of eaayMflci^
tion and of easjr'tpaBsmission.^

On what what were the^ supposed to depend ?

How is the undulatory theory applied to-cipiaia the •elenrad riegff

What is siippoud^to cbtormf^e tbe^coloarjrdfleeted hy^iurfiMesipeaeiT-
ing light with a perpendicular incidence }

• See PoweU's Elem. TjFeati^-Sii^^^i, pp. fl8«*4M

DOrBLB R£FIU€TiaV OF LIGHT. 408

iituB interstices between tliem, determines tiie colour they efadl
^ect and transmit d:t a peipendicular incidence. Thus, if tbe
molecules and interstices be less in size than the inlwral at whi<^
total transmission takes place« or less than that which corresponds
to the edee of the centraa black spot in the reflected rinfs, a me-
dium m^e up of such atoms and interstices will be perfectly
transparent. If greater, it will reflect the colour corresponding
to its thickness.*'*

214. There are several yery curious optical phenoiaeDa arising
firoin the interference of the n^ys isi light, besides the ccdours e^
hibited by thin plates ; and among Ihese may be ineladed the v»»
liable colours of fine fibres and striated suifaces. *< Fine fiJbiw
and stride give beautiful cekmrs by interference^ when single, be^
tween rays rejected firem their opposite sides; and when maa^
are placed together, more complex colours ase piodueed by thcai
combined interferences.

215. ** A striking example of this kind is seen in the irii htittmB^
invented by Mr. Barton, the surfinoe of which k coveied widi
minutely-engraved parall^ lines, in some instances ftot more thi«
one l(M)00w of an inch apart. A phenomenon very Mmilar is
tibat of the colours exhib^ed by the sur&oe of motheiHDf«peasl.
This substance, when examined by a pewerfied mieroseofe, is
found to present a surface covered wiith minute ntrie arranged is
pazallel waving lines.":]'

Double M^aeticn nf JA^iL

"216. Hepeated instances have been already adduced of ihe ap(>
pearan6e of double or multiplied images of bodies viewed through
transparent media ;4: but these phenomena axe all conformable to
the common law of optics, which indicates the correspondence
between the angles of incidence and of refiraction or reflection^
and the relation of &eir sines to each other.

217. There are, however, many cases in which a different efiecl
is produced by the transmission of Tight through certain tranfi|>^
tent substances, as some kind of salts and crystsdUneiyparsL, plates

When voifld s medium be found perfectly trangparent?

What other pfaenome^ besides that of coloured rings depend on the
•nrterferenoe of rays of light ?

To whatpnrpose m the aits. h« the colorifie dfect of grooved sarigees
^oMk applied ?

What natoral jnbstance eihibits Ae^ost of kideBeesee in consequenpe
pf possessing a striated surface ?

^•how many difierent vays may miilfipTied images ^ pred t i eed ?

What n the peculiar dTect of those aabstances vhich are denomlniited

* Ency doped. Metropol.— Mixed Sciences, toI. ii. p. 580.

t Powell's Elem. Treatise on Qptics,p. UO. Secalso Papenin the
Philosophical Transactions, 1814 and 1 829,7foy Sir'D. Brevater \ and Ihstr.
of Papers inPhilos. Trani.» vfLX^JS^JSO^JXA^JWl^xt^u^W^mjf

^ See los;, this tr^ato.

404 OPTICS.

of which hayinsr ponllel snrfeces, when any object is viewed
throagfh them exhioit a doable image, instead of a single one like
similar plates of glass. Such bodies are called doubly refratiing
nUfBtances^ and the property they possess, doable refraction
A B C D

318. This mode of refraction mi^ be experimentally demon"
Btrated by means of a small plate of Iceland spar, or crystallized
carbonate of lime, not more than i of an inch m thickness. If a
plate of glass be placed oyer either or all the preceding figures. A,
b, C, D, each will appear singly, as to the naked eye ; but if a
plate of Iceland spar be held aboye one of the figures, a double
image will be perceiTed,"as two dots, two circles, or two lines in-
BtesM of one.

219. The distance between the two images will depend on
the thickness of the plate of spar. If it be i of an inch thick,
the images will be so near toother that the little circle B wUl
look like a fi^re of 8. There is, however, another circumstance
which will influence the relative separation of the images ; and
that is the position of the plate ; for if it be laid flat on 3ie paper
and slowly turned round horizontally, one of the images will be
perceived to revolve round the other ; so that the circle will in
one position appear thus 8, and in another thus oo ; and the lines will
coalesce and aiverge successively, as the plate is made to revolve.
S20. In explanation of this phenomenon it may be stated that
a ray of light on entering into the transparent spar becomes di-
vided into two portions, one of which follows the ordinary law of
refraction, as to the ratio of the sine of the angle of incidence to
that of the angle of refraction, while the other undergoes a sepa-
rate refraction, according to a new and extraordinaiy law. 'Hie
Iceland spar consists of rhomboidal crystals, masses of which
are always reducible by natural cleavage into exact rhomboids,
having each of their faces equal and similar rhombs. These
are the forms of the molecules into which the mass can be sepa-
rated by continued subdivision ; and in every one of these rhom-
boids the short diagonal is called the optical axis.

2^1. Thus in the annexed figure the diago-
nal line O represents the axis of the rhomboi-
dal solid A B. Now if a ray of light is trans-
mitted through a crystal in the line of its op-
tical axis no double image will be formed, and
the ray will be refiract^ simply according to
the ordinary law of the proportional sines ; for
in this case the ordinary and extraordinary rays,

Bj what nibstance may this property be exemplified ?
what kind of images are seen through plates of Iceland spar ?
What cireumstance determines the amount of separation f
On what do the relaUvo poiitions of the two imagea depend ?

D0I7BLT BEFRACTINO CRYSTALS. 406

M they hare been termed, will coincide. But in all other eases
tiie law is essentially different, the ray becoming divided, and
ofbe part of the pencil will be refracted, according to a law of a
very singular and complicated nature.

223. A plane passing tfirough the axis is called a principal sec-
tion ; and if a ray be incident, so that the ordinary refraction takes
place in the plane of a principal section, then for all ^e inci-
dences, the ordinary ray having its index of refraction constant,
the extraordinary ray will also be in the same plane, though with
an index of refraction which varies according to its position. If
the ordinary refraction be in a plane perpendicular to the axis, the
extraordinary ray will also in tnis case be in the same plane, and
the index of refraction of the ordinary rat remaining of course
constant, that of the, extraordinary ray will also be constant.*

223. Hence it appears diat both the ordinary and extraordinaiy
rays hare a certain relation to the optical axis of the crystal; "all
the phenomena being the same, as though some power emanat*
ing from that axis had produced that extraordinary refraction, liy
separating a portion of the light from the original ray in its trans-
mission through the prism, and attracting it towards the axis, or
repelling it from it. Sometimes the extraordinary refraction is nega*
tive, or a deilection further from the axis, as in the Iceland spar;
and sometimes it is poaitive^ or a deflection nearer to the axis, as
in common quartz crystal; but it is always with the axirthat the
angle of extraordinary refraction is made.**f

224. A considerable number of crystalline substances are found
to possess analogous properties, though with some modifications^
depending on their peculiar structure. Thus some crystals have
only one axis of double refraction, while others have two or more
such axes. Dr. Brewster ascertained that all those bodies which
crystallize in the form of the rhomboid, the regular hexaedral prism,
the octaedron with a square base, and the right prism with a
square base, have but one axis of double refraction; some, like
the Iceland spar, haying negative axes, as tourmaline, alum-stone
sapphire, emerald, and phosphate of lime ; while a smaller num-
ber, as quartz, zircon, and oxide of tin, have positive aXes.

226. Among the crystals which have two axes of double re-
How is the phenomenon in qaestion explained ?

What is the form of crystal m the Iceland spar?

What is meant by the optical axis of such a crystal ?

What is meant by its principal section ?

How will the two refractions take place where the ordinary refraelioD
is made the plane of a principal section ?

How when it is made in a plane perpendicular to the axis ?

When is the extraordinary refraction neffoUve^ and yifhen ponthe P

How do crystals of diffei*ent substances vary from each other in their
doubly refractine power ?

MThat forms or erystal have but one axis of doirt>le refraction ?

•" Powell's Elementary Treatise on Optios, p. 181.
Bouluigt w Scieaoe> p. 148.

406 OPTICS.

fraction, are glaoberite and sulphate of iron. But with lespeef
to these bodies, as also the crystals with many planes of donble
refraction, and those with circular doable refraction, the effects
follow a very complicated law ; and M. Fresnel made the remark-
able discoTery that in such cases neither of the images is refracted
according to the ordinary law, but that both undergo a deviation
from their original plane, exhibiting a sort of complicated double
refraction.*

226. The property of double refraction was first discovered in
the Iceland spar, by Erasmus Bartholin, a Danish philosopher,
towards the close of the seventeenth century ; it was particularly
investigated by the celebrated Huygens ; and the subject has in
our own times acquired a peculiar interest in consequence of its
intimate connexion with polarization.

227. Concerning the nature of Polarization of Lights we can
only afford room for Sir David Brewster's concise account of the
discovery of this property of liffht, which was made by M. Mains,
colonel of the imperial corps of engineers, who, in 1810, published
a most valuable memoir on double refraction, for which he gained
the prize offered to the writer of the best work on that topic, by
the Institute of France.

228. M. Mains, '^ having accidentally turned a doubly refract-
ing prism to the windows of the Palace of the Luxembourg, which
were at the time illuminated by the settin? sun, he was surprised
to observe that one of the double images of the windows vanished al-
ternately during the rotation of the prism ; and afler various fruitless
speculations on the cause of this singular j^enomenon, iie was
conducted to ^e great discovery, thai Ught reflect^ at a particular
angU from transparent bodies^ is polarized like one of the rays pro-
duced by double refraction.

229. ** This singular result opened a wide field of inqoiry tophi-
losophers : and we successive labours of Mains, Arago, Biot,
Fresnel, and Cauchy, in France ; Seebeck and Mitscherlich, in
Germany; and Youuff, Herschel, and Airy, in England--{)resent
a train of research 'than which,' as a distingnished philosopher
remarks, *■ nothing prouder has adorned the annals of physical
science since the developement of the true system of the uni-
verse.' "f

What eryttals have more than one axis of this kind ?

What did Fresnel diacover in renrd to the two imaj^s formed by cryi-

tals with many planes of doable refraction ?
By whom was double refraction discovered ?

* From what is its greatest importance deriyed ?
What inyestigation led Malus to the discovery of polarization f
What incident first opened the way to this discovery }
What is the general result at which be finally arrived ?

• See Sir J. Herscbel*B Discourse on the Study of Natural Philosophj,
pp. 30—33.

t Report OD the recent Progress of Optica ; by Sir D. Brewster, in Re-
port of the British Association for 1832, p. 314. For further informatioa

1. JmI?

INVENTIOK OF SPECTACLES. 407

OPTICAL INSTRUMENTS.

230. There are two principal kinds of optical instraments ;
namely, those which may be more properly styled dioptrical, as
they consist of one or more lenses, their effects dependingr on the
refraction of light; and those called catadioptrical instruments, in

f the construction of which lenses and mirrors are combined, and
hence telescopes of this description have been termed reflecting
telescopes, to distinguish them from other telescopes, whose pow-
ers depend on refraction alone.

231. The perfection of these instruments must consist in the
excellence of the lenses and mirrors of which they are formed, and
of the accuracy of their arrange^ient, so that the axes of the re-
snective glasses may be situated in a right line. They must be
placed one behind the other, at distances exactly calculated with
reference to their several foci. The e^e must also be placed at a
fixed point for observation. That lens in a telescope or microscope
which is nearest the observer is named the eye-glass ; and the
lens or mirror which is turned towards the object to be examined
is named the object-glass. The eye-glass is usually fixed in a
tube, and so arranged that its distance from the object-glass may
be varied according to circumstances.

Spectacles,

232. The employment of convex or common spectacles, or a<
least of single convex glasses to assist the sight, must have been
coeval with the knowledge of the magnifying power of convex
lenses. The invention of spectacles has been ascribed by some
to Alessandro Spinaf an Italian, who died in 1313 ; * and accord-
ing to others the inventor was a Florentine nobleman, named Ar-
mato Salvini, who died in 1317. f It may not improbably be in-
ferred, from these statements, that the mode of adapting two con-
vex lenses to a frame, so as to form a pair of spectacles, originated
about the close of the thirteenth century. But the magnifying

Into how piany classes are optical instronvsnts divided ?

On what principles of construction is this distiuclioD founded ?

On what does their excellence depend ?

n what positions roust the several glasses of a telescope be placed in
respect to each other ?
. What names are given to the several glasses ?

At how early a period were spectacles invented ?

relative to Double Refraction and Polarization of Light, the reader is re-
ferred to the Treatise on those subjects published by the Society for the
Diffusion of Useful Knowledge ; and to the very valuable Essay on Light,
by Sir John Herschel, in the Encyclopsedia Metropolit^na, of which a
French translation, enriched with 5fotes, by MM. Yerhulst and Quetelet,
has been printed at Paris.

* y. Redi Epistola ad Falconerium.

t y. Acta Lipsientia, Ann. l74Xk

408 OFTICft*

properties of eonrex lenses or some similar transparent bodies was
certainly known at an earlier period, though we are ignorant of
the precise manner in which they were used.

233. There is a very remarkable passage in a treatise of Roger
Bacon on "The Secret Works of Art and Nature," in which he
says, " Transparent bodies may be so figured tiiat one thing may
be made to appear many, and one man an army ; and several suns
and moons may be rendered visible at pleasure. * * * •
* * Thus also things which are afar off may be brought near,
and on the contrary ; so that from an incredible distance we might
Tead very small letters, and distinguish the numbers of things col-
lected together, though extremely minute; and make the stars ap-
pear when we please.* Thus it is thought that Julius Cesar,
irom the sea-coast of Gaul, observed by means of very large glasses
(jpfcuia), the disposition and site of the camps and towns of
Britannia Major."j

234. This celebrated writer also thus expresses himself relative
to the refraction of light, in his " Opus Majus :*^ ** Greater things
than these may be performed by refracted vision. For it is easy
to understand, by the canons before-mentioned, that the ereatest
things may appear exceedingly small, and contrarily. For we
can give such figures to transparent bodies, and disperse them ia
•uch order, with respect to the eye and the objects, that the rays
shall be refracted and bent towards any place we please, so that
we shall see the object near at hand, or at a distance, under any
angle we please; and thus from an incredible distance, we may
read the smallest letter, and may number the smallest particles of
dust and sand, by reason of the greatness of the angle under which
we may see them : and on the other hand, we may not be able to
see the greatest bodies close to us, by reason of the smaliness of
the angle under which they may appear. For distance does not
affect this kind of vision, except by accident, but the quantity of
the angle does. And thus a boy may appear to be a giant, and a
man as big as a mountain ; because we may see a man under as
large an angle as the mountain, and as near as we please. And
thus a small army may appear to be a very great one, and though
very far off, yet seem very near us ; and contrarily. Thus like-
wise the sun, moon, and stars may be made to descend hither in
appearance, and be visible above the heads of our enemies ; aod
many things of a similar nature may be efifected which would as-
tonish unskilful persons."

235. From the manner in which Bacon, in the preceding pas-
sages, notices the effects of refracting substances in modifying the

What evidence is derived from ancient authors, proving that lenses
were known before the invention of spectacles ?

* This must apparently be understood of a telescope, or some such in-
atrument, by means of which the stars may be seen in broad daylight.

t Epist. Fr. R. Bacon, de Secretis Operibus Artis et Natorse, et de
Kollitate Magias. Hamburg. 1572. Cap. 5. 2h ExperientUt Ftrwpectivk
•^rtificialibui.

EFFECT OF dOETEX SPECTACLES. 400

power 6f Visioii, it can hardly be donbted that siagliB lehses, tt
least, were sometimes used for other purposes tiian Siose of mem
experiment; ^though the general employment of spectacles to as-
sist the Yisual organs of aged persons or others, may be dated
from the beginning of the fourteenth century, or just after the pe-
riod when they are said to haye been. invented at Flcurenee.

336^ Th«re are two distinct kinds of spectades, namely, tfaoas
with conyex glasses, which magnify objects, or bring their ima^
neajcer to the eyes ; and those wuh coacave passes, which diminish
the apparent sise of objects, oi extend tine limits of dtiatkiet Tision.

237. In old persons the transparent cornea becomes moee flat-
tened than in youth, and probably the crystalline humour under-
goes a corresponding alteration, in consequence of which the rays
coming from objects do not conyerge to a focus, so as to form a
distinct image on the retina, unless they are relatively at a con-
siderable distance from the eye. Hence it happens, as may be
often observed, that aged persons when they attempt to read or
examuie a minute object, without spectacles, are usually obliged
to hold the book, letter, or other object at arm's length. Such
long-sighted individuals are termed presbyteaJ^ The manner in
which they may be assisted by convex glasses may be illustrated
by the annexed diagram.

238. Let C D be supposed to represent a sectiva of the crystal-
line lens, and A B a similar section of a spectacle lens, then the
object 0, at about six inches from the eye, will form a perfect
imagfe on the retina, at R; but if the latter lens be remoTcd, the
object at the same distance will become confused, and in order to
obtain a. proper view it must be withdrawn to treble or perhaps
four times that distance, and if it be Tory small, the unassisted eye
may not be able to distinguish it at any distance.

239. Those called short-sighted persons are such as have the
transparent cornea unusually prominent, and therefore the rays

How are we to suppose that ungle leases had been used before the time
of Bacon ?

How many kinds of speotacles are employed ?

What is tne effect on the size aod apparent distance of obieeta of those
rhich have convex glasses ?

What is the effect of concaye spectacles ?

What parts of the eve undergo changes from age ?

To what expedient do persons tlius affected have reeonrse in reading?

What form of spectacle lenses do these changes render necessary 7

Explain the figure illustvatiBg the effect ef siwh lenses,

* From ths Greek nfMv^i^ an old qmd,

2M

410 OPTICS.

from objects entering their eyes conTeree to a focus befoTe they
reach the retina, unless any object be placed Tery near the eye.*
Where this peculiarity of vbion exists but in a slight degree,
it is rather an adrantaffe than otherwise, as the individuals are
thus gifted with a kind of microscopic sight; for they can see
smaller objects than are commonly discerned by odiers, and are
merely obliged to hold them reladvely nearer to the eye. Distant
objects, howeyer, can only be seen confusedly ; and hence the ad*
Tantaee such persons deriye from concave spectacles. Hie na-
ture of the assistance which these glasses afiord will appear from
eonsidering the following diagram.

S40. Let C D, as before, represent a section of the crystalline
lens, then the rays from the object O will be rendered somewhat
diyergent in their passage through liie concave glass A B, so that
the effect of the prominent cornea on them will be diminished, and
they will form a perfect image on the retina at R; whereas if the
concave glass were removed, the rays would come to a focus
before they reached the retina, and diverging again the image
would be confused.

241. Common convex spectacles and reading-glasses, especi-
ally if they magnify considerably, have the defect of deforming
more or less objects not viewed through the centre of the lens.
For the rays which issue from distant objects and reach the eye
throoffh the borders of a lens, falling on it obliquely, are more re-
fracted than the other rays, and hence the images become confios-
ed. To remedy this inconvenience Dr. WoUaston proposed em-
ploying concavo-convex lenses, with the concave sides turned
towards the eyes; and spectacles thus constructed, called peri-
scopicj* spectacles, if accurately made, and adapted to the pecaliar
decree or long-sightedness which they are intended to relieve,
will be found far superior to those constructed as usual wiUi
double convex lenses.

Whftt is the eause of short-tiehtedness ?
What advantage is possessed by short-sighted persons }
What Inconveniences do they suffer ?

Where is the image formed in the eye of a short-sighted person ?
Drav and explain the diagram illustrating the effect of concave spM^
tacles.
What defect have convex spectacles of high magnifying power ?
How did Dr. Wollaston propose to remedy this defect }

* £(hort-sighted persons are called in Latin My^pe%^ from the Greek
Mv«f to wink or halt-shut the eyelids, and o^', the eye.'
t From the Greek ii«p», about, around, and Zx»9ri«, to look.

INVENTION OF THE MICROSCOPE. 411

243. The Esqaimanx, inhabiting* a country coTered with snow,
would be subject to a weakness of vision approaching to blindness
but for the method they take to guard their eyes from the constant
stimulus of the bright white light reflected from the objects around
them. For this purpose they use a sort of spectacles which they
call snow-ei/ea, formed of small pieces of wooa or bone, with'a nar-
row slit in the middle, which aie fixed near the eyes, by strings
or thongs passing round the head, so that no light can reach the
eyes, except that which enters through these apertures. These
rude instrument^ not only protect the wearers from the excess of.
light, but also enable them to distinguish more readily distant
objects.*

343. Persons whose sight is so much impaired that thej find spec-
tacles nearl]^ useless may derive great benefit by viswing objects
through conical tubes without passes, but having only a small
aperture at the end furthest from the eye, and blackened in the
inside. Such tubes may be fitted into a frame and worn like spec-
tacles ; and they may be rendered more serviceable by being so con*
structed as to be lengthened or shortened, and have the apertures en-
larged or diminished at pleasure.

The 3£eroaeope,f

344. The transition from the use of a single lens to assist
vision to that of combinations of lenses for viewing small objects
may be conceived to be by no means difiicult; yet it appears that
three centuries elapsed between the invention of spectacles and
that of the microscope. Huygens attributes the invention of the
latter instrument to Cornelius Drebbel,^ about 1621; others to
the famous GralUeo, or to F. Fontana, a Neopolitan ; and it is
extremely probable that the idea may have occurred to different
persons engaged in scientific pursuits, about the same period.

245. The simple microscope is merely a single convex lens of
high magnifying power ; and it may consist of a globule of glass,
formed by holding a small fragment of flint glass on a piece of
iron wire, flattened at the end, in the flame of a spirit-lamp, and
letting it drop, when fused, on a sheet of paper placed to receive
it. Tlie globule must then be fitted into an aperture drilled in a
small plate of brass; or if the glass fragment be placed in the
first instance over such an aperture in a thin plate of platina, it

What expedient do the Esquimaux adopt to screen dieir eyes from the
excessive light reflected from the snow ?

What expedient mav be adopted when the eyes are too much weakened
to be aided by spectacles ?

How long did the invention of spectacles precede that of mieroaeopes ?

To whom has the Invention of tne microscope been attributed ?

What is the construction of the single microscope ?

How may lenses for such a microscope be constructed ?

• y. Sigaud de la Fond Elem. de Phys., t. iv* p. 19S.

^ From the Greek MiKpo$, minute, and x%oinm.

i See Treatise on Pt/ronotntcf, No. 51. Hoygens Dioptrics, p. SSI.

4»

OPTICS*

may be fined by exposing it to the flune, and beooming fixed In
tke litde hole, it will form m microscope leady mounttS. Such
micToscopee must necessarily hare very short Ktci, and can there*
fore be used only for- examining extremely minute objects. Tlie
magnifying power of lenses are inversely as their fowH lengths :
thus a couTex lens whose focal distance is 8 inches, will increase
the linear dimensions of the iaofe of an object 5 times ; and a
leas tibe focal distance of which is 1-10 of an inch will magnify
an object 100 times as to linear extent, and 10,000 times as to 8o>
peificial extent.

846. The compound microscope
most consist of two or more co&-
▼ex lenses. The objeet^ass is a
small lens of very short focus ; and
there may be <me or scTeral eye*
glasses. Among the most usoal
forms is the microscope with diree

glasses : bat Tarions modifications
afe been adopted, witii a view ts
" the improvement of theae inatm-
ments, by forming both the eye-
pieces and the ol^edvoea of two or
more glasses.

247. The effect of the eonipouhd
microscope may be described hy
means of the acoompa&jring dia*
mm, which represents the object A
B placed a little bevond the ob|ect-
glass C D ; then the rays issuing
from it would form an image at A^ B^
while the lens E F diminishes the
convergence of these rays, whence
it follows that the image is formed at A'' B^'. This latter image
becomes the immediate object of vision, seen by the eye throng
the lens 6 H, and therefore at A'" B''', greatly magnified.

Tke Telescope.*

U BtnifCTUie TILE8CQPI8.

348. The invention of the telescope is usually stated to have
taken place about 1590 ; but it is manifest from the writings of

B"^|-..~

To what purpote is the nie of m microioope thus Mnttructed

ittvlimitml?

Of ho V many lenses mast the eompound raieroseope be eomposed /

Whieh is the more eOmnon form of this UMtnunent ?

Draw and «9iplaiB a diagram representing the em^tial parts of ths
eompoand mioroseope.

What is the purpose of the seoond lens f

ht wbat position is the image seen with veferenee to that of the objeeti

How early did the iaventkm of the telescope oeear ?

■ ' ' - ■ —

* Fram the Cireeh Ts^«« afar* and £««»««.

THE REFRACTING TELESCOPE. 413

Roger Bacon, alieady referred to, as well as from other sonzoes
of intelligence, that the effect of combinations of optical glasses
must have been ascertained bj experiment, long before that period,
thoagh the arrancrement of them so as to form telescopes, and
their general application to the pur])oses of science may be dated
from 3ie time just mentioned. Accident is supposed to hare led
to the discovery of this important instrument, which has been va-
riously attributed to Zachai^ Jansen, or to John Lippersheim, who
were Dutch spectacle-makers; and the improvements made on
these perspective as they were styled, by Galileo, John Baptist
Porta, Simon Marius, and others, may account for their being some-
times represented as the inventors of the telescope.

349. The most simple kind of refracting or dioptrical telescope
is that termed the astronomical telescope, consisting of two con-
vex lenses, an object-jrlass and an eye-glass, the foci of which
concur in the same point

'^•li's.

. Let A B represent rays from some distant object, as a star, then
the image formed by the object-glass C D, being viewed through
the eye-glass 6 H, will have its apparent diameter ma^ified ac-
cordingly. Thus, if the object-glass have a magnifying power
equal to 10, and that of the eye-glass be equal to 6, the object
will be magnified to 10 X 6»60 times^ With such a telescope
the image will be formed inverted with respect to the object; but
as it is only used, as its name implies, lor surveying celestial
bodies, this defect is of no importance.

250. The terrestrial telescope invented by A. de Rheita, differs
from the preceding in bein&r furnished with two additional eye-
pieces, so that it has in all nmr glasses ; and thus the images of
objects appear erect, and the instrument is adapted for viewing
ships, buildings, &c.

251. The effect of common lenses in producing spherical and
chromatic aberration, and the consequent imperfections of such
telescopes as those just described, have been already pointed out ;
as alsa the methods of correcting such errors, with reference to
the principles on which achromatic lenses are constructed ; and
therefore the subject need not here be further noticed. For the
description and developement of the properties of various modifi-

' What mime was riven to the original instmrnents f
What is the simpleBt form of the instrument ?
Represent this by a diagram.

How will the magnifying power of the simple telescope be computed I
How will the image be situated with respect to its obiect?
How does the terrestrial telescope differ from the celestial ?
What is the purpose of the two additional glasses in this instrument ? ^

3 M 3

JM 6PTIC8.

eatiottB of dioptneal telescopes, we must refer the reader to tfaa
works mentioxied at the end of this treatise.

n. BSFLECTING TELESCOPES.

353. Newton, as elsewhere stated, despairing of the discoveiy
of a meUiod of forming achromatic lenses, directed his attention
to the improYement of the catadioptric telescope, inrented by
Professor James Gregory, in which an image formed by means
of a concaye mirror, or speculum, is viewed, after a second refieo*
tion, through a oonrex lens or eye-glass.

A.

854. The preceding diajpram shows the general consteactioD
and effect of the Newtonian reflecting telescope, in which the
concave metallic speculum C D receives the rays issuing from the
object A B, which it renders convergent, and thus forms a revers-
ed image in the plane mirror E F, inclined at an angle of 45 de-
crees; and this image being reflected to (f e, at the focus of the
fens or eye-iglasB 6 H, is seen through the aperture before it by
the observer.

954. In the original or Gregorian telescope, the image is viewed
by looking towaraS the object, as in the refracting telescope ; and
tnere are other modifications of this instrument, as those of Cass^
grain and Herschel ; for descriptive notices of which the works
mentioned at the end of the treatise may be consulted.

77ii Camera Obaeura,

255. The manner in which images may be formed in a camera
obscura, or darkened chamber, has been already described; but
there is an instrument in a portable form, and adapted for immedi-*
ate use, which bears the same designation, as its effects depend
upon the application of the same principle to practice. There are
also various modifications of the portable camera obscura, amonff
the more convenient of which is that represented in the anne3L^
figure.

