THE LIBRARY

OF

THE UNIVERSITY
OF CALIFORNIA

LOS ANGELES

1

THE

ECONOMY OF*NATURE

EXPLAINED AND ILLUSTRATED

ON THE

PRINCIPLES

O F

MODERN PHILOSOPHY.

B V

G. G R E G O R Y, D.'D.

AUTHOR OF

£SSJTS HISTORICAL AND MORAL, &c.

IN THREE VOLUMES.
WITH FIFTY-SIX PLATES.

The. SECOND EDITION, with confidcrable Additions.

VOL. I.

LONDON:

*»INTED FOR J. JOHNSON* N° 72, ST. *AUL*S «HUEC« TARP.

ADVERTISEMENT

vM

TO THE SECOND EDITION.

'THE favourable reception winch this work "has ex~
perienced from the public having rendered a fecond
edition neceffary Jooner than was expefted, but lit tie time
has been afforded for new difcoveries in philojophy ; yet
the additions and alterations in this edition will be found
tonfiderable, and I flatter myfelf they will be thought im-
provements.

A very long chapter has been added on the Mechanic
Powers, and two whole chapters on the Reflexion and
Refraction 'of Light , intended to make the Jcience of
optics plain and intelligible to readers unacquainted with
mathematics.

Whatever of novelty has occurred in Jiience fince the
ft? -ft publication of the work has been carefully added-,
many omijjions have been Jupplied \ and by the kind at"
tention of feveralfcientific friends, Jome errors, which had
efcaped in the fir ft imprejfion, have been corrected.

G.G.

Chapsl-ftreet, Bedford-row,
Feb. 1798.

857313

TO THE

LORD BJSHOP OF LANDAFF.

MY LORD,

IT is feldom of much confequence to be
informed concerning the circumftances
which have fuggefted any literary under-
taking. If, however, it is not a fubjeft of
curiofity to the public, it is at leaft of gra-
titude to me, that it was the perufal of your
Lordfhips two firft volumes of Chemical
Eflays, that firft convinced me of the prac-
ticability of making philofophy popular, and
induced me to project the prefent publi-
cation.

Your Lordmip will, I fear, difcover that
this is not the fum total of my obligation
to your incomparable work; but that I have
freely ufed, and almoft abufed, the liberty
your Lordmip was fo kind as to grant me
of extracting from it. In every point of
view, therefore, whatever merits the Econo-
my of Nature may be found to poiTefs, are
A 3 ultimately

DEDICATION.

ultimately to be attributed, not to the author
whofe name appears in the title page; and
I can only fatisfy my own mind by making
a public acknowledgment of my obligations,
and by fubfcribing myfelf, with the utmoft
refpecl: and fmcerity,

Your LORDSHIP'*

Ever grateful Servant,

G. GREGORY.

Chapel Street,
February 1798.

PREFACE

TO THE FIRST EDITION.

THE want of a popular treatife of philofophy, one
which might ferve as a proper introduction td
natural hiftory ; to explain to general readers the. great
principles and operations of nature; to give, in a
united view, the discoveries of the moderns on thefe
, important fubjedts, firft fuggefted to me the prefent
undertaking.

It is now many years fince I projefted this work;
and I intended to have termed- it, "The Phiiofoph/
of Natural Hiftory." — In that title I have been antir
cipated ; but my plan, though long fince announced
very amply to the public, has not yet been anticipated,
and the work is Hill as much wanted as when I firit
conceived the intention of undertaking it.

To diftinguifh certainty from conjecture is the mofl
difficult tafk of the fcholar ; a tafk which few find lei-
fure, fortitude, or attention to complete. In the pre-
fent imperfect (late of knowledge, when I fay certainty ,
I perhaps would confine the refearches of human, wildom
within too narrow limits; and probability *mny be the:
more fuitable expreflion, which muft, indeed, compre-
hend no inconfide rable portion of our difcoveries in
nature. To feparate, therefore, the probable from the
fanciful, was my firft object j and, if I was not appre-
henfive of being thought too alluming, I would add,
the ujeful from the fpeculative. I have obferved, that
in all fciences the principal difficulties arife from cer-
tain controverted and difputable points, which are of
little importance in themfelvcs, and which, as they are
not eltabliflied upon competent evidence, are not eafy
to be comprehended.

A 4 To

Vlll PREFACE TO THE

To expect much of novelty in the following pages,
would be to expect falfehood and abfurdity. One
man, even with the unparalleled powers of a Newton,
is able in the courfe of his life to make but few dif-
coverics of importance ; and after the toil of centuries,
it would be extraordinary if much of what is really
true was left to be difcovered. If 1 have liicceeded in
placing in a clear and perfpicuous light the obfervations
of others ; if I have collected, and arranged in a lucid
t>rder, the leading truths in the different branches of
philofophy, I have performed a great tafk ; but this I
dare not flatter myfelf 1 have been able to accomplifh.

Imperfect, however, as the work mud, I am con-
fident, ftill appear — it is yet the labour of fome of the
moft valuable years of my life, with the afliftance of
fome learned and excellent friends, whofe kindnefs in
thefe inftances I (hall have prefently to acknowledge
more at large. Let thofe who may be difpofed to
complain that more has not been done, only reflect on
the difficulty of what has been effected, and I flatter
my fell* they will receive with candour an attempt, in
which not to have fucceeded would icarcely reflect
difgrace on talents fupcrior to mine.

I have endeavoured to lay open the whole book of
nature to my readers. I commence with the fii ft prin-
ciples of philofophy, the laws of matter and motion,
•with an enumeration of the moft fimple or elementary
fubftanccs. I proceed from thefe to explain the nature
and phenomena of that moft active and fubtile of ele-
ir.cnts, heat or fire, which is fo intimately connected with
all other fubftances. The theory of light, and colours, fo
immediately dependant on the preceding fubject, fuc-
cecds; and this is followed by afhort treadle of electricity.
The different fpecies of airs, and the atmofpherical
phenomena, are next treated of; thefe are fucceeded
by a defcription of the earth and mineral kingdom,
and the moft remarkable phenomena conncdeu with
them, fuck as volcanoes, earthquakes, &c. The na-
.ture and compcfkion of water, with a ftiort account of

miliaal

FIRST EDITION. 1*

mineral waters, and of the general properties of that
fluid, occupy the next department of the work.

From thefe fubje&s I have proceeded to what is
called the vegetable "kingdom, including what is known
on the nature and theory of vegetation. The animal
economy fucceeds ; and that as little as poffible might
be wanting to complete the courfe of elementary
knowledge, I have concluded by a (ketch of the human
mind. This latter part will connect properly with my
Eflays Hiflorical and Moral, published fome years ago,
and which contain the great outlines of my fentiments
on moral and policical philofophy.

As it was my defirc to make this treatife as plain
and clear as poffible to unlearned perfons, I have to
apologize to my more fcientific readers for the occa-
fional repetition of the fame principles and obfervations.
Having been, in forne meafure, all my life engaged in
the bufmefs of educations I have feen the neceffiry of
frequently recalling the attention of young perfons to
principles already proved and eftabliihed, in order to
enable them to underftand what is to be taught. In
giving the hiftory of different fciences alfo, many fac~ts>
. and obfervadons are naturally anticipated j and yet it
becomes abfolutely neceffery to confirm, illuftrate, and
apply thefe in a more extenfive manner, in treating of
the fciences at large.

If it is afked, for whofe ufe this work is defigned ?
I anfwer, for all whofe curioiity would lead them to
take a general furvey of nature — for all, in particukr,
who wifhy to underftand the elements and principles of
natural hiftory. I conceive alfo, that it will not be
unufeful to the younger ftudcnts of medicine, fince it is
intended as an eafy introduction to general fcience ; and
fince it comprehends all the fjrft principles of chemiftry
and phyfiology. V/ith the more enlightened clafs of
female readers, I cannot but flatter myfelf that the
work will be favourably received, as I really had their
entertain! nent and information principally in view in
compiling it ; and they may depend upon it, that there
is not a fmgle expreflion in the whole that can rea-
fonably oiTcnd the mofl delicate and model! car.

X PREFA'CETOTttE

To Tome perfens, who, I muft obferve, have rather
more zeal than knowledge, ftudies fuch as thefe iray
appear rather inconfiftent with the clerical profeffion
and the fcicoce of theology, afcience cxtenfive enough,
I confefs, to occupy the life of a man. I might reply
by a fimple fact, that / ne-v.er yet have been enabled to
£v$%> by tbt exft'cife of my fro/cffion^ a livelihood for myjelf
tnd family ; and it muft appear a hard cafe to confine
fhc whole attention of any man to what will not furnifh
him with the neceflaries of life ; yet the great bulk of
my previous publications (without excepting my
Effays) have been in the direct line of my profcffion.
1 do not, however, reft my apology upon this argument,
but I muft fay, that in publifhing the prefent work, J
believe I am not lefs efTentially ferving the caufe of
religion, than if I had been employed in compiling
a treatife of divinity. Next to the ftudy of the fcrip-
tures, there is none which fcems to lead the human
mind fo directly to a knowledge of its Creator, as the
ftudy of nature. In an age therefore when atheifm is
publicly profeffed by fome, and privately but fcduloufly
.difieminated by others, I cannot but hope that a work
like the prefent may have fome good effects ; and%
though I have not, like an eminent philofopher of the
clerical profefiion, termed it a phyfico- theology, the
reader will perceive that this application of v the hiftory
of nature has not been forgotten.

As I have no great caufe to be intoxicated with my
fucccfs in life , and as I am verging upon that period
when man has little to hope or fear in this world, I feel
that it is no affectation to fay, I am not extremely fo-
licitous for literary fame ; yet I will not difiemble, that
I would, if poffible, deprecate on the prcfent occafion
the feverity of criticifm, both becaufe I would wifh
my publimer to be indemnified, fince my very limited
means will net admit of publifhing on my own account ;
and becaufe I would not wifh thqfe friends who have
generoufiy afforded me affiftance in the prefent work,
to fuffer any uneafinefs from the harfhnefs of cenfure,
6 or

FIR.ST EDITION. Xl

or by my felicitations to have £>een drawn into a dif-
agreeable predicament.

It remains to do juftice to thefe friends. ce To ren-
der honour to whom honour is due j praife to whom
praife," i^ the part not only of the chriftian, but of
every honeft roan. In the optical part of this work I
have been materially aflifted by a gentleman of known
and dittingu'fhed abilities, who taught publicly for a
feries of yeatb the feveral branches of natural phi-
lofophy, but who will not permit me to make my ac-
knowledgments in a more particular manner. For,
I m;j.y fay, the whole of the animal economy, I am
indebted to my valuable and fcientific friend DC. Bel-
cher, of Maidftone, as well as for moft eficntial affift-
ance in the mineralogy and the vegetable fyftem, and
for reviling and correcting feveral other parts. It
would be impoiTible to fpecify the authors from whorn
I have extracted my materials : I have inferted re-
ferences • as frequently as I could with convenience.
In fome inftances the reference was neglected in the
copying of my original notes; and in fome, the facts
were commonly known, and diffufed through a mul-
titude of authors.

CONTENTS,

BOOK 1.
QT THE GENERAL PROPERTIES OF MATTER.

CHAP. I. — page i.
Of Matter. in general.

fxplaiuttion of Terms.-"- Whether all Matter is radically the fame.—*
General Properties of Matter. ^~ Quantity of Matter in the ijniijerjt.

CHAP. JI. — page 6.

Of Elements.

OltiKions of the Ancients. -*-New Arrangement. —•Enumeration tfjimpl$
SubJIancef, according to the neiy Philofopby.~-Aerial Subftances. —
Earths.—Mttals.

CHAP. III. — page 10.

Of the Extenfion, Solidity, and Divifibility of Matter.
Ixtenjion the only Quality tffential to Matter.— Solidity, what.-— Infi-
nite Dwijibility. — Animalcule imperceptible to our Senfes.-r-Extrem$
Rarity of l>ight.~~Nerujtonian Paradox.

CHAP. IV.— page \6.
Of Attradion and Repiilfion.

fiiii Kindt sf Attraction. — CokeJton—Combination~-Cryfta[lizatii>*
(xplained. — Gravitation — Specijic Gra-vity, what. — Magnetic aet£
ttraction.— Repuljion.

CHAP. V.— page 31.
Of Motion and Reft,

ea Theory of Motion and Reft. — Yit inerti#. — Laivt of
Motion.

CHAP. VI. — page 35.
Of Magnetifm.

Of xaturat and artificial Magnets.— Magnetic Powers. — Attraction^
—Rekulfion. — Polarity. — Declination. — Dipping ef the Magnetic
ion of the Magnetic Power.

C H A r.

N T E N T

CHAP. VII.— page 5 5.
Of the Mechanic Powers.

Machines.— The Lever.— The Pulley.— The movefile Pi£-
. — Of Wheels. — Clockwork. — Beji Mode of conJlruSing tfo
Wheels of Carriages.— The Wheel And Axle. — Tht Crane.— Tbt
Capflan.—The Crick or Jack.— The inclined Plane. — Motion of
Carriages up an inclined Plane, &c. — The Wedge.— Tbt Screw.—*
The perpetual Screw.

BOOK II.

OF THE NATURE OF FIRE,

CHAP. I.— page 86.
Hiftory of the Difcoveries relative to Fire anel Heat.

Opinions of the Ancients.— Of Bacon, Boyle, and Newton.-*- Of How*
terg, Sgr-atvefend) and Lemery.-^ 'Invention of the Tbermotne-ter*.-***-
Opinion of Botrbaave.— Great Difcovery of Dr. Bl*cL

CHAP. II.— page 93.
Of Fire (Caloric) and its Properties.

Inquiry whether Heat or Fire is a Sue/ft ance or Quality.— Fire a $x$>~
fl once. —Application of this Doclrim.-^— Analogy between Heatanl
Light. — QbjeSions.-Properties of Fire or Caloric', Minutenejs
of Particles ; at trailed by all Bodies. — Conducting Powers -of dif-
ferent Bodies, — Caufe cf Fluidity. — Why Heat is produced by
flacking Lime, and by certain Mixtures of cold Subjlances. — Freez-
ing of Water by the Fire Side explained. — Fire the moft elaStf
of <dl' Bodies.

CHAP. III.— page 108.

Of Expanfion.

Experiments proving the expanfeve Force of Fire, or Caloric.— Infirm
mentsfor mtafuring Degrees of Heat.— Thermometers.— Dr. BlacVt
Mode of meafuring high Degrees of Heat. — Mr. Wedgwood 'j.

CHAP. IV.— page 1 1 6.

Of Fluidity.

Caloric equally the Caufe of Expanfeon and Fluidity.— Phenomena of

Bodies poffing from a folid to a fluid State, and the contrary .—In-

tenfe Cold of the Southern Hemifphere explained.— Diflin^iion between.

>'* and Fllli'iitJ*"~Exteriments M*ftrative of the Dodrine of

afr CONTENTS.

CHAT. V — page 123.
Of Boiling Vapour, &c.

/ from cannon Fluids.— Specific Gravity of

yapoltr.—In iu'hcf Manm Dr. Black was hd in firm bis Theory
of latent Htai. — Immtnfe 'Force cf Papo-ir.*— Boiling. — All Fluids
toil in a lefs Temperature in Vacua, than ur.cier the Preljure of the
A'.mbfpbcre. — "Experiments.-— Phenomena, of Boiling and "Evaporation
€xplaincd.-—Why Water extinguishes F!ame.~~'Sptmt6lneffus £i-af era-
tion.'—Pkenomena cf Deivs, Mijls, &c.

CHAP. VI. — page 140.
Of Ignition and Combuftion.

Ignition, ivbaf, bo*iv produced. — Burning of Phofpborus.—I/tf!ammA~
t>le Air. — Culinary Fires. — Lamps, &c.-—Why Flame afcen >. —
Theory of Argand's Lamp.-—BtJl Form of Grates, Stoves, (ffr. —
Combuftion produced by feme Sub/lances without a Communication
•with the Atmofpherc. — Gtih-fotuikr, &c. — Iron made to burn like
a. Candle. — Spontaneous Ignition. — Curious Facts. — Quantity tf
Hear excited infufmg different Bodies.— Scale of Heat.

BOOK III.

CHAP. L— -page 155. ^
Hiltory of Difcoverics concerning Light, &c.
Opinions cf tie Platonics. — Of A;-'tjlc>le.~Of Albazen. — Of Roger
Bnccn. — The 'Invention -of Sptfiaeles. — Trcatife efMaurclycus on Vi-
f.&n~z—L'>ri* f>na — Rcc/cn that the Sun's Image appears

• an angular Aperture. — Lifuen-

t.'jn cf the Camera Qbfcura. — Conjeflures of Fletcher an the Rain-
bo-iv. — Invention of the Telefcope.—Suppofid to be by Zacbarias
• Janfcn. — Galileo.*—Kefl<!r?— Invention cf tbe Micrcfcopc. — Tycbt
Eraf.t.— Reformation cf Jifcorted Inmges.—* Snellius and Hcrtenjius.
—Dcj'cartcs. — Scbeintr. — Pfftkity of Light difcovere(tj~-Boylt*s

— Nekton; bis Dif-

c o ; n. — On 7?;; . -Bolng-niar. SfcKC.—- 'BalJ-

rr. — Melville* — DoUuntl.
— De laMotte,—~Dc!aval.

CHAP. IT. — page 172.
Of the Nature of Light.

/'., . '• ' '/"I;^.— Theory cf a vibrating Me-

/,,. Drfcarte!, & c,—-Objc8iff)!s to this theory. —

v minttte Parti chs projtfltd from the luminous

Body.— -Inquiry rrfpccling the Identity of Light and Fire.— Experi-
DftK:s of Mr. BovU.— U'hy Light is net always accompanied 'with

CONTENTS. 3t».

that. "—Velocity of Light.— Light aliuay* moves in Right Lines.—
Rarity of Light,— —Force or Momentum of Light,— Mitchell's Expg~
riment.— Inquiry ho~jj far we Sun's Magnitude is diminished by tht
EmiJ/ion of Light.— Light fubjeft to the ordinary Laws of Nature.
'—Light attracled by the Bolognian Phofphori.—The fatne Property
in Diamonds, and other Precious Stones.

C H A i». III. — page 185.
General View of the Phenomena of Reflexion.

Bottles do not reflett the Whole of tht Light that falls on them.
—Law of Reflexion. — Reflexion from plane Surfaces. — Reflexion from
convex Surfaces. — From concave Surfaces. — Phenomena of Rcflexicn
from plane Mirrors 'explained. — from convex Mirrors. — From con-
cave Mirrors. — Prominent Image from a concave 'Mirror.-— Tht
cylindrical Mirror. — The Rectification of dijlorted Figures by this
Mirror.— The conical Mirror.

CHAP. IV. — page 200.
General View of the Phenomena of Refraction.
Laws of RefraEiion. — Degree of RefracJion which Light fujfers froit
different Mediums. — Common Phenomena of Refraflion. — RefraQiox
by j'pherical Surfaces — By convex Surfaces — By concave Surfaces
— Of Lenfes. — Convex Lenfes. — Concave Lenfes. — Different Refran-
gibility of the Particles of Light. — Experiment vjitb the Prifm.

CHAP. V. — page 216.
Of the Principles of Catoptrics.

Places cf Images in plane Rffleclors. — Why a Mirror only Half 'the .
Size of an ObjeS exhibits a perfefl Image of the Whole.— Place*
of Images made from Reflexion by fpherical Surfaces.— —Mode of de-
termining the Foci of refleaed Rays from fpherical Surfaces.— S;zt
and Proportions of Imc.ges in fpherical Refleclors. — Phenomena, cf
(oncave and convex Speculum! explained.

CHAP. VI. — -page 227.
Of tlie Principles of Dioptrics.

Dioptrics relate to tbt Ejf'efts of Refraftion.—In what M&nntrto find
the actual Situation of an Object feen in a different Medium.-^^The
apparent Situation of an Objerf feen through a Glafs Window dif'
fcrent from the real one.— The Ejfecls of tranf parent Media nJoitb
fpherical Surfaces on the Rays of Light which are tranfmitted through
them. — Rules for finding the Focus of Rays pajfmg through fuch Me-
diums\— Theory of Lenfes. •^Plano-convex and plano-concave Lenfes.
—Rulee to flnd the Focus of a Lens.— -Focus of diverging Rajs in-
tercepted to a Lent.— Focus of a Sphere.

CHAP.

CONTENTS.

CHAP. VII. — page 24.1.
Of Vifion.

:e deferred as an optical InJ}rument.—Drft£ti of the Eye.— Short
and weak Sight. — The former remedied by double csnca-ve, and the
latter by double con-vex, Glares. — How the Faculty of feeing correct-
ly is acquired. — Apparent Magnitude depends en the Dijlance of Ob •
jiJ.'s. — Vifual Angle. — Phenomena of Fijion. — I'/hy the dijlant
Parts of an horizontal Plain, and of the Sea, appear elevated. —
Why a long Wall appears curved. — Why a high Tower appear $
falling to an Eye beneath. — Belt of Saturn, — Phenomena of Fijian at
tonne fled with Motion. — Why the Moon cppears to move injlead of
the Clouds which pafs over its Face. — Why a Circle viewed ob-
lia^uely appears oval.— How Glajfes ajjlji the Sight.

CHAP. VIII.— page 250.
Of Telefcopes and other Optical Inilruments.

Principles on which thefe Injiruments are conJtru8edt—Defec~ls.—Tele-
fcope of Galileo.— Micrbfcope.—Refcfllng Telefcope.— Camera Qb-
J'cura.— Magic Lanthorn.

CHAP. IX.— page 260.
Of the different Refrangibility of the Rays of Light, and of

Colours.

Errors a»d Inconveniencies in optical Injiruments from the Refrafliott
of Light. — Neivton, while attempting 'to remedy thefe, difcovers a
ne~w Property in Light. — Phenomena and Caufe of Colours.'— Achro-
matic TelcJ'cope. — The Eye an achromatic optical Inftrument. — Ex-
periments en Colours.— Cauft of the permanent Colour of opakt
Bodies.

CHAP. X. — page 276.

Of the Rainbow, and other remarkable Phenomena of Light.
Of the primary and feccndan Rainbow. — Why the Phenomenon affitmit.
the Fcrm of an Arch. — At what Angles the different Colours are
apparent. — Lunar Rainbow. — Marine Bow. — Coloured Bowsfeen
on the Ground. — Halo or Corcna. — Curious Phenomena Jeen on the
yfy of the Cordtleras. — Similar Appearance in Scotland. — Parhelia,
or Mock Suns.— Singular Lunar Phenomenon.— Blue Colour of the
Atmcj'phere. — Red Colour of the Morning and Evening Clouds.—
Colour of tht Sea.

CHAP. XI. — page 293.
Of the Infleftion of Light.

Retrofpecl of the Dotfrine of Rejleclion. — Nature of Inflexion. — New-
ton's Experiments. — Analogy between this Property and Refraflion.
— Curious Ef efts from this Property.

BO«K

CONTENTS. xvii

BOOK IV.
OF ELECTRICITY.

C M A P. I.— page 297.
Hiftory of Difcoveries relative to Eledlricity.

Origin of the Name. — Hdkv far Electricity was kncvjn- to the An-
tients.—Mr. Boyle. — Otto Guericke. — Dr. Wall. — Mr. Ha-ivAfiee.
Mr. Grej's Difcoveries. — M. Du Fay's. — Subfequent Difcoveries of
Mr. Grey. — Improvements of German Philofophers. — Ley den Phial. —
Electrical Battery. — Spirits fired by Electricity conduced through
the River Thames. — T<wo Species of Eleilricity difcovered. — Dr,
Franklin's great Difcoveries.

CHAP. II. — page 307.
General Principles of Eledtricity.

Analogy between Caloric, or Fire, and the electrical Fluid.— The Argu*
ments on the contrary Side. — '-Conjectures concerning the Nature of
this Fluid.— -Means of producing tlcttrical Phenomena.— Conductors
and Non-conductors. — Inftruments employed in Electricity.

CHAP. III. — page 323.

Of the Vitreous and Refmous, or Poiitive and Negative Electricity.

Diliinction in the attractive Powers of certain Electrics,— Theft
Effects found to defend not on the Nature of the Sutftance, but the
Roughnefs or Smocthnefs of the Surface.*— Theory of Two dijtinft
Fluids. — Franklin's Theory. — Difficulties attending it,

CHAP. IV. — page 329.
The Leyden Phial, Electrical Battery, and other Parts of the

Apparatus.

Theory of the Leyden Phial. — Its Ufe in EleSrichy. — Defcription of
the beft Apparatus of this Kind. — The Charge refides in the Glafs.
— Curious Experiments -u-ith the Leyden Phial. — Electrical Baftery.
—Injlruttions relative to it.— Experiments nuilh the electrical Bat~
ttiy. — Electrical Bells. — Electrophar us. —Electrometer.

CHAP. V.— page 338.

Eledlrical Phenomena.

Phenomena of Attraction and Repuljion. — Electrical Atmofphere.— Dif-
ferent Effects on different Subjtances. — Eleftrical Cohefion.— Experi-
ments on Silk Stockings.' — On the Evaporation of Fluids. — yegeta-
tion. — Animal Per/fir ation.— Inflammation of Spirits.— Animals
killed by Electricity.— Curious Phenomena in vacua.*— Recapitulation
cf Principles.

VOL, I. a CHAP,

XT 111

CONTENTS,

CHAP. VI.— page 345.

Of Thunder and Lightning, Meteors, Water-Spouts, &c.
fheory cf Lightning. — Defcription of a Thunder Storm.— Obfervatior.s
relative to the EleBricity of the Atmofphere .— Melting of Metals by
the Cold Fufion a vulgar Error. — Condudors of Lightning.— How
to be fafe in a Thunder Storm. — Application of EleSricity to other
atmofphericalPknomena.-r-Rain.—HwL— Snow.— Meteors. — Wa-
ter-fpouts.

CHAP. VII.— page 359.

Medical Eledricity.

Declaration of the Abie Nollet on this Subjefl. -Mr. Adams an Ad-
vocate for Medical Eleflricity .— Mode of Application.— Dijeafes
to which it may be applied.— Apparatus mojl proper for Medical
Purpofes.

BOOK V.

OF AIR.

CHAP. I. — page 362.
Hiftory of Difcoveries relative to Air.

Vagut Notions of the early Chemijis. — Van Helmont.—Choak and Fire
Damp.— Mr. Boyle. — Difcoveries of Dr. Hales. — Of Dr. Black.—
Of Dr. PrieJlhy.—Of Mr. Cavendifo. — Of La-voider. — Vital or
dephlogijiicated Air difcovered by Dr. Prieftley. — Compojition of
Water and of Nitrous Air difcovered by Mr. Cavendijh.

CHAP. II. — page 371.

Of Oxygen Gas, or Pure, Vital, Empyreal, or Dephlogifticated Air.

Explanation of Terms.- — Reafons for the different Names aligned to
this Fluid. — How procured. — From CaLes of Metals.— By Vege-
tation.— From Water. — Properties of Oxygen Gas A powerful

Agent in the Syftem of Nature.^How ejfentialto Flame and Life. —
Various Mode: provided by Nature for furnijhing a Supply (f this
Fluid.

CHAP. III.— -page 382.
Azotic Gas, or Phlogiflicated Air.

Azotic Gas is the unrrfpirable Part cf the Atmofpbere.-—HoT.v pro-
cured.— Air Bladders of Fijhes filed with it. — Iti Propertits.-^
Azote tbt Ba/is of nitrous jcjd.

CHAP.

CONTENTS.

CHAP. IV. — page 385.
Of Carbonic Acid Gas ; Fixed or Fixable Air.
fhe Bajis of this Air is the elementary Matter of Charcoal.— Combined
<with Oxygen — Modes of -producing it.— Fermentation*— Quantity
contained in different Kinds of Wines.— Choak Damp. — Properties of
Fixed Air.— Great fpecific Gravity.— May be poured out of a Vejftl
like Water.— Re/tfts Putrefaction.

CHAP. V.*— page 391.
Inflammable Air or Hydrogen Gas,

This Gas forms the Bajis of Water.— Proportion of Hydrogen and Oxy-
gen luhich enter into the Composition of Water. •^Modes of procuring
inflammable Air.~—Ignes Fatui.—*Fire Damp in Mines —Lighteft of
all Fluids."— Remarkable Properties.— -Vfe in Air BaUoonst<-~C.uriou$
aerial Fireivor&s.

CHAP. VI.— page 397,
Nitrous Air or Gas.

Nature of this Fluid.— Ho-iv produced. — /// Properties.— Rejifts Putre?
fa£tion.-^-Abforbs and condenfes pure Air, — 'The Eudiometer.

CHAP. VII. — page 403.

Of Hepatic Gas.

Nature of this Gas.— 'Means of producing it.— Its Properties.— A chief
Conjiituent of Sulphureous Mineral Waters.— -Turns Metals black.—
How decompofed, &c.

CHAP. VIII. — page 404.
Of Atmofpheric Air.

Atmofbhere compofed chiefly of Two Kinds of Air.— Contains alfo Fixed
Air, and occafeonally other Subftances. — Effefls of this Mixture on
Metals and Purple Dyes. — Means of purifying the Atmojphere from
Fixed Air, and putrid Vapours. ---Effects of Moifture contained in
Air. — The Hydrometer.— Cold in the higher Regions of the Atmn-
fphere.—'Caufe.

CHAP. IX.— page 416.

Of the Weight, Elasticity, and other General Properties of the Air,
General Properties of Air. — Caufe of its Elafticity* — Opinions of the
Antients. — Torricellian Experiment.— Barometer.— The Air Pump.
— Weight and fpecif.c Gravity of Air.- — Immenfe PreJJure of the
Almofphere* — CompreJJtbility of Air.— -Cupping GlaJJes. — EffecJs of
the Air's Elajiicity. — Air Guns, &c.— Motion of Particles in Bo-
fties. — Nature of the Atmofphere,— "Its probable Height,

CHAP.

CONTENTS.

CHAP. X.— page 440.

Of Sound.

tmfiJertd in Three Points of View.-— Caitfed ly a Vibration I'M
tb' Parts of Bodic;. — Propagated by cni undulatory Motion of the

jjir. . ^d by Experiment . — Glaj/es broken by an Effort of

- Elajl'.c Fluids not the o>:!> Meant of tranfmitiing Sound. —
Water or folid Bodies convey it. — Velocity cf Sound. — Experiments
tn this Subjefi. — Echoes. — Whifpering Gallery.

C ii A P. XI. — page 450.

Winds.

Diffirtni Qpinians concerning the general Caufe of Winds.— Of Genera!
'or Trade Wcudi. — Of Monfoons — Oj Sea and. Land Breezes. —
Cattjes -of tbcfe. — Variable Win/Is. — Storms. — Hurricanes. — The
Harmattan. — The Sirocco Wind.— -The S ami el.— Moving Pillars of
Sand.— ¥ be Simoom. — Whirl-winds. — W at erfpouti,— Tornadoes,

CHAP. XII. — page 475.

Of the Heat of the Atmofphere, and Igneous Vapours.
Qbjeclf cf Meteorology as a Science. — Partly anticipated. — Tempera-
ture.— Heat of the Earth.— Effefis of the Sun's Rays on different
Mediums. ^Difference with re/pea to temperature between Land
and Water. — E/etis of Clouds on the Temperature* — Of Evapora-
tion.— Unufual Cold, hmv produced in Summer find Winter. —
Aqueous Meteors. — Igneous Meteors. — Fire Balls.— Shooting Stars.
—Ignes Fatui.

CHAP. XIII. — page 496.
Of the Prognoftics of the Weather.

Jmperfefi State of this Branch of Science.— Progmjtics of Wtatotrfnm
the pre-vious State cf thf Scajon.—Frc;n the Undulations of the At-
mcfpbere.~-Fr6m the Barometer.— F 'n m Fosrs.—~Frcm Clouds.— -
From Profptfis. — From the Dw. — From the $Jy+±From the Moon.
—From tbt Wind.

CHAP. XIV.~page 507.

Aeroftation.

Hijlory of derogation. — Difco-very of Air Balloons by M. M. Montgal-
Jier. — Firji Balloon exhibited at siiinoni '}'.— Balloon filled 'with in-
.. tamable Air exhibited at Paris. — Pilatit lit Rozinr ajands in a.
Balloon. — Firfl Balloon exhibited in England. — Afent of M. Lu-
nar Ji — Voyage cf M. Bland <ard and f>r. JrjJ'rLs acrofs the Chan-
ntL— -Unfortunate Cat ajlrophe of M. M. de Rozicr find Romain.—

Mr. Baldwin's Defcr:ftion rf the Projfrrtfrom a Balloon Prin-

dples of Aerojtation. — Modes cf filling Balloons. — Ufe 'to which tbej
beeve been, applied.

BOOK I.

:OF THE GENERAL PROPERTIES OF
MATTER.

CHAP. I.

OF MATTER IN GENERAL.

'Explanation of Terms.- — Whether all Matter i; radically ike fams.—~
General Properties of Matter.— Quantity of Matter in the Uni~jerfs.

THE word MATTER (maferia, which fome lexi-
cographers have derived from tnafer^ a mother)
denotes, in ics primitive fcnfe, that unexplained fome-
thing, from which all thofe things which are objects
of our fenfes are formed.-

The term 'body is fometimes Confounded with that
of matter; buc they are eflentially different t Body
(bobije) is of Saxon origin. It is explained by the
Latin words ftaturay peffits, truttcus\ and fignined the
•perjcn or form of a 'man, or other creature: whence it is
plain that it ought to be confined to expfefs a fubftance
pofTeiTmg form or figure.

Subftance^ both in its etymology and application, ap-
proaches nearer to the meaning of the former of thefe
terms. It is well known to be compounded from the
Latin prepofidon Jub (under), and the verb flare (ro
Hand.) It confequenly implies that which fufports or
ftands under the different forms and appearances which
are prefented to our fenfes. It is ft ill, however, ufed
in a diftincl: and more limited fenfe than matter. It
is generally indeed ufed with the article, to fignify a

VOL. I. B diftincl:

a Suppcfed Homogeneity of Matter. [Book I.

diftin<5t or definite portion of matter; whereas matter in
the abftract implies a more confufed and general idea
of folidity and extenfion, with little or no regard to
figure, proportion, or quantity.

Thefe words are of fuch common and frequent ufe
in philofophy, that it appeared neceffary to have a
competent notion of their force and meaning, particu- .
larly in a chapter which profeffes to treat of the firft of
them : and I have generally found etymology a fafer
and eafier mode of communicating knowledge than
definition.

That the whole matter, of which this univerfe of
things is compofed, is efientially the fame, and that the
apparent differences which .fubfift in different bodies
depend altogether on the particular diftribution ordif-
pofition of the component particles, is an opinion
which has been entertained by fome philofophers of
the higheft reputation. The wonderful apparent tranf-
mutations which take place in the different proceffcs
and operations of nature do, it mud be confeffcd, ac
firft fight countenance this hypothefis. A plant will
vegetate and become a folid fubftance in the pureft
water *. The generation of ftones in the earth, the
various phenomena of petrifactions, and a multitude of
other fuels, contribute greatly, on a fair confideration,
, to diminifh the abfurdity of the alchemifts, who feem
chiefly to have refted on this hypothefis (viz. that all
matter was intrinfically the fame) their hopes of con-
verting the bafeft materials by the efforts of art into
the mod fplcndid and valuable of fubftances.

Mr. Boyle diftilled the fame water about two hun-
dred times, and at the cad of each diftillation found a

* See the Experiments of Mr. Boyle and Van Hehnont,
Book VJII. c. iii.

9 frefli

Chap, i.] Mr. Boyle's Experiments. 3

frefh depofit of earth. M. Mar^raff repeated the ex-
periment with ftill greater caution. By means of two
glafs globes, which communicated with each other, he
preferved the water while in the (late of vapour from
all contact with the air; and on repeated diflillation, a
quantity of earth of the calcareous * kind was depo-
fited at the conclufion of each procefs.

The extreme rarity and minutenefs of the particles
into which different fubftances may 'be refolved, im-
parts a ftill greater degree of probability to this hypo*
theft's ; and in general, the more any body can be di-
vided, the fimpler it appears in its component parts.

We mud however be cautious of admitting opinions,
which are not fanctioned by the direct teft of experi-
ment; and however plaufible the opinion, the accurate
obfervations of modern philofophy have fuggefled
fome objections to the homogeneity of matter, which
without further difcoveries it will not be eafy to fi-
lence.

Whatever phenomena may appear to indicate a
tranfmutation of bodies, or a change of one fubflance
into another, we have the utrnoft reafon, by the la teft
and beft experiments, to believe them merely the ef-
fect of different combinations-. Thus the converfion
of water and air into a folid fubflance, fuch as the body
of a plant, is merely an apparent converfion, for that
folid fubflance may, by an artificial procefs, be refolved
again into water and air, without any real change in
the principles or elementary particles of which thofe
fluids are compofed: and the formation of ftones, and
the phenomena of petrifactions, are accounted for upon
much eafier principles than that of 'tranfmutation* ,On
the other hand, the utmoit efforts of ohemiflry have

* Earth of lime,

B 2 never

4 No tfranjmutation of Principles. [Book!,

never been able to proceed farther in the analyfis of
bodies than to reduce them to a few principles, which
appear eflerdally different from each other, and which
have never yet been brought to a more fimple form.
Thus the matter of fire, or light, appears totally dif-
ferent from that of all other bodies; thus the acid and
alkaline principles can never be brought to exhibit: the
fame properties ; nor can even the different fpecies of
earths be converted into the fubftance of each other.

If hypothetical reafoning was to be admitted on this
occafion, it would probably appear more agreeable to
the analogy of nature, to ftippole that different fub-
ftances are formed from the different combinations of
a few fimple principles in different proportions, than
that the very oppofite qualities of fome of the rareft
and moft fubtile fluids, mould depend wholly on the
different form or modification of the extremely minute
particles which enter into their competition.

It is proper however to obfcrve, that on this fubject
there has hitherto appeared no dec: five experimental
proof on either fide. The imperfection of all human
efforts, and perhaps of the human faculties themlelves,
have hitherto confined our inveftigations to the pro-
perties of a few fubftances, the limpleft which chemi-
cal analyfis has been able to obtain, a/nd which for that
reafon are denominated ELEMENTS.

There are fome properties which are accounted
common to all matter, and which from their import-
ance will require to be feparately treated of: thefe are

SOLIDITY, EXTENSION, and DIVISIBILITY ; ATTRAC-
TION', MOTION-, and REST.

The quantity of matter which is contained in the
whole univerie may probably be much lefs than com-
mon obfervation would lead us to fuppofe. The fub-
lime mathematics of Newton diftated the aftonifhing

proportion :

Chap, i.] Quantity of Matter in the Unherfe. 5

propofition : c that the whole globe of earth, nay that

* all the known bodies in the univerfe may, as far as

* we know, be compounded of no greater a portion of
f folid matter than might be reduced into a globe of
•' one inch only in diameter, and even lefs*!'

* Rembijrton's Vieu- of Sir I, Newton's ]?hilofophy, 356,

B CHAP,,

[ 6 ] [Book I.

CHAP. II.
OF ELEMENTS*.

Opinions of the Jncientis—New Arrangement. — Enumeration of finale
SubJIances, according to the new Pbilcfophy.~— Aerial Subftances. —
Earths.— Metals.

WHEN we take a general farvey of nature, we
find that notwithftanding the apparent variety
of creatures, with which the univerfe abounds, every
natural body which has hitherto come within the limits
of our infpection may be reduced into a few diftinct
kinds of matter: and though we probably have not as
yet difcovered the ultimate and moft fubtile principles
of which bodies are compounded; yet we appear to be
juftified in calling the moft fimple fubftances which
we have been able to dif cover as entering into the
competition of bodies by the name of ELEMENTS.

Ariftotle, and after him moft of the ancients, admit-
ted four different elements, fire> air, earthy and water,
It is evident, in the firft place, that in this enumeration
the falts are omitted, the exiftence of which can no
more be doubted than that of any of the others. Se-
condly, it was found necefiary in the progrels of fci-
encej not only to admit a Jaline, but a fulphureous or
inflammable principle. I might add, that we are war-
ranted by no experiments, which have as yet been
;o public, in fuppofing that there exifts but one
<ie Ipecies of earth ; and later experiments have

f From CiEOj or E LEO, to create.

determined

Chap. 2.] Water a 'compound Subftance, 7

determined that water as well as air is a compound
fubftance *.

It is, therefore, necelTary to adopt a new arrange-
ment, though it is more than probable that future dif-
coveries may render it in fome meafure nugatory.
Future difcoveries may perhaps demonftrate that many,
if not moft, of the follovying fubftances will yet admit
of fubdivifions ; or they may demonftrate that the con-
.ftituent principles of bodies are (till fewer. They may
demonftrate that all earths have originally the fame
bafis, and are only altered by different combinations
with vital air or other fubftances ; the alkaline falts
may be in reality a fpecies of earth, which alfo derives
its diftinguifhing qualities from a union with fome fub-
tile matter in a certain proportion. Thefe, however,
vare points on which we have at prefent obtained no
experimental evidence, and which for that reaibn we
are not authorized to affirm.

The mofl fimple fubftances' hitherto difcoyered may
be refolved into,

1. Fire, heat, or caloric j including light and the
electric fluid.

2. The bafis of pure, vital, or dephlogifticated air,
the oxygen of the French cbemifts.

3. Hydrogen, or the bafis of inflammable air.

4. Azote, or the bafis of nitrous acid.

5. Sulphur, the balis of vitriolic acid.

6. Phofphorus, the bafis of phoiphoric acid.

* I fhall not perplex the reader with the exploded vifions of
chemiits during the iail two centuries, with their phlegm or watry
principle,, their mercury or aciive and fpiritous principle, their
faput mortuurn, their fpiritus retfor, and a quantity cf uielefs and
alraoft unintelligible jargon.

B 4 7. CoaJ

8 Enumeration of Simple [Book t.

7. Coal * or Carbon, the bafis of fixablc air.

8. The unknown radicals of the muriatic, fluoric,
and boracic acids.

9. The fixed alkalis.

10. Earths.

11. Metals.

"The different earths which are as yet known to phi-
.Jofophers are,

1. Calcareous, or earth of lime,

2. Silicious, or earth of flints.

3. Argillaceous, or clay.

4. Magnelia.

5. Barytes, or ponderous earth.

To thefe later mineralogifts have added,

6. The Scottjlh, or Strpnthian earth,

7. Thejargonic.

8. The adamantine.

Whether all or mod of thefe may not be refolvable
into the five principal earths may yet be doubted ; and
thus far is certain, that all of them are exceedingly
fcarce: the Scottifti however is the moft common.

The metals again are fubdivided into.

1. Arfenic.

2. Molibdena.

3. Nickel.

4. Cobalt.

* More properly charcoal, for our common pit-coal is a hete-
•fogeneous mafs, containing much foreign matter, as may be feen
Book VI. c. 37. I have however employed the generic term
ccal, becaufe charcoal, though much the purefl of thefe bodies, is
in this country coftfidered not as a natural but a faftitious fub-
ftance.

5. Bifmuth,

Chap. 2.] Elementary Subftances* y

5. Bifmuth.

6. Antimony.

7. Zinc.

S. Tungftein.

9. Manganefe,

19. Tin.

11. Lead.

12. Iron.

13. Copper.

14. Mercury.

15. Silver.

16. Gold.

17. Platina.

To this lift late difcoveries have added four new
femimetals, viz. Uranite, Sylvanite, Titanite, and Me-
nachanite. — But of their nature little is known: and it
is with fome hefitation that I infert their names in the
vocabulary of fimple elements.

In the prefent ftate of natural knowledge, however,
moft of the above may be confidered as diftincl: ele-
mentary fubftances, fmce they are found to be un-
changeable or unconvertible into other fubftances,
though they may be, and generally are, combined with
Others. This fact, v/as it not for the advantage of
claflification, would therefore oblige us to admit in na-
ture, inftead of eleven, at leaft forty diftind, fimple,
and elementary fubftances.

CHAP.

io ] [BookL

CHAP. III.

OF THE EXTENSION, SOLIDITY, AND
DIVISIBILITY OF MATTER.

Extenjion the only Quality ejjential to Matter. — Solidity, ivbat. — Infi-
nite Divijibility. — A;iimalculee imperceptible to our Senj'cs. — Extreme
Rarity of Light.— Newtonian Paradox.

T7X1iENSIONis a property fo obvioufly eiTential
•^-' to whatever occupies fpace, that it is accounted
the firft and moft indifpenfable attribute common to
all matter. It is indeed the only property which we
can pofitively fay is dfential ta matter, fince all the
others that have been fpecified are fo be underftood
with fome limitation, and do not appear to be common
to all Bodies whatever.

We have no idea of Jolidity^ but that which is fur-
nifhed by the refiftance which we find in a body to the
entrance of any other body into the place it occupies
till it has left it *. This property of matter therefore
neceffarily includes neither impenetrability nor hard-
nefsj the amazing porofity of bodies militates againft
the one idea, and the almoft infinite divifibility of mat-
ter againft the other. Indeed nothing can be more
inconfiftent than to fpeak of the abfolute folidity and
impenetrability of the ultimate particles of matter, and
afterwards to enlarge upon its infinite divisibility; both
of thefe are fafls which are totally undetermined by
experiment or obfervation ; and when we fpeak of the

* See Mr. Locke's EflJ on Hum, Under. B. ii. c. 4.

actual

Chap. 3 .~\ Subftances extfting In tie Pores of other Bodies. 1 1

actual properties of matter, we ought, I apprehend, to
confine ourfelves to the teftimony of our fenfes.

How improperly the idea of impenetrability is ap-
plied to any bodies which we are acquainted with, is
evident from the aftonifhing eafe and velocity with
which the electric fluiJ pafles through the denfeft bo-
dies, and from the continual paffage of light through
a Variety of Jubilances, which afford to our fenfes the
moft perfect idea of folidity, through glafs, and dia-
mond, and other precious ftones; the focus of a burn-
ing mirror, which augments the denfity of the fun's
rays upwards of three thoufancl times, may be received
in glafs or water without producing any effect : even
fubftances of fenfible bulk and folidity can exift within
the pores of other bodies. Thus quickfilver exifts
within the pores of either filver or gold; in fact a mix-
ture of mercury and filver is considerably heavier than
an equal bulk of either of thofe metals. The com-
mon bell-metal is a mixture, of copper and tin j and
though the latter is fpecifically lighter than the former,
yet bell-metal is confiderably heavier than an equal
bulk of copper itfdf ; which is an evident proof that
the particles of the one actually enter into, and are de-
pofited in the pores of the other.

The dwifibility of matter is alfo received with limi-
tation by thofe who contend for the exiftence of atoms
or firft principles of bodies j fince, if their exiftence is
admitted, there muft neceflarily be fome parts or ;or-
tions of matter indivifible, and confequently it cannon
be admitted as a property inherent in a]\ matter.

The human faculties are loft in the p rfuit, and
the human understanding in the contemplation of the
actual ciivifibility of matter. The fmallcft animalcula
which is brought within our notice by the microfcope
poflbfles organized parts, blood and other fluids ne-

ceflarv

j a Great Tenacity of Cotton , Silk, &c. [Book I.

ceflary for the fupport of life, yet how infinitely re-
moved are thefe from our infpection ! Some idea,
however, may be formed of the aftonifhing power of
matter in this refpect, from inftances which are fur-
nilhed by all the moft common experiments of philo-
fophy. A pound of even fo grofs a fubflance as cot-
ton, may be fpun into a thread upwards of one hundred
miles in length * j and Mr. Boyle fpeaks of a thread
of filk 300 yards in length, which weighed no more
than three grains and a half. This, however, is ftill
furpafTed by the amazing ductility of gold ; fixteen
ounces of which would completely gild a wire fuffi-
cient to circumfcribe the whole globe of the earth,
though that quantity of the metal might be contained
ill a cube of not more than an inch and a quarter in
diameter. The metallic particles are yet more mi-
nutely divided in the acid folutions. A fmall piece
of the fait of filver, which is filvcr already divided
3nd united to the nitrous acid, not larger than a com-
mon pin's head, will tinge a quart of water of a milky
colour j and even the hundredth part of a grain of
copper will impart a fenfible blue to a pint of the fame
fluid.

It is well known that camphor, mufk, and other
odoriferous fubftances will emit particles that fhall
powerfully affect the organs of fcent, and fhall com-
municate their peculiar fragrance to the furrounding
air for a confiderable fpace of time, without any per-
ceptible decreafe of weight. The extreme rarity of
the elaftic fluids is a further proof of the divifibility
of matter. Gunpowder, when exploded, expands to
244 times its bulk when in a foiid ftate ; and wa-
ter in the (late of vapour occupies a fpace 1800

* Repofuory, vol. ii, p. 52.

times

Chap. 3-] Extreme Mimitenefs of certain Particles, ij

times greater than in its fluid form *. A particle
of light has been eftimated, on apparently a well

conducted calculation, at — — — c

30,83 i, 230, 1 28,000 of a

grain f-

We fhall indeed .ceafe to wor-der at fuch a calcu-
lation, when we confider that by means of this fluid
the unparalleled wonders of the microfcopic world are
made cognizable to our fenfes. The fcarf-fkin of the
human body is faid to be compofed of minute fcales
refembling thofe of fifties, two hundred of which may
be covered by a grain of fand ; under thefe fcales there
lie concealed a number of pores, or excretory duels,
through which the perfpirable matter is fuppofed to
ifiue, and one hundred and twenty of fuch pores in. a
direct line extend to only one tenth of an inch. If
fuch therefore is the organization of the human body,
what fhall we think of the organized parts of thofe
animals which are themfelves one thoufand times too
imall to affed the human eye without the aid of art ?
Animalcule, however, have been diicove^ed nearly
one hundred times fmaller than thefe, many thoufands
of which may dance upon the point of a fine needle <
indeed Lewenhoek calculates that one thoufand mil*
lion of fuch animalcula: as are difcover.ed in common
water would not equal in magnitude a grain of com-

* This fac"l may at any time be proved by an eafy experiments
Take a common flafe, and let it ,be exaftly weighed ; fill it with
water, and then let it be weighed again — after the water is emp-
tied, there \vill necefi'arily be a little moifture adhering to the
iides ; put the flafk before a fire to evaporate the moifture, and
when the whole of the water difappears, cloife the flafe, "vr.d
weigh it again, and you will then have the weight, and conis-
quently the bulk of vapour, compared with that of water.

f Botidoin on Light. Memoirs of the American Acad. Vol. I. »
p. 198.

mon

!4 Particles of Bodies not in Conlatf. [Book I.

mon fand. When therefore we confider that fuch
animalcule are pofll'ffed of organized parts ; a heart,
ftomach, bowels, mufcles, tendons, nerves, glands, &c.
we feem to approach in idea the infinite divifibility of
matter *.

It is, on the other hand, next to an abfolute cer-
tainty, that the particles of the hardeft and moft com-
pact bodies are not in actual contact with each other,
fince all fuch bodies are known to contract with cold ;
which could not be the cafe, if their parts were already
as clofe as they could be to each other.

It would be foreign to the defign of this work to
enter into thore calculations and demonftrations by
which mathematicians have attempted to prove how-
matter may be divided, to infinity. Let it fuffice to
lay, that on the principles which have been advanced
in the courfe of this chapter, the'fagacity of Newton
has demonft rated, that the leaft portion of matter may
'be wrought into a body of any affigned dimenfions,
how great Jbevcr, and yet the pores of that body be
none of them greater than any the fmalleft magnitude
propofed at pieafure ; notwithstanding that the parts
of the body fhall fo approximate, that the body icfclf
fhall be hard and folid. His manner of demonftra-
tion is this — fuppofe the body to be compounded of
particles of fuch figures, that when laid together the
pores found between them fhall be equal in fize to
the particles themfelvcs ; how this may be effected,
and yet the body remain folid, is not difficult to un-
•ad; and the pores of fuch a body may.be made
fcf any propofed degree of fmalinefs. But the folid

* If the reader \viflies to fatisfy himfelf concerning the nature
and animal functions of a variety of thefe wonderful exigences, I
mull refer him to my late much tallied friend Mr. Adams's ex-
cellent Treadle on the Microscope.

matter

Chap. 3.] Newton's Argument on Divifibility, &c. 1 5

matter of a body fo framed will take up only hall" the
fpace occupied by the body ; and if each conftituent
particle is compofed of other fmalier particles, ac-
cording to the fame rule the folid parts of fuch a
body will be but a fourth part of its bulk ; if every
one of thefe leffer particles again are compounded in
the fame manner, the folid parts of the whole body
will be but one eighth of its bulk ; and thus by con-
tinuing the compofition, the folid parts of the body
may be made to bear as fmall a proportion to the
magnitude of the whole body as fhall be defired, not-
withftanding the body Ihall, by the contiguity of its
parts, be capable of being in any degree folid *.
— When thefe fafts are confidered, the hypothecs of
the fame incomparable philofopher, concerning the
fmall quantity of folid matter contained in the uni-
verfe, as noticed in a preceding chapter, appears lels
incredible.

* Pembertoa's View, 355.

CHAP.

[ 1 6 ] [BookL

C II A P. IV.

OF ATTRACTION AND REPULSION.

)?ive Kinds of AttraSicn.—Cobefion-^-Combination — Cryftallixatim
explained. — Grai-i'.aiioii — Specif £ Gravity, what. — Magnetic and
elettrical Attraction. — Rep:d..

IT has been found by experience, that all matter, of
whatever kind, is fubject to certain general laws;
and the principal of thefe are attraction and repul-
fion. Five different kinds of attraction have been
enumerated by modern philofophers. i. The at-
traction of cohefion ; 2. Of combination, or, as it is
called by chemilb, elective attraction j 3. Gravity j
4. The magnetic attraction; and, 5. The attraction
bf electricity. Whether the fame principle acts in
ail thefe cafes; Or whether each of thefe effects de-
pends upon a diftinct caiife; human fagacity has not
been able to difcover ; nor is there any inftance in
which the principle of attraction feems to approach to
the nature of a general law, except in that of gravity j
and yet even this is not without an exception j fmce
by every experiment that we liave hitherto been able
to make, there is no reafon to believe that the element
of fire, or heat, is fubject to the common laws of gra-
vitation. Unlefs, therefore^ it could be proved that
the principle of attraction is the fame in all thefe cafes
that have been enumerated, we, perhaps, arc fcarcel7
correct in confidering it as a general property of
matter; and even fuppofing the caufe to be the fame,
it rriay, after all, belong rather to fome particular fpe-
cies of matter^ which acts upon or impels all other
bodies, than to mdtter in general.

I. The attraction of COHESION may be obferv-
cd in aliroft all the 'common operations of nature,

and

Chap. 4.] Attrattim of Cobe/jcn. 17

and is exemplified by a variety of eafy experiments.
Two leaden balls, having each a fmooth furfacc, if
ftrongly compreffed together, will cohere almoft as
flrongly as if united by fufion j and even two plates
of glafs, if the furfaces are even and dry, will require
fome force to feparate them *. By the fame law of
nature, the particles of even fluid bodies, in which the
attraction is \ neceffarily weaker than in folid fub •
fiances, indicate a difpofition to unite.

The drops of dew that appear in the morning on
the leaves of plants, affume a globular form, from the
mutual attraction between the particles of water.
Small portions of quickfilver, when brought near to
each other, will run together, and affume the fame
globular appearance. Alfo, by the fame law, a veffel
may be filled with water, mercury, or any other fluid,
above the brim, and the fluid will be obferved to rife,
in a convex form.

To this principle we may very properly refer what
is ufually termed capillary attraction. Thus, if a fluid
is contained in a veffel not full to the brim, it will al-
ways be attracted to the edges of the veffel, and will
affume a concave form. Thus, alfo, if two plates of
glafs, at a fmall diftance from each other, are im-
merfed perpendicularly in water, the fluid will rife
above its level between the two plates, and the height
to which it rifes will bear a certain proportion to the
diftance of the plates. A capillary tube is a tube with
an exceedingly fmall bore, and by the fame law which
raifes the water between the plates of glafs, a fluid
will rife to a confiderable heighth in one of thefe
tubes. Both thefe experiments will anfwer equally
well in the vacuum of an air pump, which proves that

* See the late ingenious Dr. Enfield's Inftitutes of Natural Phi-
lofophy.

VOL, I. C the

1 1 Eleftive Altratthn. [Book I.

the effect is not owing to the prefiure of the air. In
the fame manner, alfo, by the fame law, fluids will
afcend in the cavity of a fponge, in the interftices of
linen cloth, or any porous body.

II. The attraction of COMBINATION, or chemical
or elective attraction, is in many refpects analogous
to the attraction of cohefion. Like the latter, it feems
to depend on the minute particles of bodies being
brought nearly into contact with each other ; and in-
deed fo nearly alike are the effects of thefe two fpe-
cies of attraction, that if they are different in prin-
ciple, it is difficult to fay which is the moft eflential
to the cohefion and folidity of bodies *. Chemical
attraction may probably be no other than the attrac-
tion of cohefion acting in a free and unrefifting me-
dium, fmce its only diftinguifhing characteriftic is the
difpofition which bodies in folution indicate to unite
with certain fubftances in preference to others. To
make this clear by an experiment — If a quantity of
filver is added to a quantity of aqua fortis, the cohe-
fion of the particles of the filver will be ckftroyed, and
they will unite forcibly with thofe of the aqua fortis.

* The two fpecics of attraction are well defiued by Bergman :
that which he calls the attraction of aggregation, I clafs under
that of cohefion; that which he calls compofition, I call combina-
tion. « When an increa'fe of mafs only takes place, the nature of
the body remaining ftill the fame, this cifed is denominated the
attraction of aggregation. But heterogeneous fubftances, when
mixed together, and left to themfelves to form combinations, are
influenced by difference of quality rather than of quantity.
This we call attraction of comj>cfition, and when it is exerted in
forming a mere union of two or more fubftances, it receives the
name of attraction of folution ovfufion, according as it is effected
either in the moift or dry way. When it takes place between
three refpedively, to the exclufion of one, it is faid to be a/.v-
glt eleSi'vt attraction, and when between two compounds, .a dou-
ble, &c.' Berg, on Elcc. Atu. i.

The

Chap. 4.] Suppofed fr an/mutation of Metals. 19

The fluid will, however, remain perfectly clear, the
particles being fo extremely minute, that the rays of
light will fuffer no interruption in paffing through
them. If, however, to this folmion of filver a quan-
tity of mercury or quickfilver is added, the aqua for-
tis will be attracted by the mercury, and the filver will
be precipitated, or thrown to the bottom of the veflei
in which the fluid is contained ; if, again, copper is-
added, it will affume the place of the quickfilver, and
if to this folution of copper a bright piece of iron (for
ruil would exclude the acid from coming in contact
with the metal) is introduced, the acid will imme-
diately quit the copper and feize upon the iron, a
quantity of which being diffolved in the fluid, the
cop'per will be depofited in its place on the furface of
the bar of iron *. The iron may afterwards be dif-

placed

* ' This experiment explains to us, in a very fattsfaftory manner,

' the nature of" that tranfmutaticn of iron into copper y which travellers

' have been fo much furprized at. Agricola fpeaks of waters in

' the neighbourhood of Neivfol, in Hungary, which had the pro-

' petty of tranfmuting the iroa which was put into them into cop-

' per f - In the year 1673, our countryman, Dr. Brown, vifited a

' famous copper mine at Herrn-Grundt, about feven Englifh miles

' from Ne-ivfel; he informs us that he there fawtvvo fprings, called

' the old and new ziment, which turned iron into copper. This

' workmen fnewed him a curious cup made of this tranfmtited

' iron 4 it was gilt with gold, had a rich piece of filver ore faften-

* ed in the mickile, and the following infcription engraven on the

' outfide:

' Eifen ware icb, kupfer bin icb,
' Silver trag ich, gold bedeckt mich,

' Copper I am, but ircn was of old,

' Silver I carry, covered am with gold J,

« It was even at that time, he fays, contended by fome, that there '
' was no real tranfmutation of iron into copper, but that the

f Agnc. Fof. L. ix. p. 347.

\ Brown's Travels, ed. 1687, p. 69*

C 2 * z'roent

2o Solution and Mixture. [Book I.

placed by the addition of an alkaly This fpecies of
attraction is called combination, ^ecaufe the particles
of two bodies by thofe means become fo intimately
united or combined, that they cannot be feparated but
by the addition of a third body, which has a greater
attraction for one of the component bodies that they
have for one another, and it is called elective attrac-
traction and affinity, from the fuperior tendency in
fubftances to unite with certain bodies in preference
of others. In all cafes of elective attraction it is ne-
ceffary, that at lead one of the bodies mould be in a
fluid ftate.

It is evident that all folutions muft be the effect of
an elective attraction. Byfolution I mean the difper-
fion of the particles of a folid body in a fluid in il> equal
a manner, that the compound liquor (hall be perfectly
and permanently tranfparent. In this cafe, therefore,
it is plain that the particles of the fluid muft have a
ftronger attraction for the particles of the folid body
than they have for one another. A folid body may
indeed, by mere mechanical means, be minutely dif-
perfed through a fluid, but the compound in this cafe
will be opake and muddy, and if fuffered to remain
at reft, a fediment will immediately be depofited.
Thus, if chalk or clay is incorporated with water, they
will impart to it their peculiar colour, and the fluid

ziment water, containing vitriol of copper, and meeting with
the iron, depofited its copper; and it feems as if he would have
acceded to this opinion, could he have told what became of the
iron. It is now very well underlined what becomes of the iron ;
it is taken up by the water, and remains fufpendcd in it, in the
place of the copper; io that this tranfmutation is nothing but a
change of pjace ; and as the copper is precipitated by the iron,
fo the iron might be precipitated by pot-aft, or any other fub-
ftance which has a greater affinity with the acid of vitriol than
iron, has/ Watfon's Chem. Eff. p. 234. to 2^6.

will

Chap. 4.] Marks of Chemical Union. 21

will be rendered in fome degree opake ; but if com-
mon fait, or blue or green vitriol is added to water,
the fluid will Hill remain perfectly tranfparent, though
tinged with the peculiar colour of the fait. The for-
mer therefore is termed a mixture, the latter zjolution.
When a fluid has received fo much of any foiid body
that it will not diflblve a particle more, it is faid to be
Jaturated.

The marks of chemical combination in bodies have
been accurately defined by a correcl: and ingenious
philofopher. The firft is a fyecific gravity exceeding
that of the beavieft ingredients of the compound. Though
he properly obferves, it does not necefiarily follow,
that where fuch denfity is wanting a chemical union
does not exift ; fince the peculiar ftrufture of the com-
pound, which does not admit water into its vacuities,
may prevent this property from being remarked ; or a
quantity of water may enter into a competition natu-
turally heavier than water, and yet cannot be always
made fenfible.

Secondly, Tranfparency is always a mark of cne-
mical combination. Such union, however, is alfo
fometimes confident with opacity, as that effect may
fometimes arife from a mere mechanical arrange-
ment of parts, from the interpofition of fome mat-
ter not properly combined, or from too great thick-
nefs.

Thirdly, Cryftallization proves that the parts have
been very minutely divided, and in general com-
bined with the menftruum (or fluid in which the bo-
dies have been difiblved). Other fubftances, how-
ever, may fometimes intrude themfelves into the cry-
ftallized bodies, though not chemically combined with
them.

Fourthly, A difficulty of diflblving the compound
C 3 body

ft* Cryfiallizaticn. [Book I.

body in the menftruum, or fluid, which is a pro-
per folvent for one or both of the component Jub-
ilances *.

It may be proper, before the conclufion of this fec-
tion, to add a few words concerning one of the moft
curious effects of the attraction of combination, name-
ly, cryflallization. The word cryftal is derived from
cryos (froft), zndfallo (to contract) ; it was originally
confined to a particular diaphanous ftone refcinbling
clear ice, and was probably afterwards extended to all
bodies which were tranfparent, and had their particles
difpofed in a regular manner, particularly the different
fpecies of falts. It is now expreflive of that regular
order or dilpofition, in which the particles of inert
bodies arrange themfelves on pafling from a fluid to a
folid flate. This difpofition of the particles is, how-
ever, by no means the fame in all fubftances, but va-
ries almoil infinitely in different bodies. Thus com-
mon fait cryftallizes into a cubic form, falt-petre into
that of oblong pillars with fix fides, cubic nitre into
the rhomboidal form, vitriolared tartar and Glauber's
lalt into a mafs of four or fix fides. Each fpecies of
fait preferves its peculiar form however frequently the
procefs of diflblving it is repeated, and equally in the
fmalleft mafles which the microfcope renders vifible,
and in the largeft which art or nature have been able
to produce f .

It

* Kirwan's Mineralogy.

•}• ' If what has been faid relative to cryftallization be not per-
« fe<EUy intelligible to the reader, I would advife him to make the
' following eafy experiment, which will give him a better notion
« of the matter than a thoufand words. Into a bafori full of boil-

* ing water, put as much faltpetre as the water will take up; if

• the faltpetre was purified, the tranfparency of the water will not
' be injured, it will llill appear to be a homogeneous fluid: when

« the

Chap. 4.] Moft Mineral Bodies cryftallized. 23

It would perhaps be no rafh affertion to fay, that
the whole mineral kingdom appears in a cryftallized
ftate ; and this adds greatly to [the probability that
chemical combination or affinity is the great principle
which has acted in the formation of all bodies. The
caufes of this peculiar diftribution of parts are not to
be demonftrated, and on fo abftrufe a fubjecl, all that
we are able to perform is to produce fome probable
conjectures.

The old and fanciful chemifts and alchemifts, who
remarked the curious figures which faline fubftances
affumed during their cryftallization, imagined that the
falts ftill retained the vegetative powers of the plants
from which they were produced, and even thought

« the water will take up no more faltpetre, then he may conclude

* that it is faturated : let it fland without being ftirred, till it
' grows cold. As it cools, a great many cryftals, all of the fame
' fhape, may be feen {hooting out from the fides and bottom of

* the bafon, and increafing in fize tiil the folution becomes quite
' cold. When no more cryftals can be formed by that degree of
' cold which prevails in the apartment where the experiment is

* made, pour the liquor from the folid cryftals ; this liquor is ftill
« faturated with faltpetre; and in order to make it part with more
' of its faltpetre, fome of the water which keeps it diflblved mufl
' be evaporated : upon the taking away a part of the water, a

* correfpondent part of the faltpetre lofes the power by which it
' is fufpended, and ought, upon that prefumption, inftantly to fall
' to the bottom : yet it muft be remembered, that the water from
' its increafed heat during the evaporation, is able to fupport
' more faltpetre than if it was co)d; and therefore the faltpetre
' will not begin to cryftallize, notwithftanding the lofs of part of
' its menilruum, till the remainder begins to cool. By repetition
« of this procefs of evaporation and cryftallization, we may obtain
< all the faltpetre which was at firft diffolved, as. no portion of it
' can be evaporated with that degree of heat which is ufed in
4 . evaporating the water.'

Watfou's Chem. Eff. p. 90 to 92.

C 4 they

*4 Gravitation. [Book I.

they could obferve the form of the plant in the cry-
ftallized mafies. Later philofophers have afcribed to
the primary parts of bodies a certain property which
they call -polarity (as analogous to that property of the
magnetic needle) and which difpofes them to fhoot out
in certain directions *. A more probable opinion ap-
pears to be, that the minute particles of each cryftal-
lizing body are of fuch a form that the fides, which
approach in contact, difpofe them in a particular di-
rection.

From this regular arrangement of the parts it refults
that homogeneous bodies poflefs always an equal den-
fity in all their parts ; and in mod cafes, if nature is
interrupted in the procefs, the concrete will be imper-
fectly formed. So nice and critical is the arrange-
ment of the parts in fonorous bodies, that it is faid the
fmalleft vibration of the air occurring during the ope-
ration of calling a bell, or racher while the metal is
fettling in the mould, even the barking of a dog, will
injure the tone f .

III. The attraction of GRAVITATION materially dif-
fers from the two preceding fpecies of attraction, fince
it requires neither the particles of the bodies, nor the
bodies themfelves, to be brought into immediate con-
tact, but acts at confiderable diftances, and in this re-
Jpect it is analogous to the attraction of magnetifm
and electricity.

The moft obvious effect of gravitation is the gene-
ral tendency of bodies to the furface, or perhaps to the
center of the earth. It appears to be one of the great
laws of gravitation, that the attraction of bodies is in
proportion to the quantity of matter they contain.
The earth, therefore, being fuch an immenfe aggre-

* Jones's Phyfiolog. Difquif. 22. f Ibid.

gate

Chap. 4.] Laws of Gravitation. 25

gate of matter, is fuppofed to deftroy the effect of this
attraction between fmaller bodies, by forcibly compel-
ling them to itfelf. The attraction of mountains, how-
ever, upon the balls of pendulums has been found, by
repeated obfervations, to be very confiderable *.

The efficient caufe of this fpecies of attraction is as
much a fecret as all the other great principles of na-
ture. Some philofophers have fuppofed gravity to be
one of the inherent properties of matter ; others have
afcribed it to the*~agency of a fubtile fluid; while others,
with more modefty, and probably with more truth,
have had recourfe to the immediate agency and inter-
pofition of the divine power.

We are generally on fure ground when we defcribe
effects. Ignorant as we neceffarily are of the caufes
or inftruments by which the fupreme governor of the
univerfe effects his purpofes, an attentive obfervation
will commonly furniih us with the obvious mode1 in
which they generally take place. What philofophers
term the laws of nature, are no other than the modes
or forms in which her operations are ufually effected •,
and this is precifely the cafe with what are called the
laws or properties of gravitation.

Firft, It appears that the gravitating force being
proportioned always to the quantity of matter, all bo-
dies gravitate from equal diftances with equal velocity,
except prevented or impeded by fome refilling me-
dium. Thus, though a guinea and a feather will not
fall to the ground with equal velocity in the open air,
becaufe of the refiftance of that fluid; yet if the air by
any means is removed, as in the vacuum of an air
pump, they appear to fall at the very fame inflant of
time: for though the guinea contains confiderably

* Nicholfon's IntroJ. to Nat. Phil. V. i. p. 26.

more

26 Specific Gravity. [Book I.

more of folid matter than the feather, and confequent-
ly requires a more confiderable force to put it in mo-
tion, yet it appears that the attractive power being
proportioned to the quantky of matter, its velocity is
equal to that of a body which requires lefs force to put
k in motion.

Secondly, The attractive force of bodies is recipro-
cally as the fquares of the difcances. Thus, if a body
is of the weight of one hundred pounds at the diftancc
of ten diameters of the earth, at half that diftance it
would have four times that weight, or the force of gra-
vity would be exerted upon it in a quadruple ratio,
and fo in proportion as it approaches the body of the
earth.