What led Newton to the exftmlnatioo and improvemeut of th« Gre-
gorian telescope ?

What is the nature of that instrument ?

Explain the diagram relating to the Newtonian modifioatioo of Grego-
ry'* instruroent

How is thf portable camera obscura constructed ?

What is employed as a screen to receive the images in thia app^
ratas?

THE MAOIC LAKTERN. 419"

256. It consists ofasqaaie box A,
with a circular aperture in front,
into which is fitted a short tube, a,
having at its extremity a convex
lens. .This tube is made to slide
backward or forward, so that it msy
be adjusted to the proper point for
near or distant objects. Then the
ra^s O P, preeeeding from any ob-
ject passing through the lens, will form an inverted image in the
posterior focus of the lens, which being received on a reflecting
mirror E, inclined at an angle of 45 degrees, will be thrown on a
plate of ground glass at ^e top of the box. The image thus formed
may be traced on the rough surface of the glass, by a black lead pen-
cil or crayon of red French chalk, and afterwards taken off on paper;
or tiie figures may be drawn on tracing-paper placed on the ground
glass, through which they will be resulil? perceptible. The lid of
Sie box, X, has two side wings, which being raised when the in-
strument is in use, will exclude the supermious light, and thus-
TMidbr the images distinct.

357. The eamcra hadoy invented by Doctor Wollaston, is an
instrument analagous in its effects to Ae preceding, but of smaller
dimensions, and therefore more convenient for many purposes, as
for delineating distant objects, and for copyinor or reducing draw-
ines. It consists essentially of a quadrangular glass prism, by
which the rays from an object are twice reflected, and thus form
an image on a plane placed below it. The prism is fitted horiion-
tally to an axis on which it turns, so that it may be placed in m^
proper position ; and the brass frame of the instrument nas usually
two lenses adapted to it, a concave and a convex one, the former
to be used by short-sighted persons, and the latter for long^ights.
There are various improvements and modifications of the camera
lucida, the best of wkich appears to be that contrived by Signor
Amici.

T%e Magic Lantern,

258. As an amusing ^ well as instructive optical machine,,
there is hardly any superior to the magic lantern, invented by
Father Kircher. It is composed, as shown in the margin, of a
square tin box, containing a lamp, behind which is placed a
metallic concave reflector ; and in front of the lamp is a plano-
convex lens, which receives on its plane surface the reflected light
of the lamp, and concentrates it on the object, which is mag-
How may the images be made permanent ?

What purpose is raeeted by the lid and its teetoral Bide-pieees repre*
tented in the figure }

What are the essential parts of Wollaston's ea/nera luddaf

By whom was the magie lantern invented ?

What Is the purpose of the mirror in this apparatus ?

- Whi«h teM magnifies the image in this instrument i

416

OPTICS.

nified by another lens fitted
to the extremity of a tabe
projecting from the lantern.
The objects are painted on
thin plates of glass, which
may be passed through a
narrow opening in the tube
between the two lenses.
This tube must be double
one end Inoving within the
other, so that me tube car-
rying the outer lens may be
^wn backward or forward,
till the object is in the con-
jugate focus of that lens. Then if it be turned toward a yertical
screen, a magnified image will be formed; and the further the
lantern is withdrawn from the screen, the larger will the object
appear; but when the distance is considerable it becomes indis-
tinct.

259. Several years ago an exhibition took place, conducted by
M. Phillpsthal, and called the PhafUasmagoria* resembling in the
general principle on which it was founded, the magic lantern, bat
rendered more imposing, by having the objects painted on a larger
scale, and the figures being projected on a transparent curtain of
gnmmed tafifeta, by which the machinery was concealed from the
spectators. Images to represent ghosts, skeletons, and other ob-
jects, singular or appalling, were thus displayed, and for a time
formed an attractive source of popular amusementf

7^ Solar Mieroseope,

360. This instrument differs from the magic lantern principally
in the nature of the objects exhibited by it, and the manner in
which they are illuminated* This purpose is effected by admitting
the rays of the sun into a darkened room, through a lens placed in
an aperture in a window-shutter, the rays being received by a
plane mirror fixed obliquely, outside the shutter, and thrown hori-
zontally on the lens. The object is placed between this lens and

On what are the objects painted ?

What is the necessity for a sliding-tube in the magic lantern ?
On what circumstance will the size of the image depend ?
In what respects did Philipstlial*s phantasmagoria differ from the
magic lantern r
How are objects illuminated in the solar microscope ?
Bv what means are the sun's rajrs rendered horizontal for toe purpose

of this exhibition ?

I ' ■ -

• From the Greek 4>civTei(/»s, a spectre, and 'A^tp*, an assembly.

t See Young's Lectures on Natural Philosophy, 1807, vol. i. ; Brews-
ter's Natnral Magic ; and likewise the Repertory of Arts and Manufac-
tures, First Series, No. 95, in which is a copy of the specification of the
phantasmagorian machinery, for which M. Fhilipstbal took out a patent

WORKS IN THIS DEPARTMEHT^ 417

another smaller lens, as in the common microscope; and the mag^
nified image thus formed is to be received on a screen, as in the
jl case of the magic lantern. The mirror is sometimes kept in its

^ due position to reflect the sun's rays in a constant direction by a

^ species of clock-work called a heUoatai^

261« Mr. Greorge Adams, an eminent opticiaii, inTented an ni-
1^ strument, which lie called the lucemal microacope^ so constructed

I; as that objects could be illuminated by the light of the lamp ; and

thus the microscope could be used at any time, or in any situatioiu
An improvement on this mode of displaying highly magnified im-
ages of minute objects has recently been adopted, by employing
the splendid light produced hj the combustion of oxyffea and
hydrogen gases on lime ; and mstruments have been fittM up for
public exhibition, presenting some of the most curioAs and inter*
esting phenomena with wmch optical science has made as w>
quainted.

How eao the beam of loUr light be kept steadily on the objeet, doee
the toil U itself apparently in motion ?
What is the peeuliaritjr of the Ineernal mieroseope ?
How is the oxy-hydrogen Uow-pipe applied to mteroseopie exhihttiaas^

WorkB m the department of Optics,

PowelPs Treatise on Experimental and Mathematical Optics.
Oxford. 1833.

Cambridge Physics, treatise on Optics, by Prof. Farrar.

Brewster's Treatise oc Optics, wi& an Appendix by Professoi
Bache. Philadelphia edition, 1833.

Library of Useful Knowledge, treatise on Optical Instruments

Brewster's Treatise on New Philosophical Instruments.

Coddington's Treatise on Optics, part ii.

Loyd on Light and Vision.

HerschePs Treatise on Light.

Biot Traits de Physiquey torn. It

EJ.ECTRICITY.

*. Aa^ono thi p) jsical sciences, there is, perhaps, no other so
immedidtely and completely the result of the researches of modem,
«nd especially of contemporary philosophers, as Electricity. It
is true that the ancients were acquainted with one of its grand
characteristic phenomena, namely, the property which some bodies,
under certain circumstances, possess of attracting various other
bodies. Thus Plato, Theophrastus, Dioscorides, Pliny the El-
aer, and other Greek and Roman writers, state that Amber* may
be made', by rubbing it, to attract very light substances, mucn
in the same manner as the loadstone attracts iron. Thej were
even aware that a similar property belongs to jet, belemmte, the
emerald, jasper, and some other precious stones. f And notices
occur in the writings of the ancients concerning other natural
phenomena now known to depend on electricity; but all these
are reported as isolated facts, which they never referred to a com-
mon cause, nor proposed any theory to explain and illustrate thera.

3. The firet attempt towards a generalization of phenomena
which had been so long before observed, and to so little purpose,
was made towards the end of the sixteenth century, by Dr. Wil-
liam Gilbert, a physician who wrote a very curious and original
treatise on the Magnet, and being led by analogy to make experi-
ments on the attractive property of amber, he found that the power
it possessed of attracting light substances, was one which might
be induced by friction in several other bodies; and he thererore
regarded it as ori^nating from a common cause.

3. In the following century the subject was further investigated
by Boyle, Otho Guericke, Sir I. Newton, an^ othere ; but though
they accumulated facts, they were not such as were of a nature
on which to found general principles ; and what was known of
electricity by no means deserved the appellation of science.

4* In the early part of the last century. Dr. Hauksbee, a phy-
sician, made many electrical experiments, from which he ascer-
tained that glass was a substance in which the property of electric
attraction could be most readily excited by friction; and that some
other bodies, especially metals, treated in the same maimer, mani-
fested no electric power whatever.

What faet eoneerning electricity appears to have been known to the an-
cients?

From what does the science derive its name ?

In how many substances had the ancients observed electrical properties?

Who first attempted a generalization of electrical phenomena f

What names occur among the cultivators of electricity in the seveo-
teenth century ?

* From HxiicTpoy, the Greek name of Amber, the tei*m Electricity ii
derived,
t y. Mussehenbroek Institationes Physicss, 1748, 8vo, pp. 198, 199.

418

ELECTRIC PHENOMENA. 410

6. Bnt the ffrand discovery, which led to the claftoification of
all material bodies under two divisions, as beinff either condnctors
or non-condnctors of electricity, was made by Mr. Stephen Grey,
a pensioner at the Charterhouse, London, who died in 1736. This
gentleman, having occupied himself with various experiments,
partly suggested by the researches of Hauksbee, in attempting to
ascertain now far the electric influence could be proposated verti-
cally by means of a iinejconnecting two bodies, fonnd that when
an ivory ball was suspended from an electrified glass tube, by a
silk cord, the electric influence would be distinctly manifested
by the ball at the alstance of more than 700 feet ; but when a
metal wire was used to suspend the ball, it gave no signs of elec-
tricity whateyer«

6. It likewise appeared that glass, horsehair, amber, and resin,
as well as silk, ana in general all those bodies which can be ren-
dered electric by friction, also possess the property of insula-
tion, or preventiufir the escape of electricitjr; while metals, wood,
linen, and water, liave no such efiect, suffering electricity to escape
through them into any other bodies with which they may come
in contact.

7. It had been piisyiously observed that light was oflen given
out in the passage of electrici^ from one body to another through
the air, when, m 1744, Dr. Lndolf, of Berlin, discovered that
ether could be set on fire by sparks produced by friction from a

flass tube : and in 1746, the aiscovery was accidentally made at
icyden, that the electric influence could be accumulated in a hot*
tie partly filled with water; and, by making a communication
between the water and exterior surfhce of the bottle, what is termed
an electric shock might be communicated : whence a bottie or jar
with a metallic coating, which has the same efiect with water,
has been termed the ** Leyden Phial."

8. These discoveries led the way to thpse of Dr. Franklin, who
experimentally ascertained what had been before conjectured, that
lightning is an electrical phenomenon. The mode in which he
conducted the investigation was by raising a kite, during a thun-
der-storm, in June, 1752, and having attached a key to me lower
end of the hempen string, and insulated it by fiaisteninfir it to a
post by means of silk, he found that when a thunder-cloud hstd
appeared for some time over the kite, electricity was received by
it and conveyed through the string to the key, which gave oat

Who first divided bodies into eondaotors and non-eonduetors ?

By what experiments was Grey led to his great division of natural
bodies?

Eimmerate the bodies of eaeh class as he arranp^ed them.

How early was it discovered that electricity might inflame combusti-
bles ?

When and where was the principle of electrical condensation disco-
vered?

What name was given to the apparatns by which it was effected f

Who discovered ue identi^ or lightning and electricity ?

Describe the manner in which this was dSected.

eleetrle spaiks, on the knnckles of the hand bein^ presented to
it. Science ia also indebted to Franklin for the conetmction of a
theory to account for Uie phenomena of electricity, which, with
some modifications, is still regarded as affording the moet satis-
£ictorr mode of explaining them.

9. The exhibition of phenomena apparently depending on eleo-
trictty, by the volnntaiy action of animals, in the case m the tor-
pedo uid some other fishes, which communicate a kind of electric
shock to those who touch Uiem, had long been known, when 6al-
Tani, professor of anatomy at Bc^ogna, m 1790, observed that te
contact of metals with the nerres and muscles of a frog, recently
killed, produced convulsive motions, which might, for some time
after the death of the animal, be renewed at pleasure, by repeating the
. application of the metals. These singular phenomena, with oUiers
of an analogous kind, were at first supposed to depend on some
peculiar action of metals and some other bodies on the nerves of
animals; and regarded as constituting the foundation of a new
science, to ndiich, in honour of the original discoverer, was ap-
propriated the appellation of Galvanism.

10. Some philosopliers, noticing the apparent connexion oi these
appearances with the benumbing power sf the torpedo, and the
relation that seemed to exist between the effects and those ansing
from electricity, ascribed the former to some peculiar modification
of the electric influence, to which they gave the designation of
animal electricity. However, the important discovery by Pro-
fessor Volta, of Pavia, of the electric effect of certain arrange-
ments of different metals, forming what has been since called a
voltaic pile, and sometimes a Galvanic pile, and that of the simi-
larity of the effect of electricity accumulated from bodies excited
in the usual manner by friction, with the effect of such a pile, in
causing the chemical decomposition of water and metallic oxides,
contributed to the introduction of more correct views of the na-
ture of electrical and galvanic phenomena, as all depending on
the various operation of the same causes, and as belonging to the
same science.

11. Among the latest 'discoveries in natural philosophy are cer-
tain singular and important facts which afford grounds for extend-
ing the theory of electricity so as to include 3ie rationale of all
those phenomena previously regarded as belonging to the separate
science of magnetism, which, nowever, from its connexion with

What did Franklin effeet for the general explanation of electrieal phe-
Bomena ?

What knowledge of electric action excited by animals had preceded
die discoveries of Gslvani ?

How early did this philosoj^er make his grand diseovery f

In what elementary facts did that discovery consist ?

What name was at first applied to the phenomena observed by Galvani ?

What investigations led to the change of views in regard to the troe
■atare of Galvanisia ?

What has the saieBce of electricity been of late yeara extended to oom-
prize i

THBORT or 'ELTS&hlClTr, 42l

l!he art of navigation, and itd a{)plication to practical purposed,
will form in some measure a distinct subject ot investigation.

12. The term electto-maornetism has been adopted to designate
this class of phenomena ; and that of electroKshemistry has been
used with reference to the effect of the electric influence on the
chemical composition of bodies : the manner in which bodies ai^e
affected by the irre^lar distribution of heat, inducing in them or
dissipating electricity or majgnetrsm, has been made the Subject
of research, and provided with a peculiar appellation, in that of
thermo-electricity ; there seems also to be some mysterious Con-
nexion between the electric or ma^etic influence and li^t : so
^at it must be obvious that the science of electricity affords a
most extensive field for research ; and that it is so intimately con-
nected with other branches of natural philosophy, as to claim the
closest attention from those who are interested in the progress of
physical science.

13. Within the limited space to which this sfcetch is restricted,
it will be impossible to attempt more than a cursory view of the
most striking and essential phenomena of electricity or electro-
magnetism, with a few illustrative experiments and observations
which may furnish correct ideas of the present state of our know-
ledge, and- enable the young inquirer to study wiA advantage
Works of greater extent and deeper research.

14. Electricity may be investigated under several points of v!eW.
1. With reference to the sources of electric influence. 2. With
respect to its cause, including the developement of the hypothe-
sis of electric fluids, and the properties ascribed to them. 3.' The
distribution of the electric fluid in bodies imbued with it. 4. The
action of electrified bodies On those which are in their natural state ;
and the phenomena of accumulated electricity. 5. The production
of electricity by the contact of different substances ; or. Galvanic
electricity. 6. The production of electricity by heat; or, thermo-
electricity. 7. The phenomena of electric currents ; or, electro-
magnetism.

15. To these'might be added several other heads of inquiry, as
regarding the effect of elisctricity on the living bodies of animals,
in health or disease ; the investigation of the natural electricity of
marine animals, as the torpedo and gymnotus electricus ; the
chemical effects of electricity; and the nature of atmospheric
electricity, or the causes of lightning, hail, the northern lights, and
Other meteorological phenomena.

TVie Fundamental Properties or Mode of Action of the EUetfie Fhdd,

16. Some of the usual effects of electric influence, such as the
attraction of light bodies by glass tubes excited by friction, the

What is meant by the terms electro-magnetism and eleetro-cheroistrtf
What is meant by the term thermo-electricity ?
Under how many and what aspects may electridty be regai(te«*
What iocidenul inquiries are connected with its malQ.brandies ot 01 -
ttestieatioB ?

8N

4S2 SLBCTBICITY. ^

production of sparks of fire nnder certain circumstances, and other
phenomena, have been mentioned as owing their origin to a eom-
mon cause, the investigation of which forms the subject of that
branch of natural philosophy, constituting the science of electri-
city. Like Che essential causes of light and heat, that of electri-
city can onlj be inferred firom observation and experiment.

17. But m order to trace with accuracy the operations of this
powerful a^ent, and elucidate its mode of action, some hypotheti-
cal principle may be advantageously assumed, by means of which,
the phenomena may be connected and accounted for, as resulting
firom its bfluence under any given circumstances. Hence the
existence of an ethereal fluid, either identical or analagous with
that on which depend the phenomena of light and heat, may be
admitted ; and the term electricity or electric fluid may be em-
ployed to designate it.

18. But we should carefully avoid considering it as a palpable
form of matter, the existence of which can be directly demon-
strated. Instead of which, it should be regarded as merely a con-
venient method of explaining certain appearances, and showing
their mutual relations, so that we may be enabled to contemplate
them in connexion with each other.

19. Dr. Franklin advanced a theory of electricity by means of
which he accounted for the phenomena as depenaing on the ac-
tion of a particular fluid, existing in all bodies, and of which each,
according to its capaci^, possessed a relatively greater or smaller

Suantity. When this fluid is in a state of equilibrium, qr equally
istributed among two or more bodies in communication with each
other, it is quiescent, and no particular effects are perceived ; bat
if the equilibrium be destroyed, as by the contact of a body in a
different electrical state, a new distribution takes place, ancl vari-
ous phenomena may arise from the passage of electrici^ from one
body to another. Thus the phenomena were supposed to depend
on the excess or defect of the electric fluid ; those bodies which
were overcharged with it having a tendency to impart it to others,
and those in which it might be less abundant to receive it.

20. It further appeared that bodies in a similar state of electri-
city, whether of excess or deficiency, always attracted each other;
while bodies similarly electrified constantly repelled each other;
the terms positive and negative electricity were therefore adopted to
designate the states of bodies as to the quantities of electnc fluid
contained in them ; those in which it was supposed to exist in

How can we arrive at a comprehension of the cause of electrical phe-
nomena P

What assumption is it necessary to make in speaking of electric action ?

How are we required to restrict the meaning of the term electric fluid f

What sapposilion was adopted by Dr. "Franklin to account for electrical
effects?

What was meant by electrical equilibrium, according to that theory ?

What obvious phenomenon was observed to occur between bodies io the
two opposite states ?

When was a body said to hepoHtvoely electrified ? when negatvodyf

P08XTIYB AND NEGATIVS CLECTRICmr. 428

excess being tenned positively electrified bodies, and those in
which the quantity was relatively deficient negatively electrified
bodies.

21. This theory acc<5unts satisfactorily for some of the most
important phenomena, but there are others to which it appears
to be inapplicable ; in consequence of which, though once gene-
rally received, it is now almost entirely abandoned, and has been
replaced by an hypothesis originally proposed by Mr. Symmer,
an American philosopher, who ascribed the appearances observed
to the existence of two kinds of electric fluid, and their separate
or united influence under various circumstances.

22. According to this system all bodies in nature contain elec-
tric fluid ; and the earth itself is to be regarded as an immense
reservoir of electricity. This fluid is supposed to consist of a
combination of two distinct ethereal essences, which neutralize
each other ; and it is only when they are separated that electrical
phenomena are observed. Thej may be separately collected, and
thus made to display their distinct properties ; but they manifest
a strong disposition to reunite, and it 4s principally at the instant of
reunion that the most striking appearances are exhibited ; for their
combination paralyzes their several powers, and the' compound
fluid becomes perrectly quiescent and ineffective.

23. To these fluids English philosophers have generally given
the names of positive and negative fluids, borrowing in part the
phraseology of Franklin. In France, however, the former has been
termed the vitreous fluids because it is that which is commonly

S reduced by the friction of glass ; and the latter has received tlie
esiffnation oi resinous fluids as it is in the same manner exhibited
by the friction of resin or sealing-wax : though, as will be subse-
quently shown, the positive or vitreous fluid may be rendered
active by rubbing resin, aind the resinous fluid on the contrary
produced from the friction of glass ; the effects depending partly
on the nature of the substances applied to the glass or resin re-
spectively, and being modified by the relative temperature of
bodies, and other circumstances.

24. One of the most simple yet at the same time important
experiments to show the effect of bodies in different states of
electricity may be performed by means of a glass tube, about
three feet in length, and three-quarters of an inch in diameter; on
rubbing which with a dry silk handkerchief, it will become ex-
cited with positive electricity; and if a light downy feather, quite
clean and dry, suspended firom a silk thread, be held near the

How does the system of Symmer differ from thftt of Franklin ?

What tendency have Uie two opposite electricities in regard to a neu-
tral condition ? ^

What names do English writers commonly apply to the two electrici-
ties?

Wliat terms are in use in France and other countries on the continent
of Europe ?

What experiment demonstrates the effect of oppositely electrified bo»
dies >

484 SLXcnuciTr.

tabe, it will be immediately attracted and adhere to it; bat if it
be then withdrawn, still held by 4he silk line, and not suffered to
come in contact with any other body, it will be found, on agsun
hanging it near the tube, to be repelled, instead of being attracted
as before.

35. These appearances aie to be explained, as depending on
the feather having been imbued with negative electricity in the
first instance, and therefore becoming attracted by the positively
electrified tube, which having communicated a portion of its
electricity to the feather, sufficient to neutralize its former electri-
city, and brin^ it into the positive state, both bodies beeome
similarly electnfied, and therefore mutual repulsion takes place,
manifested by the fbather, as being by far the lighter body, flying
off from the tube.

26. Let a laige stick of red sealing-wax be rubbed with dry warm
woollen cloth, and it will be perceived that the suspended feather
presented to it will be first attracted and then repelled, as in the
former case. But if the feather, after having been positively
electrifie^ by contact with the excited glass tube, be presented to
the sealing-wax, it will not be repelled, as it would be by the
tube, if again presented to it, but would be more strongly attracted
by the s^insr-wax, than when in its natural state, plainly de-
ndonstrating £at since it has been positively electrified by the
g^ass, the sealing-wax which now attracts it must be in a nega-
tive state. This exneriment may be reversed by presenting 3ie
feather in its natural state to the excited sealing-wax, and then
bringing it near the fflass tube, by which it would be instantly
attracted ; for having oeep negatively electrified by contact with
the sealinflr-wax, it attaches itself to the positively electrified tnbe.

27. It has been observed that whenever electricity is excited
by the friction of one body against another, both kinds are pro-
duced, the one body becoming negatively, and the other positively
electrified. Thus when glass is rubbed by silk or fiannel, nega-
tive electricity is excited in the rubber, while the ^lass becomes
positive ; and if sealing;-wax or resin be rubbed with fiannel or
woollen cloth, the negative electricity excited in the rosin or wax
will be accompanied by the developement of positive electricity
in the woollen. Polished glass acquires positive electricity from
friction with almost all substances except the back of a cat, which
renders it negative ; but if grround glass be rubbed with silk or
any of those substances which excite positive electricity in smooth
glass, it will become negatively electnfied, and the rubbing bodies

In what manner is the experiment to be explained ?

What difference in the electric state of a fight insulated body will be
manifested after touching it with excited sealiug-wax, from that wbieh
arises from the use of a glass tube ?

What two effects must always be produced when two bodies are rubbed
tQeether to produce electricity ?

Enumerate some of the substances which, when used together, take op*
posiCe electrical states.

What sttbstaaoe may render polished glass negative f

COKDircTORd AND NON-CONDUCTORS. 42&

will be positively electrified . So sealing-wax, when robbed a^inst
an iron chain, if the surface of the former be smooth, it will be
excited with negative electricity ; but if its surface be previously
roughened with scratches, it will become positively electrified.

28* Hence it appears that the excitement of one kind of electri-
city or the other depends much on the surfaces of bodies ; and
therefore it may be conceived that the electric fluid is chiefly dis-
posed on the external parts of solid bodies. As two surfaces
rubbed against each other acquire opposite kinds of electricity, it
might be expected that they would attract each other, and that is
always found to be the case. If a black and a white ribbon, each
about a yard in length, and perfectly dry, be applied together, and
then drawn several times between the finser and thumb, so as to
rub against each other, they will be found to adhere, and if sepa-
rated by pulling one end from the other, they will fly together
a^in. While uiey remain united they mani&st no sign oi elec-
tricity ; for being in opposite states, they neutralize each other ;
But if completely separated, each will exhibit its peculiar electri-
city, those bodies being attracted by the one ribbon which are
repelled by the other.

29. When the experiment is made in a dark room, flashes of
light are perceived from the surfiices of the ribbons, together with
a rustiing noise. The black ribbon in this case will be found to
be negatively electrified, and the white ribbon positively. By
taking ribbons from the same piece and of equal length, and draw-
ing one of them lengthwise at right angles across the other, the
former will acquire positive and the latter negative electricity.
The friction of liquids or gases against solid bodies will excite
electricity ; and the efife^ts of contact, pressure, or friction of any
one body against another will in some degree produce the same
effect, the appearances being variously mc^ified according to cir-
cumstances.

30. The following substances become positively electrified if
rubbed with either of those mentioned aner them, and on the
other hand when one of these substances is rubbed with either of
those named before it, the substance rubbed becomes negatively
electrified: 1. The back of a cat. 2. Polished glass. 3. Wool
and woollen cloth. 4. Feathers. 5. Dry wood. 6. Paper.
7. Silk. 8. Gum lack. 9. Ground glass.

31. It has been already stated that some kinds of substances
freely transmit electricity to bodies in contact with, them, or suffer
it to escape through them ; while others retain it, or prevent its

On what does the particolar state which any body shall take, when
rubbed, appear to depend P

How may the developemeDt of electricity by differently coloured rib-
bons be exhibited ?

Why will not two pieees of silk, when oppositely electrified and placed
together, show signs of electricity ?

How may liquids and aeriform bodies be made to excite electricity ?
* State the order in which the several electrics become either positiTC or
negative according to the substance with which they are rubbed.

2 n2

4M SLECTRICITT*

passage: the fonner are nanaed ctmdueton of electricity^ and tlM^
latter nonrconductors,

33. Among solid bodies, the metals are generally excellent eon-
doctors, and yet they appear to be very unequal as to their powers
of conduction ; linen, straw, and wood charcoal are likewise good
conductors ; while fflass, resins, sulphur, silk, wool, sugar, fat,
and various other substances are either non-conductors, or possess
the conducting power in a very imperfect degree. Most kiads of
wood when qaite dry, and animal fibres deprived entirely of the
juices they naturally contain, become nearly absolute non-conduc-
tors ; but in their fresh state they conduct electricity freely, doubt-
less in*ponsequence of the liquid matter with which they are pene-
trated. Hence the bodies of men and other animals suffer the
electric fluid to pass tlirough them with great facility.

33. All liquid substances, except fat ous, are good conductors,
though not equally so ; for essential oils, and spirit of wine do
not conduct electricity so readily as water; and the latter fluid
has its conducting power augmented by combination with acids
or salijie substances.

34. Air and all gaseous fluids, when free from moisture, are bad
conductors, and the more dense they are the greater will be their
resistance to the passage of electricity through them. Atmos-
pherical air, theretore, in dry weather becomes a non-conductor;
out when charged with moisture, as from a fog, the electric fluids
traverse it more readily. Temperature also, as mifht be concluded,
influences its conducting power, which is greatly augmented by
heat.

35. Those bodies which are good conductors of the electric fluids
may be excited by friction as well as the non-conductors, but the e^
fects produced will depend on the eiroumftances in which they are
placed. Thus a cylinder of brass, or any other metal, grasped by
the hand,if rubbed with silk or flannel, will be perfectly inert, not
displaying any attractive power, like rubbed ^lass or sealing-wax,
when brought near to a tight feather. But if a handle of baked
wood or glass be fixed to a metal cylinder, so that it may be held
without touching the cylinder itself, and the latter be rubbed with a
dry silk handkerchief, or piece of flannel, it may be readily excited,
generally ms^nifesting negative electricity, and it will act on the
leather accordingly. In such a case the electricity la prevented
&om passing ofifirom tjiie metal, through the body of the person who

Wliat gives riwe to the dittinotion of bodies into eooduttors and Don-
conductors ?

What bodies are among the best conditctora?

What are son^e of th^ bqdies which pay chanee theii: character a»-
cording to the state hn which they happen t6 be tried ?

What liquids are I;md conductors of electricity ?

What power of conduction has di*y air ?

What influence has heat on th^ conducting power of air ?

Under what circomstaoces may a metallic cjliuder be excited bv nik-
bing? J ^

INDtTCTIpli OF BLECVRICITY. 01%

boMs it, by th9 uisatating or DQn«c<mdvicting liaadle of dry wood or

glass.

36. From the power which the most perfect non-cond actors

Sossess of preventing the escape of the electric fluids from con*
actors supported by them, they have been termed inmlatora, and
^ they most readily exhibit electricity by friction, they have alsa
been called electrics^ while the appellation of rum^'dedHea has been
applied to the metals and other freely-condncting substances.
These terms, however, can hardly be considered as correct, since
bodies differ more essentiallv in their power of retaining electri-*
city than in their capacity for receiving it | and hence the more
obvious distinction between conductors and non^Ksonductors.

37. In makinff experiments relative to the accumulation or trans*
fer of the electnc fluids, It is necessary to use instruments so con-
structed as that a conducting body may be supported and thus in-
sulated by means of a non-conductor. On this principle is formed
that necessary part of electrical apparatus called the prime conduce
toTy usually consisting of a brass cylinder fixed horiaontally by
one or more rods or thick tubes of glass to a wooden stand,

38. It may be inferred from the experiment with a glass tube
and an insulated feather, that any boay capable of free motion, on
approaching another body powerfully electrified will be thrown
into a contrary state of electricity ; and thus a feather brought
near to a glass tube excited by friction is attracted by.it, and there^
fore previously to its touching the tube negative electricity must
have been induced in it : ana on the other hand, if a feather be
brought near excited sealing-wax it will be attracted, and conse-*
quently positive electricity must have been induced in it before
contact. Hence it appears that electricity of one kind or the other
is generally induced m surrounding bodies by the vicinity of a
highly excited electric. This mode of communicating electricity
by approach is styled induction,

39. When an electrified body thus causes electricity in another
by induction, the effect extends only to that part of the surface
of the latter body immediately opposite to the former, while the
other extremity will exhibit a contrary state of electricity.

This may be shown

■^^ -^ O B by means of a brass

wire A, in the annexed
figure, moving freely oo
a pivot, and supported
by a glass tube E ; and a
= brass cylinder or con-
ductor fi, similarly sup-
Whence has the term inntlator been derived }
To what is electric applied ?
« How are we to arrange ooiiduotors and non-conductors for the purpose
of showing the accumulation or electricity ?

Into wliat electrical state is any body brought by being placed near eve
which has already been electrified ?
How is this exemplified ^

428 ELSCtRtCITT.

ported, and placed within a few inches of the eirtreiiiity of ^0
wire C, carryingr a small ball of pith of elder.

40. If the conductor be positively electrified, the ball C will
become negative, as may be shown hj approaching to it an ez«
cited stick of sealing>wax, by which it will be repelled ; while
the ball D will be attracted dy the sealing-wax, and mnst there-
fore be in a positive state of electricity. In this case the wire A
is said to be in an tUctro-polar state^ having a negative pole C oih

emte to the positively electrified conductor, and a positive pole
, at its opposite extremity. Such an arrangement might be car-
ried to any extent. Thus if another brass wire similarly insulated
and armed with pith balls w^Te to be placed near the extremity D,
tiie ball opposite to it would be negatively electrified, that at the
other end positively, and so on.

'41. The instrument above described, or mounted brass wire
with its balls, forms a convenient electroscope,* to indicate the
electrical states of bodies ; and as such it was proposed by the
French philosopher, Hauy. On the same principle depends the
action or the more simple electroscope, consisting of two small
pith balls suspended by a fine linen thread or silver wire to the
extremity of an insulated conductor. When such an instrument
is electrified, the two balls necessarily acquiring the same kind of
electricity will separate from each other ; and the nature of their
electricity may be ascertained by presenting to them an excited
glass tube, which, if they are positively electrified, will make
Siem more divergent, if negatively will draw them nearer ; and
with a stick of excited sealing-wax, the reverse effects would take
place.

42. A more delicate instrument for estimating the kind of eleo*
tricity is that called Bonnet's gold-leaf electrometer, composed
of two small slips of gold leaf suspended within a glass jar, which
by their diverorence or collapse on the approach of an electrified
body to a brass ball connected with them by a wire passing through
the neck of the jar, indicate that its electricity is similar or con-
trary to that of the gold leaves. An arc of a circle graduated
may be so placed as to show the relative extent of the divergence of
the leaves, according to the degree of electricity in the body pre-
sented to the electrometer.

43. Another very delicate electrometer is that called the electric
balance, invented by M. Coulomb ; in which the force of electri-

What name is given to this mode of exciting electricity P
In what state ai*e tlie opposite ends of a conductor thus electrified ?
Describe the apparatus bj which this effect is demonstrated.
In what state is the body electrified by induction said to be ?
Can a body electrified by induction communicate an electrical excite-
ment to another insulated body }

What use did Hauy make of the wire and balls insulated on a pivot?
What other similar apparatus depends on the same principle ?
Describe Bennet's gold-leaf electrometer. *

* From the Greek Hx<xrpov, (see p. 418), and Zxesriv, to observe.

BLECTmU. tUCHINE. AV

oal rapolaiotu and BttnotiiKiB, U neBaDied bj-tiw lonloB i>f a
wire } and others have been contrived, bj means of which ths
Bjnount of electric repplsion maj be ascertained and raeasured on
a graduated scale. From experiments with the electric balanea
it haa been conctaded that the inflaence of eleetricity, like Aat
of gravitation, is id the inveraa ratio of the sqnarea of the diatuicea
of the aetiug bodie*

Bkttneal Tiutrummtt and ExperinunU.

44. Electricity is nsnalljdereloped, in order to show its eflecta,
by the friction of glass. The earlier electricians, in the proBecu-
tion of their researches, merelj used elua tabes o( olber noa-
conductors, held in one hand and rubbed with silk or flanneL
Dr. Haukabee made an improvement on this tedious process, hj
arranging a glass globe so that it might be made to revolve con-
tinuafiy on an axis; and Professor Winkler of Leipsic, contrl-
buted ^eatly to render the apparatus useful and convenient, by
affixing a cushion of soft leatlwt stuffed with horsehair, so IhtU by
the pressure of a spring it might rub against the revolving globe.

45. Such an arraogeraent as that just described constitutes an, <
electrical machine; but subsequent experimentalists have made.
many alterations; and among the most simple and yet advantage-
ous roodilications of this instrument may be reckoned that invented
by Mr. Naime, a mathematical inatrument maker, as represeated
in the following figure. It consists of a glass cylinder, C C, fiooi

10 to 16 inches in diameter,
and about twenQr inches in

) length, supported, so that it
may turn on its axis, on two
pillars of glass, fixed to a
I wooden stand. Two metallio

conductors, P N, equal ia
length to the cj^linder, and
about one third of its diameter,
are fixed parallel witii it, on
either side upon two glass pil-
lars, which are cemented into
two separate pieces of wood,
sliding in grooves solhatfliey
may ^ respectively adjusted
at any distances from the i^-
linder required. To one of
these condootors, N, ii attaohed n oariiion en inch and a half
vide, and abont as long as the cylinder, against which it may be
made to press by means of a bent spring; and to the upper fixt

On Hhat prineiple wii Coulnntb't Ionian balinee sanUrudwl }
How i> cleetriflity moM e-ommoDi^ developed ^
. How VM thii efiesled bj the earlf electriciBm }
Wbul iraproTenjenU were nude by Hsuk.bee and WLnklei?
Describe tlie euential piMf of Nmrae'i drntriMl nuMhhte.

480 BLSCTRIOtTY.

of it is sewed a flap of oiled silk, which extends loosely OTer the
cylinder, to within an inch of a row of brass pins or pointed wires
proceeding from the side of the opposite conductor. The conduo-
tor to which the cushion is attached is called the negative con-
ductor, and the other, which by means of its points collects elec-
tricity from the glass, is named the positive conductor, and also
the prime conductor. The cylinder may be made to revolye, in
the direction of the silk flap, simply by a winch fitted to it, or by
a multiplying wheel, W.

46. In order that Uie machine may be worked with the greatest
effect, the cylinder and every other part must be made perfectly
clean and dry ; and as may be supposed, it displays the greatest
power when the air around it is quite free from moisture. To
augment the efiicacy of the machine, it is usual to apply to the
cushion an amalgam of zinc and tin, made by melting together one
part of tin and two of zinc, and mixing them in a heated iron
mortar with six parts of hot quicksilver ; and after the compound
has been reduced by trituration to a powder, it must be made into
a stifi* paste, with pure hog's lard.

47. When it is requisite to obtain positive electricity, the
cushion or negative conductor must be connected with the wooden
iBtand of the machine by a chain or wire ; and thus the electric
«c[uilibrium of the rubber is restored, by the earth, as fast as it is
disturbed by the action of the machine ; but the opposite positive
conductor being insulated cannot return to a state of equilibrium
except by the action of the wire. If it be required to produce ne-
gative electricity, the cushion must be insulated by removing the
chain, and attaching it to the prime conductor P, whence the posi-
tive electric fluid will pass to the earth, and the conductor N will
become negatively electrified.

48. There is another form of the electrical machine, consisting
of a circular glass plate, fitted up so that it may be made to re-
volve between two rubbers. It is a powerful instrument; and,
when properly made, is easily adaptea for producing positive or
negative electricity. In both forms of the machine the quantity
of electricity developed in a given time will depend, other things
being equal, on the extent of surface rubbed, and the goodness of
the insulation by which the reunion of the two electricities can be
prevented. The intensity or striking distance of electricity, in a
machine of either form, must depend on the distance between the

What names are given to the two condaetors with which that machine
18 furnished ?

What is the object of the wheels and band in this apparatus ?

What precautions are necessary to insure the efficacy of an electrical
machine?

What sobstance is applied to auement the action ?

What adjustment of^parts will yield positive electricity?

How may the negative electricity be exhibited ?

On what will the guantUy of electricity depend, in any maehine of a
given form ?

To what will the kUenaiitf be proportioned ?

ELECTRICAL PHENOMENA. 491

robber and the collectinflr points, that is, in general, on the
ter of the plate or cylinder.*

49. M. Beudant has described a machine that has the advan-
tage of being less costly than those of glass, and exempt from injury
b^ accident. It may be constraeted by taking two yards of var-
nished taffeta, and sewing together firmly, with a nat seam, the
two ends, so as to make it like what is called a jack towel ; and
it is then to be stretched over two wooden rollers, one of which
being turned with a winch, the taffeta will pass continuously over
them, cushions of hare or cat skin being placed so as to rub against
it; and a conductor with points may be placed near its surrace to
collect the electricity proauced. j

60. When an electrical macnine, as above described, with a
glass cylinder, has been properly prepared, and during a dry state
of the atmosphere, if the cylinder be made to revolve with a cer-
tain degree of velocity, sparks and vivid flashes of light will be
perceived passing over the surface of the glass, from the cushion
to the conductor; and if the knuckle be presented to the conduc-
tor, sparks, with a sharp report, will prooc^from it to the knuckle,
causinff a peculiar and slightly disagreeable, but momentary sensa-
tion. The light is supposed to be occasioned by the sudden com-
pression of the air, by the transit of the electric fluid ; and it is
accompanied by the developement of heat, for gunpowder, alcohol,
fulminating silver, and other highly inflammable bodies may be
set on fire by means of the electnc spark.

61. The operation ef the electrical machine depends on the
fflass becoming positively electrified b^ friction against the rub-
ber, when the cylinder or plate is put m motion, and the rubber
or cushion consequently becoming negatively electrified. The
positive electricity thus acquired by the glass is regularly attracted
and carried off by the metallic points of the prime conductor, in
which it becomes accumulated. But if both conductors be insulated,
so that the cushion connected Mrith the negative conductor cannot
continue to derive electricity from the earth or snrroundingr objects,
it will soon cease to afford electricity to the other conductor by
means of the glass cylinder. In order, therefore, that the supply
may be kept up, it is requisite that either the cushion or the con-

What construction has Beodant propoted for an electrical machine f
What is supposed to be the cause or electrical light ?
On what does the opei-ation of the machine depend f
What takes place when boUi conductors are insulated }

* For an experimental investigation of this and other fubjects con-
nected with the action of electrical machines, the reader is referred to
the 85th volume of Prof. SiUiman^s American Journal of Science, p. 57.
—Ed.

t Traite Elem. de Phyt., p. 570. The idea of Beudant is tometimea
realized in the action of roacninery driven by broad leather bands moving
rapidly over puUies. As there is a considerable amount of friction, and at
both leather and wood become dry and warm, eleetrical ^arki may be
obtained.— Ed.

4M SLficntiCfrT*

d^tot should eommtmioate widi the earth, or with ihe floor, by
•ome good conducting' medium, as a metal chain or wire.
• 6^. Hence it appears that ^e electricity of either conductor
must be extremely weak, when both of them are insulated ; thai
if one conductor alone be insulated, the power of the o^er will be
proportiooallj augmented; that the cushion and the fflaes must
ikwvyt be in opposite states, the one bein^ positive and the other
ftegative; and that the opposite electricitaes ars exactly in that
proportion which will cause them when combined to neutralize
eacti other. The effects produced by the positive condaotor, or
that opposed to the cylinder, will be similar to those of an excited
glass tube ; and the effects of the negatiye conductor, or that con-'
•ected with the cushioa wiU correspond with thoM of an ex-
cited stick of sealing-wax.

63. If two suspended pith balls be attached to either conductor,
they will be observed to repel eaoh other^ manifesting Uie same
kind of electricity ; but if one ball be attached to the positive, and
another to the negative conductor, they will attraet each other. If,
however, the two conductors be connected by a metal rod, their
opposite electricities will neutralize each other, and no signs of
eMier state will be exhibited.

54. The passaffe of a spark indicates the annihilation of the
opposite states of electricity previously existing in the bodies
between which the spark passes, and which has been already
shown to be the effect of induction on the approach of bodies to-
ward* each other. Thus, the knuckle, when presented to the posi-
tive conductor, becomes negatively electrified; and when the
opposite electricities thus induced become sufficiently intense, the
appearance of the spark announces that the state of excitation is
terminated.

55. The most important phenomena depending on the principle
of induction are those arising from the accumulation of electricity.
This is what takes place in using the electrical jar, or, as it has
been termed, the Leyden phial, the property of which was acci«
dentally disoovered by Professor Mnsschenbroek, of Leyden, or,
according to some writers, by M. Conens. Its mode of action
may be readily exhibited by taking a glass bottle nearly filled
with water, and placing it in a basin of water; a chain or rod of
metal must be passed into the bottle below the surface of the wa-
ter, and continued from it to the positive conductor of an electrical
machine, and another chain must have one end immersed in the
water of the basin surrounding the bottle, and the other end trail-
ing on the floor, or connected with the cushion.

Whai; will be the electrical cooditioa of both conductor* vhcn both are
insulated ?

In what pitoportion are the oppo^te etectricitiec always foUnd ?

What phenomenon will two piih ballc eahibit when taapeaded la cither
condoctop of the machine ?

3 That doet the passage of a spark indicate ?
bw may the Leyden phial, in its simplest form, be exbibttadf

TBI UTBBN PHIAL. 411

6. On tDrnbg tbe maohine, the Blectrieal fluid leeeiTad hj tli«
eondactor wilt pass flom it bj means of the chalo dt lod to t)ia
interior of the bottle, where it will be ooeuroulated; aad in (vdei
to diaoharge It, a caminaDieatlon must be mode between ike rod
BF chain proceeding from the bottle, and that Iptmened in the
basing and thna the confined elactrlcll; will make ila Mca^ A
person graaping the latter chain wilk one hand, and tonoluBg the
other Of the eoadnotor with which it i» eonneeted vitk the other
band, would receive the whole charge o( the plual, eonathutlBg
what la termed en electric ihook.

57. It vaa in this manaeT that MuMclienbTaek aadoubt«dl;r
heoame piaodoally aoqualnl«d with the- effect of aoeumulatM
eloMriclty ; and the eeaBation be expedeDced bo atron^ inipiM»<
ed lura that, in a letter on the subject which he ^dreaBed to
Reaumur, he aaid Ike crown of France would be but a feeble in-
ducement to expose himself to the hecerd of reoeiring- aiMli
another ahock.* The aenHtion oauaed bj the diaeharge of an
eleotric jat is not, however, bo formidable as mi^t he Buppoaed
from the al^rm of the alleged diaooverer; and unleaa the jv bo
targe and highly chained, the ahock will only occasion a momen-
taiT painihl feeling, much resembling that caused b; aaddenljr
Hriklng the elbow gainst a hard aubsluiee, but more tranusat.

58. A more convenient form «f the Lejden ohial Aan that Juat
desoribed conaisle of a wide mouthed jar, cootea oataide and instda
with tinfoil, to within about two inches of the top) hanng a
WMden cover, fitted into ^e qiouth like a cork, and pierced so
thai a ationg brass wire anj pass through the cover, lenniaatiBB'
below in a chain in oontaet with the inner coating of the Jar, and
luving at the other cud a braas kaob or ball. A jar or bottia wlA
a aatrow neok, as represented in the margin, mej be used, hot as

In that case it c^ be coated onlj on
the oatside, it must be filled with
Boiae neiallio aohstauoe, aa raar-
enry, or steel filings, as high as the
coating B reariiea; or moderately
I warm water may he poared into it
whenever it is wanted for UBe.t

G9. A Jac may be eleoirliied bj'
placing it qear the posiEive oondnc-
tor of a machine, with whiiA tha
kneb A must be in eontaot; and tiien,
on turning the cylinder, die electric
4aid hUI pass troni the condaotoi

to the jar, in which it will bAOOina

■eoumutated ) and ia onler ta dia-
How l» the diwhirffn pf the pfal m be etteeted I
• LlhM Uitt. Puiot. do Prog, de li PhTihiue, t. iii. p. 1*1. '"

t Far ■ detcriplion of * aaaTEnlenl method of filing a meuU'ie ooatlMr'

484 ELECTKIC1T7.

charge it a bent or jointed wire maat have one extremity placed
against the outer coatinff of the jar, and the other beinff ad-
Tanced towards the kn^, nearly the whole charge will h^
cape from the inside of the jar, through the wire, to the outer
coating. A curved brass wire, called a discharger, is sometimes
fitted up with a knob at each end, 0,D, and a glass handle ; but
the jar may be safely discharged by the bent wire only, as the
fluid will pass wholly through it without affecting the person who
uses it. In charging the jar care must be taken that the exterior
coatlngbe allowed to take the state opposite to that of the tn^e-
rtbr. This may be perhaps most conveniently effected by con-
necting the outside of a jar or battery when undergoing the op^
ration, immediately with the rubber of the machine ; then coo-
necting the prime or positive conductor with the interior the
charging will proceed with entire success, thouffh the jar, the
machine, and even the person who works it be perfectly insulated
from the ground. By connecting the rubber with the interior,
and the positive conductor with the exterior, the battery will be
charged internally with negative electricity.

60. As the^effect of the electrical jar will be proportioned in
part to the quantity of coated glass it contains, and in part to the
thinness of the fflass, it must 1^ obvious that its power will great-
ly depend on its size. Very large jars, however, would be
awkwud, inconvenient, and liable to be broken by slight shocks
if very thin, as well as highly expensive. Hence means have
been contrived for combining an^ number of jars, bo that they
may be all chargred at the same time, and discharmd with equal
&cility as a sin^e jar. This may be effected by forming a con-
nection between all the wires proceeding from the interiors of
the jars, and also connecting all their exterior coatings; and such
an arrangement is styled an electrical battery. The discharge of
electricitv from such a combination is accompanied by a loud re-
port ; and when the number of the jars is consiaerable, animals may
be killed, metal wires be melted, and other effects be produced by
the discharge of the battery, analageus to those of lightning.

61. By means of an electrical machine a vast number of curious
and interesting experiments may be performed, a few of which
may be here described.

The effect of electricity in producing the divergence of tufts of
hair is sufficiently amusing. This may be shown bv placing a
person on a stool with glass legs, so that he be perfectly insulated.

What aeeount did the discoverers of the Leyden phial give of their
mtion on receiTine the shock ?
■ What form of this apparatat is mott oonveiiient in praetiee ?

How may such a jar be ohamd ?

What apparatus may be used in the discharge ?

On what two circumstances in the constmction of a jar vill its eflkicncj
dmend ?
. Uow is the necessity of, using very large jari obviated f

What is an electrical battery ?

ELECTRICAL BXFXIIIMENT8.

43ft

and making him held in his hand a brass rod, the other end of
which toucnes the positive conductor; then on taming the ma-
chine, the hairs of the head will diverge in all directions. The
eame effect may be more perfectly exhibited by means of an arti-
ficial head, of small dimensions, with hair glaed to it, and fixed
on a brass wire, which is to be placed on the conductor.

63. The electrical bells (eanllon electrique, as designated by

the French) consist of a number of
small bells, as represented in the annex-
ed figure, suspended from the conduc-
tor by brass chains, with a ball to act
as a clapper han^ng bv a silk thread,
between every two bells, one of them
beinff connected with the table, so that
its ehsctricity is neutralized as &st as it
is received. Thus the insulated ball
will vibrate backwards and forwards al«>
temately striking the electrified and non-
electrified bell, when the machine is put
m motion.
63. The dancing figures, as shown in the margin, may be cut

out of writinff paper ;. and such figures, or
any other light bodies, placed on a brass
plate B, connected with the ground, and
naving another brass plate A, suspended
at a little distance above it, from the prime
conductor, will rapidly dance when the up-
per plate is electrified.

64. The effect is obviously caused by
the figures being attracted by the electrified

Elate and immediately after repelled, and
eing robbed, of their acquired electricity
by £e lower or non-electrified plate, they
rise again to receive a new charge,. and

__ thus the dance is continued.

■ 65. The manner in which buildings are

injured when struck by lightning, or the accumulated electricity of
the atmosphere, may be instructively elucidated by means of the
apparatus delineated in the following figure, called a Thunder-
house. It consists of a triangular piece o? mahogany, which may
represent one end of a house or bam : in the centre a small square
piece is fitted loosely into a corresponding cavity ; and diagonally
across the, moveable square passes a brass wire, C D. When this
instmment is used, the brass knob A must be brought near to the

How is the diverjgence of the hair, and aimtlar effect! on the peraony
prodaced by eleetrifieation ?

Describe the electrical chime of bells.

How is the electrical dance to be explained ?

What apparatus illastratet the effect of li^htniog on oljects ilirlueh k
cacoanters?

489 xupcTSiciry.

knob of a clialrg«d jdv^ "wiik the outside of
which ie connected a chun attached to the
brass wire B$ tfans the jar will be dis»
charg^9 aad its eieetrieity will pus
thtottgh the knob and wire A to B;
but the inlerraption occasioned by Ike
position of the square in the centre urill
canse it to be forcibly diiven from its
place. If, however, its position be alter-
ed, so that the wire C D may communi-
cate with A alid B, forming a part of the
same electric circuit, the fluid will pees
through the wire C D Withoat displaicing the square.

66« It is thus that the highest point of points of a building being
stmek by lightning, if the passage of the electric fluid be inter-
ntptedt by non-cottdttcting or imperfeetlt conducting bodies, they
may be msplaced widi violence, injured, or destroyed; but if the
electric fluid can pass readily through a good conductor, as a thick
metal rod, it will be conveyed Into 3ie earth without haaard of the
safety of the building. Hence the utility of conductors affixed M
lowers and other loil^ edifices. In praotioe, the lightning rod
must be furnished with a sharp point instead of the bml exl&ited
at the lop of the model.

GALVANISM*

67. Thk effects of eleetneity depending on the acetunulation of
the electric fluids by the friction of non-conductin? bodies having
been pointed out, we shall next attempt to e^plam those nheoo-
mena which apnear to be caused by circulating currents or those
fluids, produoea by the contact of bodies in different states of
electricity, and especially by the contact of metals and other good
conductors. Phenomena of this nature constitute the objeets of
that branch of physical science termed Galvanism or Galvanic
electricity, from the discoveries of Professor Galvani of Bologna;
and sometimes Voltaism, or Voltaic electricity^ from the s&s^
duent researches of Professor Volta of Pavia, who made great ad-
ditions to our knowledge of the sub}eot» with i^eretice both te
&cts and theory.

68. The earliest notice which has been observed of any phe-
nomenon attributable to Galvanism, occurs in a work ^titled **A

In vbat form roust the termination 6( k lightnings fOd be ?

What ii meant by galvanism f

From vhom does that science derive its name ?

^

ANIMAL ELECTRICITY. 437

General Theory of Pleasures^" published in 1?^, by John George
Sulzer, a German writer of some eminence on philology and meta«
physical philosophy. He states that when two pieces of different
metals are applied to the upper and under surfaces of the tongue, and
then brought into contact^ a peculiar taste will be perceived. Suleer
made an abortive attempt to account for this curious fact, which
seems to have attracted no particular attention till a later period,
when further discoveries led to the inference that it ought to be
regarded as depending on electricity.

69. Professor Galvani, already mentioned, about 1790, acciden-
tally made the discovery that the transmission of a small quantity
of electricity through the nerves of a frog, shortly after the deatik
of the suiimal, would excite muscular contractions in its limbs.
And he afterwards -fotind that similar contractions could be pro-
duced, by torching the muscles of the leg of a dead frog with one
metal, and the nerves belcmging to them with another, and then
bringing the metals into contact.

70. This singular effect of electricity may be experimentally
exhibited, by preparing the hind limbs of a frog
as represented in the margin. The skin being
removed, the crural nerves, C D, may then be
perceived issuing from the spine, A 6, and re-
sembling two white threads ; a silver wire, E, it
to be passed under the nerves, and a small plate
of zinc, F, to be laid on the muscles of the thipfas ;>
then on bringing the metals into contact, either
directly, or by a bent silver wire passing from
one to the other, ^e limbs will be effected with
convulsive twitching, which may be re-excited

at pleasure for some time, by suspending and renewing the contact
of the metals.

71. Similar phenomena may be produced by treating in this
manner any animals ; but cold-blooded animals, as froffs, toads,
serpents, and fishes retain their excitability longer than those with,
^varm blood, though experiments made with the latter, under
proper arrangements, have a more imposing appearance. Live am-
majs also display signs of sensibility to the mfluence of galvan->
ism; and experiments may thus be made with live flounders,
which may readily be procured in any place near the sea-coast.
If a flounder be laid in an earthenware plate, on a slip of zinc,
and a piece of silver or gold placed on its back, on connecting
the zinc with the other metal, by a bent wire, strong muscular
contractions will be excited in the fish. * ,

Wlmt phenomena attributable to electrie currents exoited by nieCali
were observed by Sulzer ?

What was the nature of Galvani 's original discover^ ?

What was the second point ascertained by his experiments?

How may this be exhibited ? ■

What classes of animals are best adapted for exhibiting the effect of
muscular contraction after death ?

How may the Galvanic effect be prodaced in the case of a living animal t

d o2

4M SLHCTttlOtTT*

79k In these and aiialtifoaB experimenUi it is requisite tliat Uie
separate pieces of metal should be of different kinds | and the
effects are most strikinff when one metal is readily soluble in
Aoids^ as is the case wiSi zinc^ and the other difteultly solubley
as silver, gold, or platina. Hence two insolnble metals, as gdd
atid platina, applied as above directed, have hardly any efifect;
while gold, platina, silver, or copper, may be advantageously op*
vosed to Bine, tin, or iron, to form a galvanic drouiti It mas*
be observed that the effect is chiefly momentary, and the cotiml'
sive motions take place at the instant of rae contact of the
metals; but the phenomena may be renewed by separating the
metals and repeating their oontact with ea(& other.