It would perhaps have been more correct to have
fpoken of what is commonly called fpecific gravity in
treating of the denfities or porofity of bodies, but the
rea'fon why it was omitted on that occafion will pre-
fently be apparent. The truth is, we have no mode
of determining the dcnfiry of bodies, but by the firfl
of thefe laws of gravitation, which have juft been no-
ticed. For fmce the force of attraction which nature
exerts upon all bodies, is in proportion to the quantity
of matter which they contain, it follows of courfe, that
if, of two bodies equal in bulk, the one is heavier than
the other, that body is poflHTed of greater denfity, or
contains more matter in the fame compafs.

Thcfpecifa gravity is therefore the very fame thing
with the denfity of bodies, and has relation to the quan-
tity of folid matter which different bodies contain in
the fame bulk. J t is alfo called relative or compara-
tive gravity, becaufe we judge of it by comparing one
body with another. If bodies are equal in bulk, it is
evident their fpeciiic gravities may be eafily deter-
mined by a common balance, and hence fluids, or any

fubftances

Chap. 4.] tiydroftatic Balance. Tf

fubftances that may be eafily reduced to the fame bulk
or form may eafily be weighed and compared. By
weighing accurately a determinate quantity of any fluid,
an ounce for inftance, in a phial, and marking precifely
the fpace which it occupies in the phial, the weight of
the fame quantity of any other fluid may eafily be had
and compared with the former.

The fpecific gravity of bodies which are not, nor
can eafily be reduced to an equal bulk, is not to be
obtained by any method equally obvious to imphilofo-
phical perfons. A method ,- however, has been invent-
ed for determining the fpecific gravities of folid bodies,
whatever their figure or dimenfions. As it is an ob-
vious principle, that every body when immerfed in a
fluid muft difplace a quantity of the fluid equal to its
own bulk, and the refiftance which it meets with from
the fluid will be found exactly equivalent to the weight
of the fluid fo difplaced ; hence if any fluid, as water
for inftance, is taken as the ftandard of connparifon, it
will be eafy to determine the fpecific gravity of dif-
ferent folids by weighing them firft accurately in air,
and afterwards weighing them in water, and comparing
their lofs of weight in this latter fluid, which will be
in exact proportion to the fpace which they occupy.
To make this clear by an experiment; fuppofe it was
neceffaryto determine the fpecific gravities of any two
metals, lead r-nd tin for inftance, I take a certain quan-
tity of the former, and weighing it carefully in air, I
find its weight amounts to thirty-four ounces; on
weighing it again in water, 1 find it weighs but thirty-
one ounces, that is, it has loft three ounces of its
weight, or in other words, the fame bulk of water
would weigh three ounces ; the fpecific gravity of lead
is therefore to that of water as 34 to 3 or as 1 1 -i to i.
On weighing a certain quantity of tin, I find again that
t it

a8 How to find Specific Gravity. [Book L

it amounts to fifteen ounces, and on weighing it in
water it appears that it has loft two ounces of its
weight. The fpecific gravity of tin is therefore to
that of water as 15 to 2, or as 7 { to r, confequently
the comparative gravities of the two metals are 117
to 7 I *.

In the common tables of fpecific gravities, the
weight of water is eftimated at i , and that of other
fubftances is exhibited in the fame ratio. To deter-
termine therefore the fpecific gravity of any fubftance
heavier than water, weigh any given quantity of that
fubftance in air in a common balance, and afterwards
weigh it in water, carefully noting its lofs of weight ;
divide the whole abfolute gravity, or weight in air of
the fubftance, by its lofs of weight in water, and you
will have its fpecific gravity.

IV. The attraction of MAGNETISM only differs from
that of gravity in its operations being limited to par-
ticular fubftances. The magnet is an ore of iron, and
its property of attracting certain portions of that me-
tal at moderate diftances is well known. Like the at-
traction of gravitation, that of magnetifm bears a pro-
portion to the diftance, and probably to the quantity
of matter (I fhouki fay of magnetic matter) in the at-
tracting bodies. But the properties of the magnet are
Ib curious and important in nature, that they well de-
ierve a diftinct chapter.

V. The attraction of ELECTRICITY is alfo analogous
to that of gravity in the property of acting upon bo-
dies at a certain diftance ; but it differs from it in its
operation being confined to a particular ftate of thofc
bjdies, that is, when excited by friction. But this pe-

* Nicholfon's Philofophy, V. ii. p. u.

culiar

Chap. 4-] Eleftrkal Attraction* 29

culiar fpecies of attraction will be more amply treated
of in a fucceeding part of the work.

There is a property fuppofcd to be incidental to
matter, which is oppoiite to this of attraction7, and
which is therefore denominated REPULSION, It is a
maxim of the Newtonian philofbphy, that where the
iphere, or power of attraction, terminates, that of re-
pulfion begin*. In the inftance which has been already
adduced of the round drops of dew upon the leaves of
plants, it is fuppofed not only that there exifts an at-
tractive force between the particles of the fluid, but a
repulfive force between them and the leaf on which
they are fufpended. That the drops are not in actual
contact with the leaf is evident from their white or
pearly appearance ; for this appearance refults from
the copious reflection of white light from the flattened
part of the furface contiguous to the plant ; and it is
well known that this effect could not take place, un-
lels there was a real interval between the under furface
of the drop, and the contiguous furface of the plant *,
The fact is alfo evident from another circumftance;
the drop is not found to have the fmalleft adhefion to
the leaf, but rolls off in a compact body with the great-
eft eafe, which it could not do if the fluid was in actual
contact with the leaf; or if there fubfifted any degree
of attraction between them.

In the' fame manner needles or other light metallic
bodies will fwim on the furface of a fluid. Flies walk
upon water, and oil obftinately refufes to mix with
that and other fluids. Hence the feathers of water
fowl, which are covered with a thin coating of fubtilc
oil, actually repel the lurrounding water.

* Prieftley's Optics, p. 4;*.

This

30 Repulfion. [Book I.

This principle of repulfion has, however, been dif-
putcd by fome late philofophers ; and all thefe efFe6ts
have been accounted for by them, by fuppofing, not
that there exifts a pofitive principle of repulfion in the
water, the oil, &c. but: that the attraction of the leaf,
of the water, &c. for the contiguous bodies, is not fuf-
ficient to deftroy the attraction which the particles of
homogeneous fluids poflefs for each other.

The repulfion of magnetifm and electricity, will be
treaced of in thofe parts of the work which are appro-
priated to thefe fubjects.

CHAP. V.
OF MOTION AND REST.

Newtonian Theory of Motion and Reft. — Vis inertia. «— Laws of
Motion.

E S I D E S the principles of gravitation and re-
pulfion, there are other laws to which all matter
in certain circumftances appears to be fubjed ; thefe
are termed by modern philofophers the laws of motion,
fince they relate to that change of place or fitualiori
of bodies which is denominated motion, to the force
which is neceffary to this effect, and the velocity which
is given to moving bodies by the application of this
force or power.

An attentive and judicious obfervation of the ufual
courfe of nature, enabled Sir Ifaac Newton to reduce
the general principles or Jaws of motion to the three
following axioms. There appears little neceffity to
illuftrate them by particular inftances, fince they are
confirmed by conftant and univerfal experience ; and
however the application of thefe principles to the mo-
tions of the heavenly bodies, or to thofe departments
of nature which are out of the reach of our obferva-
tion, may be contefted, their truth and utility, with
refpect at lead to thofe bodies with which we are beft
acquainted and have the moil intimate connexion, will
fcarcely admit of difpute.

I. All bodies are perfectly indifferent to motion and
reft. In other words, a body, if once at reft, will na-
turally

3 2 Laws of Motion. [Book I.

turally remain fb, unlcfs difturbed by fome power act-
ing upon it j and a body in motion will continue that
motion in the fame direction, and with the fame ve-
locity, unlefs flopped or impeded by fome external
caufe *.

The firfl part of this propofition is evident from
every part of nature, fmce no part or portion of in-
animate matter appears capable of giving itfelf any
degree of motion. The latter part of the propofition,
namely, that a body will continue its motion for ever,
unlefs prevented by external force, it is not fo eafy to
illuflrate by experiment, fince we are not able to pro-
duce any fpecies of motion which is not in fome de-
gree counteracted by the force of gravitation, or by'
fome refilling medium. The conclufion, however,
appears to be fairly drawn, fmce the lefs the obflruc-
tion which is oppofed to any body in motion, the lon-
ger the motion continues; thus a ball will continue
longer in motion on a fmooth than on an uneven fur-
face, whence we may reafonably infer, that if all ob-
ftacles were completely removed, motion once com-
municated would never ceafe f.

This property of refiflance in matter is termed, in
technical language, its vis inerti<e.

II. The alteration of the flate of any body, whether
from reft to motion, or from one degree of motion to
another, is always proportional to the force which is
imprefled, and in the direction of that force.

By this law, the degree of force is fuppofed to be
meafured by the greatnefs of the body which it can
move with a given velocity. Thus a power which
could give to a certain body fuch a degree of celerity

* Pemberton's View, p. 29.

f See Enfield's Inftit, Pbilofophy, p. xi, 12.

in

Chap. 5.] Motion of the Heavenly Bodies. 33

in its motion as to enable it to pafs in one hour the
length of one thoufand yards, would give to another
body, half as great as the former, twice the degree of
velocity, and would enable it to pafs in the fame time
the length of two thoufand yards *. Hence the quan-
tity of motion is always eftimated by the fwiftnefs of
the motion, and the quantity of matter which is moved.
If A and B, bodies of equal fize, move with equal ve-
locity, their quantity of motion is equal; but if A
contains twice as much folid matter as B, and moves
equally fwift, it pofielTes a double quantity of mo-
tion f.

It follows evidently from the fame law, that if a
new force is imprefTed upon a body in motion, in the
direction in which it moves, its motion will always be
increafed proportionably to the acceffion offeree, how-
ever frequently repeated.

It follows alfo, and may be proved by a very eafy
experiment, that if a new force is imprefTed upon a
body not in4 the direction in which it moves, but in an
oblique direction, the body will take a direction nei-
ther exactly the fame as that in which it was proceed-
ing, nor yet in the direction of the new force which is
imprerTcd upon it, but a direction between both. On
this is grounded the commonly received opinion con-
cerning the motion of the heavenly bodies. The cen-
trifugal force is that which is fuppofed to have been
imprefied upon them at their firft formation, and
which would carry them forward in a direct line ; this
is counteracted by the force of gravitation (or centri-
petal force) which always inclines them to that body
round which they revolve ; the confequence of thefe
two forces acting in different directions is, that the

* Pemberton's View, p. 29, 36.
f Elements of Nat. Phil, by Mr, Locke, chap, i.
VOL. I, D bodies

34 Re-aftion is equal to Attion. * [Book I.

bodies alvtays move in a curve, or orbit, which is more
or lei's elliptical as one of thefe forces happens to be
predominant.

III. The third law is, that re-action is always equal
to action *. In plain terms, the refiftance of a body
at reft, which is acted or preffed upon, acts againft a
moving body with a certain degree of power, and pro-
duces the fame effects as would have been produced
by a certain degree of active force exerted in a direc-
tion contrary to that of the moving body. Hence it
follows, that any one body acting upon another actually
lofes as much force as it communicates, as will be evi-
dent, if with a fmall bullet fufpended from a firing we
ftrike another bullet which is at reft, or from obferv-
ing a ball in motion on a billiard- table ftrike another
which is at reft on the table ; in both which cafes the
ftriking body will lofe half its quantity of motion, and
that quantity of its motion which it lofes will be com-
municated to the other body f.

This law is an effect of the vis inertia of matter,
and is extended to all cafes where there is a refifting
body. When a load is drawn by a horfe, the load re-
acts againft the motion of the horfe, and the progref-
fion of the animal is as much impeded by the load, as
the motion of the load is promoted by the efforts of
the animal. The ringer which preffes againft any fo-
lid body is preffed by thnt body; but in elaftic fub-
ftances the effect is mod apparent.

* Pemberton's View, p 31.
f Kr.Scld's Inllitutes, p. 12, 15.

[ 35 ]
CHAP. VI.

OF MAGNETISM.

Of natural and artificial Magnet's.— -Magnetic Powers.— Attra8ion>
— -"-Re tuition. — Polarify.* — Declination.— —Dipping of the Magnetic
Needle. — Communication oft.be Magnetic Power.

* I ^ H E properties of the magnet are illuftrative of*
JL fo many principles and laws of nature, that
though, perhaps, not ftriftly in order, I have deter-
mined to introduce the fubjeft before the conclufion of
this preliminary book ; as fome occafions may fhortly
occur, when a reference to this topic may probably be
ufeful, if not abfolutely neceilaiy.

It is well known that every magnet is a ferrugineous
body, and that its attractive force is confined in a great
meafure to ferrugineous fubftances. Magnets are of
two kinds, natural and artificial. The natural mag-
n'et or loadftone *,• is a bog ore of iron ; artificial mag-
nets are formed either by being touched with a natural
magnet, or by other different prccefles., which will pre-
fently be explained.

The properly magnetic ores are calciform (refem-
bling a calx or cinder) and are moflly of a dull
brownifh black j-. There are reddifli magnt ts found
in Arabia ; but mod of thofe in Europe refemble
wrought iron in colour. Their hardnefs is juft furB-
cient to afford fparks with a fteel, and they are with
difficulty attacked by a file. They differ considerably

* Load (Sax.) or leading flonc, probably from its being a guide
to mariners. — Adams on Mag. p. 377.

f Kirwan's Min.— and Cavallo on Magnetifm,

D 2 in

36 Natural Magnets. [Book I.

in fpecific gravity, and feem to contain, feveral fub-
ftances befides iron in their compofition, fuch as argil-
laceous and filiceous earth. Mr. Kirwan is of opi-
nion that fulphur enters into their fubftance, as the ore
fmells of it when heated red hot : probably, alfo, they
may fometimes contain nickel, as that metal, when
purified to a certain degree, acquires the properties of
the magnet.

Natural magnets are found more or lefs in almofl
every iron-mine, of different mapes, and of different
iizes. Some old writers mention magnets that would
fwim on water *•; thefe were probably fome light,
fpongy, volcanic fubftances, impregnated with iron.
In Virginia there is a magnetic fand, which contains
about one half iron. Thofe magnets which have the
fineft grain poffefs the magnetic virtue in the higheft
perfection, and retain it longer than any other f.

The great and well-known properties of the mag-
:net are, ifl, its attractive power; 2dly, its polarity,
or difpofition to conform to the plane of the meridian j
^dly, the property of dipping or inclining to the earth;
and laftly, the power of communicating the magnetic
virtues to other ferrugineous bodies.

I. Magnets attract clear iron more forcibly than
any other ferrugineous body. The iron ores are at-
tracted more or lefs forcibly, in proportion as they are
impregnated with the metal, and in proportion as it
exifts in a metallic date. The force of the attraction
between a magnet and iron will^depend in a great
meafure ort the weight and form of the iron j but art
and obfervation have furnilhed us with no invariable
rule in thefe refpects.

• They are in general about feven limes heavier ilun water.
f Kirwan and Cavallo.

On*

Chap. 6.] Force of Magnetic Attr&fti on. 37

One magnet attracts another magnet in contact^
•with lefs force than it attracts iron ; but the attraction
between them begins at a much greater diftance than
between the magnet and iron alone.

As iron is diffufed in greater or lefs quantities
through almoft all the different bodies of which the
univerle is compofed, it is eafy to fuppofe that the
natural bodies which are fubject to the magnetic at-
traction are very numerous. The perfect metals,
gold, fiiver, and platina, as well as lead and tin, are
however total exceptions j though their calces are a
lit le attra6ted. Animal and vegetable fubftances alfo,
though they are known to contain fmall portions of
iron, feldom exhibit any difpofition to be attracted
before combuftion * j though it is afierted that moft
fubftances may be rendered magnetic in fome degree
by being expofed to the action of fire.

Iron is attracted with different degrees of force,
according to the different modes in which it exifls,
It is, however, in no ftate quite infenfible to the
magnetic power, even in the pureft calx, or in a
ftate of folution. Soft iron is the moft fubject to a
forcible attraction ; fteel is lefs fo than iron, efpecially
when hardened, and the calces of iron in different
degrees f-

Mufchenbrock, by a feries of experiments, endea-
voured to afcertain the force with which the mag-
net attracts at different diftances. vHe fufpended a
cylindrical magnet, 2 inches long and 16 drams in
weight, to one fcale of an accurate balance, and under
it he placed a cylinder of iron of the fame ihape and
bulk. The following is the force with which it at-
tracted at different diftances, eftimated by the number
of grains in the oppofite fcale.

* Cav. on Mag. f Ibid.

D 3 At

3$ Force of Magnetic Attraction. [Book I.

At 6 inches 3 grains.

3 6."

2 9.

i 18.

In contaft — 87.

This experiment would perhaps have been more,
intelligible, if it had been previoufly remarked that;
the attraction between the magnet and the iron is
always fuppofed to be mutual. If a magnet and a,
piece of iron are placed fo as to float on the furface
of water;, the magnet will approach the iron, as well
as the iron the magnet ; or if either of them are kept;
fteadv, the other will approach towards it *.

Of natural magnets the fmaller poflefs a greater at-
tractive power, in proportion to their fize, than the
larger. There have been natural magnets of not
more than 20 or 30 grains in weight, which would
lift a piece of iron forty or fifty times heavier than
themfelves ; and mention is even made of one of
about 3 grams, which lifted a weight of iron contain-
ing 746 grains, or 2.50 times its own weight f. What
is yet mure extraordinary, it not unfrequently hap-
pens, that a loadftone cut off from a large one wilt
itfelf lift a greater weight than the ftone from which it
was cut off. This circumftance may reafonably be
attributed to the heterogeneous nature of the large
loadflone ; for if we fuppofe that one part of it, viz.
that which is cut off, contains a confiderable portion
of magnetic matter, and that the remainder is impure,
of confcquence, while they remain in an united ftate,

* Adams on Magnetifra, p. 385. f Cav. on Mag. p. 36,

the

Chap. 6.] I&^ulfion of Magnets. 39

the impure part will rather obftruct the action of the
other *.

' The power of magnets is not at all times equally
active j they will at one time attract at a much greater
diftance than at others. To what this variation is
owing, is impofiibie to decide, while we remain fo per-
fectly ignorant as we are of the caufes of all the mag-
netic phenomena : probably it may depend upon the
temperature of the ftone, as the magnetic power is
always diminished by heat.

There is in magnets a natural power of repulfion,
as well as of attraction. Two magnetic bars, for in-
ftance, will attract each other if the two extremities
or poles, which correfpond in each, are brought
within the fphere of attraction ; but if the extremities
which do not correfpond are brought into contact,
they will be mutually repelled f. This circumftance
will be better underftood when the polarity of the
magnet has been properly explained. The power of
repulfion is fuppofed by fome experlmentalifts to be
weaker than that of attraction.

II. The fecond dlftinguifhing property of the mag-
net, is what is termed its polarity. In plaint terms,
if a magnet is placed in fuch a fituation that it. fhall
have liberty to affume that direction which is moft na-
tural to it j for inftance, if it is made to float on water
upon a piece of wood or cork, if fufpended by a (len-
der firing, or fupported by a pivot, as is the needle
in the common mariner's compafs, it will difpofe itfelf
longitudinally nearly in the plane of the meridian, that
is, one extremity towards the noith pole of the earth,
and the other towards the fouth. The two extremi-
ties which correfpond to the poles of the earth arc

* Cav. on Mag. p. 37. f Adams on Mag. p. 389.

D 4 called

40 -Polarity of Magnets. [Book I.

called the poles of the magnet : and at thefe extremi-
ties the magnetic virtues feem to exift in their great-
eft force, as a magnet will fupport a much more con-
fiderable weight near the poles than at any other
part *.

This property in the magnet has proved the bafis
of an invention the moft ufeful to navigation that hu-
man fagacity ever difcovered j as before this infallible
guide was enlifted in the fervice of the mariner, the
moft adventurous pilot did not prefume to truft him-
felf out of the fight of land ; confequently commerce
is much facilitated by the difcovery, arid fhipwreck is
a much lefs frequent calamity. It was not till the
thirteenth century that this circumftance was known.
Authors are generally agreed at prefent, that a Nea-
politan, of the name of John de Gioja, if not the in-
ventor of the mariner's compafs, was at leaft the firft
who made ufe of it in conducting vefiels in the Medi-
terranean -j-. Some ridiculous pretences have been
made by the Chinefe to the honour of this, as well as
of other European inventions j but the fables of that
barbarous people, as well as of their encomiafts, the
jefuits and infidels, are little to be regarded J.

Both the properties of attraction and repulfion have
an intimate connexion with this of polarity. Thus,
it is uniformly found to be the cafe, that in two mag-
nets an attractive force obtains between the oppo-
fite poles, and a repulfive force between the poles
of the fame denomination. If, for inftance, the north
pole of the one is brought near the north pole of the
other, a mutual repulfion will take place ; but if the
fouth pole of the one is applied to the north pole of the
other, they will be mutually attracted. And if, upon,

* Adams on Mag. p. 387. | Cav. on Mag.

J See Mr. Adams's iifiay, p. 409.

the

Chap. 6.] DireStiw Power of Magnets. 41

the fame principles, a magnet is cut through the axis,
the parts or fegments of the (lone, which before were
united, will now repel and avoid .each other *. If two
magnets of a fpherical form are freely fufpended,
one will conform itfelf to the other, as to the poles of
the earth. This influence of one magnet over another
is termed the directive power, and this directive power
2<5ts at a greater diftance than that of attraction. This
may be proved by placing one magnet at the bottom
of a fcale, and holding the other at the fame diftance
at which it acts in altering its direction : in this cafe
no degree of attraction will be produced f. Jf a
^quantity of iron filings are gently dufted on a magnet,
they will arrange themfelves around it in a very whim-
fical manner : this effect is only to be accounted for
from the directive power of the magnet, for the filings
by contact with the magnet afiume the magnetic vir-
tue, that is, become each of them a fmall magnet, and
arrange themfelves according to the polarity of the
original magnet J.

Neither the directive nor the attractive power of the
magnet is diminiflied by the interpofition of a foreign
body. Steel filings fcattered on a plate of metal, or
of wood, or of any body not magnetic, will be affected
by the motions of a magnet under the plate 5 and fer-
rugineous bodies are attracted with the fame eafe, and
at the fame diftance, in the vacuum of an air-pump,
as in the open air §.

Natural magnets are frequently found to have more
than two poles. That is, the poles of another mag-
net of the regular form will frequently be attra&ed by
different parts of the furface. This circumftance de-
pends on the form and heterogeneous nature of thefe

* Adams on Mag. p. 444. f Cav. p. 98. { Ib.

— Nicholfon's Phil. ii. p. 296. $ Enfidd's Inftit. p. 340.

irregular

42 Declination of the Compafs. [Book I.

irregular magnets : for a good loadftone is always of
an uniform texture, and has only two poles, which lie
in oppofite points of the furface, in fuch a manner,
that a line drawn from the one to the other would
pafs through the center of the magnet*. When a
magnet has more than two poles of equal ftrength,
the fupernumerary poles may be fo fituated that the
magn&t will not, in the technical language, traverfe j
in other words, it will not, when fufpended by a
thread, &c. or when floating on water, afTume the
ufual direction to the poles of the earth f.

The magnetic meridian feldom coincides with the
real meridian, but generally is found to vary a few de-
grees from the true direction of north and fouth. This
is called the declination of the compafs. The mag-
netic needle varies fometimes to the eaft, and fomc-
•times to the weft ; and it varies not only in different
places, but even in the fame place at different times.
The declination at London is not the fame now as it
was a few years ago J. Nay, fome very nice obfer-

vations

* Cav. p. 40. f Ib. p. 43.

t The following table fhews the mean declination of the needle
at different times in Paris and London.

Year. 'Paris. Year. Lsujcn.

O / Of

1580 n 30 E, 1576 11 15 E.

1610 8 o E. 1634 4 - 5 E.

, 1640 . 3 o E. '657 o o.

1666 o o. 1665 i 22 W.

1670 i 30 W, 1692 6 o W.

1700 8 12 W. 1730 10 15 W.

1728 14 o W. 1,756 15 15 W.

1771 91 45 W. 1774 i\ 16 W.

1776 21 47 W.

Near the equator, in long. 40° Eaft, the higheft variation, from
the year 1700 to 1756, was 17° i5'We(l; and theleafl i6°3o/W,

Jn,

Chap. 6.] Suppofed Caufe of Polarity; 43

vations feem to determine that the declination varies
at different times of the day *.

The polarity of the magnet has been attempted to
be accounted for, by fuppoiing the earth a large load-
ftone, or at leaft that a mafs of ferrugineous matter,
equivalent to fuch a loadftone,. is contained within the
bowels of the earth, to which all finaller magnets muft
neceffarily conform. Attempts have alfo been made
to explain the variation or declination of the compafs
upon firnilar principles. If the mafs of ferrugineous
or magnetic matter which the earth contains is fup-
pofed to act upon all magnetic fubftances, and if this
mafs is almoft conftantly varying its pofition and
compofition by fubterraneous fires, it is not very dif-
ficult to fuppofe, that the magnetic needle will be fub-
ject to considerable variations from thefe important
movements f .

The magnetic center is the point between the two
poles, where the magnet porTeffes neither attraction
nor repulfion. If, however, a part of a magnetic bar
is broken off at either pole, the fragment will ftill be
a complete magnet, having two poles and a center,

In lat. 15° N. and long, 60° W. the variation was conftantly
5° E. In lat. 10° South, and long. 6oQ E. the variation decreaf-
ed from 17° W. to 7° 15' W. In lat. 10° S. and long. 5° W. it
increafed from 2° 15' to I2°45'W. In lat. 15° N. and long.
20° W. it increafed from 1° W. to 9° W. In the Indian feas, the
irregularities were greater, for in 1700, the Welt variations feem
to have decreafed regularly from long. 50° E. to long. 100° E.
but in 17^6 the variation decreafed fo fait, that there was Eaft va-
riation in long. 80°, 85°, and 90° E. and yet in long. 95° and
100° E. there was Weil variation.

In the year 1775, ^n ^at- 5^° i?'S. and long. 348° 16' E. it
was o° \6' W. In lat. 2° 24' N. and long. 32° 12' W. it was
o° 14' 45" W. In lat. 50° 6' 30" N. and long. 4° o' W. it was
?9° 28° W. Enfield's Inft. Phil.

* Adams on Mag. p. 415, 416, f Nicholfon's Phil.

9 though

44 Dipping of tbe Needle. (Book I.

though it originally -might belong to a part of the
magnet which was altogether of a certain polarity *.

III. Magnets, while they attract other bodies, ap-
pear themfelves to be fubject to the attraction of the
earth ; for a magnet, or magnetic needle, when placed
fo as to be able to* act according to its native impulfe,
inclines one of iis poles a little to the earth, while the
other is proportionably elevated : this is called the
dipping of the needle, and the inclination or dipping is
found to be different in different latitudes. Near the
equator the needle affumes a pofition almoft perfectly
horizontal ; in the northern hemifphere the fouth pole
is depreffed or attracted to the earth ; and in the
fouthern latitudes the north pole of the magnet fuffen
a fimilar depreflion,

This property of the magnet is accounted For upon
die fame principle as the former, namely, by fuppof-
ing that the earth, from the quantity of ferrugmeous
matter which it contains, acts as an immenfe load-
ftone, which at its poles attracts thofe of every other
magnet fufpended above its furface. It has more
than once been repeated, that magnets attract each
other at the oppofite poles. Thus, if a fmall magnet,
or magnetic needle, is fufpended by a thread above a,
larger magnet, while its poles are at equal diftances
from the poles of che larger magnet, it will remain in a
horizontal pofition ; but if it is removed either one
•way or the other, that is, if one pole of the fmaller
magnet is moved towards the contrary pole of the
larger, it will be attracted towards the perpendicular.
This is exactly illustrative of the dipping needle, which
upon the equator remains in an equilibrium, but
inclines to the perpendicular as it approaches to

• Cav. 218.

the

Chap. 6.3 Communicated Magnetifm. 4$

the poles of the earth ; and what is (till more agree-
able to this theory is, that the dipping or inclination
of the needle is greatly increafed as it approaches
either pole.

IV. The magnetic virtue may be communicated to
any ferrugineous body.

i ft. By contact with a real magnet: and in this
way artificial magnets are in general prepared. This
property of imparting the magnetic powers is not,
however, confined to the natural magner, for artificial
magnets are capable of commtmicating it to frefli
femigineo'js bodies, and that without the leaft dimi-
nution of their own power * : and the power may in
this manner be communicated from one piece of iron
to another to infinity. A weak magnet may alfo
be rendered more powerful by the application of a
ftronger.

Soft iron acquires magnetifm with more eafe than
hard iron or Heel, but the virtue is not fo permanent*
Hard ileel will retain it for many years without dimi-
nution.

To make artificial magnets of extraordinary power,
fome addrefs is required. A fingle magnet cannot
communicate a greater degree of power than itfelf pof-
fefTes, but feveral magnets united will impart a power
equal to their united force f . It will eafily be ima-
gined, that the power imparted will be in proportion
to the approximation of the iron to the magnet. To
acquire a very high degree of magnetifm alfo, the

* It is faid indeed that the power of a magnet is increafed ra-
ther than dim'mifhed by communication. — Cav.

f Hence it is evident, that artificial may be made much Itronger
than any catural magnets v/hatever <—>&&*« on Mag. 378.

iron

46 Communicated Magnelifm. [Book t*

iron ought to remain fbme time in contact with the
magnet.

There are feveral curious phenomena which attend
.communicated magnetifm. The nature of the mag-
netifm communicated will frequently depend upon the
length of the iron bar which is brought into contact with
the magnet. If, for inftance, the north pole of a mag-
net is applied to the extremity of a long bar of iron,
that extremity will of courfe acquire a contrary virtue,
and become a fouth pole j at a part of the bar, hew-
ever, not very difcant, there will be found a new north
pole ; at fome diftance again a fouth pole 3 and fo al-
ternately, till the power is totally loft j the number of
thefe fuccefiive poles depending on the ftrength of the
magnetifm, and the length of the bar. If, however,
the bar is of a moderate length, there will be only two
regular ;poles *»

The polarity of .a bar of iron may be altered by
gradually moving the pole of a magnet along its fur-
face. Thus, if the north pole of a magnet is ap-
plied to that extremity of a magnetic bar of iron
which is the fouth pole, and moved gradually along,
the other (that is that which was the north) pole
of the bar will in that cafe be converted into a fouth
polef. ;

If a piece of wire which has been rendered mag-
netic is twifted, its virtue will be ftrangely interrupted
and confufed. In fome parts it will attr.ict, in others
it will repel ; and even in fome places one fide of the
wire appears to be attracted, and on the other fide re-
pelled, by the fame pole of a loadftone J. This and
other phenomena icem to indicate, that much of the

* Cav. pint i. c. 7. f Ib.

J Rees's Cyclop, art. Magnet \ and Adams on Mag. 399.

magnetic

C , 6,1 rrow fyonmay acquire Magnetic Pcibsr. 4/7

magnetic power depends upon the texture of the fub-
ftance which retiins it.

Every portion of iron is capable of retaining only
a certain degree of the magnetic virtue. If a ft ohg
magnet is applied to a frnall piece of fteel, the fteel,
while within the influence of the magnet, appears
powerfully magnetic ; but if the magnet is removed,
the power fubfides to a certain degree, which may-
be termed the point of faturation *. A number of
magnetic bars, however, may be joined together, fo
as to form an exceedingly ftrong compound mag-
net f.

idly. Iron is rendered magnetic merely by being
kept a confiderable time in a ikuation perpendicular
to the furface of the earth j and in this hemifphere
the lower extremity will be the north pole, and of
confequence the contrary effect will take place- in the
fouthern hemifphere. This phenomenon alfo is ex-
plained from the magnetifrn of the earth, which com-
municates its power to ferrugintous bodies, though by
almoft imperceptible degrees. Old iron ba?s in win-
dows, &c. are frequently found to be ftrongly mag-
netic J.

The moft advantageous fituation of the bar is how-
ever not directly perpendicular, but rather in the di-
rection of the dipping needle ; and indeed the mag-
netic virtue which it acquires feems to be in propor-
tion as it approaches that direction. Hard iron or
fteel acquires little or no magnetifrn from the earth,
on account of its greater infenfibility to the magnetic
influence ; but it is well known that iron hardens by

* Cav. 02. f Ib.

I Leewenhook mentions an iron crofs, which had acquired a
very ftrong polarity. — Adams, 432.

expofure

4$ How Iron may acquire Magnetic Power. [Book I.

expofure to the atmofphere ; it has been faid, there-
fore, that bars of foft iron, which have remained for a
long time in a magnetic direction, have acquired as
ftrong a power as good natural magnets *.

A bar of iron made red hot, and left to cool, or
quenched in water in the pofition of the dipping nee-
dle, acquires a degree of magnetifm proportional to
its nature, and the circumftanccs of its cooling f.

3dly. Magnetifm may be imparted to a bar of
iron, by placing it firm in thj direction of the dipping
needle, and rubbing it hard one way wi:h a polifhed
Heel inftrument J.

4thly. Any violent percuffion will impart polarity,
and the other magnetic virtues, to a bar of iron in a
vertical pofition. A few ftrokes of a hammer will
produce this effect ; and by hitting firft one end of the
bar, and then the other, the poles may be charged.
If a long piece of wire is twifted feveral times b,.ck-
wards and forwards, and then broken off at the twifted
part, the broken end will be magnetic §.

5thly. Even hard iron tools, when heated by any
briflc action, as hammering, filing, &c. acquire an
impermanent magnetilrn, and, while warm, attract
thin filings, or fmall portions of iron |i. This fact,
I am inclined to fufpect, muft depend in a great mea-
fure on the unequal texture of thole tools : if we
fuppofe them to be compofed of hard and foft par-
ticles, the latter will eafily acquire an impermanent
magnetifm.

6thly. Apparently all the three laft- mentioned
effects depend upon precifely the fame caufe j and,
perhaps, we may add to thefe, the magnetifm which

^ Cav. f Adam?, and Cav. J Nichclfon's Phil. 292.
§ Adams on Mag. 444.. - || Rc-s's Cyclop, art.

Chap. 6.] How to mcreafe tbe Power of Magnets. 49

is produced by electricity. If a bar inlaid horizon-
tally to the magnetic meridian, and fubjected to the
electric fhock, whatever may be the direction in which
the {hock enters, that extremity which is pointed to-
wards the north, will be the north pole j and if the
bar (lands perpendicular, it will follow the ufual law
of communicated magnetifm, that is, in this hemi-
fphere, the end which is next to the earth will be the
north pole *. Lightning is the mod powerful of all
natural agents in producing immediate magnetifm ;
it will, in an inftaht, render* hardened free! ftrongly
magnetic, and will invert the poles of the magnetic
needle f.

One of the moft fingular properties of the magnet
is, the increafe of power which may be added to it by
gradually increafing the weight it fuftains ; and on the
other hand, it wi)l gradually, by difufe, lofe much of
its natural ftrength £. If a magnet is hung up with a
weight of iron, as much as it will for the prefent fuf-
tain, by adding gradually, fuppofe a few grains daily,
it will at length acquire the power of attracting near
double the weight which it would have attracted at
firft. It is probable, however, from what was for-
merly remarked, that this power has a limit.

If a piece of iron, feme what more ponderous -than
a magnet will fufiain, is applied to the pole of the mag-
net, it is plain that on removing the hand the iron
muft fall. But if another piece of iron is held at fome
little diftance btlow the firft, the magnet will be able
to fupport it. The reaibn is, that both pieces of iron
being rendered magnetic, the fiiil piece is actually con-
verted into an artificial, magnet, by its contact with the
original, and its virtue is ihcreafed by the fecond piece

* Cav. .f Adams on Mag. 398. J Car. 25.

VOL, I. E of

$o Diminution of Magnetic Power. [Book I.

of iron, confequemly it is rendered capable of a greater
degree of attraction for the original magnet*. To
make this perfectly clear, it is necefiary to be obferv-
ed, that a piece of iron, brought within a certain dif-
tance of a magnet, becomes itfelf porTeflfed of all the
magnetic properties, and that part of the iron which
is neareft the magnet acquires a contrary polarity.
Thus, if a magnetic chain is compoied of feveral
pieces of iron, each piece is in itfelf a complete mag-
net, and they mutually ftrengthen the magnetic virtue
of each other j-, as all magnets in contact are known,
to do.

The magnetic virtue is DIMINISHED :
i ft. By dtfufe : particularly if the magnet is laid
amongft iron, or permitted to ruft. Magnets will
alfo be injured, unlefs they are kept together with the
oppofite poles correiponding, the ends being connect-
ed by pieces of iron j and they ought never to touch,
except when in this pofition. The fouth pole mould
always be employed in this hemifphere to lift iron ;
and a (trait magnet will be weakened, unlefs kept
•with its fouth pole to, the north in the direction of the
magnetic needle, or downwards in that of the dip-
ping needle J.

sdly. Heat weakens the power of a magnet ; and that
high degree which is called by chemifts a white heat,
, entirely deftroys it §. On this principle Mr. Canton
endeavoured to account for the daily variation of the
compafs; as fuppofing it to depend on the heating
and cooling of the magnetic fubftances within the
earth. This theory he illuftrated by the following
experiment : — About E. N. E. from a compafs he

Cav. 200. f See Cav. p. 30 and 203.

Adams on Mag. 397, 44.3. §. Cav. 35.

placed

Chap. 6J] Hew the Magnetic Power is diminljhed. 5 1

placed a fmall magnet, exactly at fuch a diftance, that
the power of the magnet at the fouth pole was jtift
fufficient to keep the north end of the needle to the
N. E. point, or 45 degrees. He contrived to heat the
magnet, by putting upon it a brafs veffel, into which
he poured about two ounces of boiling water, and as
the magnet gradually heated, he obferved, during
feven or eight minutes, that the needle moved about
three quarters of a degree weftward, and became fta-
tionary at 44 f ° ; in nine minutes more it came back
a quarter of a degree ; but it was fome hours before it
gained its former fituation, and flood at 45° *.

jd. In general the fame means which facilitate the
communication of magnetifm to ferrugineous bodies
not magnetic, tend to deprive thofe which really are
fo of the magnetifm they have acquired. Every kind
of violent percuffion weakens the power of a magnet;
and a very ftrong magnet has been entirely deprived
of its virtue by receiving feveral fmart ftrokes with a
hammer j-. This effect appears to depend chiefly on
the derangement of the particles in the magnetic bo-
dies. TJius, if a dry glafs tube is filled with iron fil-
ings, magnetifm may be communicated to the tube by
touching it with a loadftone, exactly as if it was an
iron bar 5 but the lead agitation which difturbs the
fituation of the filings will prefently expel the magne-
tic virtue J.

4th. In the fame manner the electric ihock, which
imparts the ftrongeft virtue to iron not previoufly
magnetic, v. ill diminifh, and even deitroy, the power
of a real magnet. Electricity will alfo fometimes' in-
vert the poles of a magnet §.

* Adams on Mag. 417. f lb. 443. \ Ib. 444.
5 Cav. part i. c. 7 ; and Adams on Mag. 446.

E 2 The

52 theories of Magnetifm. [Book I.

The phenomena of magnetifm (land alone among
the wonders of philofophy, unlefs we fuppofe the at-
traction of gravitation to be a fpec-ies of genera] mag-
netifm, which indifferently affects all the various bo-
dies of which this univcrfe is compofed. Certain ana-
logies have been traced, or rather imagined, between
electricity and magnetifm. Both powers are .excit-
ed by friction ; and the magnetic polarity has been
compared to the two ftates of pofitive and negative
electricity. The analogy is favoured alfo by the pofli-
bility of imparting the magnetic virtue to iron by the
electric (hock j and the Aurora Borealis, which is ge-
nerally accounted an electrical phenomenon, is fuppofed
to have fome influence on the variation of the mag-
netic needle. Thefe powers, neverthelefs, I mud
avow, appear to me effentially different. The phe-
nomena of electricity are not at all times exhibited
by electrical bodies, but merely when thofe bodies are
in a ftate of excitation. When the electrical virtue is
imparted from one body to another, the body that
imparts it lofes proportionably of its own power, but
the magnet rather increafes than diminimes its ftrength
by communication. The electric matter is vifible ;
whereas the very exiftence of a magnetic fluid is juftly
questionable ; befides that electricity, both in its na-
ture and effects, bears fo clofe an analogy to another
apparently very different power in nature, that there
can be no reafon for referring it to one with which
it appears to have a very flight, and, moft probably,
only a cafual, agreement.

The polarity of the magnet, as well as the dipping
of the needle, are in all probability mere effects of its
great property, attraction, fince they appear to be fairly
accounted for, frorfr the ftrong and peculiar attraction
which the magnet appears to have for the earth, or

rather

Chap. 6.] Theories of^Magmtlfm. $3

rather for that immenfe mafs of ferrugineous matter
which the earth contains. The attraction of the
magnet is commonly fuppofed to depend upon the
agency of a fubtile fluid which circulates around it, en-
ters the pores of the magnet itfelf, and of all the
bodies which it attracts. I cornels that the theories
which are founded upon this hypothefis appear to me
fo deficient in the only proof that ought to be admitted
in natural philofophy, I mean actual obfervation, that
I am frill inclined to account the caufe of magnetifm
as one of the undifcovered principles of philofophy.
I am not fond of indulging the imagination in its fa-
vourite propenfity to create invifible agents in order
for the fabrication of plaufibie theories, which fome
flight and cafual experiment may mortly overturn.
We appear to^be equally ignorant of the nature of
gravitation, and of the common attraction of cohefion
and combination. It is a trite remark, that there are
certain points at which the human faculties muft flop
in all our fpeculations. This would be a dangerous
tenet, if it promoted indolence, or difcouraged our
ardour in the puriuit of natural knowledge by the
only fecure path, I mean that of experiment; but it is
a faiutary maxim when applied to the imagination, and
when it only ferves to reftrain our ardour for fabricat-
ing fyftems, which have no other end but to remove
fora moment the uneafy but ufeful fenfation of doubt
and curiofity.

I lhall not therefore incumber my work with the
detail of fyftems to which I do not feel inclined to*
afient j but for a clear, and, I think, correct, ftatement
of the moft plaufibie theories concerning" the caufc? of
magnetifm, fhall content inyfelf with referring my rea-
der to an author to whom I have many obligations,
E 3 and

54 MagnetiJ'm. [Book I.

and whofe lofs, as a friend, I can nev^r diffidently
lament j and (hall direft him to confuk the late Mr.
Adams's ingenious EfTay on this fubjedl *.

* Printed in the fame volume with his Efiay on Eleftricity.
In the lame EiTay the -reader, who wifiies to entertain himfelf with
the practical and experimental part of magnedfm, will find proper
and eafy directions.

Chap. 7-1 [ 55 ]

CHAP. VII.

OF THE MECHANIC POWERS.

Sixjtmple Machines. — The Lever. — The Pulley. — The moveahle Pul-
ley,— Qf Wheels. — Clockwork.— Beft Mode of conducting the
Wheels of Carriages. — The Wheel and Axle. — The Crane.— .The
Capjlan. — The Crick or Jack,— The inclined Plane. — Motion of
Carriages up an inclined Plane, &c, — The Wedge. — The Screw.—
The perpetual Screw.

THE fcience of mechanics may poflibly be con-
fidered as in many refpedts foreign to a work,
•which, as its title implies, was intended chiefly to de-
tail and explain the operations of nature. It muft be
remembered, however, thar the adtion of the mechanic
powers is the effect of thofe laws of motion which
have been explained in a preceding chapter, and that
even fome of nature's operations can fcarcely be well
underftood, without a previous acquaintance with thofe
principles by which bodies are moved with the great-
eft facility, and by which the animal machine in para-
cular is enabled to aft.

There are fix funple machines or powers, of which
all the more complex engines are conftructed ; and
thefe are the lever, the pulley, the wheel and axle, the
inclined plane, the wedge, and the fcre-w. It has been
remarked by fome authors, that thefe fix machines
may in fa6t be refolved into two, the lever and the
inclined plane, for the pulley and the wheel and axle
may be confidered as compound levers, and the wedge
and the fcrew are only modifications of the inclined
plane,

£4 I. The

56 Diferent Kfads [Book I.

I. The LEVER is of all machines the moft fimple ;
it is a bar of iron, of wood, or of any fimilar material,
by means of which a fmall force applied to one end,
aided by a. fulcrum, or prop, placed at any part be-
tween £he middle and the other extremity, is capable
of overcoming or refitting a greater.

There are three kinds of levers. A lever of the
firft order is where the prop C (Plate I. fig. i.) is
placed between the moving power * A and the point
of refinance B. Upon this plan are conilrucled ba-
lances, fleelyards, and the moft ufual inftruments for
weighing, as well as the common inftruments for cut-
ting, &c. as fcifiars, pincers, (buffers, &c. ; and feveral
large but fimple machines for raifmg weights, draw-
ing watrr, and other fimilar purpofes. A lever of the
Jccond order is when the refilling force B (fig. 2.) is
placed between the power at A and the prop at C. :
and a lever of the third order is when the power at A
(fig. 3.) is placed between the weight or refiftance B
and the prop C.

Thefe may be confidered as the different kinds of
levers, and diey admit of a further diftinc~uon or fub-
divifion, according as the point of power and the
point of refiftance are rriore or lefs diftant from the
prop. For example, in the lever (fig. 4.) if the prop
is at a, the power at p, and the refinance at r, ir is
called a lever of the firft order with equal arms j if the
prop is at by it is a lever whofe arm of power/), is to
that of the refiftance r, as two to one; and if the prop
is at c, the arm of power is to that of refiftance as
three to one $ and the fame may be applied to the
other orders. Thus if the point of power f, in a

* That which moves the lever, or adls upon it.

lever

Chap. 7.] of Levers. 57

lever of the third order (fig. 5.) is placed at I, it is
thep a lever whofe arm of power pf is to that of re-
fiftance R, as one is to-three; for the length of the
arm of the lever is always determined by its diftance
from the prop C. But if the power P, is placed at 2,
it is then a lever whofe arm of power P, is to that of
refinance R, as two is to three.

It is the diftance of thefe forces from the prop
which determines the velocity of sheir motion, v/hich
is always in the fame proportion as the<diftances ; for
when the prop is at C (fig. 6.) one of the powers
at B, and the other at A, double the diftance fro-n
the prop, the latter power A will have a velocity
which is double that of the firft power at B. Be-'
caufe when the lever begins to move, while the point
B describes the arch B b, A will defcribe the arch A a,
which will be double the former, for arches are always
in proportion to their radii *.

The force of a moving body is the remit of its mafs
multiplied by its velocity, it follows therefore in the firft
place, as has been obferved above, that a weight act-
ing by a lever produces a force fo much the greater,
as its diftance is greater from the prop, becaufe then it
pofihTes greater velocity ; fecondly, it follows that two
equal weights, acting in oppofite directions upon a
lever, are not in equilibrio when they are not at equal
diftances from the prop ; and thirdly, that two unequal
weights, acting upon a lever, will exert equal forces
when their diftances from the prop are in a reciprocal
proportion to their refpective mafles. / Hence it fol-
lows, that whatever is gained in power is loft in time,
and the contrary.

In what has been hitherto obferved of the lever, it

* The radius is the femidiameter of a circle, or that line which
jrcceeds direftly from the center to the circumference.

has

5S The Lfirr. [Book I.

lias been prtfoppoTcd that powers acting upon it have
cither been in perpendicular directions, or in thole of
equal obliquity to the arms of the lever.

The moil advantageous pofition for a power act-
ing by means of a lever is that which is perpendicu-
lar to the arm of the lever. As in the inftance of the
lever (fig. 7.) if the power B acts in the direction b B,
it will exert the greateft polTible force ; but it will
have lefs force if it acts in the directions b D or b E.
When, however, one power is oblique to the arm of
the lever, and another power becomes equally oblique
by being in parallel directions, as the lines a p and b r
(fig. 8) then they have the fame relation as before
to each other. But if the directions are in different
degrees of obliquity, then that which departs moft
from a right angle will have the lead force ; for exam-
ple, if the power Q^(lig. 9.) prefer ves a perpendicu-
lar direction, and the or,her power an oblique one, act*
ing upon the line p c, pd> p e> orp/, the latter power
then becomes weaker in proportion as it departs from
the perpendicular direction p P.

From what has been already dated, it follows that
the power is greater, or lefs, or equal to the refiftance,
according as the diftance of the refiftance from the
prop is greater, or lefs, or equal to that of the power.
Hence in a lever of the firft order, the power may
be either greater, or lefs, or equal to the refiftance ;
in a lever of the fe'cond order, the power is always
lefs than the refiftance j and laftly, it follows that it
muft be greater in a lever of the third order : fo that
this order of lever, fo far from aiding the power as to
its abfolute force, muft, on the contrary, impede it.
Yet it is the lever of the third order which nature moft
frequently employs in the human body *. Thus when

* See Borelli de motu aninialium.

we

Chap. 7.] Atticn of the Human Arm. 59

we elevate a weight with the hand, this weight may be
conficlered as fixed to the arm of a lever, whofe prop
is at the elbow, and whofe length is confequently equal
to the diftance between the elbow and the weight.
But this weight is fuftained in this ftate by the action
of the mufcles, the direction of which is very oblique
to the arm of the lever, and confequently the diftance
of the moving power from the prop is much lefs than
the diftance of the weight from the fame prop. Hence
the effort of the mufcles muft in this cafe be much
greater than the weight or refiftance. To account for
this ftructure, it muft be remarked, that the nearer a
power applied to a lever is to the prop, fo much the
lefs is the fpace it a6ts in when it raifes the weight.
Now the fpace which the power had to occupy was
what Providence had moft to regard in the ftructure
of our bodies. It is on this account that he has placed
the direction of the mufcles at a fmall diftance from
the prop, but he has wifely made them ftrong in pro-
portion.

The prop of a lever may be regarded as a third
power, which keeps in equilibrio the motive force and
the refiftance, or which concurs with the one to ena-
ble it to fuftain the effort of the other.

In a lever of the firft order, the prop C (fig. 10.)
which is placed between the power D and the refift-
ance E, fupports a force equal to the abfolute weight
or effort of the two forces, when thefe forces are ap-
plied in a direction parallel to each other, and the
force exerted upon the prop C3 is in the direction
C I parallel to that of the two forces. But if the
power I Q_(fig. ii.) and the refiftance K N are in a
direction which inclines them towards each other, the
prop L is charged with lefs than the fum total of the
two forces, and lefs in proportion as this inclination is
greater, and the force which is exerted upon the

prop

60 -7r,v;-. [Book I.

prop L , is in the direction L M, which tends to the
point of contact M of the two forces.

It would be the fame if the powers/ and g (fig. 12.)
were in equilibrio by the inequality of their diftances
from the prop H, that is, in cafe their malTes were in
an inverfc ratio to their diltances / H and g H from
the prop. The charge upon the prop can never
be greater than the fum of the two forces, or in
other words the fum of the oppofed maiTes ; but it
would be equal to this fum if the powers were in a di-
rection parallel to each other, and it would be iefs than
this fum if the lines of direction e c , e e were inclined
towards each other, then the force upon the prop H
would be exerted in the line H I, which would tend
to the point I, where the two mafies would meet ac-
cording to th,e direction in which they act.

In levers of the fecond and third orders the prop .
fupports only a part of the 'effort of the two forces.
In other words, it acts in conjunction with the power
in levers of the fecond order, and in conjunction with
the refiftance in levers of the third order ; as when two
men carry a burden with a ftaff upon their fhoulders ;
: two men, one of whom may be regarded as the
power, and the other as the prop, only carry each a
part of the burden ; and he who is the nearell the
burden carries the greater (hurc of it, and that in pro-
portion to his nearnelsi to i:.

II. The PULLEY is a fniall wheel moveable upon its
axis, with the circumference hollowed to receive the
cord, which is attached on the one hand to the moving
power, and on the otlvr to the refitting force. The
wheel or pulley is commonly fixed in a block, or cafe,
which admits the rope or cord to pafs freely over the
circumference of the wheel, and the gorge of the pul-
ley,

VOL.1 . />. />'".

Fig. 2.

.

r

fe>£

-M

fofah

Fig. ?.

r c b

tt-t

r-t

Chap. 7.] rhe Pulley., Ci

ley, that is the hollow part of the circumference which
receives the cord is generally hollowed out angularly
and not round, fo that the cord being in fome meafure
pinched or compreffed in this angle, ir will not be liable
to glide or flip in its motion.

Pullies are commonly made of wood or metal, and
always turn upon an axis. When they are made of
wood, it is better to fix the axis to the pulley, and to
let all turn together in the fpace which fu Mains the
pulley. The movement then being performed upon
a lefs furface will be lefs impeded by friction, and if
the fpace which contains the pulley becomes larger, as
it is only the lower part which can be affected, the
aperture will be lengthened, the pulley will defcend a
little, but its circular motion will not be diminiihed;
it is not fo when the pulley turns upon its center, for
then if the aperture which receives the axis enlarges,
the enlargement is frequently not equal in all its
parts.

By means of pullies burdens are elevated with
greater eafe, and in a more commodious manner than
they otherwife could be ; more commodious becaufe
the motion is continued, 'and its direction may be
changed fo as to bring the whole force which is ap-
plied to it into immediate action, for by this means a
horfe which can only exert his force in an horizontal
direction, is able to overcome a vertical refiftance.
Burdens are moved with more eafe by pullies, becaufe
a great weight may be elevated by a fmall force pro-
perly applied. The power applied to a pulley draws
in all directions without impediment, becaufe the cord
by which it acts is always a tangent * to the circum-

* A tangent is a right line drawn perpendicular from the ex-
tremity of the radius, which touches the circumference of a circle
without cutting it.

ference

62 'Different Kinds [Book t.

ferencc of the pulley, and confequently always perpen-
dicular to the radius. The powers applied to pullies
aft more forcibly in proportion as their diftance from
the axis is greater, whether the cords rim in feveral
groves, or feveral pullies of different diameters turn
upon the fame axis ; thcfe powers therefore which
aft at the grtateft cliitance from the axis will have the
advantage over the other. Let IP, fnopofe a weight
of fix pounds to be placed at I (Plate 11. fig. i.) there
ought then to be fix pounds at H to iuibin it, be-
caufe the radii c d and c i are equal. But three
pounds placed at K will fuftain the lame weighr, be-
caufe the radius c 2 is double the radius c d-t and it
requires but two pounds in L, becaufe the radius c 3
is treble the nuiius c d.

In all thefe cafes, the pulley performs the office of a
lever of the firit order, for it may be confidered as
an affemblage of fixed levers, of which the center is
the common fulcrum. All thefe levers have equal
arms in pullies with one gorge, and they are of un-
equal arms in pullies of feveral gorges. (See fig. i.)
All thefe pullies are fixed.

It has been obfcrycd, that by means of a pulley of
many gorges (fig. i.) the actions of two unequal
powers may be rendered equal ; in the fame manner
an equilibrium or a conftant relation may be preferved
between two powers, the relative forces of which
continually change. A pulley may be ufed for this
pur^ofe, which, inllead of many concentric gorges,
has but one, but that in a fpiral form, which confe*
quently augments the diameter by degrees, according
to the proportion m which the excels of one of the
two forces augments. For example, let a pulley A
(fig. 2.) have its gorge hollowed in a fpiral form, of
which the hollow is feen at g ft b c> and the plain at
de 43 kt there be fixed in the center of this pulley a

barrel

Chap. 7.] of Patties.

barrel e or E, furnifhed with a fpring like that of a
watch. If the force of the fpring is fuch, that any
given power (a weight, for example, acting at D E)
keeps it in equilibrium; when the fpring is rolled
three or four rounds more, the fame weight acting at
g F will keep it ftill in equilibrium, if the radius E F
is lengthened in proportion to the augmentation of
force in the fpring ; what has been obferved of the
point F may be faid of all the others. Hence it fol-
lows, that thefe two pov/ers, the fpring and the weight,
will always act againft each other in a certain ratio or
proportion, even though the force of either mould be
occafionally augmented : It is upon thefe principles
that clock and watch-makers are able to calculate the
force of their fprings, weights, and pendulums, and to
adapt them with the utmcft precifion to the other
movements.

The axis c (fig. I.) of a fimple pulley can never be
charged with a greater force than that which is equal
to the fum of the two powers I and H, but it may be
fomewhat lefs. When the directions b I and i H of
two powers are parallel, that is, when the cord em-
braces half the circumference of the pulley, the axis is*
charged with a force equal to the fum of the two*
powers. But if the direction of the two powers is
oblique to each other, the axis is then charged witli a
lefs force than the fum of thofe of the two powers j
and in that cafe, the force with which the axis ii
charged is to the fum of the forces of the two powers,
as the chord of the arch embraced by the chord is to
the diameter d i ; the effort is then made upon the
axis cy in a direction which, paffing through f, tends
to the point of meeting according to the direction of
the two powers.

In all thefe cafes the force H mult be equal to the.

refiftance

$4 ?be movable Pulley. [Book I,

refiftance I, in order to keep the equilibrium. Hence
it follows, that the fimplt pulley neither aids nor hin-
ders the power; it only ferves, as has already been
obfervcd, to ke^p the power in its mod advantage -
',:•.;' ion, to change the direction of the motion,

li r it conftant.

'I ;,< y may aifo'Jbe confidered as a lever of

c , for it has all the properties of that
machin- when the refiftance R (fig. 3.) is attached to
the neck c i, and one of the ends of the cord which
paries under the pulley is attached to the fixed point
a> v;hile the cthf r is drawn or fuftained by the power
d: The pulley then becomes what is called a mc-je-
able pu^ky, and is elevated with the burden ; it con-
fequently is analagous to a lever of the fecond order
I e, of wnich the fulcrum or prop is at b, and is di-
vided into two equal parts b c, c e} by the direction
c I of the refiftance R ; it is on this account only
neceffary that the power d fhonld pcfTcfs half the force
of the refiftance ,R to keep it in equilibrium ; and
if the burden is elevated, the power d acts through
nvice the fpacc of that of^the refiftance R, and confe-
quently with double the velocity : For fuppofe the
center c of the pulley is carried to the point />, then
there only remains under the line d a the portion of
the cord which pafTes under the pulley ; the tv/o por-
tions b a and e d have then pafird^ above ; but b a
and e d> which mark the fpacc run through by the
power, are, taken together, double to c h, the fpace
run through by the pulley ; then the power has a
velocity double to that of the refiftance. In this cafe
the cord embraces half the circumference of the pul-
ley, and the directions of the two powers are parallel.
The arm of the lever, of power is then the diameter
e b of the pulley, that of the refiftance is only the ra-

Chap. 7.] Syjlem tf Pullies. 65

dius c b, Becaufe to keep an equilibrium, it is necef-
fary that the power fhould be to the refiftance, as the
radius is to the diameter*

But if the direction of the powers is oblique, as for
inftance, if one end of the cord is attached to the fixed
point £, while the other is fuftained by the power P, it
ftiil reprefents a lever of the fecond order m /, of which
the fulcrum will be at m, and which will be divided
into two equal parts m /, i /, by the direction c I of the
refiftance. Then the power P will be to the refift-
ance R as the radius c b is to the fpace I m of the
arch embraced by the cord.

If inftead of drawing the cord upwards it is necef-
fary to draw it downwards, a fixed pulley » (fig. 4.) is
placed above the moveable pulley m, which makes no
change in the effect of the power. And when the
power is not fufficient to elevate the burden, a fecond
moveable pulley is added, and another fixed one
(fig. 5.) or even a greater number, by means of which
the power has much greater effect. This fyftem of
pullies, fome moveable and fome fixed, and all em-
braced by the fame cord, is called by fome a tackle.
The fixed pullies i and 4 are fupported in the fame
neck or cafe, and the moveable pullies i and 3 by
another neck. The lower part M of the neck, which
fupports the fixed pullies, ferves as a fixed point for
one end of the cord, and it is the lower part R of the
neck which fupports the moveable pullies to which
the burden is hung.

- By means of this union of pullies, a very great
burden may be raifed by a fmall force; for ic is de-
monftrable that the force neceffary to fuftain a weight
by means of feveral pullies, is to the weight itfelf as
unity is to double the number of moveable pullies.
When the directions of the cords are parallel to each
VOL. I. F other,

66 Syftem of [Book I.

other, the powers are then (as has been before ob-
ferved) in an inverfe ratio to the velocities.

Hence it follows, that the number of pullies and
the power being given, the weight which the fyftem
of pullies is capable of fuftaining is eafily found, by
multiplying the power by double the .number of
moveable pullies. For example, fuppofe that the
power is equal to 60 pounds, and that the number of
moveable pulies is 3, 60 multipled by 6 (double the
number of 3) will be equal to 360, which is the
weight that this fyftem of pullies is .able to fuftain.

In the fame manner the number of moveable pul-
lies being given, as well as the weight which the
tackle is able to fuftain, the power is eafily found by
dividing the weight by double the number of move-
able pullies. Si?ppofe the weight equal to 800 Ib.
and the number of moveable pullies to be 4 j 800
divided by 8 (that is, by double the number of pul-
lies) gives the quotient loolbs. which is the force
neceffary to fuftain 800 Ibs. with fuch an union of
pullies *.

To

* ' It may be obferved, that in all contrivances by which

* power is gained, a proportional lofs is fuffered in time. If one

* man, by means of a tackle, can raiie as much weight as ten
' men could by their unafliited ftrength, he will be ten times as

* long about it.

' It is convenience alone, and not any aftnal increafe of fore?,
' which we obtain from mechanics. This may be illaftrated by
« the following example :

' Suppofe a man at the top of a houfe draws up ten weights,
' one at a time, by a fingle rope, in ten minutes. Let him have a

* tackle of five lower puilies, and he will draw up the whole ten
« at once with the fame eafe as he before raifed up one j but in
' ten times the time, that is, in ten minutes. Thus we fee the

* fame work is performed in the fame time, whether the tackle be

* ufcd or not; but the convenience is, that if 'the whole ten

« weights,

Chap. 7.] Bullies. 67

To find the number of moveable pullies which are
neceflary to fuftain a given weight with a given power,
the weight muft be divided by the power, and then
half the quotient will be the number fought. Suppofe,
for example, the weight to be 500 Ibs. and the power
50 j the apparatus ought to have 5 moveable pullies,
for 500 divided by 50 gives 10 for the quotient, the
half of which is 5.

In all thefe cafes it has been prefumed, that the di-
rection of the cords is parallel to each other. If it is
oblique, then the burden to be fuftained is to the
power, as the fum of the fines * of the angles, which
the cords of the moveable pullies make with the ho-
rizon, is to the whole fine. It follows then, in this
cafe, that the power muft be greater than is required
in the former cafe j the direction therefore of the cords
ought if poffible to be always parallel to each other.

To prevent the friction of the ropes one againft
another, which occafions a confiderable refiftance and
wears them much, it has been found necefiary to em-
ploy in a fyftem of pullies fome of a fmaller diameter,
which is inconvenient on account of the ftiffhefs of the
rope. It is therefore better to place the pullies of

* weights be joined into one, they may be raifed with the tackle,
' chough it would be impoffible to move them by the unaffiUed
' ftrength of one man.

* Or, fuppofe, inflead of ten weights, a man draws ten buckets
' of water from the hold of a fhip in ten minutes, and that the;
' fhip being leaky, [admits an equal quantity in the fame time.

* It is propofed, that by means of a tackle, he mall raife a bucket
' ten times as capacious. With this affiltance he performs it, but
' in as long a time as he employed to draw the ten, and therefore
' is as far from gaining on the waLer in the latter cafe as in the

* former.' Nichol. Phil. vol. i. p. 74..

* The fine is the meafure of xn angle, or a right line drawn
from the extremity of one leg to that of the other.

F 2 each

6& 1'beory of [Book I.

each tackle, the upper and the lower, parallel to each
other, to place them in a common neck, and to make
them move upon an axis common to all, as in fig. 6.
all the pullies are then of equal diameters. This kind
of tackle is in common ufe, efpecially on board mips.
The ropes, however, are not exactly parallels but this
defect is inconfiderable.

In the preceding calculations the refiftance pro-
duced by friction, and that which arifes from the (tiff- •
nefs and weight of the ropes are not regarded, on ac-
count of which it is neceflary to augment the power,
and to make it greater than I have fuppofed. It may
alfo happen, that in augmenting the number of pullies
thefe refiftances will be augmented fo much, that they
do more than compenfate for the augmentation of the
force which refults from the increafe of the number of
pullies.

Wheels, like pullies3 may be confidered as an afiem-
blage of levers. Of wheels there are two kinds : the
firft always turn in the fame fpace upon an axis fixed
to the center of the wheel, the .pivots of which turn
in holes or cavities which ferve as a prop: fuch are
the wheels of clocks, of mills, &c. Thefe kind of
wheels receive or tranfmit the movement by teeth, or
cogs placed round the circumference-.

Wheels of the other kind, that is, which turn upon
their circumference, have their center or axle-tree in a
direction parallel to the plane on which they move ;
fuch are the wheels of waggons, coaches, &c. They
have therefore two movements j the one, of their
center which advances in a right line, and the other,
of all their parts which perform a circular motion
round the center.

When wheels of the firft kind are put into action,
it is common to place upon the fame axle a great

wheel

VOL.

Plate

Chap. 7.] Wbeek. 69

wheel and a fmall one, called a pinion, and fbmetimes
a nut, the teeth of which coincide with the teeth of
another large wheel. In large machines, trundles are
often fubftituted for pinions or nuts, and perform their
office ; thefe are cylinders or fpindles parallel to each
other, and placed circularly in two plain pieces of
wood at the top and bottom. The teeth of the wheel
then catch the fpindles of the trundle as they do the
cogs of the nut or pinion. The mechanifm is the
fame in both cafes : fo that it fuffices to examine the
manner of hooking or catching of wheels and pi-
nions.

Wheels of the firft kind, thofe whofe motion is con-
fined to the fame place, may be confidered as levers of
the firft order; the arms of which are the radii of the
wheels and nuts, and which have their prop at the axle.
Let A B C (Plate III. fig. i .) be three wheels, and a b c
their correfponding pinions or nuts. The nutj or what
is the fame thing, the cylinder a fuftains the weight P ;
the wheel A, which has the fame axle as the cylinder a,
catches the nut b ; the wheel B, which has the fame axle
as the n\Qb, catches the nut c\ the wheel C, which
has the fame axle as the nut c, is drawn at its circum-
ference by the power Qj and the whole fyftem is
in equilibrium. Here the weight P a<5bs by the radii
of the nuts ; but the power Q^acts by the radii of the
wheels. Suppofe the radii of the wheels to be four
times thofe of the nuts; and that the firft are eight
inches, and the other two inches. To preferve an
equilibrium, it is neceffary that the power fhould be
to the refiftance, as the product of the arms of the
lever of refiftance is to the product of the arms of the
lever of power, that is, in an inverfe ratio of the length
of the arms of the lever ; thefe products are found by
multiplying the one by the other, that is, the radii of
F 3 the

yo Clock-work. [Book I.

the wheels and the radii of the nuts. The firft pro-
duct will be 512; and the fecond 8, in which cafe the
power Q^ought to be to the weight P, as 8 is to 512,
or as i is to 64. Hence it follows, that to preferve
' an equilibrium, whatever is the diameter of the wheels
and of the nuts, the power is to the refiftance as the
product of the radii of the nuts is to the product of the
radii of the wheels.

It appears then that this form of machines is ca-
pable of giving a great advantage to the force or
power over the refiftance ; but this advantage is necei-
v ceffarily acquired at the expence of time or velocity;
when the machine paries from a ftate of reft to that
of motion. For there is always as much left in time
as there is gained in force, and fo reciprocally.

There is often occafion, efpecially in clock-work,
that the number of the revolutions of the wheels and
that of the nuts fhould beajpa certain proportion.
This is performed by giving a convenient number of
teeth or cogs to the wheels and nuts : as for example,
if it was required that a wheel fhould make only one
revolution while a nut fhould make four, there muft
be four times as many teeth in the wheel as there are
cogs in the nut. Suppofe ABCD (fig. 2.) to be
four wheels, the firft of which A, catches the nut b
fixed to the fecond B ; this catches the nut c fixed to
the third wheel C ; this third catches the nut 4 fixed
to the fourth D j laftly, this fourth wheel catches the
laft nut e ; now to obtain the proportion between the
number of revolutions of the firft wheel A, and the
number of revolutions of the laft nut e, multiply the
number of teeth of the wheel A, by the number of
teeth of the wheel B > this firft product by the num-
ber of teeth in the wheel C, and the fecond product
8 by

Chap. 7.] AEiion of Wheels. 71

by the number of teeth in the wheel D ; then multiply
the number of cogs of the nut If by the number of
cogs in the nut c ; this firft product by the number of
cogs in the nut d; and the fecond product, by the
number of cogs of the laft nut e : the iaft products of
the teeth of the wheels and the cogs of the nuts, will
give the proportion required.

It may then be eftablifhed as a general rule, that
the number of the revolutions of the firft wheel A, is
to the number of the revolutions of the laft nut, as the
product of the cogs of the nuts is to the product of
the teeth of the wheels. Hence it follows, that it is.
not necefiary to determine the number of cogs and
teeth which each rrut and wheel fliould have in parti-
cular ; it fuffices that the proportion of the product of
all the cogs to the product of all the teeth, (hall be
fuch as is required.

By means of this kind of wheels, the action of a
power may be tranfmitted to a diftance, the direction
of the movement may be changed, and the velocity of
the powers may be varied.

Firft, if inftead of applying the nut (fig. 3.) im-
mediately upon the wheel H, a nut D is fixed to the
other extremity of the prolonged axle as far as is ne-
ceflary, then the power which acts by the handle G
may be tranfmitted to a certain diftance by means of
the nut D fixed at the extremity of the axle.

Secondly, If this nut D catches with another wheel
E, which has teeth parallel to its axle, the movement
which will be tranfmitted to it will change the direc-
tion, and become horizontal inftead of vertical.

In fine, if the wheel E has four times as many teeth
as the nut D has cogs, fmce the nut cannot move
without the vertical wheel H, it follows that the lat-
ter muft turn four times round while the horizontal
F 4 wheel

72 Of Wheel [Book I.

wheel turns round once ; and fo on the other hand, if
this makes one revolution, the nut D and the vertical
wheel H will make four in the fame time. For in-
fiance, fuppofe that to each of the great wheels II and
E, a handle G or F is fixed, and turned by a man
once in a fecond, the velocity will be four times as
great when he turns by the handle F as if he turned
by the handle G. But it is true, that in this cafe he
inuft ufe four times the force; becaufe whatever is
gained in fcrce is loft in time ; and on the contrary,
what is gained in time is loft in force j yet ic is often
advantageous to have the liberty of choice.