73. Sulzer*8 experiment be&yre noticed, may be performed by
placing a ^iece or silver, aa a half dollar, upon tl^ tongue, and
a disk of sine under it, and on bringing togeUier the edges of the
Mietals while their flat sides remain in contact with the tongue,
a peculiar taste will be perceived^ and a sensation c^proaehingr to
a slight electric shock, especially if the metallic plates have rauiei
extensive surfaces. In that case^ also, a flash of light will some-
tbnes pass before the eyes ; but this latter phenomenon may be
more certainly excited by placing one of the metals between the
apper lip and the gams, ana the other on the tongue, and bringing
meit edges in contact as beibte*

74i It has been found that when two metals ale brought into
eoniact, and then separated, they ivill exhibit opposite states
of electricity. Thus If an insillated disk of sine be laid on one of
silver or copper, and then removed bjr means of Some non-conducting
aubstance, the ainc, on being applied |o a delicate electrometer,
WiU show positive, and the silver <A copper, on the other hand,
negative electricity. Whence it may be inferred that a portion of
free electricity had been developed by the metals, and to the
passage and reuniota of the two opposite kinds are to be attributed
the convulsions of the muscles of animals when their nerves a^
in contact with them placed in a galvanic citcoiu

tfl« The etfect of the contact of diffefent metals may be exhibit^
ed by placing on the eap of a gold-leaf electrometer, a large plate
of any metal^ and sifting ovte^ it zinc filings through a copper sieve^
insulated by a slaes handle ; when it wiU be found that the leaves
will diverge widi positive eleciridty, and the sieve will become
Begatitely electrified. On repeating the experiment, but using a
sline sieve to sift copper filings, the effidct will be reversed, and the
^ectrometelr will show that 3ie copper filings aie negatively elec«
trified, while the zinc sieve Will dieplay positive electricity.

What «iiBrt(BteH«t{«B pMperliMOf th« metals employed, fliVout the sa»-

cets of this experiment ?
How may Sulz6r*» elpeHraent be cotiv^nie^tlv feneiited f
What effect may it prtxtiM&e On the organ of sight f
In what states are two metals left after having beett in eonlact with

olheff
How are the convulsiTe motions to be expUin^ f
In what Other uamier May the eff^t of eOnta^t be exhibited f -

GALVANIC CIKGLE, 449

76. A simple g&lYHnie circle may be formed, by the wparatas
repreeeated in the mar^n^ consisting of a plate
of sine, Z, and one of copper, C, immersed to a
certain depth in sulphuric acid grreatiy diluted
with water, Contained in a glass vessel. Then,
when Uie upper edges of the metals are brought
in contact, a current of electricity will tSke
place, the poeitite electric fluid circulating from
the Ssitto to the acid, from the acid to the copper,
thence again to the zinc, and so on in the direc-
tion indicated by the dafts ; the negative current
beitig in the opposite direction*

77. Various modifications of this arrangement may be eontoived:
thus, instead of making the metals communicate immediately, as
above, a wire of atiy metal may be attached to the upper eztremi*-
ties of each plate, and when the wires are brought together the
circuit of electricity will go on, but when thev are separated, it
will be interrupted. By this means the electric cunents may be
directed through any bodies, by placing them between the wires,
so that they may fcmn a part of the circuit, and various effects may
be produced. The wire connected with the zinc in this case is
called ** the negative wite," and that connected with the copper
^* the positive wire.'* By some writers they have been denomi-
Bated positive and negative rheo-phoree.*

78. The efiects of such an arrangement as that just described^
at least with small metal plates, will be but inconsiderable. Hence
Professor Volta conceived the idea of forming what may be term*'
ed a compound galvanic or voltaic circle, by arranging a mimbef
of disks of different metals, as zinc and copper, with cloth ot
pasteboard soaked in some acid or saline solution between them ;
as thus the effect might be indefinitely augmented, according to
the number and size of the disks.

79. The apparatus may be fitted up as represented in the annexed
figure, consisting of an equal number of silver or copper coins,
or flat pieces of either metal, and of similar pieces of zinc, ar-
ranged one above another, with wet pasteboard between them in
the following order : zinc, copper, wet pasteboard, denoted by the
letters Z, C, W, in successive layers throughout the series. One
end of the pile must terminate with a zinc plaje, and the other
with one of copper, with each of which wires may be con-
nected; and the whole should be made steady by fixing the

Which electrical state is taken by the eopper in a simple galyauic pair
or circuit ?

In what manner may the efRscCs of such a circuit be displayed ?

Wiiat difTerent names are ZiYtn to the coDdactors usually attached to
the opposite extremities of the galvanic arrangement?

In what manner did Volta Undertake to augment the power of the gal-
vanic apparatus ?

Describe the fiite of Yoltfti

* CurrenP-bearert^ from Piov, a current, and «of<«, to baar.

disks benreen three verttcal glaaa rods, pn^
pertj Tarnished, toi cemented iolv two thick
pieces of wood, one of which serrea u the base ■
and the other as the corei of the pile. Aity
uamber of snch piles may be united so aa to cob-
stitute e Voltaic battery, by makin|r a metallic
coDtmanicBiion between the last plate (rf* one pile
■nd Uie Erat of another, to any extent.

ficient, and forme a eonTenient instrument, s
long as the cloth or pasteboard disks betwera
' the metals retain their moisture ; bat when tfaej
I become dry, the pile is rendered compatatiTdy
inactiTe. Volta, therefore, contrired a different
UTSOgcment, to which has been siven the French deaignitioft
Courmru de Tauei, as consisting of any ntunber of glasses partly
filled with diluted acid, with a plate of ^nc and another ot cop-
per in each as before described ; and the ziuc plate in one riaat
being connected with the copt>er one in the next, throagnont,
(he circuit might be completed by wires attached to the tenui-
DBting plates.

81. But thia instrument, though not liable to the same objectioB
with the pile, was incoiiTenient, and therefore has been snper-
eedcd by varions other arraneements, among which we select fbf
dMcripUon the Galranic trou^, or as it also is (ermed, the GalTBoie
battery of Hr. Cruicshank. It may conHist of a trou^, T, eoo-
atraeted of baked mahogany, with partitions of ^Isas in the ints-
riori or it may be formed of Wedgwood ware, with interior cells,

How ii (he efflcieooj of the pile limited in regard to the time of hi

What other srrsngtment of (he elen
What [i the coDatruetioQ of the cram
DcMuibe the GalTinis batterr.

OALTAHIO TROirSH. 441

m&Ck trough eonlaiiuog ten or iweltei Th» meial plaM P P
Adapted to them are ui^fciBd by a bar of baked wood A B, so that
the whole Mt maj be let down into the trough^ or liAed out to^
gethor.

8dw Hie cells sie to be filled with water or diluted acid when
the inKtrttment is to be tised, and the plates placed in thetn^ each
eell will contain a line and copper plate, And the circalation of the
electric fluid will take place wougfaout the wholo^ w^ile wirea
proceeding from the last zinc plate on one side, and from the last
copper plate on the other, any bodies, by being placed between the
wires, will form a part of the circuit, and be subjected to the action
of the electric fluid. When the necessary experiments are com-
pleted, the plates should be llAed oat of die trough, that they may
not be too hastily corroded by the acid.

83. Several such tronffhs may be eombitifed like roltaic piles, aAer
the manner before stated ( and If very large plates be employed to
form the batterr, its power will be exceedingly increased. One
was constructed, for tile Use of the Royal Institution in London,
consisting of two hundred separate parts, each part composed of
ten double plates^ and ete^ plate containing thirty-two square
inches. The whole nudibi^r of double plated amounts to two mon-
sand, and thdt entite sutface to 128,000 square inches, or 888
square feet.

84. Several ibrlti^ of Galvanic appdratus have been invented
and applied in thil United Statas, some of Which manifest ereat
energy, combined With facility In manipulation. Among them,
those of Dr. H^e, denomiiiated the de/bigraior and th6 eakrlmotor^
deserve particular metition. llie ibtmer is composed of two troughs,
in one bf which the tine and cdpper plates ate arranged across
the trough, so ti^at each pair fortns, when unitldd, a separate par-
tition for a cell, Hhd the whole thus adjusted throughout a length
of ten feet, is to receive the acid liquor When the trough is to be
put into action. To otie edge of this trough is attached another of
the same length With ^e plane of its open side or mouth forming
a right angle with thlit of the ttotigh which contains the cells.
This is to receive the atid when the action of the deflagrator is to
be suspended, l^se troughs thus united tire hung on aa axis
passing longitudinally through the line which unitte their edges
so as to allow the liqudr to be, by the quarter of a revolution,
transferred from one troligh lo die other. Many of the brilliant
and important experiments exhibited by Dr« Hare, are shown by
means of this appatdtus.

With what are the tt\\% Mb be iUed f

What adTantage atteudt thit apparatoi io regard t^ the eorroaion of
the metals ?

How may the powfer of this sort of batteries be augmented }

What was the size df that belongiilg to the Royal tostitation }

What is the oonstMction of Hare's deflagrator ?

By what means is the aoid brought to act on the metallic plates in this
apparatus ?

How are the metals disposed in the ealorimotor ?

442

BLBCTRICITT*

86. The accompanying fignre represents the defiagrator. Thetvo
troughs containing the plates are seem at A A and A' A'« When
the open mouths of these two are in a yertical position, as seen
in the figure, those of the other two, B B and B' B% containing
the acid liquor, are in a horizontal one ; on raising the handle at
the right of the figure, so as to give each pair of troughs half a
revolution, the aid will be decanted from its receptacle, and flow
into the trough containing the plates.

THE CALOBOIOTOR. 448

86. The ealorimotOT in which a great qaantity of heat aeeompa-
nied bj little electrical tauioa is produced, consiBtB of such an or-
rangeiuent of the elements as to form in fact but one, or at most, heo
pairs of separate plates ; for all the zinc plates in one half of the

Sparalus being coonecCed tc^ther constitute but oru plate, while all
) copper ones being united, afford anothtr. The plates aie, hov-
ever, arranged in an ^temadng seriea, so as lo present their surfacea
to each other without occupying too great a apace.

The accompanying figures lepresent the anangement of parts
in the calorimotor. A and a are the cubical boxes cnntaining the
one acidulated and the other piue water ; b b b b is the WMdea

tVanie containing the linc and copper plates alternating with each
other, and from ^ to J an inch apart, T T 1 1 ste masse* of tin

h other . The smaller figure, representing a hori-
xontal section through the plates, shows the manner in wliich the

i unction between the several sheets and the tin masses is effected,
letween the letters z i the zinc only is in contact with the
masses. Between e e the copper alone Couches the tin. At the
back of the frame ten sheets of copper between e c, and ten sheets
of zinc between z x are made to communicate by a common mass
of tin, exlendinff the whole length of the fiame between T T ;
but in front, as shown in the larger figure, there is an iotantice
between the mass of tin connecting the ten copper sheets, and
What relation hu the eleotrie leaaion in the nlof imolor to that of (lie
pile or trough >
Dunribe the •everal parti of the calorimotor.

444 SLBcvBiomr

&at con&eetin? the ten sino sheets. The screw ibreeps, //, joaj
be seen on each side of this interstice, holding the wire which »
to uaderffo ignition. A wooden partition, p p^ separates the two
sets of plates of which tlM apparatus is seen to he eemposed.
The swivel at S permits the fiame to he^ swnng round aftor being
taken out of the acid in A and to be lowered into the pave water
in a I this is for the purpose ef washing off, after an experiaient,
the acid which might otherwise too R^idly corrode the plates.

88. The inrentor le^rds this as furnisung an extreme case of
jfreat heating power with low electric intensiw^, and also as show-
ng that ttie quantity of heat erolved in single nrge purs is graatef,
but its intensity less than that given out hy an equal quantity of
metallic surfoce arranged in several successive pairs.

89. Though the jaost efficient voltaie circles, whether arranged
as piles or troughs, are such as consist of plates of different metals
and layers of fluid matter containing oxy^n, 9s already described,
yet combinations may be formed of various kinds of matter, be-
sides, metals and acids, mani^sting analogous efects, though, in
most cases, with far inferior energy.

90. Dr. Baconio of Milan, constructed i^ vcdtftie pile entirely of
vegetable substances ; using disks of red beet root, two inches in
diameter, and similar disks of walnut-tree, the lal^r deprived of
their resinous matter by masceration in s, solution of oream of tar-
tar in distilled vinegar. With such a pile, using a leaf of scurvy
grass as a conductor, he is stated to have produced contraptions of
the muscles of a dead hof. Other expenmentalis^ hfive finmed
voltaic piles wholly of aninial substances.

91. MM. Hachette and Besormes composed piles of layers of
metallic plates separated by masses of common paste made of
flour and mixed with marine salt (muriate of soda). This, whieh
has been iiqproperly called the dry pile, appears to o^e its effiei-
ciency to the attraction of moisture from the air by the salt con-
tained in the liiyers of paste. Professor Zamboni of Verona, made
a pile with dis^s of paper gilt on oil^e side* and coated on the oAer
With layers of U^ck oxide of pft^ngnnese. ^flAl^ into ^. peste with
heiney.

99. The most simple ^irrangemeiit of this kin4 i« that caUed
Deluo's electric cbluo^n, consisting of disks of paper eofered with
gold OF silver leaf, and simiiai disks ^f i^^minated ?ino, properly
arranged. Mr, 6. J. Siingev eoastmeted fii^ inst|ument in tia>
manner eomposed of twenty thwiisand pair of disks inclosed in a
tube of gla^s of suitable diameter, h<iving at each e^ a brass cap«
perforated by a screw i&s ike purpose of preiaiag V^e^ier the

l¥liat does it demonttrate with respect to the heat furnished br a swgie
pair ftomptred with that giyen out by the same amoant of SMtal in other
»rraage«ie(^s ?

W^at BBStef Uls were used b^ Haeof^ id the eoaitnietloB of hit h«t*
terr ?

What materials did Hachette and Desormes employ^

What were adopted hj Znroboni ? what by Delue ?

What account is given of Singer^ oehimn ^

THE VOLTAIC PILE. 448

dtsks, a wire being attached to either screw, so that onb might be
in contact with the zinc, and that at the other end with the other
metal. Each extremity or pole of sach a column will affect the
electrometer, and exhibit electrical attractions and repulsions.

93. If two .upright electrical columns be placed near each oihct
With their poles in opposite directions^ and their upper extremiUeS
(connected, while a small bell is attached to the lower end of each
column, and a brass ball is snspended between them, it Will atter^
nately strike either bell, and tiie ringing thus caused may be kept
up for a g^reat length of time. Sir J. Herschel mentions his havinff
seen su(£ an apparatus in the study of Deluc, which had continuei
in action for whole years.*

94. iSome of the remarkable phenomena produced by the agency
of the electric fluids, through the voltaic pile or battery, havd

V been already noticed ; and a few additional experiments may be
adduced which will serve more strikingly to illustrate the modd
of action of voltaic electricity, and demons^te its similarity to
common electricity.

• 95. Among the effects of the voltaic pile may be mentioned the
production of sparks and brilliant flashes of light, the heating and
fusing of metais, the deflagration of gunpowder and other inflam*
mable substances, and'the decomposition of water, saline compounds
and metallic oxides.

96. The most splendid exhibition of light may be obtained b^
flxing pieces of pointed charcoal to the wires connected with the
opposite poles of a voltaic batteij. When the charcoal points ard
brought almost into contact, a vivid light and intense heat will b^
iexcited ; and on gradually withdrawing the points from each other,
a continued discbarge of electric fire will take place, forming aH
arch of light of the most dazzling brightness. If the wires be in*
troduced rato a tube partially exhausted of air, and the charcoal
points be made to approach and then recede as before, the effect
will be heightened, and the arch of light will assume a beautiful
purple colour.

97. Wires of metal introduced into the Voltaic chreuit may be
raised to a red or white heat; and #ires of moderate dimensions,
composed of the least fusible metals, as platina, speedily become
melted. The same effect is produced on some of the most refrac<^
tory stibstances, as quartz, sapphire, magnesia, and lime ; while
fragments of the plumbago or of the diamond are dissipated, under*
going a real combustion.

How nkcy sueh eohimns be eropToyed td roaintftin oscitlatioti ?

What is related of the dorability of the electric effect in such colnrons f

What arte some of the effects produced by the voltaic pile ?

How may electrical light be best exhibited by the galvanic apparatas f

What pecidiar effect is observed when the experiment is made m vaoao f

What refractory substances are fused by the battery ?

* See Discourse on the Study of Natural Philosophy, p. 343. On the
prinetple .of l)ehio'» column is eonttructed ^ eleetricail clock mentioAed
in the Treatise on MechatAct, No. 9S6.

2P

446 SLECTRICITT.

98. The chemical powers of the voltaic battery have afforded
the means for some of the most remarkable discoveries of modem
times, among which it will be sufficient to mention the decomposi •
tion of potash and soda, and the exhibition of thei^ metallic basest
by Sir Humphry Davy. But for an account of his researches, and
of the modes of effecting various other chemical analyses by means
of voltaic arrangements, we must refer the reader to the treatise on
Chemistry, in we second part of the Scientific Class Book.

99. The decomposition of water by the voltaic battery may,
however, be shortly noticed as one of the most simple yet impor-
tant processes exhibiting the chemical influence of electricity. If
two wires of platina connected with the opposite poles of a bat-
tery be passea through corks into the extremities of a glass tube
filled with water, on suffering the electric current to traverse the
fluid between the ends of the wires, it will be decomposed into
oxygen and hydrogen gases ; and if one of the wires be of iron,
or any other easily oxidable metal, the oxygen will combine with
the iron as fast as it is evolved, and the hydrogen only will appear
in the form of ^as. By a proper modification of the apparatus wi^
two platina wires, both gases may be separately collected; and
on examination it will be found that they are produced exactly in
the proper proportions to form water.

100. The spontaneous evolution of electricity observable in some
animals, and particularly in certain kinds of fishes, has been as-
cribed to galvanism ; but though the electrical phenomena exhib-
ited by the torpedo and a few other marine animals, have much
analogy with the effects of the voltaic pile or battery, the re-
searches of philosophers have not hitherto enabled us to ascertain
how far the structure of the electrical fishes may be assimilated
to the arrangement of bodies in different states of electricity,
forming the galvanic pile. The production of electric sparks and
other phenomena of a similar nature lead to ^e conclusion that
electrical excitement is a concomitant property of animal life in
general.

101. Many instances are recorded of the spontaneous display of
electric light issuing from the skin of the human body, and the
production of electricity by friction, as from the back of a cat, is a
common and well-known phenomenon. Cardan mentions a Car-
melite firiar, from whose hair sparks issued whenever it was stroked
backwards. Scaliger gives a somewhat similar account of a

What remarkable discoveries have been effected by its aid ?

In what arrangement is the decomposition of water effected by galvanic
electricity ?

In what proportion are the elements oxygen and hydrogen found to be
vhen separately collected ?

To which class of artificial electrical phenomena does that of electrical
fishes bear the strongest analogy ?

What general facts indicate a relation between electricity and the ex-
istence of animal life ?

What examples of electrical developement in the human body, and the
bodies of other animals, have been recorded ? ,

ANIMAL ELECTRICITy. 447

woman at Canmont, whose hair emitted fire when comhed in the
dark. Ezekiel di Castro, an Italian physician, in his treatise ** De
Igne Lambente," relates of Cassandra fiuri, a lady of Verona, that
when she touched her body but lightly with a linen cloth, it gave
forth sparks in abundance. Scafiger, abova quoted, mentions a
white Calabrian horse, whose coat when combed in the dark
emitted lucid sparks. Various instances of a similar nature are
recorded by Bartholin, Beccaria, Saussure, and other writers ; and
those cases of spontaneous combustion which have been related
by physicians were probably owing to the evolution of electricity ;
but of these further notice will be taken in the treatise on ChemiS'

try-

102. The electrical animals already alluded to display much
greater powers in the developement of electricity than those ex-
hibited by human beings ; and the production of the electric shock
appears in these creatures to be dependent on the will, and the
power of producing it to be bestowed on them in order that they
may be enabled to defend themselves from their enemies, or to
take the prey necessary for their subsistence. Amon? these ani-
mals the most noted is the torpedo {rata torpedo), uie peculiar
powers of which were known to the ancients, and are mentioned
by Pliny, Oppian, and other writers. These phenomena have
also been noticed by Redi, Koemper, and other modem authors;
but Dr. Bancroft appears to have first conjectured that the influ-
ence of the torpedo depended on electricity, and Mr. Walsh made
some important experiments which served to confirm this conclu-
sion. The subject has since been more fully investigated by John
Hunter, Spallanzani, Humboldt, Volta, and other philosophers.

103. The torpedo is an inhabitant of several different seas,
being found on the coast of England, in the Mediterranean, and
in TMe Bay, at the Cape of Good Hope. The weight of the
animal when full grown is about eighteen or twenty pounds. It
gives a benumbing sensation, like an electric shock, when touched,
and these effects are renewed by repeated contacts. The shock
may be conveyed, like common electricity, through an iron rod or
a wet line, but not throuffh non-conductors. The ffreatest shock
the torpedo can give is never felt above the shoulder, and rarely
above the elbow-joint; its strength depending more on the liveli-
ness of the animal than upon its size. The electric discharge is
generally accompanied by an obvious muscular action in the ani-
mal, with an apparent contraction of the superior surface of the
electric organs, and by a retraction of the eyes.

On what does the production of electric shocks in animals appear to
depend ? For what purpose is it generally employed ?

What was known to the ancients respecting the powers of the torpedo ?

By whom was the true nature of those powers first explained ?

In what parts of the globe is the torpedo found ?

How may th? benumbing effect of this animal be transmitted to the
person without an actual contact ?

On what does the force of the shock depend ?

By what effort does it appear to be produced ?

448 BLECTUOITT.

104. These fish appear to be ipreatly weakened by the enussion
of electricity, and tboee that gire shocks most readily soon be-
come exhausted and die. From dissection of the torpedo it is
foand to be provided with peculiar organs, placed on each side of
the head and gills, and connected with the nerrons system. It
has been ascertained, however, from the researches of In. Geoffroy
Su Hilaire, that a similar orsanic structore is found in other am*
mals of the raia genas, which nerertheless exhibit no electrical
power,

106. The gymnotus electricus or electrical eel, is a fish havii^
similar powers with the preceding. It is a native of the inter-
tropical regions of Africa and America, being frequently found in
the rivers and lakes of Surinam; and it was first described in 1677
by M. Richer, who was sent by the Academy of Sciences of Paris,
to make philosophical observations at Cayenne. Hiis fish (which
was dissected by Mr. Hunter), like the torpedo, possesses pecnliar
electric organs, which consist of divisions, formed by thin plates
or membranes, ranged transversely, so that in the space of one
inch there were two hundred and forty of these transverse mem-
branes. These organs are copiously supplied with nerves, and
their too frequent use occasions debility and death. It seems,
however, that they are not essential to the existence of these ani-
inals, which live and thrive after the organs have been removed.

106. Humboldt, in his ** Tableau Physique des Regions Equ»*
toriales," describes a curious method of taking the gymnoti, by
driving wild horses into a lake which abounds with those fish.
Some of these are very large, and capable of giving most power-
ful shocks, by which some of the horses are paralyzed and drown-
^ ; but the eels, at length, being exhausted by llileir own efibrts,
are taken without difficulty. This philosopl^z states^ that the
gymnottts in giving shocks does not make any motion of the
head, eyes, or fins, like the torpedo^

107. Three other electrical nshes have been mentioned besidea*
the fore$|[oing, namely, the silurus electrieus, found in tiie Nile $
the trichiurus Indious, which inhabits the Indian seas ; and the
tetraodon electrieus, discovered off the island of Joanna. Little
is known concerning the two latter ; but they all appear to possess
the same general powers of evolving electricity with tibose already
described.

108. That the various phenomena of common electricity and
galvanism, to which may be added those of magnetism, depend
on the operation of a common cause, may now be regarded as an

What is the effeet of repeated diaeharget on the fish itself.^

Where is the gymnotut eleetrious foand f

What peculiar organs has it in coromon with the torpedo ?

By what method are the gymnoti captured }

What other fishes hitherto diseof ered pottess the proper^ of giring;
electrical shocks ?

What remarkable chemieal efects ha^e been pradnced by the voltiia
battery ?

DIFFERENT KINDS OF ELECTRICITT. 449

established principle of physical science ; but the investigations
which have led to this conclusion are only of recent date, though the
experiments on which it is founded appear to be perfectly satis*
factory.

109. In the progress of his electrical researches, Dr. Faraday
found it necessary, for their further prosecution, to establish either
the identity or the distinction of the electricities excited by different
means ; and in a paper of great value, which has been published,
he has established beyond a doubt the identity of common electri-
city, voltaic electricity, magnetic electricity, thermo-electricity,
and animal electricity. The phenomena exhibited in these five
kinds of electricity do not diner in kind, but merely in degree;
and in this respect they vary in proportion to the various circum-
stances of quantity and intensity, which can be at pleasure made
to change in almost any one of the kinds of electricity, as much
as it does between one kind and another.

110. Dr. Faraday was anxious to determine the relation by
measure of ordinary and voltaic electricity ; and after various ex-
cellent experiments he found as an approximation, and judging
from magnetical force only, that two wires, one of platina and one
of zinc, each 1-18 of an inch in diameter, and placed 6-16 of an inch
apart, and immersed to the depth of &-18 of an inch in acid con-
sisting of a drop of oil of vitriol and four ounces of distilled water,
at a temperature of about 60°, and connected at the other extremi-
ties by a copper wire 18 feet lon^ and 1-18 of an inch thick (being
the wire of the galvanometer coils), yielded as much electricity
in 8 beats of his watch, or 8-150 oi a minute (3.2 sec.) as the
electrical battery ^of 15 jars) charged by thirty turns of a plate
machine 4 feet in diameter, and in excellent order. The same re*
suit was found to be true in the case of chemical force.*

111. It further appeared, from the experiments of Dr. Faraday,
that a great number of bodies which when solid were incapable
of conducting electricity of low tension, acquired by liquefaction
or fusion the power of conducting it in a very high degree. Such
are water, and several saline and other substances ; but sulphur,
phosphorus, camphof, spermaceti, sugar, and various other bodies,
including tome salts, acquire no conducting power when melted.

What have recent investigations proved with regard to the phenomena
oi Electricity, Galvinism, and Magnetism ?

In what respect did Faraday find the different kinds of electrical action
to differ ?

State the relation in point ot magnetic force and of chendcal action be-
tween a four feet plate machine and a single Gralvanic pair, with the con-
ditions of the experiment.

What is the general effect of liquefaction on the conducting power of
electrics ?

What bodies remain non-condactors when melted ?

* Encyclopedia Britannica^ 7th edition, 1834, pp. 574, 575.

2p2

450 BtBCTucmr.

MAGNETISM.

1 12. It was long since conjectared by some philosopher that a
eonnection exists between electricity and magnetism, and Aat
electric and magnetic phenomena arise from the same canse. Tlie
disoorery of the effects of the contact of metals and other y(^
taic combinations tended greatly to render the analogy more
striking ; but the grand diseorery of liie power of electric cnnents
to induce magnetism was made only in 1819, by Professor Oersted
of Copenhagen; and Mr. Faraday has more recently demonstrated
the similarity of electricity and magnetisiii, by ascertaining a
method of eliciting electrical sparks from the magnet.

113. The power of the natural magnet or loadstone to attrect
iron was known to the ancients, thon^ tfiey did not ayail them-
selves of it for any useful purpose. The loadstone is an oie of
iron, originally found in the country of Magnesia, in Asia, whenee
it derived its name;* but it is by no means uncommon in various
parts of the world. The principal varieties are those called l^
mineralogists natural loadstone, earthy loadstone, and magnetie
iron ore, all which are oxides of liron; and meteoric iron, or those
masses which appear to have fallen from the atmosphere,| priih
eipally composed of metallic iron and nickel, are in general
found to be strongly magnetic. All these bodies, as well as some
other iron ores, have long been known to possess the proper^ of
attracting metallic iron when brouffht nearly in contact wim it
The magnetic property is capable of. being communicated to sted
by touching it witn a natural magnet ; and in this manner artifietal
magnets aie formed for various purposes. When steel is touched
by a magnet it aeqnires permanent magnetism ; but soft iion treat-
ed in the same manner, mough it also hecomes magnetic, loses its
virtue as soon as it is separated from the magnet.

114. Other metallic bodies besides iron and steel »e suscepti-
ble of magnetism. This is found to be the case with nickel, co-
balt, and brass; tiie first mentioned of these metals especially
being observed sometimes to manifest a^high degree of magnetic
power. Nor is this property confined to metals, for many other
substances belonging to the mineral kingdom, as the emerald, the

What coDJeetnre was formerly made respfecting electrioity and magie-
tism }

What is.roeant bj loadstone } Where was it orieinally discovered ?

What particular varieties of miDerals belong to ue magnetic ispeetes?

In what manner and to what materials maj the magoetie propertjr te
aomrounicated ? ,

What diiFerence arises in magnetizing soft iron from that of hard sted?