As to wheels of the fecond kind, which have two
fores of motion, as thofe of carriages, the center of
which advances in a right line while the other parts
turn round it, they may be regarded as a lever of the
fecond order, the action of which is repeated as often
as there is fuppofed to be points in the circumference.
For each of thefe points is the extremity of a radius
C M (fig. 4.) fupported at one end by the ground M ;
and the other end C charged with the axle which fup-
ports the carriage is at the fame time drawn by the
power P which moves it along. So that if the plane
was perfectly level, and the circumference of the wheels
witho-ut any inequalities j if there was no friction be-
tween the axle and the nave, and if the direction of
the power conftr^ntly remained parallel to the plane,
then a fmall force would draw a very heavy carriage;
for the re/iftance which proceeds from the weight,
refts entirely upon the ground by the radius or fppke
C M, or by another fpoke which immediately fuc-
cecds. But thefe circumftances are never or very fel-
dom to be found in practice. The wheels of carriages
are frequently rounded in a coarle manner, and large
nails driven into them; the roads are uneven, or made

fo

Chap. 7.] Carriages. 73

fo by the weight of the carriages which pafs over
them. Thefe inequalities, whether of the wheels or
of the earth, therefore caufe the wheel to be fupporred
by a radius C Q_or C N, oblique to the direction of
the power C P, or to the direction of the refiftance
C M. Then rhe weight which is fuppofed to prefs at
C refills the power which cannot make it advance but
by caufing it to rife as much as the point Q^or N is
above the point M. The power is then obliged to
fuftain part of the weight of the carriage, as if it was
placed upon an inclined plasie. When, moreover, the
circumferences roll upon furfaces perfectly folid and
level, there is indifpenfably a confiderabie friction be-
tween the axle-tree and the nave.

The little elevations and deprefTions in roads change
alfo the direction of the power. A boric placed higher
or lower, by the difpofition of the ground, inftead of
making his effort in th? line C P, parallel to the por-
tion of the plane which fupports the wheels, makes it
often by the line C S or C R, that is, obliquely to the
direction of the refiftance CM, and confequently with
difadvuntage ; for a carriage which may be moved
eafily by one horfe only upon a horizontal plane, often
requires many horfes to move it up a rifing ground.

In general, the mod advantageous mode of moving
burdens in carriages over rough and uneven ground,
as roads are for the moft part, is, according to Meflrs.
Stevin, Wallis, and Deparcieux, to draw in a rifing
line, as C R j for this purpofe it is neceffary that the
axle of the wheels fhould be a little lower than the
breads of the horfes, by which means the direction of
the power approaches more to the parallelifm of each
of the final! inclined planes which form die inequalities
of the .roads.

But

74 Advantage of large Wheels to Carriages. [Book I.

But if it is impOiTible to overcome thefe difficulties
entirely, they may be prevented in part by employing
large ir.ftead of fmall wheels. For it is certain, that
fmall wheels entangle themfelves more than great ones
in the ruts and hollows in roads, as may be feen by
the fig. 5, where the radius c q of the fmall wheel,
which bears againft the ground, in riling out of a hol-
low in the road, is much more oblique to the direc-
tion of the power cp than the radius C q of the great
wheel to the direction C P. As the circumference alfo
of a great wheel meafures, in rolling, more of the road
than that of the fmall one, it turns fwifter, or makes
fewer revolutions, in pafiing over a given fpace, which
faves no inconfiderable part of the friction.

III. The WHEEL and AXLE * or windlafs, one of the
fix fimple machines, is a cylinder which turns upon its
own axis, by means of which, with a fmall force, a great
burden may be elevated by a rope which wraps round
the cylinder by the aid of a handle, or by means of cogs
or bars ufed as levers, acting on the circumference.

It is the common practice to fix at one of the
extremities of the cylinder AB (fig. 6.) levers, fuch
as E, F, G, H, by means of which the cylinder is
turned upon its axis C D, while the cord which fuf-
tains the weight a, is wrapped or wound about it. It
is eafy to fee that the effect of the wheel and axle is
analogous to that of a lever of the firft order. For,
fuppofe that bg (fig. 7.) reprefents the radius of a
cylinder, and that b P reprefents the arm of a lever,
by which the power P acts : if the length of b P is to
that of bg as 3 to i, a power of 100 pounds at P,
acting in a perpendicular direction at P b, will balancq
a weight G of 300 pounds.

* By fome called the axis in peritrochio.

Hense

Plate 3

Fi. 2.

Chap. 7.] fbs Wheel and Axle. 75

Hence it follows, that to elevate a weight by means
of this machine, it is required that the power P fhould
be to the weight G, as the radius of the cylinder hg,
is to the lever h P j or, which amounts to the fame, as
the radius of the cylinder is to the radius of any wheel
or handle by which it may be turned. If in a ftate of
equilibrium the power is lefs than the weight, and that
in the proportion of the radius of the cylinder to that
of the handle which turns it, fo in a (late of motion the
power has more velocity than the weight, and that in
proportion as the radius of the handle or wheel that turns
it is to that of the cylinder. This rule fuppofes that the
power is always perpendidular to the radius by which it
acts; for the direction of the weight is always perpendi-
cular to the radius of the cylinder, fince the cord that
fuftains it is always a tangent to its circumference.

In great efforts, ' as it is necefTary that the arms of •
the lever of power Ihould be very long; when there-
fore it is extremely inconvenient to make them fo, and
when to multiply the number of them would weaken
the head of the cylinder too much, it has been the
practice to unite the extremities of the radii or cogs by
a circumference, and form a kind of wheel to which
other cogs are adapted, by which it is turned by men ;
as may be feen in the wheels ufed at quarries and for
cranes (fig. 8.)

The capjtan is a real windlafs, and it differs only
in the pofition of the cylinder, which is vertical, where ^
as in the windlafs it is horizontal. The manner of a
power acting upon a refiftance or burden, by means
of a wheel and axle or windlafs, is entirely applicable
to the capftan, but the latter is more advantageous.
Capftans are often fixed in fliips, to raife anchors or
other burdens to which cables are fattened, \s hich are
rolled or coiled upon the cylinder.

It

76 'The Ctiftlw. [Book I.

It is eafy to perceive that the oipftan ads as a
perpetual lever of the firft or fecond order with un-
equal arms} an(i that the arm of refiftance is much
fhorter than that of the power. For the arm of the
lever by which the refiftance acls, is the radius of the
cylinder; and the arm of the lever bv which the
power acts, is the fame radius lengthened by the whole
extent of one of the crofs levers.

Ships have often two kinds of capftans on board;
that is, a double Gapftan and a fmall one. The double
one is placed upon the firft deck, and rifes about four
or five feet above the fecond deck, it is dcfigned for
the more important purpofes, as to raife the anchor,
&c. The fmall capftan. is placed upon the fecond or
third deck between the main and mizen mafts, and
ferves to work. the fails, yards, &c. on different oc-
cafions.

The crick or Jack is another machine by which a
great refiftance or weight may be overcome by a fmall
force. The fimple jack confifts of a bar of iron A B
(Plate IV. fig. i.) furnilhed with teeth in one of its
faces, and moveable in a cafe C E. The teeth of the
bar A B coincide with thofe of the nut D D, which
turns upon its axis by means of the handle M N. The
action then of the nut protrudes the bar, and confe-
quently raifes the weight placed at its head A.

When the effort which each tooth of the nut makes
in D to raife the bar, is confide red as a weight ap-
plied to a lever, it is clear that the power applied to
the handle, is to that weight as the radius of the nut
is to the arm of the handle N M. Hence it may be
perceived, that by making the radius of the nut Very
fmall, in proportion to that of the handle, a very con-
fiderable weight may be raifed or moved by a mo-
Aerate force. A fmal] portable inftrument of this Kind

is,

Chap. 7.] fhe Jack lifed by Houfe&reakers. 77

is, I underftand, commonly employed by die houfe-
breakers about the metropolis, to force open doors
or windows, or to remove locks or whatever obftruc-
tions may be oppofed to them.

IV. The INCLINED PLANE is that which forms an
angle with the plane of the horizon. This angle may
be infinitely fmall, and then it is confounded with an
horizontal line ; on the contrary, it may be a right
angle, and then the plane becomes vertical : between
thefe two extremes are comprized all the other de-
grees of inclination.

The principle on which the whole theory of the in-
clined plane is founded is this : That the time which a
roiling body takes to defcend upon an inclined plane,
is to the time in which it would defcend vertically
by its abfolute gravity from the higheft part of the
plane, in the ratio or proportion which the length of
the plane bears to its perpendicular height ; a body
therefore placed upon an inclined plane is partly fuf-
tained by the plane itfelf, and therefore a weight or
power confiderabiy inferior to that of the body is able
to fupport it in its fmiation on the plane, or even
to caufe it to afcend. On this account it is that in
making refervoirs for water, trenches in fortification,
or in clearing the earth av/ay for the foundations of
buildings, the wheelbarrows for other vehicles em-
ployed are made to afcend upon a plank or fcaffolding,
which is placed in the direction of an inclined plane.

To render this part of the fubjcct perfectly intelli-
gible, let A C (fig. 2.) be an inclined plane, then to
fuftain the body D upon this plane, and to prevent it
from failing, it is nor nece/Tary that the weights d> dy
which retain it by means of the cords D e d, fhould be
(taken together) equal to the weight of the body D,

but

7 8 fbe Inclined Plane. [Book I.

but may in fact be confiderably lefs, if thefe weights
dj dt draw in the direction D e, parallel to the inclined
plane. But if thefe weights draw in the direction
D F or D E, they necefTarily lofe a part of their force,
as will appear from what has been already advanced
on the fubject of obliquity, in treating ofpullies, &c.

Hence it is evident, that the power ads to the
greateft advantage when the line of traction, or the
line in which the body is drawn, is in the direction
D ey parallel to the inclined plane. When thus fi-
tuatcd therefore, there will be an equilibrium, when
the power is to the weight of the body, as the height
of the plane is to its bafe. In other words, the me-
chanical advantage gained by the inclined plane is in
proportion as the length of the plane exceeds its
height*. Thus if a weight of four ounces is laid on
an inclined plane, the length of which is to its height
as 2 to i, it will be counterbalanced by a weight
of two ounces drawing in the line D e (fig. 2.) pa-
rallel to the plane; or if the length of the plane is to
its height as 4 to i, the body will be fuftained by one
ounce only. Hence in drawing a cart or waggon up
hill, if the power of the horfes bears the fame propor-
tion to the weight of the waggon, as the height of the
hill to its declivity, then the waggon will not run back,
and a fmall additional fcrce will enable it to advance.

V. The WEDGE, which is alfo one of the fix fimple
machines, is of a triangular form ; the thinned part
is called the point or edge, and the thicker the head or
bafe of the wedge.

The action of the wedge agrees mo(t with that of
the inclined plane. It is made ufe of to cleave, to

* Adams's Le&ures, vol. iii- p. 295.

/ raife,

Chap. 7.] Me We^t. 79

raife, or to comprefs bodies ; and to put it in action
the blow or-ihock is commonly given with a hard
body, fuch as a (ledge or hammer, though fometimes
the prefiure of a weight is employed. The refjftance
which may be overcome by means of the wedge, of-
ten depends upon the tenacity of the parts, which is
difficult to eftimatei The percuffion which puts the
wedge into action is alfo difficult to judge of by the .
effects of preflure: on this account the theory of the
wedge is not fufceptible of great precificn. But ap-
proaches may be made to precifion, by fubftituting
powers, the abfolute force of which is known, as of
weights, and then obferving what proportion there
exifts between the power and the refiftance when a
wedge is introduced.

Let us fuppofe two rollers m, n> (fig. 3.) the, one
m attached to the cord / m e, and' the other n to the
cord n i d, each bearing a weight of lolbs. _p and r>
and paffing over the pullies / and b ; and let us fup-
pofe alfo that the bafe a b of a wedge is equal to
the half of its height c h. It will then require a pref-%
{lire of 5 ibs. to keep this wedge in equiiibrio with the
fum of the two weights, which is equal to 20 Ibs,
and a little more than 5 Ibs. to fink the wedge its
whole depth c bt without making any allowance for
friction. It is evident by the conftruction, that while
the wedge is funk its whole depth c h, the two weights
^>.and r will each rife one half of i /, which is equal
to a b, the bafe of the wedge. And as it is required,
in producing an equilibrium, that the power iliould
be to the refiftance in an inverfe ratio to tne velocity,
or to the fpace through which two bodies move in the
fame time, it is clear that in this cafe, the power
rnuft be to the refiftance as 'the half of the bafe is to
the height of the wedge. The (harper the wedge i?,

therefore,

8o Action and Force of the Wedge. [Book L

therefore, that is, the more acute the angle, the more
powerful is its action, and the greater the effects which
may be produced by the fame force.

If the wedge is employed to fplit or to cleave the
parts of a hard body which ftrongly adhere together,
its advantage is augmented in proportion as the wedge
is funk or driven deeper between thefe parts. For
fuppcfe two pieces of wcod/^ and / r (fig. 4.) firm-
ly connected together by the ftrong bandages p, u, x,
&c. all equal in ftrength, and which may reprefcnt
the adhcfion of the parts of a billet of wood ; the
wedge being placed between the two billets, acts in
fome meafure as by the arms//), fp of two angular
levers fp q, t pr, while the two other arms pqypr con-
fined by the bandages, mutually fupport each other.
If then the force of the wedge exceeds a little that of
the firft bandage p, this bandage will be- broken. The
fecond bandage «, though as tfrong as the firft, will
be broken more eafily by the action of the fame wedge,
becaufe then the arms of the lever by which it a6ts,
are lengthened by the quantity p uy and fo of the
ethers. It is doubdefs on this account that hard and
dry wood, ftones, glafs, and in general all bodies
which are very ftiffor inflexible, break with confider-
nble noifc, and cleave or crack upon the firft effectual
attempt to cut them.

All infr.rurnents defigned for cutting or ftabbing,
.as kr.ivrs, hatchets, fwords, punches, &c. are claflcd
with the wedge. In mort, they have at leaft two in-
clined planes, fometimes four or more, which form
among them 'an angle more or k-fs acute ; nails, pins,
and needles are alfo included in this clafs.

VI. The SCREW is the lafc of the fix fimple ma-
chines which we have to confider, and is a long cone

or

Chap. 7-] Male and female Screw's. 81

or cylinder A B (fig. 5.) upon the circumference of
\vhich is cut a fpiral groove or gorge CFG. The
partition C F between the rounds of the gorge is
called the thread of the fcrew ; and the diftance C G>
which there is between one thread and another, is
called the ftep or pace of the fcrew.

The thre-ad and the gorge are fitted fometimes into
a cylindrical cavity made in a piece of metal or wood*
which is fometimes called a focket, but more fre-
quently a femak fcrew, while the other is named the
male, or principal fcrew.

It is eafy to fee that the thread of a fcrew is an in-
clined plane, at the bafe of the cylinder AB (fig. 5.)
The height of this plane is the pace or fpiral of .the
fcrew, or, which is the fame thing, the diftance of one
thread from another : its bafe is the circumference of
the fcrew, and its length is eftimated by this circum-
ference and the height of the pace ; for if one of the
threads a b is developed, it will form with its pace b c9
and its bafe, or the circumference a c of the fcrew, a
triangle a b c , and a rectangle at c, of which it is eafy
to find the fide a b, fince the two others are known, as
well as the angle at c : hence by a fcrew turning in its
focket they conftitute two inclined planes Hiding the
one upon the other.

The threads of fcrews afiume different forms ac-
cording to the materials of which they are made, or
thofe into which they are to enter, or according to" the •
efforts they are defigned to make. In wooden fcrews
the thread* C, G, F, are generally angular, which add
greatly to their ftrength ; for by this form they have a
larger bafe upon the cylinder which fupports them.
This form is alfo given to the threads of thofe fmall
iron fcrews which are conical, ending almoft in a
point, and which are defigned to enter into wood, irt
, G

82 ?be Screw. [Book I.

which they form the fockets for themfclves. Drills
and gimblets may be confidered as of the fame na-
ture, the fpiral points of which enter the wood fo
much eafier in proportion as they are (harper at the
end. But with refpect to large metal fcrews which
are ufed for preffes, vices, &c. their threads are gene-
rally made fquare, in order that die friction may be in-
creafed by augmenting the furface of each thread ; for
it frequently happens that the principal effect of fcrews
arifes from the clolenefs of the friction : this form hin-
ders the- cheeks or chaps of the vice from fwerving
backwards, to which they have a natural tendency by the
re-action of the piece which they prefs between them.

Screws are ufed principally for the preffing of bo*
dies firmly againft each other, and fometimes for raif-
ing weights or burdens, or for forcing backwards or
forwards certain mattes of a determinate quantity. For
this purpofe a male and female fcrew are made ufe of,
one or the other of which ferves as a fulcrum or prop.*
Sometimes the male fcrew is fixed, while the female
fcrew is moveable ; but in both cafes the effect of the
fcrew is the fame.

When this machine is made ufe of, one of the
two pieces (the male or the female) is applied
to the refiftance which is to be overcome, and
the other ferves as a fulcrum or prop to the ma-
chine ; then by the act of turning, the focket is
made to move upon the fcrew, or the fcrew into
the focket. In fmiths' vices, for inftance, one of
the cheeks is preffed, by the action of the fcrew,
againft the other cheek ; it appears, therefore, that the
power muft move one complete round, in order to ad-
vance the refiftance one pace or fpiral of the fcrew,
that is, a quantity equal to the diftance of one thread

from

Chap. 7.] 9Cbe perpetual Screw. £3

from another. If the power is applied immediately
to the fcrew, the fpace it pafles through, or its quan-
tity of motion, is a c (fig. 5.) which is the meafure of
the circumference of the fcrew, and the motion of the
refiftance is meafured by c b, the width of one pace of
the fcrew. But as it is common to turn fcrews, efpe-
cially large ones, with levers or fomething equivalent,
in that cafe a c does not meafure the motive force of
the power; it is, on the contrary, meafured by the
circumference of the circle, of which the lever D E
is the radius. And as it is necefTaryj in order to main*
tain an equilibrium, that the powers mould be in the
inverfe ratio of their velocities, it may be eflablifhed
as a general rule in ufing fcrews, if we make no ac-*
count of the friction, thap the power is ta the refift-
ance as the height of the pace of the fcrew is to the
circumference which the power defcribes.

The perpetual fcrew differs in many refpecls from
that which has now been delcribed. It is a cylinder,
which always turns in the fame direction, its two ex-
tremities A and B (fig. 6.) being carried upon folid
pivots, fo that its a6tion is perpetuated, whence it
derives its name. The threads z h of this fcrew,
•which are generally fquare, coincide with the teeth of
a vertical wheel C h, which carries upon its axis a rol-
ler or windlafs T with a cord, to which is fattened the
burden P, which i$ required to be elevated. A very
fmall force, therefore, applied to the handle M E is
fufikient to raife a very 'confiderable burden at P,
but it requires confiderable time, from the invariable
rule in mechanics, that whatever is gained in force is
l,ofi: in velocity.

In order to find the relation between the •weight P

and the force or power Q^, it mull firft be confidered

that the weight P is counterbalanced immediately by

G 2 the

*4 ^be perpetual Screw, f Book f,

the refiftance which the thread h of the fcrew oppofes
to the tooth of the wheel, keeping the direction h g per-
pendicular to the radius C h. This thread h therefore
acls by the radius, of the wheel C b, whilft the weight
P acts by the radius of the roller or windlafs C d ; fo
that to maintain an equilibrium, the force at h fhould
be to the weight P as C d> as the radius of the roller
rs to the radius of the wheel C h. Thus the re-
lation which the weight P Ihould have to the power
QJn cafe of an equilibrium, may be expreffed in this
manner. The weight is to the power as the product
of the radius of the wheel, multiplied by the circum-
ference which the radius of the handle defcribes, is
to the product of the radius of the windlafs, multiplied
by the height of the pace of the fcrew.

The motion of the wheel being exceedingly flow in
proportion to that of the handle, it follows that a very
fmall power is capable of raifing a confiderable weight
by means of the perpetual fcrew. For example, fup-
pofe, as in fig. 8. a wheel C ht which has nine teeth,
and a fcrew which has but one thread, and which, at
each round, caufes only one to«th of the wheel to pafs -t
fuppofe the circumference of the windlafs T to be
cme foot, and the circumference which the radius of
the handle E M defcribes to be five feet ; when the
wheel C h fhall have performed an entire round, the
weight P will be railed one foot, and the fpace run
through by the power Q^will be 19 times five feet or
95 feet. . The velocity of the power Q^will then be
to the velocity of the weight P as 95 is to one; con-
fequently this power, with the effort of I Ib. is capable
of railing 95 Ib. ; and if its effort v.as equal to 30 Ib.
it would" raife 2,850105.

If the wheel C h had as many more teeth as it has,
ike radius E M of the handle were as long again,

the

Vol.. \.p.d4.

Plate 4

Fin. 3

Fig. 6\

Fig. 5.

Chap. 7.] Caufes which impede tbedftion of Machines. 8 5

the .fame power Q^would produce a double effect,
that is, ic would raife 5,700 Ibs.

But if, without changing the number of teeth in the
wheel C by or the length of the radius E M of the han-
dle, another perpetual fcrew is placed upon the axis
of the wheel inftead of the windlafs T, the thread of
which fhould catch with the teeth of a fecond wheel
of the fame number of teeth with the fi ft, and to
which fhould be annexed the windlafs T, which is to
fuftain the weight P, then the fame power Qjvould
be capable of raifmg a weight 19 times as great ; in
other words, this power, intrinfically only 30 Ibs. would
be able to raife the weight of 54,150^5.

Such are the fimple machines which have been ge-
nerally confidered as the bafes of all others.

If all the materials of which machines are compofed
were perfectly hard, perfectly polifhed, and if the ropes
which are often ufed to tranfmit the motive force from
one part of the machine to another had a perfect flexi-
bility, the theory of equilibrity, of which we have
hitherto fpoken, would be fufficient to determine, in
every cafe, the force requifite to counterbalance a
given refiflance ; and this force once found, it would
be clear, that by augmenting it ever fo little, the equi-
librium would be deftroyed and the refiftance over-
come -, but the friction of furfaces one againft an-
other, and the refiftance which cords produce by being
wrapped round pullies and cylinders, necefTarily impede
the motion of machines ; and it is extremely difficult
to eftimate, with even tolerable exactnefs, the amount
of the refiftance which may in different circum- "
fauces proceed from thefe caufes,

G 3 BOOK

[ 86 ] [Book II,

BOOK II.

OF THE NATURE OF FIRE,

CHAP. I,

HISTORY OF THE DISCOVERIES RELATIVE
TO FIRE AND HEAT.

Opinions of the Ancients.— -Of Bacon, Boyle, and Newton.— Of Horn-
bergy Sgravefend, and Ltmery. — Invention of the ^Thermometer.—
Opinion of Boerhaave.—* Great Difco-very of Dr. Black.

SO wonderful is the nature, fo extenfive is the action,
and fo formidable is the power of fire, that by
one of the moft confiderable nations of antiquity * it
•was adored, as the embodied prefence of the fupreme
God : and even in countries where the adoration was
lefs palpable and direct, fomething myfterious was al-
ways attributed to this fubtile and aftonifhing ele-
ment j and the rites and myfteries of fire were cele-
brated in temples and in groves, from 'the Ihores of
the Hellefpont to the banks of the Tiber.

An opinion feems to have been prevalent among
the early phiiofophers of Greece, that fire is the only
elementary and homogenial principle in nature, and
that from its different modifications all this variety of
different bodies is produced f. This idea is ridiculed
by Lucretius, who adopts the fyftem of Epicurus: and

* The Ferfians.— See Herod, Lib. II. c. 18.
f Lucret, Lib. 1. 636.

indeed

Chap, i.] Opinions of Bacon, Boyk, HomZerg, &c. 87

indeed the Epicureans, as well as the Peripatetics,
feem to have confidered fir : as a .diftinct elementary
fubftance, capable of combining with the other ele-
ments, but by no means the matter from which they
are originally generated.

The hiftory of error can afford but little inftrucftion,
otherwife volumes might be filled with the fantaftical
opinions which have been from time to time eatertain-
ed concerning the element of fire. On the revival of
letters and philofophy, our illuftrious Bacon, in a trea-
tife exprefsly written upon the fubject *, endeavours
to prove, that heat is no other than an inteftine motion
or vibration in the parts of bodies ; and he was fol-
lowed by mod of the philofophers of this kingdom,
during the laft century. The opinion of Bacon is fup-
ported by a variety of facts, which are adduced by
Mr. Boyle in a diflertation on the mechanical origin of
heat and cold j- ; nor does the fyftem appear repugnant
to the fentiments,of Newton; though he fpeaks of it
with that diffidence which is always obfcrvable in his
writings, when treating of facts not abfolutely demon-
ilrated by experiments of his own J.

Notwithftanding the reputation of the Englifh phi-
lofophy, this theory was received with great reluctance
abroad. The celebrated Homberg, Sgravefend, and
Lemery the younger, afTert, that fire is a diflinct fub-r
fiance or body, which enters into combination with,
all other bodies, pervades all bodies, and may be again
expelled from them by violent motion or compreffion,

* De Forma Calidi.

•f- Mr, Boyle, howerer, though thus apparently deceived with
refpeft to the caufe of heat, in another eflay reafons juftly with re-
fpeft to its effe&s. He confidcrs ice not as the preternatural (late
of water, but water as ice preternaturally thawed by heat.

Boyle on the nat. and preleruat. State of Bodies f
I Optics, 318.349.

G 4 though

88 Invention of the Thermometer. [Book II.

though the fire is certainly not generated by fuch mo-
tion *.

One of thefe philofophers (M. Lemery) indeed car-
ried his fyftem much further, and made a very near
approach to the received doctrines of the prefent day.
He afiferted, that fire is not only contained in thofe
bodies which are inflammable, but even in water itfelf.
Jce he affirmed to be the natural ftate of water ; and
he added, that the fluidity of that fubftance is a real
fufion, like that of metals expofed to the fire, only dif-
fering as to the quantity of heat neceflary to preferve
it in fufion f.

About the commencement of the laft century in-
flrnments were firft contrived for meafuring the heat
of bodies by the degree of expanfion; and this inven-
tion ieemed to give fome colour to the hypolhefis of
the German philofophers, fince it is not very clear
how a mere increafe. of motion can increafe the ex-
tent of bodies. It was long obferved, that all bodies
are expanded by an increafe of heatj and it was evi-
dent that fluid matters were affected more than folids.
The firft fubftance therefore that was employed, was
the very expansible and elaftic fluid air; a quantity of
this fluid was inclofed within a fmall tube, with a fmall
drop of oil, or fome coloured liquor, at the top, which
ferved to fhew the expanfion which the inclofed air
underwent from the increafe of temperature. As this
thermometer, however, was open at the top, it was
alfo found to be affected by the prcffure of the ex-
ternal air ; tubes hermetically -fealed were therefore
prefently fubftituted, and the coloured liquors them-
Jelves were found to be fufficiently expanfible to mark

* The reafons in fupport of each of thcfe theories will be con-
litlercd in the following chapter.
f MQIH. de 1'Acad. Roy. 1709,

the

Chap. I.] DoBritie of Bcerhaave. 89

the degrees of heat Spirit of wine was employed by j
the Florentine academicians, and oil was afterwards
made ufe of by Sir Ifaac Newton, who conftituted the
points at which water freezes, and that at which the
lame fluid boils or aflumes the form of vapour, as
extreme points of his fcale of heat. Thcfe thermome-
ters were however fuperfeded, at leaft in England and
Germany, by the invention of Olaus Roemer, after-
wards improved by Fahrenheit, who fubftituted mer-
cury in the place of the other fluids which had previ-
oufly been employed in the conftruction of thermo-
meters.

The fagacious and learned Boerhaave, both by his
own experiments and by his attention to thofe of others,
contributed greatly to the elucidation of the doctrine
of heat and fire. He was a ftrenuous afierter of the
exigence of fire as a diftinct elementary fubftance.
'Expanfion or rarefaction he confiders as the uniform
fign or criterion of its exiftence in other bodies. The
production of fire from the attrition of two hard bo-
dies, as a flint and fteel, or two pieces of hard wood,
&c. he accounts for, by fuppofmg that the parts of
thefe bodies will every moment be violently compref-
fed, which will excite in them, by their re-action, a
vibratory motion, and this will neceflfarily excite and
expel the fire which exifted latent in their pores ; and
as fire is capable of being produced in this manner by
the violent attrition or motion of all bodies, he infers
that it is prelent through every part of nature ; yet,
iwice it is expelled by the attrition or vibration of the
particles, he thinks it is clear that it does not penetrate
the integrant or elementary particles of bodies, bun
. exifts only in their pores or interftices. As fire is fup-
pofed to exift in all bodies, he proves its exiftence in
air and water; and agrees in opinion with the younger
JLemery, that ice is the natural ftate of water, and that

it

90 Great Di/covery of Dr. Black. [Book II.

it is kept in a fluid ftate by a quantity of fire which it
abforbs. .

There is a period when the minds of men are pre-
pared for the reception, as well as for the profecution,
of great diicoveries in icience. The hints, for they are
little more, which had been afforded by thefe philo-
fophers, appear to have made little imp re (lion j and
the nature of heat, fire, and fluidity feems to have been
involved in obfcurity and contradiction, till the genius
and induftry of Dr. Black, of Edinburgh, developed
a fyftem, which explains fatisfactorily a variety of the
moil curious and difficult phenomena in nature. By
a number of nice obfervations, he was enabled to de-
termine that abfolute heat or fire was abforbed by all
bodies whatever, and that it was abforbed in greater
quantities by fluid than by folid fubftances; heat there-
fore he confidered as the caufe of fluidity. He found
further, that bodies in pafiing from a folid to a fluid
ftate abforb a quantity of heat without increafing their
temperature or fenfible heat, as manifefled by the ther-
mometer. Thus, if water with a quantity of folid ice
is let over the fire, the temperature of the water will
not be increased, but will continue at the heat of 3 2 de-
grees, the freezing point, tiil every particle of the ice is
diffolved. The reafon is, that fire or heat being ablb-
lutely neceflary to impart fluidity to any body, in pro-
portion as the ice becomes fluid the fuperfluous fire is
abforbed. In the fame manner, when the fluid is con-
verted into vapour, a quantity of abfolute heat or fire
is abforbed without any inert-ale of temperature above
the boiling or vapourific point. This difcovery Dr.
Black was led to by heating water in a clofe furnace a
confiderable degree above the boiling point ; when
on opening the veflel in which the water was confined,
he found that a fmall quantity of the fluid burft out
fuddenly in the form of vapour, and the temperature

Chap, i,] Latent Heat. 91

both of the vapour and of the remaining water im-
mediately iunk to the boiling point. It was evident
therefore that the fliperfhious heat was aoforbed by the
vap-ur, and as the quantity of water which was loft
by the procefs was not great, it followed that a confi-
derable quantity of the matter of heat or fire is necef-
fary to keep water in a (late of vapour. When any
quantity of heat is expelled from a body, in fuch a
manner as to affect our touch, it is termed, according
to Dr. Black's theory, Jenfible heat; and when it is
abforbed by any body, and exifts in combination with
that body, either in a fluid or vapourific (late, it is
termed latent heat. It is alfo evident from what has
been ftated, that the opinion of thefe later philofophers
is, that heat or fire, which has alfo been called igneous
fluid, matter of beat, and lately by the French che mills
caloric., is a diftinct fubftance or fluid, which has an at-
traction for all other fubftances; that it pervades mod
bodies ; that it is the only permanent fluid in nature,
and the caufe of fluidity in all other bodies. That
not only common fluids, fuch as water, but all elaftic
fluids, fuch as vapour and air, owe their exiftence in
that (late to the prefence of heat; and that it is fubje6t
to all the laws of attraction, and is more forcibly at-
tracted by fome bodies than by others.

The fchool of Dr. Black feems to have confidered
Jight and heat as effentially different; and Dr. Scheele,
a Swedifli philofbpher, has endeavoured to prove, that
light is formed by an union of the matter of heat with
phlogifton or the inflammable principle : but this
theory is now exploded.

Upon the theory of Dr. Black, the late ingenious
Pr. Crawford * has founded a very curious fyftem con-
cerning

* I cannot mention this truly amiable philofopher, without a
fliort tribute to his memory, though it has apparently little con-

neftioa

9 1 Dr. Crawford's Theory. [Book II.

cerning the generation of heat within animal bodies,
which he confiders as derived from the air we breathe.
The air being condenfed on the lungs, the heat which
it contained in a latent ftate is abforbed and difperfed
over the animal body. — But this is a fubjedl which
properly belongs to another part of the work.

jiedlion with the fubje£l. No man was ever better calculated for
promoting ufeful fcience than Dr. Crawford. In him induftry and
perfeverance were eftablilhcd habits; and candour and caution
charafteriflic difpofitions. With all the advantages of a liberal
education, he united great natural fagacity, acutenefs, and inge-
nuity; yet the laft quality was tempered by a coolncfs and collecV
ednefs of mind, which effectually prevented his too hailily acced-
ing to the rafli conclufions of plauiible theory. With all his excel-
lence as a fcientific -man, he poilefled the gentleft of tempers,, the
moft friendly heart.— From his promifed revifion of this work, I
had flattered myfelf with great advantages; but what arc private
lofles compared with that of the public ! If, after having ferved his
country in a public capacity, the family of fuch a man mould be
left in indigence, to what a itate is the national fpirit reduced \

Chap. 1.1 [ 93 1

CHAP. II.

OF FIRE (CALORIC) AND ITS PROPERTIES.

Inquiry whether Heat or Fire is a Subftance cr Quality,— Fire a Sub-
Jiance.— Application of this Doftrine. — Analogy between Heat and
Light. — Obje&ions.— Properties of Fire or Caloric ; Minutenefs
if .Particles ; attraSed by all Bodies. — Conducing Powers of dif-
ferent Bodies. — Caufe of Fluidity. — Why Heat is produced by
flacking Lime, and by certain Mixtures of cold Subftances. — Freez-
ing of Water by the Fire Side explained. — Fire the mojl elaftic
of all Bodies.

THE element of fire is only known by its effects;
fo fubtile and evafive indeed is this wonderful
fluid, fo various are the forms which it alfumes in the
different departments of nature which it occupies, that
its very exiftence, we have feen, has been questioned
by fome philofophers.

Heat, fay thefe theorifts, is nothing more than an
inteftine motion of the molt fubtile particles of bodies.
Fire is no other than this motion increafed to a cer-
tain degree, in other words, a body heated very hot ;
and flame is no more than ignited vapour, that is, va-
pour, the particles of which are agitated in an extra.-.
ordinary degree.

In fupport of this theory it is alledged, i. That
motion in all cafes is known to generate heat; and if
continued to a certain degree, aclual ignition will be
produced, as the friction of two pieces of wood will
6rft produce heat; and afterwards tire; and the motion
of a glafs globe upon an elaftic cufhion will caafe a
Itream of fire to be copioufly emitted, adly, Bodies
which are molt fufceptible of inteftine motion, are
readily heated, jdly, Motion always accompa-
• ' nies

94- Arguments to prove Fire not a Sub/lance. [Book II.

nies fire or heat, as is evident on mixing oil of tur-
pentine and vitriolic acid ; and the heat feems in mod
cafes to bear a proportion to the degree of motion or
agitation. In the boilinp; of water, and in the hiflmg
of heated iron v hen applied to a fluid, this motion is
evidently manifefted. 4thly, If the particles of any
body are. excited to a violent decree of interline mo-
tion, by attrlrion, fermentation, &c. if they do not
actually emit flames, they will yet be difpofed to catch
fire with the ucmoft facility ; as in the diftillation of
ipirits, if the head of the (till is removed, the vapours
will inftantly be converted into flame if brought into
contact with a lighted candle, or any other ignited
body. Laftly, Heated bodies receive no acceffion of
weight, which they apparently ought to do, on another
body being introduced into their pores.

Plaufible as this reafoning appears at firft fight, the
hypothefis which afllgns exiilence to the principle of
fire, as a diftinct elementary principle, is fupported by
snore numerous fads, and by more decifive reafons ;
it accounts better for all the phenomena of nature, and
even for thofe very phenomena which are adduced in
fupport of the contrary opinion.

i ft. If it is admitted, as I apprehend it muft, by
the advocates for the contrary opinion, that the inter-
nal motion or agitation, which they fay conllitutcs heat,
is not equally felt by all the component particles of
bodies, but only by the minuter and more fubtile
particles ; and that thefe particles being afterwards
thrown into a projectile (late produce the effect of
light; thefe concefllons will almoft amount to the
eftabliftmient of the principle of fire as an elementary
principle.

idly, That fire is really a fubftance, and not a qua-
lity, appears from its acting upon other fubftances, the

reality

Chap. 2.] Arguments to prove Fire a Subftance. 95

reality of which has never been doubted. Charcoal,
in its natural ftate, contains within its pores a large
quantity of air ; but if charcoal is heated, this air is
expelled by the fire, which afiumes its place, and oc-
cupies the pores of the charcoal. The burning of
lime alfo, which deprives it of a great part of its
weight by expelling the fixable air, demonftrates that
fire, as a fubflance, enters into the pores of the lime,
and forces out thofe other fubftances which are leaft
intimately combined with it.

jdly. All the evidence of our fenfes, and many in-
dubitable experiments, prove that light, which many
fuppofe to be fire in a projectile ftate, is a fubftance.
Boerhaave concentrated the rays of the fun in a very
ftrong burning-glafs, and by throwing them upon the
needle of a compafs, the needle was put in motion by
the force of the rays, as it would have been by a bla-ft
of air, or a ftroke from fome other body. But this
experiment was purfued with Hill fuperior fuccefs, by
a late ingenious philofopher *. He conftruded an
inftrument, in the form of a fmall vane or weather-
cock. It confifted of a very thin plate of copper, of
about one inch fquare, which was attached to one of
the fineft harpfichord wires, about ten inches long.
To the middle of the wire was fixed an agate cap,
fuch as is ufed for the fmalleft mariners' compafies,
after the manner of which it was intended to turn j
and the copper plate was balanced on the other fide by
a grain of fmall fhot. The inftrument weighed ten
grains j and to prevent its being affected by the vibra-
tions of the air, it was inclofed in a glafs box. The
rays of the fun were thrown upon the plate of copper

* Mr. Mitchell. — See a fuller defcription of his inftrument and
experiments, in the Phil. Tranf. and Piicftley's Opt it?, p. 387.

from

9<5 Caloric an elementary Subftance* [Book II*

from a concave mirror* of two feet in diameter ; in
confeqnence of which the vane or copper plate, moved
on repeated trials with a gradual motion, of about one
inch in a fecond of time. This experiment I think
a iufficient demonftration, if any demonftration was
wanted, that light at lead is a fubftance. Of the iden-
tity of light, heat, and fire, I fhall have occafion after-
wards to treat.

4thly. The electric fire affects bodies with a true
corporeal percuflion * ; and that this effect is not ow-
ing to the vibration of the air, or any medium but that
of fire itfel£ is proved by many experiments in vacuo,
&c. Now, if one fpecies of fire is allowed to be ma-
terial, there feems to be no reafon why we mould
deny the fame attribute to the reft.

5thly. It is not eafy to conceive how a body can be
expanded by motion alone ; and it is much more na^
tural to fuppofe, that bodies are expanded by the in-
terpofition of an extremely active and elaftic fubftance
.bet>veen their component particles.

6thly. It is well known that there can be no igni-
tion or combuftion, that is, there can be no very high
degree of heat, without a fupply of air j a candle, for
inftance, will ceafe to burn in vacuo, or in air, the
pure part of which is dcftroyed by burning or refpi--
ration. This is a fact which cannot be accounted for
on the principle that ail heat is ho other than inteftinc
motion ; but is eafily explained it we fuppofe fire a
tliftinct elementary fubftance, which is contained in
pure air, and is yielded by the air co the force of a fu-
perior attracti

ythly. Thai heat is generally accompanied by mo-
tion, is no proof that heat and motion are the fame j

- Jones's Phrfibl. Difq. p. 8j.

oa

Chap. 2.] Caloric an elementary Principle. 97

on the contrary, nothing is mose natural than that the
entrance of an exceedingly elaftic fubftance into the
pores of another body mould excite fome degree of
inteftine motion, as well as the emiffion of the fame
fubftance, which muft occafibn fome degree of con-
traction in the particles of the body. Heat is indeed
excited by the attrition of two pieces of wood ; but
why may not the fire in that cafe be expelled from the
wood by the vibration or contraction of its fibres *,
or from the air which occafionally interpofes itfelf ? In
the fame manner a piece of lead will become hot by
hammering ; but lead, and all metals, are known to
contain a quantity of fire in a latent ftate, which indeed
occafionally caufes their expanfion or dilatation ; it
is then the more probable fuppofition, that the fire
or caloric is expelled from the lead by the hammer-
ing and contraction of the metal, and this is rendered
ftill more probable, fmce the contraction or compref-
fion of the metal in a vice will produce the fame
effect. The inftance taken from the inflammability
of the fleam of fpirituous liquors will be. perfectly ex-
plained when I fpeak of fteam or vapour ; befides
that, thefe liquors are amongft the moft inflammable
fubftances with which we are acquainted, and their
particles, being in a rarefied ftate, will be more fubject
to thofe natural forces, which in all ftates are known
to act upon them. There is no increafe of gravity
in heated bodies, becaufe of the great elafticity of ca-
loric or the matter of fire, which expands the- bodies
into which it enters, and confequently rather dimiriifhes
their fpecific gravities.

Sthly. All the other phenomena of nature are more

* If the parts of a body, containing any fluid, are made to vi-
brate ftrongly, they will in general expel a part of the fluid out of
the pores. — Nickclfin, Vol. II. p. 12?,.

VOL. I. H fads factor ily

98 Experiments of Mr. Boy k and Dr. Franklin. [Book II.

iatisfactorily accounted for, on the principle that fire
is a diftind fubftance, than on that which fuppofes it a
mere quality, depending on the tremor or inteftine
motion of bodies.

Heat and light are the only means by which we
are enabled to difcover the prefence of fire, I con-
clude, therefore, that they are both effects of the fame
caufe. The rays of the fun, when concentrated to a
certain degree, produce intenfe heatj and heat, when
violently excited by attrition, &c. if the body in which
it is excited is in favourable circumftances, will gene-
rally terminate in flame, and confequendy in the emif-
jRon of light. This hypothefis receives a ftrong con-
firmation from an experiment of Mr. Boyle. He co-
loured the furface of a large tile, one half white, and
the other black : after fuiFering it to lie for fome
time, expofed to the fummer fun, he found that while
the whited part of the furface, or that part which re-
flecled back the rays of light, remained quite cool,
the black part, which imbibed them, was grown ex-
tremely hot. He occafionally left a part of the tile of
its native red ; and, after expofing the whole to the
fun, found that this part grew hotter than the whice,
but not quite fo hot as the black part. He obferves,
that rooms hung with black are not only the darkeft
but the warmeft alfo ; and a virtuofo of unfufpected
credit aflured him, that in hot climates he had feen
eggs well roafted in a fhort time, by only blacking the
fhells, and expoiing them to the fun. This fad: was
afterwards completely eftablifhed by Dr. Franklin,
who expofed feveral pieces of cloth of different co-
lours upon the furface of fnow ; he found that the
black funlc confiderably beneath the furface, confe-
quently that it imbibed a large quantity of heat,
••whereas the white, which reflected the greater part of

the

Chap, a.] Analogy of Heat and Li git. 09

the rays of light, had imbibed fcarcely any heat what-
ever.

The only objection of any moment which has arifen
againft this doctrine is, that there exift certain bodies,
fuch as what are called the fokr phofphori, putrefcent
fubftances, and rotten wood, which emit or reflect
light, without apparently poffeffing the fmalleft quan-
tity of fenfible heat. If however we confider the ex-
treme weaknefs of the light which is emitted by thefe
fubftances, the objection will appear to have little
force. The moft concentrated moon-light, in the
focus of a concave mirror, is not more than the three
hundredth part of the intenfity of common funfhine * ;
and yet the light from thefe fubftances is not to bs
compared with that of the moon. Nay, the analogy
between heat and light receives confirmation from
thefe very fubftances ; for the property which they
poflefs of emitting light, is greatly increafed by an
accefllon of heat ; and even phofphori, in which the
light has for fome fpace of time been dormant, or in
which it is apparently exhaufted, will emit light upon
the application of heat alone f.

I conceive fire therefore, or caloric, as termed by
the French chemifts, to be the elementary principle
or caufe of heat and light. Caloric in a difengaged
ftate, or in the act of paffing from one body to another,
imprerTes our organs with the fenfe of heat ; and in a
rarefied and projectile ftate, it probably conftitutes the
matter of light. Confiftently with thefe principles, tj^e
fun may be confidered as the great fource of fire,
whence it is diftributed to all the different bodies in
our folar fyitem. On the fame ground alfo, cold is

* See a note by the ingenious translator of Fourcroy's Leftures,
Vol.1, p. 123.

f Priefiley's Optics, part iv. f. i.

H a univerfally

ioo General Properties of Fire or Caloric,. [Book II.

univerfally allowed to be a mere negative quality, and
to mean nothing more than the abfence of heat or
fire.

It appears the moil' convenient form of treating this
important fubject, firft to confider caloric or the mat-
ter of fire in its capacity of exciting heat and produc-
ing expanfion ; and fecondly, to direct our attention to
the various phenomena which it exhibits in its latent
or combined ftate, as the efficient caufe of fluidity
both in the incomprefllble and elaftic or aerial fluids.
I fhall firft enter into a brief detail of the principal
and known properties of caloric; and fhall afterwards
illuftrate thefe properties by its effects in different in-
ftances.

Firft. The particles of fire appear to be more mi-
ffx/tf.than thole of any other fubftance whatever. It
penetrates all bodies with the utmoit eaie. If the
pores of a body are difpofed in right lines, fo as to
admit the paffage of fire without impeding its velo-
city, it will be tranfmitted in the ftate of light as well
as in its ordinary ftate, when it excites the fcnfation of
heat j as is the cafe with all tranfparent or diaphanous
bodies. But there is no body, however denfe, which
will not admit this elemenl to circulate through its
pores with the utmoil rapidity. A. .piece of charcoal
fcrewed up faft in a vefTel of iron will be ignited as
effectually as in the naked fire. Thole bodies which
mod completely exclude the air, are utterly unable to
refift the entrance of caloric : for a thermometer will
rile equally in the moft complete vacuum that can
be produced, as in the open ak *. •

idly. The matter of fire is attracted more or lefs
by all bodies. When any heate^i body comes in con-

* See Jones's Pbyf. Difq. p. 38.

tact

Chap. 2.] Firejuljeft to the Laws of Attraction. 1 01

tact with a cold one, the former lofes a part of its
heat, and both of them become equally warm. If
heated iron is laid upon a (lone, its heat will 'flow into
the (lone ; if thrown into water, the hear will be dif-
fufed through the water. If a number of different
fubftances, as metals, wood, wool, &c. are brought
together into a place v/here there is not a fire, if they
are of different temperatures, that is, of different de-
grees of heat, the fire will be attracted from the hot-
teft to thofe that are colder, till a perfect equilibrium
is produced, or till they have all acquired the fame
temperature, as may be proved by applying the ther-
mometer fucceffively to each of them.

It does not appear, however, that all bodies have
an equal attraction for the matter of fire. If a rod of
iron is put into the fire for a more time, the end which
is at a moderate dii'tance from the fire will almoft
burn the hand ; but a rod of wood, of the fame length
will be confumed to afh.es at the end which is in the
fire before the other end is fufficiently heated to burn
the hand. A ball of lead and a ball of wool may be
of exactly the fame temperature by the thermometer,
but they will not appear of the fame degree of heat
on applying the hand. If they are of a temperature
below that of our bodies, the lead will appear much
colder than tije wool, becaufe it attracts the heat more
rapidly from the hand; if they are of a higher tem-
perature, the lead will appear much hotter, from the
facility with which it parts with its heat. This pro-
perty in bodies is called their conducing power ; and
thofe bodies through which the element of fire moil
rapidly circulates, are called good conductors.

The power of conducting the matter of fire feems

*o depend upon the texture of bodies, that is, upon the

H 3 contact

lOi Count Rumford's Experiments. [Book II.

contact of their parts * j hence the excefllve flownefs
with which heat is communicated to bodies of a rare
and fpcngy texture. Thus flannel, wool, and feathers,
are confidered as warm coverings, not becaufe they
pofiefs more heat in themfeJves (for they ferve to pre-
ferve any cold body in a cool ilate better than other
fubftances) but becaufe they prevent the efcape of the
animal heat from our bodies. It is a well-known
fact, that ice is generally kept in ice-houfcs in ftraw
or wool, thofe fubftances, from the rarity of their parts,
preventing the entrance of the matter of heat. On
the fame principle the ground is kept warm by fnow,
that fubftance being of a foft and fpongy texture. It
is true, it will not keep the ground warmer than the
-freezing point; but that is warm, when compared
with the intenfe cold which is occafionally experienced
in moft'northern climates.

An ingenious and accurate experimentalifl has lately
endeavoured to eftimate the conducting power of dif-
ferent bodies. The conducting power of mercury he
found to be to that of water as i,coo to 313. Hence
it is plain why mercury appears fo much hotter or fo
much colder to the touch than water, at a time when
they are evidently of the fame temperature by the ther-
mometer. Common air is a much better conductor
than the Torricellian vacuum f ; its conducting power,

* This is proved by an eafy experiment:-— If a cube and a
fphere of the fame metal are put upon a plane intenfely healed,
the heat will flow fader into the^cube ; and if the fame bodies are
previoufly heated, and expofed on a cold plane, the cube will ccol
fooneit.

f Made by filling a tube, clofed at the top, with mercury, and
emptying the upper part of the tube by immernng the lower in a
veflel filled with the fame fluid, as is the cafe in the common ba~
rometers. This is the moil perfeft vacuum we can make.

compared

Chap. 2.] Conducing Power of different Bodies. 103

compared with that of the vacuum, is nearly as 1,000
to 605.

A moid air conducts the matter of fire with much
greater rapidity than a dry air ; but the rarity or den-
fity of the air appears to have little effect upon its
conducting power.

The proportion of the conducting power in the
different fubftances which were the objects of his ex-
periments, is as follows :

Mercury --.------- 1,000

Moift air--------- 330

Water ------- - 313

Common air, the barom. at 27 incs 9 lines - 8oTVy
Rarified air, - - barom. at 6 - 1 1 lines - 8oT\V
The fame, - - barom. at i - 2 - - 78
The Torricellian vacuum - - - - -55*

From the different effects of bodies upon our feel-
ings, according to their conducting powers, arifes the
distinction which philofopbers have made between
abfolute and fenfible heat. It will be remembered, that
the fenfation of hot is the entrance of caloric or heat
into our bodies, and the fenfation of cold is its depar-
ture from them f . Thefe circumftances render the
fenfes of animals a very inaccurate meafure of heat ;
efpecially if we confider further, that much will alfo
depend upon the ftate of the organ of feeling at the
particular time. Water, at the temperature of 62°,
appears cold to a warm hand i but it will appear warm

* Sir Benj. Thompfon's (now Count Rumford) Experiments
on Heat. "Phil. Tranf. vol. Ixxvi.

f The fudden and unexpected application of an extremely cold
fubftanee to the human body, produces a fenfation very fimilar to
that of a hot on«.

H 4 . to

. ic-4 Scnj'ulicn a bad Meafure of Temperature. [Book II.

to a hand which is of a lower temperature *. Tra-
vellers, therefore, from a warm to a cold country,
\vill have lenfations very different from thofe who tra-
vel in an oppofite direction, mould they happen to
meet, as they frequently do, in a temperate climate.
It is evident that the travellers from a cold climate,
being deprived of lefs heat than ufual, will have the
fenfation of warmth ; and the others, on the contrary,
will experience a degree of cold iufficient to excite
confiderable uneafinefs.

jdly. The matter of fire wil'Iexift in a (late of com-
bination. I do not contend for the term chemical
combination, in the ftrict and literal fenfe of the word ;
it is fnfficient if it can be proved, that caloric may exift
in bodies in a latent ftate, or in a ftate not perceptible
to our fenfes. It will be found by obfervation, that
every body which exifts contains a quantity of the
matter of fire in this fixed or neutralized ftate, diiarm-
ed of all its active, penetrating, and deftruftive quali-
ties, like an acid and an alkali in combination. If
the coldeft bodies with which we are acquainted are
condenfed or brought into a fmaller compafs, a quan-
tity of caloric will be emitted. If a piece of lead or
iron is beaten with a hammer, or compreiTed in a vice,
fo as to force it to contract its dimenfions, it has been
already remarked that a degree of fenfible heat will
i>e produced.

Fluids, from their very nature and conflitutios, con-
tain a greater quantity of caloric in a latent ftate than
. folid bodies : indeed it is now univerfally admitted,
and may be eafily proved, that the fluidity of all bo-
dies is altogether owing to the quantity of fire which
they retain in this latent or combined ftate, the elafti-

* Crawford on Animal Heat, p. 5. zd, edit.

city

Chap. 2,] Phenomena on flacking Urns. 105

city of which keeps their particles remote from each
other, and prevents their fixing into a folid mafs. All
bodies, therefore, in patting from a fluid to a folid ftate,
emit a quantity of fire or hear. When water is thrown
upon quick-lime, it is abforbed by the lime, and in
this ftate it is capable of retaining a much fmaller quan-
tity of caloric than in its natural ftate ; on the flacking
of lime therefore a very intenfe heat is produced, the
matter of fire which preferved the water fluid being
difengaged and detached. If fpirit of vitriol is added
to ftrong oil of turpentine, they will condenfe into a
foiid mafs, and a great quantity of heat will be fen-
fibly emitted. If water is expofed to freeze, and a
thermometer applied to it, during the act of freezing,
or parting from a fluid to a folid ftate, it will be found
feveral degrees warmer than the air which furrounds
it, which is owing to the caloric or fire emitted by that
part of the water which is converted into ice. This
effect is ftill more apparent from the condenfation of
the elaftic fluids, which, from their rarity, contain a
greater proportion of the matter of fire,.

Upon the fame principle it will be found, on the
other hand, that when any body pafles from a folid to
a fluid ftate, the adjacent bodies will be deprived of a
quantity of their natural heat. Thus if a quantity of
aqua-fortis is poured upon folid ice, the ice immedi-
ately liquifies, and an aftonifhing degree of cold is in-
ftantly produced, even by the fire -fide : this effect is
altogether owing to the quantity of caloric which is
abforbed by the congealed water reafluming its fluid
form. This experiment will ferve to explain the fact
that a thaw is generally colder than the commence-
ment of a froft. The abfbrption of the matter of fire
is further exemplified in the inftance of bodies pafling
from the ilate of a common fluid to that of vapov.r, or

an

106 .Evaptr Alien produces Cold. [Book II.

an elaftic fluid. If a thermometer is immerfed in
ipirit of wine, in water, or in any fluid that eafily eva-
porates, and is afterwards taken out and fufpended in
the air, the thermometer will fink two or three degrees,
though the temperature of the air and water mould be
exactly the fame j the fadt is, the linall quantity of
fluid which remains on the bulb of the thermometer
is carried off in vapour, and in that cafe the mercury
within the thermometer is deprived of a certain por-
tion of its latent fire. If the thermometer is repeat-
edly dipped in the fluid, the cold which is produced
will be confiderable. If ether, which is a very vo-
latile fluid, is applied to any part of our bodies, cold
is immediately produced ; and on the fame principle,
a man may be frozen to death in very warm weather,
by expofing him to continued evaporation ; which
may be effected by throwing repeatedly upon his body
a. quantity of ether, of fpirit of wine, or of any other
fluid which is eafily evaporable. It is a common prac-
tice in China to cool wine or other liquors by wrap-
ping the bottle in a wet doth, and hanging it up in
the fun j the water in the cloth" is gradually convert-
ed into vapour, to form which the liquor in the bottle
is deprived of its latent fire. The celebrated Muf-
chenbroek was aftonifhed at the freezing of a wet
cloth which was hung up to dry, when there was nt>
appearance of froft in the atmofphere : the folution of
the difficulty is, the temperature of the air at the time
rnuft have been within fome degrees of froft, and the
temperature of the cloth was fuddenly reduced to the
freezing point by the lofs of a part of its heat from
evaporation.

Let it be remembered, that in all thefe inftanoes
there is an evident acceflion or increafe of the matter
of fire thrown into the bodies which are rendered

fluid,

Chap, 2.3 "Phenomena cf Evaporation. 107

fluid, and yet the temperature or obvious heat of the
fluids is not increafed, as may be proved by the ther-
mometer ; wherefore it is plain that the caloric exifts
in thefe fubftances in a latent or combined Hate.

4thly. The matter of fire is elaftic, as is proved
evidently from all its effects. There is indeed reafon
to believe that caloric is the only fluid in nature which
is permanently elaftic, and that it is the caufe of the
elaflicity of all fluids whiclt are efteemed fb.

From the elafticity of this element it refults that all
natural bodies can only retain a certain quantity of it,
without undergoing an alteration in their ftate and
form. Thus a moderate quantity of fire admitted into
a folid body expands it ; a ftifl larger quantity renders
it fluid j and if the quantity is ftill increafed, it will
be converted into vapour. But this, and all the other
properties of caloric, will be better underftood from
its effects. Let it fu.^ce to remark for the prefent,
that moft fluids may be converted into a flate of un-
ufual rarity, by the acceflion cf fire. Vapour is 1,800
times lefs dcnfe than water ; and thofe matters which
have a ftronger attraction for fire may by the fame
means be converted into fluids permanently elaftic.
The nitrous acid is wholly convertible into two fpe-
cies of air, oxygen and azote, or pure and phlogifti-
cated air ; and oils, refins, charcoal, and other inflam-
mable matters, will by the application of heat readily
the form of inflammabfc air.

[ jcS ] [Book II.

CHAP. III.

OF .EXPANSION.

Experiments proving the exfanjlfje Force of Fire, or Caloric, — //////•«-
Mints for meafuring Degrees of Heat. — Thermometers. ~— Dr. Black's
Made of meafuring high Degrees of Heat.— Mr. Wedgwood's.

CALORIC, as was intimated in the preceding
chapter, expands all bodies which it penetrates,
more or lefs, in proportion to its quantity, and to
the nature of thofe bodies. The expanfion of water,
even previous to its afiumingthe form of vapour, may
be feen in an eafy experiment. If a quantity of cold
water, contained in a clear flafk, is immerfed in a
veflel of boiling water ; as the heat enters, the water
in the flufk will be feen to rife in the neck tiil it
overflows.

An iron rod a foot long being heated red hot, be-
came ^0-th longer than before; and a glafs cylinder, a
fathom long, under the fame cireumftances, gained
v',.th in length. A metalline ring thus heated was in-
creafed T-|-^ in its diameter : and a glafs globe became
extended TA-0- part by the heat of the hand only ap-
plied to its furface *.

It is a well-known practice to immerfe razors, or
any inftruments which are required to cut fmooth, in
warm water j as the whole of the metal expands, the
edge is alfo proportionably expanded, and confequently
is rendered fo much finer and fmoother.

An inftrument was invented by Mr. Jones, for mea-
furing the force of expanfion, which by the flame of a

* Boerhaave's Chem. by Shaw, Vol. I. p. 299.

farthing

Chap. 3.] Force of Exparjion-. 109

farthing candle was able to lift a weight of five hun-
dred pounds, without any afliftance from the mecha-
nical powers; and he fhews that the fame infignificant
power, namely, the flame of a fmall candle, would by
the force of expanfion overcome a weight even of five
thoufand pounds, could an inftrument be conveniently
fitted up for the experiment*. Indeed, when we
confider that the force of cohefion in metals is fb
great as to enable a gold wire of one-tenth of an inch
diameter to fupport five hundred pounds weight, and
an iron wire of the fame dimenfions to fupport one
thoufand five hundred pounds, without producing any
feparation of the parts ; what mufl be the force of fire,
which can relax and even difTolve the texture of the
firmeft metals f ?

It is a fact univerfally known, that clocks and time-
pieces in general go flower in warm weather, and falter
in cold j this effect is owing entirely to the expanfion
of the pendulum, which being lengthened by the ac-
ceflion of heat or fire in warm weather, makes a lon-
ger vibration, and confequently lofes a proportionate
quantity of time ; on the contrary, the length of the
pendulum being contracted by cold, the vibrations
will be proportionably quicker, though the quantity
of time gained or loft in a fingle vibration may be ex-
ceedingly trifling; yet as the vibrations are very often
repeated, the effect will in a courfe of time be very
confiderable. An alteration of one hundred thoufandch
part of the time of a fingle vibration, will amount to
nearly that of a whole vibration in the courfe of a
dayj.

The cafes are fo numerous in philofophy and the
arts, when it is defirable to be informed of the quan-

* Jones's Phyf. Difij p. 99, ico. $ Ib. p. 98,

J Ib. p. 98.

tlty

i to Florentine Thermometer. [Book II,

tity of heat which exifts in bodies, that it foon became
an objecb of the utmoil importance, to difcover an ac-
curate method for afccrtaining it. The expanfive pro-
perty of fire was the property which moft naturally
iuggefted itfelf as likely to furnifh an eafy method of
accomplifhing this object, fince the evidence of our
fenfes affure us that, at leaft in all lower degrees of
temperature, the expanfion of bodies bears lome de-
gree of proportion to the quantity of the matter of fire
which they have imbibed.

Air, as was intimated in a preceding chapter, was
the firft fluid which was employed as a meafure of
heat and cold. A fmall tube was prepared, open at
the top, into which a quantity of coloured liquor was
introduced ; a quantity of air was left in the lower part
of the tube, below the liquor, and in proportion as this
air expanded or contracted, the heat of the furround-
ing medium was fuppofed to be increafed or diini-
nilhed. The manifeit inconvenience attending this
inftrument was, that as the upper orifice of the tube
was necefiarily left open, it was liable to be affected
by two caufes, by the natural heat of the medium, and
by the weight of the atmofphere preffing upon the li-
quor in the upper part of the tube.

The next fluid that was made ufe of was fpirit of
wine, and this, being incloied in a tube which was ex-
hauiled of air, afforded an inftrument much more per-
fect: than the former. The principal objection to this
fpecies of thermometer is, that fpirit of wine is inca-
pable of enduring any great degree either of heat or
cold, fmce it boils in vacuo at fifty-two degrees. This
thermometer is diftinguifhed by the name of the Flo-
rentine thermometer, as it was invented by fome of
the members of that academy. It was afterwards
greatly improved by the celebrated M. Reaumur, who

proportioned

Chap. 3.] NewtorfsandFahrenheiCs'fhermQmeters. in

proportioned the expansibility of the liquor to the fize
of his tube, by diluting it with water, or the contrary;
the generality therefore of thermometers made with
ipirit of wine are termed Reaumur's thermometers.

Oil was employed by Sir Ifaac Newton inftead of
fpirit of wine, as being capable of a greater degree of
expanfion, fince that fluid will bear about four times
the heat of boiling water before it boils, and in gene-,
ral a very great degree of cold is required to make it
freeze. The principal objection to Newton's thermo-
meter arifes from the vifcidity of the oil, which occa-
fions it to adhere to the fides of the veflel, fo that a
confiderable quantity of the fluid being retained by the
gkfs, when the thermometer finks, it appears to fink
lower at firft than it ought to do, according to the na-
tural temperature.

Thefe thermometers, therefore, were all of them
fnperfeded by the famous invention of Olaus Roemur,
improved by Fahrenheit, who fubftituted mercury in-
ftead of the other fluids. Mercury is found to be a
more homogeneous body than any other fluid, and
more regular in its expanfions^ befides that it is ca-
pable of exhibiting a more copious fcale of both heat
and cold.

Sir Ifaac Newton, obferving that water uniformly
froze with a certain degree of cold, and as uniformly
boiled when the heat was increafed to a certain degree,
took what is called the freezing point for the com-
mencement of his fcale, and from that to the boiling
point he counted thirty four degrees, and divided his
fcale accordingly. It is evident, however, that even
in this climate we have many degrees of cold below
the freezing point. Reaumur, therefore, though 'he
commenced his fcale alfo at the freezing point, yet
admits of feveral degrees below it, and proceeds both
3 wavs

112 Graduating Thermometers. [Book II.

ways from O; the boiling point in his fcalc is at 80*
above o. The fcale of Fahrenheit begins confiderably
below the fieezing point, at that period of cold which
is produced by furrounding the bulb of the thermo-
meter v/ith a mixture of fnow or pounded ice and lal
ammoniac or fea-fah: he divided his fcale into mi-
nuter portions than either Newton or Reaumur, on
which account it is well known that the boiling point
in Fahrenheit's thermometer is at 212°. Sir Ifaac New-
ton's thermometer is, I believe, now quite obfolete :
Reaumur's is dill tiled by many of the French, and
other experimentalifts. The degree of heat, however,
when noted on either of theie inftruments, may eafily
be computed, by remembering that 34° of Newton's
aiifwer to So of Reaumur, and to 21 2 of Fahrenheit ?
and that the freezing point, which is the commence-
ment of both the other fcales, is in Fahrenheit's at 32°
above o. /

The graduating a mercurial, or Fahrenheit's thermo-
meter, cannot, from 'what has been obferved, be a dif-
ficult talk. The mercury muft be carefully purged
from air, as that, being a more elaftic fluid, would
.ic fome irregularity in the expanfions of the metal,
or would colled: in the upper part of the tube, which
ought to be the mod perfect vacuum that can be
formed. It is well known that what is called the Tor-
ricellian * vacuum is formed by filling a glafs tube with
mercury, and then inverting the tube in a veflel of the
lame fluid, and withdrawing it flowly till the mercury
fubfides, by which means all that part of the rube
which is above the mercury will be free from air; on
withdrawing the tube out of the mercury, it is obvious
that the orifice mult be flopped with the finger or

* From Torricelli, the inventor.

fome

Chap. 3-] Beft^Form of thermometers. 113

fome other ftopper, to prevent the air from rufhing in.
When by thefe means a quantity of mercury is in-
cluded in a glafs tube with a fmall bulb, the glafs may-
be eafily clofed by applying an ignited 'charcoal and a
blow-pipe, fuch as the jewellers make ufe of, which
melts the glafs, and enables us to twift it round, in
fuch a manner as completely to clofe it againft the ad-
miiFion of air; and this operation is called hermetically
fealing it ?. In order to graduate the thermometer,
it muft be firft immerfed in a mixture of pounded ice
or fnow and fal ammoniac, and the point at which the
mercury fettles muft be marked as the commence- -
ment of the fcale, or o. It is next to be immerfed in
boiling water, and that point is to be marked 212°,
and the intermediate part of the thermometer muft be
divided into this number of degrees.

Thermometers with final! bulbs, and proportionable
cylinders, are moft ufeful, fmce a large volume of mer-
cury requires a confiderable time to heat and cool. It
is alfo accounted a favourable circumftance when that
part of the bulb which is adjoining to the tube is rather
of a conical~form.

That the thermometer is a true meafure of heat, is
proved by fome very fatisfactory experiments. If
equal quantities of hot and cold water are mixed toge-
ther, the heat of the mixture will be nearly as the mean
heat of the two component parts f. This fact was

* It is eafy to prove whether the tube is a perfeft vacuum or
no)!, after it is hermetically fealed, by merely inverting it, and ob-
ferving whether any bubble of air remains to refill the mercury's,
falling to the botcpm of the tube.

f This experiment was originally made by Dr. Brook Taylor.
It was afterwards repeated by M. de Luc, and by Dr. Crawford ;
who, from the impofllbility of conducting the experiment without
lofs of time, found the heat of the mixture always below the arith-
metical mean. Crawford on Heat, p. 1 8, e t feq. zJ edit.

VOL. I, I alcertained

ji4 Defetts of Thermometers. [Book II.

aicertained 'by a ftill more accurate experiment of Dr.
Crawford, who contrived a method of combining the
boiling and freezing points together, and found that
the degree of heat communicated to the thermometer
was as nearly as poifible the arithmetical mean *.

It is evident, from the nature of expanfion, that
thermometers might be conflructed of folid bodies.
Metallic thermometers have indeed occafionally been
made, and graduated for different purpofes ; but their
utility is ncceffarily very limited, fince folid bodies are
expanded with much more difficulty, and in a lefs de-
gree, than fluids.

Though the mercurial thermometer is fo much
more perfect, and is capable of exhibiting much higher
degrees of heat than thofe which had been in ufe be-
fore the time of Fahrenheit ; yet as mercury boils at
600°, that is, confiderably below the red heat of iron,
and as it is plain that no fluid can afford any true
meafure of heat beyond that point in which it is itfelf
converted into vapour, it is equally plain, that there
mud exifl fcveral degrees of heat which cannot pof-
fibly be exhibited by the mercurial thermometer.
Thefe degrees are very inaccurately defined by the
chemifts and artifls, according to the appearance, term-
ing them a red and white heat*, &c. To remedy the
inconveniences resulting from the want of a definite
llandard of heat above the point of boiling mercury,
leveral methods have been propofed, but there are
only two which I efteem worthy of notice.

A very eminent philofopher, who may be termed
the father of the modern doctrines concerning heat,
propofes, in order to afcertain the heat of any given
furnace, for inftance, to heat fome body (the dimen-

* Crawford on Heat, p. 47, 48.

fions

Chap. 3 .] Mode ofmeafuring high Degrees of Heat. 1 1 5

fions of which may be eafily taken) in that furnace,
and, when heated, to plunge it into a quantity of cold
water, and multiply the degree of heat by the propor-
tion which the bulk of the water bears to that of the
body heated. Thus, if a piece of iron is taken red hot,
and thrown into a quantity of water 100 times its bulk,
when the heat which v/as concentrated in the iron is
diffufed through the whole quantity of water, it is evi-
dent that the temperature of the water thus heated,
multiplied by 100, will give the heat of the iron when

' red hot.

Another mode of afcertaining high degrees of heat
has been propofed by the late Mr. Wedgwood, who
by means of a diftinguifhing property in argillaceous^
bodies, namely, that of contracting when expofed to
fire, was enabled to conftruct a new thermometer for

, this purpofe. The fenfible contraction of earthen-
ware commences at a low red heat, and proceeds re-
gularly till the clay becomes vitrified. Mr. Wedg-
wood's thermometer, therefore, confifts of a fmall por-
tion of this clay, properly baked, and fo nicely adapt-
ed to a brafs gage, that the clay is permitted to flide
along the gage in proportion as it is contracted by the
fire. He divided his fcale, from the degree of heat at
which the clay begins to contract, to the greateft de-
gree of heat he was able to produce, into 160°. By
this inflrument he found, that copper melted at 27 ° i
filver at 28° i gold at 32°} caft iron at 130*.

[ n6 ] [Book II.

CHAP. IV.

OF FLUIDITY.

»

Caloric equally tkcCewfe of Expanjlcn and Fluidity.— Phenomena of
Bodies faffing from -a jolid to a fluid State, and the contrary. — ./«-
tenfe Cold of the Southern Hemifphere explained. — Dijti nation between
Expanjion and Fluidity. — Experiments iUmftratwe of the Doflrine of
latent Heal,

IT was intimated that all bodies are capable of con-
taining only a limited quantity of caloric, without
undergoing an alteration in their external form j the
fame caufe which produces expanfion, being increafed
to a certain degree, produces a total d involution of the
parts of bodies, and reduces them to a fluid ftate ;
and a further increafe of the fame power renders them
volatile, or caufes them to be carried off in the form
of an elaftic fluid, fuch as air or vapour.

After what has been formerly ftated, it will be no
difficult matter to conceive the caufe of all thefe effects
to be the fame. The fubtile matter of fire, which
appears to be the only fubftartce in nature which is
permanently elaftic, or between whofe particles a na-
tural re'pulfion exifts, infmuating itfelf between the
particles of bodies, deftroys or rather counteracts the
natural power of attraction or cohefion, which impels
the particles of bodies to approach as nearly in con-
tact to each other as poflible. When a body is reduc-
ed from a folid to a fluid ftate, a quantity of caloric
or fire is abforbed from fome of the furrounding me-
dia. The nature of fluids would therefore be, per-
haps, not improperly defcribed, by fuppofing them to
confift of the very minute particles of the bodies from

which

Chap. 4.3 Caufe of Fluidity. nj

which they are produced kept floating in a quantity
of fire. To understand how caloric may exift in this
combined ftate, without exhibiting any of its deftructive
properties, let it be remembered, that fire, like every
other body, can be only active while in a difengaged
flate. Fire cannot excite in our organs the fenfation
of heat, unlefs it penetrates thofe organs ; if therefore
it is retained by another body by the force of a fupe-
rior attraction, it is evident it cannot affect our organs
as it would if in a (late to be attracted by them. In.
the fame manner the mineral acids (aquafortis for in-
ftance) in a difengaged ftate act with violence on al~
moft every fubftance, and corrode or ulcerate our-flefh,
when brought in contact with them ; but if united
with a body which poflefles a ftronger attraction for
them (fuch as an alkali) they will not leave that body
• to act upon any other, but are perfectly difarmed of
all their noxious qualities : thus the fafe and innocent
compound falt-petre is formed from two violently
active and corrofive fubftances, a cauftic alkali, and
the nitrous acid, or aqua-fortisj and common fait
from the fame alkali, and the muriatic acid.

Every body in palling from a folid to a fluid ftate,
or from that of a common to a rarer or elaftic fluid,
abforbs a quantity of caloric or fire, and confequently
a degree of cold is always produced by the procefs ;
and on the contrary, every body in pafiing from a
fluid to a folid ftate, or from that of a rarer to that of
a denfer fluid, emits a quantity of that fire which kept
it in a ftate of fluidity ; and by this procefs, on the
other hand, . proportionable degree of fcnfible heat is
produced.

A number of phenomena, which were before un-
explained, are now clearly illuftrated by this theory.
What is calLed the freezing mixture, it is well known
J 3 cbrififts

nS Phenomena exhibited by Bodies in [Book II.

confifls of a quantity of pounded ice or fnow with
aqua-fortis, or any faline fubftance. The immenfe
cold which is fuddenly produced by this procefs, is
owing entirely to the fudden liquefaction of the ice,
in which cafe all the adjacent bodies mud fupply a
quantity of caloric or fire, which is abforbed by the
melting ice, and retained by the fluid in a latent ilate,
or ftate of combination. Cold is produced by eva-
poration, on the fame principle ; the quantity of ele-
mentary fire which is attracted by a fluid when pafilng
into a rarer (late, and which is required to form at-
mofpheres of fire round the particles of the body, fo
as to keep them fufpended in the fluid form, is ne-
ceffarily fupplied from the furrounding bodies, and
muft be attended with a degree of cold.

The fouthern hemifphere is remarkably colder than
that of the north, and even in the midfl of fummer an
exceffive degree of cold has been found in the regions
which lie near the antarctic circle. To account for
thefe phenomena, we muft probably have recourfe to
two caufes. As there is a greater extent of water in
that hemifphere, the evaporation is confiderably greater
than in that of the north ; and as the fouthern ocean
abounds with a multitude of immenfe ice iflands, the
continual melting of the ice abforbs the matter of fire
from all the circumjacent atmofphere ; and in fact we
are informed by mariners, that the cold is confiderably
increaied by the approach of one of thefe floating
mountains of ice. Partly for the fame reafon a thaw
is obferved to be much colder than a fettled froft ;
though it is alfo to be remembered, that the atmo-
fphere is always rather inclined to damp in a thaw,
and a damp air is a much more powerful conductor of
heat than a dry one.

On the contrary, when a fluid body pafTes into a.

foiid

Chap. 4.] faffing from a folid to a fluid State, 6fr. 119

folid Hate a quantity of caloric is necefiarily extricated.
The heat produced by flacking lime has fomttimes
been fo great as to fire wood ; in this cafe it has already
been (hewn, that the component particles of the water
being abforbed by the lime, the fire which held it in
fbfion is expelled. A mixture of the eflential oils
with fpirit of vitriol produces the fame effect; the
mixture forms itfelf into a folid mafs, and the fire
which, the fluids contained is fuddenly extricated. A
quantity of water will often continue fluid at fome
degrees below the freezing point, but by agitating the
water in forms fuddenly into ice, and the caloric which
the fluid contained being ft- free, the thermometer
will rife fome degrees. The air is often obferved to
be peculiarly mild during a fail of fnow ; the reafon
is, that the caloric which the water of the fnow con~
tained is difcharged by its patting into a folid Hate,
and fen fible heat is produced. The union ofacauf-
tic alkali, which contains no fixed air, with an acid,
excites great heat, in the fame manner as when water
is thrown upon quick-lime j but if the alkali is mild,
that is, if it contains a quantity of fixed air, that fub-
ftance going off in an aerial form ablbrbs the matter
of fire, which it carries off with it, and no heat is ge-
nerated *.

It was dated that expanfion and fluidity are pro-
duced by the fame caufe : -there is, however, this dif-
ference in the effects, that in expanfion there is a re-
gular increafe or extenfion of bulk, according to the
degree of heat j whereas the tranfition from a fluid to
a folid ftate, or the contrary, is fudden j and below or
nbove a particular point of temperature, a body al-

* See Dr. Higgins's excellent Experiments and Obfervation^
P- 3*9'

I 4 way*

no "Pfanmena exhibited by Bodies in [Book II.

ways remains fluid or folid. There are, it is true,
fbnje bodies which appear in an intermediate ilate of
fluidity, fuch as wax, tallow, &c. j yet even in thefe
the point at which they become fluid is a fettled point,
though the different flages of foftnefs depend upon the
degrees of heat.

In expanfion alfo the fenfible heat is increafcd in
proportion to the effect ; but it is different in fluidity j
for when bodies are arrived at the melting point, or
point of fluidity, a large, quantity of elementary fire is
abforbed, without producing any fenfible heat, or al-
tering the temperature of the body. This abibrption
of the matter of fire frequently continues a confider-
able time, according to the fupply from the adjacent
bodies. Thus, when a thaw comes on, the heat is
often far above the freezing point; and though
the ice melts flowly, it is conftantly furrounded by
air warmer than itfelf, and conftaatly imbibing the
matter of fire from it. On the other hand, if a quan-
tity of boiling water is thrown upon ice, it will im-
mediately melt : which proves that there is no diffi-
culty in feparating the particles of ice, if a fufficient
quantity of heat is fupplied : but the reafon of thefc
facts will be rendered clearer by the following expe-
riments.

If a pound of water at 32° is mixed with an equal
quantity of that fluid at 172°, the temperature of the
mixture will be 102°, which is the arithmetical mean
between the heat of the two fluids ; but if a pound of
ice at 32° is mixed with a pound of water at 172", the
temperature of the mixture will be 32°. Hence it
appears, that in the melting of the ice one hundred
and forty degrees of heat (that is, fuclva quantity of
elementary fire as is necefTary to excite that degree
8 of

Chap, 4.] faffing from a fluid to ajcltd State, 13 c. 121
of heat) are abforbed, or reduced to a ftate of combi-
nation, fo as not to produce any effect on the thermo-
meter *.

The heat which the water abforbs in afTuming its
fluid form is again feparated by congelation, if a
pound of water at 32° is mixed with an equal quan-
tity of ice at 4°, nearly one fifth of the water will be
frozen, and the temperature of the mixture will be 32°.
In this experiment the ice is raifed from 4° to the
freezing point. It is therefore evident;, in this expe-
riment, that by the congelation of one-fifth of the wa-
ter a quantity of caloric is emitted fufficient to raife the
heat of the ice nearly twenty-eight degrees; by the con-
gelation therefore of ar whole pound of water3 a quan-
tity of caloric would be detached fufficient to raife it
five times twenty-eight degrees. The caloric which
is extricated by the congelation of the water is there-
fore precifely equal to that which is abforbed by the
melting of ice f.

There were difputes in the time of Fahrenheit, con-
cerning the rarefaction of ice, whether it depended on
the air contained in it during its fluidity. He ima-
gined, that if he extracted the air from water, he could
produce an ice heavier than water. He extracted the
air therefore from fmall glafs globes filled with water.
After expofing them to an intenfe cold, they were a
long time in freezing, though cooled greatly below
the freezing point ; but upon breaking them to exa-
mine them, the air ruihed in, which, from the fudden
fhock, occafioned the water inftantly to freeze. He
afterwards found, that fimple agitation would produce
the fame effect. If water which is freed from air, and
which is perfexftly at reft, is expofed to the atmo-
fpherewhen it is colder than 32°, it will frequently fink

* Crawford on Animal Heat, p. 72. f Ibid.

eiirht

122 Phenomena of freezing. [Book II.

eight: or ten degrees below the freezing point, with-
out undergoing any degree of congelation ; but if the
veiTel is flightly agitated, a portion of it will imme-
diately become folid, and the mixture of ice and wa-
ter will be raifed to 32°. The reafon of this increafe
of temperature in the remaining water will be evident
from the preceding experiments. By the freezing of
a part of the water, a quantity of elementary fire,
which exifled in the fluid, is expelled, by its afFuming
a folid form ; and this fire being difTufed among the
remaining water, raifes its temperature to the freez-
ing point.

Different degrees of heat are required to retain
different bodies in a fluid form. Water, mercury,
and fome other fubftance?, are kept fluid by a degree
of heat considerably below the ordinary heat of the
atmofphere ; and fo great is the degree of cold which
the latter endures, that before the experiments of Pro-
feflbr Braun, of Peterfburgh, it was not fuppofed that
it was capable of being frozen.

Chap. 5.] [ 123 ]

CHAP. V.

OF BOILING, VAPOUR, &c.

EJaftif Fluids diftinguijhed from common Fluids, — Specific Gravity of
Vapour. — In <what Manner Dr. Black ivas led to form bis Tbcorj
of latent Heat. — Imnenfe Force of Vapour.— Bailing.— ~ All Fluids
boil in a lefs Temperature in Vacua, than under the PreJJiire of the
Atjnofpbere. — Experiments. — Phenomena of Boiling and Evaporation
explained.— -~lVby Water extinguishes Flame.— Spontaneous JL*vapora~
tion.— Phenomena of De^vs, Mifts, tyc,

IF the matter of fire is accumulated to a certain de-
gree, the fubftance which is expofed to its aftion
will be converted from the ftate of a common fluid to
that of an elaflic or compreflible fluid, generally tranf-
parent, and extremely rare and light.

Vapour or ftcam, which is water converted into an
elaftic fluid, is of a fpecific gravity one thoufand eight
hundred times lighter than water; that is, a given
portion of water will, in an elaftic form, occupy one
thoufand eight hundred times the fpace it did before.
The procefs of paffing from the ftate of a common
fluid to that of vapour, is called boiling -, and the der
gree of heat at which a fluid begins to boil is called
the boiling point, which, in water, is fixed on Fahren-
heit's fcale at 212°.

When a fluid arrives at the boiling point, and pafies
from its ordinary ftate to that of vapour, the fame ef-
fect takes place as in the converfion of folid bodies
into fluids, a quantity of diloric or elementary fire is
abforbed without any increafe of temperature ; and
when an elaftic fluid is eondenfed, the fame ftre is conr

Jlantly

124 Phenomena of E oiling explained. [Book II.

Handy emitted, and fenfible heat is confequently pro-
duced. If we obferve the heating of water, we ihall
find that the heat flows into it very fad, till it arrives
at the boiling or vaporific point. Suppofe that in the
laft five minutes its heat is increafed 10°, in the next
five we fhould expe<5t that it would at lead be fix or
feven more; but this is not in reality the cafe, for
though very little of the water is evaporated, yet the
remainder is not fenfibly hotter. In order to prove
the time neceffary to convert a quanticy of water into
vapour, a number of flat-bottomed cylindrical vefiels
of iron were conftruded, into which a quantity of wa-
ter was put, at the temperature of 54°. The water
was heated to the boiling point in four minutes, but it
was not evaporated in lefs than twenty. Thus it is
evident that the water had acquired 158° of heat in
the fpace of four minutes ; znd confequently, as the
heat of the fire continued the fame, it required five
times 158° of heat to convert it into vapour. This irn-
menfe acceffion of caloric is, however, neither fenfible
in the water nor in the vapour, for if a thermometer
is applied to the (team, it will not be found hotter than
the boiling water ; it is therefore really abforbed by the
fluid, which is converted into vapour, and is retained
in the latter in a combined ftatc. When the vapour
is condenfed in the refrigeratory of a {till, the latent or
combined fire is once more rendered fenfible, for the
refrigeratory is heated much higher than the fenfible
heat of the vapour, as the heat, if accumulated, would
raife the thermometer to more^than 800°.

We are informed by Dr. Crawford, that Dr. Black,
vho is certainly the author of the prefent theory of
heat and fluidity, f was firft led to the difcovery of the
abforption of heat by aqueous vapour, in confequence
of an unexpected appearance which occurred in :<:i

experiment

Chap. 5.] Dr. Crawford's Experiments on 'Boiling. 125

experiment made with water at a high temperature,
A quantity of that fluid having been raifed in Papin's
digefter * to a temperature many degrees above the
boiling point -J-, was fuffered to communicate with the
external air, by opening a ftop-cock, upon which a
part of it was inftantly converted into vapour, and the
water at the fame moment funk to 212 {.' As it ap-
peared however that only a very fmall quantity of the
water had been carried off by this fudden evapora-
tion, it was naturally concluded that the whole of the
fuperfluous fire which the water had previoufly im-
bibed was abforbed by that part which afTumed the
form of vapour.

The fact is accounted as eftablifhed by the fame
philofopher from the following experiment. If eight
pounds of the filings of iron at 212 are mixed with a
pound of water at 32, the temperature of the mixture
will be nearly I22j the iron will be cooled 90 de-
grees, and the water heated 90. But if eight pounds
of iron filings at 300 are mixed with a pound of wa-
ter at the boiling pointj the temperature of rhe mix-
ture will continue at 212, and a part of the water will
be fuddenly carried off in vapour, which vapour itfelf
will be found to retain the fame temperature of 212.
In this experiment the temperature of the iron is low-
ered 88 degrees, without any apparent accefllon of
heat to the water j the fair conclufion is therefore that
the fuperfluous caloric is abforbed and carried off by
the vapour ||.

Vapour is an elaftic fluid, that is, it admits of being
compreffed within a compafs proportioned to the force
1 ^

* Papin's digefter is an iron vefTel, made particularly ftron^.

*J- I have been told to 412.

} Crawford on Animal Heat, p. 77. || Ibid. p. 78.

which

116 Immerffe Force of Steam. [Book II.

which compreffcs it. Its force in refitting compref-
fion, when it is accumulated to a certain degree, is
however greater than that of gunpowder, or of any
power widi which we are acquainted. Steam is there-
fore one of the moft potent and mod dangerous agents
in nature. A fmall quantity of water thrown upon
boiling oil, or introduced among metals while in fu-
fion, produces the moft formidable effects. The wa-
ter finks towards the bottom in the oil, where being
converted into vapour, by the force of its expanfion it
caufes a moft violent ebullition and explofion, and
throws the heated fluid about with incredible velocity.
Hence in cafting iron or copper velTels, if the fmalleft
particle of humidity is contained in the mold, or if the
metal meets with any liquid in its paflage from the
furnace to the mold, it will be exploded with the ut-
moft hazard to the workmen from the burning metal.
We iiave an inftance recorded in the Philofophical
Tranfac~lions, of the burfting of one of Papin's digeft-
crs containing a pint of water, which demonftrates the
amazing expanfive force of vapour. The report on
the burfting of the veffel was heard at a confiderable
diftance by a maid fervant who was milking the cows
in an adjacent field, and the fervants within faid it
fhook the houfej the veflel flew in a direct line acrofs
the room, and Ihivered an inch plank of oak in pieces:
what is moft extraordinary, no traces of water were to
be found in any part of the room ; the fire was how-
ever completely extinguifhed *.

The force of the common fleam engine is well
known f ; and an inftance recorded by Mr. Jones in

his

* Phil. Tr. Abr. vol. viii. p. 465.

f In thefe machines, the fteam is conveyed into a large cylin-
der or barrel of iron, in which a very heavy pifton of the fame

metal

Chap. 5.] formidable Effe&s of Steam. 127

his Phyfiological Difquificions, ferves well to mark its
effects. * A workman, who with fome others was em-
ployed to repair a fleam -engine at Chelfta, informed
me, that as they were bufy about it in working it to
imderftand the defect, the barrel, which was of great
capacity, and too much worn with long ufe, burft on
a hidden, and a cloud of fteam rufhing out at the frac-
ture (truck one of the workmen who was (landing by,
and killed him in a moment like a blaft of lightning.
His fellows ran up as foon as they could to give him
alTiftance, but when they endeavoured to take off his
cloaths, the fiefh came away from the bones along with
them/

Though all fluids are rendered elaftic by fire, yet
the quantity of caloric neceffary to raife them to a ftate
of vapour depends upon different circumflances. The
very nature of an elaftic fluid renders it particularly
liable to be affected by the weight which is incumbent
upon it. All fluids therefore boil with a much lels
degree of heat in vacuo, than under the ordinary prei-
fure of the atmofphere. Thus water moderately heat-
ed, and placed in the receiver of an air-pump, may
be made to boil by withdrawing a part of the air by
which it is comprefied ; and it will be obferved to
ceafe as foon as the air is returned, and the preffurc

metal is raifed : when the pifton is to fall, the fteam is fuddenly
made to collapfe by the injection of fome cold water, which, im-
mediately condenfes it, and makes a vacuum, fo that the pifton is
forced down again by the preflure of the atmofphere. By thefe
alternate rifings and fallings of the pifton, feveral of which are
performed in the fpace of a minute, the machine afls on the work
of a forcing pump, by which the water is raifed, and difcharged at
the proper place. This machine, confidering the vaft force of it,
is one of the fimpleft in the world; but, like the digefter, may be-
come extremely dangerous if .-.ny accident fhould get
the power over it.

reftored.

i:3 , '.ling in Vacua. [Book II.

rcftored *. In the moil perfect vacuum ihat we are
able to procure water boils at 90, and fpirit of wine at
52, that is at 122 below the boiling point under the
common preffure of the atmofphcre.

A pleafmg experiment is related by that elegant and
ingenious philolbpher, the prefent Biihop of Landaf}^
which is fllufrrative of the nature of boiling in general,
and particularly of what has been juft advanced. With
an intention of exhibiting a linking inftance of the in-
creafe of dhnenfions produced by heat in fluids, he
took a glafs vefTel, not unlike a thermometer in form;
the bulb contained above a gallon, the ftem had a
fmall diameter, and was about two feet in length.
This veflel he' filled with boiling water to the very top
of the ftem, and corked it dole with a common cork.
The water and the cork were at firft contiguous, but
as the water cooled it contracted, and funk vifibly in
the ftem ; and thus the firft intention of the experiment
was anfwered.' But here an unexpected phenomenon
prefented itfelf. The water, though it was removed
from the fire, though it was growing cold, and had for
fome time entirely ceafed from boiling, began to boil
very violently. When a hot iron was applied to that
part of the ftem, through which the water in contract-
ing itfelf had defcended, the ebullition prefently ceafed ;
it was renewed when the iron was removed j and it
became more than ordinarily violent, when by the ap-
plication of a cloth dipped in cold water that part was
cooled. To account for thefe appearances, it is only
cecefiary to recollect, that by the finking of the water
in the ftem a kind of vacuum is left between its fur-
face and the ccrk; the water therefore necefiarily boils
with a lower degree of heat than it would- under the
preffure of the atmofphere. The fpace- between the

* Higgins's "Experiments and Obfervations, p. 313.

cork

Chap. 5.] Experiments of .Bijhop Watfon* &c. 129

cork and the water is not however a perfect vacuum j
it is occupied either by the vapour of the water, or by
a Frnall portion of air, or by both. Heat increafes the
elafticity both of air and vapour, and thus augments
the preffure upon the furface of the water, hence the
ebullition ceafes upon the application of the hot iron.
Cold, on the contrary, diminimes the elafticity of the
air, and condenfes vapour; and thus the preflure upon
the furface being leffened by the application of a cold
cloth, the ebullition of the water became more violent.
The heat of the water when it ceafed boiling was
130°*.

An experiment of another diftinguifhed philofopher
affords perhaps a better illustration of the whole theory
which has been juft advanced. This gentleman placed
a quantity of vitriolic ether under the receiver of an
air-pump, which was fo contrived that he was able to
let down a thermometer at pleafure, without admit-
ting the external air. He no fooner began to extract
the air, than the ether was thrown into a violent ebul-
lition, at the fame time its temperature funk furpriz-
ingly. When the ether was firft put in, its tempera-
ture was about 58% but it became fo cold when boil-
ing, that a quantity of water in a veflel contiguous to
it was fuddenly frozen. The manner in which thefe
phenomena may be explained is this : — The weight of
the atmofphere being removed, the heat which the
ether contained was fufficient to make it boil. The
elementary fire which the ether loft in boiling was diC-
pofed of in forming a vapour more fubtile than the
ether itfelf; which could not, confiftently with the
principles eftablifhed, be formed without the abforption
of a considerable 'quantity of the matter of fire. Now
as it appears that water and fpirit of wine boil in va^uo

• * Chem. Effays, vol. iii. p. 162.
VOL. I. K at

1 30 fPby Fluids loll in Vacuo with [Book II*

at 122° below their ordinary boiling point, it is natu-
ral that ether, which boils in the open air at about the
heat of the human blood, (hould boil in vacuo at 24°
below o, a degree of cold fufficient to freeze any wa-
ter that might happen to be in contact with the vefTel
which contains the ether.

As the weight of the atmofphere varies fome degrees
at different times, it is evident, from thefe remarks,
that the boiling point of fluids will alfo occafionally
vary. Boerhaave fuppofes, that, according to the
"changes of the atmofphere, as marked by the barome-
ter, the heat of boiling water may vary occafionally
eight or nine degrees * ; and we find the opinion cofl-
rirmed by the accurate experiments of M. de Luc and
Sir George Shuckburg, in the courfe of which the
boiling point was fometimes lower than 205°, and
fometimes higher than 2 1 3° f.

* Boerhaave Chem. quoted by Watfon, Chem, Efl*. vol.iii. p. 157.
t This will be better underftood by exhibiting Mr. Cavallo's
table of the refult of each experiment.

Height

Heat of boiling Water,

of the

according to

M. de Luc.

j
Sir G. Shuckburg.

Parts of a

Parts of a

Deg. deg.

Deg. deg.

26

205,17

204,91

261

206,07

205,82

27

206,96

206,73

27!

207,84

207,63

28

208,69

208,25

tti

209,55

209,41

29"

210,38

210,28

*9f

210,02

211,15

30

212

212

3°l

212,79

221,85

3i

213,57

213,69

Water boiling hot cools in fix hours in th« ordinary heat of the
actmcfphere.

Some

Chap. 5-j lefs Heat than in the Air. ijt

-Some of the phenomena of evaporation and boiling,
not hitherto noticed, wiJl receive a fatisfactory expla-
nation from this theory. — ift, If a fingle drop of wa-
ter is heated to the vapourifk point, it is immediately
converted into vapour; but if the quantity is more
confiderable, the phenomenon will be varied: for, if a
quantity of water is thrown into an iron veffel heated
very hot, it will feem to run about the veflel like quick-
ill ver, but without touching the bottom or fides of the
veffel. The reafon is, that the water nearefl the bot-
tom and fides is converted into vapour, which prevents
the fluid from coming in contact with the iron; and
this is the reafon alfo that a red-hot piece of iron drop-
ped into water continues for fome little time in the
fame red-hot (late, the water neareft the iron being
fuddenly converted into an elaftic vapour, which repels
jor keeps off the reft of the fluid.

2'dly, The bubbling and hitting of boiling fluids, or
of fluids upon the point of boiling, was unaccounted!
for till Dr. Black's theory elucidated the point. In
the common mode of boiling water it is plain, that the
bottom of the fluid arrives at the vapourific point of
heat before the furface; a quantity therefore of the
fluid which is neareft the bottom of the veffel is con-
verted into vapour, vi hich forcing its way through the
fuperincumbent medium occafions that violent ebulli-
tion which always takes place when a fluid is heated to
its vapourific point. The hiffing of kettles and other
veflels, previous to their arriving at the boiling point,
is perhaps to be accounted for from thefe vapourific
bubbles in their afcent meeting with the cold water,
and difcharging their caloric, which condenfes the va-
pour before it arrives at the furface, and occafions the
feeble found which has been juft mentioned, without
K 2 any

134 Why Water extinguifies Flame. [Book IL

any considerable agitation of the fluid, or emifllon of
fteam.

3dly, The rcafon why water, either hot or cold, im-
mediately extinguimes flame, will eafily be understood
from what has been premifed. When a quantity of
water is thrown upon an ignited body, it is immedi-
ately converted into vapour, and this procefs fuddenly
deprives the burning body of as much elementary fire
as the vapour is naturally difpofed to abforb. If
therefore the water is applied in fufficient quantity, the
whole of the caloric will be imbibed by the evaporation,
and the body will be left totally deftitute of heat. For
the fame reafon it is evident, that water will prevent
the melting of metals, or of any fubftances which re-
quire a degree of heat fuperior to the boiling point, to
render them fluid: for the water, being in contact with
the other fubftance, will not permit its temperature to
increafe, and all the fuperfluous fire which would heat
it above the boiling point, if the water was not there,
is carried off by the evaporation of that fluid *.

* Perhaps it may not be quite unacceptable to the reader to
notice in this place the vulgar paradox : " That when water is
boiling in a vcflel, the bottom is cool ; but the moment it ceafcs to
boil, the bottom becomes hotter." The whole of the paradox
appears to be founded on an error of the fenfe. When a perfon
applj.es his finger -to the vefiel, though he applies it for a confider-
able time, it is not heated more than he can endure, for the blood in
the courfe of its circulation lofes fome of its heat before it arrives
at the extremities ; and till the blood in the extremities is heated
to the fame degree with that of the heart we feel no pain from
burning ; but as icon as this is effefted, the leali degree of heat
becomes painful. When the finger is firft applied to the bottom
of the veffel after it is taken olF the fire, the heat is endured for
thefe rcafons. When the boiling ceafes, it is natural to take the
fame finger (for having dirtied one, people feldom chufe to take
another) ar.d that finder, being already heated alniott as much as
it could bear, now finds the heat at the bottoai of the veflel exqui-
fitely painful.

The

Chap. 5-] Subjtances which eafily evaporate. 133

The general procefs which bodies not highly in-
flammable undergo when fubjected to the a6Hon of
fire, is firft to be reduced to a fluid (late, and then to
a ftate of vapour. There are, however, fome matters
which are converted into vapour without at all afiiim-
ing the fluid form, fuch as camphor, fal ammoniac,
arfenic, &c. Thefe, when expofed to fire, fly, ofF in
vapour, without being melted j which vapour, on con-
denfation, becomes a folid mafs again. Thefe fub-
flances may therefore be faid to have their vapourific
point below that of their fluidity -, and the reafon of
this appears to be, that their particles have a ftronger
attraction for the matter of fire than for each other.
In fad, we find that thefe fubftances may be reduced
to a fluid form by confining them in clofe veflels,
•where they may be forced to endure a greater degree
of heat than under the preflure of the atmofphere.
Camphor at leaft has in this manner been rendered
fluid ; and there is no reafon why fal ammoniac, and
all the volatile alkalies, might not be reduced to the
fame ftate j but from the great elafticity of the vapour,
the procefs has not been completed for fear of burfting
the veflels.

There are fome bodies which have never hitherto
been reduced to a ftate of vapour. Thofe earthy fub-
ftances which have been rendered fluid, have never, by
any degree of heat, been rendered volatile, and there
are fome earths which have never been even brought
into fufion. Some of the metals, particularly gold
and filver, were thought formerly to be abfolutely fix-
ed. Mr. Boyle expofed a fmall quantity of each for
two months to the heat of a glafs-houfe furnace, and
at the end of that time he found them not altered ;
they have fince, however, been compelled to emit very
fenfible vapours by the more intenfe heat of a burning-
K 3 glafs:

1 3 4 V/byJme Fluids an permanently elaft'ic. [Book 1 1 .

glafs : and as the diamond itfelf has been fubjected to
evaporation, it is not improbable that by a fufficirnt
quantity of heat every other mineral fubflance might
be fufed and volatilized.

The vapour of water, and mod other fluids, requir-
ing a degree of heat above that of the atmofphere to
keep it in a volatile ftate, is eafily deprived of its ftiper-
fluous caloric ; and its particles being no longer kept
afunder by a fuperior force, yield to the ordinary im-
pulie of attraction, and are condenfed into the (late of
a. common fluid. There are, however, permanently
elaftic fluids, which are maintained in their claftic (late
by the ordinary heat of this earth ; and thefe by ana-
logy we may conclude are ccmpofed of particles which
have a weak attradtion for each other, and are there-
fore preferved in this rare and volatile ftate by a mo-
derate portion of the matter of fire interpofed between
them : of this kind is the air we breathe, and fome
other fluids, of which I fhall prefently have occafion to
treat *.

It would be improper to difmifs this fubject, with.
out offering a few remarks on that fpecies of evapora-
tion which is termed fpontaneous, as I apprehend it
not to be efTentially different from that of which we
have been treating. It is evident that a quantity of
humidity is continually and infenfibly emitted by every
body which contains any principle of moiilure. If a
glafs of cold water, or any cold body, fuch as fmooth
marble, is expofed in a room when many people are
afiembled, its outfide will be covered with dew ; the
•walls of churches and other buildings which have not
conftant fires are covered with moifture in the fame

* As the ordinary heat of our atmofphere is fufficient to keep
water and fome other fubilances fluid ; fo it ferves to keep thefe
bodies in a volatile itate.

manner,

Chap.5«] Valour ccndenjed on cold Surfaces. 13 1

manner, and this moifcure in both cafes is produced
by the cold body (the glafs of water, the marble, or
the wall) condenfing the vapour with which the air is
charged from the breath and perfpiration of the com-
pany. Similar to this is the dew on the infide of win-
dows, which in cold weather is frequently frozen, and
affumes the forms of leaves, of trees, and of the moil
beautiful mofl~es»

Water cannot be expofed in open vefiHs without
fuffering a diminution of its bulk, and indeed in courfe
of time the whole will be exhaled. The vapour how-
ever which is thus formed is not fufficiently elaftic to
produce any of the common effects of vapour; for
water will remain in bottles corked up without forcing
the corks ; the vapour ftagnating over the furface of
the water, prevents a frefh quantity from riling : in-
deed the mere force of gravitation would in general
be fufficient to counteract the force of this fpontaneous
evaporation, was it not that the wind carries off the
quantity which h exhaled, which would otherwife be
fufpended and ftagnate over the fluid. This vapour,
it is to be obferved, proceeds always from the furface
of bodies ; and the greater the extent of furface, the
more copious the exhalation : it is obferved, therefore,
to rife more copioufly from a graffy plain, from the
pores of a fpunge, or from loofe earth, than from any
fingle furface; and it is always more or lefs in quantity,
according to the temperature of the atmofphere.

The quantity of vapour which is raifcd in this man-
ner from the earth, has been efti mated by a very fimple
yet apparently accurate experiment of the Bifliop of
Landaff. Having provided a large drinking glafs, the
area of the mouth of which was twenty fquare inches,
he placed it with its mouth downwards on a grafs plat
which was mown clofe. The fun fhone bright and
K 4 hot,

i^6 Experiments on Evapdratiw. [Book II.

hot, and there had been no rain for upwards of a month.
When the glafs had flood on the grafs plat one quar-
ter of an hour, and had collected a quantity of con-
denfed vapour, he wiped its infide with a piece of
muflin, the weight of which he had previoufly afcer-
tained, and as foon as the glafs was wiped dry the
muflin was weighed. The medium increafe of weight
from various experiments, between twelve and three
o'clock, was fix grains in one quarter of an hour from
twenty fquare inches of earth. At this rate of evapo-
ration, it is eafy to fee that, computing at feven thou-
fand grains troy -to one pint of water, and eight pints
to a gallon, not lefs than one thoufand fix hundred
gallons of water would be raifed from one acre of
ground in twenty-four hours. It may well be fuppofed
thar the quantity will be (till greater when the ground
has been drenched with rain. In order to prove this,
the fame judicious philofopher made two other expe-
riments, one of them the day after the ground had
been wetted by a thunder-mower; and to afcertain
the circumftances more exactly, he took the heat of the
earth by a thermometer laid on the grafs, which in the
firft experiment was 96', when the evaporation was at
the rate of one thoufand nine hundred and ieventy-
three gallons from an acre in twelve hours. The other
experiment was made when there had been no rain for
a week, and when the heat of the earth was 1 10°: this
experiment gave after the rate of two thoufand eight
hundred gallons from an acre in twelve hours; the
earth was hotter than the air, being expofed to the re-
flexion of the fun's rays from a brick wall *.

It is the vapour which is exhaled in this manner
from the earth, which forms thofe mifts fo commonly

* Watfon's Chera. E%s, vol. iii. Eff. 2, '

obferved

Chap. 5.] Mijts, Dews, &c. 137

obferved in marfhy grounds. If a hole is broken in
ice, we may obferve a mift rife from it j the water be-
ing warmer than the air, emits a vapour, which the
cold condenfes and renders vifible. It is the fame va-
pour which, when condenfed by the cold of the night,
forms the dew which is obferved in fmall globules,
like pearls, upon the leaves of plants *. When the
weather has been intenfely cold, if a thaw fuddenly
comes on, the walls of houfes are all covered with
dew ; for thefe thawing winds, coming from a hotter
part of the world, bring with them a quantity of va-
pour, which is condenfed by the cold fubftances, when
it comes into a more northern climate.

Various theories have been propofed to account for
the afcent of vapour. One of the moft plaufible is
that which attributes it to the attraction of the air; but
this theory is in a great meafure deftroyed, by the

* This beautiful appearance has not efcaped the poets; the
Hebrew poets in particular have made the belt advantage of the
beauties of nature. The following is Buchanan's paraphrafe of a
part of the cxxxiii Pfalin.

" Ut aura fuavis balfami, quum funditur

Aaronis in facrum caput,
Et imbre lano proluens barbam & finus

Limbum pererrat aureum:
Ut ros, tenella gemmulif argentiis

Pingens Sionis gramina ;
Aut verna dulci inebrians uligine

Hermonis intonfi juga."

Sweet as the od'rous balfam pour'd

On Aaron's facred head ;
Which o'er his beard and down his bread

A breathing fragrance flied.
As morning dew on Sion's mount

Beams forth a Jther ray ;
Or Jludt nvith gems the verdant pomp

That Hermon's tops difplay.

ccnfideracio-

J 3 5 JJieut of y<-pour. [Book II.

Confideration that vapour will afcend in vacuo. The
electric fire has alfo been called in to account for this
phenomenon j but if the electric fire is no other than
common fire, or fome particular modification of it>
there is no reafon to believe that the fpontaneous
evaporation depends upon any other principle than
that which has been already ftated as the caufe of the
formation of common vapour from boiling water.

The fact appears to be, that there exifts fuch an
attraction between the particles of caloric and thofe of
water, that whenever ji portion of the former in a dif-
engaged ftate meets with any of the latter, they mime"
diately unite. Hence, when water is heated beyond
the temperature of the armofpherc, it naturally yields
up a quantity of its fuperfluous fire to reftore the equi-
librium, and" this fire always carries with it a quantity
of the fluid medium in the form of vapour. When
ihcfe vapours firft afcend, they are in an invilible
{late ; and they muft be in fome degree condenicd to
enable them to reflect the folar rays, ib as to become
vifible. This frequently takes place when they reach
the higher and colder regions of the atmofphere, or if
they happen to meet with cold winds in their progrefs
thither : they then appear to us in the form of clouds.
A ftill greater degree of cone1.?- n&tion renders them too
heavy to be ilipported by the atmofphere, and they fall
down in the form of rain, fnow, or hail, according to.
the circumftances of their difiblution.

Agreeable to this theory is the common obfrrva-
tion, that in very cold weather vapour becomes vifi-
ble almoft as foon as it is formed : thus in froil the
breath, which is vapour from the lungs, is always vi-
fible. M. de Maupertuis faw in Lapland the warm
vapour of a room converted into fnow upon opening
the door to the external air j and in a crowded afTem-

My

Chap. 5.] Phenomena of Valour. . ij^

biy at Peterfburgh, the company fuffering from the
clofenefs of the room, a gentleman broke a window
for relief; the confequence of which was, that the
cold air ruining in, caufed a vifible circum agitation of
a white fnowy fubitance *.

Agreeably alfo to the fame principles it is evident,
that air is a fluid which has a fironger attraction for
the matter of fire than water, but that by means of
this infenfible evaporation the interfaces of the air, if
I may Ib exprefs myfelf, are filled with a quantity of
vapour, which being extremely rare, and being equally
difFufed, is invifible to us. If, however, a ftream of
cold air is introduced from any quarter, the caloric,
which is united with the water in the form of vapour,
will flow into the cold air to reftore the equilibrium,
and the vapour will be condenfed. The condenfa-
tion will fometimes be only fufficient 'to produce the
appearance of clouds, but at fome times it will be fuf-
ficient to caufe rain, which will fall in greater or lefler
quantities in proportion to the quantity of moifture in
the atmofphere, and the degree of cold in the con*
denfmg medium.

* Edin. Phil. Tr. Vol. I. p. 48.
Like words congcal'd in northern air."

HUDIBRAS.

[ 1 40 ] [Book IL

CHAP. VI,
OF IGNITION AND'COMBUSTION.

Jgnition, ivhat ; hew produced,— Bunting of Phofphorus.— Inflamma-
ble Air. — Culinary Fires. — Lamps, &c. — //7y Flame a/cex-s. —
y^eory of Argand's Lamp.—Beji Form of Grates, Stoves, t>V.—
C.cmbuftion produced by fame Subftanccs without a Communication,
n'jith the Atmofpbcre. — Gun-powder, lff<r. — Iron made to burn like
a Candle. — Spontaneous Ignition. — Curious Fatfs. — Quantity of
Heat excited infuf.ng different Bodies. — Scale of 'Heat '.

IGNITION is that ftate of bodies in which the
matter of fire or caloric is rendered active, and
obvious to the fight, by the emiilion of light; in other
words, when both light and heat are at once emitted
by any body, it is faid to be ignited. Ignition does
not imply combuftion, as the latter indicates a change
or diflblution of texture in the inflammable body j
whereas fome of the metals, and many of the earths,
may be ignited without being confumed. — But this
is a fubject which it will be neceifary to treat more
at large, when I come to fpeak of inflammable fub-
flances.

In all cafes of ignition or inflammation the matter
of fire is 'detached from fome body in which it pre-
vioufly exiiled in a latent ftate. The fubftance with
which fire is moft copioufly combined, and from
which it is moft eafily detached, is air. If therefore
any fubftance can be found in nature which has a
greater attraction for the bafis of air than that has for
the matter of fire, the union of thefc two fubftances
will detach a quantity of the fire which had exifted in a
latent or combined ftate \ and ignition, and combuf-
tion,

Chap. 6.] Accenfan of Pkojptorus. 141

tion, will be produced. Of this nature is the matter
of phofphorus, and that very common or almoft uni-
verfal fubftance, which is diftinguiihed by the name of
hydrogen, or inflammable air. Thus if a quantity of
phofphorus is expofed to the atmofphere, it will abforb
a confiderable quantity of the pure part of the air, and
by the union be converted into phofphoric acid. In
the mean time the caloric will be detached from the
air i and, provided the air is well charged with heat,
ignition or accenfion will be produced on the farface
of the phofphorus. Inflammable air (or bodies con-
taining that principle) has, however, either a weaker
attraction for air than phofphorus has, or its particles
have a flronger attraction for each other. It is ne-
cefiary therefore that it fhould be prefented to the air
in a rarer ftate ; and a degree of internal agitation, and
even a third attractive power, are required to effect
the union of the two fubflances, and the detaching of
the fire from them. Thus, for inftance, if pure air is
mixed with a quantity of inflammable air, the electric
fpark, or a fmall quantity of fire in fome form or other,
muft be introduced to effect their accenfion. In this
cafe a double attraction takes place. The pure and
inflammable airs unite together, or are condenfed into
a fluid, and the matter of fire, which is introduced,
carries with it the fire which is detached from the two
airs, and thus a complete ignition is produced. In
the ordinary procefs of burning, when a quantity of
inflammable or combuftible fubftances are heaped to-
gether, and fire introduced among them, by the ac-
tion of the fire the inflammable part is firft expanded
from its folid ftate into a ftate of inflammable vapour,
it corhes neceflarily into contact with the pure air of
ihe atmoiphere, and the action of the fire ftill conti-
nuing;

qjcends. [Book II.

nuing, the matter of fire is detached from the air, and
combuftion enfues in the fame manner as in the for-
mer cafe, when the pure and inflammable airs are fired
by the electric fpark.

Hence there can bs no combuftion without a fup-
ply of pure air ; and from this confiderction moft of
the phenomena of combuftion may be explained. In
a common coal fire, if the coals cake or adhere in fuch
a manner that the inflammable part cannot come in
contact with the external air, the fire is neceflarily ex-
tinguifhed. Flame is ignited vapour; but as that
part only which comes in contact with the air can be
ignited, that part of the inflammable vapour, which is
not confumed, takes the form of fmoke and foot, and
adheres to the fide or top of the place or veflel which
contains the fire. The flame of a common lamp or
candle may be confidered as a tube of fire, in the
hollow of which the inflammable vapour is inclofed.
It afiiimes a conical form, in confequence of the gra-
dual confumption of the vapour, which is leflfened in
quantity as it rifes, and 'confequently is contracted in
its dimenfions. A confiderable quantity of the va-
pour, however, ftill efcapes in the form of fmoke, as
muft be evident to any obferver, and as is decidedly
evinced by holding a paper, or any other covering,
over the flame, in which cafe a quantity of foot will
prefently be collected.

•If thefe principles are clearly underftood, it will no
longer be a fubject of wonder that all flame naturally
afcends. Vapour is confiderably lighter than air, and
flame is no other than ignited vapour. Thus in light-
ing a common lamp or candle at an ignited bar of
iron, or any other burning body, the wick of the
candle muft be applied to the lower furface of the
ignited body, and not held above it, becaufe then the

vapour,

] Argand's Lamp. 143

vapour,, which the heat extracts from the candle, cocncs
in contact with the burning body, and accenfion takes
place* It fometimes indeed may happen, that the
lamp or candle may be lighted by holding it above
the ignited bar ; but it is obvious, in that cafe, that a.
quantity of the oil or tallow firlt drops on the burn-
ing body, and is then converted into vapour and
flame, fo that at the time of the accenfion taking
place, the wick is actually furroundcd with flame,
above as well as below.

To remedy the immenfe wafte of oil, which, ac-
cording to the common 'conftruction of lamps, was
difpofcd of, unconfumed, in the form of fmoke anc}
foot, was the great object of that very ingenious in-
vention, the patent lamp of M. Argand. I recollect,
ibme years previous to M. Argand's invention, I turn-
ed my attention to the procuring of a vivid flame,
without a wafte of oil, or the offenfivenefs of fmoke.
I obferved that, the fmaller the wick of a lamp, the
brighter in general was the flame, and for this plain
reafbn, that in thefe cafes a greater furface than ufual,
proportionably to the quantity of vapour, was expofed
to the air. My fcherne was, therefore, to procure a
lamp with a number of very fmall wicks, between each
of which there was to have been an orifice or chim-
ney, which might introduce a current of air, and keep
the flajne proceeding from each wick diftinct. M-.
Argand's, I muft confefs, is a great improvement upon
this idea. By means of a thin circular wick, through
the middle of which a current of air is introduced by
a funnel, he produces a very thin flame, and confe-
quently e'xpofes a very large furface of the oily va-
pour to the contact of the air. As there is, however,
a ftrong attraction between the particles of fire, there
would be danger of the flame uniting from all the fides
of the lamp, at a certain height above the funnel, and

fc

144 Common Fires. [Book IL

fo forming a conical flame like that of a common
candle, was it not that this effect is prevented by a
tube of glafs, with which he furrounds the flame,
•which, when warmed, counteracts the attraction, which
the different fides of the circular flame would have
for each other, and fo preferves the current of air
free and without interruption.

The fame principles will apply to the conftrudtion
of common fires. The great object mould be, to ex-
pofe as large a furface of the fuel as pofilble to the
air, or rather, if poffible, to introduce a ftrong and
difTufed current of air through the fire. On this prin-
ciple the air furnace is conftructed. It is well known
that thefe furnaces confift of an aperture or afh-hole
under the fire, and a high vent, funnel, or chimney
above, and that the door, by which the fuel is intro-
duced, is kept clofed, unlefs it Ihould be occafionally
opened for the purpofe of diminifhing the heat. By
means, therefore, of the high vent or funnel, the air
above is rarefied and rendered lighter, and confe-
quently the air below prefTes in through the am pit j
and if the bottom of the grate is kept clear, a ftrong
current of air circulates directly through the fire, and
fupplies it conftantly with this neceffary ingredient.
On fimilar principles die regifter ftoves are conftruct-
cd. In thefe the vent or chimney coming very near
the grate, the air below it is forced through the fuel,
and by enlarging or contracting the vent, the current
of air is increafed or diminifhed. The bath and pan-
theon ftoves -are alfo found to produce more heat in
proportion to the quantity of fuel than common open
grates, becaufe they are ufually fet high, and the afh-
pit or aperture below being clofed on three fides, a
confiderable part of the air which enters it is forced
up through the fire. The fame reafons will alfo fatis-
factorily account for the effects of a common bellows,

winch

thap. 6.J

which brings a fupply of pure air to unite with the
inflammable matter contained in the coal.

The inflammable and pure air, which are apparently
confumed by the procefs of combuftion, are in reality,
by their union, converted into water, as will be evinced
by fixing an alembic head, or any good recipient, to
the top of Mr. Argand's lamp, in which a quantity of
water will prefently be found, but no foot.

There are, however, certain fubilances, fbch as
gunpowder, &c. which will burn with a very fmall
fupply of air, as when included within the barrel of a
gun, and cloiely wadded. To explain this, it mufc be
premifed, that nitre, or fome of the ingredients of fuch
compofitions, contains a large quantity of the bafis of
pure air : and it mud be remembered, that air confifts
of a certain matter or bafis, which is expanded by a
union with the matter or element of fire, but which is
alfo capable of exifting in a more condenfed ftate.
Let it alfo be remembered, that though air, which is
a compound body, will not penetrate metallic fub-
ftanees, yet the element of fire, or caloric, will pene-
trate them, or any other fubftance with which we are
acquainted. One of the ingredients of air, therefore,
is contained in the nitre or gunpowder, and the other
(the fire or caloric) cannot be excluded. When there-
fore the matter or elementary part of the air, is fet
free from the nitre by accenfion, it immediately meets
with the matter of heat or fire, and becomes embodied
into the form of air, and thus an adual fupply of that
material is generated, though the air of the atmofphere
h nearly excluded *.

According

* Gunpowder is a mixture, which in an hundred parts contains
about 75 of nitre, 9! of fulphur, and 1 5f of charcoal. The effects
»f gunpowder depend on the fudden production of a quantity of

VOL. L L »ir>

146 Bodies more or lefs difpofedto Ignition. [Book II.

According to the different properties of bodies,
they are more or lefs difpofed to ignition. Iron is
ignited with great difficulty ; on the contrary, not

only

air, which takes place from the decompofition of the nitre. Nitre
is compofed of a fixed alkali and of nitrous acid, which is itfelf a
compound of the bafes of azote and oxygen. The ingredient firft
inflamed is the fulphur, which fets fire to the charcoal. The nitre
is equally difperfed among all the particles of combuftible matter,
and as its quantity is by much the greateft, each particle of fulphur
and charcoal is furrounded with nitre. When combuftion, there-
fore, is once excited in the mafs, the oxygen afforded by the nitre
carries it on with great rapidity. The oxygen, being withdrawn
from the azote, caufes it to aflame an aeriform ftate, and by being
attracted by the charcoal converts the latter into fixable air. The
fulphur alfo attracting fome of the oxygen, but not fufficient to
reduce it to the ftate of a fluid, is partly converted into volatile
vitriolic acid, the fmell of which is very perceptible. The gun-
powder, therefore, is in an inftant converted into three kinds of
air, which occupy the fpace of the folid matter. What remains
after combuftion is a liver of fulphur formed by the union of fome
portion of that fubftance with the fixed alkali of the nitre.

The efte&s, however, of this mixture of nitre, fulphur, and coal,
are trifling, in companion with thofe of another preparation called
fulminating powder, This is made by triturating in a hot marble
mortar, with a wooden peftle, three ounces of nitre, two ounces of
very dry fixed fait of tartar, and one ounce of flowers of fulphur,
till the whole is very accurately mixed. If a drachm of this pow-
powder is expofed to a gentle heat, in an iron ladle, it melts, and
foon after produces a detonation as loud as the report of a cannon.'
* This phenomenon, which is fo much the more aftonifhing, be-
caufe its efteft is produced without inclofmg the powder in any in-
ftrument, as is done with gunpowder, may be explained, by ob-
fcrving, I. That it does not fucceed, but by gradually heating ths
mixture, fo as to melt it. 2. That if fulminating powder is
thrown on ignited charcoal, it only detonates like nitre, but with
very little ncife. 3. That a mixture of liver of fulphur with nitre,
in the proportion of one part of the former, and two of the lat-
ter, fulminates with more rapidity, and produces as loud a report
as the composition of fulphur, nitre, and alkali : hence it appears,
that when fulminating powder is heated, liver of fulphur is formed
* before

Chap. 6.] Comluftion of Iron. 147

only the pholphorus of K.unkei, but the pyrophori,
which are made of three parts of flour, or any vege-
table matter convertible into charcoal, and one of alum,
will immediately ignite, on being expofed to the air
in the ordinary heat of our atmofphere. In this cafe
the pyrophorus, which is of a light fpongy texture,
prefents a large furface containing a quantity of in-
flammable matter to the atmofphere, and the union
of the two fubftances immediately fucceeding, the
matter of fire is emitted, and ignition takes place.

Every means, indeed, by which pure air may be
attracted and condenfed, will produce flame. It was
obferved in the preceding paragraph, that iron was
ignited with difficulty ; yet if a very fmall iron wire is

before the detonation takes place ; and this faft is fufficient to ex-
plain the whole appearance. When cryftallized nitre and liver of
fulphur are expofed to the adYion of heat, inflammable or hepatic
gas is difengaged from the latter, while the fait gives out vital
air. Now thefe two, which together are capable of producing a
ftrong inflammation, as we have obferved in the hiflory of inflam-
mable gas, are fet on fire by a portion of the fulphur. But as the
thick fluid they are obliged to pafs through prefents a oonfiderable
obftacle, and as the whole takes fire at the fame inftant, they ftrike
the air with fuch rapidity, that it refifts in the fame manner as the
chamber of a muiquet refills the expanfiort of gunpowder. A
proof of this is obfervable in the effecl: the fulminating powder has
on the ladle in which it explodes. The bottom of this veflel is
bulged outwards, and the fides bent inwards, in the fame manner
as if it had been a&ed on by a force directed perpendicularly
downwards, and laterally inwards.' — Fourcrqy's Chem. v. ii. p. 388.
Gold precipitated from its folution in aqua regia by means of
rolatile alkali, conftitutes a fubftance called fulminating gold, the
effe&s of which are flill more tremendous than thofe of the pre-
ceding compofitions. An extremely fmall portion of it is fuf?.-
cient to produce alarming, and even fatal effedls ; and what ren-
ders it ftill more dangerous is, that a mere blow, or a flight degree
of fridion, are furacient to ignite it. With refpeft to the caufe of
the explofive power of this fubftance, it will be explained when I
treat of gold itfelf.

L 2 conveyed

148 Heat emitted by Condtnfaticn of Fluids. [Book II.

conveyed into a clofe bottle filled with this air, with a
fmall piece of tinder or any combiiftible matter upon
it, to which fire has been communicated, the wire will
be obferved to burn, after the other combuftible mat-
ter is confumed, with a clear and bright flame, and if
there is a large quantity of pure air, the whole of the
iron will be converted into a calx.

It has been amply demonftrated, that the conden-
fation, not only of pure air, but of every fluid, is at-
tended with the emiffion of heat or elementary fire ;
and even the partial condenfation of a fluid, or the
reduction of it from a rarer to a denfer ftate, will
produce the fame effe<5b. Thus air and vapour are
rarer fluids than water, and their condenfation into
water always produces fcnfible heatj thus fixable air
is a denfer fluid than atmofpherical or pure airj.
and when a quantity of the latter is by any procefs
converted into the former, a quantity of fuperfluous
caloric is confequently emitted. This is the cafe in
all fermenting bodies, which abforb a large quantity of
the pure ai-r of the atmofphere, and emit that denfe
acid fluid, which always hangs over their furface like a
vapour, and is univerfally known by the name of fix-
able air: and this procefs is always attended with heat,
that is, with the feparation of a quantity of elementary
fire. The accenfion or ignition of the mafs depends,
however, on the fpeedy emiiTion of the matter of fire,
that is, upon the violence of attraction between the
two fubftances which occafipns the condenfation.
When fulphur, iron -filings, and water, are mixed to-
gether snd kneaded into a pafte, the air is rapidly at-
tra&ed, and the mafs becomes fo hot as to take
fire. Play-ricks and other fermenting mafTes are fre-
quently fired by this kind of fpontaneous procefs,

If

Chap. £.] Spontaneous Inflammation.. 149

If one of the fub fiances contains a large quantity of
the bafis of pure air, and a- ftrong general attraction
e.y.ifts between the fubftances, a fimilar effect will enfua
Thus, if aqua fords, or ftrong nitrous acid, is poured
upon oil of turpentine, the attraction between the in-
flammable part of the oil and the pure air, which the
nitrous acid contains in abundance, will be fo violent,
that the whole will be inftantaneoufly converted into
flame. The fame effect is produced from a mixture
of black wad (an ore of manganefe, containing much
oxygen or pure air) with common linfeed oil. If a
•quantity of nitrated copper allb, or rhe fak which is
formed by the folution of copper in the nitrous acid,
is moiftened, and inclofed in a piece of tin-foil, the fait
melts or deliquefces, nitrous fumes are emitted, and
the mafs fuddenly burfts into a flame. This effK. is
undoubtedly occafioned by the ftrong attraction of the
tin for the nitrous acid, by which the fire is extricated
in fb rapid a manner as to produce inflammation.

The effects of fpontaneous inflammation are chiefly
feen in the mineral world ; and to this caufe is to be
attributed a variety of the mod formidable phenomena
of nature, fuch as volcanoes, earthquakes, .&c.

M. Lavoifier defcribes an apparatus for afcertaining
the quantities pf heat extricated during the combuftion
of different fubftances. This contrivance refL on the
propofition, that when a body is burnt in the center
of a hollow fphere of ice, and fupplied with air at the
temperature of -32°, the quantity of ice melted from
the infide of the fphere becomes a meafure of the re-
lative quantities of heat difengaged. With this appa-
ratus, phofphorns, charcoal, and hydrogen gas, gave
the following refults :

One pound of phofphorus melted one hundred pounds
pf ice.

L One

150 Heat extri-cated in Combuftion. [Book II.

One pound of charcoal melted ninety-fix pounds,
eight ounces.

One pound of hydrogen gas melted 295105. 9 ounces,
3* drams.

As a concrete acid is formed by the combuftion of
phofphorus, it is probable that very little heat or ca-
loric remains in the acid, and, confequently, that the
above experiment gives us very nearly the whole quan-
tity of elementary fire contained in the oxygen gas.

M. Lavoifier had found, by a former experiment,
that one pound of phofphorus abforbs one pound eight
ounces of oxygen during combuftion j and fince, by
the fame operation, one hundred pounds of ice are
melted, it follows, that the quantity of caloric con-
tained in one pound of oxygen gas is capable of melt-
ing 66 Ibs. 10 ounces, 5 drams, 24 grains of ice. By
the aid of this fimple contrivance, M. Lavoifier has
been able to afccrtain, with apparent accuracy, the
quantity of caloric difengaged in mod of the common
proceffes of combuftion *.

* ' From the combuftion of phofphorus, as related in the fore-
going experiments, it appears, that one pound of phofphorus re-
quires i Ib. 8 oz. of oxygen gas for its combuftion, and that z Ibs.
9 oz. of concrete phofphoric acid are produced.

' The quantity of caloric difengaged by the combuftion of one
pound of phofphorus, exprefled by the number of pounds of ice
melted during that operation, is 100.00000.

The quantity diiengaged from each pound of oxygen, during
the combuftion of phofphorus, exprefled ,4n the fame manner,
is - - ... •••'-• 66.66667.

The quantity difengaged during the formation of one pound of
phofphoric acid, 40.00000. The quantity remaining in each
pound of phofp'.ioric acid - - - o'.ooooo *.

* We rjere fuppofe the phofphoric acid not to contain any calo-
ric, which is not flnftly true ; but, as I have before obferved, the
quantity it really contains is probably very fmall, and we have not
given it a value, for want of a fufficient data to go upon.

In

Chap. 6.] Experiments on Combuftion. 15 1

In the combuftion of one pound of charcoal, 2 Ibs. 9 oz. \ gros,
10 grs. of oxygen gas are abforbed, and 3 Ibs. 9 oz. i gros, logrs.
of carbonic acid gas are formed.

Caloric, difengaged during the combuftion of one pound of char-
coal - - - - -- - - 96.50000 f .

Caloric difengaged during the combuftion of charcoal, from each
pound of oxygen gas abforbed - - 37.52823.

Caloric difengaged during the formation of one pound of car-
bonic acid gas - -v- - - - 27.02024:

Caloric retained by each pound of oxygen after the combuf-
tion ------_. 29.13844.

Caloric neceffary for fupporting one pound of carbonic acid in
theftateofgas - - - - -• • • - 20.97969.

In the combuftion of one pound of hydrogen gas, 5 Ibs. iooz.
5 gros, 24 grs. of oxygen gas are abforbed, and 6 Ibs. looz.
5 gros, 24 grs. of water are formed.

Caloric from each Ib. of hydrogen gas ... 295.58950.

Caloric from each Ib. of oxygen gas - - - 52.16280.

Caloric difengaged during the formation of each Ib.

of water 44.33840.

Caloric retained by each Ib. of oxygen after com-
buftion with hydrogen -------- 14.50386,

Caloric retained by each Ib. of water at the tempe-
rature of Zero (32°) -------- 12.32823,

' When we combine nitrous gas with oxygen gas, fo as to form
nitric or nitrous acid, a degree of heat is produced, which is much
lefs confiderable than what is evolved during the other combina-
tions of oxygen ; whence it follows that oxygen, when it becomes
fixed in nitric acid, retains a great part of th'e heat which it pof-
feffed in the ftate of gas. It is certainly poffible to determine the
quantity of caloric which is difer.gaged during the combination of
thefe two gaffes, and confequcntly to determine what quantity re-
mains after the combination takes place. The firft of thefe quan-
tities might be afcertained, by making the combination of the, two
gaffes in an apparatus furrounded by ice ; but, as the quantity of
caloric difengaged is very inconfiderable, it would be neceffary to
operate upon a large quantity of the two gaffes in a very trouble-

f All thefe relative quantities of caloric ,are exprefled by the
number of pounds of ice, and decimal parts, melted during the fe-
veral operations,

L A. feme

X$2 Heat extricated in Combtiftion. [Book I! ,

fome and complicated apparatus. By this confideration, Mr. de la
Place and I have hitherto been prevented frpm making the at-
tempt. In the mean time, the place of fuch an experiment may
be fupplied by calculations, the refults of which cannot be very far
from truth.

'. Mr. de la Place and I deflagrated a convenient quantity of
uitre and charcoal in an ice apparatus, and found that twelve
pounds of ice were melted by the deflagration of one pound of
i-.itre. We (hall fee, in the fequel, that one pound of nitre is cora-
pofed, as under, of

Pot-alh 7 oz. 6 gros 51. 84 grs. =

Dry acid 8 i 21.16 =±= 4700.16.

The above quantity of dry acid is compofed of
Oxygen 6 cz. $ gros 66.34 grs. — 3738.34 grs.
Azote i 5 25.82 = 961.82.

* By this we find that, during the above deflagration, 2 gros i \. gr
of charcoal have fuffered combuiHon, along with 37.38.34 grs. or
6 oz. 3 gros, 66.34 grs. of ozygen. Hence, iince 12 Ibs. of ice
were melted during the combuftion, it follows, that one pound of
oxygen burnt in the fame manner would have melted 29.58320 Ibs.
of ice. To which the quantity of caloric, retained by a pound of
oxygen after combining with charcoal to form carbonic acid gas,
being added, which was already afcertained to be capable of melt-
ing 29.13844^3. of ice, we have for the total quantity of caloric
remaining in a pound of oxygen, when combined with nitrous gas
in the nitric acid 58.72164; which is the number of pounds of ice
the caloric remaining in. the oxygen in that ftate is capable of
melting.

' We have before feen that, in the ftate of oxygen gas, it con-
tained at leaft 66.66667 ; wherefore it follows that, in combining
with azote to form nitric acid, it only Icfes 7.94502. Farther ex-
periments upon this fubjeci are neceflar.y to ascertain how far the
refults of this calculation may agree with direft faft. This enor-
mous quantity of caloric retained by oxygen in its combination
into nitric acid, explains the caufe of the great difengagement of
caloric during the deflagrations of nitre; or, more ftriftly fpeaking,
upon all occafions of the decompofition of nitric acid.

' Having examined fcveral cafes of fimple combuftion, I mean
now to give a few examples of a more complex nature. One pound
of wax-taper being allowed to burn ilowly in an ice apparatus,
melted 133 Ibs. 2 oz. 53 grps of ice. According to my experi-

ments

Chap. 6.] AScale efEeah 153

jnents in the Memoirs of the Academy for 17 $4, p. 606, one pound
pf wax- taper confifts of 13 92. i gros, 23 grs. of charcoal, and
2 oz. 6 gros, 49 grs. of hydrogen.

f By the foregoing experiments, the above

quantity of charcoal ought to melt - - 79-39390 Ibs. of ice;
and the hydrogen mould melt - 52.37605

In all 131.76995 Ibs.

* Thus, we fee the quantity of caloric difengaged, from a burn-
ing taper, is pretty exactly conformable to what was obtained by
burning fe parately a quantity of charcoal and hydjrogen, equal to
what enters into its compofition. Thefe experiments with the ta-
per were feveral times repeated, fo that I have reafon to believe
them accurate.' Lav oifieS s Chemijlry.

A SCALE OF HEAT.

The firft part of this table is taken from Mr. Wedgwood's fcale,
according to his clay pyrometer, the reft is by Fahrenheit's
fcale.

• Fahr.
Extremity of the fcale of Mr. Wedgwood's ther7 "

mcmeter — — 32277° 240°

Greateft heat of his final 1 air furnace — — 21877 J6°

Caft iron melts — — 17977 *3°

Greateft heat of a common fmith's forge — 1/327 125

Welding heat of iron, greateft — — *3427 95

- - ..... , leaft — — 12777 90

Fine gold melts — - — 5237 32

Fine filver melts — . •— — 4717 28

Swedifh copper melts — — 4587 27

Brafs melts — — — — 3807 21
Heat by which his enamel colours are burnt

on — — — 1857 6

Fahr.

Iron with a white fparkling heat — — 2780°

Iron with a heat almoft white — — — 2080

The heat of live coals without blowing, perhaps about 1650

Jron with a glowing red by day-light — — 1600

*54 Scale of Hgat. [Book IT.

Iron juft red-hot by day. light — _» T ^^

Iron juft red-hot in the dark — — _ Iooo

Greateit heat of lead in fufion — g2O

Colours of iron arc burned oft* -— _ goo

Mercury boih, by fome placed at 600 — — 700

Polifhed iron takes \f\ill blue — ^oo

Polifhed iron takes a purple — — . . — 66O

Linfeed oil boils, by fome at 600 — 620

Lead melts — — .— — 610

Polifhed iron takes a ftraw colour — 60^

Oil of vitriol boils — — — 5^5

Brafs takes a blue colour — — — CQO

Bifmuth melts — — — — A£Q

Tin- foil and bifmuth melt — — — 450

Tin melts — — — 408

Equal parts of tin and bifmuth melt — *— 283

Equal parts of tin, lead, and bifmuth melt — - 220

Water boils — — • — — 2iz

Brandy boils — — — — 190

Rectified fpirit of wine boils — — — 175

Serum of blood and white of eggs coagulate — 156

Bees-wax melts — — — 142

Grcateft heat of water which the hand can well bear — 114

Heat of the Sirocco wind at Palermo, in Sicily — 112

Violent feverifh heat -!— — — 108

Heat of the flcin of ducks, geefe, and pidgeons — 106

Heat of the fkin of cats, dogs, meep, &c. — — 103

Heat of the human body in health — — 98

Heat of a hive of bees — — — 97

Sultry weather — — — — 75

Ordinary fummer heat — -r- — 65

Water juft freezing, or ice juft melting — — 32

Milk freezes — — — 30

Vinegar of ordinary ftrer.gth froze at — — 27

Strong wines froze at — — — 20
A mixture of fnow and fait finks the thermometer to — o

Greateft natural cold obferved in England — 3

Weafc fpirit of wine froze — — — 33

Mercury freezes about — ^— — 39

As mercury contrcfts irregularly on freezing, no cold below this
can be obferved by the -thermometers now in ufe.

Chap, i.] [ 155 3

BOOK III,

CHAP. I.

HISTORY OF DISCOVERIES CONCERNING
LIGHT, &c*.

Opinions of the Platonics.— -Of Arijiotle. — Of Alhazen.— Of Roger
Bacon. — The Invention of Spectacles. — Treatife ofMaurolycus on Vi-
Jton.—Long and /hort Vijion. — Reafon that the Sun's Image appears
round, though the Rays pafs through an angular Aperture. — In<ven-

- tion of the Camera Obfcura. — Conjectures of Fletcher on the Rain-
borvu.— Invention of the Telefcope — Suppofed to be by Zacharias
Janfcn. — Galileo. — •Kepler.'— Invention of the Micrcfcope.—'Tycho
Brake.— Reformation cf diftorted Images.'-* Snellius and Hortenjiuf.
—Defcartes, — Scheiner. — Velocity cf Light difcovered. — Boyle's
Difcci>eries en Colours. — Grimaldi. — Gregory.— — Newton ; his Dif-
coveries on Cohan. — On Refrangibility. — Bolognian Stone. — Bald-
nuin's Phojphgrus, &c.— -Bradley.-— Bouguer.— — Melville.^— T)ollanJ,
— De la Motte.—Dela<val.

H E mod ancient hypothefis, which leads to the
JL true theory of light and colours, is that of the
Platonics, namely, that light, from whatever it pro-
ceeds, is propagated in right lines ; and that when it
is reflected from the furfaces of polifhed bodies, the
angle of reflexion is equal to the angle of incidence.

To

* The unfcientinc reader is earneftly requefted to give parti-
cular attention to the following fhort axioms and definitions,
which will enable him not only better to underftand this chapter*
but all the fuccecding ; and if in the coiufe of this book any diffi-
culty

156 Opinions ofArijtotle3 Roger Bacott, &c. [Book III.

To this may be added the opinion of Ariftotle, who
fuppofed that rainbows, haios, and mock funs, were

occafioned

culty mould occur, he will probably find it removed by referring
to this note.

1. Lip, lit is a matter, the particles of which are extremely fmaJI,
wlii-ii, by ftriking en our vifual organs, gives us the fenfation of
feeing.

2. The particles of light are emitted from what are called lumi-
nous bodies, fuch as the fun, a fixe, a torch, or candle, &c. &c. ;
it is reflected or fent back by what are termed opake bodies, or
thofe which have no power of affording light in themfelves.

3. Light, whether emitted or reflected, always moves \v\ftrait or
JireB lines, as may eafily be proved by looking into a bent tube
which evidently obftruds the progrefs of the light in direft lines.

4. By a ray of ligiit is ufua'ly meant the leail particle of light
that can be either intercepted or fcparated from the reft. A
learn of light is generally ufed to expreis fomething of an aggre-
gate or mafs of fight greater than a fingle ray.

5. Parallel rays are fuch as proceed equally diftant from each
other through their whole courfe. The diftance of the fun from
the earth is fo immenfe, that rays proceeding from the body of
that luminary are generally regarded as parallel.

6. Converging rajs are fuch as, proceeding from any body, ap-
proach nearer and nearer to each other, and tend to unite in a
point. The form of rays thus tending to a union in a fingle
point has been compared to that of a candle extinguimer ; it is in
faft a per fed cone.

7. Diverging rays are thofe which, proceeding from a point,
continue to recede from each other, and exhibit the form of an in-
verted cone.