What other substances besides steel and iron are sosceptible of being
artificially magnetized ?

* In the Greek language the loadstone Is- called Umyv»*
t See Treatise on Mechamc»t No. 88.

EI.ECTRO-MAON£TISM. 451

niby, the garnet, and name other precioias etones are stated b^r
Cavallo* to be susceptible of magiietie attraction. More recent
researches have led to the detection of magnetism in a great va^'
riety of bodies, including*iglas8, chalk, bone, wood, and other kinds
of animal and vegetable matter. And since it may be concluded
that magnetic attraction is oply a peculiar mode of action of the
electric fluid or fluids, there can be no reason to doubt that its in-^
fluence in particular circumstances must be as.extensiye as that of
electricity, and consequently that all kinds of matter are subject
to it.

115, The attraction of iron is to be regarded as only one of the
peculiar ^Sects of magnetism, bat there is another which though
less imposing and obvious, i» highly important : namely, the po^
larity of magnetic bodies, or that tendency they possess, whea
capable of fi^ motion, to assume such a position that one par-
ticular part, as one extremity of an iron rod suspended horizontal-
ly, shall be directed towards the northern regions of the earth, and
the opposite extremity towards the southern regions. On this
property depends the utility of the mariner's compass, which es-
sentially consists of a maffnetie needle suspended on a pivot,, so
that it may turn horizontally without obstruction. Such a needlci
if the box containing it be placed on a level surface, will generally
be observed to vibrate more or less, till it settles in such a direc-
tion tiiat one of its extremities or poles will point towards the
north, and the other consequently towards the south. If the po-
sition of the box be altered or reversed, the needle will always
tarn and vibrate again, till lis poles have attained the same direc-
tions as before.

116. All magnets and magnetic bars have a north and a south
pole; and if the north pole of one magnet be presented to the
soutii pole of another, attraction takes place between them ; but if
two north poles or two so^th poles of different magnets be made
to approach, they repel each other. If the north pole of a common
bar magnet be presented to the south pole of the needle of a com-
pass, the latter will be attracted, and may thus be drawn from
Its proper direction, which it will recover as soon as it is left at
liberty ; and on the contrary, if a similar pole be presented, as the
north pole of the mafnet to the north pole of the needle, the latter
may be repelled, and thus driven from its true direction, to which
it will return when the disturbing object is withdrawn.

Whftt general eonclosion follow* the researches of recent eiperiment-
en on this subject ?

What effect besides simple attraction is an attendant of magnetizing ?

What practical purpose is sobserred by this properly of magnetized
bodies ?

Describe the manner in whysh this is uiplaed.

How do poles of the same and those of opposite names respectively af-
fect each other ?

• See Philos. Trans, for 1^:86 and 1787; and Carallo's Treatise on
Magnetism, 1787, p. 73.

452 ELECTMCITY.

117. When a piece of iron not magnetic is bronght in contact
with a common magnet, it will be attracted by either pole; bat the
3iost powerful attraction takes place when both poles can be ap«
plied to the surface of the piece of iron at once. It is on this ao
count that artificial magnets are often bent into the form of a horse-
shoe, the north pole bemg usually marked by a line or point to
distinguish it.

118. Having thus stated the most common phenomena of mag-
netism, the reader will be prepared to understand the nature of the
connexion between electricity and magnetism as deduced from the
researches of Oersted, Ampere, Faraday, and other philososhers.
It appears that a metallic wire forming a part of a voltaic circuit
exercises a peculiar attraction towards a magnetic needle. Thus if
a wire connecting the extremities of a voltaic battery be broaght
over and parallel with a magnetic needle at rest, or with its poles
properly oirected north and south, that end of the needle next to the
negative pole of the battery will move towards the west, and that
whether the wire be on one side of the needle or the other, pro-
vided only that it be parallel with it.

119. If the connecting wire be lowered on either side of the
needle, so as to be in the horizontal plane in which the needle
should move, it will not move in that plane, but will have a ten-
dency to revolve in a vertical direction, in which, however, it will
be prevented from moving in conseauence of the manner in which
it is suspended, and the attraction ot the earth. When the wire is
to the east of the needle, the pole nearest to the negative extremi-
ty of the battery will be elevated, and when it is on the west side
that pole will be depressed. If the connecting wire be placed be-
low the plane in which the needle moves, and parallel with it, the
pole of the needle next to the ne^tive end of the wire will more
towards the east; and the attractions and repulsions will be rela-
tively contrary to those observed in the former case. The con-
necting wire will be equally efficient whatever be the metal of
which it is composed ; and even a small tube filled with mercory
will answer the purpose. The interruption of the circuit by water,
unless it be earned to a great extent, does not prevent the action
of the connecting wire; and its influence, like that of conmion
magnetism, penetrates all bodies not too thick, whether conductors
of electricity or non-conductors.

120. If an unmagnetized steel needle be placed parallel with
the connecting wire of a voltaic battery, and nearly or quite in
contact with it, the two sides of the needle become endued with

Under what circumstances is the most powerful exertion of magnetio
force displayed /

What form must be given to the mag;net in order to exhibit this ?

What effect proceeds from placing over a compass needle, and parsllel
with its direction, a wire connecting two poles or a voltaic batteiy?

In what direction will the two poles of the needle move when the wire
is on a level with the needle and parallel in direction f

What change of tendency will arise from carrying the wire below tbe
needle ?

£L£CTRO-MAONSTIC CURRENTS. 458

opposite kinds of magnetism ; one side being attracted by the
north pole of a magnet, and the other side by the south pole.
But it the needle be placed -at right angles to the connecting
wire, it will become permanently magnetic, one of its extremities
pointing to the north pole and the other to the south, when it is
suspended and suffered to vibrate undisturbed.

121. Magnetism may be communicated to steel by means of
electricity uom an electrical machine, evidencing the identity of
the cause of attraction in the different cases; but the voltaio
battery is more conveniently adapted to the purpose of rendering
steel magnetic.

123. Among the various arrangements for the superinduction
of magnetism in steel bars, one of the most efficient and useful is
by inclosing the bar within the coils of a conducting wire twisted
into a helix or corkscrew form, by wrapping it round a glass tube.
It will then in some degree represent a polar magnet, and a bar
of steel introduced into the <*'en^ cavity of the helix will speed*
ily become highly magnetic. The wire should be coated with
some non-conducting substance, as silk wound round it, as it may
then be formed into close coils without suffering the electric fluids
to pass from sur&ce to surface, which would impair its effect. If
such a helix be so placed that it may move fireeiy, aer when made
to float on a basin of water, it will be attracted and repelled by
the opposite poles of a common magnet, forming a kind of vdtaio
mwaet. M. Ampdre describes such an apparatus under the ap^
pellation of an £lectrodynamio Cylinder.

123. K a magnetic needle be surrounded by coiled wire covered
with silk, the transmission of a very minute quantity of electri-
city through the wir^ will cause the needle to deviate from ita
proper direction. A needle thus prepared, therefore, forms an in*
strument adapted to indicate trifling degrees of electricity pro?
duced by the contact of metals, by slight changes of temperature,
or bv any chemical action of one body on another. The magRetio
needle thus applied has been termed an Electro-magnetic Multi*
plier.

124. Professor Henry and Dr. Ten Ejck have availed them*
selves of the influence <^ voltaic electricity on iron, under the
arrangement above described, to form magnets whose powers
ure most extraordinary. Those gentlemen first constructed an
electro-magnet capable of supporting the weight of about 750
pounds ; and they have since formed another which will sustain

Throng what labstaneet may die voltaic current be transnutted with*
out affecting its influence over the magnet ?

How may magnetism be communicated by the electrical current ?

Is the magnetizing power limited to electricity from any particular
soarce?

What arrangement gives the greatest iacUi^ in producing the umgjsm ■
tism of steel Imrs ?

How may the voltaic magnet be constructed ?

What name has Ampere given to that apparabis ?

How is the electro-magnetie multiplier formed I

464 ELECTRICITr.

2063 pounds, or nearly a ton. It consists of a bar of soft iron
bent into the form of a horseshoe, and ** wonnd with twenty-six
strands of copper bell-wire, covefed with cotton threads, each
thirty-one feet long : about eighteen inches of the ends are left
projecting, so that only twenty-eight feet of each actually sur-
round the iron ; the aggregate length of the coils is therefore 728
feeL Each strand is wound on a little less than an inch : in the
middle of the horseshoe it forms three thicknesses of wire, and
on the ends, or near the poles, it is wound so as to form six thick-
nesses.''

125. With a battery of 4.79 square feet, the magnet sapported
the weight already stated, 3063 pounds. The effects of a larger
battery were not tried. It induced magnetism in a piece of soft
iron so powerfully as to raise 156 pounds. When two batteries
were employed, so that the poles could be rapidly reversed, it was
observed that while one of the batteries was removed, the arma-
ture, with the weights suspended from it, amounting to 89 poaods,
did not fall, though the magnetic influence must for a moment
have been interrupted. This seemingly surprising phenomenoD
is readily explained by adverting to the obvious consideration,
that the interruption and renewal of the voltaic circuit, and conse-
quent magnetic attraction, occupied too short a space of time to
admit of the armature becoming sufficiently detached firom the
poles of the magnet for it to sink beyond its influence, before the
circuit was again completed ; whereas, in ^neral, its action ceases
as soon as the circuit of electricity is entirely broken, affording a
striking illustration of the nature and causes of magnetism.*

126. If any further evidence had been requisite to prove the
analogy between electricity and magnetism, it might be derived
from the discovery recently made by Mr. Faraday, of the possi-
bility of eliciting electric sparks from the common magnet.

127. One arrangement for effecting this, consists of twelve
sheer-steel plates, connected together, in the form of a horseshoe;
with a keeper or lifter made of the purest soft iron. Around the
middle of die keeper is a wooden winder, having about 100 yards
of common threaded bonnet-wire, the two ends, composed of four
lengrths of the wire twisted together, being carriea out widi a
vertical curve of about j of a circle ; one of these twisted ends
passing beyond each end of the keeper, and resting on the re-

' ' Wliat eztraordinaiy results have been obtained in the inductioa of mag^
netism by voltaic currents ?

Wliat apparent anomaly was observed by Messrs. Henry and Ten Eyck
after breaking the voltaic circuit ?

How is it to be exphkined ?

^ What discovery illustrates most forcibly tlie analogy between electri-
city and magnetism ?

What form of magnet has been found roost convenient for thispurpoie?

Under what arrangement and operation of the apparatus are electric
sparks elicited by the magnet ?

* Billiman*s American Journal of Science.

ELECTRO-MAONETIC PHENOMENA. 455

spective poles of the magnet. A small wooden lever is so fixed
as to admit of the winder and keeper heing suddenly separated
from contact with the magnet, when a beautifnl and brilliant
spark is perceived to issue from that extremity of the wire which
first becomes separated from the magnet. By means of this elec-
tro-magnetic spark gunpowder may be inflamed.

128. Some researches have been made relative to electro-mag-
netism by Dr. Ritchie, Professor of Natural Philosophy in the

fUniversity of London. One of hip experiments was the continued
rotation of a temporary magnet on its centre by the action of |)er-
manent magnets. This effect is produced by suddenly changing
the poles of the temporary magnet, and thus at the proper mo-
ment converting attraction into repulsion. The instrument used
consists of a series of soft iron cylinders, having ribbons, or ra-
ther bands, of copper surrounding them, in a similar manner as
in the apparatus for showing the detonation of oxygen and hy-
drogen gases by the electro-magnetic spark. The cylinders are
made to revolve rapidly opposite the poles of the permanent mag-
net, 80 that before one current of electricity ceases the other com-
mences its action. By a peculiar arrangement of the apparatus.
Dr. Ritchie succeeded in obtaining a series of sparks from the
common magnet, forming a complete circle, appearing in the dark
like a lucid ring of the finest diamonds.*

129. The ma^eto-electrical machine of Mr. J. Saxton, an in-
genious mechanic of Philadelphia resident in London, has been
constructed by Mr. I. Lukens of Philadelphia, in a very neat and

Eortable form, and serves to demonstrate the nature of the reaction
etween magnets and electrical currents. It consists of a horse-
shoe magnet capable of supporting about 10 pounds laid horizon-
tal with the two poles at the same level. Throu^^h the bend of
the- magnet and between the two poles passes horizontally a spin- ^
die, carrying at the posterior part, next to the bend, a small
toothed wheel acted upon by another of larger diameter turned
by a crank. This spindle also carries at the anterior, and just
beyond the poles of the magnet, a piece of soft iron bent into the
form of a horseshoe, the arms of which are at the same distance
apart as those of the stationary magnet. This is connected to the
spindle through the intervention of a disk of brass, so that in re-
volving the soft iron magnetic poles come successively in contact
with those of the permanent magnet.

130. The former is thus successively magneti^sed and neutral-
ized, each complete revolution performing the operation twice
over, reducing its two ends to the condition of north and south
poles alternately.

«

What method was employed by Ritchie to produce continued rotation
of a temporary magnet ?

In what manner is the electric spark elicited in Saxton's apparatus ?

What is the succession of magnetic slates in which the keeper is found
in this apparatus ?

* New Monthly Magazine for July, 1833, p. 366.

45<l SI.ECTIUCITT.

riie meaiw of minifestlng these two BtatoB BonMSts of » wound
copper wire encircling the keeper, and having its two ends ter-
mmatisg, the one in a copper disk on the spindle eiterior to the
keeper, and the other in a small crose-head apon the same azu.
Tite disk leTolTes, having a smsll part of itt lower rim immened
in mercury. The eros»-head alternately dips its two ends into 1^
nroe cnp, and at tiie moment of rising out of it eihibits a bril-
liant spark. The whole is supported on a neat mahogany fiame."

131. Theancompaaying figure represents Mr, Baxton'^mBgneto-
dactric inacbine. ■

M is the horseshoe magnet, composed of three Sal magneti
nniled, and is about 9 or 10 inches long; a is tlie axis on vtatk
■ levolres the keeper K, to which it is connected throligfa the inter-
vention of tlie brass disk d, and at the other end the pinion h set
in motion by the tooth wheel and winch H. Round the keeper K
are wound several coils of wire, iv, all terminating in the two se-

faiBte polar wires n and p, of which the ft»mer ia made to pass
ongitudinallj through the wooden axis o on ■ line with a, bat
connected with the Keeper by the lectangulai piece of brass r,
and then serves as an attachment for the little cross-head i, while
the latter pasBes along the outside of the wooden axis, and jdns
the copper disk e.

^ is a nearly spherical glaas cup, 3J inches in diameter, with
its mouth turned towards the magnet to receive the end of o widi
the copper disk and cross-head. This cup is sunported on a Bteq
of glass moveable up and down, and capable of being fixed at the

ELECTRO-MAONBTEC AFFAIUTUS.

487

required height by the screw a, Aie lower pert of the enp at m con-
tains mereury I e is a nnt and screw to keep the cfup and stand in
filace.

132. Professor Heniy of Princeton, New Jersey, has constraeC-
ed an apparatus for exbibtting in a temporary magnet a recipro-
'Ciating motion, the soft iron magnet with its coils of wire being
•ospended like the beam of a steam en^ne, on an axis, and fiur-
miidied with projecting wires which dip mto mercurial caps oon-
meeted with a voltaic battery at each end of the apparatus. The
wires are so arranged as to diange the poles of the soft ma^et
^ CTery alternation in the movement. Each end of the soft iron
ibar, I, plajs between the pc^es of a permuient magnet curved
into an elliptical form as seen at M M in the 'figure.

133. The north poles of the permanent magnets are both np-
ward, and when the projecting wires at either end dip into the
cup, the corresponding end of the soft iron becomes a south pole,
and is repelled by the south pole of the ma^et below it, while
the elevated end being made a north pole is likewise repelled by
the north pole of the other permanent magnet. These repulsions
are so vigorous as to raise the wires out of the cups, and the mo-
mentum given to the bar throws the apparatus beyond the hoii-
zontal position, so that the wires at the opposite end dip into their
appropriate cups, and the magnetism of the soft iron bar being in-
stantly reversed, the operation is .repeated.* The zinc element of
each galvanic pair is marked z and the copper c. The poles of
the two elliptic magnets are indicated by N and S respectively.
It will be understood that the coil of wire is continuous, and all

In what manner has an alternating motion been prodneed by eombined
voltaie and magnetic influenee ?

To what stiites are the poles of the temporary magnet meoetsively re-
duced ?

How is the soft mi^et made to raise its connecting wires from the
mercurial cups in Prof. Henry's apparatus ?

IVhat is the best form of apparatus for exhibiting vivid galvanic sparks ?

On what circumstances does its efficacy appear to depend ?

v«« — '' •> tw , I » I ■ I I I III II I I . r, ,

* See, for a description of this apparatus, SiUiman's Amer. Journal
j>f Science, vol. xx. p. 348. The above figure was kindly furnished to the
editor by Prof. Henry*

458 ELECTRicmr.

in the same direction, and that one of each pair of projecting
wires is the immediate prolongation of the heHx, while the other,
a straight line, comes from the opposite end of the bar, being sol-
dered to the wire which there terminates the coil. The reversing
of the magnetism will easily be understood from observing that
each end of the helix, as P r, dips alternately into a cup from the
copper, and then into one from the zinc element of the gralvanie
pairs G 6. This neat and ingenious apparatus will coptinue in
action for a long time, limited indeed only by the durability of
materials in the galvanic circuits, and their power of furnishing
a supply of electricity. It is far more energetic than Delac's
pendulum, or any similar apparatus depending on the action of
what is called the dry pile.

134. Professor Henry has also made some interesting obser-
vations on the power of voltaic conductors to exhibit sparks pro-
portioned to their lengths, breadths, and relative arrangement of
parts, from which it appears that a ribbon of copper coiled into
a spiral* gives a more intense spark than any other arrangement
yet tried, and that an increase of length and of breadth in 3ke rib-
bon gives an increase in the effect, but the limits of this increase
are not yet ascertained.t

135. The identity ot the electric influence under its various
modifications— whether as arising from the excitement of elec-
trics on non-conductors by friction, from the contact of bodies in
different states, the one being positively and the other negatively
electrified, from the action of heat, from compression ; and in its
more anomalous forms, as in the production of meteorological
phenomena, of animal electricity, or of magnetism, from circulat-
ing currents of the electric fluids — may be regarded as having
been satisfactorily demonstrated, in consequence of the experi-
mental researches and important discoveries of modern philoso-
phers.

136. Most of the topics of inquiry just mentioned have been
already noticed in this treatise, the plan of which prevents the
introduction of more detailed information, for which the reader
may have recourse to works of greater extent, and to such as
are exclusively appropriated to the discussion of the branch of
science now under review. But the peculiar effects of currents of
electricity on metallic substances, and especially steel, inducing
magnetic attraction and repulsion, and the application of the mag-
netic needle to the purposes of navigation, demand some further
notice, without which this compendium of science would be im-
perfect.

137. The general properties of the magnet, whether natural or
artificial, and the affinity between contrary poles, and antipathy

What general truth, in regard to the different kinds of electrieity, may
now be considered as demonstrated by modern experiments ?

* See Treatise on Mtechanics^ No. 90S, note.
t See Joorn. of Franklin Inst, vol. xv. p. 170.

TERRESTRIAL MAGNETISM. 469

between those which are similar, as in the case of bodies posi-
tively and negatively electrified, have been already noticed. Nap
tural magnets or mineral loadstones, though sometimes possessing
strong magnetic power, are not in all respects so well adapted for
practical purposes as bars of steel artificially magnetized ; and
the latter are therefore used in the construction of the mariner^s
compass, and other instruments.

138. There are many methods of inducing permanent magnetism
in steel ; but one of the most simple and efifectual consists m pass-
ing a strong horseshoe magnet over bars previously hardenea and
f prepared. ** If bar magnets are to be produced, the bars must be
aia in a longitudinal direction, on a flat table, with the marked
end of one bar against the unmarked end of the next; and if horse-
shoe magnets are required, the pieces of steel, previously bent into
their proper form, must be laid with their ends in contact, so as
to form a figure like this cs? , observing that the marked ends
come opposite to those which are not marked ; and then, in either
case, a strong horseshoe magnet is to be passed with moderate
pressure over the bars, taking care to let the marked end of this
magnet precede, and its unmarked end follow it, and to move it
constantly over the steel bars so as to enter or commence the pro-
cess at a mark, and proceed to an unmarked end, and then enter
the next bar at at its marked end, and so proceed.

193. Afler having so passed over the bars ten or a dozen times
on each side, and in the same direetion, as to the marks, they will
be converted into tolerably strong and permaneht magnets ; but if,
after having continued the process for some time, the exciting
magnet is moved but once over the bars in a contrary direction,
or if its S. pole should be permitted to precede after the N. pole
has been first used, all the previously excited magnetism will dis-
appear, and the bars will be found in their original state. This
seems to show an effect of circulation rather than of any internal
mechanical arrangement ; and from the circumstance of a stronger
power in proportion being produced in thin plates of steel than in
thick ones, and the acquired magnetism being diminished by rust,
filing, or grinding, it appears that the virtue communicated is more
external than internal."*

140. That a suspended magnet will become fixed in such a di-
rection as if its opposite poles were attracted by certaiti points of
the earth, not very distant from the north and south poles respec-
tively, was known at an early period, but it is somewhat uncertain

Whftt kinds of mag;net8 are best adapted to the purposes of naTigation f

What is the method of producing bar magnets r

How are horseshoe magnets placed in order to be magnetized ?

In what manner iAay a magnetized bar be neutralized, and its poles re-
versed ?

What facts favour the supposition tbat magnetism is chiefly confined to
ttie surface, of a bar }

1

* Report of Mr. Millington's Lectures at the Royal Institution in 1818,
published in Journal of Science, vol. vi. pp. 82, 83.

4B0 luenticnT*

wlieii Bsfigston ftttst ftTailed themselres of tfaiB property of tfie
magnet^ in order to discover the potots of the compass in doadj
wiBsther, when neither the sun hy day nor the stars by night can
ttlbrd them any assistance. Some writers state that Marco Polo,
the Venitian traveller, about 1360, introduced amons the Ita&ans
the nse of the mariner^s compass, having learnt it from the Chi-
nese ; but it is more commonly regarded as Ae invention of Flavio
di Gioja, a native of Amalfi, m the kingdom of Naples, who says
that he used it in the Mediterranean Sea in the thirteenth century.

141. The compass which was first employed by European sesp
men, about the period just mentioned, appears to have been a very
rude instrument, consistinflr of pieces of the natural loadstone,
fixed on cork or light wood, so that it might float on die soriace
of water, in a dish on which were marked the cardinal points of
the compass. At present die mariner's compass is more aoca
rately constructed, under various forms adapted to peculiar puiv
poses ; but in all cases composed of a small flattened magnetie
steel wire, or needle, carefully suspended on a pivot in a hoii*
zontal direction, so that it may vibrate and revolve with tiie least
possible degree of friction ; sikI when intended to be used on board
a ship, it is made to hang in a frame which preserves its horison-
tal position independent of the motion of tne vesseL A card it
placed below the magnetic needle, on which aore described two
circles, one divided into 360 degrees, and the other marked whh
the diirty-two points of the compass ; and thus the direction of
the magnetic pmes in any given situation may be ascertelDed and
noted.

142. There are several circumstances which interfere with the
regular action of the magnetic needle, and to which, therefoie, i^B
attention of the mariner^must be directed in making observattons,
and performing calculations founded on diem, so as to obtain es<
act mformation. These are chiefly the ^^dip*' of the magnetie
needle, its ^« secular" and *Miumal variation," and that anoma-
lous variation that long puszled navigators, but which is now
supposed to depend on the attraction of the iron used in the Oon-
strucdon of a ship, or any other pordons of that metal which it
may contain, acting on the compass and disturbing its resnlar
opendon. The dip of the needle is a tendency manifested by
either pole to lose its balance except near the equator, the north
pole sinking as if heaviest on the iKNrth side of the equator, and
the south pole on the south side. As it is of importance to the

To whom has been ascribed die diaooverj of the direedfe inflneiMe of
the earth apon suspended magnets ?

How early was this principle applied bj Giqja?

Of what did the compass then consist ?

What is the form of the mariner's compass at present used ?

In what manner is its card divided ?

How many circumstances interfere with the regular aedoa of the
pass?

What is meant by the dip of the needle ?

How is its amount to lie ascertaiuvd ?

VARIATION OF THE OOMPASS. 461

sailor to be able to estimate the extent to which the compass may
be thus affected in any situation, an instrument is provided for
the purpose, called a ** dipping needle,'' in which the magnetic
wire is suspended in a vertical direction.

143. It has been already observed that the magnetic poles of
the earth, or those points towards which the poles of a compass
are directed, do not exactly coincide with the poles on which the
earth performs its diurnal revolution; and this deviation of the
magnetic from the true meridian, is termed the variation of the
compass. It appears to have been first discovered, or rather ac-
curately observed by Sebastian Cabot, in 1497 ; and in the seven-
teenth century, Heiiry Gellibrand ascertained that the variation
itself is subject to a secular alteration. Thus when the variation
was first noticed at London, the needle pointed to the east of the
true meridian ; in 1657 thpre was no variation, the needle pointing
exactly north and south ; it then progressively veered westward,
having, as is supposed, attained its utmost western declination
about 1818, when it had reached 24 deg. 36 min. W. ; and it now
appears to be annually verging towards the east.

144. Hence it seems not only that the earth's poles of revolu-
tion do not correspond with its magnetic poles, but also that the
latter are not stationary, the line of no variation, which passed
through London in 1657, now crossing the continent of North
America ; to account for which it has been conjectured by some,
that the north magnetic pole revolves round the north pole of the
earth in about 644 years, and consequently, in 1979, the line of
no variation will again cross the island of Great Briain, as it did
in 1657; for if the period of revolution of the magnetic pole be
644 years, half that period, 322 4-1657 = 1979 will indicate
nearly the next return of no variation, while others supposing that
the earth's magnetism is due to the electric currents excited by
the heat of the sun, and that these currents produce magnetizing
effects, the resultants of which are in the points of greatest cold,
have conceived that it is to these points that the norSi pole of the
needle is directed, and that as such points may vary somewhat
from age to s^, the direction of the needle must vary with them.

154. The diurnal variation of the maornetic needle was first no-
ticed by Mr. George Graham, who gave an account of his obser-
vations to the Royal Societjr in 1722. It amounts to several mi-
nutes of augmentation or diminution of the secular variation, at
any given place, in a day ; and it appears to be occasioned by the
influence of the sun's light or heat, or perhaps by both. Its quan-

What is meant by the variation of the needle ?
Is the yariation constant, when we compare it through long periods of
firoe ?
Who discovered the tecular alteration ?

Give the history of this alteration as observed in Great Britain.
When was the daily variation discovered ?
What is its amoaht ? '
On what does it appear to depend f

2 q2

4M SIACTRXCITT*

tity is likewiM afieeted by the m^mds, being matt eonsidenUe
during the summer than in the winter.

146. The intimate connexion* between electricity and magnet*
ism, evidenced by the yeiy important discoyeries recently made^
affords abundant reasons for belieTin|^ that the polariUr of tiie
magnetic needle must be liable to Tanations, from the inflnenoe
of certain natural phenomena. Tims some observers, and espe-
cially Captain Franklin, have stated that the action of the needle
is impeded by Anrora Borealis, the appearance of which seems to
be dependent on electricity; and it has lonr since been remarked
that atmospheric electricity often powerfully atfects the maeneL*

147. Another curious fiict is the induction of magnetism By tiie
exposure of a steel wire or needle to the violet ray of the solar
SpHBctrum. These and other phenomena recently observed cei^
tainly indicate such a connexion between heat, light, electricity,
and ma^etism, as affords grounds for regarding them as probably
depending on a common cause ; and the very curious discoveries
which have been already made, and the striking analogies ob-
served between the operations of nature under diSerent ciicum-
8tance8,furnish abundant inducement to contemporary philosophers,
tad indeed to all who feel an interest in the advancement of sci-
ence, to pursue the track already opened, witii the fair prospect
that the assiduous inquirer will be amply rewarded for his time
and attention to these most important topics of investigation.

Whftt ooe»sioiud oeeurreneet inflaenee the aetion of the magnetie
Medic?
What relation bei been diKOvered betveen l^ht and magnetUmP

* <■ We htkve intunces,*' mjt Profetsor Winkler, ** that tna^tie nee-
diet have aeqaired an inverted direetion by the riolenee of m flaib of
lightning, the north pole eoming to be the ioiilh."--£2m. qfMO^ PhUf .
vol. i. p. SSI.

Work* in the depatfment of EketrieUy.