.9. A fmall objeft, or a fma.H fingle point of an objeft, from
which rays of light diverge or indeed proceed in any direction, is
fometimes called the radiant, or radiant point.

o. Any parcel of rays, diverging from a point, confidered as fe-
parate from the reft, is called a pencil of rays.

10. The foe us of rajs is that point to which converging rays
tend, and in which they unite and inte,rfe£l or crofs each other.
It may be confidered as the apex or point of the cone ; and it is
called the focus (or fire place) becaufe it is the point at which
burning- glaffes burn mod intenfely.

11. The virtual or imaginary focus is that fuppofed point be-
hind a mirror or looking-glafs, where the rays would have natu-
rally united, had they not been intercepted by the mirror.

Chap, i.] Opinions of Ariftotle^ P^oger Bacon, &c. 157

occafioned by the reflexion of the fun's beams in differ-
ent circumftances. We have reafon to believe, that the

ufe

12. Plane mirrors or fpeculums are thofe reflecting bodies, the
furfaces of which are perfectly plain or even, fuch as our common
looking-gla'fles. Convex and concave mirrors are thofe the fur-
faces of which are curved.

j 3. An incident ray is that which comes from any body to the
reflecling furface ; the rejttQidraj is that which is fent back or re-
flected.

14. The angle of incidence is the angle which is formed by the
line which the incident ray defcribes in its progrefs, and a line
drawn perpendicularly to the reflecting furface; and the angle of
reflection is the angle formed by the fame perpendicular and the

a b

V/

reflected ray, thus, * c ; where a is the angle of incidence,
b the angle of reflpclion, and c the reflecting furface.

15. By a medium opticians mean any thing which is tranfparent,
fuch as void fpace, air, water, or glafs, through which confe-
quently the rays of light can pafs in itrait lines.

1 6. The refrattien of the rays of light is their being bent, or
attradlad out of their courfe in paffirtg obliquely from one medium
to another of a different denfity, and which caufes objects to ap-
pear broken or diftorted when part of them is feen in a different
Kiedium. It is from this property of light, that a ftick or an oar
which is partly immerfed in water Appears broken.

17. A lens is a tranfparent body of a different density from the
furrounding medium, commonly of glafs, and ufed by opticians to
collect or difperfe the rays of light. They are ingeneral either con-
vex; that is, thicker in the middle than at the edges, which colled
and by the force of refraftion converge the rays, and confequently
magnify ; or concave, that is, thinner in the middle than at the
edges, which by the refraction difperfe the rays of light, and dimi-
nifh the objedls that are feen through them.

1 8. Vifion is performed by a contrivance of th:.s kind. The
cryllalline humour, which is feated in the fore part of the hu-
man eye, immediately behind the pupil, is a perfedl convex
lens. As therefore every objeft is rendered vifible by beams
or pencils of light which proceed or diverge from eve*y radi-
ant point of the objeft, the cryftalline lens collects all thefe di-
vergent rays, and caufes them to converge on the back part of
the eye, where the retina or optic nerve is fpread out ; and the
points where each pencil of rays is made to converge on the re-
tina.

i$$ Opinions ofAriftot1e> Roger Bacon> &c. [Book III.

life of convex glafTes, both as magnifiers and as burn-
ing glafles, was not unknown to the ancients, though
the theory was not underftood. The magnifying
power of glafles, and fome other optical phenomena,
were alfo largely treated of by Alhazen, an Arabic phi-
lofopher of the twelfth century. Thefe obfervations
were followed by thofe of Roger Bacon, who demon-
ftrates by actual experiment, that a fmall fegmenr, of
a glafs globe would greatly afllft the fight of old per-
fons ; and from the hints afforded by thefe two philo-
fbphers, it is not unreafonable to conclude, that the
invention of fpectacles proceeded. Concerning the
a6bual author of this ufeful invention, we have no
certain information ; we only find, that it was gene-
rally known about the beginning of the fourteenth
century.

In the year 1575, Maurolycus, a teacher of mathe-
matics at Meffina, publiflicd a treatife on optics, in

tina, are exadlly correfpondent to the points of the object from
which they proceed. As however, from the great degree of con-
vergence which this contrivance will produce, the pencils of light
proceeding from the extreme points of the object will be made to
crofs each other before they rench the retina, the image on the re-
tina is always inverted. (See Plate XX. fig. 39.)

19. The magnitude of the image painted on the retina will,
therefore, it is evident, depend on the greatnefs or obtufenefs of
the angle under which the rays proceeding 'from the extreme
points of the object enter the eye. For it is plain, that the more
open or obtufe the angle is, the greater is the tendency of thefe
rays to meet in a point and crofs each other : and the fooner they
crofs each other, after paffing the ctyftalline lens, the larger will be
the inverted image painted on the retina. (See Plate XX. fig. 40.)
The 'uij'ual angle, therefore, is that which is made by two right
lines drawn from the extreme points of any object to the eye ; and
on the meafure of that angle, the apparent magnitude of every
vifible object, wili depend.

20. Tne prtftn ufed by opticians is a triangular piece of fi»e
glafs, which has, the power of feparating the rays of light.

6 which

Chap, i.] theory ofjhort and imperfect Vifwn.

which he demonftrates, that the cryftalline humour of
die eye is a lens, which collects the rays of light pro-
ceeding from external objects, and throws them on the
retina, or optic nerve. From this principle he was
led to difcover the reafon of what are called fhort and
imperfect fight, In the one cafe, the rays converge
too foon ; in the other, they do not converge foon
enough. Hence fhort-fighted perfons are relieved by
a concave glafs, which caufes the rays to diverge in
fome degree before they enter the eye, arid renders it
more difficult for them to converge fo faft as they
would have done after entering the cryftalline hu-
mour ; hence, too, he proves that a convex lens is of
ufe to perfons who have weak, but long fight, by
caufing the rays to converge fooner, and in a greater
quantity, than would otherwife happen. He was the
firft, alib, that folved a problem, which had caufed
much perplexity in the ancient fchools, refpecting the
fun's image appearing round, though the rays that
form it are tranfmitted into a dark room through an
angular aperture. He confidered, that as the rays of
light are conftantly proceeding, in every direction,
from every part of the fun's difk *, " they muft be
crofiing each other from the extreme part of it in
every point of the aperture ; fo that every Tuch point
will be the apex of two cones, of which the bafe of the
one is the fun's difk, and that of the other his image
on the oppofite wall." The whole image, therefore,
•confifts of a number of images, all of which are cir-r
cular j the image of the fun formed of thofe images
muft be circular alfo ; and it will approach .the nearer
a perfect circle, the fmaller the aperture, and the more
diftant the image.

Nearly about the fame time, Johannes Baptifta

* The face of the fun.

Porta,

i6o ConjeKures on the Rainbow. [Book 111,

Porta, of Naples, invented the camera obfcura ; and
his experiments upon that inftrument convinced him
that light is a iiibftance, by the intromifiion of which
into the eye vifion is performed ; for it is proper to
mention, that before his, time the opinion was afmofl
general, that vifion depended upon what was termed
.vi/ual rays> proceeding from the eye. In this the fyf-
tem of Porta correlponds nearly with that of" Mauro-
lycus : but it ought to be remarked, that the difcove ~
ries of each of thefe two philofophers were unknown to
the other. He fhews, moreover, that a defect of light
is remedied by the dilatation of the pupil, which con-
tracts involuntarily when expofed to a ftrong light,
and opens when the light is faint and knguid.

One Fletcher, of Breflau, in 1571, endeavoured to
account for the phenomena of the rainbow, by a dou-
ble reflexion and one refraction -, but Antonio de Do-
minis, whofe treatife was published in 1611, was the
firft who came near to the true theory. He defcribes
the progrefs of the ray of light through each drop of
the falling rain ; he fhews that it enters the upper part
of the drop, where it fuffers one refraction 3 that it is
reflected once, and then refracted again, fo as to come
directly to the eye of the fpectator j why this refrac-
tion 'mould produce the different colours was referved
for Sir I. Newton to explain.

The litter end of the fixteenth century was illuftri-
ous for the invention of telefcopes. It is generally
allowed to have been cafual. That effect of refrac-
tion, which caufes the rays of light, in paffing through
a denfe medium thicker in the middle, to converge
to a point, and allb that which takes place when they
pafs through one thicker at the extremities, had been
long obferved j and the afliftance which convex and
concave glaffcs afforded to the fight had brought them

into

Chap; J.] Ga!iko,Kepkr>&f. x£j

into common ufe. The inventor of the tele'fcope is
not certainly known. The moft probable account is,
that one Zacharias Janfen, a fpectacle maker of Mid-
dleburgh, trying the effect of a concave and convex
glafs united, found that, placed at a certain diftance
from each other, they had the. property of bringing
diftant objects apparently nearer to the eye *. Tele-
fcopes were greatly improved by Galileo, who made
one to magnify thirty-three times, and with this he^
made all his wonderful aftronomical difcoveries.

The rationale of telefcopes was, however, not ex-
plained till Kepler, who defcribed the nature and the
degree of refraction, when light parted through denfer
or rarer mediums, the furfaces of which are convex or
concave, namely, that it correfponds to the diameter
of the circle of which the convexity or concavity are
portions of arches. He fuggefted fome improvements
in the construction of telefcopes, which, however,
were left to others to put in practice.

To the Janfens we are alfo indebted for the difcovery
of the microfcopej an inftrument depending upon
exactly the fame principles as the former. In fact, it
is not improbable that the double lens was firft applied
to the obfervation of near but minute objects, and
afterwards, on the fame principles, to objects which
appeared minute on account of their diftance.

Much attention was given by Kepler to the invef-
tigation of the law of refraction ; but he was. able to

* An account which is very commonly received is, that fome
©f his children playing in his fhop with fpe&acle glafles, perceiv-
ed that when they held two of thefe glafles between their fingers,
at a certain diftance from each other, the dial of the clock ap-
peared greatly magnified, but in an inverted pofition. From this
their father took the idea of adjufting two of thefe glailes on a
board, fo as to move them at pleafure.

VOL. 1. M advance

162 fycbo Brahe> &c. [Book IIL

advance no nearer the truth than the obfervation, that
when the incident ray does not make an angle of more
than thirty degrees with the perpendicular, the refract-
ed ray proceeds in an angle which is about two-thirds
of it. Many difputes arofe about the time of Kepler
(i6t>o) upon this fubject, but it appears that little was
effected by them in the caufe of truth.

Kepler was more fuccefsful in purfuing the difco-
veries of Maurolycus and B. Porta. He demonftrated
that images of external objects were formed upon the
optic nerve by the foci of rays coming from every
part of the object; he alfo obferved, that thefe images
are inverted ; but this circumilance, he fays, is recti-
fied by the mind, which, when an imprefllon is made
on the lov/er part of the retina, confiders it as made
by rays proceeding from the higher parts of the object.
Habit is fuppofed to reconcile us to this deception,
and to teach us to direct our hands to thofe parts of
objects from which the rays proceed. Tycho Brahe,
obferving the apparent diminution of the moon's difk
in folar eclipfes, imagined that there was a real dimi-
nution of the difk by the force of the fun's rays j but
Kepler faid, that the difk of the moon does not ap-
pear lefs in confequence of being unenlightened, but
rather that it appears at ether times larger than it
really is, in confequence of its being enlightened. For
pencils of rays from fnch diftant objects generally come
to their foci before they reach the retina, and confe-
quently diverge and fpread when they reach it. For
this reafon, he adds, different perfons may imagine the
difk to be of different magnitudes, according to the;
relative goodnefs of their fight.

In the fixteenth century alfo many improvements

were made in perfpective j the ingenious- device, in

particular, of the reformation of diftorted images by

2 concave

Chap. i>] Defearfes, Scheiner, csV. . 163

concave or convex fpeculums was invented, but it is
uncertain by whom.

The true law of refraction was difcovered by Snel-
lius, the mathematical profeflbr :it Leyden j but not
living to complete it, the difcovery was publimed and
explained by Profeflbr Hortenfius. Some difcoveries
of lefier imporrance were made at this time, among
others by Defcartes, who very clearly explained the
nature and caufe of the figure of the rainbow, though
he was able to give no account of the colours j he
however confidered the fmall portion of water, at which
the ray ifiues, as having the effect of a prifm, which
was known to have the property of exhibiting the
light, tranfmitted through it, coloured.

In 1625, the curious difcovery of Scheiner was pub-
lifhed at Rome, which afcertains the fact, that viiion
depends upon the images of external objects upon the
retina. For taking the eye of an animal, and cutting
away the coats of the back part, and prefenting diffe-
rent objects before it, he.difp)ayed their images diftinct-
ly painted on the naked retina or optic nerve. The
fame philofopher demonftrated by experiment, that
the pupil of the eye is enlarged in order to view re-
mote objects, and contracted when we view thofe which
are near. He fhewed, that the rays proceeding from
any object, and palling through a fmall hole in a pafte-
board, crofs one another before they enter the eye$ for
if the edge of a knife is held on the fide next the eye,
and is moved along till it in part covers the hole, it
will firft conceal from the eye that part of the object
which is fituated on the oppofite fide of the hole.

Towards the middle of the feventeenth century the

velocity of light was difcovered by fome members of

the Royal Academy of Sciences at Paris, particularly

Cafmi and Roemer, by obferving the eclipfes of Ju-

M 2 piter's

164 Bcyk, Grimaldi t Gregory > &c. [Book III.

piter's fatcllites. About the fame time Mr. Boyle
made ,his experiments on colours. He proved that
jfnow {lid not affect the eye by a native, but reflected
light, a circumftance which, however, at this day, we
ihould fcarcely believe was ever neceffary 10 be proved
by experiment. By admitting alfo a ray of light into
a dark room, and letting it fall on a fheet of paper, he
tiemonftrated, that white reflected much more light
than any other colour \ and to prove that white bodies
reflect the rays outwards, he adds, that common burn-
ing-glaiTes will not, for a long while, burn or difcolour
white paper j on the contrary, a concave mirror of
black marble did not reflect the rays of the fun with
near fo much power as a common concave mirror.
The fame effect was verified by a tile, one half of the
lurface of which was white, and the other black.

Some experiments were made about this time on
the difference of the refractive powers of bodies ; and
the firft advance to the great difcoveries by means of
the prifm was made by Grimaldi, who obferved, that
a beam of the fun's light, tranfmitted through a prifir,
inftead of appearing round on the oppofite wall, exhi-
bited an oblong image of the fun. Towards the clofe
of this century the reflecting telefcope was invented by
our countryman James Gregory.

The reader will foon perceive how very imperfect
all the preceding difcoveries were in comparifon with
thofe of Sir I. Newton. Before his time, little or no-
thing was 'known concerning colours; even the remark
of Grimaldi refpecting the oblong figure of the fun,
made by tranfmitting the rays through a prifm, was
unknown to our great philofopher, having been pub-
Jifhed only the year before. This, however, it appears,
was the firft circumftance which directed the attention
of Newton to the inveftigation of the theory of co-
lours.

Chap, i.] Newton's Difcoveries on Colours. 165

lours. Upon meafuring the coloured image, which
was made by the light admitted into a dark chamber
through a prifm, he found that its length was five
times greater than its breadth. So unaccountable a
circumftance induced him to try the effect of two
prifms j and he found that the light, which by the firft
prifm was diffufed into an oblong, was by the fecond
reduced to a circular form, as regularly as if it had
paffed through neither of them. After many conjec-
tures and experiments relative to the caufe of thefe
phenomena, he at length applied to them what he calls
the experimentum crucis. He took two boards, and
placed one of them clofe to the window, fo that the
light might be admitted through a fmall hole made in
it, and after paffing through a prifm might fall on the
other board, which was placed at about twelve feet
diftance, and in which there was alfo a fmall aperture, in
order that fome of the incident light might pafs through
it. Behind this hole, in the fecond board, he alfo
placed a prifm, fo that the light, after patting both the
boards, might fuffcr a fecond refraction before it
reached the wall. He then moved the firft prifm ia
fuch a manner as to make the feveral parts of the image
caft upon the fecond board pafs fucceffively through
the hole in it, that he might obferve to what places on
the wall the fecond prifm would refract them. The
confequence was, that the coloured light, which form<-
ed one end of the image, fuffercd a refraction confi-
derably greater than that at the other end ; in other
words, rays or particles of light of one colour were
found to be more refrangible than thole of another.
The true caufe, therefore, of the length of the image
was evident, fince it was proved by the experiment,
that light was not homogenial, but confided of diffe-
rent particles or rays, which were capable of different
M 3 degrees

1 66 Further Experiments of Newton. [Book III,

degrees of refrangibility, according to which they were
transmitted through the prifm to the oppofite wall.
It was further evident from thefe experiments, that as
the rays of light differ in refrangibility, fo they alfo
differ in exhibiting particular colours, fome rays pro-
ducing the colour red, others that of yellow, blue, &c.
and of thefe different-coloured rays, feparated by means
of the p:ifm according to their different degrees of re-
•frnngibility, the oblong figure on the wall was com-
pofed. But to relate the great variety of experiments,
by which he demonftrated thefe principles, or the ex-
tenfive application of them, would lead me too much
into detail; let it fuffice to lay, that he applied his
principles to the fatisfaftory explanation of the colours
of natural bodies, of the rainbow, and of moft of the
phenomena of nature, where light and colour are con-
cerned ; and that almoft every thing which we at pre-
fent know upon thefe fubjec~ts was laid open by his ex-
peri men rs.

His oblervations on the different refractive powers
of different Jubilances are curious and profound ; but
chemiftry was at that period fcarcely in a ftate fuffi-
tiendy advanced to warrant all his conclufions. The
general refult is, that all bodies feem to have their re-
fractive powers proportional to their denfities, except-
ing fo far as they partake more or lefs of inflammable
or oily particles.

The difcovery of the different refrangibility of the
component rays of light fuggefted defects in the con-
flrudlion of telefcopes, which were before unthought
of, and in the creative hand of a Newton led to fome
no lefs extraordinary improvements in them. It is
evident, that fince the rays of light are of different re-
frangibilities, the more refrangible will converge to a
focus much fooner than the lefs refrangible, confe-

quently

Chap, i.] Refleffing felefcope, &c. 167

quently that the whole beam cannot be brought to a
focus in any one point, fo that the focus of every ob-
jed-glafs will be a circular fpace of confiderable dia-
meter, namely, about one fifty- fifth of the aperture of
the telefcope. To remedy this, he adopted Gregory's
idea of a reflector, with fuch improvements as have
been the bafis of all the prefent inftruments of this
kind.

When a fcience has been carried to a certain degree
of perfection, fubfequent difcoveries are too apt to be
confidered as of little importance. The real philofo-
pher will not, however, regard the difcoveries on light
and colours, fmce the time of Newton, as unworthy his,
attention. By a mere accident, a very extraordinary
property in fome bodies of imbibing light, and after-
wards emitting it in the dark, was obferved. A Ihoe-
maker of Bolognia, being in quefl of fome chemical
fecret, calcined, among other things, fome Hones of a
particular kind, which he found at the bottom of
Mount Peterus, and cafually obferved, tha" when thefe
{tones were carried into a dark place after having been •
cxpofed to the light, they poffefled a felf-illuminating
power. Accident afterwards difcovered the fame pro-
perty in other fubftances. Baldwin of Mifhia, diflblv-
ing chalk in aqua-fortis, found that the refiduum, after
diftillation, exactly refembled the Bolognian ftone in
retaining and emitting light, whence it now has the
name of Baldwin's phofphorus j and M. Du Fay ob-
ferved the fame property in all fubftances that could
be reduced to a calx by burning only, or after folution
in nitrous acid. Thefe fe&s feem to eftablifh the ma-
teriality of light.

Some very accurate calculations were made about

the year 1725 by Dr. Bradley, which afforded a more

M 4 convincing

1 6 S Bouguer and Mchille. [ Book 111.

convincing proof of the velocity of light, and the mo-
tion of the earth in its orbit. Nor muft we forget M.
Bouguer's very curious and accurate experiments for
afcerta'ming the quantity of light which was loft by
reflexion, the moft decifive of which was by admitting
into a darkened chamber two rays of light, one of
•which he contrived mould be reflected, and the other-
fall direct on the oppofite wall j then by comparing
the fize of the apertures, by which the light was ad-
mitted (that through which the direct ray proceeded
being much fmaller than that through which the, re-
flected ray was fuffered to pafs, and the illumination
on the wall being equal in both) he was enabled to
form an exact eftimate of the quantity of light which
was loft. To prove the fame effect with candles, he
placed himfelf in a room perfectly dark, with a book
in his hand, and having a candle lighted in the next
room, he had it brought nearer to him till he could juft
fee the letters, which were then twenty-four feet from
the candle. He then received the light of the candle
reflected by a looking-glafs upon the book, and he
found the whole diftance of the book from the fource
of the light (including the diftance from the book to
the looking-glafs) to be only fifteen feet; whence he
concluded, that the quantity of direct light is to that
of reflected as 576 to 225 ; and fimilar methods were
purfued by him for meafuring the proportions of light
in general *.

The fpeculations of Mr. Melville, concerning the
blue fnadows which appear from opake bodies in the
morning and evening, when the atmofphere is ferene>

* See an accurate defcription of M. Bouguer's infiruments,
Hijl. tf Optics, Per. vi. f. 7.

are

Chap, i.] Dollond, Martin, and De la Motte. 169

are far from uninterefting. Thefe phenomena he at-
tribute's to the power which the atmofphere poflfeiTes
of reflecting the fainter and more refrangible rays of
light, the blue, violet, &c. and upon this principle he
alfo explained the blue colour of the fky, and fome
other phenomena.

The fame period produced Mr. Dollond's great im-
provement in the construction of telefcopes. It coa-
fifts in ufing three glafles of different refractive powers,
crowo and flint glafs, which correct each other. The
great difperfi6n of the rays which the flint-glafs pro-
duces, is the effect of the lead, and is in proportion to
the quantity of that metal, which is ufed irrrts ccrmpo-
fition. Mr. Martin found the refractive powers -of
different glalfes to be in proportion to their ipecific
gravity.

Several difcoveries and improvements have been
made fince the time of Newton in that branch of optics
which relates more immediately to vifion; but thefe,
being rather foreign to the chief fubje6t of this chap-
ter, I fhall not detail. One difcovery only. I mail men-
tion, becaufe it not only is curious in itfel£ but becaufe
it led to the explanation of feveral circumftances relat-
ing to vifion. M. De la Motte, a phyfician of Dant-
zick, was endeavouring to verify an experiment of
Schemer, in which a diftant object appeared multiplied
when viewed through feveral holes made with the
point of a pin in a card, not further diftant from one
another than the diameter of the pupil of the eye; but
notwithftanding all his labour, he was unable to fuc-
ceed, till a friend happening to call upon him, he de-
fired him to make the trial, and it anfwered perfect-
ly. This friend was mcrt-fighted ; and when he ap-
plied a concave glafs clofe to the card, the object,

which

170 Ddaval. • [Book III.

•which Teemed multiplied before, now appeared but

The laft, though not lead fuccefsful adventurer in
this branch of fcience, is Mr. Delaval, who, in a paper
read before the Philofophical Society of Manchefter
in 1784, has endeavoured, with great ingenuity, to ex-
plain the permanent colours of opake bodies. The
majority of thofe philofophers, who have treated of
t and colours, h.ive, he obferves, fuppofed that cer-
t'i ' odies or furfaces reflected only one kind of rays,
a:id therefore exhibited the phenomena of colours; on
tie contrary, Mr. Delaval, by a variety of well con-
ducted experiments, evinced, that colours are exhibit-
ed, not by rerkfted^ but by tranfmitted light. This
he proved by covering coloured glaffes and other tranf-
parent coloured media, on the further furface, with
fome fubftance perfectly opake, when he found they
reflected no colour, but appeared perfectly black. He
concludes, therefore, as the fibres or bafes of all vege-
table, mineral, and animal fubftances are found, when
cleared of heterogeneous matters, to be perfectly
white, that die rays of light are in fact reflected from
thefe white particles, through coloured media, with
which they are covered ; that thefe media ferve to in-
tercept and impede certain rays in their.pafTage through
them, while, a free paflage being left to others, they
exhibit, according to thefe circumftances, different co-
lours. This he illuftrates by the fact remarked by
Dr. Halley, who, in diving deep into the fea, found
that the upper part of his hand, when extended into
the water from the diving b-11, reflected a deep red
colour, while the under part appeared perfectly green.
The conclufion is, that the more refrangible rays were
intercepted and reflected by particles contained in the

fea- water.

Chap, i.] On the Colours of Opake 'Bodies 17 $

fea- water, and were confequently reflected back by the
under part of the hand ; while the red rays,, which
were permitted to pafs through the water, were in the
fame manner reflected by the upper part of the hand,
which therefore appeared of a red rofe colour. Thofe
media> our author thinks, tranfmit coloured light with
the greateft flrength which have the ilrongefl refrac-?
tive power.

,[ 172 ] [Bock III.

C I! A P. II.
OF THE NATURE OF LIGHT.

Various Opinions on the ASivn cf Light.— 'Theory of a vibrating jl/e-
dium\ fuppcrted by Dcfcartes, £sV.»— Objections to this Theory. —
Light conjijis of infinitely minute Particles projected from the luminous
Body. — Inquiry refpeSling the Identity cf Light and Fire. — Experi-
ments of Mr. Beyle.— Why Light is not always accompanied with
Heat. — Velocity cf Light. — Light always moves in Right Lines. —
Rarity of Light. — Force or Momentum of Light.— Mitchell's Expe-
riment. — Inquiry bo=w far the Sun's Magnitude is diminijhed ly the
Emzjfion cf Light. — Light fubjeft to the ordinary Laws of Nature.
—Light attradtd by the Bolognian Phofphori.—Tke fame Property
ia. Diamonds, and other Precious Stones.

NUMEROUS opinions have fucceflively been
adopted concerning this wonderful fluid. It
has been fometirjnes'confkkred as a diftinct fubftance,
fqmetimes as a quality, fometimes as a caufe, fre-
quently as an effect; by fome regarded as a compound,
and by others as a fimple fubftance. Des Cartes and
other philofophers of high repute have imagined that
the fcnfation which we receive from light is to be at-
tributed entirely to the vibrations of a fubtile medium,
or fluid, which is diffufed throughout the tiniverfe^
and which is put into action by the impulfe of the fun.
In this view they confider light as analogous to found,
which is known to depend entirely on the pulfations
of the air upon the auditory nerves; and in fupport of
this opinion, it has been even lately urged'*, ift. TKat

* Sre Dr. Franklin's woiks; and Pro feller Boudoin's Memoir,
Tranji;c>lcni cf American Ace. dan*;., Ye!, i.

fomc

Chap. 2.] Objections to the Theory ofDefcartes, &c. 173

ibme diamonds, on being rubbed or chafed, are lumi-
nous in the dark. 2. That an electric fpark, not
larger, but much brighter, than the flame of a candle,
may be produced, and yet that no part of the electric
fluid is known to efcape, in fuch a cafe, to diftant
places, but the whole proceeds in the direction to which
it is deftined by the hand of the operator. Weaker
or ftronger fparks of this fluid are alfo known to differ
in colour ; the flrongeft are white and the weakeft red,
&c.

To this opinion, however, there are many prefllng
and, indeed, infurmountable objections, id, The ve-
locity of found bears a very fmall proportion to that of
light. Light travels, in the fpace of eight minutes, a
diftance in which found could not be communicated in
feventeen years; and even our fenfes may convince
us, if we attend to the explofion of gunpowder, &c.
of the alrnoft infinite velocity of the one compared
with that of the other, adly, If lightdepended alto-
gether on the vibrations jof a fluid, no folid reafon can
be affigned why this fluid fhould ceafe to vibrate in
the night, fmce the fun mud always affect forne pare
of the circumambient fluid, and produce a perpetual
day. 3dly, The artifice of candles, lamps, &c. would
be wholly unnecefiary upon this hypothefis, fmce, by
a quick motion of the hand, or of a machine contrived
for this purpofe, light might on all occafions be eafily
produced. 4thly, Would not a ray of light, admitted
through a fmall aperture, put in motion, according to
this theory, the whole fluid contained in a chamber ?
In fact, we know that light is propagated only in right
lines, whereas found, which depends upon vibration,
is propagated in every direction. 5thly, The fepara-
tion or extenfion of the rays, by means of the prifm,
can never be accounted for by the theory of a vibrat-
ing

574 Light doss not depend on Vibration. [Book III.

ing medium. 6thly, The texture of many bodies is
actually changed by expofure to the light. The juice
of a certain fhell-fifh contracts, it is well known, a
very fine purple colour, when permitted to imbibe the
rays of the fun ; and the ftronger the light is the more
perfect the colour. Pieces of cloth wetted with this
fluid become purple, even though inclofed in glafs, if
the folar light only is admitted } but the effect is to-
tally excluded by the intervention of the thinneft plates
of metal, which exclude the light. Some of the pre-
parations of filver alfo, fuch as luna cornea, will re-
main perfectly white, if covered from the light, but
contract a dark purple colour when expofed to it j and
'even the colour of plants is derived from the light,
fmce a plant which vegetates in darknefs will be per-
fectly white. As colour is imparted by light, fo it is
alfo deftroyed by it. It muft have fallen within the
obfervation of every reader, that filks, and other fluffs
of delicate colours, are greatly by the action of

light. Experiments "have been made upon the fame
fluffs by expofmg them to both heat and moifture in
the dark, and alfo by expofmg them to the light in the
vacuum of an air pump, and it was found by all thefe
experiments, that the change of colour was to be af-
cribed to the action of light *. 7thly, With refpect
to the emiffion of light by diamonds and other ftones,
it is eafily accounted for upon other principles j and
the arguments founded upon the electric fpark not
being fenfibly diminifhed will meet with a fatisfactory*

* <f It was conje&ured by fome, that the rays ef the fun di/J
perfed thofe parts of the bodies on which colours depended ; but
Bonzius obfcrved, that when ribbons were expofed on white -pa-
per, the colours vaniihed from buth their fides, but that nothing
could be found ncarth,e places where they had lain." Prie/ikft
Optics, 381.

folntion

Chap. 2.] Nature of Light. 17$

folution by confidering the extreme rarity of light, and
the minutenefs of its particles.

It is, therefore, almott univerfally agreed by the
moderns, that light confifts of a number of extremely
minute particles, which are actually projected from the
luminous body, and aft by their projectile force upon
our optic nerve. Concerning the nature of thefe par-
ticks, or rather of the matter of which they confift,
there is lefs unanimity in the philofophical world.

It is an opinion fupported by the mod refpectable
names, that light is a fubftance perfectly diftinct from
the matter of fire, and which excites ignition when
concentrated by a burning glafs, merely by its mecha-
nical force upon the matter of heat. Others of equal
eminence have contended, that light is no other than
fire in a projectile date. Fire, according to thefe phi-
lofdphers, is produced by the accumulation or con-
centration of the particles j light is the effect of the
rapid projectile motion of the fame particles. Fire
or heat may therefore exiit to a considerable degree,
as it is found to do occafionally in metallic bars, with-
out being fufficiently difengaged to afiurne the projec-
tile (late, and to' be forcibly emitted or projected from
the burning body. The fame matter may alfo exiil
in its active and projectile itate, or, in other words, ia
the form of light, but too much diffufed to produce
the fen fation of burning, or to effect the diffclutton of
any body, or the reparation of its parts by combuftion.
To the great elafticity of the matter of fire, fo obvious
in all the phenomena of fluidity, both thefe effects are
afcribed. When a quantity of this matter is intro-
duced into any folid body, the repulfion which exifts
between its particles will occafion the difiblution of
that body ; and when it becomes perfectly free and dif-
engaged, the fame repuHion will caufe a quantity of

its

'All "Bodies tvbxt mud} healed firne. [Book 111=?

its particles to be emitted with a rapid projectile force,
and will produce the effects of light *. The light of
the fun is conlidered as " the fame matter propelled
by the fame powers, in greater quantity and with
greater vigour -J-." The atmofphcre through which
light has to pafs in proceeding from the fun to us is in
ail probability of infinitely greater purity than ours,
and confequently, according to the laws of motion,
the projectile force of the rays of light is very little
diminifried in their pailage from the great fource of

light and fire.

The theory which is here advanced appears to reft
chiefly on the great fact of the fun's rays, when con-
centrated by a burning glafs, producing all the pheno-
mena and effects of elementary nre. There are, how-
ever, other facts which countenance the opinion. Sir
Ifaac Newton obferves, that all bodies, when heated
beyond a certain degree, emit light, and mine. Light
alfo accompanies the electric fire, whenever it exifts
in a difengaged or projectile ftate. The abfbrption ot"

* « When we confider that the elafticity of aeriform fluids is
owing to the matter of fire; that a cannon ball is propelled only
by the excefs of the repellant powers of parts of this matter, above
the forces with which they are attracted by the grofs parts of gun-
- powder, or of the airs emitted during the deflagration of it; that
the velocity of the ball is incomparably lefs than it would be if" it
were mot through an unreiiiling medium; and that even this lat-
ter velocity would be an inadequate reprefentation of that, with
\vhich the parts of the fiery matter are fhot forth, in the firft in-
flant cf their liberation from the combufable body which held
them clofely approximated ; we find the natural powers already de-
fcribed, fufficicnt in themfelves, for the projection of thefe parts/
with all the velocity experienced in the light of flaming bodies on
earth, and to every diftance at which it has been perceived.'

Higgini1! Experiments and Obfer -vat ions, p. 338.

•j- Higgins's Experiments and Obfervations, p. 339.

light

Chap. l.~\ Why Light does not always produce Heat. 1 77

light by the darker colours is alfo found to have an
extraordinary effect in the production of heats but
the experiments to this effect have been already re-
lated *.

That light (or more properly according to this
theory, the matter of fire) does not always produce
a&ual hear, is accounted for from the minutenefs
of the particles, and the extreme rarity of the fluid.
It was before ftated that light muft be exceedingly
concentrated to produce ignition. A plane1 -mirror re-
flects the light in too difFufed a ftate ; but a concave
mirror collects and converges the particles to a point,
and is therefore capable of producing igninon $ and
yet we have feen that the light of the moon will not,
in the moft condenfed ftate, produce the lead degree
of heat, though the moft delicate thermometers mould
be employed. The light which is emitted by putre-
fcent fubftances, by the glow-worm, and fome other
infects, is analogous to the light of the moon in this
refpect; it is of too faint and rare a nature to produce
heat or ignition. In the fame manner, when the rays
of light pa& through a tranfparent medium, they fuo
ceed each other at an immenfe diftance f. I£ there-
fore, the rays concentrated by the moft powerful burn-
ing glafs are made to pafs through a phial containing
'fpirit of wine, or through any other tranfparent in-
flammable fubftance, the latter will not be fet on fire j
but if there is any opake body (as a fpoon or other
veffel) .placed under the fpirit or the tranfparent body,
which by intercepting may ferve to accumulate the
particles of light, the fpirit or inflammable matter
will be immediately inflamed. Conformably to thefe

* See p. 76. f That of 1,000 nuks.

VOL. I. N prin-

17 3 Velocity of Light. [ B ook II I .

principles it is found, that the atmofphere is not
warmed by the mere paffage of the fun's rays through
it, but chiefly by the heat which is collected by the
earth, and which is thence imparted to the air. Thus
the air, at the fummits of high mountains, is always
cold, bccaufe they are too much elevated above the
general furface of the earth to derive any confiderable
advantage from this circumftance.

When thefe fafts are fairly confidered, ihe fyftem
which fuppofes light to be a modification of the matter
of fire, or a combination of that element with fome
unknown principle, muft be allowed to be at lea-ft
probable. It has, however, been cuftomary to confider
ciiftindtly the properties of light, without a reference
to its analogy with the matter of fire, and in this
mode it will be necefiary, on the prefent occafion, to
proceed. .

• The firft remarkable property of light is its amaz-
ing VELOCITY. In the fhort fpace vionejecond a parti-
cle of light traverfes an extent of one hundred and f evenly
tkoujand miles *, which is fo much fwifcer than the
progrefs of a cannon ball, that the light is enabled to
pafs a fpace in about eight minutes, which could not
be palled with the ordinary velocity of a cannon ball
in lefs than thirty-two years f. The velocity of light
is allb found to be uniform, whether it is original,
as from the ftars, or rtiiecled only, as from the
planets.

The mode of calculating the velocity of light is a
branch of atlronomy rather than of natural hiftory. It
"will fuffice therefore in this place to remark, that by
mathematical obfervations made upon the tranfits of

•» Nicholfon's Phil. vol. i. p. 258. -J- Ibid, p. 257.

•x

Venus

Chap. 2.] Light propelled in right Lines. - 179

Venus in 1761 and 1769, the diameter of the earth's
orbit was found to be about 16^,636,800 geographi-
cal miles. When, therefore, the earth happens to be
on that fide of her orbit which is oppofite to Jupiter,
an eclipfe of his fatellites, or any other appearance in
that planet, is obferved to take place fifteen or fixteen
feconds later tha^ it would have done if the earth had
been on that fide of her orbit which is neareft to Ju-
piter *. From the very accurate obfervations of Dr.
Bradley, it appears that the light of the fun paflfes
from that luminary to the earth in eight minutes and
twelve feconds.

The next property of light, to which it is proper
to advert, is, that it is detached from every luminous
or vifible body in all directions, and conftantly moves
in RIGHT LINES. It is evident that the particles of
light move continually in right lines, fmce they will
not pafs through a bended tube, and lince if a beam
of light is in part intercepted by any intervening body,
the fhadow of that body will be bounded by right
lines paffing from the luminous body, and meeting
the lines which terminate the interceding body: This
being granted, it is obvious, that the rays of light mud
be emitted from luminous bodies in every direction,
fmce, whatever may be the diftance at which a fpec-
tator is placed from any vifible object, every point of
the furface which is turned towards him is vifibie to
him, which could not be upon any other principle.

The RARITY of light, and the minuteness of irs par-
ticles, are not lefs remarkable than its velocity. If
indeed the Creator had, not formed its particles infi-
nitely fmall, their exceflive velocity would be deftruc-
tive in the higheft degree. It was demcnftrated, that

* See Newton's Optics, 1. ii. p. 3. prop, u, Prieftley's Op-
tics, p. 1 40. Nicholion's Phil. vol. i. p. 135.

N 2 light

I So Mimttenefs and Rarity of Light. [Book III.

light moves about two million of times as faft as a
cannon ball *. The force with which moving bodies
ftrike, is in proportion to their mafles multiplied by
their velocities ; and confequently if the particles of
light were equal in bulk to the two millioneth part
of a grain of fand, we mould be no more able to en-
dure their impulfe than that of fand fliot point blank
from the mouth of a cannon f. The minutenefs of
the rays' of light is alfo demonftrable from the facility
with which they penetrate glafs, cryftal, and other
folid bodies, which have their pores in a rectilinear
direction, and that without the fmalleft diminution of
their velocity, as well as from the circumftance of
their not being able to remove the fmalleft particle
of microfcopic duft or matter which they encounter
in their progrefs. A further proof might be added,
jhat if a candle is lighted, and there is no obftacle to
obftruct its rays, it will fill the whole fpace within two
miles around it alrroft inftantaneoufly, and before it
has loft the lead fenfible part of its fubftance J.

To the velocity with which the particles of light
are known to move may, in a great meafure, be at-
tributed the extreme rarity an.d tenuity of that fluid.
It is a well-known fact, that the effect of light upon
the eye is not inftantaneous, but continues for a con-
iiderable time §. Now we can fcarcely conceive, a
more minute divifion of time than the one hundred
and fiftieth part of a fecond. If, therefore, one lucid

* A cannon ball flies with the velocity of about a mile in eight
feconds. ' Nichol foil's Phil. vol. i. p. 257. During the late fiege
of Gibraltar, there were two boys, who ufed to be Rationed on the
works, and whofe quick fight enabled them to gi\re notice to the
workmen of the approach of a ball from the enemy's works.
*~*DttnfawattF>t Gib,

f Nicholfon's Phil. vol. i. p. 257. J Enficld's Phil. 131,

§ Nicholfon's Phil. vol. i, p. 25^.

point

Chap. 2.] Force or Momentum of Light. 181

point of the fun's flirface emits one hundred and fifty
particles of light in one fecond, we may conclude that
this will be fufficient to afford light to the eye with-
out any feeming intermiffion ; and yet, fuch is the
velocity with which light proceeds, that flill thefe
particles will be at leaft one thoufand miles diftant from
each, other *. If it was not indeed for this extreme
tenuity of the fluid, it would be impoffible that the
particles fhould pate, as we know they do, in all di-
rections without interfering with each other. In all
probability the fplendour of all vifible objects may be •
in proportion to the greater or lefs number of parti-
cles, which are emitted or reflected from their furface
in a given fpace of time ; and if we even fuppofe three
hundred particles emitted fuccefnvely from the fun's
furface in a fingle fecond, ftili thefe particles will follow
each other at the immenfe diftance of above five hun-*
dred miles.

That light is, however, not deftitute of FORCE or
MOMENTUM; has been proved by the experiment of
Mr. Mitchel, already mentioned f. On that experi*
ment the following calculation is grounded. If the
inftrument weighed ten grains, and the velocity with
which it moved was one inch in a fecond, the quantity
of matter contained in the rays which fell upon the in-
ftrument in that time was equal to the twelve hundred
millioneth part of a grain j the velocity of light ex-
ceeding the velocity with which the inftrument moved
in that proportion. The light in this experiment was
collected from a furface of about three fquare feet,
which reflecting qnly half what falls upon it, the quan-
tity of matter contained in the rays of the fun incident
upon a fquare foot and half of furface is no more than

* Priellley's Optics, p. 385. t See p. 95.

• N 3 one

1 8 2 Waf.e of Light from the Sun's Body. [ Book III.

one twelve hundred millioneth part of a grain. But
the denlVy of the rays of light at the furface of the
fun is greater than at the earth in the proportion of
45,000 to i. There ought therefore to ifiue from
one fquare foot of the fun's furface in one fecond
T^-s-e-s- part of a grain of matter to fupply the coh-
fumption of light j that is at the rate of a little more
than two grains a day, or about 4,752,000 grains, or
670 pounds in 6,000 years, which would have fhort-
ened the fun's diameter about ten feet, if it was form-
ed of matter of the denfity of water only *.

Thus we fee there are little grounds for any rea-
fcnable apprehenfions concerning the body of the fun
becoming exhavifted by the confumption or wafte of
the matter of light, if the immenfity of his diameter
(878,808 £nglifh miles) is confidered. It is, how-
*ever, not impoffible that there are means by which the
fun jnay be enabled to receive back again a part of
that Jight or fire which he is continually emitting ; it
is not impoitible that this world, a/id the other planets,
may have a power of reflecting back a certain portion
of their light within the fphere of the fun's attraction,
or ihat the fixed ftars or funs may have fome power
of replenishing one another. After all, we have no
right to fuppofe our world, or the fyftem of which it
makes a part, defigned for an eternal duration ; its
exiftence is doubtlefs proportioned to the ends which
were intended to be accompiifhed in it ; but with
refpect to the period of it: termination, there is no
chain, of moral or phyfical reafoning which appears to
conduct to any fatisfactory conclusion.

Notwithftanding the minutenefs of the particles of
light, and the amazing velocity with which they are

* PriefUey's Optics, p'. 389.

projected,

Chap. 2.] Bohgnian Pbofphorus. 183

projected, they are found, by a variety of experiments,
to be fubject to the fame laws of ATTRACTION that'
govern all other bodies. ' On this principle the ma-
jority of phiiofophers have explained the phenomena
of the Bolognian flone, and what are called the folar
phofphqri.

The difcovery of the Bolognian phofphorus, as re-
lated by Mr. Lemery, has already been detailed. The
property of imbibing and emitting light is not, how-
ever, confined to one fpecies, but is common to all
the varieties of that mineral, which is called ponderous
ipar.

The light which they emit bears an analogy to that
which they have imbibed. In general, the illuminated
phofphorus is red ; but when a weak light has been
admitted to it, or when it has been received through
pieces of white paper, the emitted light is of a pale
white *.

It has been already remarked f, that an artificial
phofphorus may be obtained from all fubflances which
can be reduced to a calx by burning only, or by folu-
tion in the nitrous acid. Some diamonds, however, as
well as emeralds and other precious ftones, are found
to have the fame property without any chemical pre-
paration ; and a diamond has been known to retain
its virtue of emitting light, after being buried in wax
fix hours J. In fact, Beccaria has obferved, that ai-
med all natural bodies have the power of imbibing
light, and of emitting it in the dark. To metals and
water, however, he could not communicate the flighted
degree of this property -} but he found that although
water in its fluid Hate could not be made to mine

* Prieftley's Optics, p. 363, 364. | See Chap. i.

4. Prieftley's Optics, p. 367.

]Sf 4 fa

184 Diamonds, &c. imbibe Light. [Book III.

in the dark, ice and fnow had this property in a re-
cm ark able degree *.

The light which is emitted from putrid fubftances
and rotten wood, alfo that of ignes fatui, and other
fimilar meteoi-s, proceeds from a different caufe, and
will be explained in another part of this work, to
which the fubject more properly appertains.

To the principle of attraction Newton has alfo re-
ferred the molt extraordinary phenomena of light, re-
fraction and inflexion. That incomparable philofopher
has alfo fhewn that light is not only fubject to the law
of attraction, but of repulfion alfo, fince it is repelled
or reflected from certain bodies ; a property which
was long known, but was never underftood till he gave
us a fyftem, which accounts for the general operations
of nature, upon fewer and fimpler principles, yet in a
manner much more fatisfactory than any which had
preceded. But to enumerate and briefly explain thefe
general properties would extend this chapter, to an im-
proper length, and would probably in fome meafure.
tend to confufe the ftudent.

* Prieftley's Optics, p. 368, 369, 370.

Chap. 3.] [ 185 ]

CHAP. HI.

GENERAL VIEW OF THE PHENOMENA
OF REFLEXION.

Opale Bodies do not reflet! the Whole of the Light that falls on them.
—Law of Reflexion.;— Reflexion from plane Surfaces. — Reflexionfrcm
convex Surfaces.-— From concave Surfaces. — Phenomena of Reflexion
' from plane Mirrors explained.— r-From convex Mirrors. — From con-
cave Mirrors. -^-Prominent Image from a concave Mirror .f^-The
cylindrical Mirror^ — T/ie Rectification of diftorted Figures by this
Mirror. — The conical Mirror.

IT has been already intimated that the rays of light,
which proceed from any luminous body, move al-
ways in ftrait lines, unlefs this direction or motion
is changed by certain circumftances, and thefe are re-
flection, refraction, and inflexion. Of the firft of thefe
it will be proper to treat in this chapter, becaufe by
purfujng the fubject in this order, it will I conceive be
more eafily comprehended by the unfcientific reader.

A common paftime of children with a piece of glafs
oppofed to the fun, and cafting by means of it a vivid
ipot of light in various places at will, proves that the
rays of light may be reflected by certain bodies ; and
more accurate obfervation will convince us, that every-
body that is not luminous in itfelf is made vifible to
qur fenfe by reflected light.

We are not to fuppofe, however, that every opal^e
body reflects the whole of the light which falls upon
it. On the contrary, it is only a limited portion which
is regularly reflected according to the known law of
reflexion ; .another portion may be confidered as ab-
forbed by the body, or as rendered latent by feme

caufe.

i86 The Angle of Reflexion [Book III.

caufe which we cannot at prefent explain, and the
quantity which is thus loft or abforbed differs accord-
ing to the nature and circumftances of the reflecting
iurface.

The great law of reflexion, and which ferves to ex-
plain all its phenomena, is this, that the angle of re-
Jlexion is always equal to the angle cf incidence. It was
already intimated, that by the angle of incidence is
meant the angle made by a ray of light with a per-
pendicular to the reflecting furface at the point where
the ray falls ; and by the angle of reflexion, the angle
which the ray makes with the fame perpendicular on
the other fide *.

The angle of reflexion being thus in all cafes equal
to the angle of incidence, it is evident that the power
which caufes this reflexion is always the fame. No
furface however has hitherto been found, which has
not fome inequalities in it to be difcovered by the
microfcope, and yet thefe inequalities do not? affect the
law thus difcovered of re fleeted rays. It is therefore
uriiverfally concluded, that the power which produces
this effect in the direction of the rays acts at fome dif-
tance from the reficcting furface. Innumerable con-
jectures have been propofed to explain this pheno-
menon, but it muit be confc-ffed that even the fag.i-
cky of a Newron was unable to develope and fully
explain this reflecting power. Tie however attributes
it to the general principle of repulfion.

A ray of light falling perpendicularly on a plane
furface is reflected back exactly in the fame direction

* See note on the beginning of Chap. I. and Plate V. fig. I.
The angle of refl-jxicm \viH alfo be found equal to the angle of in-
cidence on plane furfaces, if meaiured from the ivfu-cting furface,
waich is indeed very coramcn in books of philofcpay acd tre;itiit"-»
on <j\

in

Chap. 3.] equal to the Angle of Incidence. 187

in which it came to the reflecting furface ; rays falling
obliquely obferve the general law of reflexion, and
their angle of reflexion is exactly equal to the angle
of incidence. In Plate V. (fig. i.)/Vis a ray of light
falling perpendicularly on the plane furface a by and it
is reflected back exactly in the fame direction j e c is a
ray falling obliquely on the furface at c3 and it is re-
jected in the direction c d, making the angle of re-
flexion c d P exactly equal to the angle of incidence
c e P, as may be feen by the infpection of the figure *.

Parallel rays falling obliquely on a plane reflecting
furface are reflected parallel, converging rays are re-
flected with the fame degree of convergence, and di-
verging rays equally diverging. In other words, plane
furfaces or mirrors make no change in the natural dif-
pofition of the rays of light.

A mirror is a body the furface of which is poliihed
to fuch a degree as to reflect moft copioufly the rays
of light. In Plate V. fig. i, 2, 3, are plane mirrors:
in fig. 2, the rays d b and c a, which are parallel, after
having reached the furface a b are reflected, the one
"towards b and the other towards k, and in both in-
fiances the angle of reflexion is evidently equal to the
angle of incidence.

The rays d b and c a (fig. 3.) are convergent, an4
without the interpofition of the mirror would unite in
the point E ; but being reflected they unite in the op-
pofite point F, the angle of reflexion with refpect tc*
each being ftill equal to the angle of incidence, as may
be feen by drawing perpendiculars to the points a
and b.

* The reader will fee that the angles d c b and ec a, made
u-'uh the lines of incidence and reflexion and the reflecting fur-
face, are alfo equal.

The

1 8 S Reflexion from plane Surfaces. [Book III.

The rays db and c a (fig. 4.) are on the contrary
divergent, and after reflexion towards b and k pr<:: :.-rve
exactly the fame diftance from each other, as they
•would have had if they had proceeded without inter-
ruption towards F and E, the angle of reflexion being
with rcfpect to each ray itill exactly equal to the an-
gle of incidence.

Thus it is that .plane furfaccs reflect the; rays of
light j but the effects are materially different when the
furfaces are convex or concave, though the fame law
flill obtains with rcfpect to thefe. From a convex
furface parallel rays when reflected are made to di-
verge j convergent rays are reflected lefs convergent,
or are even made to diverge in proportion to the cur-
vature of the furface compared with their c'^ree of
convergence j and divergent rays are rendered more
divergent. Thus, it is the nature of convex furfaces
to fbatter or dilperle the mys of light, and in every
inftance to impede their convergence. From a con-
cave ' furface, on the contrary, parallel rays when re-
fieded are made to converge ; converging rays are
rendered more convergent ; and diverging rays are
made lefs divergent, or even in certain cafes may be
made to converge.

To underftand this part of the fubject, it is neceifary
to be aware that all curvilinear furfaces are compofed
cf right lines infinitely ihorr, or prints -, and the reader
will recollect that only thofe rays which fill perpendi-
cularly on a reflecting furface are reflected back in the
fame direction. Ail curves are arches or fegments of
circles ; if therefore any curvilinear or fpherical fur-
face .is prefentcd to a number of parallel rays, it is
evident that only that ray which Rrikes the fpherical
furface in fuch a direction that it would proceed in a
right line to the center of that circle, of which the re-
flecting

Chap. 3-1 Reflexion from fy&er teal Surfaces. 189

fleeting furface is an arch or fegment, can be faid to
fall perpendicularly upon it, of which the reader may
convince himfelf by drawing a ftrait line with a ruler
at any point of a given circle or curve. All the reft
of the parallel rays therefore falling on the fpherical
furface will fall obliquely upon it, and will confe-
quently be fubject to the general law of reflexion, and
the angle of their reflexion will be. equal to the angle
of their incidence.

Perhaps the fubject will be rendered (till plainer it,
purfuing the idea thrown out in the preceding para-
graph, that all curved are formed of a number of ftrait
lines infinitely fiiort, and inclining to each other like
the ftqnes in the arch of a bridge, I prefent to the
reader the figures 5, 6, 7, which may be imagined fo
many mirrors bent or inclined in the form which is
reprefented in the plate. The rays a b and c d (fig. 5.)
which are parallel, are from their different points of
incidence rendered divergent in h and e\ the angle of
reflection with refpect to each being equal to the an-
gle of incidence.

In figure 6 the rays a b and c d are convergent, and
would without the interpofition of the reflecting fur-
face b d unite in m ; but according to the fame prin-
ciple they now proceed to unite in /, which is more
diftant from the reflecting furface than the point m ;
and it is evident, that if the curvature of the two
branches of the reflecting furface b and d was greater
they might be reflected parallel or even divergent. la
the fame manner in fig. 7, the rays a b and c d which,
without the interpcfition of the convex furface b d>
would diverge but very little at m> become after re-
flexion much more divergent at / ; and the angles of
reflexion will be found in all thefe cafes exactly equal
to the angles of incidence, if meafured from the re-

fleet ine

1 90 D cults Image in common Mirrors. [Book III.

fleeting furface produced or lengthened, as at f g
and i k.

Let now figure 8 reprefent a concave mirror form*
ed upon the fame principles as thofe which we have
been examining of the convex kind. The rays a b,
c d, which were parallel before reflexion, and which
make their angles of reflexion equal to their angles of
incidence (mealured for convenience in this figure from
the reflecting furface produced) become evidently con-
vergent at the point /j upon the fame principles in
fig. 9, the converging rays a b and c dy which would
not have united before they reached the point m> are
now after reflexion united at /, which is much nearer
the reflecting furface. In fine, the divergent rays a b
and c dm fig. 10, which would have become more di-
vergent at m, had they not been intercepted by the re-
flecting furface, become convergent after reflexion,
and are found actually to unite at o.

Mirrors are formed either of metal or of glafs,
which is plated behind with an amalgam of mercury
and tin. The latter are moft in common ufe, but
they are improper for optical inftruments, fuch as
telefcopes, &c. becaufe they commonly prefcnt two
images of rhe fame object, the one vivid and the other
faint, as may be perceived by placing the flame of a
wax taper before a common looking-glafs. The rea-
fon of this double image is, that a part of the rays arc
immediately reflected from the anterior furface of the
glafs,. and thus form the faint image, while the great-
eft part of the rays penetrating the glafs are reflected
by the amalgam, and form the vivid image.

From the principles laid down in the courfe of this
chapter, moft of .the common phenomena of reflexion
may be explained. In PLANE MIRRORS the image ap-
pears of its natural fize, and at the fame diftance be-
hind

. 10.

Chap. 3-] Reflexion from plane Mirrors. 191

hind the glafs as the object is before it. To under-
ftand perfectly the reafon of this, it will be neceflary
to advert to the fubject of vifion as explained for-
merly in a note. It will be remembered, that by the
fpherical form of the eye, and particularly by means
of the cryilalline humour which is placed in the middle
of it, the rays of light are converged, and thofe from
the extreme points of the object crofs each other, fo as
to form an inverted image on that part of the optic
nerve which is called the retina. The apparent mag-
nitude of objects will conlequently depend upon the
fize of the inverted image, or, in other words, upon
the angle which the rays of light form, by entering
the eye from the extremities of any object.

As therefore the angle of reflexion is always equal
to the angle of incidence, it will be evident on the
infpection of Plate VI. fig. i, that the converging
rays K m, L », proceeding from the extremities of
the object K L, and falling on the mirror a b, are re-
flected to the eye at e with the fame degree of conver-
gence, and confequently will caufe the image k I to
be feen under an angle equal to that under which
the object itfelf would nave b^en feen from the point
/ without the interpolation of the mirror. The image
appears alib at a diftance behind the mirror equal to
that at which the object (lands before it. For it muft
be remembered, that objects are rendered vifible to
our eyes not by a fingle ray proceeding from every
point of an object, but that in fact pencils or aggre-
gates of divergent rays proceed from every point of
all viiible objects, which rays are again by the mecha-
nifm of the eye converged to a point on all thofe parts
of the retina whjre the image is depicted. The point
from which the rays diverge is called the focus of di-
vergent rays 3 and the point behind a reflecting furface
from which they appear to diverge is called the -vir-
tual

i$2 $eft Pcfitionfor Lcoking-Gla/es. [fiook Ilf.

tual focus. As therefore the angle of reflexion is ex-
actly equal to the angle of incidence, it is evident,
that the virtual focus will be at the fame diftance be-
hind the mirror as the real focus is at before it. Thus
in fig. 2, the diverging rays c h will after reflexion
appear to diverge from the point g which is behind
the mirror a b, and that point for the reafons affigned
(viz. no alteration being made in the difpofition of
the rays but only in the direction) will be at an equal
diftance behind the mirror with the luminous point c
before it.

As every part of the image appears at a diftance
behind the mirror equal to that at which the object
(lands before it, and as the object K L (fig. i.) is in-
clined or out of the vertical pofition, the image k I ap-
pears aifo inclined. Hence it is evident, that to ex-
hibit objects as they are without any degree of dif-
tortion, looking-glafles mould be always hung in a
vertical pofition, that is, at right angles with the floor.
of the apartment.

It is clear, hov/ever, from what has preceded, that
-the cafe muft be very different with thofe mirrors, the
lurfaces of which are fpherical, whether convex or
concave. Of the former, it has been (hewn that their
property is to fcatter and difperfe the rays of light,
to render thofe divergent which were parallel, to di-
nrinifh the convergence of converging rays, and to
augment the divergence of thofe which diverged be-
fore. T^he firft obvious effect of thefe mirrors, there-
fore, muft be to exhibit the image of the object which
is oppofed to them fmaller thnn it is in reality. For
the angle under which the rays ftrike the eye of the
obferver, mufj: necdlarily be fmaller in proportion to
the convexity of the mirror. Suppofe, for inflance,
the objed C D, Plate VI. fig. 3, placed before the

convex

Chap. 3.] Phenomena of convex Mirror >. 193

convex mirror a b j the two rays C e and D d> which
proceed from the extremities of the- objed, and which,
without the interpolation of the mirror, would converge
at/, are refleded lefs convergent, and unite at it form-
ing an angle much more acute than they would other-
wife have done. The confequence, therefore, of the
vifual angle being fo much more acute is, that the
image g b is proportionably fmaller than the objed it-
felf.

The fecond effed of this difperfion t>f the rays isj
that the image appears at a lefs diftance behind the
glafs than it would have done in a plane mirror. To
underftand this effect, it is necefiary again to advert
to a principle of optics, which has been juft dated, viz.
that objects are rendered vifible not by a fingle ray of
light proceeding from every point of the objed, but that
from every minute point of the furface of every vifi-
ble objed pencils of divergent rays proceed, which are
<again converged on the retina of the fpedator's eye.

Suppofe then G (fig. 4.) a luminous point of
any vifible objed, from which a pencil of diver-
gent rays proceed, and fall upon the convex mir-
ror a b. Thefe rays, agreeably to the nature of thefe
mirrors, are reflected more divergent, and have their
fiditious point of re-union (or virtual focus) g much
nearer to the eye and to the furface of the mirror than
they would otherwife have. The image, therefore, as
may be feen in the figure, inftead of being at a dif-
tance behind the mirror equal to the diftance at which
the objed ftands before it (as would be the cafe in a
plane mirror) will appear at a fmaller diftance, and
this diftance will always be diminifhed in proportion
to the convexity of the mirror.

For the fame reafons an objed of a certain fize,
placed either perpendicularly or obliquely before a

VOL. I. Q convex

194 Principal Fccus of concave Mirrors. [Book III.

convex mirror, will neceflarily appear curved or bent,
becaufe the different points of the object are not at
equal diftances from the furface of the mirror. All
thefe effects will be very apparent from infpecting
one of thofe fmall glafs globes, lined with the common
amalgam for making looking-glaffes, which are fome-
tirnes fufpended in old-fafhioncd apartments. In thefe
the company feated in the room, or round the table,
are reprefented by very minute images, which appear
not at a certain diftance behind as in plane looking-
glafies, but very near the furface of the mirror, and al-
ways in fome degree curved or diftorted.

The effects and phenomena of CONCAVE MIRRORS
will obvioufly, from what has been faid, be the direct
contrary to thofe of the convex kind. The furface of
concave mirrors is generally fpherical (or in the form
of a globe) though that is not always the moft conve-
nient form for optical purpofes, but it is that which is
lead difficult to the workmen.

The general effect of concave mirrors is, we have
already feen,-to render the rays more convergent. The
point in which the converged rays unite is called the
focus of converging rays, but this focus cannot be the
fame for all the rays incident on a concave furface.
The parallel rays a I, de (fig. 5.) are converged by
the mirror at the point F, which is diftant from the
mirror one-fourth part of the diameter of that circle,
of "which the mirror is a part or fection ; and this is
the point which is called the focus cf parallel rays, and it
is the real or principal focus of the mirror. The con-
verging rays/£, h /, are reflected upon the fame prin-
ciples more convergent, and unite at the point K,
nearer to the furface of the mirror than the principal
&cus. In fine, the divergent rays R m and R o, which
proceed from the point R, beyond the principal focus,

unite

Chap. 3.] Phenomena of concave Mirrors. 195

unite at the point P. But if the point of divergence
was nearer the mirror than the principal focus, as for
inftance at K, they would ftill be reflected divergent*
and'would proceed one towards / and the other to-
wards b.

Plane and convex mirrors exhibit, as -has been al-
ready mentioned, the image behind the glafs or mirror,
and in a fituation conformable. to that of the object;
but concave mirrors produce this effect only when the
object is. placed between the principal focus and the
mirror, and £hen the image is larger than the -object.
Let AB (fig. 6.) be the object placed • before the
concave mirror E F, and nearer to the mirror than
its principal focus. The two rays A <?, Rf> which pro •*
ceed from the extremities of the object, and -which,
without the interpofition of the mirror, would converge
at d, are reflected more converging, and unite at £> j
and making an. angle greater or more obtufe than they
would otherwife have done, the image a b is confe-
quently greater than the objtct.

This image too appears 'at a greater diftance behind
the mirror than the object is at before it. The reafon
of this will appear, if we fuppofe A (Plate VII. fig. i.)
a point of any object placed nearer to the mirror than
the principal focus F, whence a pencil of divergent
rays proceeds, and falling on the mirror, are (accord-
cording to the principles before laid down) reflected
lefs divergent, and confequently have their virtual oi'
imaginary focus at a greater diftance than if the objedt
had been placed before a plain mirror.

If, on the contrary, the object is placed farther from
the mirror than the principal focus, as for inftance at e,
the rays eb^edy being only moderately divergent, when
they come in contact with the mirror, are reflected con-
vergent, and will represent at E an image of the ob-
O 2 ject,

1 96 Prominent Image from concave Mtrrtr. [Book III.

ject. If the eye, therefore, is withdrawn to a fufHcient
diftance (to o for example) for the rays to crofs each
other, it will perceive the image at E between the
mirror and itfelf. The reafon of this depends upon
what has been already ftated. Every object is ren-
dered vifible to us by pencils of divergent rays from
every point of that object ; it therefore ceafes to be vi-
fible if thefe rays become parallel or convergent ; and
this happens when the object is not nearer to the
mirror than the principal focus. To render, there-
fore, an object thus fmmted vifible, it is neceflary that
the eye fhould recede fo far beyond the place of the
image E, as to allow the rays to crofs each other, and
meet the eye in a ftate of divergence.

The image is in this cafe always inverted. Such is
the image ba of the object A B (fig. 2.) From this
property of the concave reflector to form the image of
an object, in thefe cafes, before the reflector, many de-
ceptions have been produced, to the great furprize of
the ignorant fpectator. He is made to fee a bottle,
half full of water inverted in the air without lofing a
drop of its contents ; as he advances into a room, he
is tempted to exclaim with Macbeth, " Is this a dag-
ger that I fee before me !" and when he attempts to
grafp it, it vanifhes into the air.

A variety of fimilar appearances may be reprefent-
td, which are all produced by means of a concave
mirror, having an object before it ftrongly illuminat-
ed, care being taken that only the rays of light re-
flected from the object fhall fall upon the concave
reflector, placed in fuch a manner that the image (hall
be in the middle of the adjoining room ; or, if in the
fame room with the object and reflector, a fcreen muft
be placed fo as to prevent the fpectator from difco-
vering them. A hole is then made in the partition
4 between

Chap. 3.] 'The cylindrical Mirror. 197

between the two rooms, or in the fcreen, through
which the rays pafs, by which the image is formed.
The fpectator then, when he cafts his eyes upon the
partition of the fcreen, will, ia certain fituations, re-
ceive the rays coming through this fmall aperture.
He will fee the image formed in the air ; he will have
no idea, if not previously acquainted with optics, of
the nature of the deception j and may either be amuf-
ed, according to the inclination of his friends, with
tempting fruit, or be terrified at the fight of a ghaftly
apparition.

Since it is the property of a concave mirror to caufe
thofe rays which proceed in a parallel direction to its
furface to converge to a focus, and fmce the folar jays,
from the immenfe diftance of that body, may be con-
fidered as parallel, concave mirrors prove very ufeful
burning glaffes, and the focus of parallel rays, or prin-
cipal focus," is their focus or burning point.

CYLINDRICAL MIRRORS, fuch as that reprefented
in figure 3, are employed more for the purpofe of
amuiement than of philofophy. They are called mix-
ed mirrors, becaufe they produce at the fame inftant
the effects of plain and of convex mirrors. Suppofe,
for inftance, G F (fig. 4.) to be the height of fuch a
mirror, and A E an object placed before, or rather be-
low it ; all the rays, which proceed from the points
A, B, C, D, E, falling on the furface G F of the mirror,
and reflected to the eye at O, will reprefent the images
of thefe different points at a, b, £, dy <?, as they would
be reprefented in a plane mirror \ and with refpect to
thefe the dimenfions of the object will not be altered
in the correfponding image. But fmce the mirror is
alfo curved, if we luppofe the fpace ^, /,jy (fig. 5.)
to reprefent a part of its circumference, the rays A q, ,
L r, M s, N /, O AT, P z, F y, being reflected to the
O 3 eye

ReRif cation of diverted Figures [Book 1 II.

eye at Z, will exhibit all thefe points A, L, M, N,
&c. within the fpace af, which will in this direction
diminifh confiderably the dimenfions of the image,
according to the principles already explained in
treating of the convex mirror *. The fame will
take place with refpect to all the points of the object
which are vifible within the lines B Q^G, C R H,
D T I, E S K, concentric to the furface of the mirror.
Thefe parts muft therefore be very much extended in
the drawing or defign, if a perfect image is to be re-
prefented in the mirror. Diftorted drawings of this
•kind are common in the mops of the opticians, which,
on -a cylindrical mirror being placed on the board or
•drawing, difplay perfect figures. The principle of thefe
will, however, be very eafily underftood from what has
been now ftated.

The CONICAL MIRROR is reprefented in fig. 6,
and this is alfo confidered as a mixed mirror ; for, as
well as the cylindrical, it produces at once the effects
of a convex and a plane mirror. Suppofe, for inftance,
the angle C K F (fig. 7.) to reprefent this mirror,
and the lines C K, F K, two of the right lines which
compofe it. Thefe two lines would then anfwer to
two plane mirrors inclined towards each other, and
the rays proceeding from the points A, B, C, falling
on the furface at g, b, /, and reflected towards the eye
at O, would reprefent thefe points as if at the bafe
of the mirror in the oppofite order a, b, c j and the
fame obfervation will apply to the points D, E, F,
. which are reprefented at d> e, f, as well as all thofe
which are in the circles A H D, B I E, C G F. But

* Viz. by dimimlhing the convergence of rays, and confequently
reducing the fize of the image in proportion to the convexity. In
the cylindrical mirror, it muft be obferved, that it is !h the breadth
•nly that this diminution takes place. *

as

\\)\..\.p.-ia8.

{'/'Iff 7.

Chap. 3.] by cylindrical and conical Mirrors. 109

as there do not proceed from each point fimple rays
of light, but pencils of rays, they are modified in this
mirror upon the fame principles as in the convex
mirror, and confequently the image will appear fmaller
than the object, and nearer to the eye than in the plane
mirror.

Hence it will be evident, that we may fee in the
center the image of whatever is painted on the exte-
rior circumference A H D, and the extremities of the
image will be formed from the interior circle C G F ;;
and as the curvature or convexity of the mirror is
greater towards the apex or point of the cone, it follows,
that that which is the moft extended in the object will
be the moft compreffed or concentrated in the image.
Thus the dark part of the board (Plate VIII. fig. i.)
is intended to reprefent in the mirror an ace of fpades ;
and the points a, by £, d, <?,/, g, fcc. which are neareft to
the mirror, form the outer circumference of the image,
and the points i, 2, 3, 4, 5, 6, 7, 8 of the external cir-
cumference of the board unite in the cerrter of the
image almoft at an imperceptible point.

[ 200 ] [Book III,

CHAP. IV.

GENERAL VIEW OF THE PHENOMENA
O F R E F R A C T I O N.

Laws of Refraction. — Degree of Refraftion which Light fujfers front
Jiffertnt Mediums."— Common Phenomena of Refraction.— Refraflioit
by fpherical Surfaces— By convex Surfaces— By concave Surfaces,
— OfLenfe s.— Convex Lenfes.— "Concave Lenfes. — Different Refran-
gibilitj of the Particles of Light. — Experiment with the Prifm.

IT has been proved that light, like every known
fubftance, is fubject to the laws of attraction; it
has been intimated too, that even its propenfity to
move in a direct line is, in certain cafes, overcome by
this fuperior influence; and that the direction of the
rays of light is changed in paffing from one medium
to another. The fpace in which a ray of light moves
is called a medium, whether pure fpace, air, water,
glafs, or any other tranfparent fubftance ; and when a
ray is bent out of its natural courfe in paffing from one
medium to another, it is faid to be refrafted or broken,
probably from the broken appearance which a ftaff,
&c. exhibits when part of it is immerfed in water.

There are two circumftances eiTential to refraction ;
i ft, That the rays of light mall pafs out of on-e me-
dium into another of a different denfity, or of a greater
or lefs degree of refiftance. 2dly, That they pafs in
an oblique direction.

The denfer the refracting medium, or- that into
which the ray paffes, is, the greater will be its refract-
ing power j and of two refracting mediums of the fame

denfity,

Chap. 4'] Angles of Incidence and Refra^kn. 201

denfity, that which is of an oily or inflammable nature
will have a greater refracting power than the other.

The angle of refraction depends on the obliquity of
the rays falling on the refracting furface being fnch al-
ways that the fine of the incident angle is to the fine
of the refracted angle in a given proportion.

The incident angle is the angle made by a ray of
light, and a line drawn perpendicular to the refracting
furface at the point where the light enters the furface;
and the refracted angle is the angle made by the ray
in the refracting medium with the fame perpendicular
produced. The fine of the angle is a line which ferves
to meafure the angle, being drawn from a point in
one leg perpendicular to the other.

In pafling from a rare into a denfe medium, or frora
one denfe medium into a denfer medium, a ray of
light is refracted towards the perpendicular, that is, fo
that the angle of refraction fnall be lefs than the angle
of incidence; on the contrary, in pafiing from a
denfe medium into a rare medium, or from one rare
medium into a rarer, a ray of light is refracted from
the perpendicular. Thus, in paffing from empty fpacc
into air, or any other medium whatever, the ray is
bent towards the perpendicular, and in paffing from
any other medium into pure fpace, it is bent the con-
trary way, that is, from the perpendicular ; the fame
effects will take place in paffing from air into glafs,
and from glafs into air, &c.

To render this perfectly clear, let us have recourfe
to Plate VIII. fig. 2. If a ray of light p C paries from
air into water, in the direction ^>C, perpendicular to the
plane D*/, which feparates the two mediums, it fuffers
no refraction, becaufe one of the eflentials is wanting
' to that effect, viz, the obliquity of the incidence.

But

3O2 Refrafting Power of different Mediums. [Book III.

But if a ray AC paffcs obliquely from air into wa-
ter, inftead of continuing its conrle in the direct line
C B, it takes the direction C a, and approaches the
perpendicular p P, in fuch a manner that the angle of
refraction PCa is lefs than its angle of incidence
$C A.

If the ray came in a more oblique direction, the re-
fraction would be ftill greater; fo that in all cafes,
where the mediums are the fame, the angle of refrac-
tion will always be found to bear a regular and con-
.ftant proportion to the angle of incidence; or, to fpeak
in technical language, the fine of incidence is to the
fine of refraction in a given ratio, and this ratio is dif-
covered by experience. Thus, when a 'ray paffcs out
of air into water, the ratio is as 4 to 3.
out of water into air, as 3 to 4.

air into glafs, as 3 to 2 *.

glafs into air, as 2 to 3.

air into diamond, as 5 to 2.

diamond into air, as 2 to 5.

The refraction of light, we have already feen, is at-
tributed by Sir Ifaac Newton to the principle of at-
traction; and perhaps one of the moft fatisfactory
proofs of this theory is the known fact, that the change
in the direction of the ray commences, not when it
comes in contact with the refracting medium, but a
little before it reaches the furface, and the incurvation
augments in proportion as' it approaches this medium.
Indeed no principle will account for the phenomenon of
light pafling more eafily, that is, more directly, through
a denfe than through a rare medium, but that of at-

* There are feme differences in the refra&ive powers of dif-
ferent glafTcs, according to the nature of the materials ; but thefe
are too minute to deferv« notice in treating of general principles.

traftion,

Chap. 4.] Bottom of a River feems nearer than it is. 203

traction, fmce it is found by univerfal experience, that
the attraction of all bodies is in proportion to their
denfities.

In pafiing from a denfe.into a rare medium, how-
ever, there is a certain degree of obliquity at which
the refraction is changed into reflection. In other
words, a ray of light will not pafs out of a denfe into
a rare medium, if the angle of incidence exceed: a cer-
tain limit, but will be reflected back. Thus a ray of
light will not pafs out of glafs into air, if the angle of
incidence exceeds 40° 1 1, or out of glafs into water,
if the angle of incidence exceeds 59° 20.

As the rays of light, in pafling from a denfe medium
to a rarer, are refracted from the perpendicular, in fad!
are bent or inclined towards the eye of the fpectator, who
looks at an object in the denfer medium while Handing
at its fide, the reafon will be clear why the bottom of a
river appears to us nearer than it really is * ; and why
an oar, partly in and partly out of the water, feems
broken. Let Qj?; a (fig. 3.) reprefent an oar^ the part
m QJbeing out of, and the part ma being in the water3
the rays diverging from a will appear to diverge from
b nearer to the furface of the water, every point in m a
will be found nearer to the furface than its real place,
and the part ma will appear to make an angle with
the part Qni. On this account alfo, a fifli in the wa-
ter appears much nearer the furface than it actually is;
and a fkilful markiman, in fbooting at it, will aim con-
fiderably below the place which it feems to occupy. t

On the fame principle a common experiment is ex-
plained. Put a Ihilling into a bafon, and walk back
from it till the fhilling is juft-obfcured by the fide of

* If the fpeftator ftands on a bank, juft about the level of the
water, it is about one-third deeper than it appears.

the

2O4 Real and apparent rifmg of the Sun> &JV. [Book III.

the bafon ; then by pouring water into the bafon, the
fhilling inftantly appears 5 for by what has been faid
above, the object, being now in a denier medium, is
fnade to appear nearer to its furface.

As the refraction muft in all cafes depend on the ob-
liquity of the ray, that part of any body which is moft
immerfed will feem to be moft materially altered by
the refraction. When, however, the object extends to
no great depth in the water, the figure is not materially
diftorted; but if the object is of a considerable fizc, or
extends to a great depth, thofe rays which proceed
from the more diftant extremities come in a more
oblique direction on their emergence into the air, and
they confequently fuffer a greater refraction than the
reft. Thus a ftrait leaden pipe appears near the bottom
of a deep water to be curved, and a flat bafon feems
deeper in the middle than near the fides.

To thefe laws of refraction is to be attributed the
difference between the real and the apparent rifmg of
the fun, moon, and ftars, above the horizon. The ho-
rizontal refraction is fomething more than half a de-
gree, whence the fun and moon appear above the ho-
rizon when they are entirely below it. From the ho-
rizon the refraction continually decreafes to the zenith.
Refraction is incrcafed by the denfity of the air, and
confequently it is greater in cold countries than in hotj
and it is alfo affected by the degree of cold or heat in
the fame country.

Parallel rays, if refracted, preferve thtir parallel dU
rection both in entering and in pafiing out of a refract-
ing medium, provided the two fur faces of the refract-
ing medium are parallel. The two ray*, E A, EA,
(fig. 4.) after refraction, while they approach the per-
pendiculars pp, continue parallel as before, the reafon
of which is evident on the principles already efhblilh-

VOL. i. p. 10 4-.

Fig. 2.

Chap, 4.] Qbjffts in Water magnified. 205

ed, for the ray AC, (fig 7.) on coming in contact
with the furface of the refracting medium E F, does
not continue its courfe in the {trait line C b, but being
refracted at the point of contact C, it approaches the
perpendicular P p, and comes out at a.

After coming out of the refracting medium, if we
fuppofe the furface G H parallel to E F, it ought to
proceed to B, having deviated from the perpendicular
in the fame degree in which it approached it on its firft
refraction, and thus it continues parallel to the line C B,
which is that in which it would have proceeded, if ic
had not been intercepted by the medium.

This parallelifm cannot fubfift if the two furfaces
KL, HI, (fig. 8.) are inclined, as in the figure, be-
caufe the ray entering at ay and emerging at b, the ob-
ject A will be feen from the point B at e, which is out
of its true place.

Converging rays become lefs convergent in pafling
from a rare to a denfer medium, as from air into wa-
ter ; and on the contrary, their convergence is -aug-
mented by paffing from a denfe to a rarer medium, as
from water into air, (See fig. 5.) In the fame man-
ner, diverging rays become lefs divergent in paffing
put of a rare medium into one which is denfer, and
their divergence h increafed by paffing out of a denfe
into a rarer medium. (See fig. 6.) This fact is A
necefiary confequence of the general law of refraction ;
but it will fatisfactorily explain why an object under
water appears larger to an eye above the furface than
it really is; and why all objects appear magnified feen.
through a mift ; for in all thefe cafes, the converging
rays, by which we fee the extreme points of the object,
and which during their pailage through the water, &c,
were refracted towards the perpendicular, on their
emergence into the air are made more fuddenly to

converge,

ic6 Refraction by ccr.vex Surfaces. [Book lit.

converge, and confequently thevifual angle is rendered
more obtufe.

It is evident, that when parallel rays fall upon a
SPHERICAL SURFACE, that ray only which penetrates to
the center or axis will proceed in a direct courfe, for
all the red muft neceflarily make an angle more or lefs
obtufe, in proportion to their diftance from the c^n-
ter * ; they are therefore rendered convergent or di-
vergent according to the nature of the medium on
which they are incident. If they fall on the CONVEX
SURFACE of a medium denjer than that which they leave,
as in paffing from air into glafs, they will converge, as
may be feen in Plate IX. fig. i. where that pheno-
menon is reprefented ; for the parallel rays, h /, fg,
(tig. 6.) falling in an oblique direction on the refract-
ing medium, terminated by the convex furface Ez£,
they will be refracted, and will each refpectively ap-
proach the perpendiculars z'C, or gC, and will confe-
quently have a tendency to unite towards the axis
AB.

It is however proper to remark, that the point at
which they join the axis A B will be diftant from the
furface of the refracting medium in proportion as the
point on which they fall on the convex furface is dif-
tant from that axis, becaufe the more diftant that point
is, the more oblique is the incidence of the ray. Thus
the ray bi joins the axis at k j but the ray/g does not
meet it till it arrives at D.

Rays already convergent, falling on the convex fur-
face of a denfe medium, will be acted upon differently
according to circumftances.

» See what was obferved on this fubjeft in the preceding
chapter, p. i8S.

If

Chap. 4.] Coavergijie Rajs on a convex Surface. 207

If their convergence is exactly proportioned to the
convexity of the furface, they will not fuffer any re-
fraction j (fee fig. 2.) becaufe in that cafe one of
the efTentials is wanting to refraction, viz. the obliquity
of the incidence, and each ray proceeds in a direct line
to the center of that circle, of which the convex fur-
face is an arch or fegment. For inflance, the rays eft
and dhy (fig. 7.) which tend to unite at C, the center
of the convex furface, may be confidered as. perpendi-
cular, being the radii of the circle.

If the rays have a tendency to converge before they
reach the center of the convexity, xhey will then be.
rendered lefs convergent ; for inftead of converging to
a point at b (fig. 3.) they will converge at B. The
reafon of this is evident, for the ray ib (fig. 7.) which,
if not intercepted, would meet the axis at k, nearer the
lurface of the refracting medium than the center of con-
vexity C, being refracted towards the perpendicular or
radius dC, meets the axis only at o.

If, on the contrary, the rays have a tendency to con-
verge beyond the center of the convexity, they will
then, by the law of refraction, be rendered ftill more
convergent, as in fig. 4, where their point of union,
if not intercepted, would be c, but where, by the influ-
ence of the refraction, they are found to converge at C.
For the ray gb (fig. 7.) the tendency of which is to-
wards /, is refracted towards the perpendicular dCf
and joins the axis zip.

If diverging rays fall on the convex furface of a
denfer medium, they are always rendered lefs divergent,,
as in fig. 5.; and they may be rendered parallel, or
even convergent, according to the degree of divergence
compared with the convexity of the refracting furface,
on the principles already explained.

If

2C& Refraftlon by convex Surfaces. [Book III.

If rays pafs from a denfe to a rarer medium, the fur-
face of the denfe medium being convex *, in this cafe
parallel rays become convergent ; for the parallel rays
de, gi (fig. 8.) when they reach the convex furfacc
e D /', inftead of continuing their direct courfe, are re-v
fracted from the perpendiculars aC> bC, and converge
at*.

Converging rays are alfo rendered more convergent.
Thus the rays /<?> ni, which, without any ehange in the
medium, would have proceeded in the direction m and
0, in confequence of the refraction which they fuffer,
and which bends them from the perpendiculars a C,
b C, unite at p.

Diverging rays, if they proceed from the point C,
the center of convexity, faffer no refraction, becaufe,
for the reafons already affigned, they may be confidcr-
cd as perpendicular to the refracting furface, and con>-
fequently they are deficient in one of the caufes of re-
fraction, the obliquity of incidence.

If they proceed from a point which is nearer to the
furface than the center of convexity, fuch as r, they
will be refracted from the perpendiculars aC, &C, and
will be rendered more divergent towards x andj.

If, on the contrary, the diverging rays come from a
point, fuch as qy beyond the center of convexity, they
will be rendered lefs divergent, for inftead of going to-
wards z and z, they will be refhclcrd from the per-
pendiculars a C, b C, towards/ and h.

When rays pafs from a rare into a denfe medium,
and the furface of the denfe medium is CONCAVE, then
parallel r&ys are rendered divergent, as in Plate X. fig. i.
for the parallel rays ab, de (fig. 5.) are refracted

* The furface of the rare medium consequently being coa-
cave.

towards

VOL. I.

Mate n.

Chap. 4.] Refrafiion by concave Surfaces. 209

towards the perpendiculars /C and g C ; and are con-
fequently divergent.

Converging rays falling on the fame concave furface
will be rendered lefs convergent, as in fig. 2. For
the rays ab, de (fig. 6.) which would have converged
at O, if their progrefs had not been intercepted, will
be refracted towards the perpendiculars /C and g C,
and will unite only at i. If the convergence was lefs,
they might by the refraction be rendered parallel or
even divergent.

Diverging rays proceeding from the center of con-?
cavity will not fuffer any refraction, for the reafons al-
ready afligned.

If, however, diverging rays proceed from any point
nearer the refracting furface than the center of concav-
ity, they will be rendered lefs divergent, as in fig. j.
For the two diverging rays kb and ke (fig. 7.) inftead
of proceeding to d and h, are refracted towards the per-
pendiculars/C and^C.

If, on the contrary, which is the mod general cafe,
the diverging rays proceed from a point more diftant
from the furface than the center of concavity, their di-
vergence will be increafed as in fig. 4. For the di-
verging rays Ib and le (fig. 7.) which tend towards
m and n, are refracted towards the perpendiculars /C
and g C, and become more divergent than they would
otherwife have been.

When rays pafs from a denfe into a rarer medium,
and the denfe medium is terminated by a concave fur-
face, then

Parallel rays become divergent; for the parallel rays
de, gi (fig. 8.) when they reach the concave furface
e D /, inftead of continuing their courfe in the direct
lines towards /and b, proceed towards m and^>, being

VOL. I. P refracted

Convex burning Qaffes. [Book III.

refracted from the perpendiculars Ca, C£, acd arc
confequently divergent.

Converging rays, if their point of convergence is pre-
cifely a,t C, the center of the concavity e D /, will not
fuffer any refraction, becaufe they are perpendiculars,
as already explained, therefore have no obliquity of in-
cidence. If, on the other hand, the rays tend to a point,'
fiich as n, nearer to the furface than the center of the
concavity C, then they are rendered more convergent,
for the rays q <?, r i, which naturally tend to that point,
are refracted from the perpendiculars C e, C /', and con-
verge at o, nearer the concave furface.

Laftly, if the converging rays tend to a point /, which
is beyond the center C, they are rendered lefs conver-
gent. For the rays se, //', which would naturally
unite at that point, are refracted from the perpendicu-
lars Ct, d, and unite at k3 which is more diftant
dill.

Diverging rays in the fame circumftances are ren-
dered more divergent. For the rays E <?, E./, diverg-
ing from the point E, inftead of proceeding towards «
and xy are refracted from the perpendiculars, and are
directed towards y and z.

From the property which all fpherical convex fur-
faces have of rendering parallel rays pafiing out of a
rarer medium convergent, glafles made in this form
are very commonly ufed as burning glafies; and as the
fun's rays, proceeding from fo vail a diftance, may be
confidered as parallel, the focus of parallel rays will of
courfe be their burning point.

A LENS is a tranfparent body of a different denfity
from the furrounding medium, and terminated by two
furfaces, either botl* fpherical, or the one plane and
tlie other fpherical, whether convex or concave. They
are therefore generally diftinguifhed by their forms,

and

A'O.L.J./'. MO.

Chap. 4-1 Convex Lenfes. 211

and are called plano-convex, or plano-concave; or
double convex or double concave; a lens which has
one fide convex and the other concave, is called a
menifcus, or concave- convex lens. See Plate X.

fig- 9;

It is evident that in lenfes 'there may be almoft an

infinite variety with refpecl: to the degree of convexity
or concavity, for every convex furface is to be confi-
dered as the fegment of a circle, the diameter and ra-
dius of which may vary to almoft an infinite extent.
Hence, when opticians fpeak of the length of the ra-
dius as applied. to a lens, as for inftance, when they fay
its radius is 3 or 6 inches, they mean that the convex
furface of the glafs is -the part of a circle, the radius of
which, or half the diameter, is 3 or 6 inches.

The axis of a lens is a ftrait line drawn through the
center of its fpherical furface; and as the fpherical
fides of every lens are arches of circles, the axis of the
lens would pafs exactly through the centers of that
circle, of which its fides are. arches or fegments.

From what has been already ftated in- the former
part of this chapter, it is obvious that the certain effect
of a CONVEX LENS muft be to render parallel rays con-
vergent; to augment the convergence of converging
rays; to diminifh in like manner the divergence of di-
verging rays, and in fome cafes to make them parallel
or even convergent, according to the degree of diver-
gence, compared with the convexity of the lens. In
what is called a double convex lens, this effect will be
increafed in a duplicate proportion, fince both furfaces
will aft in the fame manner upon the rays ; and fince
it has been proved, that parallel or convergent rays
have their convergence equally augmented by being
•incident on the convex furface of a denfe, or the con-
cave furface of a rare medium. Thefe gUfles then
P 2 muft

212 Magnifying Glajes. [Book lit.

mufl necefftrily have the effect of magnifying glades,
fmce by the convergence of the rays the vilual angle is
rendered more obtufe, and confequently the image
which is depicted on the retina muft be proportionably
larger.

The mode of rinding, upon mathematical principles,
the focus of parallel rays or principal focus in thefe
glades, will be explained in a fucceeding chapter!} and
it may be eaiily found, though not with equal exact-
nefs, by holding a iheet of paper before the glafs when
expofed to the rays of the fun, and obfcrving the dif-
tance of the paper from the glafs, when the luminous
fpot on the paper is very final), and when it begins to
burn ; or when the focal length does not exceed three
feet, the focus may be found by holding the lens at
fuch a diftance from the wall oppofite a window fafli,
that the image of the fafh may appear diftinct upon
the wall. The principal focus, or as it is often called
the focus, of a double convex lens is at the length
of the radius, or fernidiameter of that circle which is
formed by the convexity of cither of its furfaces.

From this property in convex Icnfes of rendering all
rays in fome degree convergent which fall upon their
•furfaces, it is evident that in all fuch cafes there muft:
be a point or focus, where rays proceeding from the
.extreme point of any object muft crofs each other; and
confequently an inverted image of the object will be
exhibited at any diftance beyond that point. This
may be elucidated by a very eafy experiment, viz. by
holding a common reading or magnifying glafs be-
tween a candle and a flieet of paper fufpended on the
wall, at a proper diftance, when the image of the
candle will appear on the paper inverted j and the rea-
. fon of this is extremely clear, for it is evident that the
upper rays, after refraction, are thofe which proceeded

from

Chap. 4-] Concave Lenjes. 213

from the under part of the luminous body, and the un-
der rays are thofe which come from its top. The rays
are therefore only inverted, and the image remains un>-
impaired.

From the fame property, convex lenfes will caufe
many rays to enter the eye which would other wife
have been fcattered or difperfed, and therefore objects
feen through them appear clearer and more fplendid,
than when viewed by the naked eye. If, however, the.
glafs is very thick, fome of the rays which enter it will
be reflected or fent back, and confequently the bril-
liancy of the image will fuffer lome diminution.

, A large object feen through a lens which is very
convex, will appear deformed ; and this proceeds from
the refraction not being equal at all points in fuch
cafes. The fame caufe operates allb to render forne
parts T>f the image indiftinct, while others are diftinct
and clear. Thus the extremities of the image feen
through a lens of a very fhort focus are commonly
confufcd and indiftinct, becaufe the refraction at the'
edges of the lens do not agree with that of the middle
parts. This defect in optical glafles* has in fome mea-
fure been remedied by the ingenious invention of Mr.
Dolland, of which we mail have afterwards to treat.

The effects of a CONCAVE LENS are directly oppofite
to thofe of the convex lens. In other words, by filch
a glafs, parallel rays are rendered divergent, converg-
ing rays have their convergence diminiihed, and di-
verging rays have their divergence augmented in pro-
portion to the concavity of the lens. Thefe glafles
then exhibit objects fmaller than they really are, for by
caufing the rays to diverge, or more properly by di-
minifhing the convergence of the rays proceeding from
the extreme points of the object, the vifual angle is
rendered more acute, and the image painted on the
P 3 retina

214 Different Refrangilility [Book III.

retina is fmaller than it would have been, had thefc
rays not been intercepted in their natural progrefs ;
and by the divergence of the rays the object is repre-
fented with lefs clearnefs than it would otherwife have
had, fince from this caufe a lefs quantity of light in
fact enters the pupil of the eye. All concave lenfes
have a negative or virtual focus, which is a point cor-
refponding with the divergence of parallel rays inci-
dent on the furface of the lens.

Light is, however, not fo fimple a fubftance as it
may be fuppofed upon fuperficially confidering its
general effects ; it is indeed found to confift of particles
which are DIFFERENTLY REFRANGIBLE, that is, fomc
of them may be refracted more than others in palling
through certain mediums, whence they are fuppofed
by philofophers to be different in fize. The common
optical inilrument, called a prifm, is a triangular piece
of glafs, through which, if a pencil or collection of
rays is made to pafs, it is found that the rays do not
proceed parallel to each other on their emergence,
but produce on an oppofite wall, or any plain furface
that receives them, an oblong fpectrum, which is va-
rioufly coloured, and it confequemly follows that fome
of the rays or particles are more refrangible than
Others.

The fpectrum thus formed is, perhaps, the moft
beautiful object which any of the experiments of phi-
lofcphy prcfent to our view. The lower part, which
confifts of the leaft refrangible rays, is of a lively red,
which, higher up, by infenfible gradations, becomes an
orange ; the orange, in the fame manner, is fucceed-
cd by a yellow -, the yellow, by a green j the green,
by a blue ; r.fter which follows a deep blue or indigo ;
and Jaftiy, a faint violet.

In the two fucceeding chapters the principles which

have

C hap. 4.3 ef the Rays of Light. 2 1 $

have been juft introduced to the notice of the reader
will be further explained and elucidated -, but as the
application of the general doctrines of reflection and
refraction to optical fcience can only be underftood
upon mathematical principles, I have thought it pro-
per to diftinguifh thofe chapters by the fcientific and
technical words, catoptrics and dioptrics. In the mean
time, the majority of readers will find fufficient to la-
tisfy their curiofity, and to afford them a general view
of the nature and effects of this wonderful fluid in the
preceding obfervations, and they may therefore pro-
ceed immediately to the feventh chapter, which treats
of vifion and optical glares.

[ 2i6 ] [Book III.

CHAP. V.

OF THE PRINCIPLES OF CATOPTRICS,

Places cf Image* in plane Refleftors.-^-tt'hy a Mirror only Half the
Size of an Qbjctt exhibits a perfefl Image of the Whole.- —Places,
of Images made from Reflexion by fpherical Surfaces.— -Mode of de-
termining the Foci of reflected Rays from fpherical Surfaces.— S^xe
and Proportions of Images in fpherical Refleflors.— Phenomena of
concave and convex Speculum* explained*

BY the application of mathematical principles to
the few fimple fafts with which experiment fur-
nifties us, concerning the reflexion and refraction of
light, the phenomena of vifion have been reduced to a
fcience ; and every particular which it becomes ne-
cefiary to know either with refpeft to the fimple
effects of light on the human eye, or the ufe of glafies,
may be calculated by certain rules with the minuteft
exaftnefs. This complex fcience is called OPTICS (or
the fcience of vifion) and it may be fubdivided into
two branches, called in fcientific language catoptrics
and dioptrics.

The firft of thefe relates to the theory of reflex
vifion, and fupplies us with rules, principles, and
modes of calculation, by which all the effefts refult-
ing from the reflexion of light may be determined
and explained ; and of this it will be proper to treat,
before we proceed to the other ftill more important
branch of optical fcience.

The whole of the theory of catoptrics is founded

upon,

Chap, 4.] Catoptrics. 217

upon a plain and fimple principle, which has been al-
ready explained, viz. that the angle of reflexion is al-
ways equal to the angle of incidence. Thus let Q^a
Plate XL (fig. i,) be a point from which rays diverg-
ing fall on the reflecting furface A B, and let QJD,
QJi, be two incident rays. At D, E draw the per-
pendiculars D C, E F to A B, and make the angles
C D G, H E F equal to QJD C,. QjL R and the rays
QJD, QJ£ will be reflected by the furface in the di-
rections D G, E H.

The point Q^, from which the rays diverge, is
called the focus of diverging rays j and as, after re-
flexion, the rays appear to have diverged from a point
behind the furface, that point is called the focus of re-
flected rays. To find this point, produce the lines
G D, H E till they meet the perpendicular drawn
from Q^on the reflecting furface produced, if necef-
fary. Let QJVE q be this perpendicular, which G D
meets in q j then, fince QJD C is equal to GDC,
QJD M is equal to G D B, but G D B is equal to
M D q ; in the two triangles QJD M, M D ^, there
are two angles in the one equal to two angles in the
other, and one fide M D common to both, therefore
Q^M is equal to M g. The fame may be proved
alfo of the interfection of the lines H E q and Q^M q.
Therefore the focus of rays reflected by a plane fur-
face is at the fame diftance behind the furface, as the
focus of diverging rays is before it.

If, inftead of rays diverging from one point they
diverge from feveral, the correfponding foci will be
found in the fame manner. Let QJR. (fig. 2.) be a
furface, from every point of which draw perpendicu-
lars to the reflecting furface as before, and q r will be
the image of QJR, or all the rays diverging from

2 1 S Image In flam Reflefttjrs. [Book III.

QJR. will, after reflection, appear to have diverged
from q r.

Every object placed before a reflecting furface has
its correfponding image. If the object is a plane
furface, the image will alfo be a fimilar plane furface ;
if the object is a curvilinear furface, the image will
correfpond to it ; and in all cafes it is found in this
manner, by perpendiculars drawn from the object
to the reflecting furface, or the reflecting furface pro-
duced.

To fee any object, the eye muft be fo placed that
fome of the rays of light diverging from the object
juay fail upon the eye j and if, by looking upon a re-
flecting furface we fee an image, we Ihould, if our
judgment had not been corrected by experience, con-
ceive an object to be placed behind the furface from
which thefe rays diverged. Now, as an object may
be placed in fuc'h a fituation before the reflecting fur-
face that no rays can be reflected to our eyes, we
fhall not always fee an object by reflection, and the
places of the object, the fpectator, and the reflecting
furface, mufl be taken into consideration. Let QJR
be an object before the reflecting furface A B, q r its
image, as before (fig. 2.) and O the place of the fpec-
tator. Join O A, O B, and produce the lines O A,
OB indefinitely to T and P, unlefs the image lies
within the lines A T, B P the object will not be feen
by reflexion. Let/£ without this fpace be the image
of F G, and join OfOg, and fmce thefe two lines do
not any where cut the reflecting furface, it is evident
that by looking on the furface we fhall not fee the ob-
ject. We fhall fee part of QJR, becaufe part of q r
lies within the fpace abovementioned, and to find the
part of QJl which is vifible by reflexion from the
point s, where O P cuts £ r, draw s S perpendicular
S t*

Chap-. 5 .] Refleftor Half the Size of the Otjeff. 2 T 9

to A B produced. Then the ray S B will be refiect-
ed in the direction B 0. Now join O q cutting A B
in D, and join QJ). The ray QJ) will be refleded
in tire direction D O, and the part of the object vifible
by reflexion will be feen in part of the reflecting fur-
face only D B. All the reft being fuperfluons as to
this objecl. Thus we can always find by what rays,
and by what part of a reflecting furface, an object is
feen. The limits of the fpace in which an objecl:
muft be placed to appear vifible by reflexion are, on
thefe principles, eafily determined. Join O B, O A,
and make the angles I B K, L A E equal to OBI,
O A L, then every objecl: placed within the lines B K,
A E indefinitely produced, will be vifible at O by re-
flexion.

Thus, when we are placed before a looking-glafc
in a room, part of the room only is vifible -, as we
walk backwards and forwards other parts appear and
dif;ippear in fucceflion, and fome parts of the room are
never feen in the glais.

When a perfon (lands before a looking-glafs of the
fame dimenfions with himfelf, hir. image appears to
occupy the half of it, or, in other words, a looking-
glafs of half his dimenfions is capable of mewing him
the whole of his figure. Let A B (fig. 3.) be an ob-
ject placed before the reflecting furface gb i of the
plane mirror C D -, and let the eye be at o. Let A b
be a ray of light flowing from the top A of the objecl:,
and falling upon the mirror at h : and b m be a. per-*
pendicular to the furface of the mirror at b, the ray
A b will be reflected from the mirror to the -eye at o,
making . an angle mho equal to the angle A km:
then will the top 'of the image E appear to the eye in
the direction of the reflected ray o b produced to E,
where the right line ACE, from the top of the ob-

1 20 Rtfleflor Half the Size of the ObjeH. [Book III.

jcct, cuts the right line o h E, at E. Let B i be a ray
of light proceeding from the foot of the object at B
to the mfrror at /, and n i a perpendicular to the mir-
r.or from the point /', where the ray B / falls upon it ;
this ray will be reflected in the line i o, making an
angle n i 0,, equal to the angle B in, with that perpen-
dicular, and entering the eye at o : then will the foot
F of the image appear in the direction of the reflected
ray oi, produced to F, where the right line BF cuts
die reflected ray produced to F. All the other rays
that flow from the intermediate points of the object
A B, and fall upon the mirror between b and i, will
be reflected to the eye at o ; and all the interme-
diate points of the image E F will appear to the
eye in the direction of thefe reflected rays produced.
But all the rays that flow from the object, and fall
upon the mirror above hy will be reflected back above
the eye at o j and all the rays that flow from the ob-
ject, and fall upon the mirror below /, will be re-
flected back below the eye at o : fo that none of the
rriys that fall above £, or below i, can be reflected to
the eye at o ; and the diftance between h and / is
equal to half the length of the object A B, if the cye^
or o, is in the line A B produced : for then A b will
be equal to k 0, and A h is equal to b E. Therefore
Jj E is equal to o h, and o b is one half of o E3 and
confequently i b (from fimilar triangles) is equal to
one half of E F or A B.

In rooms where looking-glafles are placed parallel
and oppofite to each other, a perfon looking into one
fees feveral images of himfelf •, for rays will be re-
flected from one glafs to the other, and each image
becomes an object to the other glafs. If, inftead of
be'.ng parallel, the glafles were placed oppofite to each
other,, but making an acute angle, there will be fe-

veral

\"~O\..\./>.220.

Plate JJ.

./'/>/. .7.

c: IF

k

C hap. $.] Places of Images made by Reflexion. 2 If

veral images of the fame object appearing to be placed
in a circle, whofe center is the vertex of the acute
angle, and radius the diftance of the object from that
vertex. Let M N O P, Plate XII. (fig. 4-) two fur-
faces produced, meet in b, and let QJbe an object, and
<7, c its images in the refpective glafTes. Join bq,
b Q^> and the triangles Q^a b, q a b, having twd fides
and an angle rcfpectively equal, the third fide Q£ is
equal to q b. So b c is equal to b Qj and in the
fame manner the images of q, and c found in the op-
pofite glaffes, will be equidiftant from b.

The places of images, made by the reflexion of the
rays of light from plane furfaces, are cafily determin-
ed: but when rays are reflected by curvilinear fur-
faces, the diffisulty of determining the place of the
image is confiderably increafed. I fhall endeavour to
fhew the manner of inveftigating this fubject in the
fimpleft cafes.

Let A B (fig. 5.) be a fpherical furface, of which C
is the center, reflecting the rays of light both on the
concave and convex fide ; and let QE, a ray of light
parallel to the radius C D, be incident on the furface
at E. After reflexion on the concave fide, the ray
will proceed in the direction E q> making the angle
y E C equal to QJE C ; but the ray Q^E reflected by
the convex furface will proceed in the direction E K,
making the angle K E I equal to the angle Q^E I.
The greater the diftance of E, the point of incidence,
is from D, the vertex of the furface, the farther will the
interfection of the reflected ray and the radius C D be
from the center of the furface. Since QJi, is parallel
to C D, the angle QJE C is equal to the angle E C q ;
therefore the angles q E C, q C E are equal to each
other, and confequently q C is equal to q E. If E is
very near to D, q D and q E will be very nearly equal

to

tii Places of Images. [Book HI.

each other, and the point q will then be very near -to
T, the point bifecting the radius of the fur face. The
parallel rays then falling upon the concave fu'rface1
very near to D will converge, after reflexion, very
nearly to the point T, and that point may be confi-
de red, and is confidered, as the focus after reflexion
gf ihoie rays ; the aberration of every other ray, or the
diilance q T fhall be afterwards confidered. The pa-
rallel rays falling on the convex fide will alfo, after
reflexion,, appear to have diverged from this point T,
without any very material error. We may lay it
do-wn, therefore, as a principle, that rays falling upon
a reflecting furface will, by the concave fide, be made
to converge to a point bifecting a radius drawn pa-
rallel to them, and by the convex fide will be reflected
fo as to appear to diverge from a point bifecting the
radius drawn parallel to them.

Let now (fig. 6.) the rays diverging from a cer-
tain point be intercepted by a fpherical reflecting fur-
face, and let Q^be that point, and A B the furface of
•which C is the center ; and let q E be the reflected
ray. Draw C m parallel to q E, and C ;/ parallel to
Q^E. By the principle above mentioned a ray di-
verging from the point m will, after reflexion, cut the
parallel radius Cn in #, bifecting the radius in that
point ; and, if a ray diverges from «, it will, after re-
flexion, cut the parallel radius C m in m, bifecting that
radius; therefore C;;;,»Cw, CT are equal. Since
the triangles Qjn C, C n q are fimilar, Qj» : m C, or
C T : : C », or C T : 7; q. The nearer E is to D, the
nearer will the points m and n be to T j Q^nt will be
nearly equal to QJ, and q n to q T. Therefore the
focus of rays, after reflexion, will be found, without
very material error, by. faying, as QJT : C T : : C T
: T q. Calling therefore T the principal focu^, its

diftance

Fja. 4.

Fta, (!.

Chap. 5-] Rules for finding the Focus of fefleaed Rays. 223

diftance from the center of the furface will be a mean
proportional between its diftances from the foci of di-
Yerging and reflected rays.

In Plate XIII. fig. 7. the proportion is in the fame
manner demonftrated, by faying, that rays appearing
to diverge from w, were reflected by the furface in-
tercepting rays converging to ;/, and vice ver.fa.

The foci of diverging and reflected rays are always
on the fame fide as the principal focus. The greater
the diftance of Q_ from T, the lefs is the diftance
of q from T ; as, Q, approaches to T,-y recedes from
it j they medt together when rays are reflected by a
concave furface in the center. When the focus of
diverging rays is between the center and the princi-
pal focus, the focus of reflected rays is on the other
fide of the center. When Q^is ki T, the reflected
rays are parallel ; when QJs between T and the fur-
face, the rays appear to diverge from a point on the
other fide of the furface. When rays are reflected by
the convex furface, the focus of the reflected rays is
always between the principal focus and the furface.

Having found the focus of reflected rays for a fingle
point, we can, as before, find the fituation of the image
of any object, by confidering the object as made up
of innumerable foci of diverging rays. Let QJR.
(fig. 8.) reprefent an object before a fpherical reflector,
then join QJ3 D, and in the line QJ3 find the point
q, the focus of rays after reflexion, by the proportion
laid down in the preceding inftance. In the fame
manner find the point r, and, if neceflary, find the
correfponding foci to other points in the object Q^R,
then q r is its image. This image will be either erect
or inverted, according to the nature of the reflector,
and the pofirion of the object. Firft, if the reflector
is a fpherical concave, as in fig. 8, and the diftance of

the

224 CorYcfrondehce of linage with Oljetf. [BooIcIIL

the object from the furface is greater than half the ra-
dius of the reflector, the image will be inverted, and
on the fame fide of the reflector with the object. If
the diftance of the object from the reflector is lefs than
half the radius, the image will be erect, but on the
other fide of the reflector. This is feen in fig. 9,
\vhere q r reprefents an object in the fituation above-
mentioned, and QJl is its image, idly, The image
of an object before a convex fpherical reflector is al-
ways erect, as may be feen in fig. 9.

In plane reflectors images correfpond, and are fi-
milar to their objects. It is not fo in fpherical re-
flectors, by which an image is made fometimes greater,
fometimes fmaller than the object. The concave re-
flector has the power of diminifhing and magnifying.
"When the diftance of the object from the reflector is
greater than the radius, the image is always lefs than
the object, for q r : QJl : : C q : C Qj and in that
cafe C q being lefs than C Q^, q r muft be lefs than
QJR. When the object is between the center and
the principal focus, the image is greater than the ob-
ject, for now q r being the object, QJl is its image,
and C q being lefs than C Q^, the object muft be lefs
than its image. When the object is between the
principal focus and the reflector the image is greater
than the object; for fuppofing qr (fig. 9.) to be the
object, and QJl the image, q r : QJl : : T q : T Qj
bur T q being lefs than T Q^, q r muft be lefs than
QR. When an object is placed before a convex
fpherical reflector, the image is lefs than its object ;
for (fig. 9.) T Q^ being greater than T qt QJl muft
be greater than q r.

To find whether an object may be feen in a re-
factor by a fpectator in any fituation, we draw lines
from the eye to the image, and, if thefe lines are cut
by the reflecting furface, the image is vifiblc, and the

part

/^J* •

Chap. 5.] Beautiful and deformed Images. 025

part of the reflecting furface intercepted between th:fe
fines is that part which reflects the rays to the c^e.
Let O be the eye of the fpectator, Fig. 8, 9, loin O qy
O r> and produce themj if neceilary, till they cut the
reflecting furface in m and #, iiien m n is the pa^t of
the reflecting fur :::u.e :n which the image is feen } and
the rays Qj;;> R n, refined in the direction m O, n O,
are thofe by which the extreme parts of the object are
leen.

This would be ftrictly true in all cafes, if rays pro-
seeding from an object made ^it always vifible and
clear -, but we are accuilomed from our infancy to de-
termine on the nature and pofition of objects by rays
'diverging from them. To fee, therefore, by reflected
rays, they mud appear to the eye to diverge from
the image, which will not be the cafe when the eye
is at a lefs diftance from a concave reflector than the
image. In that cafe our vifion is confufed, the image
is behind us, and we can form ito conception of it.
But this will be farther explained when I come to
treat on the nature of vifion.

Upon the principles now laid down, we fee the rea-
Jbn of thofe beautiful and deformed images made by
'objects placed before fpherical reflectors, as alfo the
changes produced in tfcem by their various pofitions
with refpect to the reflector. When a perfon is at a
greater diilance from a concave fphefical refle6tor than
the radius, he perceives an image, for in{tance> of him-
felfj much diminiihed, itanding upon its head before
the reflecting furface in the air ; as he walks towards
the center^ the image walks towards him, increafing in
magnitude j as he walks from the center to the prin-
cipal focus, his image appears confufed, and he cannot
afcertain any of its parts -, as he walks from the prin-
cipal focus, to the furface, the image is again clearly
VOL. I; Q vifible,

a 26 Ufe cf Convex Mirrors to Travellers. [Book III,

vifible, erect, greater than hjmfelf, but walking to-
wards him, and diminifhing conftantly, till both object
and image me^t together in the reflecting furface.

The pnenomena of convex fpeculums are different,
and in moft refpects oppofitc, to thofc of the concave
ipeculum. When a perfon looks in a convex fphe-
rical reflector, he fees an image of himfclf, erect, but
diminiflied As he walks towards the reflector, the.
image appears to walk towards him, oonftantly in-
creafing in magnitude, till they touch each other in
the refle&ing furface.

From this property of diminifhing objects, fpherical
reflectors are in great requeit with all lovers of pic-
turefque fcenery. Small convex reflectors are made
in the mops for the ufe of travellers, who, when fa-
tigued by ftretching the eye to alps towering on alps,
can by their mirror bring thefe fublime objects into a
narrow compafs, and gratify the fight by pictures which
xthe art of man in vain attempts to imitate.

Chap. ^.] t 227 ]

CHAP. VL

<DF THE PRINCIPLES OF DIOPTRICS.

Dioptrics relate to the Effect; of Refra3ion.—In what Manntr to find,
the aSual Situation of an Qbjecl feen in a different Medium.— Thf
apparent Situation of an Qbjefl feen through a Ghifs Window dif-
ferent from the real one.—~The Effects of tranfparent M:dia <iuitb
.fpherical Surfaces on the Rays of Light vbbicb are tranfmitted through
them.— Rules for finding the Focus of Rajs pajjlng through fucb Me-
diums.-—Theory of Lenfes.— Plano-convex and flano-c onca-Tjs Lenfts.
—Rules to find the Focus of a Lens. — Focus of diverging Rays in-
tercepted to a Lens^—^Focus of a Sphere,

TI^E word dioptrics is compounded of two Greek
words, which mean to fee through-, and this
branch of the fcience of optics furniihes us with rules
and principles to determine with accuracy the effects
of tranfparent mediums upon the rays of light, what-
ever may be the form of the furface through which
the light is made to pafs. Thus, in an extenfive fenfe,
idioptrics would include the whole theory of optical
glafles, and even of vifion itfelf*. For the convenience
of the ftudent, however, I have thought it bed to treat
ieparately of thefe fubje&s, and {hall therefore in this
ch ipter include only the general principles.

As the whole of dioptrics is founded on the laws of re-
fra<£tion, it will be necefTary to recur to what was advan-
ced on that fubjecl: in our fourth chapter, and to recollect
that the angle of refraction is in a given ratio to the angle
of incidence. Let H F for inftance be a ray of light
incident on the furface AB, (Plate XIV. Fig. 10.) of
fc denfe medium ABCD,.fuppofe it to be glafs fur-
i rounded

228 Angles of Incidence and Refraklhn. [Book III,

rounded by air. Draw E F M perpendicular to A B,
and make G F M fuch an angle, that the fine of
HFE fhall be to the fine of M FG as 3 to 2, and the
ray H F will in the glafs move in the direction G F.
When the ray comes to G it fufFcrs another change in
its direction by moving into air, and to find this di-
rection, draw I G N perpendicular to C D, and make
L G N fuch an angle, that the fine of F G I fhali be
to the fine of N'G L as 2 to 3, then the ray will move
in the direction G L. Thus the whole progrefs of the
ray is found to be in the direction H FG L, and by the
fame rule its progrefs through any number of mediums
might be found.

The direction G L (in Fig. 10.) is parallel to the
direction H F ; for the angles M F G, F G I, N G K,
are equal ; and fince the fine of M F G is to the fine
of MFI as the fine of N G K is to the fine of NGL,
the angles NGL, MFI, are equal, and confequently
the angles NGL, N I O, are equal. There-fore the
lines HO, GL, are parallel.

1 Let FG (Fig. ic.) be a ray in a denfe medium in-
cipient on G, and its direction after emergence be G L.
The greater the .angle FGI is, the greater will be the
angle N'G L. Suppofe N G L to be a right angle,
then the fine of FGI is to the radius as the fine of in-
cidence to the fine of refraction, and according to the
law of refraction for the given medium, the limiting
angle of incidence will be found for a ray to emerge.
When the angle of incidence is greater than this angle
thus found, the incident ray will be reflected buck in
die direction abc> as was explained in a -preceding
chapter.

Let rays diverging from a point Q^(Fig. 1 1, 12.)
after refraction, move in the medium ABCD. To a
pcrfon in this medium they will not appear to have*

diverged

Chap. 6.] Real and apparent Places of Objeffs. 229

diverged from Qj but from a point near to or farther
from him, according as the medium, in which he is, is
denfer or rarer than the medium in which the point
of diverging rays is fituated. Let QJ be an incident
ray proceeding after refraction in the direction I M,
cutting QJ) a perpendicular to the furface A B in q,
q will be the point from which the rays appear to di-
verge. In the triangle QJ qy QJ \\q\\ fine ofl^O
: fines of I QJ3 ; that is, fmce QJ3 is parallel to I P
:: fine of refra6Uon : fine of incidence. If I is very
near to O, the lines QJ, ql, will be very nearly equal
to QJD and q O, and a perfon being placed in the di-
rection QJD produced will conceive that the rays di-
verged from q, when QP : qQ :: fine of refraction :
fine of incidence.

Upon this principle we can find the actual fituation
of any object feen in a medium different from that in
which we are, or feen through different mediums.
Let Q^R (Fig. 13, 14.) be any object feen by a pcr-
Ibn in the medium ABCD. Then make QJE : ^E
and R F : r F : : fine of refraction to the fine of inci-
dence, a,nd the object will appear to be at q r nearly,
if the perfon was in the direction QJi produced. Let
O be the place of the perfon's eye in any other fitua-
tion, and join O r, O q, then the object is feen by rays
refracted within the furface mn, and Q^O, R;/O, arc
the directions nearly of the extreme rays by which the
object is vifible.

As we are accuflomed to fee objects frequently
through thin panes of glafs, it may, to prevent milap-
prehenfions on this fubject, be neceffary to fhew what
changes take place in the apparent fituation of thefe
objects from the intervention of fuch a medium.

Let ABCD (F?g. 15.) be a pane of glafs, QJ}. an

object feen through ita whofe apparent place, fo.und by

Q.3 the

4jo Place of an Olje ft feen through a Window. [Book HI.

the preceding rules, is xr> and let Qinop reprefent
the progrefs of one of the rays diverging from Q^
Then, from what has been before obferved, xp is pa-^
rallel to Qni. Therefore q m : m o : : Q^ : Qj, that
is, when m is very near to E,

qE : EF :: ,Q^ : Q#}
or, ^E: Q^q :: EF : Qjc.

But fuppofing I : R to reprefent the ratio of the fines
of incidence and refra6tion of a ray pafling into the
glafs,

?E: Qj>::I : I — R,
.-.EF:Q* ::I:I — R;

that is, the interval between the furfaces of the pane is
to the diftance between the real and apparent places
of the object as the fine of incidence to the difference
between the fines of incidence and refraction. For
glafs this ratio- is nearly that of 3 to a ; therefore I : I
— R : : 3 : i, and Q^ will be therefore one third of
E F ; if the pane of glafs is a tenth of an inch thick,
an object fren through it will not appear to be a thir-
tieth part of an inch out of its real place ; a change
which is too fmall to be taken notice of in Common
life.

A ray paffing through a medium bound by plane
furfices inclined to each other, is bent towards the
thicker or thinner part of the medium, according as
the medium is denfer or rarer than that by which it is
furrounded, and the place in which an object will ap-
pear to be is found by a very eafy conftruction. Let
ABC (Plate XV. Fig. 16.) be a glafs prifm, QJR an
object feen through it by an obferver at O, From Q^
draw QJi perpendicular to the firft fiirface A C, and
let q be the focus of rays refracted by that furface ;
fr-.>m q draw <?E perpendicular on AB, the frcond fur-
face produced, if neceflary, and fuppofing q then to be

Vol.. I .

Plate 14.

„

\ /''if/. 10.

\

AJ \ B

. Q

51 B 4

1> B

Chap. 6.] EjeBs of fpberical Surfaces. 231

the focus of diverging rays falling on A B, let x be the
focus of rays after refraction found by the proportion
before laid down, or by joining H E, and drawing Qj<r
parallel to it, In the fame manner the apparent place
of R will be found, and a perfon at O will fee the ob-
ject QJR apparently in fx, The point Q^he fees by
the ray QJ? G O, and the point R by the ray R L,
MO.

When the furfaces are fpherical, a change is made
in the apparent places of objects no lefs remarkable
than that which we have obferved in objects placed
before convex or concave mirrors. To underftand the
reafon of thefe appearances, it is necefTary to examine
the progrefs of a ray in the fimplcft cafes, and thence
to proceed to the more difficult.

Let ABFD (Fig. 17, 18.) reprefent a medium,
rarer or denfer than the furrounding .medium, and.
bounded by a fpherical furface A E B3 and let the ray
G H parallel to I E a ray parting through the center
of the arch A E B be refracted at H, to or front the.
perpendicular, according to the nature of the medium.
The fine of the angle C HG is to the fine of the angle
CI1 1 in a given ratio, but CI : IH :: S. CHI Ts.
C H G, therefore J H : I C in the given ratio of the
fine of incidence to the fine of refraction depending on
the nature of the mediums. The nearer H is to E the.
nearer will the ratio of I H to I C be to that of I E :
I C, and consequently by finding a point I in the line
C K produced, fuch that IE may be to 1C in the given
ratio of the fine of incidence to the fine of refraction,
all the rays parallel to I E, which are refracted by the
convex furface A E B (Fig. 17.) will after refraction
converge to I, or a fmall fpace very near it. The
greater the diftance of H from E, the greater will be
the diftance of the intei feel ion of the refracted ray and

line

232 Effect* ofjpberica! Surfaces, [Book lit

I'm* 1C from the poihts I and M. This point I is
called the focus of refracted parallel rays.

In the, fame manner M is the focus of rays coming
out of the denfe medium, and CM : E M :: fine R ;
fine I.

Parallel rays incident on the convex furface of a
' denfer medium, or the concave furface of a rarer me-
dium (Fig. 17.) converge after refraction; and on the
contrary, if they fall on the convex furface of a rarer^
or the concave furface of a denfer medium (Fig. 18.)
they diverge after refraction. The focus of parallel
rays thus found is called the principal focus, as for-
merly explained.

Let CL(Plate XVI. Fig. 19.) be the focus of di-
verging rays incident on the furface AB of a denfer*
medium ; to find the focus after refraction draw QJ:
an incident ray, and fnppofe T to be the focus of rays
parallel to C Q^ incident on the concave furface A E B,
and make C P equal to C T, from I draw I q parallel
to C P, and q will be the focus of refracted rays. Let
/ be the focus of rays parallel to Qj3 incident on the
convex furface, and make Cp equal to C/. Then,
. fince a ray parallel to C P incident on the concave fur-
face would after refraction converge ,to P, a ray di-.
verging from P will after, refraction go parallel to C P.
Now the courfe of the ray QJ is the fame, whether it
is confideied as diverging from Q^or P, therefore the
direction of one of the rays diverging from Q^will be
in the line QJ q. Suppofe" now the ray q 1 to be turn-
ed back, its progrefs ,will be the fame as if it had di-
verged from p; but all rays diverging from p, and in-
cident on the concave furface, move after refraction
parallel to Cp, p being the focus of parallel rays inci-
dent on -the other furface, therefore the ray p I muft
after refraction move parallel to C/»j but its direction

mull

A"bL.I

Chap. 6.] Convergence of Rays explained. 233

imift necerTarily be I Qj therefore I Q^is parallel to
Cp. We have hence two fimilar triangles QJ3 C,
Cpq and QJP : PC :: Cp :pq. Now if I is very
near to E, Qj3, PC, Cp, pq will be very nearly equal
to QT, TC, C /, tq, and by making as QJT to TC,
fo C t to / q, we fhall find a point q near to which all
the rays diverging from Q^will by refraction be made
to converge.

Hence QT varies inverfely as tq-t that is, the
greater the diftance of Q^from T, the lefs will be that
of q from /, for C T and C /, remain invariable in the
proportion, however the pofition of Q^may be va-
ried.

The points Q^and q are always on oppofite fides of
T and /.

If the point Q^was at fuch a diftance, that the rays
diverging from it might be confidered as parallel, q
and / would coincide. As Qjwas brought nearer to T,
q would recede from t ; when Qjmd T coincide, little
q will be ho longer in the line E qy but the refradled
rays will now be parallel. As Qjnoves from T to E,
q appears at a great diftance from E on the fame fide
of the furface with Q^and overtakes it at E.

Diverging rays incident on the convex furface of a
clenfer medium, or the concave furface of a rarer me-
dium, are made to converge or diverge according to
the fituaticn of their foci with rcfpecl to the principal
focus. When they are incident en the convex fur-
face of a rarer medium, or the concave furface of a
denfer medjum, their progrels may be feen in Figures
20, 21.

The ray QJ diverging from Q^will be affected in

the fame manner as if it was fuppofed to converge to

P the principal focus of rays incident on the concave

furface •, but a ray converging to P, will by refra&ion

9 of

Concave Surfaces. [Book III,

of the convex furface be made to proceed in a direc-
tion parallel to C P, therefore q I will be parallel to
CP. Again, a ray incident on a concave furface con-
verging to qt may be confide, red as converging to pt
the focus of parallel rays on the convex furface, and
therefore by refraction of the concave furface, it will
be made to proceed in a direction parallel to C/>.
Hence as before the triangles QJ?C, qpC are fimi-
lar, and the fame proportion is deduced QJT : T C : :
/C : tq.

Rays diverging from any point, and intercepted by
the convex furface of a rarer or concave furface of a
de nfer medium will by refraction, we have before feen,
be made always to diverge more. Upon the fame prin-
ciples, and in the fame manner, the effects of fpherical
furfaces on converging rays is fhewn, which are ex-
actly oppofite to thofe of diverging rays, and a learner
may profitably exercife himfelf by trying the effect on
paper on rays converging or diverging, refracted by
the convex or concave furface of different mediums.

Having thus difcovered the progrefs of rays of light
diverging from any point, and intercepted by a refract-
ing fpherical furface, we (hall find no difficulty in ac-
counting for the apparent places of objects feen in dif-
ferent mediums bounded by fpherical liirfaces.

Let QM (Plate XVII. Fig. 22.) be an object in a
glafs medium, and q the focus of refracted rays diverg-
ing from Q. > m the focus of refracted rays diverging
from M. Then q m will be the image of QM. Lee
O be the place of the fpectator, and join O q, O m,
then r s is the part of the glafs through which he fees
the object, and Qr O, M s O are the extreme rays by
which it is feen. Let a denfe medium be now bound-
ed by a convex furface, Fig. 23. and an object QJM
be at fuch a diftancc from it, that its image fhaU be

Vol. I . />.234-

FlCf.

Dense

.'

Chap. 6.] Spherical refrafting Surfaces, &c. 235

qm> and the place of the fpectator O. He will fee only
part of the object correfponding to n m, and through
the part of the furface A s} and the object will appear
to him inverted.

If the eye is placed nearer the furface than the image
is, the object will appear confufed ; for the rays ftrik-
ing the eye will then be converging to a place behind
it : but as I have been rather prolix on this fubject in
the cafe of objects before reflecting mirrors, it will not
be neceflary to purfue it farther, as on the fame prin-
ciples the reader will with eafe determine the part of
an object feen in any medium, bounded by a fpherical
furface, the part of the furface through which it is feen,
and the rays by which it is feen. The truth of fome
of thefe principles may be experimentally fhewn by
objects placed in glafs. vefiels, with concave or convex
furfaces, filled with water, but as we cannot form a
medium rarer than that in which we live, the cafes of
objects placed in a rarer rrtedium muft remain efta-
blifhed on the fixed bafis of mathematical truth here
laid down. If we were indeed to rarify the air in a
hollow glafs globe, we might obferve, perhaps, the
changes made in the apparent places of an object, ac-
cording to the fucceffive degrees of rarefaction ; but
ftill the object would be feen through one medium
much denfer than the furrounding atmofphere, and be-
fore we can examine the apparent fituation of an ob-
ject placed in one medium, which is feparated from
another by a medium of a different denfity from either,
and bounded by a concave and convex furface, we muft
endeavour to account for the appearances which are
daily before our eyes, namely, the changes made in the
apparent places of objects by the interpofition of a denfe
/ubftance bounded by fpherical iurfaces,

In

23 6 Theory of Lenjes: [Book I II.

In the chapter which rivaled generally of refraction,
the fubject of LF.NSL . .perflcially con-

:cij the tlieory oft-. icniains now to be

invefligated, and from their great importance in the
fcience of optics, it will be ntcdfary to fpeak of them
fomewhat in dct

The different forms of lenfes have been already
mentioned; but it is necefuiry to premife, that in in-
veitigating the properties of a lens, we cdnfidcr its
thicknefs as very inconfiderable, and that in every fpe-
cies there is a poin", through whieh if a line is drawn
in any direction^ and interfered by the furfaces of the
lens, a ray refracted by one furface into this line will,
after the fecond refraction, emerge parallel to its firft
direction.

Let AI0B0 (Plate XVII. Fig. 24, 25.) reprefcnt
a convex or concave lens, the radii of whofe furfaces arc
equal, and draw C 1, ci from the centers C, c, paral-
lel to each other, and join I /. Suppofe now I / to be
a ray of light within the lens refracted by both furfaces
at I and i. Since the radii are parallel, the angfes of
incidence are equal, and confequently the angles of re-
fraction are equal, and the rcfra6ted rays muft make
equal angles with the incident ray I /, that is, they mufl
be parallel to each other. A ray, therefore, incident
on I, and proceeding in the direction I i, will, after re-
fraction at ij proceed in a direction parallel to its firft
direction. In the fame manner any other ray incident
on one furface, and proceeding in die Irne joining two
parallel radii, will, after refraction at the feccrid fur-
face, emerge parallel to its fiifV direction. But the line
joining two parallel radii will always pafs through the
fame point m; therefore all rays pafiing through this
point m will, after- refraction at the fecond furface, pro-
ceed parallel to the direction, which they had before

the

VOLJ . //. 236.

Fig. 23.

Chap.- 6.] Plano-convex and concave Lenfis. 23?

the firft refrafHon. This point m is the center of the
lens, and in the two cafes before us it bifects the thick-
nefs of the lens ; far fmce the triangles C I m, dm are
firnilar, C I : ci :: Cm \ cm and as C I and ci are
equal C ;;/ and cm are equal, and confequently nm and
mo are equal. If the radii' were not equal, the center
of the lens is neareft to that furface whofe radius is the
leaft, and its place may be accurately found by the
preceding oroportion.

In Plate XVIII. Fig. 26, 27. reprefent two !enfes>
the one v/i h a plane and a convex furface, the other
with a plane and a concave furface. In both cafes the
center of the lens will be at m in the fpherical furface^
for the point may be confidered as in the tangent to
the circle at m, which is parallel to A IB, therefore the
ray I / makes equal angles of incidence at the points I
and z, and confequently the angles of refraftion will bd
equal. From the proportion aifo difcovered in convex
or concave lenfes, the fame truth is evident; for as we
increafe the radius of A I B, the point m (Fig. 24, 2,5.)
approaches nearer to » ; and as this radius may be in-
creafed without limit, the diftance of mn may be.de-
creafed without limit, fo that evidently the nearer the
circle approaches to a plane figure, the nearer will be
the approach of m to ;/.

Plate XVIIL Fig. 28. reprefents a lens with one
furface concave the other convex, in which cafe the
point m will be without the lens.

Having found the" center of a lens, we are next to
find its principal focus or point, from which parallel rays,
after refraclion, appear to diverge, or to which they
converge. Let AB (Plates XVIII. XIX. Fig. 2.9,
30, 31, 32; 33, 34) reprefent a lens, whofe center is E,
and the centers of the llirfa'tes R and r, and let q E G
be drawn parallel to the incident rays ; then as the di-

rewlions

23 8 t-low to find tbt Focus of a Lens. [ Book 1 1 L

rections before and after refraction of a ray patting
through the center of a lens are parallel, q EG will re-
prefcnt the courfe of one of the incident rays without
fenfible crron Parallel to E G draw B R, and in B R
produced find the focus V of parallel rays incident on
the furface B ; therefore by the firft refra&ion the pa-
rallel rays are made to ftrike the fecond furface A*
converging to the point V. Join r V* Then one of
the rays, which after the firft refraction moved within
the glafs in the direction r A, will pafs through the fe-
cond furface A, without any refraction, in the direc-
tion AV, fmce r A is perpendicular to the fecond fur-
face. But E G is the courfe of another ray alfo after
the fecond refraction, and G, the point of interfection
of thefe two rays> will be the focus of the lens, near
to which all the other rays will interfect each other*
For all the rays incident on the fecond furface, con-,
verging to the point V, by what has been proved of
rays refracted by a fingle fpherical furface, will be
made to converge to a point between A and V* We
have found the point G, as above, for a double convex
lens, but the fame mode of reafoning applies to other
lenfes, and a fingle infpection of the figjre will fhew
whether the rays converge or diverge after the firft re-
fraction.

With E as a center, and EG as radius, defcribe the
arc GF. Then if the direction of the parallel rays is
changed, the focus will always be in the arc GF, and
if the incident rays are parallel to rR, the ?xis of the
lens, the principal focus is in F. A double convex
and plane convex lens, we have already fecn, make pa-
rallel rays to converge; a double concave and plane
concave make them diverge. A lens with a concave
and convex furface make them converge or diverge,
according as the furfaces do or do not interfect each

other,

VOL .'I . />. 138.

Fit/. 27.

Ft,/. /;/.

Chap. 6.] Focus cf diverging Rays. 439

other, or in general it may be faid of all thefe lenfes,
that the parallel rays converge or diverge after refrac-
tion according as the middle of the glafs is thicker or
thinner than its extremities.

Upon the fame principles alib may be found the
foci of lenfes, fuppofing them to be compofed of fub-»
fiances rarer inftead of deafer than the furrounding
medium.

Having found the principal focus of a lens, we fhall
with the fame eafe find the focus of rays diverging
from a point, and intercepted by a lens, as we did
when they were intercepted by a fingle furface. Let Q
(Plate XIX. Fig. 35.) be the focus of diverging rays
incident on the lens A B, and let F, /, be the principal
foci of rays incident on the furfaces B or A, in a con-
trary direction, and let Qj reprefent an incident ray,
With E as a center, and E F radius, defcribc the arc
F G interfering Qj in G, join EG, and draw Aq pa-
rallel to E G, then with E as a center, and E/ radius,
defcribe the arc fg interjecting iq in g, and join E£.
The ray QJ will be affected in the fame manner, whe-
ther it is considered as diverging from Q^or G; but
*fmce rays parallel to E G, incident on the furface B,
are by the refraction of the lens made to converge to
G, a ray diverging from G will, after the refraction of
the lens, move parallel to E G ; therefore the ray QJ
ttill be made by the lens to move in the direction Aq.
Again, if the ray was turned back, it might be confi-
dered as diverging from g, and by the lens it would be
made to move in the direction /Q^, parallel to E<?.
Hence we have two triangles, QJ3 E, Egy, fimijar
to each other, and QG : GE :: Eg : gq-t that is, the
nearer * is to E, the nearer will this proportion be to
that of QJF : F E : : E/ : fqt a proportion fimilar i«
that which we found before with fingle furfacejs.

The,

240 How to find the -Focus of a Sphere. [Book lit.

The points Qjmd q will be on the oppofite fides of
their refpeftive foci. The greater QF is, the lefs will
be qf, and vice verfa ; and the apparent places of ob-
jects viewed through thefe glaffes will be found in the
fame manner as with fingle furfaces. It will be fuffi-
cicnt therefore only to refer the reader to Plates XIX.
XX. Fig. 36, 37. which he will eafily undcrftand with-
out farther explanation.

The principal focus of a fphere is found with the
fame facility as that of a 'lens. Let M (Plate XX.
Fig. 38.) be the principal focus of rays incident on the
convex fufface A O, then bifed M D in I, and I will
be the principal focus of the fphere. For let the my
G A be refracted by the convex furface to B, and at.
B by the concave fufface to I, and let IB,- G A pro-
duced meet each other in K. Now iuppofe that the
ray A B within the fphere emerged at both places A
slnd B, then fihce the4 angles of incidence C AB, C B A
are equal, the angles of refraction, and the difference
between the angles of incidence and refraction, will
be equal; therefore KAB is equal to KB A, and K A
equal to K B ; but the triangle BIM is fimilar to the
triangle K B A, therefore I B is equal to I M j and as
B approaches to D, the nearer will I B be equal to ID,-
that is, I D to I M, and confequently the principal fo-
cus will bifedt D M* or the leaft diftance of the princi-
pal focus of rays refracted by the convex furface from
the fphere.

Having thus found the principal focus of the fphere,
I mail leave the reader to find the apparent places of
objects feen through it, as the mode to be purfned has
already been fufHciently dcfcribcd, and the proportion
is fimiiar to that difcovered in the confidenmon of
Jenfes.

Vol..! ./>. •• /<>.

F"J-

/%. J/.

Chap. ;.] [ 241 ]

CHAP; VII.

OF VISION.

fhe Eye defcriled as an optical InJlrument.—Defefls of the Eye.— Short
and weak Sight.— The former remedied by double concave, and the
/•after by double convex, Glaff'es. — How the Faculty of feeing correSl -
ly is acquired. — Apparent Magnitude depends on the Diftance ofQ{f>
jefls.—Pifual Jingle. — Phenomena of ViJion.—^Why the d+fta**^
Parts of an horizontal Plain, and of the Sea, appear elevated.—
Why a long Watt appears curved.*— Why a high Tower appear!
falling to an Eye beneath, — Belt of Saturn.— Phenomena of Pi/ion at
tonnefted with Motion. — Why the Moon appears to move in/lead of
the Clouds ivhich pafs over its Face.— Why a Circle viewed ob-
liquely appears oval. — How GlaJJts ajjijl the Sight.

TH E ftructure of the eye will be confidered irt
another place * ; it will be fufficient therefore at
prefent to treat it as a fimple lens, which has the
power of refracting the rays of light, incident upon itj
on the retina, whence we derive all our ideas of fight,
and by repeated experience correct the errors which
by that organ alone might have been produced. Let
A B (Plate XX. Fig.J9.) rcprefent a fection of the eye>
MN being the pupil, AqpB the retina or optic nerve
expanded over the internal furface of the eye, and let
qp be the image of QP made by the lens M N. The
action produced on the part of the retina q.p occafions
a fenfation, from which we derive by the fight our
knowledge of external objects.

Now if the lens M N was fixed, the image of two
objects at different diftances from the eye would not
fall on the retina, and therefore it is wifely provided

* Book IX. Chap. 41. Senfe of Sight,

VOL, I. R by

242 Short and long Sight. [Book III.

by our Creator, that we fhould have the power of
adapting the lens to the diftance of the objeft, fo that
its image fliould.be always upon the retina. This is
the cafe with the generality of mankind j but all per-
fons have not this power j for the eyes of fome are fo
conftrufted, that the rays of light either converge too
foon or too late j that is, the image is made in fome
place between the retina and the pupil, or it would
be made behind the retina, if the retina was removed ;
hence thefe perfons cannot {ee objefts diftinctly, and
to remedy thefe defects glafies are ufed, which in the
one cafe make the rays diverge, and in the other cafe
make them converge. Thus for Ihort-fighted perfons
as they are called, double concave glafies ; for long-
fighted perfons, double convex glaffes are ufed. By
means of the concave glaffes, the rays incident on the
eye are made to diverge more ; and confequently the
eye, which before made them- converge too foon, will
now be able to form the image on the retina. By the
convex glafles the rays are made to converge, and con-
fequently the image, which would othcrwife have been
behind the retina, is now formed in its proper place
on the retina.

It is a long time, probably, before the child is able
• to ufe all the mufcles by which the pupil is contracted
or expanded, fo that the image fhould fall exactly up-
on the retina, and then the ideas formed by the fight
muft be exceedingly inaccurate. At firfl all objects
will appear equally to touch the eye, and the apparent
magnitude of the objedl: will depend on the part of the
retina covered by die image. By degrees thefe ideas
are corrected, and the hands inftrucl:, after fome diffi-
cult experiments, the eye. - The child difcovers that
objects are at a diftance from its body, and that fych
as make the fame angle at the pupil are not always of

the

Chap. 7.] How we judge of Diftance. 243

the fame real magnitude ; hence it learns by degrees
to combine together the angle under which an object
is feen and the diftance, and, according to its future
employments in life, thefe ideas w.ill be combined to-
gether with greater or lefs accuracy. The judgment
of one perfon, accuftomed to diftant objects, will be
very correct, while a perfon employed in the nicer
works of art will be continually deceived in looking at
the fame diftant objects, and the contrary.

An object will affect us differently, I have faid, ac-
" cording to the angle under which it is feen, and its dif-
tance. In figure 40 let AB be an object viewed di-
rectly by the eye Q_R. From each extremity draw
the lines AN and B M, interfering each other in the
cryftajline humour at I. Then draw the line I K in.
the direction in which the eye is fuppofed to look at
the object. The angle A IB is then the optical or vi-
fual angle, and the line I K is called the optical axis,
becaufe it is the axis of the lens or cryftalline humour
continued to the obje6t.

The apparent magnitude of objecls then, depending
thus on the angle under which they are feen, will evi-
dently vary according to their distances. Thus dif-
ferent objects, as A B, C D, E F, the real magnitudes
of which are very unequal, may be fituated at fuch
diflances from the eye as to have their apparent mag-
nitudes all equal ; for if they are fituaj/bd at fuch dif-
tances that the rays AN, B M, mail touch the extre-
mities of each, they will then appear all under the fame
optical angle, and the diameter M N of each image on
the retina will confequently be equal.

In the fame manner objedls of equal magnitude,

fituated at unequal diftances will appear unequal. For

let A B and G H, two objects of equal fize, be placed

before the eye at different diftances I K and I S ; draw

R 2 the

244 Wh' an extended Plain appears elevated. [B ook III.
the lines G P and H O, eroding each other in I ; then
O P, the image formed by the object G H on the re-
tina, is evidently of a greater diameter than the image
M N, which reprefents the object A B ; in other words,
the object G H will appear as large a$ an object of the
diameter T V fituated at the fame place as the object
AB.

Hence it follows, that objects fituated at different
diftances, whole apparent magnitudes are equal, are to
each other as their diftances from the eye ; and by the
fame rule, equal objects fituated directly before the
eye, have their apparent magnitudes in a reciprocal
proportion to their diftances.