A Popular Treatise on the subject of Electro-magnetiBm, by
By Jacoo Green. Philadelphia.

Cumming's Electrodynamics.

Cambridge Physics, treatise on Electricity, Magnetism, and
E lectro-magnetism.

Library of Useful Knowledge, treatises on the same subjects.

Singer on Electricity, 1 vol. 8vo.

Priestly on Electricity, 1 vol. 4to.

Franklin's Philosophical Papers, 1 vol. 8vo.

Faraday's recent Researches in the T^sactionS of &e Royd
Society.

BecquereU in the Annales de Chimie

Thompson on Heat and Electrici^.

BerzeUu8*8 phemistry, (Fr.) voL i. article Eli«trioil)r. Paris
edition

INDEX.

ABBRkATiON of the fixed ttan, p. 9i3

ChromatiCf . . 393

Absolute ■!»»», . . • 15

Action and rewftion equal, . . 22

insular in machine*, . 104

Accelerated motion, . . . 96

Acihrbmitic tel^acopta, * 392

Aodustica, genetal account o^ . 231

books on, . . . 276

A^lafages increase the flow of

watet, 170

.^lolian hariH .... 260
Aeriform fluids, . .15
hoWconflned, . 16
dilate equally, . 301
Aeiolites, . . . ; . 51
Aeronaut^ legtilate dwir eleva-
tion, 37

Aerostatics, . . . . 218

Aerial images ihconbavetoiRon, 961

Air, propenies of, . . . 180

currents of, produced bf &11

of water, . . .HI

columns o^ in tubes, . . 246
ccNnpressibili^ oil . . 181
compositioh o( . • . 183
compressed, • • < • 193
comoines with wat^r, . . .194
diy, a bad conductor of ele*.

tricity, .... ^26
elasticity of, . . . . 185
expansion of, by heat, . . 292
gravity of) .... 182

Sin described, • • . 205
ermometer, . 294

vessel of a fordliff pomp, • 218 '
weight of, . . . 182, 196
pump, construction of, . . 186
vacuum in, . 195

mode of Action in, . 188
Albinos, ..... 374
Ampere's electio-magnetic eiqiie-

riments, 453

Angles ofincidence and reflection, 28

of friction, . . .111

of visioli, .... 342

different kinds of, . . 10

An^lar veleci^, . . 57

Animal power, .... 11|^

structure, . . .80

Anvfl supported on the body, . 25

Aplanatic lenses, . 395

AqheductB, Roman • j»i 149

Arc of Qscflhtion, ... 61

Archimedes' screw, . .174

mechanical ftats, . 107

experiments oo

Hiero's crawn, . 150

ArfMi of stability, : ... 64

Aristotle, error of, . . 43

Amott*s hydrostatic bed, . . 135

Artesian welh^ . .144

AMnmomy, mora perfect than

other sciences, 19

how far known, . 18
Atmosphere, height of a unifoim, 206
extent of, . 20^

density jof, hf La-
place, . .808
adhesion, produced
by, . 209
Atmospheric refhiction, . . 365

Ass, physical power oC • .119

Attraction of gravitation, . . %

' electric and tnagnetic^ IS

capillary, . • 1^

oonesive, in liquids, . 183

ofmetalsfbrheat,

magnetic, . •

Attwood*! machine, .

Audibility, ....

Aurora borealis, effects of, .

Automaton, 266

Axisofaletis, . . . . 368
Axle and wheel, . & • .84

.461
. A
.236

469

Bacon, Roger's idea of flying, . 894
formed spekldng

figures, . . 871
knew the uto of
lenses, . 4xjcf

Baoonio's yegetable electric pfle, 444
Ball cannon, how brought to rest, 19
ivoiy, comprsssea by colli-
sion, . . .' . 24
billiard, moved by springs, 83, 100
ofwax, proves foite of gravity, 48
weight of; at the centre or
the earfii, .... 49

Ballast, how stowed,
Balance for weighing,
fidse,
Dankh, .
hydrostatic.

463

181
81
8d
88

161

464

INDEX.

Ba]loon» why it ascends, p. 37

inflated with hot air, . 221
with coal gas, . 223
calculations respecting, . 223
afifords means of deter-
mining aerial tempe-
ratures, . . 283
Bamberg bellows, . . 192
Barker's mill, .... 176
Barlow on seasoninff timber, . 290
Baiometer inventeo, . . . 199
wheel, . . .202
water, . . . 203
compound, . . 203
Barton's iris buttons, • . . 403
Battery, voltaic, . . . 440
Bed, hydrostatic . . .135
Beer, rormenting, eflecto^ . .194
Bell, diving . . .227
Bellows, hydrostatic, . . .133
smith's, .... 192
BemouilU on human strength, .116
Beudant's silk electrical machine, 431
Bevell'd wheels, . . . . 102
Bianchi's salvanic clock, . .114
Billiard table, motions on, . .29
Biot's experiments on sound, . 240
on vocal sound, 263
or repetitions in
pipes, . . 270
Birds, winp of, . . .81
Black, Dr. on melting ice, . 305
Blind person restored to sight, 378
Bladders ibr swimming, . 158
air, in fishes, . 159
Boat moved by oblique force, . 35
rowing of a, . . .79

flyinff, 224

Bodies, Mling laws of, • 43

projected upwards, . 52

elftftic and mei^stic, • 24

Bones of the ear, . . • .232

Books on acoustics, . . .276

mechanucs, . . .121

heat, .... 332

optics, . . . 417

electricity, . . . 462

hydrostatics, . . 165

hydraulics, . . .179

pneumatics, . . . 230

Boiler, Perkins'. . . . .312

Bolognian phosphorus, . « 337

Bossut on flowing liquids and jets, 169

Bottle imps, . . . .191

Branch's hydrostatic press, . 134

Breast wheel, . * .175

Brevyster on double refraction, . 405

Bridge, iron Jirched, raised by

he^i, . : . . . .289
Briot's coining press, . . .105
Boujrancy, centre 0^ . • .161

Buchanan on hnman labour, j). 116
Bucket supported on the edge en
a table, . . .69

CfBsar, Julius, employed specula, 408
Cadet de Vaux on preserving

lamp glasses, . . .890

Caloric, 278

engine, .... 323
Calorimeter, Lavoisier's, . . 304
Calorimotoir, Hare's, . . 441, 443
Camel, power of the, . . .119
water, .... 162
Camera lucida, .... 414
obscura, . . 373, 414

Canal locks, .... 146
Candle may be shot through a

[dank, . .23

Canton on compression of water, 125
Capacity for heat, . . . 304
Capillary attraction, . . 163

Capstan 85

double, . . . 87

Cargueros, 117

Catoptrics, ..... 347
Cataracts, . . . * . 145
Cavendish on the density of the

globe, 41

Celsius' Thermometer, . . 295

Central forces, .... 56

Centrifugal forces, . . 56, 57, 58

Centripetal forces, • . 56, 57

Centre of action, . . . 72

gravity, . . 65, 67

pressure, . .137

curvature of mirrors, . 355

Chantry's observations on hot air, 326

Change of motion i»oportionate

to force, 28

Chantepleur apparatus, . 213

Charles first mflated balloons

with hydrt^en, . . . 221
Chimborazo, gravitative attrac-
tion of, . . . . .40
Chladni's experiments on sound, 240
da do. 251

Chromatics defined, . . . 384

Chain pump 172

Circle, divisions of, . . .9
Circus, motions in, . . .34
Cistern, flowing of water from, . 136
resulated by a ball-cock, 160
Circuit, ffalvanic, . . . 439
Clock, electrical, . . .114
Coaches, when liable to be over*

turiied, 68

Cochlion, .... 174

Cog, hunting, • ' . • .101
C(«8; . . . . . .87

Cohesion of liquids, . . 129

Coining press, .... 104

INDBX-

465

ColOUfly

number of,

breadth o4 in the ipao*

trum,- . • •
of chin plalwaiid rin^i,
Golnram, electrical, .
ComboBtiblea, «

Comtrastion, a aotifee of moving
power,

O.40S
384
386

990
999
445

S87

114

Componenti of oUiqne finoea^ 91,92

Compaaa needles,

variatienB of the, .
Compoaition of finroea, firequent,
Compenaation pendulunai, .
Oompreaaibility of air, .
Oo&e. double, to ahow centrea of

gravity, .
Concords in music,
Gontracted Tein^ .
Concert pitch.
Conductors of heat,
Convex mirror, .
spectacles.
Conductors of electricic3F,

ConvuMve notidae by galvwim, 498

flattened in aged penoliB,
Cords^ tenrioBOt, •
CorkS) used in swiUBiing, .

Cbrdage, stiffness eC •
Couronne de tasses, .
Coulomb on human labour,
Crickets, notes o^ . .
Cmickshank's battery, .
Curved surAoes, notioat oo.
Curvilinear motions, .
Cuvier's experimenii on the votoe

ef animals,
Cydoid, the curve of isochioDistti
Cylinder «lectrieai machine.

Gallon on liquid expansion,
Daniel on the pyrometer, .

on qnaatitf ef nin ftll<

ing, . .

Danish balance, .

Davy's discoveries by calvunsm,

on heating by uietion,
Deflagralor, Hare's, . . 441,
Delnc on me nanmmn demity

of water, • • • «
Delisle's thermometer,
Density defined^ .

related to temperature,
Desaguliers on human, labour.
Descent of bodies from the mpon,
iWxterity, feats of,
Diameter, polar, of the earth,
Dickenson* Captain, nse of diving
beUs, . ^^ . . .

460
461
93
103
181

67
S46
167
S53
320
356
410
426

375

409

88

158

108

440

116

237

440

54

65

865

61

430

301

298

142
88

446
286
442

308

896

147

308

116

46

71

46

Diihienilial therfmuttelir , . ^.999
Diminutionofotgectabydislaiiee, 380

Dioptrics, 368

Dipping needle, . . » . 461
Direction of gravitetion» « . 41
lineo^ . . 70

DispeiaionoflightbyleMMs, .390
Disehaige of liquids through oii>

fices^ . * . .169

Distances meaiured by sound, . 238

Diving beU, .887,288

Diving, deep, efleeton the'ear, 138, 838

jackets, ... * 829

Distillation, 310

Dott employed to labour, . 119

DoUond's achromatic lensei^ « 394
Double refraction, 409

vision, . . * . 377
Draaoing friction, .110

DnBoelr inventor of the tbermo>

meter, . 899

supposed inventor of
microscopes^ . .411
Driving vdieel, .... IQO
Dmm of the ear raptured by di-
vine, . . 238
caiMiUe of ten-
sion, . 237
Dromedary, pswer oil . . 119
Droviming, how to prevent, . 157
Druids, ..... 98
Duhamel <m mosiGal glasses, . 851
Dynamica defined, ... 17

Ear, musical, .... 248
£arth*s attraction, ... 39
Earth supposed to be perfinated, 46
EbuUition, . . . . .309
in vacuo, . . . 310

Echo,. 267, 268

Eckeberg's extraordinary ftals, . 25
Eclipse of the sun, . 346

Ediplic, obliquity oC causes the

seasons, 281

Eflkaency of wheel work, . . 87

the inclined plane, 98

the screw and lever, 96

compound machines, 106 /

the hydraulic ram, . 178

Eflicaeioiis rays, .... 396

Elater noctilncus, . . 336

B\Ba!deity common lo all matter, . 23

a measure of gravity, . 4>8

of the human oody, . 85

of steam, . 316

as a moving power, . 113

of air. .... 185

experiments CO, • . 191

Electricity, 418

excited by fiictioiH « 484
kindsoC - ' •485

466

INDEX.

Electricitvrkindt of^l idflntical, ^449

Electrical animalt, . 447

balance, . .428

chime of belli, . . 435

dance, . 435

jar, • > • • 4«)4

machine, . . 4S9, 431

Electric fluid, . . .421

positive and nega-

tiTe, . . .'433

Electria, 427

Elecuoluminom ether, .16

Electromagnetism explained, . 421
Electromagnetic cylinder, . . 453
induction, . 452
EHectiometer, .... 428
Electrqpcope, .... 428
Elements, four, apparatus of, . 149
Elephants, atrength of, . .119
walk on tight rope, . 71
Engine, steam, .... 313

fire 218

caloric .... 323

Equator, centrifugal force at, . 58

Equilibrium in a oalance, . . 29

hv cords and pullies, 29

M lamps suspMided, 30

• stable and unstable, 71

of floating bodies, . 161

Ericsson's caloric engine, . . 323

Esquimaux use mow eyei, . .411

ventriloquists, . . 275

Ether, boiling, freezes, water, . 309

luminiferous, . . . 339

elecCroluminous, . . 16

Eulenstein, improved the Jews'-

harp, 258

Euler on Barker's mill, . .176

Eustachian tube, .... 231

Evans', Oliver, steam-engine, . 318

Evaporation, rate o^ . . 308

of ice, . . .308

Expansion of bodies, . . 279,288

liquids, . . . 300

in freezing, . . 303

Experiment, its value to physical

science, . .13
on cmnpositifm c£

forces, . .33

on bodies falling in

mcuo, . • .38

wiUi the air-pump, . 190

Extraordinary rainbows, . . 398

Eye, structure of, . . . 374

Fahrenheit's thermometer, . . 294
Faraday, gasses condensed by, . 179
magneto^lectric sparks, 454
on vibrating plates, . 252
Fata Morgana, . . » . 353
Fat persons float easily, . . 156

Feather and guinea appaimtiii f. 38 <
Fecundating powders m the air, 220
Firmus, feats of strength 1^, . 25
Figures, jjeometrical, . . . 10

Fire engine 218

Fishes, air bladdeif in, . . 159
. electric, .... 448
Flea, comparative strength of, . 113
Float boards, direction oC . - 175
Floating of bodies on water, .155
Floatation emjdoyed to laiae

weights, 160

Florence academy, • .125

Flow of water through apertures, 167
Fluid, electric, .... ^1
aeriform,. . . . .15
imponderable, . . .16
magnetic, how used, . 16
viscous, .... 128

Fly wheel, 104

Flying, art of, . . . 220,224
Foci conjugate, .... 370
Focus of a mirror, . . . 355
lens, .... 370
Forcing pump, .... 217
Forks, tuning, differ in different

nations, 254

Force of traction, . . 118

or impetus, how esliniated, 28

.of gravitation, . . .50

Forces, parallelogram of, . . 31

. central, .... 56

. composition o£ . . 31

union ot, in one line, . 38

resolution of, . . .35

Fordyce on ponderability of heat, 277

Fountain, Hero's, . . . 193

Fountains, submarine, . . 144

intermitting, . . 214

Four elements,. .... 149

Franklin on leaminc to swim, . 157

pouring oil on the sea, 171

electrical kite o( . . 286

theory of electricity by, 438

Freezinff, artificial mode of, . 306

French horn, ..... 259

Friction, 20

action oC . 108^ 109, 110
angle o^ . . .111
wheeU ...» Ill
Froffs electrified after death, . 437
Friuts, shrivelled, expand in va-
cuo, ...... 190

Fulcrum, ..... • .75

Fusee, 86

Gamut, musical, .... 846

Galileo on falling bodies, . • 43

discovered the law of oa-

dilation, . • .61

Gaises, specific gravity of| . .154

INDBX<

467

GangeSf rain, their constroctimi, p. 142
Garipuy, application of siphons, . 214
Gay, Luasac, balloon aacenaion, . S^2
Gaasea, dUOferent kinds oC • • 183
their power of ooiiducting
sound, .... 239
Gattoni abbate, .... 260
Gaivani's discovery in electricity, 419

Galvanism, 436

GeoQietricalllnea, ... 9
GUbert, Dr. . . . . .418

Giant's harp, .... 260

Glasses broken by sound, . . 251
Globular form ofliquid masses, . 165

Glottis, 262

Glow worm, 336

Goat employed in mechanical la-
boor, 120

Gold, globe of, compared to the

earth, 39

Gongs used in China, . . . 256

Governor for machinery, . .105

Gray's electrical discoveries, . 419

Gravity, cause of, unknown, . 11

affects all bodies, . 20,38

centre of, . . .66

prevents perpetual mo-

tion, . . • . 20
a general moving force, . 36
i9 in proportion to masses, 39
gives velocities inversely

as masses, . . . 39
influence of, by Cavendish, 4L
its direction, . . 41, 42
ibrce of, increases with

the heiffht, . . . 42
IS invenely as squares of

distances, . '. .* ^
counteracted by centri-

fugal force, . 59

determined by pendulums, 63
of mountains, . . . 40
experiments
on, . .40
mtensity of, at London, . 50
centres of, how deter-
mined, . . . .67

acts as a moving power, 1 13
specific . . 147,162
centre of, in liquid
masses, .... . 166

causes the flow of liquids, 168
of air, .... 182

Grindstones split by centrifugal
force, ... . . .59

Guinea and feather experiment, . 38
Guitar, structure and use of^ . 255

Hammer, a mechanical power, . 78
.how diiven upon its
handle, '. . .26

Hammer, water, . p,\dQ

Hammering produces heat, . 287

Handmill, 104

Hare's litrameter, .... .150
deflagrator and calorimotor, 441

Harmonica, 256

Hawksbee's electrical discoveries, 418

Hearing trumpet, . . . 275

Heat, a moving power, . .114

cauae of, . . . . 277

. sources of, . . . . S80

sources of, within the earth, 284
produced by friction, . . 285
evolved by compression, . 287
latent,. ... . .303

specific, .... 304

propagati(m of, . . . 320
reflection of, . . . 329
Hebatock. to raise carriages, . 83
Heights measured by ialling

bodies, . 51
barometer, 201

Heliostat, 417

Helix defined, .... 95

Hemispheres, Magdeburg, . . 197
Henry s mode of magnetizing soft

iron, . • . 454

vascillating magnet, . 457

induction of electricity, . 458

Hero's fountain, .... 193

Herbert on expansi<m, . . . 300
Herschel.on solar heat, . .280
on thin plates, . . 400
Hiero*s crown, fraud in, detect-
ed, . . . . . .150

High pressure steam-engine, . 317
Hildreth on the cicadae, . 261

Hogshead burst by a small tube, 133
Home, Sir E. on the fly's foot, . 209
Hollow cylinders, strength of, . 113
Horn, French, .... 257

Horse power, standard of, • . 118
Horses, strength of, . . .118
Humours of uie eye, . . .375
Human body, stability of, . . 70
Humboldt's description of car^

gueros 117

Humboldt on equatorial climates, 282
Hunter's screw, .... 97

Hunting cog, . ... . . 101

Huygens' arc of isochranism, . 61

Hydraulic machines, . . . 172

. ram, .... 1T7

Hydraulics, science ot, . . 166

books on, . . . 179

Hydrogen applied to aerostation, 221

Hydrometer, 163

Hydrostatic balance, . . • .162

paradox, . . . 132

preas, • 134

Hydroatatics, science o£, ^ • • 122

46§

tilXlf • • • • Jk

•vaporated at low tampei^^
ture,

JCaUHM MNUTt « • ■ •

InagQa, double bjr rafractiMi, .

tuiiuod Of lonaei, •

iR a dark room,

in plana niffoia» .

in a concave miiror^
impenetrability of air, .
Imponderable fluids, .
Imppemiooi, dui«bilil)r oC •
Impi» bottle, ....
lnoandeMenoe» ....
Inddence, angle o^
inolined planei, ....

mOlifHIB ODt

Indoctifm of eleotficity,
Index of rafraetixMi,
Inertia, nature of,
IneoeveiUe fluids.
Inflaming- point of vapours,
Inffenbooc, Dr* on eonduotion of
neat, -....•
loseeli, saunda piodueed bjr,

^esof^ . . . .
Insensible spots on flie retina, .
Interftnnoe of liquid partides, .
Insulation, electrical, •
Invariable stratum, .
Invisible lady, . . . .
fiis buttons, ....
Isoctmnisn of pendulnms, •
Italian musical scale, .
Ivory oompfessed by coUiaiaii, .

laoobi, BarlMora, ventriloquist,
Jar, Leydeo,.
lew's>liaffp^ . •

-Kaleidosoope* s tm c tur e of, .
Kempelen*8 autxxnalDD,
Killamey, echo at,
Kite, paper, its mode of aetion,
Knift used by droggista, .
Vrataeenttein s vocal tubes, .

( •

iAb3rrinthoftfae

liabour. human, .

liacerta oeckOk .

tedder, now mised, .

Lama, power of, .

Lamp glasses, sinumbml, .

Lapwing, ....

Laplaoe on density of air, •

tittentheat,

Xiarynz, ....

I^be, tuming» •

Iaws of eunruittear nocion,

109

308

404

404

378

340

360

360

180

16

381

191

879

364

91

53

487

3i5

80

16

387

381
861
373

383
167
487

871
403

61
848

84

874
433
867

,350
865
869
886
79
864

831

116

800

80

180

367

861

808

306

868

SO

88'

. 79
188-104

Lawioflklltagbodfoi^^. p.40
motion, bv AUvwjod, . til
Leeden ball eqosi to the ataa»<

phere, M

Leaning towwa, .

Lemon squeezers.

Lengths of pendnlnni»

linnseSi bmnmg, •

optical, .

oonvez, .

concave,. .371

achromatic, . .894

Levelling instrument; . .139

. 74
. 75
. %
. 78
. 78
. 81
.438
.817

Lever, propenies wi, .
meae of its action^
theory of).
eedeieeC •
progressive,
compound,
Levden phial,
lifting fNunp,
Light and heat, eauseof, ooknomi, 18
interrupted by the air. . 334
transmuwion ofc . . 3%
velocityof, . 338^343,344
movee m right lines, . . 339
intensity o&ow<liBiinidioi, 340
theory oC • . . 386
Lightning identical with electri-
city, . . . 419
roda, . ■ .436
Liquid substances defined, . . 15
preasara on container, . 137
Liquids, propertiee of, . .128
and solids, bow diflerent, 123
form oC in large masMS, 124
weight and pressure oC • 129
equal piess£e ofi . .136
' difleient in density, . 148
impact oC .172
conduct heat imperlbctly, 328
produce electneity by
friction, . . .425
Liquon botded in condeiued air, 227
Litrameter of 'Dr. Hare, . ISO
LoadsiDiie, natural, .450
Locks on canals, . . . 146,147
Locusts, . .861
LudolTs eleetrical discoveries, . 419
Lukens' magneto-electric ma-
chine 455

Luminous bodies^

Lunar bow

Lyon's account of arctic dog%
Lyre, account of the, .

398
119
854

Machine, A ttwood*s . . . 51
effects oil how investi-
gated, - * 2
eomplei^ •98

INDEX.

469

Macliine, compound, how esti-
mated, . . p. 105
usefulness o^ . .112
Mainspring of a watch, . . 86
Magic lantern, .... 415
Magdeburg hemispheres, . ' .197

Magnetism, 450

terrestrial, . . 459
Magnetizing, .... 459
Matus discovered polarization, . 406
Maskeleyne, experiments of, . 41
Ma«»ses and velocities produce

momentum, . . . .21
Mast-head, ball dropped from, . 34
Materials used in the arts, . .112
Maximum density of water, . 902
Mechanics, nature of, > . . 17
necessi^ of, . .17
Mechanic powers, . . 72

Media, refracting, . . . 364
Medium, resistance of, to motion, 19
Mechanical use of the gases, . 184
Melville Island, . . . .282
Meniscus lens, .... 372
Mercurial gauge, . . .187
Metals, polished, bad radiaturs, . 331
Metronome, Maelzell*s, . . 65
Micrometer screw, . . .98
Microscope, simple, . . .411
compound, . . 412
solar, . . . 416
lucernal, . . . 417
Middle C of the piauo forte, . 249
Mill, Barker's, . . . .176
Minerals luminous after heating, 338

Mirage, 351

artificial, . . . 352

Mirror, plane, .... 349

concave, effects of, . . 359

Chinese, .... 357

burning 328

Mixtures, calorific, . . 288

frigorific, . . . 307
Mobility defined, . . . 18
how estimated, . .18
Moccia*s power of floating, . 156
Malard restoring the position of

walls .289

Momentum, how estimated, . 72
of power and resistance, 78
Montgolfier's hydraulic ram, . 177
balloon, . . .221
Moon seen through clouds, . . 18
phases of the, . . 346
Morveau on Wedge wood's pyro-
meter, 298

Motion requires an active cause, . 19
a£racted b^ inertia, . . 21
different kinds of, . . 27
direction of, « . .27

Motion iccelereted,

p, 36, 47

on inclined planei, . ~ .52
rotatory, .... 99
conversion of| . . . 101
of li(}uids, how caused, . 166
in distant bodies, . . 380

Moving bodies, .... 25
force, oblique, . . 35
powers, .... 113

Muschenbroek'sjar, . . . 432

Naime's electrieal machine, . 429
Natural phUoeophy, object of, . 14
forces producing motion, 72
Nature supposed to abhor a var

cuum, 199

Navigati<m of riven, . . . 146
Newcomen's engine, . . 314

Newton's laws of curvilinear mo-
tion, . ' . . .28
cause of planetary mo-
tions, . .38
theory of light, .386
error respecting disper-
sion, . 393
experiments on colour-
ed rings, . . . 400
Niagara, cataract of. . . .145
Nighthawk, noise of, . . * 261
North pole, weight of a body at

the, 45

Notes, musical, .... 247
Nut crackers, .... 79

Oblique action, .... 88
Octave organ pipes, . . . 244
Oersted, water compressed by, . 126

discovers electromagnet-
ism, .... 425
Oil, effect of, in calming npples, . 171
Opaque bodies, .... 407
Optical axis, .... 404

instruments, . . . 407
Optics, science of, . . . 333

works on 417

Orbitsof the eyes, . . .375
Ore, a mode of raising, . . 86
Organ {upes, length of, . . 244
Oscillation of pendulums, . . 59
centre of, . . . 64
Otto Guericke's air-pump, . . 190
hemispheres, . 198
Overshot wheel, . "^ . . .175
Oxen, power of, . . . .120
Oxyhydrogen microaoope, . . 417

Pabos, power of^ . . • . 120

Paganini, 255

Papin's digester, . . . .311
Parachute, .... 223

2R

470

Pkndox, hydnntatie, .
PtraMlene and Fariielia,
Ptoillelogram of forces,
Ptrallelopiped, tides of.
Parallel motioa of Watt,
pMcml, discovery <^ in hydn)sla<
tics,

oo bArometric measipe-
ments, . . . •
experimenii on liquid co-
lumns, .
PendalniBs, oscillatioa ot^
weight of,
length oC
compensating,
Penuoibra, .
Percussion and premire compa-

leo, • • • • •
Perkins, steam boiler of,

oo oomoresBibility,
on coiuiensation of air,
Penm on human labour,
Persian wheel,
Phases of the moon, .
Phantasmagoria, .
Phantaamasoope, .
Philosophical mquiry, limits oC
Philosophy, nataral,'arrangement

of^ .
defined,
Phusphoreacence of matter,
of the sea,
Physical sciences, province o^
Piesometer of Perkins,
Pincushion and cannon ball com-
pared, • • ■
Pile, electric,
voltaic.
Pini<m and wheel.

INDE^.

Plane, inclined.

Plaster of Paris, mirrors made of.
Plates, colours of thin,
Pliiiy on speaking nightingales,
Pluviameter or rain*gauge, .
Pneumatics, science of.
Polarization of li^ht, .
Polarity, magnetic, by light,

Polyearons

Pompeii, lead pipes used in.
Porters, strensth of) .
Potter's wheel, .
Power or impulse defined, .

when counterbalances re-
sistance,

Powers, mechanic, divisi<m oC, 72, 73

moving, .
Press, hydrostatic.
Pressure on curved surfaces,

and percussion, .

centre oC •

13S

353

30

33

108

134

801

904

60

61

68

103

345

93
312
196
180
118
173
346
416
382

13

14

11
335
337

14
127

23
444
439

99

53,91,92

328

399

266

142

179

406

462

10

140

117

59

21

105

lis

134
55
93

137

Pressure, bjdroslatic . . p, 191
or air on the body, . 906
Prince, Dr. improved the mr-^pmap, 196
Printing press, Russel, . 88

Prisms, defined, . . .11

refracting, . 369
Proteus, supposed meaning oC . 14
Pumps, 814

with holes in the sactioii
pipes, .... 216

liffing. . . . .816
Pulley deacrOied, .89,90

compound, . . .91
Pupflofthe eye dilated, . . 376
Puy de dome, experimenii on, • 901
Pyionomics^ . . • . 877

Rack and pimoo, . 102

Radiation, .... 397,330
Radius of curvature, . .57

Railway planes, .... 98
Rain, how retarded, . . • 141
quantity of estimated, . 141
gauge, .148

quantity of in difierent coun-
tries, .... 143
Rainbow explained, . . . 396
Raro&ction, degree of, . 188— >I95
Rays, efficacious, .... 396
refracted by drops, . . 396
Reaumur's thermometer, . . 294
Receiver fixed to air-pump^ . 197
Reciprocating motion, . . . 101
Regulation of machinery, . 108, 103
Reindeer, labour performed l^, . 190

Remora, 810

Rennie on strength of granite, . 112
Reed suspended by nair and

broken, 96

Reflected motion, . • .28

Reflection of sound, . . . 266

of liffht, . .348

by thin plates, . . 401

atmospheric, • . 351

from concave surftcee, 358

from convex suriacea, 354

Refraction of light, . . . 363

vatAeot, . . .364

incfex of, . . . 365

double, . . . 403

negative and positive, 405

Refractive power. . . . 391

Refrangibiljty of 06I0UT8, . . 388

Resiliency 24

Resistance defined, . 81,72

as related to power, . 21
Resinous electricity, . . . 423
Resonances, .... 853
Resolution of forces, . . .35
Resultant of oblique fixces, 31

INDEX.