This laft propofition muft, however, be received with
lome allowance j for it is only applicable to very dif-
tant objects, and to thole where the fenfe is not cor~
' reeled by the judgment. For if the objects are near,
we do not judge of their magnitude according to the
vifual angle. Thus, if a man of fix feet high is feen
at the diftance of fix feet under t'he very fame angle as
a dwarf of only two feet high at the diftance of two
feet, ftill the dwarf will not appear as large as the man,
becaufe the fenfe is corrected by the judgment.

In moft cafes, however, where the diftance is confi-
dcrable the rule will be found accurate ; and as it has
its foundation in nature, moft of the phenomena of vi-
fion will be explained by having racourfe to the prin-
ciples laid down in this chapter. If the eye is placed
above a horizontal plain, the different parts of this
plain will appear elevated in proportion to their dif-
tance, till at length they will appear upon a level with
it. For in proportion as the different parts are more
diftant, the rays which proceed from them form angles
with the optical axis I K more and more acute, and at
length become almoft parallel. This is the reafon

why,

Vol.. [./> 2 U

riff. 3A

Chap. 7.] Why a long fir ah Wall appears curved. 245

why, if we ftand on the fea-fliore, thofe parts of the
ocean which are at a great diftance appear elevated;
for the globular form of the earth is not perceptible to
the eye, and if it was, the apparent elevation of the fea
is far greater than the arch which a fegment of the
globe would form within any diftance that our eyes are
capable of reaching.

For the fame reafon, if a number of objects are
placed on the fame plain and at {he fame height below
the eye, the more diftant will appear taller than the,
others ; and if the fame objects are placed on a fimilar
plain above the eye, the more diftant will appear the
loweft.

The diftant parts of a long wall, for the fame rea-
fon, appear to a perfon who ftands near one end to
curve or incline towards him. In the fame manner the
high wall of a lofty tower, feems to a fpectator placed
directly under it to bend over him, and threaten him
with inftant deftruction. If any perfon inclined to
make the experiment, will lie down on his back in a
fituation of this defcription, at the diftance of five
or fix feet from the wall of which he contemplates
the tremendous height, he will immediately be made
fenfible of the phenomenon.

If the diftance between two objects forms an infen-
fible angle, the objects, though in reality at fome dif-
tance from each other, will appear contiguous. This
is afligned by fome aftronomers, as the reafon why the
ring or belt of Saturn appears as one mafs of light,
while they contend that it is formed from a number ot
little ftars or fatellites ranged within a certain diftance
of each other.

If the eye is carried along, as in a boat, without be-
ing fenfible of its own motion, the objects which arc
ilationary on each fide, will appear to move in a con-
R 3 trary

246 Moon fe ems io move inftead of Clouds. [Book III.

trary direction. Thus we attribute to the fun and the
other heavenly bodies, a diurnal motion, which only
affects the earth which we inhabit.
. If two or three objects at a confiderable diftance,
and on which the eye of. the fpectator is fixed, move
with equal velocity paft a third object which is at reft,
the moving objects will appear to be actually at reft,
and that which is really ftationary vnll appear in mo-
tion. Thus the clouds which pafs over the face of
the moon appear at reft, while the moon itfelf appears
to proceed rapidly along in an oppofite diiection.
This happens, bccaule the eye which is fixed upon the
clouds follows their motion mechanically, and there-
fore the moon appears to move and not the clouds, as
in the boat we do not pe/ceive its motion, but con-
ceive the banks are retiring behind us.

If the center of the pupil, that is, the optic axis is
directed along the furface of any flender object in a
perfectly right line, this line will appear only a point,
becaufe, in fact, the extremities only are vifible.

An extended and cliftant arch, viewed by an eye
which is exactly in the fame line, will appear a.s a plane
furface, becaufe all the pares appearing equally diftant,
the curvature will not be perceived.

If a circle is viewed obliquely it will appear an oval,
becaufe the diameter which is perpendicular to the eye
is fhortened ; in other words, the rays which proceed
from the extremities form an angle fo much the more
acute as the obliquity is greater ; on the contrary, the
diameter which is parallel to the eye is apparently ex-
tended.

To fee any object it has been cbferved (Fig. 39.)
that an imprelnon muft be made upon the retina ; but
to the eye, as to the other fenfes, it happens, that an
imprcflion does not always produce a lentatio.n. There

is

Chap. 7.] How 'very minute Oljeffs are made vijible. 247

is an angle q L,p of a determinate magnitude*, which
varies indeed in, the eyes of different perfons, but is
fuch that any object making a lefs angle in the pupil
does not produce a lenfation on the retina of fuch a
nature that a perfon can have a determinate idea of
the object. This angle is in general about half a mi-
nute of a degree ; and each perfon may difcover the
ftrength of his fight by taking an object of a determi-
nate length, and removing it to fuch a diftance that
it mail juft ceafe to be vifible, or appear only as a
point. -

When objects are beyond that diftance, they confe-
quently are invifible to us ; and if by any art we can
make them appear as if they were within this diftance,
they will then appear to us as other objects feen under
the fame angle, and at the fame diftance. The fimpleft
contrivance, where the object is either too minute or
too diftant, is to interpofe between the eye and the ob-
ject a glafs or lens, which by its magnifying power,
or, in ftill clearer terms, its power of converging the
rays of light, mall enlarge, or render more cbtule the
angle under which it is prefented to the naked eye,
and which muft of courfe proportionably enlarge the
image depicted on the retina. To find by what fort
of glafs or lens an object is to be feen at any diitance,

* If the diflance of any object from the eye is fufficiently great
• for the rays to fall nearly parallel on the pupil, the name objed is
feen more enlarged and dilHndl the nearer it is brought to the eye,
becaufethe image of any object on the retiaa will be gre.v
lefs in proportion to its apparent magnitud. : when the object is
top near the eye it continues to be enlarged, but is confufed. The
leaf: diftance is about fix inches.

The eye is capable of difiinguifhing objedls that fubtend an
angle of half a minute of a degree, in which cafe the image on the
retina is lefs in breadth than the ^-£r6 p.ut of an i =
object, fuppofing it fix inches difta'it3 i-bout the TiVj part, of an
inch broad. And all fraaller objects are invifible to the naked eye.
R 4 we

248 Caution in the Ufe of Spettaclcs. [Book III.

\ve muft take into confideration the diftance of the ob-
jedl, and the diftance of diftinct vifion, from which it
•will be eafy to find a lens of fuch a focal length as will
make the object appear at the diftance required *.

By want of attention in the choice of glaffes, a de-
fect in fight may be considerably increaied, for the eye
may be ftrrined to an accommodation with the glafs.
Short-fighted perfons, as they advance in years, are
often found to improve in fight; long-fighted perfons,
on the contrary, find their fight impaired by age. For
the convexity in the pupil of both diminifhcs with
years -j- ; in one cafe it was too great, and confequently
the dintmution was beneficial ; in the other cafe it was
already too fnnall, and the diminution muft be confe-
quently prejudicial.

* For in Plate XXI. Fig. 41, 42, the triangles Q_A 7, E g $ ,
are fimilar ; therefore,

QA : Q^? :: Eg : Ey, or

QE : CL? :: E/ : Er,

That is, the focal length of the glafs is equal to the reftangle un-
der thediftances of ciiitindl vil.on,and the given object divided by
the difference ''of thefe djftances.

If in the cafe of (hort-fighted perfons, the object is at fuch a dif-
tance that the rays coming from it to the eye may be confidered
as parallel, QJi and Q^ may be confulered as equal without any
material error, and the focal length of the glafs will then be equal
to the diltance of difuixft vifion.

•{• This' is the genera'ly received opinion, but from fome ob-
fervations lately communicated to the world in the Philofophical
Transactions, by Dr. Hofack, there is reafon to .believe, that the
convexity and concavity are not changed, as was generally ima-
gined, but that the mufcles of the eye grow weaker, like other
mufcies, by age, and confequently are nov able, as in eaily life, to
vary the difiance bet'.vccn the retina and anterior furface of the eye,
fo as to make it coirefpond to the diilance cf the object. See
Book IX. Chap. 41.

With

Chap. 7.] DefeRs of Sight, how remedied.

With a fmgle glafs the defects in fight, with refpe<5t
to many objects either too near or at too great a dif-
tance, for the perfons labouring under them, are re-
medied i but there are cafes where the object is fo far
diftant or fo minute, that though its outline may reach
the eye, its parts muft ftill, even with the aid of a
fingle lens, be indiftinctly perceived. The art of man
has difcovered a remedy, in a great degree, to this im-
perfection, and by means of a combination of glaffes
has opened a wide field for his refearches into the won-
ders of nature ; he can now trace the limbs of an in-
fed invifible to the naked eye, or he can make the ce-
leftial objects appear to him as if their diftance had
been on a fudden diminifhed by mhny millions of
miles.

[ "250 ] [Book III.

CHAP. VIII.

OF TELESCOPES AND OTHER OPTICAL
INSTRUMENTS.

Principles on which tbefg Inftrumenis are nnJiru£ied%—-Dcft:?li. — Telt-
fcope of Galileo. — Micro/cope.— RefleQing Telescope.— Camera O£-
Jcura.—Magu Lantbcrn.

LE T QJP (Plate XXI. fig. 43.) reprefent a very
diftarit object, and let the rays coming from it,
before they fall upon the eye, be intercepted by two
convex lenfes, placed at a diflance from each other
equal to the fum of the focal lengths. The lens A B
is called the object-glafs, from its being oppofed to the
object ; C D the eye-glafs, from its being neareft to
the eye. Since the object is at a very great diftance,
the image made by refraction, q p, will be made at a
diflance from the object-glafs equal to its focal length,
and confeqnently the image is' diftant from the other
glafs exactly its focal length, and the rays diverging
from any point of the image will, after refraction by
the eye-glafs, move parallel to the line drawn from
that point through the center of the glafs. The pro-
grefs of the rays then, by which the object is feen, is
cafily traced. A ray P £ will be refracted by, the ob-
ject-glafs in the direction P A^D, and by the eye-
glafs in the direction D O parallel to p E. The angle,
therefore, urHer which the image is feen, is equal to
qF.p, ,and the angle by which the object would have
been feen by the naked eye is equal to qFp; con-
feqnently the magnitude of the object feen by the
:! eye is to its magnitude, feen through the glaftes,

as

C hap. 8 .] Principle of the telefcope. 2 5 1

as q F/> to q Ep, that is, as q E to q F, or as the focal
length of the eye glafs to the focal length of the object-
glafs *.

By this fimple combination of glafTes an obj eel: ap-
pears inverted 3 but this is of no confequence to altro-

* < When the diftance of the object is very confiderable, the
effects may all be referred to the fame diftance, and a telefcope
may be faid to enlarge an object jaft as many times as the angle
under which it reprefents it is greater than th:it under which it ap-
pears to the r:j.ked eye. Thus the moon appears to the naked eye
under an angle of about half a degree; confequently a telefcope
magnifies 100 times, if- it reprefents th? moon under an angle of
50 degrees ; if it magnified 200 times, it would exhibit the moon
under an angle of 100 degrees ; and the moon would appear to
occupy more than half the vifible heavens, of which the whole ex-
tent is or.ly 180 degrees.

' It is a common exprelTion, that telefcopes bring objects nearer;
but this exprefiion is equivocal, admitting of two different fignifi-
cations. The one is, that, looking through a telefcope, we efti-
mate an object to be as much nearer to us as it is magnified by
the telefcope. But I have already fhewn, that we can form no
certain eftimate of the diirance of an object but by the judgment,
and that our judgment deceives us when the objects are beyond a.
certain distance ; and in the prefent inftance, lofmg all thofe fub-
jects of comparifon on which it is founded, will deceive us more.
The other meaning applied to, the expreffion is, that the telefcope
reprefents the objects as large as they would appear if we were fo
much nearer to them : this latter meaning is more conformable to
the truth than the preceding, for the nearer we approach to an
objefl the larger is the vifual angle. When you look, however,
at a well-known object, as a man, at a great diftance, and he is
feen under a larger angle, we are led to think him fo much nearer,
when he would really appear under a greater angle; but with
refpect to objects lefs known, as the fun and moon, there can be
no eilimation of diftance.

' One principal end of telefcopes is to enlarge or multiply the
3Pgle under which objects appear to the naked eye, and they
are eftimateil according to this effect, and are faid to magnify
live, ten, or any other number of times, according to the nature
and cqnftruction of the telefcope.' — Adams's Le<3. vol. ii. p. 483.

nomers,

252 Principle of tbe felefcope. [Book III.

nomers, and for objects on land feveral contrivances
are ufed to rectify this appearance.

The quantity of the object vifible depends upon the
rr.riiitude of the eye-glafs. Let Ap reprefent an ex-
treme ray refracted by the object-glafs j if the e'ye-glafs
is of fuch a magnitude as to intercept it, the whole
of the object will be feen ; if the eye-glafs is too fmall
for this purpofe, join D A, C B, the extremities of
the lenfes, and the part of the image cut off by thefe
lines will mew the proportional part of the object
which is invifible.

Since the focal lengths of two lenfes are fufceptible
of any proportion whatever, it might feem that no-
thing more was necefiary than to take two lenfes of
determinate focal lengths to make us intimately ac-
quainted with the mcft diliant of the heavenly bodies;
but after a certain length the difficulty of managing
thefe glafies becomes infurmountable ; and for diftant
objects on the earth, when the magnifying power is
more than a hundred, the vapours on the earth would
render vifion obfcure.

The breadth of the object-glafs is of no confequence
as to the magnifying power, for whatever it may be,
the image will be equally formed at the diftance of
its focal length ; but the brilliancy of the image will
be increafed by the breadth, as a greater number of
rays will then diverge from every given point of the
image.

To mnke the image appear erect, two other con-
vex lenfes are required, of equal focal lengths. The
rays emerging from the eye-glafs are intercepted by
the firft of thefe lenfes, and are made to converge to
points at the diftance of its focal length j thence they
diverge, and being intercepted by the other lens at the
diftance of its focal length, they are made to proceed

parallel

Fig. 4-2.

V

\

"

Chap. 8.] Re/rafting and Galilean ' fekfcope. 253

parallel to the lines drawn from each point through
the center of the lens. Thus the firft image of QJ3
(fig. 44.) is qpy the fecond K I. The apparent mag-
nitude of the object is not changed by thefe glaJTes,
and depends, as before, on the focal lengths of the ob-
ject-glafs and lens neareft to it. The brilliancy of the
object, however, will be diminifhed, fince feveral rays
will be loft in their paflfage through the two additional
glaffes.

An inflrument made with glafles combined together
in this manner, and inferted in a tube, is called a
refracting telefcope ; the latter word implying, ac-
cording to its fignification in the Greek language, the
property of feeing objects at a diftance. In placing
the glafles in this tube, care muft be taken that the
axes of the lenfes coincide, or, as .it is evident from
cur principles, indiftinct vifion only will be pro-
duced.

Inliead of the two additional glafles, Galileo chang-
ed the convex eye-glafs into a concave one, by which,
as to the magnifying power, the fame effect was pro-
duced. Thus (fig. 45.) qp being the image of QJP,
by placing a double concave lens between this image
and the object-glafs, the rays converging to p ap-
pear to diverge from R, and I R, the image feen
by the eye at Q^, is made erect. In this cafe, the
nearer the eye is placed to the glafs, and the greater
the pupil, the more the object will be feen.

A microfcope, or an inflrument formed to inipect
minute objects, is conftructed exactly on the fame
principles. A globe of glafs, or double convex lens,
will, from what has been faid, anfwer the purpofe ;
or, a minute object, feen through two convex lenfes,
will be magnified in the proportion of the focal length
6 of

254 Me Microfcope. '• t [Book III.

of the objeft-glafs to that of the eye-glafs *. In all
cafes, in whatever manner we combine our glades to

* Microfcopes are conftruaed in t\vo different modes. The
one is, by the interpofition of a convex lens between the objeft
and the eye, to fender it diftinft at a lefs diftance than fix inches,
by which means its apparent magnitude increafes as the diftance
is diminilhed ; and the other is, by placing the object fo with re-
fpecl to a convex lens, that its focal image may be much greater
than itfelf, and contemplating that image inftead of the object.
The firft are called fimple or fingle microfcopes, and the latter
compound or double. The former is conftrufted in this manner :
fuppofe a fmall object fituated very near the eye, fo that the angle
of its apparent magnitude may be large; .then its image on the
retina will alfo be large ; but becaufe the pencils of rays are too
divergent to be collected into their foci on the retina, it will be
very confufed and indiftinct. Then let a convex lens be inter-
pofcd, fo that the diftance between it and the. object may be equal
to the focal length at which parallel rays would unite, and the rays
which diverge from the object, and pals through the lens, will af-
terwards proceed, and confequently enter the eye, .parallel ; they
will therefore unite, and form a diftindt image on the retina, and
the object will be clearly feen, though if removed to the diftance
of Cx inches, its fmallnefs would render it invifible. The moft
convex lenfes, having the fhorteft fecal diflance of parallel rays,
muft magnify the moft ; for they permit the object to approach
nearer the eye than thofe do which are flatter.

A drop of water is a kind of microfcope, from its convex fur-
face ; for, if a fmall hole is made in a plate of metal, or other thin
fublbmce, and carefully filled with a drop of water, fmall objects
may be feen through it very diftindl, and much magnified.

In the compound microfcope, the image is contemplated inftead
of the object ; it is of two kinds, the folar and the common dou-
ble microfcope ; in the latter, the image is viewed through a fingle
lens In the fame manner as the okjeJl in a fingle microfcope. The
folar microfcope is conftructed by placing a convex lens oppofite
a hole in a darkened chamber, and placing the object at a proper
diftarce from the lens, the pencil of light will converge to a focus
on a fcreen, and the pencil which proceeds from the other point
will converge to another locus, and the intermediate points of the
objedl will be formed into a picture, which will be as much larger
than the objedl in proportion as the diftance of the fcreen exceeds
that of the image from the lens.

difcover

Chap. 8.] . RejleBing telefcope. 25$

difcover the magnifying power, it is neceffary on7.y to
compare together the angles under which the object is
feen through the eye-glafs, and that under which it
would 'have been feen by the naked eye.

Inftead of lenfes only, for reafons hereafter to be
mentioned, a combination has been formed of refiect-
ing furfaces and lenfes, and from the names of the
inventors, thofe of the greateft ufe are now called the
Newtonian, Gregorian *, and Herfchelian telefcopes.
The reflecting telefcope on the Gregorian principle,
which is the mod common, as it is found to be the moft
convenient, is conftructed in the following manner.

At the bottom of the great tube (PlateXXII. fig. 46.)
T T T T is placed a large concave mirror D U V F,
whole principal focus is at *», and in the middle of
this mirror is a round hole P, oppofite to which is
placed the fmall mirror L, concave toward the great
one, and fo fixed to a ftrong wire M, that it may be
removed further from the great mirror, or nearer to
it, by means of a long fcrew in the iniide of the tube,
keeping its axis ftill in the fame line P m n with that of
the great one. Now, fince in viewing a very remote
object, we can fcarcely fee a point of it, but what is,
at lead, as broad as the great mirror, we may con-
fider the rays of each pencil, which flow from every
point of the object, to be parallel to each other, and
to cover the whole reflecting furface D U V F. But
to avoid confufion in the figure, we (hall only draw

* The difference between the Newtonian and Gregorian tele-
fcope is, that in the former the fpe&ator looks in at the fide
through an aperture upon a plane mirror, by which the rays re-
flecled from the concave mirror are reflected to the eye glafs,
whereas in the latter, the reader will fee that he looks through
the common eye-glafs, which is in general more convenient, and
therefore that is the telefcope which is. now in the moil univerfal
repute.

two

256* Refletfing Telefcope. [Book III.

two rays of a pencil flowing from each extremity of
the object into the great tube, and trace their pro-
grefs through all their reflexions and refractions to the
eye /at the end of the fmall tube //, which is joined
to the great one.

Let us then fuppofe the object A B to be at fuch
a diftance, that the rays C may flow from its lower
extremity B, and the rays E from its upper extre-
mity A ; then the rays C falling parallel upon the
great mirror at D, will be thence reflected converg-
ing in the direction D.G, and by croffing at I in the
principal focus of the mirror, they will form the upper
extremity. I of the inverted image I K, fimilar to the
Jower extremity B of the object A B, and patting on
to the concave mirror L (whofe focus is at n) they
will fall upon it at g, and be thence reflected, con-
verging in the direction gN, becaufe gm is longer
than g ny and paffing through the hole P in the large
mirror, they would meet fomewhere about rt and from
the lower extremity b of the erect image a b, fimilar
to the lower extremity B of the object A B. But by
pafling through the plano-convex glafs R in their way,
they form that extremity of the image at &. In the
fame manner the rays E, which come from the top of
the object A B, and fall parallel upon the great mirror
at F, are thence reflected, converging to its focus,
where they form the lower extren ity K of the invert-
ed image I K fimilar to the upper extremity A of the
object A B, and thence pafiing on to the fmall mirror
L, and falling upon it at b, they are thence| reflected
in the converging ftate h O ; and going on through
the hole P of the great mirror, they would meet fome-
where about q> and form there the upper extremity
a of the erect image a bt fimilar to the upper extre-
mity

Chap. 8.] Reflecting Telefcope. 257

mity A of the object AB ; but by paffmg through the
convex glafs R in their way, they meet and crofs
fooner, as at a> where that point of the erect image is
formed. The like being underflood of all thofe rays
•which flow from the intermediate points of the object
between A and B, and enter the tube T T, all the
intermediate points of the image between a and b will
be formed ; and the rays pafiing on from the image
through the eye- glafs S, and through a fmall hole
t in the end of the leffer tube //, they enter the eye/j
which fee* the image a b (by means of the eye-glafs)
under the large angle c e d> and magnified in length
under that angle from c to d.

In the bed reflecting telefcopes, the focus of the
fmall mirror is never coincident with the focus m of
the great one, where the firft image I K is formed,
but a little beyond it (with refpect to the eye) as at
n ; the confequence of which is, that the rays of the
pencils will not be parallel after reflexion from the
fmall mirror, but converge fo as to meet in points
about q> e, r, where they would form a larger upright
image than a b, if the glafs R was not in their way,
and this image might be viewed by means of a fingle
eye-glafs properly placed between the image and the
eye ; but then the field of view would be lefs, and
confequently not fo pleafant ; for that reafon the glafs
R is itill retained to enlarge the fcope or area of the
field.

To find the magnifying power of this telefoope,
multiply the focal diftance of the great mirror by the
diftance of the fmall mirror from the image next the
eye, and multiply the focal diftance of the fmall mirror
by the focal diftance of jche eye-glafs j then divide the
product of the former multiplication by that of the

VOL. I, S latter,

us « • Dr.HerfcMsfelefcepe. [Book III.

latter, and the quotient will exprefs the magnifying
power *.

The immenfely powerful telefcopes of Dr. Herf-
chel are on a different conftruction. This afTiduous
aftronomer has made feveral fpeculums, which are fo
petfect as to bear a magnifying power of more than
fix thoufand times in diameter on a diftant object f .
The object is reflected by a mirror as in the Gregorian
telefcope, and the rays are intercepted by a lens at,
a proper diftance, fo that the obferver has his back
to the object, and looks through the lens at the mirror.
The magnifying power will in this cafe be the fame as
in the Newtonian telefcope, but there not being a fe-
cond reflect or, the brightnefs of the object viewed in
the Herfchelian is greater than that in the Newtonian
telefcope.

There are feveral amufing optical deceptions, which
are effected by a proper combination of plane or con-
vex glaffes. My limits will not admit the notice of
more than two of the amufing kind, namely^ the ma-
gic lanthorn and the camera obfcura. The former
is a microfcope upon the fame principles as the folar
microfcope, and may be ufed with good effect for
magnifying fmall tranfparent objects ; but in general
it is applied to the purpofe of amufement,- by calling
the fpecies or image of a fmall tranfparent painting on
glafs upon a white wall or fcreen, at a focal diftance
from the inftrument.

Let a candle or lamp C (fig. 47.) be placed in the
infide of a box, fo that the light may pafs through the
plano-convex tens N N, and ftrongly illuminate the
object OB, which is a tranfparent painting on glafs, in-
vert jd and moveable before N N, by means of a flicj-

* Fergufou's Le&ures, p. 235.
f Sec Phil. Tranf. 1784.

ing

Fig. ft>.

r-O|

Q

ff. 49

M

Chap. 8.] Camera Otycura. 259

ing piece in which the glafs is fet or fixed. This il-
lumination is ftill more increafed by the reflexion of
light from a concave mirror S S, placed at the other
end of the box, that caufes the light to fail upon the
lens N N, as reprefented in the figure. Laftly, a lens
L L, fixed in a Hiding tube, is brought to the requifite
diftance from the object O B, and a large erect image
I M is formed upon the oppofite wall.

The camera obfcura has the fame relation to the
telefcope, as the folar microfcope has to the common
double microfcope, and is thus conftructed :

Let C D (fig. 48.) reprefent a darkened chamber
perforated at L, where a convex lens is fixed, the cur-
vity of which is fuch, that the focus of parallel rays
falls upon the oppofite wall. Then if A B is an ob-
ject at fuch a diftance, that the rays which proceed
from any given point of its furface to the lens L may
be efteemed parallel, an inverted picture will be formed
on the oppofite wall ; for the pencil which proceeds
from A will converge to a, and the pencil which pro-
ceeds from B will converge to bt and the interme-
diate points of the object will be depicted between
* and b *.

' * Nicholfon's Natural Philof. vol. i. p« 347-

[ ft6o ] [Book III.

CHAP. IX.

OF THE DIFFERENT REFRANGIBILITY
OF THE RAYS OF LIGHT, AND OF CO-
LOURS •.

Errors and Incwveniencies in optical Injlrumenti from the Refraclion
of Light. — Newton, while attempting to remedy thefe, difco'vers a
ne-M Property in Ligbt. — Phenomena and Caufe of Colours. — Achro-
matic Telefcope. — The Eye an achromatic optical Inftrument. — Ex-
periments on Colours.— Caufe of the permanent Colour of opaht
Bodies.

IN the preceding chapters we have feen, that in
finding the foci, fome errors naturally arife by re-
fraction from every furface whatever, and by reflexion
from all fpherical furfaces. When parallel rays arc
refracted by a lens (as in Plate XXII. fig. 49.) the far-
ther the ray is from its center, the greater will be its
deviation from the point F which we called the focus.
This deviation M F is called the longitudinal aberra-
tion, and F N its latitudinal aberration ; and as we fee
on a piece of paper a fmall circle formed by the rays
of the fun intercepted by a convex lens, its magni-
tude depends on its radius, or the latitudinal aberra-
tion FN. This aberration depends on the aperture
of the glafs A B, and it does not increafe in the fim-

* This fubjeft has been partly anticipated in the ift and 4th
chapters of this book, in which it was neceflary to give a fuper-
ficial account of the difcoveries concerning light, and of the na-
ture of refraftion. The reader will, however, have no caufe to re-
gret a little repetition, as the fubjeft is not very clear to a learner,
and cannot be too forcibly imprcffed on the mind.

pic

Chap. 9.] Aberration. 261

pie proportion, but the triplicate of the femi-aper-
ture*. Thus, if P V and p V reprefent the fame
glafs with different apertures P H, p E3 the latitudi-
nal

* Let K a, L b (Plate XXV. fig. 59.) be two parallel rays very
near to each other, incident on the concave refracting furface
A C B at a, b, it is required to find m, the interfection of the rays
after refraction. Draw E I, E H, the fines of incidence and re-
fraction ; and from the point H draw H G perpendicular to the
radius E b, and from G draw G m parallel to the incident rays,
and cutting the refracted ray £/*in m, m is the point required.

For A a En a m c + ac m=:a m c-\-c E b+c If E .-. A a E —
c b E~<z m e-\-c E £— increment of the angle of refraction, and
c E ^rrincrement of the angle of incidence .•. increment of the
angle of incidence : increment of refraction : : a E b : a E b-^-a m b.

.•. a E b : a m b : : increment of incidence : increment of refrac-
tion — increment of incidence.

But when the fines of angles are to each other in a given ratio,
the increments of the angles vary as their tangents.

a E b : a m b : : tangent of incidence : tangent of refraction —
tangent of incidence.

But H G is the tangent of refraction H£ G, and OG is the
tangent of incidence O b G.

.-. a E b : a m b : : O G : H O.

With m as center, and m b radius, defcribe the arc b d, then

. ab bd

a. E b : a m b : : -— : - —

E6 bm

Now*J: £</::££: JH.

Hence b M : b H : : O G : H O, and G m joining the points
G and m is parellel to the incident rays, and confequently by
drawing from G a parallel to the incident rays, the point of inter-
fedtion of the refracted rays is determined.

Hence, fince b H— b E xcof. of E b H, the radius being unity,
the diftance of the interfection from the point of incidence is
found, by making as the difference of the tangents of incidence
and refraction to the tangent of incidence, fo is radius of the fur-
face multiplied into the cof. of refraction, to the di-ftance of the
point of interfection from the point of incidence.

The diftance m of the interfection m g from the axis C F, varies
as the cube of the femi-aperture of the fpherical furface.

S -? For

a6a Aberration. [Book III.

nal aberration N F in the firft cafe will be to that of
w F in the fecond as the cube of P H to the cube
of p E.

Philofo£>hers, confidering the errors to which they
were thus expofed by the fpherical form of their

For mgs=D G— EG X Sin. GED:=E GxS.
EG — HExCos. HEG— HExS. H4E.
3. li I, E.

. EH.

but S. H b E varies, as S. E b I and E b is a ccnftant quantity.

.-. mg varies as sTLT^Tj3, that is as JIT,3, that is as
Semi- aperture]3.

If the line mg be now fuppofed to move parallel to itfelf on the
axis C F, and to be made proportional in every place to the cub«
of the femi- aperture, a curve will be formed to which the refracted
rays will be tangents, and as the adjacent rays crofs each other in
the points m, or the extremities of the ordinate ; the light in thefe
points will be ilronger than within its area ; and the curve thus
formed, which experiment mews to us in various inftanccs, i»
called a cauftick.

All the rays incident on the furface b C will pafs within the
fpace mg, and consequently if the rays of the fun are refraded by
a concave furface, and received by an opake body perpendicular to
the axis at the diftance C^, a circle of light will be formed, whofe
denfity is greater at the circumference and lead at its center. But
thcugh all the refrafted rays will pafs through the area of a circle
at the d'ilance C^, whofe diameter is m /, they will, at a greater
diftance from the furface, pafs through a much fmaller circle »/,
which, when it is the leaft, is called the circle of lead diffufion,
and in this circle the denfity of rays, and ccnfequently the heat,
is the greateft. In this circle the denfity of the rays will be the
greatelt in the center, and it decreafes between the center and the
circumference to<a place where it is a minima n, and confequently
increafes again to the circumference. The inveftigation of this
property would carry us too far into the abiUufe mathematics ; but
what hm.-'bpen faid fufficiemly fliews the nature of the diftufion of
the rays u ' ht from the figure of the furface, which is now
Known to occaiion a much greater error, in proportion to that
arifing fiom refrangibility, than was flippofed by cur nift phi-
Ipfojpher.

glafles,

Chap. 9.] Newton* t Dijcovsry by means of the Prifm. 263

glarTes, employed their thoughts a long lime on vari-
ous modes to bring the rays of light more accurately
to a focus. The greateft of 'philofophers, Newton
himfelf, was endeavouring to make a reflector of a
parabolical form, which, if he had fucceeded in his
attempt, would manifeftly have obviated this incon-
venience, when his thoughts were turned into another
channel by a difcovery, which taught him that the
errors arifmg from the fpherical form of his glafTes
were trifling, compared with what muft arife from his
newly difcovered property of the rays of light. Each
ray, notwithftanding the- exceeding minutenefs of its
breadth, was now found to be compounded of feven
other rays, from whofe various combinations all the
beautiful colours in nature originate.

The experiment on which this difcovery is founded
is thus defcribed by Newton himfelf.

£ In a very dark chamber, at a round hole F (Plate
XXIII. Fig. 50.) about one-third of an inch broad,
made in the fhut of a window, I placed a glafs prifm
ABC, whereby the beam of the fun's light, S F,
which came in at that hole, might be refracted up-
wards, toward the oppofite wall of the chamber, and
there form a coloured image of the fun, reprefented
at P T. The axis of the prifm (that is, the line paf-
fing through the middle of the prifm, from one end of
it to the other end, parallel to the edge of the refract-
ing angle) was in this and the following experiments
perpendicular to the incident rays. About this axis
I turned the prifm flowly, and faw the refracted light
on the wall, or coloured image of the fun, firft to
defcend, and then to afcend. Between the defcent
and afcent, when the image feemed ftationary, I ftop~
ped the prifm and fixed it in that pofture.
. * Then I let the refracted light fall perpendicularly
S 4 upon

S*4 Coloured Image made ly the Prifm. [Book III,

upon a fheet of white paper, M N, placed at the op»
polite wall of the chamber, and obferved the figure
and dimenfions of the folar image, P T, formed on
the paper by that light. This image was oblong, and
not oval, but terminated by two rectilinear and pa-
rallel fides and two femicircular ends. On its fides it
was bounded pretty diftinctly j but on its ends very
confufedly and indiftinclly, the light there decaying
and vaniihing by degrees. At the diftance of i8|
feet from the prifm, the breadth of the image was
about 2f inches, but its length was about lo-J inches,
and the length of its rectilinear fides about eight
inches j and A C B, the refracting angle of the prifm.,
•whereby fo great a length was made, was 64 degrees.
With a lefs angle the length of the image was lefs,
the breadth remaining the fame. It is farther to be
obferved, that the rays went on in ftrait lines from
the prifm to the image, and therefore at their going
put of the prifm, had all that inclination to one
another from which rhe length of the image proceed-
ed. This image P T was coloured, and the more
eminent colours iay in this order from the bottom at
T to t e top at P j red, orange, yellow, green, blue,
ind go, violet, together with all their intermediate de-
grees, in a continual fuccefiion, perpetually varying.'

Our philofopher continued his experiments, and by
making the rays thus decompounded pafs through a
fecond prifm, he found that they did not admit of far-
ther decompofition, and that objects placed in the
rays producing one colour always appeared to be of
that colour. He then examined the ratio between the
fin -o of incidence and refraction of thefe decompound-
ed rays, and found that each of the feven primary co-
lour-making rays, as they may be called, had certain
limits wkfiin which they were confined. Thus, let

Chap. 9.] Separation of component Rays, 26$

the fine of incidence in glafs be divided into fifty equal
parts, the fine of refraction into air of the leaft and
moil refrangible rays will contain refpectively 77 and
78 fuch parts. The fines of refraction of all the de-
grees of red will have the intermediate degrees of
magnitude, from 77 to 77-5-, orange from 77 v to 77!,
yellow from 77t to 777, green from 777 to 777,
blue from 777 to 77^, indigo from 77^ to 77^-, and
violet from 77^ to 78.

According to the properties of bodies in reflecting
or abforbing thefe rays, the colours which we fee in
them are formed. If every ray falling upon an ob-
ject was reflected to cur eyes, it would appear white ;
if every ray was abforbed it would appear black ; be-
tween thefe two appearances innumerable fpecies of
colours may be formed by reflexion or tranfmiffion of
the various combinations of the colour-making rays *,
Jf the rays alfo of light were not thus compounded,

every

* The original or component rays of light are feparable from
each other, not only by refra&ion, or by varying the angle of in-
cidence on a reflecting furface, but are likewife at like incidences
more or lefs refleftible, according to the thicknefs or diftance be-
tween the two furfaces of the medium on which they fall. They arc
alfo liable to be turned out of their direct courfe by approaching
within a certain diftance from a body, by which means a fepara-
tion enfues, the rays being more or lefs deflected as they differ in
colour. Of thefe circumftances it will be proper to give fome
account.

If a convex glafs or lens, or a portion of a fphere, is laid upon
another plane glafs, it is evident that it will reft or touch at one
particular point only ; and, therefore, that at all other places be-
tween the adjacent furfaces will be interpofed a thin plate of air,
the thicknefs of which will increafe in a certain ratio, according
to the diftance from the point of contact. Light incident upon
fuch a plate of air is difpofed to be tranfmitted or refle&ed ac-
cording to its thicknefs ; thus, at the center of contact, the light
is tranfmitted, and a black circular fpot appears j this fpot is en-
vironed

266 Defeft in Optical Glaffes, [Book III.

every objeft would appear of the fame colour, and
an irktbme uniformity would prevail over the face of
nature.

We have feen that when rays of light were refrnct-
ed by any fur face, the focus after refraction depended
on the ratio between the fines of incidence and re-
fraction, and that this focus was not a mathematical
poinr. The reader will naturally infer, that the aber-
ration muft be confiderably increafed, when for each
order of colour-making rays a different ratio between
thefe fines muft be afflimed. Common obfervers,
without underftanding the caufe, may be made fen-
fible of this by noticing the colours of objects feen
through a telefcope; and from the difficulty of the
fubjeft it mignn feem impofiible that any remedy
fhould be applied to the inconvenience. Yet who
fhall fet bounds to the fagacicy of man ? Mathema-
ticians could point out certain combinations of forms
and refrangible powers by which the rays might come
colourlefs, as the white-making rays are commonly
called, to the eye j and a celebrated optician of our
times, Mr. Doliond, has had the merit of realiz-

•vironed by a circle, the colours of which, reckoning from the in-
ternal part, are blue, white, yellow, reel ; then follows another cir-
cular feries, viz. violet, blue, green, 'yellow, red; then purple,
blue, green, yellow, red ; green, red ; g.ecoilh blue, red ; greenifh
blue, pale red ; greenifli blue, reddiih white.

Thefe are the colours which appear by reflexion. By the tranf-
muted light the following fcri?s are feen. At the center, white,
then yeliowlfh red, blaclc, violet, blue, white, yellow, red, Sec. ;
fo that the tranfmitted light at any thicknefs, in!L-a J of white, ap-
pears of the compounded colour which it ought to have after the
fubtraclion of fonie of the conftituent colours by reflexion; after
which feries, the colours become too f.iint and diluted to be
difcerned. It is curious to obferve, that the glaflb will not come
into c.-ntacl without a confiderable degree of prcflurc. Nicbolfon's
^ vol. i. p. 282.

Cnap. 9.] from the Ref Tangibility of Light. 267

ing, in great meafure, their theories. By making a
compound lens of three different fubftances of differ-
ent refrangible powers, the rays of light, which were
difperfed too much by one convex lens, are brought
nearer to an union with each other, and the telefcopes
made with an object glafs of this kind are now com-
monly ufed, and well known by the name of achro-
matic telefcopes -, the word achromatic being ufed by
that pedantry which infects mod of our philofophers,
who love to give a Greek word, unintelligible to the
greater part of their readers, inftead of the equally fig-
nificant term in our own language, colourlefs.

1 The object-glarTes of Mr. Dollond's telefcopes arc
compofed of three diftinct lenfes, two convex and one
concave j of which the concave one is placed in the
middle, as is reprefented in Fig. 51. where a and c
fhow the two convex lenfes, and b b the concave one,
which is by the Britifh artifts placed in the middle.
The two convex ones are made of London crown
glafs, and the middle one of white flint glafs 3 and they
are all ground to fpheres of different radii, according
to the refractive powers of the different kinds of glafs,
and the intended focal diftance of the object glafs of
the telefcope. According to Bofcovich, the focal dif-
tance of the parallel rays for the concave lens is one-
half, and for the convex glafs one-third of the com-
bined focus. When put together, they refract the rays
in the following manner. Let ab, ab (Fig. 52.) be
Itwo red rays of the fun's light tailing parallel on the
firft convex lens c. Suppofmg there was no other lens
prefent but that one, they would then be converged
into the lines bey be, and at lail meet in the focus q.
Let the lines gh, gb, reprefent tv. o violet rays falling
on the furtttC.e of the lens. Theie are aifo refracted,
and will meet in a focus j but as they have a greater

268 Achromatic Telefctsps. [Book HI.

degree of refrangibility than the red rays, they muft of
confequence converge more by the fame power of re-
fraction in the glafs, and meet fooner in a focus, fup-
pofe at r. — Let now the concave lens dd be placed in
fuch a manner as to intercept all die rays before they
corne to their focus. If this lens was made of the fame
materials, and ground to the fame radius with the
convex one, it would have the fame power to caufe
the rays to diverge that the former had to make them
converge. In this cafe, the red rays would become
parallel, and move on in the line 00,00: but the con-
cave lens, being made of flint glafs, and upon a fhorter
radius, has a greater refractive power, and therefore
they diverge a little after they come out of it; and if
no third lens was interpofed, they would proceed di-
verging in the lines opt, opt-, but, by the interpofi-
tion of the third lens ovo, they are again made to con-
verge, and' meet in a focus fomewhat more diltant than
the former, as at x. By the concave lens the violet
lays are alfo refracted, and made to diverge : but hav-
ing a greater degree of refrangibility, the fame power
of refraction makes them diverge fomewhat more than
the red ones; and thus, if no third lens was interpofed,
they would proceed in fuch lines as Imn3 Imn. Now
as the differently coloured rays fall upon the third lens
with different degrees of divergence, it is plain, that
the fame power of refraction in that lens will operate
upon them in fuch a manner as to bring them all to-
gether to a focus very nearly kat the fame point. The
red rays, it is true^ require the greateft power of re-
fraction to bring them to a focus •, but they fall upon
the lens with the lead degree of divergence. The
violet rays, though they require the lead power of re-
fraction, yet have the greateft degree of divergence j.

and

VOL. I .p. 268.

Chap. 9.] the Eye an achromatic Inftrumtnt. -269
and thus all meet together at the point #, or very near-
ly fo *.'

The more we inveftigate the works of nature, the
greater reafon have we to admire the wifdom of its
auth6r, and that wonderful adaptation of our organs,
in the minuted particulars, to the general laws which
pervade the univerfe. The fubject before us affords
a ftriking inftance to corroborate1 this remark. We
have hitherto fuppofed the eye to be a lens capable
only of enlarging and contracting, and confequently,
from the defcription now given of the rays of light, it
muft be incapable of obviating the confulion which
muft arife from their different degrees of refrangibi-
lity. But here the ufe of that wonderful ftructure of
parts, and the different fluids in the eye, is clearly feen.
The eye is, in fact, a compound lens. Each fluid has
its proper degree of refrangible power. The fhape of
the ienfes is altered at will, according to the diftance
of the object; and the three fubftances having the pro-
per powers of refrangibility, the effects of an achro-
matic glafs are without difficulty performed by the
eye, whofe mechanical ftructure arid judicious arrange-
ment of fubftances it is in vain for the art of man to
imitate.

From what has been ftated, the principal pheno-
mena of colours may, without much difficulty, be ex-
plained.

If all the different coloured rays which the prifm
affords are re-united by means of a concave mirror,
the produce will be white; yet tl)efe fame rays, which,
taken together, form white, give, after the point of
their re-union, that is, beyond the point where they
crofs each other, the fame colours as thofe which de-

* Encyclop. Brit. vol. xiii. p. 354.

parted

270 Theory of Colours. [Book III.

parted from the prifm, but in a reverfed order, by the
eroding of the rays j the reafon of which is clear ; for
the ray being white before it was divided by the prifm,
muft neceffarily become fo by the re-union of its part.^
which the difference of refrangibility had feparated,
and this re- union cannot in any manner tend to alter
or deftroy the nature of the colours j it follows then
that they muft appear again beyond the point of crof-
fing.

In the fame manner, if we mix a certain proportion
of red colour with orange, yellow, green, blue, indigo,
and violet, a colour will be produced which refembles
that which is made by mixing a little black with white,
and which would be entirely white if fome of the rays
were not loft or abforbed by the groflhefs of the co-
louring matter.

A colour nearly approaching to white is alfo formed
by colouring a piece of round pafteboard with the dif-
ferent prifmatic colours, and caufing it to be turned
round fo rapidly that no particular colour can be per-
ceived.

If to a fingle ray of the fun divided by the prifm,
which will then form an oblong-coloured fpeftrum, a
thick glafs deeply coloured with one of the primitive
colours is applied, for example red, the light which
pafies through will appear red only, and will form a
round image.

If two thick glalTes, the one red and the other green,
are placed one upon another, they will produce a per-
fect opacity, though each of them, taken feparately, is
tranfparent, bccaufe the one permits the red rays only
to pafs through it, and the other only green ones,
therefore when thefe two glaffes are united, neither of
thofe kind of rays can reach the eye, becaufe the firft
permits only red rays to pafs, whereas die fecond re-
* ceives

Chap. 9.] Colours ofopake Bodies. <ijt

ceives only green ones, which are the only rays it can
tranfmit.

If the rays of the fun are made to fall very obliquely
upon the interior furface of a prifm, the violet-colour-
ed rays will be reflected, and the red, &c. will be
tranfmitted ; if the obliquity of incidence is augment-
ed, the blue will be alfo reflected, and the other tran£-
mitted ; the reafon of which is, that the rays which
have the rnoft refrangibility are alfo thole which are the
eafieft reflected *.

In whatever manner we examine the colour of a
fmgle prifmatic ray, we mall always find, that neither
refraction, reflection, nor any other means, can make
it forego its natural hue; but if we examine the artifi-
cial colouring of bodies by a microfcope, it will ap-
pear a rude heap of colours, unequally mixed. If we
mix a blue and yellow to make a common green, it
will appear moderately beautiful to the naked eye; but
when we regard it with microfcopic attention, it feems
a confufed mafs of yellow and blue parts, each particle
reflecting but one feparate colour. -

To determine the caufe of the permanent colours of
opake bodies, a feries of experiments was inftituted
by Mr. Delaval, as noticed in the hiftorical part of this.
book. He prepared a great variety of coloured fluids,
which he put in phial bottles of a fquare form. The
backs of thefe phials he coated over with an opake
fubftance, leaving the front of the phial uncovered, and
the whole of the neck. On expofing them to the in-
cident light, he found, that from the parts of the phials
which v/ere covered at the back no light whatever was
reflecte \ but :t was perfectly black, while the light
tranfmitted through the uncoated parts of the phials

* Briflbn, Traite-EIem. de Phyfiguc, tora. ii. page 361.

was

272 Colours of opake Bodies. [Book III.

was of different colours. The fame fluids, fpread
thinly on a white ground, exhibited their proper co-
lours -, the light indeed being in this cafe reflected from
the white ground, and tranfmitted through a coloured
medium. It is almofl unnecefiary to add, that when
fpread upon a black ground they afforded no co-
lour.

The fame experiments were repeated with glafs
tinged of various colours, and the refult was perfectly
the fame. When thefe glaffes are of fuch a thinnefs,
and are tinged fo dilutely that light is tranfmitted
through them, they appear vividly coloured; when in
larger maffes, and the tinging matter more denfely dif-
fufed through them, they are black ; when the tranf-
mitted light, in the tranfparent plates of thefe glaflljs,
was intercepted by covering the further furface, they
appeared black.

From thefe different phenomena Mr. Delaval clearly
deduces thefe remarks — ift, That the colouring par-
ticles do not reflect any light, adly, That a medium,
fuch as Sirlfaac Newton has defcribed, is diffufed over
the anterior and further furfaces of the plates, where-
by objects are reflected equally and regularly as in a
mirror.

When a lighted candle is placed near one of thefe
coloured plates, the flame is reflected by the medium
diffufed over the anterior furface j the image thus re-
fie&ed refemb'es the flame in fize and colour; for it is
fcarcely fenfibly diminifhed, and is not at all tinged
with colours : if the plate is not very maffy, or too
deeply tinged, there appears a lecondary image of the
flame, reflected from the further furface of the glafs,
and as the light, thus reflected, pafles back through the
coloured glafs, it is vividly tinged.

The fecondary image is ids than that which is re-
flected

Chap. 95] Colours of-opake Bodies. 273

fleeted from the anterior furface. This diminution is
occafioned by the lofs of that part of the light which ia
abforbed in patting through the coloured glafs.

The next object of this ingenious philotbpher was to
obtain the colouring particles pure and unmixed with
other media. To this end he reduced Feveral tranf-
parent coloured liquors to a folid confiftence by eva-
poration, and in this (late the colouring particles re-
flected fio light, but were entirely black.

To determine the principle on which opake bodies
appear coloured, it is therefore only necefTary, in the
firft place, to recollect, that all the coloured liquors
appeared fuch only by tranfmitced light; and, adly,
that thefe liquors, fprcad thinly upon a white ground,
exhibited their refpedtive colours; Ke therefore con-
cludes, that all coloured bodies, which are not tranf-
parent, confift of a fubfcratum of fome white fub-
ftance, which is thinly covered with the colouring par-
ticles.

On extracting carefully the colouring matter from
the leaves, wood, and other parts of vegetables, he
jjpund that the bads was a fubftance perfectly white.
He alfo extracted the colouring matter from different
animal fubftances; from flefli, feathers, &c. whence
the fame conclufion was directly proved;

Flefh confifts of fibrous veflels-, containing blood,
and is perfectly white when divefted of the blood by-
ablution ; and the florid red colour of the fleih pro-
ceeds from the light which is reflected- from the -white
fibrous fubftance, through the red tranfparent covering
formed by the blood.

The refult was the fame from an examination of the
mineral kingdom.

Some portions of light are reflected from every fur-
face of a body, or from every different medium into
VOL. I. T \vhich

274 Caufe of the "permanent [Book III.

which it enters. Thus, tranfparent bodies reduced to
powder appear white, which is no other than a co-
pious reflection of the light from all the furfaces of the
minute parts, and from the air which is interpofed be-
tween thefe particles.

The general appearance of a flrong infufion of co-
chineal is black; but when agitated, its furface is co-
vered with a red froth. The reafon is, that the light
is reflected from the globules of air inclofed in each
of the bubbles which conftitute the froth, and is tranf-
mitted through the films of red liquor which cover
them. Several vitreous fubftances in like manner ap-
pear black in a folid mafs, but when powdered, of a
different colour. The action of thefe powders on the
rays of light is the effect, of the difcontinuance of their
parts, and the air being admitted into the interfaces,
the light is tranfmitted through the thin tranfparent
particles of the glafs, which give it that tinge the
powder exhibits. If oil, inftead of air, intercedes
the interftices of powdered fubftances, in proportion as
it approaches to the denfity of the fubftances them-
felves, and as it exceeds air in this refpect, it renders
the colour proportionably darker. " Thus when in-
digo, and other tranfparent paints, are united with oil,
the air is expelled from their interftices, and the oil
which is admitted in its ftead, from the nearnefs of its
denfity to chat of the powder, reflects no fenfible light,
fo that the mafs, which confifts of fuch uniformly denfe
media, is black." " When fmooth furfaces of dark-
coloured marble or flate, or any other polifhed fub-
ftance, are fcratched, the air enters into the interftices
which are opened by this operation, and according to
the excefs of its rarity over that of the mattes which it
intercedes, it reflects a whiter or lighter coloured hue.
. By

Chap. 9.] Colours of opake Bodies. 275

By polifhing the furface alfo, the air is removed from
them, and the dark hue is reftored."

From thefe experiments he concludes, " that vege-
table, animal, and mineral coloured matter is tranfpa-
rent j that it does not reflect colours, but only exhibits
them by tranfmiffion ; that opake coloured bodies
confift of tranfparent matter covering opake white par-
ticles, and tranfmitting the light which is reflected
from them."

With refpeft to the femi-pellucid fubftances, fuch
as the folution of lignum nephriticum, &c. which ap-
pear of one colour by incident and another by tranf-
mitted light, he fays, " they confift of pellucid media*
through which white or colourlefs opake particles are
diffufed." Thefe white particles, he adds, are difpofed
at fuch diftances from each other, that fome of the in-
cident rays of light are capable of patting through the
intervals which intercede them, and thus are tranfrr.it-
ted through the femi-pellucid mafs. Some forts of
rays penetrate through the mafles, whilft other fortsj
which differ from them in refrangibility, are refracted
by the white or colourlefs particles. Thus, when pel-
lucid colourlefs glafs is melted with arfenic, the arfenic
is thereby divided into minute opake particles, which
are equally diffufed through the glafs. If only a fmall
quantity of arfenic is ufed in this compound, the white
particles are thinly difieminated in it. When glafs of
this compofition is held between the window and the
eye, it exhibits a yellow or orange tinge •„ when viewed
by incident light it is blue. The yellow or orange
arifrs from the lefs refrangible rays, from the mixture
of which that colour refults. The mere refrangible
are reflected back by the white particles.

[Book III,

CHAP. X.

OF THE RAINBOW, AND OTHER REMARK-
ABLE PHENOMENA OF LIGHT.

Gf the primary and fecondary Rainbow. — Why the Phenomenon affumci
the Form of an Arch. — At what Angles the different Colours are
apparent. — Lunar Rainbow. — Marine BO~M. — Coloured Bowsfeen
»n the Ground.-*- Halo or Corona. — Curious Phenomena fsen on the
I~6P of the Cordileras.— Similar Appearance in Scotland.— Parhelia,
or Mock Suns.— Singular Lunar Phenomenon.'— Blue Colour of the
.Atmofphere. — Red- Colour of the Morning and Evening Clouds.—
Colour of the Sea.

SINCE the rays of light are found to be decom-
pounded by refracting furfaces, \ve can no longer
be furprifed at the changes produced in any object by
the intervention of another. The vivid colours,
which gild the rifing or the fetting fun, mufl necef-
farily differ from thofe which adorn its noon-day
fplendor. There muft be the greateft variety which
the livelieft fancy can imagine. The clouds will
afiume the moll fantaftic forms, or will lour with the
darkeft hues, according to the different rays which
are reflected to our eyes, or the quantity abforbed
by the vapours in the air. The ignorant multitude
will neceflarily be alarmed by the fights in the hea-
vens, by the appearance at one time of three, at ano-
ther of five funs, of circles of various magnitudes
round the fun or moon, and thence conceive that
fome fatal change muft take place in the phyfical or
the moral world, fome fall of empires or tremendous
earthquake, while the optician contemplates them

merely

Chap. 10.] ¥be Rainbow- 277

merely as the natural and beautiful effects produced
by clouds or vapour in various mafTes upon the rays
of light.

One of the mod beautiful and common of thefe ap-
pearances deferves particular inveftigation, as, when
this fubject is well underflood, there will be little dif-
ficulty in accounting for others of a fimilar nature,
dependant on the different refrangibility of the rays of
light. Frequently, when our backs are turned to the
fun, and there is a fhowc-r either around us, or at fome
diftance before us, a fpecies of bow is feen in the
air, adorned with all or fome of the feven primary co-
lours. The appearance of this bow, in poetical lan-
guage called the iris, and in common language the
RAINBOW, was an inexplicable myftery to the antients ;
and, though now well underftood, continues to be the
fubject of admiration to the peafant and the philo-
fopher.

. We are 'indebted to Sir Ifaac Newton for the ex-
planation of this appearance, and by various eafy ex-
periments we may convince any man that his theory
is founded on truth. If a glafs globe is fufpended in
<• the ftrong' light of the fun, it will be found to reflect
the different prifmatic colours exactly in proportion
to the pofition in which it is placed ; in other words,
agreeably to the angle which it forms wiih the fpec-
tator's eye and the incidence of the rays of light.
The fact is, that innumerable pencils of light fall upon
the furface of the globe, and each of thefe is feparated
as by a prifm. To make this matter (rill clearer, let
us fuppofe the circle B A W (Plate XXIII. Fig. 53.)
to reprefent the globe, or a drop of rain, for each drop
may- be confidered as a fmall globe of water. The
red rays, it is well known, are lead refrangible ; they
wiL therefore be refracted, agreeably to their angle of
incidence, to a certain point A in the moft diftant
T 3

378 Phenomena of the [Book III.

part of the globe j the yellow, the green, the blue4
and the purple rays will each be refracted to another
point. A part of the light, as refracted, will be tranf-
mitted, but a part will alfo be reflected ; the red rays
at the point A, and the others at certain other points,
agreeably to their angle of refraction.

It is very evident, that if the fpectator's eye is placed
in the direction of M W, or the courfe of the red-
making rays, he will only diftinguifli the red colour j
if in another flation, he will fee only by the yellow
rays j in another by the, blue, &c. : but as in a fhower
of rain there are drops at all heights and all ditlances,
all thofe that aye in a certain pofidon with refpect to
the fpectator will reflect the reel' rays, all thbfe in the
next flation the orange, thofe in the next the gretn,
&c.

To avoid confufion let us for the prejent imagine
only three drops of rain, and three degrees of colours
in the fection of a bow (Plate XXIV. Fig. 54.) It
is evident that the angle C E P is lefs than the angle
B E P, and that the angle A E P is the greateft of
the three. This largeft angle then is formed by the
red rays, the middle one confifts of the green, and
the fmalleft is the purple. All the drops of rain,
therefore, that happen to be in a certain pofition to
the eye of the fp£ctator, will reflect the red rays, and
form a band or femicircle of red ; thofe again in a
certain pofition will prefent a band of green, &c. If
lie alters his ftation, the fpectator will ftill fee a bow,
though not the iame bow as before ; and if there arc
many fpectators they will each fee a different bow,
though it appears to be the lame.

There are fometimes feen two bows, one formed as
has been defcr.bed, the other appearing externally to
embrace jhe primary bow, and which is fometimes

' called

Chap. 10.] Rainbow explained. 279

called a fecondary or falfe bo\v, becaufe it is faincer
than the other j and what is moft remarkable is, that
in the falfe bow the order of the colours appears al-
ways reverfed.

In the true or primary bow, we have feeh that the
rays of light arrive at the fpectator's eye after two re-
fractions and one reflexion ; in the fecondary bow,
the rays are fent to our eyes after two refractions end
two reflections, and the order of the colours is reverf-
ed, becaufe in this latter cafe the light enters at the
inferior part of the drop, and is tranfmitted through
the fuperior. Thus (Fig. 55.) the ray of light which
enters at B is refracted to A, whence it is reflected to
P, and again reflected to W, where, furTering another
refraction, it is fent to the eye of the fpectator. The
colours of this outer bow are fainter than thofe of the
other, becaufe, the drop being tranfparent, a part of
the light is tranfmitted, and confequently loft, at each
reflection.

The phenomenon aflumes a femicircular appear-
ance, becaufe it is only at certain angles that the re-
fracted rays are vifible to our eyes. The lead refran-
gible, or red rays, make an angle of forty- two de-
grees two minutes, and the moft refrangible or violet
rays an angle of forty degrees feventeen minutes.
Now if a line is drawn horizontally from the fpecta-
tor's eye, it is evident that angles formed with this
line, of a certain dimenfion in every direction, will
produce a circle, as will be evident by only attaching
a cord of a given length to a certain point, round
which it may turn as round its axis, and in every
point will defcribe an angle with the horizontal line
of a certain and determinate extent.

Let H O, for inftance (Fig. 53.) repre'fent the ho-
rizon, B W a drop of rain at any altitude, S B a line
T 4 drawn

Why the Phenomenon [Book II L
drawn from the fun to the drop, which will be parallel
to a line S M drawn from the eye of the fpedtator
to the fun. The courfe of part of the decompounded
ray S B may be firft by refra6r.ion from B to A, then
by reflection from A to W, laftly by refraction from
W to M. Now all drops, which are in fuch a fitua-
ticn that the incident and emergent rays S B, M W,
produced through them make the fame angle, S N M
will be the means of exciting in the fpectators the
fame idea of colour *. Let M W turn upon H O as
an axis till W meets the horizon on both fides and
the point W will defcribe the arc of a circle, and all
the drops placed in its circumference will have the
property we have mentioned, of tranfmitting to the eye

a par-

* Half the angle between the incident and emergent rays is
pqual to the difference between m times the angle of refradion
and the angle of incidence j m being equal to the number of
reflations added to unity. B C L=C B N-f C N B, and alio
B C L=C A B + C B A~z. C B A.

.-. C B N-f C N E — 2. C B O.

.-. C N B= 2. C B A — C B N-

C N B is half the angle between the incident and emergent
rays, and 2 = //j, there being in this cafe only one reflection ; and
by purfuing the enquiry in the fame manner when the number
pf reflections is increafed, it will appear that C N B always equals
m. CBA — CBN.

This angle C N B, if the angle of incidence increafes from no-
thing, firft increafes and then decreafe?, therefore m C B A —
G B N will in lome places be a maximum ; that is, where m times
the fluxion of C B A, is equal to the fluxion of C B N.
• Let C B A=A and C B N==B.

.-. A : B : : i : m
and radius being equal to unity

S A S B S A SB

cof. A ' co). 'B ' ' cof. A ' col. B
: tang. B : : .
Jang. A ; tang. B : : j

J>ut tang. A : tang. B : : S' A - S' B
cof. A ' col. jj.

Pence

Chap. 10.] affumes the Form of a Bow. 281

a particular colour. When the plane H M W A is
perpendicular to the horizon, the line M W is di-
rected to the vertejc of the bow, and W K is its alti-
tude.

Hence the problem is reduced to a quefticn to find two angles
whofe fines and tangents fhall be to each pther in a given ratio.
Let *:=cof. of A.
= cof. of B,

•== Tan. A.

— K

Hence the co-fine of one angle being found, its fine is given, and
from thence the fine of the other angle, fince they are in a given
ratio to each other.

Thus, according to the nature of the bow, whether primary,
fecondary, &c. the greatcft angle between the incident and emer-
gent rays is found ; but in this cafe the rays entering juft above
or below the point where the incident ray makes the greateft angle
between the incident and emergent rays muft, after emerging from
the drop, proceed nearly parallel to each other, and confequently
a number of rays of one colour will fall upon the eye, diverging
from the point where the angle between the incident and emer-
gent rays is the greateft, and produce the appearance of that co-
lour at the thus, determined height in the Ikies.

This

282- Secondary Rainbb'Jj. [Book III.

This altitude depends on two things, the angle be-
tween the incident and emergent- rays, and the height
of the fun above the horizon ; for fmce S M is pa-
rallel to S N, the angle S N M is equal- to N M I,
but S M H, the altitude of the fun, is equal to K M I,
therefore the altitude of the bow W M K, which is
equal to the difference between W M I and K M I,
is equal to the difference between the angles made by
the incident and emergent rays and the altitude of the
fun.

The angle between the incident and emergent rays
is different for the different colours, as was already
intimated ; for the red or leaft refrangible rays it is
equal to 42° 2' j for the violet, or moft refrangible, it
is equal to 40" 17' -, confequently when the fun is
more than 42° ?! above the horizon, the red colour
cannot be feen -, when it is above 40° 1 7', the violet
colour cannot be feen.

The fecondary bow, as I have faid, is made in a
fimii.ir manner, but the fun's rays fuffer, in this cafe,
two reflections within the drop. The ray S B is de-
compounded at B and one part is refracted to A,
thence re Reeled to P, and from P reflected to W,
where it is refracted to M. The angle between the
incident and emergent rays S N M is equal as before
to N M I, and N M K, the height of the bow, is equal
to the difference between the angle made by the inci-
dent and emergent rays and the height of the fun. In
this cafe the angle S N M, for the red rays, is equal
to 50° 7', and for the violet rays it is equal to 54° 7";
confequently the upper part of the fecondary bow will
be feen only when the fun is above 54° 7' above the
horizon, and the lower part of the bow will be fern
only when the fun is 50° 7' above the horizon.

lit

Mats 2-1.

Chap. 10.] Lunar and -Marine Bows. 483

In the fame manner innumerable bows might be
formed by a greater number of reflections within the
drops ; but as the fecondary is fo much fainter than
the primary, that all the colours in it are feldom feen,
for the fame reafon a bow made with three reflec-
tions would be fainter frill, and in general altogether
imperceptible. Since the rays of light, by various re-
flections and refractions, are thus capable of forming,
by means of drops of rain, the bows which we io fre-
quently fee in the heavens, it is evident that there will
be not only folar and lunar bows, but that many
finking appearances will be produced by drops upon
the ground, or air on the agitated furface of the water.
Thus a lunar bow will be formed by rays from the
moon affected by drops of rain, but as its light is
very faint in comparifon with that of the fun, fuch a
bow will very feldom be feen, and the colours of it,
when feen, will be faint and dim. I was once a fpee-
tator of a lunar bow, in the courfe of a pedeflrian ex-
pedition by moonlight in the autumnal feafon. The i
night was uncommonly light, though fhowery, and
the colours much more vivid than I could have con-
ceived ; indeed I have feen rainbows by day not more
confpicuotis". There were not, however, fo many co-
lours diftinguiihable as' in the folar bow.

The marine or fea bow is a phenomenon fome-
times obferved in a much agitated fea ; when the
wind, fweeping part of the tops of the waves, carries
them aloft, fo that the fun's rays, falling upon them,
are refracted, &c. as in a common fhower, and paint
the colours of the bow.

Rohault mentions coloured "bows on the grafs,
formed by the refraction of the fun's rays in the
morning dew.

Dr.

284 Kaloy cr Corona. [Book III.

Dr. Langwitb, indeed, once Taw a bow lying on
the ground, the colours of which were almoft as lively
as thofe of the common rainbow. It was extended
feveral hundred yards. It was not round, but ob-
long, being, as he conceived, the portion of an hyper-
bola. The colours took up lefs fpace, and were much
more lively in trnfe parts of the bow which were
near him than in thofe which were at a diftance.

The drops of rain defcend in a globular form, and
thence we can eafily account for the effects produced
by them on the rays of light ; but in different flutes
of the air, inftead of drops of rain vapour falls to the
the earth in different forms of fleet, fnow, and hail.
In the two latter dates there cannot be a refraction of
the rays of light, but in the former ftate, when a drop
is partly in a congealed and partly in a fluid form,
. jhe rays of light v/ili be differently affected, both from
the form of the drop and its various refracting powers.
Jrlence we may expect a variety of curious appc-arances
in the heavens, and to thefe drops, in different dates,
we may attribute the formation of haios, parhelia, and
many orher phenomena, detailed in the phiiofophical
tranfactions, or in the hiftories of every country.

The HALO, or CORONA, is a luminous circle fur-
rounding the fun, the moon, a planrt, or a fixed ftar.
ItTs fometimes quite white, and fometimes coloured like
the rainbow. Tiu;fe which have been obferved round
the moon or ftars are but of a very fmall diameter ;
thofe round the fun are of different magnitudes, and
fometimes immenfely great- When coloured, the co-
lours are fainter than thofe of the rainbow, and ap-
pear in a different order, according to their fize. In
thofe which Sir Ifaac Newton o'ofervcd in 1692, the
order of 'the colours, from the infide next the fun, was
jn the innermoft, blue, white, red ; in the middle, pur-

Chap, i o.] Extraordinary Phenomena of Light. 285

pie, blue, green, yellow, pale red ;• in the outermoft,
pale blue and pale red. Hugens obferved one red
.next the fun, and pale blue at the extremity. Mr.
Weidler has given an account of one yellow on the
infide and white on the outlide. In France one was
obferved, in which the order of the colours was,
white, red, blue, green, and a bright red on the out-
fide *.

Artificial coronas may be made in cold weather, by
placing a lighted candle in the midft of a cloud of
fteam ; or if a glafs window is breathed upon, and the
flame of a candle placed at fome diftance from the
window, while the operator is alfo at the diftance of
fome feet from another part of the window, the flame
will be furrounded with a coloured halo.

I was once witnefs to a very pleating phenomenon.
The full moon was partly obfcured behind the ikirt of
a very thin white cloud, which, as it grew thinner to-
wards the edge, had the full effecT: of a prifm in fepa-
rating the rays of light, and exhibited the colours of
the rainbow in their proper gradations. •

When M. Bouguer was on the top of mount Pi-
chinea, in the Corclileras, he and fome gentlemen who
accompanied him, obfcrvcd a mod remarkable pheno-
menon. When the fun was juft rifing behind them,
and a white cloud was about thirty paces from them,
each of them obferved his own (hadow (and no other)
projected upon it. All the parts of the fnadow were
idiftinft, and the head was adorned with a kind of glo-
ry, confiding of three or four concentric crowns, of a
very lively colour, each exhibiting all the varieties of
the primary rainbow, and having the circle red on the
outfide.

» Prieftley's Hift. of Opt. p. 597.

Similar

286 Parhelia, or [Book III.

Similar to this appearance was one which occurred
to Dr. M'Fait, in Scotland. This gentleman obferved
a rainbow round his fhadow in a mid, when he was
fituated on an eminence above it. In this firuation
the whole country appeared to be immerfed in a vaft
deluge, and nothing but the tops of hills appeared here
and there above the flood ; at another time he obferved
a double range of colours round his fhadow*.

The PARHELIA, or mock funs, are the moil fplen-
did appearances of this kind. We find thefe ap-
pearances frequently adverted to by the ancientr, who
generally confidered them as formidable omens. Four
mock funs were feen at once by Scheiner at Rome,
and by Mufchenbroeck at Utrecht; and feven were
obferved by Hevelius at Sedan, in 1661 f.
. The parhelia generally appear about the fize of the
true fun, not quite fo bright, though they are faid
fbmetimes to rival their parent luminary in fplcndor.
"When there are a number of them they are not equal
to each other in brightnefs. Externally they are
tinged with colours like the rainbow. They are not
always round, and have fometimes a long fiery tail
oppofite the fun, but paler towards the extremity.
Dr. Halley obferved one with tails extending both
ways. Mr. Weidlcr faw a parhelion with one tail
pointing up and another downwards, a little crooked ;
the limb which was fartheft from the fun being of a
purple colour, the other tinged with the colours of the
rainbow J.

Coronas generally accompany parhelia, fome colour-
ed and others white. There is alfo in general a very

• PrieiHey's Hift. Opt. p. 600. f lb'ld> P- 613.

I Ibid. p. 614.

large

Chap. 10.] Mock Suns. 287

Jarge white circle, parallel to the horizon, which pafTes
through all the parhelia ; and, if it was entire, would
go through the center of the fun; fometimes there are
arches of fmaller circles concentric to this, and touch-
ing the coloured circles which furround the fun ; they
are alfo tinged with colours, and contain other par-
helia.

One of the molt remarkable appearances of this kind
was that which was obferved at Rome by^Scheiner, as
intimated above, and this may ferve as a fufficient in-
fiance of the parhelion.

This celebrated phenomenon is reprefented in Plate
XXV. Fig. 56. in which A is the place of the' ob-
ferver, B his zenith, C the true fun, A B a plane paf-
fing through the obferver's eye, the true fun, and the
zenith. About the fun C, there appeared two concen-
tric rings, not compleat, but diverfified with colours.
The leffer of them, D E F, was fuller, and more per-
fect ; and though it was open from D to F, yet thofe
ends were perpetually endeavouring to unite, and fome-
times they did fo. The outer of thefe rings was much
fainter, fo as fcarcely to be difcernible. It had, how-
ever, a variety of colours, but was very inconltant.
The third circle, KLMN, was very large, and en-
tirely white, pafTing through the middle of the fun, and
every where parallel to the horizon. At firll this circle
was entire ; but towards the end of the phenomenon
it was weak and ragged, fo as hardly to be perceived
from M towards N..