471

RMultant of three or more forces, p. 32
Retina of the eye, . . .341
Ricochet motion on water, . . 128
Rider in a circus, . . .34
Rings, coloured of Newton, . 400
Rigidity of cordage, . . .111
Ritchie 8 magneto-electric appa-
ratus, 455

Rockets, sky, . . . . 225
Rolling friction, . . . .110
Roman aqueducta, . . 140

Romans understood hydrostatic

presBure, 140

Rope pump, .... 173

ferry boat, .... 35

Rowing, labour in, . . .117

Rozier and Romain, . • . 222

Romfoid, 278

on heating by friction, . 286

calorimeter oil . . 305

Russel press, .... 88

Sadler the aeronaut, . . . 222
Sails set to receive a side wind* > 35
of a windmill, . . .35
Saint Bernard, mount, . . ~ . 283
Send on vibrating plates, . . 252
Santorio*s thermometer, . . 293
Sappharina indicator, . • . 337
Saxton*s electro-magnet, . . 55
Scale of musical composition, • 245
diatonic, .... 246
Scales for weighing, . . . 81
Scoriae float on mmted metals, . 158
Scott, (Sir W.)t insensible to mu-

sic, 242

Screw, properties of, . . .94
Hunter's, .... 97
efliciency of, . . .95
Sealinff-wax, electrical state of, . 424
Seconds' pendulum, . . .64
Seeds conveyed throug^h the air, 218
Seesaw used as a macmne,. . 75

Shadows, 345

Sharp castings of iron, . . 302
ShucKburgh on specific gravity, . 152
Sight, how assisted, . . .15
Siphon, invested with mercury

and water, . . . 149

principle of the, . . 212

Wirtemberg,. . . 213

Slinff, principle of, . . 56, 58

Smoking through the ears, . . 232

Smoke ascending in air, . . ^

descend when cooled, . 37

and balloon near each other, 38

Smoothness, only apparent, . . 109

Snow eyes, used by the .^Bsqui-

mauz, . . '. . 411

Siiow melted by black earth, . 331

Soda water,

Solid bodies, . .

figures.
Solids immersed in fluids,
pressed bv liquids.
Soniferous undulations.
Sonorous bodies, .
Sound, medium of,

inaudible in vacuo,

0.194
. 15
. 10
. 138
. 171
.235
. 243
. 233
.233

propagated through solids, 234
vibrations caused bjf^ . * 234
diminished by distance, . 237
inversely as square of dis-
tance, .... 238
Telocity of, . . .238
conducted by water, . 239
transmitted by solids, . 240
interference o^ . . 231
reflection of, . . .266
concentrated by a sail, . 270
Space, relative, defined, . .15
Spaces described in fallin|f, . 48
as square of velocities, . 50
relative to power and re-
sistance, . . . 106^
Sparks from voltaic apparatus, . 445
Speaking trumpet, '. . 278,

Specific gravity, by the siphon, . 150 '
table o£ . . 153
of persons, '. 155
Specific heat, .... 304

Spectacles, 407

Spectre of the Brocken, . . 361
Spectrum, solar, .... 385
Spina, inventor of spectacles, . 407

Spiral 95

Spirit, level, .... 139
Sponge, principle of its action, . 164
Stable equilibrium, . . .71
Stability, area of, . . . .70
Statics defined, . . . .17
Statera, Roman, .... 82

Steel-yard, 82

Steamboat, iron, .... 160

Steam, temperature oC . . 31 1

engmeof Watt, . .312

engine of Evans, . .319

boilers projected upwards, 225

Stewart on ventriloquism, . . 273

Stockenschneider on heating, . 286

Stonehenge, .... 92

Strength of materials, . • .112

Strings, musical, .... 243

Sun blind, 85

the fountain of heat, . . 280
influence of, on climate, . 282
Surfoces, convex motions on, . 55
Sultzer's discoveiy on metallic

eflTects 427

Suspension, points of, . . .8]

473

INDEX.

SwinmiDgf how learned, .

importance ol^ .

syringe, exfaautting,

(f •

n.196
. ]57
. 187

TiUe of fiaUnf bodies, . 48, 51

■pecific gravities, . .153

IVkleofpulliei, . . 91

Tantalus' cup, . . . .213

Tate liqueur, . . .212

Telescopes, achromatic, . 398

refracting, . .413

f reflecting, . .414

Temperature, .... 282

remarkable, . . 285

Temperatures obtained by Daniel, 299

scales of^ . . 298

Tenerifle, peak oC, , . , 379

Tension, 112

of cords, •88

Tlianmatrope of Dr. Paris, . . 381
Tliermoaieter, .... 292
scales, ... 298
mercurial, . . 297
differential, . . 299
Time of falling bodies, . 48

related to mechanical ac-
tion, .... 107
Tbbacoo pipe, experiment with, . 69

Toggle imnt 88

TDrricelli invented the barome-
ter, 199

Torricellian tubes, > . . . 201

Tournaments, collision in, . .23

Torpedo, explosive, . . . 230

electric, . . . 448

Towers, leaning, . . . .69

Traction, force of, . • .118

Transmitting motion, . . .87

Transparent bodies, . . . 333

Transparency of media, .' 334

Tread wheel, .... 86

Tuba stentoriphonica, . . . 273

Tubes, safetv, Watson's, . . 159

capillary,. . . . 164

long, retard flowing water, 170

of adjutage, . . .170

Tunics of the eye, . . . 374

Tuning forks, .... 254

TwiUght, 366

Undulations, theory of. . . 338
Uniformly moving bodies, . . 27
Unstable equilibnom, . .71

Vacuum, perfoct, unattainable in

the air-pump, . . 195

nature's aohorrence of, 199

Vaporization, .... 307

Variation of gravity, measured, . 59

the compass, . . 461

Velocity before and after impact, p. 21
of moving bodies, • . 25
measured by time and

space, .26

degrees ot, produced by

ttlling, . . .42
requirea to project bo-
dies to the moon, . 46
aot^uired, 47, 49

on mclined planes, . 54«
angular, . . .57
maximum in a jet of

water, . . . 168
of light,. . . .342
of sound, . 238

Vem contracted in flowing liquids, 167
Venturi, marsh drained by, . 171
Vent hole in casks, .211

Vessels, two, encountering, ruin-
ous to the smaller, . . .23
Vibrating figures, .68

Vibrations of musical strings, . 235
of sonorous bodies, . 243
numbers oC . 248

visibly demonstrated, 250
of luminiferous ether, 387
Vision, organs of^ . 373

at different distances, . 376
distant, . . . .379
Voice, human, .... 262
Volta modified Galvani's theory, 420
Von Mengen, Baron, a ventiuo-

quist, 275

Vowel sounds formed by pipes, . 265

Walcot, Dr. mimicry by, . .273
Walrus, foot of, . . . . 210
Water barometer, . . . 203
compressibility of, . .124
compressed by Perkins, . 128
daily supply of, in London, 140
collected in basins under

ipionnd, .... 145
resistance of) to pressure, . 152
distilled, weights of, . . 153
camel, . . 160

discharged from a basin, . 170

wheels 174

works, air-vessel 6^ burst, 177

rising in a pump, . 200

held m an inverted tumbler, 211

Watt's jointed bars. . . 102, 316

estimate of horse power, . 118

steam-engine, , . . 314

sun and planet wheels, . 101

Wedffe explained, . . .93

Weighing machine, . . .83

Weight of 'body carried to tfas

m^n, . .44

as a moving force, . 72

INBEZ.

478

Weight, of liquidi, • . p. 129
diminution oC by immer*

•ion, . 130

rant, . 132

of air, . . .182,184

Wedf(ewood'i pynmetor, . . 297

Welb, Artesian, .... 144

WheatBtone on aoand, . . 258

Wheel ramored fiom a carriage, 77

and asle, .... 83

worli, efficiency oC • .87

and pinion, . • .99

driving, .... 100

toothed, . . . .100

win and planet, • . 101

Peruan 173

overshot and breait, . 175
barometer, • . 202

Wheeli, fiiction, . • • .111

Whirling; table 57

Whispenng gallery, . • • 270
White's piHley, .... 91
Wick ranes oil by capiUarity, . 164
Wind instnimenis, • . . 256
pipe of animals, . . 262
Wirtemberg siphon, . . 213

Wollaston on sounds inaudible, . 237
Wollaston's ooncavo-convezleniies,410
Woriuofraferance on mechanics, 121

hydraulics, 179
acoustics, . 276
pyronomics,332
optics, . 417
electricity, 463
hydrosta-
tics, . 165
pnettniatics,230

THB E]n>«

y

Work§ PubllAlied by £dward C. Blddle.

MANUAL OF CLASSICAL LITERATURE, from the Ger-
man of John J. Eschenbarg. With Additions by Professor Fiske
of Amherst College. The work comprises five parts: — 1. The
Archeology of Greek and Roman Literature and Art. d> The Greek
and Roman Classic Authors. 3. The Gfeekand Roman Mythology.
4. The Greek and Roman Antiquities. 5^ Classical Geography
and Chronology. — ^Third Edition.

Mb. Edwakd C. Biddlb,

Sir,— At yonr requent I have examined the " Manual of Glassical Literature,
from the German of J. J. Eschenburg, Professor in the Carolinum at Brunswick,
with Additions," and am prepared to state^ without reserve^ that I consider it the
best assistant to the classical student of all the worlcs of the kind that have ever
met my eye. It ought to be in the hands, not only x)f every tyro in the commence-
ment of his classical career, but should find a place in the library of every lover of
Grecian and Roman literature. It is a most Valuable acquisition to the academies
and colleges of our country. With great pleasure I recommend it to the patronage
of a liberal public Very respectfully.

Sir, yount, &c. SAML. B. WTLIE,

Universitpy Ma^ S5, 183&. Fice-Provo9t of tke University of Pm%.

We cheerfully concur ki the above opinion of Dr. Wylie.

JOHN FROST.
WILLIAM RUSSELL.

JiVom tUt. B. B. Baekta^ Profetsor of Clasneal Literature in Bnmm UnivergUif.

** The Manual of Classical Literature*' is, in my opinion* the most valuable work
of the kind which has yet been given to the public. It goes farther towards the
supply of a want which teftchers have long felt, than any similar work with which
I am acquainted.

From Rev. J. TbtU, au£kot (ffthe **8tudeni*s Manual,** and the "Sahbath-aehool Teacher.**

This book ought to be in the library of every professional man, the physician,
the lawyer, and the clergyman. There is an amount of information condensed in
this volume, which amazes one who has known the toil of trying to gather up in-
formation in his study. No professional man can afford to lose what he must lose
if unacquainted with this work. And as to students, I have no doubt they will
gladly obtain it. Professor Fiske has made himself a benefactor to our young men,
and they wBl do ii^ustice to themselves, not to follow in the path which he has
opened.

From A. 8. Padtardy Profetsor of the Latin and Ghreek Languagea and CUunad JJi/^

raJture m Bovodoin College.

The American student has now access to important sourees of information, from
which he has hitherto been, for the most part, excluded. In regard to the labours
of the translator, especially in the additions he has made to the work, I very
cheerfully respond to the general sentiment which has been expressed in fkvour
of their great value.

From Mr. John D. OgtOty, A. JIT, Editor of Horner^ VtrgU, Lempriere*s ClaeeiealDie-
tionary, ^e.; and Prcfeeeor of Latin and Greek Languagee in Rutgers CoUegSy Jfew
Brunswick, JV*. J.

I have for several years, in the course of my teaching, felt the want of a Manual
like Eschenburg*8, but had little expectation that the want would be so soon and
so well supplied. I examined the work when it first appeared, and determined to
adopt it as a text-book in the department under my charge. The favourable opi-
nion, which was the result of my first perusal, has since been confirmed by daily
use with my classes ; and I am well assured that its popularity in our colleges and
classical schools will more than realize the expectations of its able editor, and
abundantly reward the enterprise of its publishers.

From the Rev.' Solomon Peck, formerly Professor of Latin and Hebrew in Amherst Col'
lege, and late Professor of Classical Literature and Philosophy in Brown Unicertity.

E8chenburg*8 ** Manual of Clastical Literature,*' translated, with additions, by
Professor Fiske, will be foand a truly valuable help in the study of the Ancient
Classics. The original work <has for many years enjoyed distinguished fhvour
with German scholars ; and the English copy has been prepared with due regard
to neatness and accuracy. The additions appoar to have been made with good
Judgment, especially in the-ikmrtment of Greek literature. As an introduction
to classical authors, I am acquainted with no work of equal merit. It is compre-
hensive in its plan, and its materials are select, and judiciously arranged.

WorftJi PnbUsliecl bjr Edward C. Riddle.

JProm C. H. JSUtmj A. M.y Ohoirmmn of ike Eaumitmig CemmiUee of tke JiwuntmmJSS'
9oeuaionfor the Supply of Teachers, ami Prindpal of the PhUadUphia. FemaU JBgh
School,

Blr,— I have with care looked over a ve#y yalaable work, lately from your jwen,
** Eschenburg'a Manual of Classical Literature," and I close the Tolume with feel-
ings which prompt me to state to you, in a few words, my opinion of its merits.
Its title is sufficiently indicative of its contents, but without examination, no scholar f
would suppose that in- about MO pages are comprised ftill, but conciBe and able,
treatises on the following subjects : — Archeology of Greek and Roman Literature '
and Arts, History of Greek and Roman Literature, Mytholoi^y of the Greeks and
Romans, Greek and Roman Antiquities, and Classical Geography and Chronol<^y.
A glance at these subjects will show, that if sufficiently exact, this Manual wUl
supirty the place of some four or five volumes, which the diligent student finds it
useful often to consult. The portion devoted to the view of the Classical Authors
may seem too limited, and yet all that can be easily retained in memory, i. e. the
most important focts, are given. In other respects I am not disposed to wish it
enlarged by the addition of a single paragraph. The fact that tnis Manual has
gone through seven or eight editions in Germany, a country, most of all, celebrated
for classical attainments, is of itself no mean commendation of its excellence ; and
it is somewhat singular that three or four eminent classical scholars, in distant
parts of our country, were engaged in the translation of it at the same time, un-
known to each other : so general is the conviction of its utility among us. Though
Professor Fiske very modestly comes before the public as a translator of the work
only, it will be found that many and very important additions and useful altera-
tions are made. Besides what is necessary on the subject of the value of Greek
and Roman coins, there are interesting additions to the text of Eschenbnrg re-
specting the remains of Athens and Rome, and a condensed view of the sacred
writings, and the writings of the early Christians, as found in the Greek language.
The whole of part five is also added. Professor Fiske deserves much from our
scholars for this excellent epitome, and I have little doubt that he will be gratified
by its extensive circulation and use. It is well adapted to our high schools and
academies, as well as indispensable to the college student, unless, indeed, he
would have the trouble to refer often to Adams, Lempriere, Urquhart, and others.
In every public and private library it deserves a place, and will no doubt find one,
when the work becomes generally known.

Very respectfully, CHARLES HENRY ALDEN.

July 7, 1836.

ThefoUotoing extratU are from a critical notice of the ^^Manualy" pubKshed m the Bi-
blical Repontorjff Andovery Mas*.

** Eschenburg's Manual of Classical Literature'* has long had a high reputation
in Europe, having gone through seven or eight editions in German, and one in a
French translation. The author zealously extended a taste for English literature
in Germany, having translated the works of Bumey, Shakspeare, &c. Among
his publications, the one now first presented to the American public, and which
has been adopted as the basis of public and private instruction in the major part
of the colleges and universities in Germany, is designed to form a complete manual
of the most essential aids in reading the classical authors. The matter, in the
American dress, is arranged under five parts, or heads : — Part I. Archeology of
Literature and Art. Part II. History of Ancient Literature, Greek and Roman.
Part III. Mythology of the Greeks and Romans. Part IV. Greek and Roman An-
tiquities. Part V. Classical Geography and Chronology. The volume is divided
into about 600 paragraphs, for the sake of convenient reference. These are printed
in a larger type, and are for the most part a translation from Eschenburg. In-
serted between many of these paragraphs are a large number of references, ex-
planatory remarks, iffustrations, k.c., nearly all from the pen of the translator.
In these aidditions. Professor Fiske has rendered more complete the great design
of the work, in that which constitutes its peculiarity, and distinguishes it from
other works in the language.

As to the need of such a work as this of Eschenburg, there can be but one opi-
nion. Some Valuable detached sources of information may be found, like Potter's
Antiquities ; but no comprehensive, copious, and at the same time select and dis-
criminating manual on the subject has been within the reach of the mass of students.
The statement of the contents of the work of Eschenbnrg just given, will fUmis^
some idea of the comprehensive nature, as well as the scientific arrangement of the
topics ; both of which are characteristic of the volume. The number of works
referred to, the various sources and materials for fUrthe'r illustration and investi-
gation, are Very great. While these will not impede the progress of the young
student, being for the most part thrown into a small and separate type, they will
Airnish the advanced scholar cluf|s and hints for more extended and profound rer-
search. The references are not merely to German works, but to English publica-
tions, and frequently to important articles in our periodical Reviews. The manner
in which the translator has executed his work needs no commendation f^om us.

I-^ I I 11 ■■ «I 1 1 1 1 !■ I ■ t H I ■ Ill !■■ !■*>— ^il— —!<

Works PubUslied by Kdward C. Blddle.

To an acquaintance with the German language, he adds the practical experience
derived from the many years in which he has been employed in classical instruc-
tion in two of our principal colleges. The volume will find a place in our college
text books ; in our academies and higher schools ; and in many private libraries,
it will fill the same place in classical literature which the works of Jahn do in bi-
blical. A part of the translation is by Professor Cruse, late of the University of
Pennsylvania ; and Fart V. is not the original German.

From the Boston Recorder.

We have no hesitiation in saying, this is the most comprehensive and valuable
work of the kind which has appeared in the English language. Eschenburg was
one of the most distinguished scholars of Germany . Six editions of his work were
published before his death, (in 1820,) to each of which useful improvements were
made under his own eye. A French translator of the work remarks, ^^ it is suffi-
cient ehcomium on the book, that it had been adopted as the basis of public and
private instruction in the major part of the universities and colleges in Germany."
The present volume is divided into five parts : I. Archeology of Literature and
Art. II. History of Ancient Literature, Greek and Roman. IIL Mythology of the
Greeks and Romans. IV. Greek and Roman Antiquities. V. Classical Geography
and Chronology. The work is divided into sections of great convenience for re-
ference. The intervals are occupied with notes, illustrations, and references, by
Professor Fiske. These are very numerous and valuable, as they render more
complete the design of the work, and furnish a vast amount of important matter in
a small compass. The notes and references do great honour to the tf ansiatori as
an accomplished, judicious, and diligent scholar.

EXTRACTS FROM LETTERS ADDRESSED TO THE TRANSLATOR.

Frwn Rev. Edzoard RobinsoHj.late Professor Extraordinary at the Theological Semi-
nary ^ Andover.

I formerly had occasion to make considerable use of the original " Manual" of
Eschenburg; and have ever regarded it as the best work of the kind extant. It
is the production of an elegant aiid philosophical mind, perfectly at home in its
acquaintance with the subjects of which it treats. It was therefore with great
pleasure that I learned your intention of translating and preparing the work for
the benefit of American students ; not only because I had entire confidence that
you would do it well, but also because you would thus in a good measure fill out
what has hitherto been a blank in English literature.

fVom hia EaceUeney Edward Everett, formerly Professor ^f Greek Uterature tit Har- '

vard University.

I am acquainted with the work in the original, and have always regarded it as
one of the best of the class. I know of no volume which contains so much infor-
mation, in every department of classical literature. I have, of course, had very
little time, since I received your translation, to form an opinion, by actual exami-
nation, of its merits ; but as far as I have looked into it, and after a cursory peru-
sal of a few of the leading chapters, I feel warranted in saying that you have aug-
mented considerably the value of the work. I regard your translation of it as an
important service rendered to the study of classical literature.

The foUovoing is from Mr. Sohrmon Stoddard, lately a Teacher in Tale College, and in
the JVetc Haven Oymnasium, and one of the authors of the Jfeto Latin Ctrammar.

Professor Fiske has rendered an important service to the cause of classical learn-
ing, by his translation of the "Manual" of Eschenburg. The original work con-
tains a large amount of valuable matter in a comprehensive and convenient form ;
and the additions of the translator are Judicious and important. As a whole, it
ftirnishes such a storehouse of information to the classical student as is not other-
wise accessible to him, except in large and numerous volumes. I cordially recom-
mend it to the attention and the study of teachers and scholars.

The foUotnng is from a letter from Rev. Moses Stuart, Professor of Sacred Literature

in the Theological Seminary, Andover.

As to the valtu of "Eschenburg," there can, I think, be but one opinion among
competent judges. We surely have no work in English which will compare with
it. I hope that it will be introduced, and made a necessary part of apparatus, in
every Latin and Greek school and in every college in our country. The additions
which you have made in the notes, and in Part V., will surely be deemed an im-
portant part of the book^ for American students. ■ If minute inveltigators in Bibli-
ography, Mythology^ &c., should discover some errors in your book, you must not
be disheartened, but rather encouraged to go on with your plan. In a work of such
a nature, to avoid all erpror in the innumerable iisurts and dates which are stated, is
out of the question.

TT"^

^!irw*«*^f^w^«»w^

| | * |||

■*•

Workfl PubUsliecl bj Edward €. Biddle.

JOHNSON'S MOFFAT'S NATURAL PHILOSOPHY.— A
System of Natural Philoaophy designed for the use of Schools and Acaide>
Biies, on the basis of Mr. J. M. Moffat^ eomprising Mechanics, Hydrostatics,
Hydraulics, Pneumatics, Acoustics, Pyronomics, Optics, Electricity, Gal-
vanism and Maenetism : With Emendations, Notes, Questions tor Ex-
amination, &c. &.C. By Prof. W. R. Johnson.

Tlie title of the above woik has been cbanged from ** Scientific Class Book,
Part I."

JOHNSON'S MOFFAT'S CHEMISTRY.— An Elementary Trea-
tise on Chemistry, together with Treatises on Metallurgy, Mineraloey,
Chrystallography, Geology, Orjrctology and Meteorology, designed for
the use of Schools and Academies ; on the basis of Mr. J. M. Moffat :
With Additions, Emendations, Notes, References, Questions for Ex-
amination, &c. &c. By Prof. W. R. Johnson.

The title of tbe above worI( bas been changed Arom *' Scientific Class Book,
Part II."

The Board of Controllers of the Public Schools of the First School District of
Pennsylvania, at a meeting hejd March 8, 1842, authorized the introduction into
the Grammar Schools of the District, of the above works by Prof. Johnson.

Ma. Edwasd C. BinoLE,— PhUaiapkiayJwu 93, 1835.

I have carefully examined your *^Scienti^ Class Book, Part I.,'* and find it what
has for some time been much wanted in our academies and high schools. The
emendations, notes, and additional illustrations, are important, and what might
be ezpectedyfrom one so perfectly at home, both theoretically and practicaBy, in
the range of Natural Philosophy, as Mr. Johnson is extensively known to be. The
list of works for reference will be appreciated by intelligent teachers. I have in-
troduced It aa a Text- Book, and commend it cordially to the notice and examina-
tion of others. CHARLES HBNRT ALDEN,

Principal of the Philadelphia High School for Young Ladies.

Ma. EnwAnn C. BinoLR, tth Mnuk fOdy 1835.

Sir,— r have examined the first part of the Scientific Class-Book Just pnUished
by you, and cheerfblly express my opinion, that, for accuracy and compreheaeive-
ness, this work contains a system of principles and illustrations on the subject on
which it treats, superior to any book of the same sise and price intended for the
use of schools.

As this volume is the first of a series on the Mechanical and Physical Sciences,
the public may confidently expect that the successive parts, when completed, will
oonstitttte a consistent set of treatises peculiarly adapted to the present wants of
places of education. JOHN M. ](£A6T.

We cheerfliHy concur in opinion whh tbe above recommendations.

JOS. P. ENGLES,
HUGH MORROW,
WM. A, GARRIGUES,
M. SOULE,
JACOB PEIRCE,
BENJAMIN C. TUCKER,
T. O. POTTS,
WM. CURRAN,
8. BICKNELL,

D. R. ASHTON>
EL. FOUSE,

C. FELTT,

THOMAS BALDWIN,
JOHN STOCKDALE,
URIAH KITCHEN,
THOMAS H. WILSON,
SHEPHERD A. REEVES,

E. H. HUBBARD,
WILLIAM McNAm,
JAMES CROWELU
J. O'CONNOR,

WILLIAM MARRIOTT.

RIAL LAKE,

BENJAMIN MATO,

JAMES P. ESPY,

REV. SAML. W. CRAWFORD, A. M.,

Principab of the Acadl. D^. of tbe

University of Pennsylvania.
THOMAS McAD^M,
CHARLES MEAD,
JAS. E. SLAC^,
L. W. BURNET,
WM. MANN, A. M.
CHAS. B. TREGO,
WM. ROBERTS,
THOa. COLLINS,
SAML. CLENDENIN, '
AUGUSTINE LUDIN6T0N,
JNO. D. GRISCOBf,
N. DODGE,
JOHN HASLAM.

JWw rorJk, JWy, 1635.
Having examined tbe First Part of the Scientific Class-Book, we feel Justified hi
concurring in the above fbvourable recommendations.

EDW. p. BARRY, QAVID SCHUPER^

J. M. ELY, F. A. STREETER,

JOSfePH McKEEN, CHARLES W. NICHOLS,

JONATHAN B. KIDDER, THOMAS McKEE,

Works Publlslied by Edward ۥ BIddle.

PATRICK 8. CASSADY,
WM. R. ADDINGTON,
RUFU8 LOCKWOOD,
NORTON THAYER,
JOHN OAKLEY,

6. I. HOPPER,
J. B. PECK,
S. JENNER,
RICHARD J. SMITH.

From JSkxoMder D. Bache^ A. JIf., Professor of A'atitral PhUoeophf and C&Miistry,

University of Pennsylvania,

Me. Evwasd C. Biddlb,

Sir,— I have examined, with much pleasure, the first part of the << Scientific
Claee-Book/* The additions of the American editor appear to me to have well
adapted the book for use in schools and academies. Its utility to the general reader
has no doubt been increased by the same labours. Very respectfully, yours,

September 16, 1835. A. D. BACHE.

Prom JV. W. Fiske, A, JIf., V. D» JIf., Professor^ Amherst College^ Mass,

Mr. Edward C. Biddle,

Sir,— The "Scientific Class-Book'* appears to me,Judgin$r A'om the portions I
have yet found time to read, a very excellent work. A vast amount of the most
interesting and valuabl^nowtedge )m brought into a small compass, and is gene-
rally presented in a very clear and happy method. I hope it will obtain extensive
circulation, as I know of nothing better adapted for common instruction in the
sciences which are treated in the part I have seen.

Very respectfully, I am yours,

Septmher 21, 1835. N. W. FI8KE.

In the opinion expressed by Professor Fiske, respecting the " Scientific Class-
BooIe, Part I.," I can most cheerfully concur. E. S. SNELL, A. M.,

Professor of Mathematics and Natural Philosophy, in Amherst College,

Massachusetts.

JSVotii Ren. David R. j^utfttn, A. JIf., Principal of Monson Academy.

I fully agree with Professors Fiske and Snell, in regard to the " Scientific Class-
Book," and shall adopt it in the institution of which I have the charge.

D. R. AUSTIN.

Professor Johnson has rendered the public an invaluable service in his ** Scien-
tific Class-Book." It is a treasure of useftil knowledge, happily adapted not only
to the wants of the student, but not less so to the general reader. There is so
much intrinsic merit in this volume, so much of what every youth of every grade
in the country should, in some sense, be familiar with, that I am sure it needs only
to be known to ensure it a wide circulation. Aside from its peculiar merit as a
class-book for the higher schools, I would say to every young man in the United
States, about to engage in the business of life, Let the Scuwt^fic Class-Book he your
eonstant companion. E. H. BURRITT.

Jfew Britain^ Conn.y Dec. 7, 1835.

From Rev. W. C. Fowler, A. JIT, C. A. S., Professor Middlelmry College^ Vermont.

The ** Scientific Class-Book*' is admirably adapted to the use of high schools
and academies, as an introduction to the principles of physical science. It is neither
a meagre sketch on the one hand, nor on the other is it overloaded with fkcts.
The principles are distinctly announced, and the illustrations and proofk are inte-
resting and satisfkctory.

FremAOeHHoplane, A. M., Professor of Maihematies and Miotural Philosapkyy WU-

Uams Cottege.

A work like the ^* Scientific Class-Book," edited by Professor Johnson, has been
fbr some time called for by an increasing taste for scienoC, and a higher standard
of popular education. Such works ought to meet the popular demand, and to ele-
vate still higher the standard of attainment. Both these objects, I think, ax« ade-
quately secured in the present work. I cheerfully recommend it.

fVilUamstown, Mass.y February 22, 1836.

From Aare/n JV. Sjfciiiii«r, Esq., A.M., Prindpal of a Seleet Classieat School, JV*e» Agtoen,

Connecticut,

After three months' use, I have no hesitation in saying, that I thtok the '* Scien-
tific Class-Book" the best work with which I am acquainted for popular and prae-'
tlcal instruction, when the object is to convey asefal and interesting information
without mathematical demonstrations. Its arrangement is good, and its plan ex-
tensive, embracing Almost aU the toptes of Physical Science. The great number
of Ikcts, experiments, and illustratkMW by drawings, &c., render it a highly attrac-
tive book to the pupil. I cheerftdly recommend it as tiM l>e8t and most complete
work I have seen for what it is intended, viz. ** A ftmiliar Introduction te tke
Prineiplefl of Physical Science."

Workfl Publlslied bjr £dward €• Riddle.

From AugtLMtuM W. Smitk, A. M.y Profeuor of Jfatural Pkilooopky and

fVeologan Unhenitiff MtddUtown^ Conn,

An examination of the *' Scientific Ciau-Book, Part I./' pablighed by yon, haa
left a very favourable impression. Of the excellencies of this work, there is one
which establishes its claim to public fkvour, and will most certainly secure for it a .
speedy triumph over works of similar grade and pretensions. I allude to the m- (
troduction of many scientific Acts and principles which have liitherto been buried I
in the voluminous and inaccessible records of learned societies, or are of too recent I
developement to have been earlier imbodied in any popular work. It appears to
me to be one of the very few popular odentiflc works which are not dignified by
their title, and one of the still smaller class whkh possess the merits of a pablK
bene&ction. AUGUSTUS' W. SMITH.

March 17, 1835.

From laaae Webb^ Esq.y A, M.

I Ailly concur in the opinion of the ** Scientific dass-Book, Part I.," as ex-
pressed by Professor Smith. ISAAC WEBB;

Enraet from a Report madt to tke Ijffeenm of Teachers, of PhUadelphia.

Your Oommittee are of opinion that the book (Scientific^lass-Book) in qaestion
is, in almost every respect, superior to the books now in use, on the subjects it
embraces. They submit the following reasons as the ground of their preference : —
1. The different subjects are presented to the student in such a manner, that,
without some effort on his part, he cannot understand them ; but with that effort,
he is richly rewarded with an ample fund of valuable facts, arranged, explained,
and classed in accordance with the recent improvements in physical science. S.
At the foot of each page the editor has introduced a few questions so judiciously,
as to induce the important habit of attention and reflection, without which, to an-
swer them would be impossible ; thus affording one of the best tests of the actual
amount of acquirement which the student has made. S. The work never seems to
lose sight of the great importance of making all science subservient to the happi-
ness of man. This, it appears to your Committee, it has done in a high degree, by
showing to what a great extent the successful prosecution of the arts depends on
science. 4. The editor appears to have spared no pains in the effort not only to
render the work in a high degree instructive, but at the same time to introduce
such interesting (because practical) illustrations, as to make it a very |rieasaat
book for those for whom it was designed. In conclusion, your Committee have
seldom seen a work, intended for youth, in which there is so little to regret and
so much to approve, as that submitted as the subject of this report.

From JV*. Dodge, A. M., Member of the Examining Committee of the American Aseod'

ation for Supply of Teachers.

I have examined with as much care as my leisure would permit your *' Scientific
Class-Book, Part IL," and shall introduce it into my seminary as a text-book, for
the subjects of science which it embraces. lam AiUy convinced, that the scientific
course presented in these volumes, is decidedly superior in systematic form, as
well as compass, to any extant in the English language. N. DODGE,

Principal of Harmony Hall Female Seminary.

From Colonel James M Porter, President of Board of Trusteee, LefayOts CeUegt,

Easton, JsTorthampton Co., Pennsylvania.

In this age, wherein utility is the true test of value of publications, *< the Scientific
Class-Book" must meet with public favour, because it so ftilly deserves it. I
would recommend it for use in schools, as admirably adapted for the purpose of
instructing youth in the principles of the physical sciences ; and master mechanics
would advance their own interests and promote the knowledge of their appren-
tices, and consequently the value of their services, by placing the work in their
hands for perusal ; for ** every mechanic art is the reduction to practice of scientific
principles," and the better the principles are understood, the more perfect will be
that reduction to practice. J. M. PORTER.

Mktston, Pa., April 6, 1836.

From Mr, Cleanthes FeU, M. A.

I have carefully examined the second part of **the Scientific Class-Book," and
it appears to me to deserve the patronage of those concerned in the education of
youth. It is, indeed, in my opinion, the very book so long needed ; I, therefore,
cheerfully recommend it to parents, guardians, and teachers throughout the United
States.

From Charles Henry Alden, A. M., T^ettdker, PhUaddphia,
Bin. Edward C. Biddle,

The surest test of the excellenceof a book,— its extensive adoption and me;— has
Pcen applied, and successfVilly, to the "Scientific Class-Book, Part I. ;" and the

Works Publisbed by £dward €• Blddle.

success of "Part II.,'* which you have Just published, is therefore not to be
doubted. Given to the public under the supervision of the same accredited scholar
as the former volume ; enriched by additional illustrations ; in many places emend-
ed, and containing a valuable list of bibliographical notices, it can, with propriety,
be commended to the use of schools and academies, as well as to private families,
as a most valuable manual. The treatise on Chemistry, though necessarily very
short, embraces a perfect outline of the science, and contains the most recent dis-
coveries. The tracts on Metallurgy, Mineralogy, Chrystallography, Geology,
Oryctology, and Meteorology, are nowhere more lucidly and attractively explained.
This volume ought to accompany Part I., wherever that is adopted ; indeed, in
my opinion, it is more deserving of public favour.

The style and execution of the "Scientific Class-Book, Part II.," as a produc-
tion of your press, is highly creditable.

February 10, 1836.

From John M. Keagf, M. !>., Profeasor eUet of Dkkinson CoUege,

After an examination of the second volume of the " Scientific Class-Book,'' I
feel a pleasure in stating that it fully sustains the character given of the previous
part, as an excellent compend on the subjects of which it treats. The Chemistry
and Metallurgy, the Geology, and History of Fossils, and the sketch of Meteor-
ology, are particularly clear and comprehensive, to be comprised within the limits
of a single duodecimo. JNO. M. KEAGY.

Pkiladelpkia, February 15, 1836.

JFVom Professor BeeJfc, Rutgers College^ JVW0 Brunsvieky JV*. J.

' "The Book of Science," by Mr. J. M. Mofiat, which forms the basis of the pre-
sent volume, (Scientific Class-Book,) has already become extensively and de-
servedly popular in Engl9.nd. Professor Johnson, the American editor of these
volumes, has greatly improved them by correcting many of the errors contained In
the original works, and by the addition of many interesting notes, of a set of ques-
tions for examination, lists of works for reference, &c. They are very properly
styled ** A Popular Introduction to the Principles of Physical Science." On each
of the subjects treated of, there is an amount of information in these volumes
which is seldom found in elementary treatises of this description ; while this
information is set forth in such a manner as peculiarly to engage the attention of
the pupil. In their composition, the best authorities have been consulted, and
" due acknowledgments have been made wherever they seemed to be required."
These works are indeed what they purport to be— Scientific Class-Books ; and Pro-
fessor Johnson deserves well of the friends of science for the labour which be has
devoted to the preparing of them for the American public.* If the friends of educa-
tion are really in earnest in the business of improvement, these books will soon
take the place of those incorrect and defective treatises on the various branches
of physical science which most unfortunately are now so generally adopted.

nionunaieiy are now so generally adopted.

ji

Refuge, near Meehanie^urg, Pa.y June 15, 1896.
' Scientific Class-Book," Parts I. and II. As tt

Sir,— I have examined your " Scientific Class-Book," Parts I. and II. As the
result of my examination, I am happy to state that in these books I found a work
well adapted to, and much wanted in our schools. The editor. Professor Johnson,
has evinced a sound judgment in the additions made ; and you, as publishers, have
conferred a lasting favour upon the public in giving this judicious work circulation,
and I trust it will be generally introduced in all our schools and fltmilies. I can
recommend it as one of the best works extant, on the physical sciences. I shall
cordially use my influence to give the work an extended introduction into schools,
lyceums, and families. J. D. RUPP,

Agent for the Pa. Lyceum.

From C. H. Anikony, Esq., City Surveyor, (TVoy, JV*. F.,) and Leeturer on th4 JiTatural

and Experimental Sciences.

As a teacher of the Natural and Experimental Sciences, I have often felt the
need of some works in all respects adapted to the present state of science in this
country. My beau ideal of such a work is flilly realized in the ** Scientific Class-
Book," parts First and Second ; and I have lost no time in introducing them into
my school.. Part First is excellent; but Part Second I consider as the best text-
book in general science ever published in the English language.

From Samuei Jones, Ji. M., of Pkiladelpkia.

I have already given the First Part of the " Scientific Class-Book" my approval t
and now, after having tested the utility of the Second Part, I am fully prepared to
endorse the favourable opinion expresfied by others of its value.

Works Publlfllied by Edward ۥ BIddle.

AN ETYMOLOGICAL DICTIONARY OF THE ENGLISH
LANGUAGE, — On a Plan entirely new. By John Oswald, Author of
the " Etymological Manual of Enelish Language," and '* Outlines of En-
glish Grammar." Revised and ImproYed, and especially adapted to the
pQipose of teaching English Composition in Schools i^id Academies. By
J. M. Keagy.

The Board of Controllers of the Pnblic Schools of the First Sehool District of
Pennsylvania, at a meeting held March 8, 1842, authorized the introduction of
Oswald's Etymological Dictionary into the Grammar Schools of the District.

Mb. Edward C. BiohLS,

Sir,— In republishing *' Oswald's Etymological Dictionary,** enriched as it is by
Che sensible and well written " Introduction" of Dr. Keagy, you have done a real
service to the cause of found edmeation. It is the best work of the kind (designed
for schools) that I have yet seen, and it must have an extensive circulation. For
in every well regulated school taught by competent masters, etymology will form
a prominent branch of study as long as there is an inseparable connexion between
clearness of thought and a correct use of language.

Yours respectfuUy, C. D. CLEVELAND.

We ftUly concur in the above.

J. M*INTYRE,
JAMES B. ESPY,
JNO. SIMMONS,
B. W. BLACKWOOD,
E. H. llUBBARD,

E. NEVILLE,

F. M. LUBBREN,
WM. A. 6ARRI6UES,
WILLIAM MARRIOTT,
RIAL LAKE,
THOS. T. ASPELL,
A. MITCHELL,
CHARLES MEAD,
WM. MANN,
WILLIAM M'NAIR,
JOHN STEEL,
BENJAMIN MAYO,
JOHN HASLAM,
CHAS. HENRY ALDEN,
THOMAS EUSTACE,
W. CURRAN,
BENJAMIN TUCKER,
M. 15. HURLBUT,
T. G. POTTS,
CHARLES ATHERTON,
HENRY LONGSTRETH, A. M.

SAMUEL CLENDENIN,

E. FOUSE,

THOMAS CONARD,

HENRY BILL,

THOMAS BALDWIN,

U. KITCHEN,

DANIEL MAGINIS,

JOHN EVANS,

JOSEPH P. ENGLES,

J. W. ROBERTS,

BARTRAM KAIGN,

JNO. D. GRISCOM,

RICHARD O. R. LOVETT,

AUGUSTINE LUDINGTON,

WM. B. ROSE,

NICHOLAS DONNELLY,

C. R. FROST,

WILLIAM ALEXANDER, A. Bf.

M. SOULE,

J. KAPP,

JOHN STOCKDALE,

REV. SAML. W. CRAWFORD, A. M.,

Principal of the Acadl. Dept. of the

University of Pennsylvania.
THOMAS H. WILSON,
THOMAS A^«ADAM.

Pnm Mr. William MtuiseU, A M,y author of an JShridgmont of Mama' Latin Grammar^

Teacher, t^e.

Oswald's "Etymological Dictionary,*' revised by Dr. Keagy, is a work which
will be found invaluable in all schools in which attention is paid to the systematic
study of the English language. The plan and arrangement of this manual are such
as to bring under a single glance the etymology of all cognate terms, in addition to
that of the particular word which happens to occur in any instance ; and the ex-
tent to which this classification is carried, enables the student to command a sur-
vey, as it were, of the capabilities of our language, in the expression of whole
classes of ideas. Oswald's Etymological Dictionary possesses, in this respect, an
advantage over other works of its class ; as most of these are restricted to a mere
alphabetic arrangement of words, in consequence of which it becomes exceed-
ingly difficult to obtain a complete view of any series of derivations.

I am happy to have the opportunity of introducing the Dictionary in my school,
as I shall find it a useful substitute for oral instruction, in parsing lessons, both in
Latin and English ; having been accustomed to require a statement of the deriva-
tion or composition of every word in such lessons before that of its inflection or
other variations. The use of this work will not, therefore, cause me any extra
arrangement of classes, while it will be of equal assistance to my pupils and my-
self. Other teachers may find it convenient to introduce the book in the same or a
similar way. The merits of the work itself, however, are such as to render it con-
ducive, in the highest degree, to all purposes of instruction connected with lan-
V^^i and I have no doubt that it wiU be adopted in all schools in which an ac-

8

&•'

Works Publislied by £dward C. Blddle.

curate knowledge of etymology is deemed important. Dr. Keagy's ]M-eliminary!
essay on tbe forms of thought as giving origin to those of expression, will greatly
enhance the value of the work to all teachers who place any reliance on the phi-
losophy of instruction. WM. RUSSELL,

No. 03, South 8th street, Philadelphia.

From Mr. J. H. Brown, TVocAer, PkUadelphia.

a

The ** Etymological Dictionary*' of Oswald, needs no commendation when it is
known that its merits have been such as to induce Dr. J. M. Keagy to revise and
improve it for the use of schools and academies.

The merits of the work will bear testimony in fovour of the ability of Mr. Os-
wald for the present undertaking; while the extensive philological researches of
Dr. Keagy, his devotion to the cause of education, particularly to the study of lan-
guage, and his success as a teacher, leave no room to doubt the merit and utility
of the present work.

No one aiming to make himself master of the English language, should bsuwith-
out a copy of the present work, fox daily examination and reference.

J. H. BROWN,

May 16, 1836. No. 52 Cherry street.

*<The Etymological. Dictionary by Dr. Keagy on the basis of Oswald,*' appears
to me highly adapted to remove many of the difficulties with which youth have to
contend in their earlier attempts at composition. Those who have had the slightest
experience in teaching, must be aware how utterly inadequate our ordinary dic-
tionaries are to the wants of the pupil ; and even were his Judgment sufficiently
matured to make the necessary discrimination, the time requisite for searching the
larger dictionary^ could not well be spared from other studies. While the work,
however, presents many important advantages to the learner, it proposes neither
to supersede tbe exercise of his judgment, nor to secure in every instance a just
application of the language without labour and care. From the ease with which
reference is made to principles, in the arranging of the words according to their
genera, thereby enabling the pupil to acquire the significatioh of a whole class of
words with comparative ease ; and in the fkcilities afforded to the mere English
scholar for obtaining a radical acquaintance with his own language, the Etymolo-
gical Dictionary offers decided advantages to the pupil, and must prove a valuable
auxiliary to the teacher. JAMES 600DFELL0W,

Teacher, Sansom street.

From Charles Henry Alden, A. JIf., Cltakmnn of Examiidng CommUeeofihe Jhiuriean

Aasoeiationfor tko Supply of TeacMrs.

Mb. Edward C. Biddle, —

I have examined with great interest your *' Etsrmological Dictionary," and I am
convinced that its use will prove of immense benefit to pupils and students of every
age. While its prominent design is to flirnish a correct knowledge of our lan-
guage, it will serve also as a most admirable apparatus for mental discipline. To
the teacher who is not acquainted with the Latin and Greek languages, this work
is invaluable ; and even to the classical scholar, the number of derivatives placed
after the several roots, will suggest shades of signification invaluable to him who
is desirous of expressing his thoughts in definitive terms.

Dr. Keagy'a Introduction is such as a mind like his might be supposed to pro-
duce. Successfully devoted to elementary instruction for several years, and hav-
ing given his attention very much to what may be called the philosophy of educa-
tion, he has here put together a series of fitcts, and Arom them deduced principles
of primary interest to all, especially to parents and teachers. The worlc ought to
be adopted as a text-book in our high schools, and be possessed and daily used by
our students in college.

From J, B. Walker^ A. B., TBoekery of PkUadelphia,

.... M/-^-_.U*.. «|l«.._^1^._t»l Tkl..**......— ...^ ««.. v..

»»

Such a book as ** Oswald's Etymological Dictionary of the English Language
has long been a desideratum. I am gratified to find that this excellent work, im-
proved and rendered more practically uaefal by the labours of Dr. Keagy, has at
length been given to the public. It is well fitted to exercise the pupil's powers of
discrimination and judgment, and to aid him in acquiring a thorough knowledge of
the English language. It commends itself to the consideration and adoption of
teachers.

9

Works Publislied by Edward G. filddle.

MAURY'S NAVIGATION A New Theoretical and PracUcal

Tfeatise on Navigation, in which the Auxiliary Branches of Mathe-
matics and Astronomy, composed of Algebra, Geometry, Logarithms,
Plane and Spherical Trigonometry, the Motions of the Heavenly
Bodies, Tides, Variation of the Compass, &c. arc treated of. Also,
the Theory and gaost simple Methods of finding Time, Latitude and
Longitude by Chronometers, Lunar Observations, Single and Double^
Altitudes, are taught. Together with a New and Easy Plan fbr Find-
ing Diff*. Lat. Dep. Course, and Distance. By M. F. Maury, Passed
Midshipman, U. S. Navy.

" U. S. Jf. S., J\reiD York, January 19, 1836.

** Dear Sir,— I have had much pleasure in the perusal of your " New Theoretical
and Practical Treatise on Navigation;" the plan and arrangements of which are
original ; it contains little or nothing superfluous, and every part of it appears to
be as clear and intelligible as the nature of the subject will admit. Such a work
has long been wanted in our Naval Schools, and on board our vessels of war. I
intend to make use of it in the Naval School on this station ; and 1 reconamend it
to be used by all the professors of Mathematics and Nautical Science in the Navy
of the United States. Yours Respectfully, EDW. C. WAAD,

^ Passed Midshipman M. F. Maury, Prof Math. U. S. Navy.**

U. S. Navy."

" U. S. Jfavy Yard, Chtaport, March 7, 1836.

*<I have examined a Treatise on Navigation written by M. F. Maury of the U. S.
Navy; and have no hesitation in recommending it to the students of that science.
The explanations are clear, the rules are illustrated by many examples, and the
new arrangementof someof the tables exemplify the calculations of the navigator.
Mr. Maury ia deserving of great credit for that work, and I wish him every
succeBS. P. J. RODRIGUEZ.

*< Jfavy Departmtntj AprU 9, 1836.
*' Sir,— I have to request that yon will add the *' New Theoretical and Practical
Treatise on Navigation," by M. F. Maury, Passed Midshipman, to the list of books
Airnished vessels of the navy going to sea. 1 am respectfully yours,

"Com. John Rodqbrs, Signed, H. DICKERSON."

** President of the Board of Navy Commissioners."

FRENCH LESSONS FOR BEGINNERS.— L'ABEILLE
POUR LES ENFANS, ou Lemons Frangaises, lere Partie; a
I'usage des ecoles.

Several compilations of short and interesting French tales have been lately
offered to the i>ublic. In all of them, however, expressions are found, which,
although familiar to the ear of a Frenchman, offend that of a carefhlly educated
American child. It is true that the French do not consider **Mon Dieu!" swear-
ing ; with them, it is equivalent to " Gracious !" or " Oh, dear !" but it is certainly
desirable that the eye and the ear of the pupils of schools in this country should
not become accustomed to such expressions. They have therefore been careftilly
excluded from this little work, as well as every thing of an unchristian tendency.
It is designed for the flrst reading book. The style is simple, the sentences short,
and containing few idioms, inversions, or difficulties. At the end of each page is a
translation of the idiomatic expressions it contains, and of the words used in an
acceptation not given in the dictionary.

From J. O. de Sotevy Ji.A.^ Professor of French^ Spaniahy and Italian, Philadelphia.

I have examined "L'Abeille.pour les Enfans," published by Mr. Edward C.
Biddle of this city, and am so much pleased with the pure and chaste style of the
selection, that I shall use it in my instructions with the younger pupils.

J. 6. DE SOTER.

THE NEW AMERICAN SPEAKER.— Being a Selection of
Speeches, Dialogues, and Poetry, for the Use of Schools. By
Thomas Hughs.

Fi'om the Rev. 8. B» How, D.D., late President of Dickinson CoUege ; and Reo. Dr,
fVestbrookf Principal of Female Seminary, and Rector of Rutgers Cottle Orummar
School.

"*The New American Speaker' contains judicious selections from the writings
of different authors, and is well adapted to the use of our Schools."
Mw Brunswiekj February 17, 1836.

ID

Worlig PnJblisli^ Af Edward C. Blddle.

SPEECHES OF PATRICK HENRY, FISHER AMES, WILLIAM FIMCKNET>
AND OTHERS. .

SPEECHES OP PHILLIPS, CURRAN, GRATTAN, AND EMMETT.

SPEECHES OF CHATHAM, BURKE, AND ERSKINE, to which are added
the Arguments of Mr. Mackintosh in the case of Peltier, selected by a Member
of the Philadelphia Bar.

SELECT SPEECHES of the Right Honorable George Canning. Edited by Ro-
bert Walsh, Esq., with a Biographical and Critical Introduction, by the editor.
1 vol. 8vo.

SELECT SPEECHES, of the Right Honorable William Huskisson, and of the
Right Honorable William Windham. Edited by Robert Walsh, Esq., with a
Biographical and Critical Introduction, by the editor. 1 vol. 8vo.

LIBRARY OF ORATORY, comprising the above five volumes, uniformly bound.

FAMILY BOOK OF DEVOTION, comprising Daily Morning and Evening Prayers
for four weeks, a Sermon on Contemplation and an Evening Prayer for every
Sunday in the year : and an Appendix of Prayers for particular occasions, with
an Introduction on the Importance of Family Religion. By the Rev. Herman
Hooker, A. M.

HOME BOOK OF HEALTH AND MEDICINE, being a popular Treatise on the
means of avoiding and curing diseases, and of preserving the health and vigour
of the body to the latest period : including a full account of the Diseases of
Women and Children, &c., by a Philadelphia Physician.

FOX'S BOOK OF MARTYRS, 2 vols, in one, 8vo , 60 engravings.

DICK'S WORKS, 8 Vols., fine edition

DICK'S PHILOSOPHY OF A FUTURE STATE.

DICK'S PHILOSOPHY OF RELIGION.

DICK'S CHRISTIAN PHILOSOPHER.

DICK'S IMPROVEMENT OF SOCIETY.

DICK'S ESSAY ON COVETOUSNESS.

DICK ON MENTAL ILLUMINATION AND MORAL IMPROVEMENT OF
MANKIND.

DICK'S CELESTIAL SCENERY.

DICK'S SIDEREAL HEAVENS.

THE LIFE OF WILLIAM COWPER, Compiled fVom his Correspondence and
other authentic sources of information. By Thomas Taylor. New edition.

LETTERS TO AN ANXIOUS INQUIRER.
Introduciory Essay, by Dr. Bedell. /

WU^ERFORCE'S PRACTICAL VIEW OF THE PREVAILING RELIGIOUS
SYSTEM OP PROFESSED CHRISTIANS, in the Higher and Middle Classes,
contrasted with Real Christianity. With an Introductory Essay, by the Rev.
Daniel Wilson, A. M., late Vicar of Islington, now Bishop of Calcutta.

BICKERSTETH'S HARMONY OF THE FOUR GOSPELS, designed for the
use of Families and Schools, and for private edification.

PROGRESSIVE EXPERIENCE OF THE HEART, under the Discipline of the
Holy Ghost, from Regeneration to Maturity. By Mrs. Stevens.

CHRISTIAN'S DEFENSIVE DICTIONARY. Being an Alphabetical Refutation
of the General Objections to the Sacred Scriptures. By Dr. Sleigh. 1 vol. 12mo.

SELECT POEMS. By Mrs. L. H. Sigourney. 4th edition. With Five fine En-
gravings.

CHRISTIAN LIBRARY, 2 vols. imp. oct., containing the following works— Me-
moir of the Rev. Robert Hall. History of the Reformed Religion in France.
Taylor's Life of Cowper. Fergus on Nature and Revelation. Viller's Essay on
the Reformation. History of the Civilization and Christianization of South Af-
rica. The Christian Remembrancer, by Ambrose Serle. Rafile's Tour. The
Church of God, by Robert Wilson Evans. Historical Sketches on the Missions
of the United Brethren. Lectures on the Law and Gospel, by Dr. Tyng. Fair-
holme's Geology of Scripture. Lectures on Portions of the Psalms. A Por-
traiture of Modern Scepticism. Memoirs of Miss Mary Jane Graham. The
Personality and Oflice of the Christian Comforter. History of the Reformation
in Spain. Fanaticism. History of the Crusade against the Albigenses. The
Life of Bishop Wilson. Sermons by the Right Rev. Joseph Butler. Sermons
by the late Rev. Robert Hall. With Critical Notices of various works.

Many of the above works are out of print, except in this form,

- ' ■ .,.'.,■ „■„. . — —

11

By T. Carlton Henry. With an

Workfl PubUsHed bjr Bdward C. Blddle,

IN PRESS.

A BIBLIOGRAPHY OP ZOOLOGY, with Blbliograpliical
Sketches of the principal Authors. By William Svirainson,^
F. R. S. L. S. First American Edition^ with numerous Illustia-
tlons and Corrections by S. S. Haldeman^ Member of the Aca-
demy of Natural Sciences of Philadelphia, Author of '^ A Mono-
graph of the Limniades, or Freshwater Univalve Shells of North
America,'^ &c. Embellished with a Portrait of Thomas Say.

This important work, by an English naturalist o^ high standing, contains the
names of about five hundred writers on Natural History, with a complete list of
their works, and numerous biographical sketches.

Such a Work has long been a desideratum, both among professed naturalists
and students of natural history, especially in this country ; where it has hitherto
been almost impossible to ascertain what works have been written upon particu-
lar branches of the subject.

To the present edition, the American editor has added about one hundred names,
including an extended list of American authors; and many important German,
French and Italian names.

IN PREPARATION FOR THE PRESS.

A GEOGRAPHY OF PENNSYLVANIA, designed for the
use of Schools and for the general reader.

Such means have been taken for the collection of information as the publisher
belieVeH Will enable him to issue a work embracing, in a condensed form, a view
of the physical character, climate, population, pr^uctions, wealth, internal im-
provements, state of education, ice. &c., throughout the State, and in each county.

The services of a gentleman, eminently qualified for the office, have been se-
cured, to edit the work, which the publisher hopes may prove Worthy the patronage
of the public.

"■*

I

L

12