In the interfeclion of this circle, and the outward iris
G K I, there broke out two parhelia, or mock funs,
N and K, not quite perfect, K being rather weak, but
N ihone brighter and ftronger. The brightnefs of the
jniddle of them was fomething like that of the fun, but

towards

288 The Parhelion. [Book III.

towards the edges they were tinged with colours Tike
thofe of the rainbow, and they were uneven and rag-
ged. The parhelion N was a little wavering, and fent
out a fpiked tail N P, of a colour fomewhat fiery, the
length of which was continually changing.

The parhelia at L and M, in the horizontal ring,
were not fo bright as the former, but were rounder,
and white, like the circle in which they were placed.
The parhelion N difappeared before K j and while M
grew fainter, K grew brighter, and vanifhed the laft of
all.

It is to be obferved farther, that the order of the co-
lours in the circles D E F, G K N, was the lame as in
the common halo-'s, namely red next the fun, and the
diameter of the inner circle was alfo about 45 degrees ;
•which is the ufual fize of a halo.

Parhelia have been feen for one, two, three, and
four hours together ; and in North America they are
faid to continue fome days, and to be vifible from fun-
rife to fun fet. When they difappear, it fometimes
rains, or fnow falls in the form of oblong fpiculas *.

Mr. Wales fays, that at Churchill, in Hudfon's Bay,
the rifing of the fun is always preceded by two long
ftreams of red light. Thefe nfe as the fan rifes; and
as they grow longer begin -to bend towards each other,
till they meet directly over the fun, forming there a
kind of parhelion or mock fun.

Thefe two ftreams of light, he fays, feem to have
their fburce in two other parhelia, which rife with the
true fun j and in the winter feafon, when the fun never
rifes above the haze or fog, which he foys is conftantly
found near ihe horizon, all thefe accompany him the

• Prieftley's Hift. Opt. p. 614 to 617.

whole

Chap. 10.] Blue Coloilr of tie Sky. » 289

whole day, and fet with him in the fame manner as
they rife. Once or twice he faw a fourth parhelion
under the true fun, but this, he acids, is not com-
mon *.

The caufe of thefe is apparently the reflection of the
fun's light and image from the thick and frozen clouds
in the northern atmofphere, accompanied alfo with
fome degree of refraction. To enter upon a mathe-
matical analyfis of thefe phenomena would be only te-
dious, and very foreign to our purpofe. From what
has been faid upon this fubject it is evident, that all
the phenomena of .colours depend upon two proper-
ties of light, the refrangibiiity and reflexibility of its
rays.

The blue colour of the atmofphere has been beauti-
fully accounted for by Mr. Delaval, in the experiments
already detailed. The atmofphere he confiders as a
femi-pellucid medium, which abounds in volatile and
evaporable particles, difengaged from natural bodies
by feveral operations, as fermentation, effervefcence,
putrefaction, &c. Thefe particles differ greatly in
denfity, &c. from the air, and, as they reflect a white
light, may be confidered as fo many white particles
diffufed through the pellucid colourlefs air. In this
refpect the atmofphere is fimilar to the femi-pellucid
medium, which is formed by a mixture of arfenic with
glafs. In both thefe fubftances, whilft the white par-
tieles are rarely diffeminated through the tranfparent
medium, the lefs refrangible rays are tranfmitted
through the intervals which intercede the particles f ,

but

* Prieftley's Hift. Opt. p. 617. ,

f On this account, diitant mountains covered with fnow (which

it is well known reflect all the rays of the fun) appear, when the

VOL. I. U air

290 Red Colour of Morning Clouds y &c. [Book III.

but the more refrangible rays are intercepted and re-
flected by the particles, and the mixture of thofe rays
produces a blue colour.

In air, as well as in the folid femi-pellucid media,
\vhen the white particles are more denfely arranged,
the intervals which intercede them are diminifhed, and
in this Hate of the atmo.fphere a great proportion of
all the rays are reflected, fo as to produce the effect of
perfect whitenefs, or at leaft an approach towards it.
Thus, when the part of the atmofphere, which is near
the furface of the earth, is occupied by grofs vapours,
this mixture of air with aqueous or orher particles is
\vhite: fuch is the common appearance of fogs. When
fuch vapours are elevated high in the atmofphere, and
form clouds, they reflect the white light of the fun,
and appear white, whenever its incident rays fall on
them entire and undivided; and as the reflecting par-
ticles are not equally diffufed through every part of the
pellucid air, of which the atmofphere principally con-
fifts, it frequently happens that large tracts of air are
only furniftied with fuch a portion as qualify them to
reflect a blue colour, while others are fo denfely ftored
as to form clouds.

Of the red and vivid colour of the morning and
evening clouds Mr. Melville has fuggefted a caufe
upon fimilar principles, which we muft at leaft allow is
ingenious and probable. He fuppofes, as well as Mr.
Delaval, that a reparation of the rays is made in paf-
fing through the horizontal atmofphere, and that the
clouds reflect and tranfmit the fun's light, as any half
tranfparent colourlefs body would do ; for as the at-
mofphere reflects a greater quantity of blue and violet

air is thick, and [the fun nearly cppofite them, of a warmer co-
lour than they otlierwife would, and more approaching to yellow
or orange.

rays

Chap, i o.] Green Colour of the Sea. i$\

rays than of the reft, the fun's light tranfmitted through
it inclines towards yellow, orange, or red, efpccially
when it pafles through a long tract of air ; and in this
manner the fun's horizontal light is tinctured with a
deep orange, and even red, and the colour becomes
ftill deeper after fun-fet; hence he concludes, that the
clouds, according to their different altitudes, may af-
fume all the variety of colours at fun-rifing and letting,
by barely reflecting the fun's incident light as they re-
ceive it.

The green colour of the fea may alfo be accounted
for in the fame manner. Sir Ifaac Newton, and others,
have fuppoleel that this effect was produced by the re-
flective power of the water; but that this is not the
cafe is manifeft ; for when fea water is admitted into a
reiervoir, which does not exceed a few inches in depth,
it appears pellucid aud colourlefs.

Dr. Halley, in the diving- bell *, obfcrved, that when
he was funk many fathoms deep into the fea, the up-
per part of his hand, on which the fun fhone directly
through the water, was red, and the lower part a blue-
ifh green. On thefe phenomena Mr. Delaval obferves,
that the fea water abounds with heterogeneous particles,
many of which approach fo near in denfity to the wa-
ter itfelf, that their reflective power muft be very weak,
though, as they are not quite of the lame denfity, they
ftill muft have fome degree of refle6tive power. Al-
though thefe, therefore, may be invifible when fepa-
rately viewed, yet when the forces of a great number
of fuch minute bodies are united, their action on the
rays of light becomes perceptible, fome rays being re-
flected by them, whilft others are tranfmitted through
their intervals, according to the quantity of reflective

* Newton's Opt. 1. i. Part ad.

U 2 matter

292 Opacity and Green Colour of the Sea. [Book III.

matter which the rays arrive at in the internal parts of
the water.

The opacity of the fea, caufed by the numerous re-
flections from its internal parts, is Ib confiderable, that:
it is not near fo transparent as other water ; the reflec-
tive particles, therefore, which are difperfed through
the mafs of fea water, have confequently a greater re-
flective power than thofe which are difperfed through
the atmofphere. Inftead, therefore, of reflecting a de-
licate blue, fuch as that of the fky, the fea water, by
acting upon a greater portion of the more refrangible
rays, exhibits a green colour, which we know to be a
middle colour produced by the mixture of blue rays
with fome of the lefs refrangible, as the yellow or
orange.

With refpect to the phenomena remarked by Dr.
Halley, it is eafy to conceive that the light, when (trip-
ped of all the more refrangible rays, fliould produce a
rofe colony fuch as that he obferved on the upper part
of his hand ; on the contrary, that which illuminated
the lower part of his hand confifted partly of rays re-
flected from the ground, and partly of thofe which
were reflected from the internal parts of the fea water,
which, we have feen, are chiefly blue and violet ; and
the mixture of thefe produced the greenim tinge which
the Doctor remarked *, and which common experience
mews is the predominant colour of the ocean.

* Delaval on the caufts of colours in opake bodies, vol. ii»
Manch. Mem.

Chap, ii.] [ 293 ]

CHAP. XI.

OF THE INFLECTION OF LIGHT.

F.c';-:ftetf of the Doffrine of Rtflb&Htm. — Nature of Inflection — Neva-
ton's Experiments, — Analogy between this Property and Refraftion.
- —Curious Ejfecls from this Property.

f p -S HE direction of the rays" of light is changed, as
JL we have feen, in their approach to certain bo-
dies, by reflection and refraction, and confequently we
muft admit that there is fome power in thefe bodies
by which fuch effects are univerfally produced. If
reflection was produced (imply by the impinging of
particles of light on hard or elaftic bodies, or if they
were in themfclves elaftic, the fame effects would fol-
low as in the impulfe of other elaftic bodies $' but the
tingle of incidence could not be equal to the angle of re-
flection, unlefs the particles of light were perfectly elaf-
tic, or the bodies on which they impinged were perfectly
elaftic. Now we know that the bodies on which thefe
particles impinge are not perfectly elaftic, and alfo that
if the particles of light were perfectly elaftic, the dif-
fufion of light from the reflecting bodies would be very
different from its prefent appearance ; for as no body
can be perfectly poiiihed, the particles of light v/hich
are fo inconceivably fmall would be reflected back by
the inequalities on the furface in every direction; con-
fequently we are led to this concliifion, that the reflect-
ing bodies have a power which acts at fome little dif-
fance from their furfaces.

If this reafoning is allowed to be juft, it neceffarily

follows, that if a ray of light, inftead of impinging on

U 3 a body,

294 Experiment of Newton. [Book III.

a body, fhould pafs fo near to it as to be .within the
fphere of that power which the body pofiefles, it muft
necelTarily fuffer a change in its direction. Actual ex-
periments confirm the truth of ^his pofition, and to the
change in the direction of a particle of light, owing to
its nearnefs to a body, we give the name of inflec-
tion.

From one of thefe experiments, made by Sir Ifaac
Newton, the whole of this fubject will be eafily im-
derftood. At the diftance of two or three feet from
the window of a darkened room, in which was a hole
three-fourths of an inch broad, to admit the light, he
placed a black meet of pafteboard, having in the middle
a hole about a quarter of an inch fquare, and behind
the hole the blade of a (harp knife, to intercept a fmall
part of the light which would otherwife have pafied
through the hole. The planes of the pafteboard and
blade were parallel to each other, and when the pafte-
board was removed at liich a diftance from the win-
dow, as that all the light coming into the room muft
pafs through the hole in the pafteboard, he received
what came through this hole on a piece of paper two
or three feet beyond the knife, and perceived two
ftreams of faint light fhooting out both ways from the
beam of light into the fhadow. As the brightnefs of
the direct rays obfcured the fainter light, by making,
a hole in his paper he let them pafs through, and had
thus an opportunity of attending clofely to the two
ftreams, which were nearly equal in length, breadth,
and quantity of light. That part which was neareft to
the fun's direct light was pretty ftrong for the fpace of
about a quarter of an inch, decreafing gradually till it
became imperceptible, and at the edge of the knife it
fubtended an angle of about twelve or at moft four-
teen degrees.

Another

Vol.. I .

Plate Id.

I
!

Chap. IT.] Inflexion of Light. 295

Another knife was then placed oppofite to the for-
mer, and he obferved, that when the ciiftance of their
edges was about the four hundredth part of an inch,
the ftream divided in the middle, and left a fhadow
between the two pares, which was fo dark, that all
light palling between the knives feemed to be bent
afide to one knife or the other; as the knives were
brought nearer to each other, this fhadow grew broad-
er, till upon the contact of the knives the whole light
difappeared.

Purfuing his obfervado'ns upon this appearance, he
perceived fringes, as they may be termed, of different
coloured light, three made on one fide by the edge of
one knife, and three on the other fide by the edge of
the other, and thence concluded, that as in refraction
the rays of light are differently acted upon, fo are they
at a diftance from bodies by inflection ; and by many
other experiments of the fame kind he fupported his
pofition, which is confirmed by all fubfequent experi-
ments.

We may naturally conclude, that from this property
-of inflection fome curious changes will be produced
in the appearances of external objects. If we take a
piece of wire of a lefs diameter than the pupil of the
eye, and place it between the eye and a diftant object,
the latter will appear magnified -(Fig. 57.) Let A be
a church fleeple, B the eye, C the wire. The rays by
which the fleeple would have been otherwife feen are
intercepted by the wire, and it is now feen by inflected
rays, which make a greater angle than the direct rays,
and confequently the fleeple will be magnified.

In nearly fhutting the eyes, and looking at a candle,
there appear rays of light extending from it in vari-
ous directions, like comets' tails -, for the light, in paf-
iing through the eye-lames, is inflected, and confe*-
U 4 * quently

296 Inflexion of Light. [Book III.

quently many feparate b^ams will be formed, diverging
from the luminous object. The power cf bodies to
inflect the rays of light pafiing near to them will pro-
duce different effects, according to the naCure of the
rays acted upon j ccnfequently a fcparation will take
place in the differently refrangible rays, and thofe
fringes, which were taken notice of by Sir Ifaac New-
ton, will appear in other objects which are feen by the
means of inflected rays. From confidering thus the
action of bodies upon light, we come to this general
conclufion, for which we are indebted to our great phi-
lofopher, that light, as well as all other matter., is acted
upon at a diftance; and that reflection, refraction, and
inflection, are owing to certain general laws in the par-
ticles of matter, which are equally neceflary for the
prefervadon of the beautiful harmony in the objects
neareft to us, as to produce by their joint action that
great law by which the greater bodies in their fyftem
are retained in their refpective orbits.

Chap, i.] [ 297 ]

BOOK IV.

OF ELECTRICITT.

CHAP. I.

HISTORY OF DISCOVERIES RELATIVE TO
ELECTRICITY.

Origin of the Name.- — Ho-tv far Eleflricity f was known to the An-
tients.—Mr. Boyle. — Otto Guericke.-~Dr. Wall— Mr, Hawk/bse.
Mr, Grey's Difcocueries. — M. Du Fay's. — Subfequent Difcweries tf
Mr, Grey. — Improvements of German Philofophers. — -Ley den Phial,
Eleflrical Buttery.— -Spirits fired by EleSricity conduced through
the River Thames.'. — Tnv-j Species of Eleflricity dif.o*vsred.—Dr.
Franklin's great Difccveries.

TH E attractive power which amber, and other
electric bodies, acquire by friction, was long
known to philofophers ; jind it is almoftunneceflary
to remark, that this branch of fcience derives its name
from £Af>c7pov (electron) the Greek word for amber.
The ©ther electric properties were {lowly difcovered.
Mr. Boyle was the firft who had a glimpfe of the
electric light ; as he remarked, after rubbing fome
diamonds in order to give them the power of attrac-
tion, that they afforded light in the dark.

Otto Guericke, burgomailer of Madgeburg, made
an electric globe of fulphur, and by whirling it about
in a wooden frame, and rubbing it at the fame time
with his hand, he performed various electrical expe-
riments.

298 Otto Guerlcke, Wall* ,- lee. [Book IV.

riments. He ad.ied to the (lock of knowledge the

O

diioovery, th;i once attracted by an excited

electric v, .d by it, and not attracted again till

it had touched fome other body. Thus he was able
to keep a feather fuipended in the air over his globe
of fuiphur ; but he obferved, that if he drove i: near
a linen tiiread, or the flame of a candle, it inrhntly
recovered its propenfity (if I may ufe the exprefiion)
for approaching the globe again. The hiffin0 noife,
and the gleam of light which his globe afforded, both
attracted his notice.

Thefe circumftances were, however, afterwards ac-
'curately remarked by Dr. 'Wall, who, by rubbing am-
ber upon a woollen fubiiance in the dark, found alfo
that light was produced in considerable quantities, ac-
companied with a crackling noife; and what is ftill
more extraordinary, he adds, " this light and crack-
ling feems, in fome degree, to reprdent thunder and
lightning."

Mr. Hawkfbee firft obferved the great electric
power of glafs. He conftructed a wooden machine,
which enabled him conveniently to put a glafs globe
in motion. He confirmed all the experiments of Dr.
Wall. He obferved, that the light emitted by the
friction of electric bodies, befides the crackling noife,
was accompanied by an acute fenfe of feeling when
applied to his hand. He fays, that, all the powers of
electricity were improved by warmth, and diminifhed
by moifture.

Hitherto the diftinction between thofe bodies which
are capable of being excited to electricity and thofe
•which are only capable of receiving it from the others,
appears fcarcely to have been fufpected. About the
year 1729, this great difcovery was made by Mr.
Grey, a penfioner of the Charter- Houfe. After fome

fruitlefc

Chap, i .] Drfcovertes of Mr. Grey. 299

fruitlefs attempts to make metals attractive by heat-
ing, rubbimg, and hammering,' he conceivl a fuf-
picion, that as a glafs tube, when rubbe4 in the dark,
communicated its light to various bodies, it might
poffibly at the fame time communicate its power of
attraction to them. In order to put this to the trial,
he provided himfelf with a tube three feer five inches
lon-g, and near an inch and one-fifth in diameter;
the ends of the tube were flopped by cork ; and he
found that when the tube was excited, a down feather
was attracted as powerfully by the cork as by the
tube itfelf. To convince himfelf more completely,
he procured a fmall ivory ball, which he fixed at firfl
to a flick of fir four inches long, which was thrufl into
the cork, and found that it attracted and repelled the
feather even with more vigour than the cork itfelf.
He afterwards fixed the bill upon long flicks, and
upon pieces of brafs and iron wire, with the fame
fuccefs; and laflly, attached it to a long piece of
packthread, and hung it from a high balcony, in which
flate he found, that by rubbing the tube the ball was
conflantly enabled to attract light bodies in the court
below.

-His next attempt was to prove, whether this power
could be conveyed horizontally as well as perpendi-
cularly j with this view he fixed a cord to a nail which
was in one of the beams of the ceiling, and making
a loop at that end which hung down, he inferted his
packthread, with the ball which was at the end of it,
through the loop of the cord, and retired with the
tube to the other end of the room ; but in this flate
he found that his ball had totally loft the power of at-
traction. Upon mentioning his difappointed efforts
to a friend, it was fuggefted, that the cord which he
had ufed to fupport his packthread might be fo coarfe

as

3co Conductors and Non-Conduttors. [Book IV.

as . to intercept the electric power, and they accord-
ingly attempted to remedy this evil by employing a
(ilk ftring, which was much ftronger in proportion
than a hempen cord. With this apparatus the expe-
ment fucceeded far beyond their expectations. En-
couraged by this fuccefs, and attributing it wholly to
the finenefs of the filk, they proceeded to fupporr the
packthread, to which the ball was attached, by very
fine brafs and iron wire, but, to their utter ailonifh-
ment, found the effect exactly the fame as when they
, ufedv the hempen cord ; the electrical virtue utterly
pafted away ; while on the other hand, when the pack-
thread was fupportcd by a filken cord, they were able
to convey the electric virtue feven hundred and fixty-
five feet.

It was evident, therefore, that thefe effects depended
upon fome peculiar quality in the filk, which difablcd
it from conducting away the electrical power, as the
hempenVord and the wire had done. This, probably,
immediately led to die difcovery of other non-con-
ch icting bodies; and bairt ro/in, glq/s, &c. were pre-
iently made ufe of to infulate the bodies which were
electrified. The next obvious improvement was to
electrify feparate bodies, by placing them upon non-
conductors; and in this manner Mr. Grey and his
friend Mr. Wheeler electrified a large map, a table
cloth, &c. &c. In the latter end of the iame fism-
mer, Mr. Grey found that he could electrify a rod as
well as a packthread, without inferting any part into
his excited tube, and that it only required to be placed
Dearly in contact with the apparatus.

Mr. Grey proceeded to try the effects of electricity

upon animal bodies. lie fufpended a boy on hair

lines in a horizontal polition, and bringing the excited

tube near his feet, he found that leaf brafs was attracted

2 very

Chap, i.] Experiments of Du Fay. 30 i

very vigoroufly by the head of the boy. He found
alfo, that he could communicate electricity to fluid
bodies, by infulating them upon a cake of rofm ; and
obferved, that when an excited tube was held over a
cup of water, the water was prefently attracted, in a
conical form, towards the tube ; that the electic matter
paffed from the tube to the water with a flight flafh
and a crackling noife -, -and that the fluid fubfided with
a tremulous and waving motion.

After this period the fpirit of philofophy in this
branch was no longer confined to England. M. Du
Fay, intcndant of the French king's gardens, added to
the (lock of difcoveries. He found that all bodies,
except metallic, foft, and fluid ones, might be made
electric by firft heating them, and then rubbing them
on any fort of cloth. He alfo excepts thofe fub-
ilances which grow foft by heat, as gums, or which
diflblve in water, as glue. In purfuing Mr. Grey's
experiments with a packthread, &c. he perceived thau
they fucceeded better by wetting the line. To prove
the effects of this wonderful agent on the animal body,
he fufpended himfelf by filk cords, as Mr. Grey had
fufpended the boy, and in this fituation he obferved,
that as foon as he was electrified, if another perfon
approached him, and brought his hand, or a metal
rod, within an inch of his body, there immediately
iflircd from it one or more prickling (hoots, attended
with a fnapping noife j and he adds, that this experi-
ment occafioned a fimilar fenfation in the perfon who
placed his hand near him : in the dark he obferved,
that thefe ihappings were occafioned by fo many fparks
of fire.

Mr. Grey, on rerunning his experiments, imme-
diately concluded from that of M. Du Fay, in which
a piece of metal drew fparks from the perfon elec-
trified,

302 Ley den Phial difcovsred. [Book IV.

trificd, and fufpended on filk line.-, that if die perfon
and the metal changed places the off! ct woflld be ;he
fame. He accordingly fufpenueci a piece of metal
by filk threads near his excited tu^e^ ; he

drew fparks from it at pleafure. This was the origin
of metallic condu&ors. Mr. Grey fufpedted that Jie
electric fire might be of the iume nature with thun-
der and lightning.

To the philofophers of Germany rve are indebted
for moft of the improvers !•".> in thr c!t Jtrical appa-
ratus. They revived the ule of the gio'>e, vvLkh had
been invented by Mr. Hawkfbee, which was after-
wards fuperfeded by a cylinder, and to which they im-
parted a circular motion by means of wheels, and ufed
a woollen rubber inftead of the hand. By the great
force alfo of their machines, they were able to fire
fome of the moft inflammable fubftances, fuch as
highly rectified fpirits, by the electric fpark.

But the moft lurprizing difcovery was that which
immediately followed thefe attempts, in the years
1745-6 ; I mean the method of accumulating the
electric power by the Leyden phial. M. Von Kleift,
dean of the cathedral of Carnnin, was the firfl who
found that a nail or hrafs wire, confined in an apothe-
cary's phial, and expofed to the electrifying glafs or
prime conductor, had a power of collecting the elec-
tric virtue fo as to produce the moft remarkable
effeds -s he foon found that a fmall quantity of fluid
added to it increafed the power; and fucceffive elec-
tricians found, that fluid matter, or any conducting
body confined in a glafs veflel, had this power of ac-
cumulating and condenfing (if I may ufe the expref-
fion) the electric virtue. The fliock which an elec-
trician is enabled to give by means of the Leyden
phial is well known ; and this was foon followed by

another

Chap, i.] Nature of the eleftric Matter. 303

another improvement, that of forming what is called
the electric battery, by increafing the number of phials,
by which means the force is proportionably increafed.
By thefe means the electric Ihock was tried upon the
brute creation, and proved fatal to many of the fmaller
animals, which appeared as if killed by lightning. By
thefe means alfo the electric matter was conveyed to
great diftances ; by the French philofophers, for near
three miles ; and by Dr. Watfon, and fome other
members of the Royal Society, it was conveyed, by
a wire, over the river Thames, and back again through
the river, and fpirits were kindled by the electric fire,
which had paffed through the river. In another expe-
riment by the fame gentleman, it was found that the
electric matter made a circuit of about four miles
almoft inftantaneoufly.

The next difcovery refpects the nature, or rather
the origin, of the eleclric matter. Dr. Watfon was
firft induced to fufpect that the gkfs tubes and globes
did not contain the electric power in themfelves, by
obferving, that upon rubbing the glafs tube while he
was (landing on cakes of wax (in order to prevent, as
he expected, any of the electric matter from difcharg-
ing itfelf through his body on the floor) the power
was fo much leffened that no lhapping could be ob-
ferved upon another perfon's touching any part of his
body j but that if a perfon not electrified held his hand
near the tube while it was rubbed, the fnapping was
very fenfibie. The event was the fame when the
globe was whirled in fimilar circumftances j for if the
man who turned the wheel, and who, together with
the machine, was fufpended upon filk, touched the
floor with one foot, the fire appeared upon the con-
ductor j but if he kept himfelf free from any commu-
nication with the floor, no fire was produced. From

thefe

304 STWy ofM. Du F [Book III.

thefe and other dccifive experiments Dr. Wat Ton con-
cludes, that thefe globes and tubes are no more than
the firit movers or determiners of the electric power.

M. Du Fay had made a diftinction of two dif-
ferent fpecies of electricity, one of which he called the
vitreous, and the other the refmous electricity ; and foon
after the difcovery of the Leyden phial, it was found,
that by coating the outfide of the phial with a conduct-
ing lubftance, which communicated by a wire with
the perfon who difcharged the phial, the fhock was
immenfely increafed ; and indeed it appeared, that the
phial could not be charged unlefs fome conducting
Nlubfrance was in contact with the outfide. Dr. Frank-
lin, however, was the firft who explained thefs pheno-
mena. He fhewed that the furplus of electricity,
which was received by one of the coated furfaces of
the phial, was actually taken from the other ; and that
one was poTefTecl of lefs than its natural fhare of the
electric matter, while the other had a fuperabundance.
Thefe two different dates of bodies, with refpect to
their portion of electricity, he diftinguifhed by the
terms plus or pofitive, and minus or negative j and it
was inferred from the appearances, that bodies which
exhibited what M. Du Fay called the refinous electri-
city, were in the ftate of minus, that is, in the ftate of
attracting the electic matter from other bodies, while
thofe which were podefied of the vitreous electricity
were bodies electrified plus, or in a itate capable of
imparting electricity to other bodies. By this difco-
very Dr. Franklin was enabled no increafe the electric
power almofl: at pleafure, namely, by connecting the
outfide of one phial with the infide of another, in fuch
a manner that the fluid which was driven out of the
firft would be received by the fecond, and what was
driven out of the fecond would be received by the

»' third,

Chap, i.] Dr. Franklin. 30$

third, &c. and this conftitutes what we now call an
electrical battery.

But the mofl aftonifhing difcovery which Franklin,
or I might fay any other perfon, ever made in this
branch of fcience, was the demonftration of what had
been (lightly fufpected by others, the perfect fimilarity,
or rather identity, of lightning and electricity. The
Doctor was led to this difcovery by comparing the
effects of lightning with thofe of electricity, and by re-
flecting, that if two gun- barrels electrified will ftrike at
two inches, and make a loud report, what muft be
the effect of ten thoufand acres of electrified cloud.
Not fatisfied, however, with {peculation, he conftructed
a kite with a pointed wire fixed upon it, which, during
a thunder ftorm, he contrived to fend up into an
electrical cloud. The wire in the kite attracted the
lightning from the cloud, and it defcended along the
hempen firing, and was received by a key tied to the
extremity of it, that part of the firing which he held
in his hand being of filk, that the electric virtue might
Hop when it came to the. key. At this key he charged
phials, and from the fire thus obtained he kindled
fpirits, and performed all the common electrical ex-
periments.

Dr. Franklin, after this difcovery, conftructed an
infulated rod to draw the lightning from the atmo-
fphere into his houfe, in order to enable him to make
experiments upon it ; he alfo connected with it two
bells, which gave him notice by their ringing when
his rod was electrified. This was the origin of the
metallic conductors now in general ufe.

It was afterwards difcovered by Mr. Canton, that
the pofitive and negative electricity, which were fup-
pofed to depend upon the nature of the excited body,
and therefore had obtained the namejs of refmous and

VOL. I. X vitreous,

Hiftory ofDifcoveries, &V. [Book IV.

vitreous, depended chiefly upon the nature of the fur-
face j for that a glafs tube, when the polifhed furface
was deltfoyed, exhibited proofs of negative electricity
as much as fulphur or fealing wax, and drew fparks
from the knuckle when applied to it, inftead of giving
fire from its own body ; when the tube was greafed,
and a rubber with a rough furface was applied to it,
its pofitive power was reftored, and the contrary, when
the rubber became fmooth by friction.

At this period it may not be improper to clofe my
(ketch of the difcoveries relating to electricity ; fmce
the fole object of thefe narratives, in this work, is to
conduct the reader to a more ready apprehenfion of
the fcience, it would be ufelefs to lead him into the
minutias of it before he was made properly acquainted
with the general principles.

Chap. 2.] [ 307 ]

CHAP. II.

GENERAL PRINCIPLES OF ELECTRICITY.

Analogy between Caloric, or Fire, and the e leEirical Fluid.— The Argu*
ments on the contrary Side.— Conjectures concerning the Nature oj*
this Fluid.— —Means of producing elefirical Phenomena.'— Condu^iors
and Non-ccnduffon.—Inftruments employed in Eleftricity.

FROM the brief account, which has been given, in
the preceding chapter, of difcoveries relative to
this branch of fcience, the reader will be in a confi-
derable degree prepared to infer, that electricity is the
action of a body put in a (late to attract of repel light
bodies placed at a certain diftancej to give a flight
fenfation to the Ikin, refembling in fome meafure that
which we experience in meeting with a cobweb in the
air j to fpread an odour like the phofphorus of Kun-
kell ; to dart pencils of light from the furface, attend-
ed with a fnapping noife, on the approach of certain
fubftances ; laftly, that the body put in this flate is ca-
pable of communicating to other bodies the power o£*
producing the fame effects during a certain time.

The electric power is indubitably the effect of fome
matter put in motion, either within or round the elec-
trified body, fmce if we place either our hands or face
before an excited tube of glafs, or before an infulated
conductor which is electrified, we fhall perceive ema-
nations fenfible to the touch, and if we approach
nearer, we (hall feel it diftinctly, and hear a weak noife j
in the dark we perceive fparks of vivid light, efpecially
from angular points j we fee emitted pencils of rays,
X 2 or

308 Eleftiic Matter. [Book IV.

or fmall flames of divergent flame j it is certain, there-
fore, that fome fubtile matter put in motion is alone
capable of making thefe imprefllons upon our fenfes ;
and we may conclude, that every electrified body is
cncompafTed by fome matter in motion, which is, with-
out doubt, the immediate caufe of all the electrical
phenomena, and which we term the electric matter or
fluid.

Thus far, and no farther, are we warranted in af-
firming, on the only evidence to be admitted in philo-
fophy, that of experiment, fact, and obfervation. There
is, however, in man, a curiofity that prompts us to
look beyond effects, and a difpofition that leads us to
theorize, even on the moft difficult fubjects. Let us,
however, do it with diffidence and caution. What
then is this electric matter ? or whence does it derive
its origin ? It apparently proceeds not from the elec-
trified body, for that fuffers no fenfible diminution. It
depends not on any property inherent in the air of the
atmofphere, for three obvious reafons ; firft, becaufc
electrical phenomena may be produced in a ipace
from which the air has been moft carefully exhaufted.
Secondly, Becatife the electrical matter has qualities
which are not inherent in air; it penetrates certain bo-
dies impervious to air, fuch as metals j it has a fen-
fible odour j it appears itfelf inflamed ; it is capable of
inflaming other bodies, and of melting metals ; effects
which air cannot produce. Thirdly, It tranfmits its
motions with confiderably more rapidity than that of
found, which is a motion of the air the moft rapid that
we are acquainted with.

It is generally agreed, that the electric matter has
a ftrong analogy with the matter of heat and light. It
appears indeed, that nature, who is lo very ceconomi-
cnl in the production of principles, whiift (he multiplies

their

Chap. 2.] Its Analogy with Heat and Light. 309

their properties fo liberally, has in no cafe eftablifhed
two caufes for one effect. We may apply this remark
to the electric matter ; and the more we inquire into
the properties of the electric matter, and thofe of the
matter of heat and light, the more mall we difcover of
this analogy between them, and the more probable will
it appear, that fire, light, and electricity depend upon
the fame principle, and that they are only three dif-
ferent effects from the fame matter or efTence.

i ft. Of all the means neceffary to excite the matter
of heat, there is none more efficacious than that which
is moft neceflary to produce electricity, namely, fric-
tion, adly. As fire, or caloric, extends itfelf with
more facility in metals and humid bodies than in any
other fpecies of bodies, fo metals and water are con-
ductors of electricity in the fame manner as they are
of heat, and, in general, the fame conductors are found
equally good for both. jdly. Fire, or caloric, is the
moft elaftic of all bodies, and is confidered by moft
philofophers as the principal caufe of that repulfion
which takes place between the particles of bodies, of
which the ftrongeft inltance has already been given in
explaining the caufe of fluidity; and to a fimilar caufe
the electric repulfion may be referred. 4thly. The
pulfe and perfpiration of animals are increafed by elec»-
tricity as by the actual application of heat, and the
growth of vegetables is promoted by it *. fthly. Ac-
tual ignition is produced by the electric fluid : thus it
is a common experiment to fire fpirit of wine by the
electric fpark; inflammable air is fet on fire by the
fame means in the commc-'i electrical piftol .; and even
gunpowder may be exploded by a fpark from a power-
ful conductor. 6thly. Metals are melted by electri-

* Count Rumford's experiments, Phil: Tranf. vol. IxxvL

X city,

3 TO Analogy between Light and Eleftricity. [Book IV,

eity, and moft inflammable fubflances are affc6ted by it
as by 'common fire, but in a weaker degree *. ythly.
The light emitted by the electrical apparatus has all
the properties of that which is emitted from the fun,
the compofition differing in fome refpects, according
to circumftances, as to the predominancy of certain
rays, the light in different inftances inclining to blue,
red, white, &c. according to its intenfity. Sthly. The
motion of light is exceedingly rapid, whether it is re-
flected or refracted ; in the fame manner the electric
fluid is found to move with almoft infinite velocity, for
it has been proved by experiments, that a cord twelve
hundred feet long has become inftantly electric in its
whole extent f. The Abbe Nollet has communicated
the electric mock to two hundred perfons at the fame
time, or at the lead perceptible inftant.

Notwithstanding thefe confkkrations, it muft be
confefled that there are fome facts which feem to indi-
cate that the electric fluid is not purely and fimply the
matter of heat or light unmixed with other fubflances j
for i ft, we have obferved, that the electric matter has
the property of affecting the organs of fcent, which
belongs neither to light nor heat.

2dly. It is well known alfo, that an accumulation of
the matter of fire or heat increafes the fluidity of all

* Mr. Kinnerfley made a large cafe of bottles explode at once
through a fine iron wire; the wire at firft appeared red hot, and
then fell into drops, \vhich burned themfelves into the table and
floor, and cooled in a fpherical form like fmall fhot. Artificial
lightning, from a cafe of about thirty-five bottles, will entirely de-
ftroy brafs wire of one part in three hundred and thirty of an inch.
Metals may alfo be revived by the electric fhock ; and Slg. Bec-
caria melted borax and glafs by it. Prieftley'E Hift. Eledt. vi. 34;
r—343- Seeds of clubmofs (lycopodium) were fired by itj alfa
aurujn fulminans, ib. 343.

rf- Memoires de 1'Acad. des Sci. 1733, p. 247.

bodies,

Chap. 2.] Difference between Heat and EkRricity. 311

bodies, and prevents them from congealing, whereas
congealed fluids may be highly charged with electri-
city i nor does it appear to have the fmalleft effect in
increafing their fluidity.

3dly. Heat fpreads in every direction, whereas the
electrical fluid may be arrefted in its progrefs by cer-
tain bodies, which, on that account, have obtained the
name of non-conductors. The Torricellian vacuum,
on the contrary, affords a ready paflage to the electric
fluid, but is a bad conductor of heat *.

4thly. Whenever the matter of heat penetrates bo-
dies, it warms as well as expands them. The electric
fluid does not produce thefe effects j bodies, however
long they may be electrified, become neither hotter to
the touch, nor more extended in dimenfions,

fthly. The fmgular property of adhering to certain
conductors, without diffufing itfelf to others, which
may be even in contact with them, fo obfervable in the
electrical fluid, is a property not common to caloric,
or elementary fire. Thus we have feen that ipirits were
fired by an electric fpark drawn by a wire through the
water of the Thames, and large pieces of iron wire
have been heated red hot, while immerfed in water, by
an electrical explofion f.

6thly. With refpect to the identity of light and
electricity, it fliould alfo be recollected, that light per-
vades glafs with the greateft facility, whereas that fub-
ftance is penetrated by the electrical fluid only in cer-
tain circumftances, and with the utmoft difficulty; if,
therefore, it (hould be admitted that the bafis of the
electric matter is radically the fame with the matter
of heat or light, it muft alfo be admitted, that it re-

* Count Rumforl's experiments above quoted, •
t Jbid,

X tains

3 1 2 Means of producing Elcftruity. [Book IV.

tains fome other matter in combination with it, of the
nature of which we are as yet uninformed ; and it is
probably this combination of foreign matter which
difables it, in ordinary cafes, from penetrating glafs.
Let it, however, be carefully remembered, that this is
fpeculation and conjecture, and that we at prefent know
nothing of a certainty concerning the electrical fluid,
but fome of its effects.

Electrical phenomena are produced by friction, and
by communication. In general, bodies which electrify
the beft by friction, electrify the word by communi-
cation (except glafs in certain circumftances) and on
the contrary, fubftances which electrify the bed by
communication electrify the worft by friction. I (hall
begin with thofe experiments which gave rife to the
principal technical terms made ufe of in this fcience.

If a dry glafs tube is rubbed with a piece of dry
filk, and if light bodies, as feathers, pith balls, &c. are
prefented to it, they will be firft attracted, and then re-
pelled. The beft rubber for a fmooth glafs tube is a
piece of black or oiled filk, on which a little amalgam
has been fpread; fealing wax, rubbed with new and
foft flannel, will produce the fame effect. By this fric-
tion an agent or power is put in action, and this power
is called the electrical fluid ; a certain quantity of this
fluid is fuppofed to exift latent in all bodies, in which
ftate it makes no impreflion on our lenfes, but when
by the powers of nature or of art, this equilibrium is
dcftroyerl, and the agency of the fluid is rendered per-
ceptible to the fenfes, then thofe effects are produced
which are termed electrical, and the body is laid to be
electrified.

If a homogeneous body is prefented to the excited
tube, fo as to receive electricity from it, and the elec-
tricity

Chap. 2.] Conductors and Ntn- conductors. 313

tricity remains at or near the end or part prefented,
without being communicated to the reft of the body,
it is called a non-conductor or electric ; but if, on the
contrary, the electricity is communicated to every
part, the body is called a conductor, or non-electric.
A body is faid to be infulated when it communicates
with nothing but electrics.

A conductor cannot be electrified while it commu-
nicates with the earth, either by direct contact or by
the interpofition of other conductors, becaufe the elec-
tricity is immediately conveyed away to the earth.

A mutual attraction is exerted between a body in a
ftate of electricity and all non-electric bodies, which,
if not large and heavy, wi,ll pafs rapidly through the
air to the electrified body, where they remain till they
have, by communication, acquired the fame ftate,
when they will be repelled. If aa uninfulated con-
ductor is at hand, it will attract the fmali body when
electrified, and deprive it of its electricity, fo that it
will be again attracted by the electrified body, and re-
pelled as before, and will continue to pafs and repafs
between the two, till the electric ftate is entirely de-
ftroyed.

The following fubftances are reckoned among the
principal conductors of the electric fluid:

Stony fubftances in general,

Lime-ftone, marbles,

Oil of vitriol,

Allum,

Black pyrites,

Black lead in a pencil,

Charcoal,

All kinds of metals and ores,

The fluids .of animal bodies,

All fluids, except air and oils.

Electric

314 Conductors and Non-conduftors. [Book I\r.

Electric bodies, or thofe fubftances which emit this
fiuid, are the following :

Amber, jet, fulphur,

Glafs, and all precious 'ftones,

All refmous compounds,

All dry animal fubftances, as filk, hair, wool,

paper. &c.

M. Achard, of Berlin, has proved by experiment,
that certain circumftances will caufe a body to conduct
electricity, which before was a non- conductor. The
principal of thefe circumftances are the degrees of
heat to which the body is fubjected. This gentleman
agrees in opinion with M. Euler, that the principal dif-
ference between conductors and non-conductors con-
fifts in the fize of the pores of the conftituent parts of
the body.

It muft be obferved, that electrics and non-electrics
are not fa ftrongly marked by nature as to be defined
with precifion j for the fame fubflance has been diffe-
rently claffed by different writers ; befides, the electric
properties of the fame fubftance vary according to
changes of circumftances j thus a piece of green wood
is a conductor, and the fame piece, after it has been
baked, becomes a non-conductor j when it is formed
into charcoal it again conducts the electric matter ; but
when reduced to afhes is impervious to it. Indeed, it
might perhaps be generally laid, that every fubftance
is in a certain degree a conductor of this fiuid, though
fome conduct it \vith much more facility than
others.

The inftruments ufed in electricity are of five kinds ;
firfl, tubes of glafs, or cylinders of fealing wax j the
fecond confift of a fingle winch or of a multiplying
wheel, by means of which, globes, cylinders, and plates
of glafs, of fulphur, or of fealing wax, are made to

turn

Chap. 2.] Inftwments ujed in Ekfirkity. 315

turn round •, thirdly, metallic conductors, or fubftanees
charged with humidity; fourthly, electric bottles,
commonly called Leyden phials j fifthly, eledric bat-
teries.

The firft electrical machine made ufe of was a tube
of glafs, which, being electrified by friction, was then
put in a (late to communicate electricity to other bo-
dies. The beft glafs for this purpofe is the fine white
Englilh cryftal. The moil convenient dimerifions for
thefe tubes are about three feet of length, twelve or
fifteen lines of diameter, and quite a line of thicknefs.
It is of little importance whether the tube is open or
clofed at the extremities ; yet it is necefTary that the
air within fhould be in the fame ftate as that without j
for this reafon the tube fhould at lean: be open at one
end j but care muft be taken left dirt fhould be ad-
mitted into the infide, for that would confiderably im-
pede its effects. If, notwithftanding thefe precau-
tions, the tube receives either dirt or moifture, fome
dry and fine fand fhould be introduced into the infide,
and it fhould be afterwards cleaned out with fine dry
cotton.

When it is intended to electrify a tube, it is only
neceffary to take the end in one hand, and to continue
to rub the tube with the other hand from one end to
the other until it affords marks of its being fufficiently
charged with the electric fluid. This friction may be
performed with the naked hand when it is dry and
clean, otherwife with a piece of brown paper, or wax-
ed taffeta. When .the tube has been rubbed in this
manner, the circumambient air being dry, if light fub-
ftanees are prefented to it, they will be firft attracted
towards it and immediately afterwards repelled.

The electric fluid may be excited in nearly a fimi-

lar

3 1 6 Large Eleftrical Machine. [Book IV.

lar manner, by rubbing a flick of fulphur or fcaling
wax.

Thefe tubes being but fmall, the electric fluid pro-
duced by thefe means is but feeble in its effects. We
have feen that a method was contrived to turn a globe
of glafs upon its axis, by means of a machine with a
winch or multiplying wheel ; this method admitted of
a larger furface, and the friction was performed with
greater eafe, by means of a rubber being placed clofe
to the revolving globe.

To conflruct a machine fufficiently large for all the
purpofes of electrical experiments, M. Briffon directs
that the wheel RO (fee Plate XXVI. Fig. i.) fhould
be at leaft four feet in diameter, and be turned round
in a ilrong and folid frame H I C D. He directs fur-
ther, that there mould be two handles M, m, fo that
two men may be employed at once in certain cafes, to
give a fufficient friction to the globe to augment the
effects. The globe S ought to be carried round be-
tween two fmall ports N, which ought to be ib placed
that they may be drawn farther from or nearer to the
wheel, in order to admit the cord to be moved com-
modioufly whenever it is contracted or extended. It
is allb neceffary that one of thefe fmall polls mould be
moveable, that it may be placed either nearer to or
farther from the other, fo that globes of different dia-
meters may be placed in the machine; the cord of the
wheel R O mould communicate immediately with the
pulley P of the globe S.

When this machine is ufed for the purpofes of elec-
tricity, the globe S fhould be turned according to the
order of the cyphers i i 3, and its equator rubbed
with a leathern cufhion fluffed with horfe hair, this
may alfo be done by the hands when they are clean

and

Vol.. I . /'.j

Ti.

Fig. 3

. 4.

Chap. 2.] Small Eletfrical Machine. 317

and dry. A bar of iron (AB, Fig. 2.) infulated \vith
the cords of filk j, j, is placed over the globe S,
this bar fcrves as a conductor to the electric fluid *.

A machine of a fimpler conftruction has been in-
vented in this country, and is represented in Plate
XXVII. Fig. i. In this inftrument a circular plate of
glafs is employed inftead of a globe. The plate P p,
is bored through the center, and mounted on an axis,
a a3 of copper or hard wood, to which is fixed the
handle, a b. The axis is Supported by two vertical
polls of wood, m, n, to which are appended four
cufhions, / z, formed according to the preceding di-
rections, and which ferve by their friction to excite
the plate.

Before the plate a metal conductor, E D, is placed
horizontally, having two arms or branches, A B, alia
of metal, each terminating in a fmall globe or knob,
which may be brought within a convenient diflance
of the plate to receive the electrical fluid. The con-
ductor itfelf is infulated by two glafs pillars, F G.

The advantages of this machine are, that it may be:
made portable, and is of fo fimple a conftruction, that
any gentleman in the country, after procuring a plate
of a reafonable thicknefs from a glafs houfe, may, by
the aid of a common cabinet-maker, conftruct one
for his own ufe ; the conductor may be equally infu-
lated by rofin, wax, filk, or any other electric or non-
conducting fubftance.

This machine is, however, feeble in its operations,,
.compared with thofe constructed with globes or cy-
linders. The moil powerful, and yet the moft fim-
ple, of thefe that I have feen, are thofe defcribed by

* Briflbn, Traitc elcmentaire de Phyf. torn. iii. p. 305.

my

318 Elefiricd [Book IV,

my late valuable friend, Mr. Adams, in his treatife of
ekdricity.

Fig. i. and 2, Plate XXVIII. reprefent two electrical
machines of the moft approved conftruction ; the only
difference between them is, the mechanifm by which
the cylinder is put in motion.

The cylinder of the machine, Fig. i. is turned round
by two wheels, a b, c dy which act on each other by a
catgut band, part of which is feen at e and/.

The cylinder in Fig. i. is put in motion by a fimple
winch, which is lefs complicated than that with a
multiplying wheel (Fig. 2) : as, however, both ma-
chines are fo nearly firnilar, the fame letters of refer-
ence are ufed in defcribing them both. ABC repre-
fent the bottom board of the machine, D and E the
two perpendicular fupports, which fuflain the glafs cy-
linder F G H I. The axis of the cap K paffes through
the fupport D ; on the extremity of this axis either a
fimple winch is fixed, as in Fig. i. or a pulley, as in
Fig. 2 *. The axis of the other cap runs in a fmall
hole, which is made in the top of the fupporter E.

O P is the glafs pillar to which the cufnion is fix-
ed ; T a brafs fcrew at the bottom of this pillar, which
is to regulate the preflure of the cufhion againft the
cylinder. This adjufting fcrew is peculiarly advan-
tageous: by it the operator is enabled to leffen or in-
creafe gradually the preffure of the cufhion, which it
effects in a much neater manner than it is poffible to
do when the infulating pillar is fixed on a Hiding
board.

* Mr. Adams, in his Leftures on Nat. Philofophy, obfervcs,
that machines turned by a fimple winch are lefs liable to be out
of order than thofe which are turned by a multiplying wheel, and
may alfo be excited more powerfully. Adams's Lefi. vol. iv.

On

YOI..J

Fiq. 1.

Chap. 2.] Machines. 319

On the top of the pillar O P is a conductor, which
is conne&ed with the cufhion, and this is called the
negative conductor. In both figures this conductor
is fuppofed to be fixed clofe to the culhion, and to
lie parallel to the glafs cylinder. In Fig. i. it is
brought forwards, or placed too near the handle, in
order that more of it may be in fight, as at R S j in
Fig. a. the end R S only is feen.

Y Z (Fig. I. and 2.) reprefents the pofitive prime
conductor, or that which takes the electric fluid im- '
mediately from the cylinder, L M the glafs pillar by
which it is fupported and infulated, and V X a wooden
foot or bafe for the glafs pillar. In Fig. i. this con-
ductor is placed in a direction parallel to the glafs
cylinder ; in Fig. i. it ftands at right angles to the
cylinder : it may be placed in either pofition occa-
fionally, as is moft convenient to the operator.

Previous to relating feveral circumftances, by which
a large quantity of the electric fluid may be excited, it
may be neceflary to premife, that the refiflance of the
air feems to be leffened, or a kind of vacuum is pro-
duced, where the cufhion is in clofe contact with the
cylinder ; and that the electric matter, agreeably to
the law obferved by all other elaflic fluids, is prefied
towards that part where it finds lead refiftance } the
fame inftant, therefore, that the cylinder is feparated
from the cufhion, the fire iflues forth in abundance,
becaufe the refiftance made to it by the action of the
atmofphere is lefiened at that part : the effect which
ariles from the deftruction of the attraction or cohe-
fion between the cylinder and the cufhion is a further
proof of the truth of this hypothefis. The more per-
fect the continuity is made, and the quicker the folu-
tion of it, the greater is the quantity which will pro-
ceed from the culhion.

' To

320 How to excite more effettually [Book IV.

To excite, therefore, an electrical machine effec-
tually, we muft firft find out thofe parts of the cufhion
which are prefTed by the glafs cylinder, then the amal-
gam muft be applied to thofe parts only. The line of
contact between the cylinder and cumion muft be
made as perfect as poflible, and the fire which is col-
lected muft be prevented from efcaping. The breadth
of the cumion mould not be great, and it mould be
placed in fuch a manner that it may be eafily raifed or
lowered.

In order to find the line of contact between the cy-
linder and cumion, place a line of whiting, which has
been difiblved in fpirits of wine, on the cylinder ; on
turning this round, the whiting is depofited on the
cumion, and marks thofe parts of it which bear
or rub againft the cylinder. The amalgam is to be
put, on thofe parts only which are thus marked by the
whiting.

Whenever the electricity of the cylinder grows Jefs
powerful, it is eafily renewed by turning back the fiik
which lies over it, and then rubbing the cylinder with
the amalgamated leather, or by altering the prefibre
of the adjufting fcrew.

A fmall quantity of tallow placed over the amal-
gam is obferved to give more force to the electric
powers of the cylinder ; or the fame end may be
effected by rubbing the cylinder with a coarfe cloth,
which has been a little greafed, and afterwards wip-
ing the fame with a clean cloth.

As air not only refifts the emiflion of the electric
fluid, but alfo diffipates what is collected, on account
of the conducting fubftances which are floating in it,
a piece of black or oiled filk mould be placed from
the line of contact to the collecting points of the prime

conductor,

Chap. 2.] Electrical Machines. 321

• conductor, and thefe points fhould be placed within its
atmofphere.

Sometimes the filk will adhere fo ftrongly to the
cylinder, when zinc amalgam is ufed, as to render ic
very difficult to turn j this may be obviated by rub-
bing a fmall quantity of aurum mufivum, or a little
whiting, over the filk, when it is wiped clean *.

There are now in ufe two kinds of amalgam : One
is made of quickfilver five parts, zinc one part, which
are melted together with a fmall quantity of bee's -
wax; the other is the aurum mufivum of the fhops.
Before either of thefe will adhere clofely to the filk it
is neceflary to greafe it, to wipe off the fuperfluous
greafe, and then fpread the amalgam.

One property of the electric fluid it will be neceflary

• The following direftions of Mr. Adams, relative to exciting
the machine, will be ufeful to the experimentalift :

' To excite your machine, clean the cylinder, and wipe the
filk.

' Greafe the cylinder by turning it againft a greafy leather, till
it is uniformly obfcured. The tallow of a candle may be ufed.

' Turn the cylinder till the filk flap has wiped off fo much of
the greafe, as to render it femi-tranfparent.

' Put fome amalgam on a piece of leather, and fpread it well,
fo that it may be uniformly bright; apply this againfl the turning
cylinder, the friclion will immediately increafe, and the leather
muft not be removed until it ceafes to become greater.

' Remove the leather, and the aftion of the machine will be
very llrong.

• The preffure of the cufhion cannot be too fmall, when the ex-
citation is properly made.

' The amalgam is that of Dr. Higgins, compofed of zinc and
mercury ; if a little mercury be added to melted zinc, it renders it
eafily pulverable, and more mercury may be added to the powder,
to make a very foft amalgam. It is apt to cryftallize by jepofe,
which fccms in fome meafure to be prevented by triturating it
with a fmall proportion of greafe : and it i.s always of advantage
to triturate it before ufing.'

VOL. I. Y to

3 * 2 P0/tf/ j * ttra 51 mojl forcibly. [Book IV.

to notice before the conclufion of this branch of the
lubjecl, and that is, that it is more forcibly attracted
by points than by balls or any blunt or rounded fur-
faces. This may be demonftrated by a variety of ca-fy
experiments, and may be feen by prefenting a metal
ball at a given diftance to a conductor in the act of
being charged, when it will be found that a metal
point prefented at a much greater diftance will draw
off the whole of the electrical matter from the con-
ductor. In the one cafe alfo (the point) the electri-
city goes off invifibly, and without noife ; in the other
cale there is both a flafli and a report.

With refpect to the modes employed by electricians
for the accumulation of the fluid, it will be neceffary
to confider them in a diftinct chapter j but previous
to this a few particulars muft be dated relative to
wnat tt termed pofitive and negative electricity.

Chap. 3.

CHAP. III.

OF THE VITREOUS AND RESINOUS, OR
POSITIVE AND NEGATIVE ELECTRI-
CITY.

DiJiinSlion in the attrafti-Tje Powers of certain EleRrics. — Tbefe
Ejfefts found to depend not on the Nature of the Subftance, but the
Roughnefs or Smoothness of the Surface. — Theory of Two diJtinB
Fluids, — Franklin's Theory, — Difficulties attending it.

IN a very early ftage of the fcicnce, we have feen,
that a diftinction was obfervcd with refpect to the
attractive and repulfive powers of certain electric bo-
dies. Thus if we electrify with the fame fubftance,
for inftance, either with excited glafs or with fealing
wax, two cork balls in an infulated flate, that is, fuf-
pended by filk lines about fix inches long, the balls
will feparate and repel each other j but if we electrify
one of the balls with glafs, and the other with fealing
wax, they will be mutually attracted. This circum-
ftance gave rife to the opinion, that two different fpe-
ci'es'of electricity exifted, and the one was termed the
vitrt'jus electricity, or that produced from glafs ; and
the other, which was produced from fealing wax, re-
fmous fubftances, and fulphur, was termed ihe refinous
electricity.

Subsequent experiments ferved to mew, that in the
common ekctrical machine, the rubber exhibited the
appearances of the refmous electricity, and the cylin-
der that of the vitreous, while the former was con-
nected with. the earth. A divergent cone or brufh of
electrical light was obferved to be the obvious mark
Y 2 of

324 Theory of two Kinds of Eleftricity. [Book IT*

of the vitreous electricity, and a fingle globular mafs
of light diftinguiHied the refmous kind. The hand or
body alfo, which approached the vitreous or glafTy
fubftance, when excited, appeared to receive the mat-
ter from the electric ; but when one of the refmous
kind was excited, the electrical matter appeared to pro-
ceed from the hand or other approaching body.

Notwithftanding, however, the names by which thefe
different forms of electricity were diftinguifhed, as the
vitreous and refmous, it was at length difcovered, that
>the different phenomena depended rather upon the
furface, than upon the nature and compofition of the
electric j for a glafs tube, when the polifhed furface
was deflroyed, by being ground with emery, and be-
ing rubbed with a fmooth body, exhibited all the proofs
of the refmous electricity, as much as fulphur or feal-
ing wax ; yet afterwards, when it was greafcd and rub-
bed with a rough furface, it refumed its former pro-
perty. It feems, therefore, to be a rule, that the
imootheft of two bodies, upon friction, exhibits the
phenomena of the vitreous electricity, for baked wooden
cylinders with a fmooth rubber are refinoufly electri-
fied, but with a rubber of coarfe flannel exhibit the
appearances of the vitreous kind, and even polilhed
glafs will produce the phenomena of the rcfinous
electricity, if rubbed with the fmooth hair of a cat's
fkin.

Amidft this embarraffing variety of experiments,
thofe philofophers who applied to this branch of fci-
ence, were eagerly employed in inventing theories to
account for thefe phenomena, and electricians are ftill
divided with rclpect to the caufe.

The eld theory of vitreous and refmous electricity,
or two diftinct, pofitive, and active powers, which
equally and ftronely attract and condenfe each other,

has

Chap.j.] Franklin 's Theory of Eleffririty. 325

has ftill its fupporters ; amongft the ableft of its de-
fenders was my late friend Mr. Adams, who, it muft
be confefied, upon this theory, has ingenioufly ac-
counted for the moft remarkable electrical pheno-
mena *.

The theory of Franklin, however, though not with-
out its defects, has more fnnpliciry, and accounts for
fads in a more eafy and more natural manner. The
principles of this diftinguiflied philofopher may be re-
folved into the following axioms :

i ft. The electric matter is one and the fame in all
bodies, and is not of two diftinct kinds.

ad. All terreflrial bodies contain a quantity of this
matter.

jd. The electric matter violently repels itfelf, but
attracts all other matter.

4th. Glafs and other fubftances, denominated elec-
trics, contain a large portion of this matter, but are
impermeable by it.

5th. Conducting fubftances are permeable-by it, and
do not conduct it merely over their furface.

6th. A body may contain a fuperfluous quantity of
the electrical fluid, when it is faid, according to this
theory, to be in a pofttive ftate, or electrified plus ; and
when it contains lefs than its proper fhare it is faid to
be negative, or electrified minus.

7. By exciting an electric, the equilibrium of the
fluid is broken, and the one body becomes overloaded
with electricity, while the other is deprived of its na-
tural fhare.

Thus, according to the Franklinean theory, that
electricity, which was before called vitreous, is now
called pofitive electricity ; and that which was termed
the refmous, is now denominated negative electricity.
* See Mr. Adams's Lectures on Nat. Phil. vol. iv.

•Y 3 It

3:6 ObjeBiom to [Book IV.

Ic is evident, that it is only in paffmg from one body
to anpther, that the effects of the electrical fluid are
apparent. When all the adjacent bodies therefore are
equally charged with electricity, no effects whatever
•will appear. The equilibrium mud, according to the
principles of Dr. Franklin, be deftroyed, that is, the
fluid muft be made rarer in fome one part, before any
of the phenomena will be exhibited. In that cafe the
denfe fluid rufhing in to fupply the deficiency in that
part where it is rarer, produces the flafh of light, the
crackling noife, and the other effects of electricity.

The different effects on rough and fmooth bodies,
when excited, have been previoufly remarked. The
Franklinean theory is, if a rough and fmooth body are
rubbed together, the fmooth body will generally be
electrified plus, and that with a rough uneven furface,
minus. Thus, in the ordinary operation of the com-
mon machine, the cylinder is pofitively electrified, or
plus, and the rubber negative, or minus. The redun-
dance of the pofitive electricity is fent from the cylin-
der to the prime conductor, and may be communicated
from it to any conducting body. If, however, the
prime conductor is made to communicate with the
earth, which- has a great attraction for the electrical
matter (and which, being one great mafs of conduct-
ing fubftances, will not permit the accumulation of the
fluid in a particular part) and if at the fame time the
rubber is in an infulated ftate, fupported for inftancc
by glafs or any electric, thefe effects will be reverfcd,
for the prime conductor will then be negatively elec-
trified, and the rubber will be plus or poficive.

This theory is, it muft be confeffed, not without its

difficulties, and jt is much to be feared, that we have

as yet no complete theory of electricity. The fact moil

difficult to be explained on the Franklinean fyflem is,

8 that

Chap. 3.] tie Frarikttman 'Theory. 327

that of two bodies ne'gatively electrified repelling each
other, for if the repulfisn in the cafe of pofitive elec-
tricity is caufed entirely, as there is reafon to believe
it is, by the electric matter, how mould a deficiency of
that matter produce the fame effect ? Attempts have
been made to explain the fact by having recourfe to
the electricity of the air, which (when not charged with
moifture) is certainly an electric or non-condu£r.ing
fubitance, and in all cafes is an imperfect electric. The
cork balls, or other light fubftances, which are electri-
fied negatively, are therefore fuppofed to be acted
upon by the pofitive electricity of the air, which pro-
duce.s an effect adequate to their being pofitively elec-
trified. This folution, however, is not quite fatisfac-
tory; though it is perhaps unphilofophical to reject an
hypothefis, which explains fome facts greatly to our
fatisfaction, merely becaufe it has not as yet explained
every thing.

Dr. Franklin fuppofed that the electric fluid is col-
lected from the earth, and this hypothefis he fupported
by the following experiment.

Let one perfon ftand on wax (or be iniulaceci) and
rub a glafs tube, and let another perfon en wax take
the fire from the firft, they will both of them (provid-
ed they do not touch eaerr other) appear to be electri-
fied to a perfon {landing on the floor ; that is, he will
perceive a fpark on approaching either of them with
his knuckle or finger ; but if they touch each other
during the excitation of the tube, neither of them will
appear to be electrified. If they touch one another
after exciting the tube, and draw the fire as before,
there will be a ftronger fpark between therrj than was
between either of them and the perfon on the floor.
After fuch a ftrong fpark neither of them difcover &ny
tkffricity*

Y 4 He

328 Eleftricity colkfydfrom the Earth. [B.ook IV.

He accounts for thefe appearances by fuppofmg the
electric fluid to be a common element, of which each
of the three perfons has his equal lhare before any ope-
ration is begun with the tube.

A, who (lands upon wax and rubs the tube, collects
the electrical fire from himfelf into the glafs, and his
communication with the common (lock being cut off
by the wax, his body is not again immediately fup-
plied.

B, who alfo {lands upon wax, parting his knuckle
along the tube, receives the fire which was collected
from A, and being infulated, he retains this additiona.1
quantity.

To the third perfon, who Hands upon the floor, both
appear electrified; for he, having only the middle quan-
tity of electrical fire, receives a fpark on approaching
B, who has an over quantity, but gives one to A, whp
has an under quantity.

If A and B approach to touch each other, the fpark
is ftronger, becaufe the difference between them is
greater. After this touch there is no fpark between
either of them and C, becaufe the electrical fluid in all
is reduced to the original equality. If they touch while
electrifying, the equality is never deftroyed, the fire is
only circulating ; hence we fay that B is electrified por
ficivcly, A negatively *.

* Mr. Adams on Ele&ricity, p. ^3.

Chap. 4.]" [ 329 3

CHAP. IV,

THE LEYDEN PHIAL.ELECTRICAL BATTERY,
AND OTHER PARTS OF THE APPARATUS.

Theory of the Leyden Phial.— Its Ufe in Ekaridty.—Defcription of
the beft Apparatus of this Kind.— 'The Charge rejides in the Glafs.
-—Curious Experiments -with the Leyden Phial.— Elecirical Battery.
—Injlruftions relative to it. — Experiments <with the eledrical Bat-
tery'.— -Electrical Bells.-r<'Ekftrapborus.-~Eletfrometer,

IN order to underftand properly the nature of what
is called the Leyden phial, or electrical jar, it will
be neceflfary to revert £o what has been faid both in
the introduction and in the preceding chapter on pofi-
tive and negative electricity. If a piece of glafs is
coated with any conducting fubftance, it may be made
to accumulate the electrical matter to a furprifing de-
gree. In this cafe one fide of the glafs, if it does not
exceed a given thicknefs, will be pofitively electrified,
and the other negatively. The form of the glafs is of
no confequence in this experiment j it may be either
flat, cylindrical, or otherwife.

The object of the philofopher being, therefore, on
many occafions, to collect a large quantity of electri-
city, by means of the furfaces, of electrics, and, as flat
plates are neither neceflary nor convenient for this pur-
pofe, he accommodates himfelf with a fufficient num-
ber of prepared jars. Thefe are made of various (hapes
and magnitudesj but the moft ufeful are thin cylindri-
cal glafs veflels, about four inches in diameter, and
fourteen in height, coated within and without (except

about

330 ' Leyden Pbial. [Book IV.

about two inches from the top) with tinfoil, or any
other conducting fubftance.

If one fide of this jar is electrified, while the other
fide communicates with the earth, it is faid to be
charged.

When a communication is formed from one fide of
the jar to the other by a conducting fubftance, .after it
has been charged, an explofion will take place, and
this is called difcharging the jar. .This phial is inca-
pable of being charged when it is infulated j that is,
when neither fide communicates with the earth. When
it is charged, the two fides are in contrary Rates, the
one being pofitively, the other negatively electrified.

A jar is faid to be pofitively electrified when the in-
fide receives the fluid from the conductor, and the
outfide is connected with the earth. It is negatively
electrified when the outfide receives the fluid from the
conductor, and the infide communicates with the
earth ; but it is neceflary that the jar charging nega-
tively fhould be infulated, becaufe the fluid is, in the
firft inftance, conveyed to the coating, and would be
immediately carried to the earth if it was not infu-
lated.

The moft ufual forms of the Leyden phial are re-
prefented in Fig. 3. and 5.^ Plate XXVI i. and its
nature may be exemplified by the following experi-
ment:

Place the phial (Fig. 5.) on an infulated ftand; bring
the coating in contact with the conductor ; turn the
machine (lowly, and after a few turns remove the phial
from the conductor ; then form a communication be-
tween the outfide and the infide of the phial, by plac-
ing one end of the difcharging rod firft upon the coat-
'ing, and then bringing the other end of the rod to the
brafs ball of the bottle ; in this cafe there will be no

explofion,

Chap. 4.] EleRrtcal Spider. 331

explofion, becaufe, both fides being infulated, the bot-
tle was not charged ; but if a chain is fufpended from
the brafs ball of the phial to the table, and the coat-
ing brought in contact with the conductor, after a
few turns of the machine remove the phial as be-
fore ; then if the discharger is applied, an explofion
will be heard, and the bottle will be difcharged ; be-
caufe, in this cafe, the infulation of the infide is de-
ilroyed by the chain, and the phial becomes capable of
receiving a charge.

That the charge of a coated jar refides in the glafs,
and not in the coating, is proved in the following
manner: let a plate of glafs between two metallic
plates, about two inches in diameter^ fmaller than
the plate of glafs j charge the glafs, and then re-
move the upper metallic plate by an infulated handie j
take up the glafs plate, and place it between two other
plates of metal uneteftrified and infulated, and the
plate of glafs thus coated afrefh will ftill be charged.
The following experiments are further ' illuftrative of
the nature of the Leyden phial.

A cork ball, or an artificial fpider made of burnt
cork, with legs of linen thread, fufpended by filk, wiil
play between the knobs of two bottles, one of v/hich.
is charged pofuively, the other negatively, and will in
a little time difcharge them.

A ball fufpended on filk, and placed between two
brafs balls, one proceeding from the outfide, the other
from the infide of a Leyden jar, when the bottle is
charged, will fly from one knob to the other, and by
thus conveying the fire from the infide to the outfide
of the bottle will foon difcharge it.

An infulated cork ball, after having received a fpark,
will not play between, but be equally repelled by two
bottles, which are charged with the fame power.

A wire

332 Ettflrifal Fly. [Book IV,

A wire is fometimes fixed to the under part of the
infulated coated phial (Fig. 5.) and be is another v/irc
fitted to, and at right angles with die former j a brafs
fly (Fig. 4.) is placed on the point of this wire j charge
the bottle, and all the time the bottle is charging the
fly will turn round j when it is charged the needle will
flop. If the top of the bottle is touched with the
finger, or any other conducting fubftarite, the fly will
turn again till the bottle is difcharged. The fly wiil
electrify a pair of balls pofitively while the bottle is
charging, and negatively, when it is difcharging *.

When a Leyden phial pofitively charged is infulat-
ed, it will give a fpark from its knob to an excited
flick of wax, but not a fpark will pafs at that time
between it and an excited glafs tube.

An additional quantity of the fluid may be thrown
on one fide of the jar, if by any contrivance an equal
quantity may be made to efcape from the other, and
not otherwife.

Electricians, in order to increafe the force of the
electric explofion, connect feveral Leyden phials toge-
ther in a box j and this collection they call an electri-
cal battery.

In this apparatus the bottom of the box (Plate
XXVII. Fig. 2.) is covered with tin-foil, to connect
the exterior coatings ; the infide coatings of the jars,
are connected by the wires a, b, c> dy e,f, which meet
in the large ball A ; C is a hook at the bottom of the
box, by which any fubftance may be connected with
the outfide coating of the jars ; a ball, B, proceeds
from the infide, by which the circuit may be conve-
niently completed. Mr. Adams gives the following
precautions to thofe who make ufe of an electrical
battery f .

* Mr. Adams's Eflay on Eleft. p. 131. f Ibid. p. 14-;.

The

Chap. 4.] Elettricai Battery. 33$

The top and uncoated part of the jar mould be
kept dry and free from dud ; and after the explofion
has taken place, a wire from the hook is to be con-
nected to the ball, and left there till the battery is to
be charged again, by which means the inconveniences
arifing from the frequent refiduum of a charge will be
obviated.

Every broken jar in a battery muft be taken away,
before it is poffible to charge the reft.

It has been recommended, not to difcharge a bat-
tery through a good conductor, if the circuit is not at
lead five feet long ; but it mutt be obferved, that in
proportion to the lengthening of the circuit the force
of the mock will be leffened.

Jars made of the green glafs, manufactured at New-
caftle, are laid to endure an explofion without a pro-
bability of breaking.

If the fpark from the explofion is concentrated, by
caufing it to pafs through fmall circuits of non-con-
ducting fubftances, the force of the battery will be
confiderably increafed. For this purpofe, caufe the
fpark to pafs through a hole in a plate of glafs one-
twelfth or one-fixth of an inch diameter, by which
means it will be more compact and powerful. By
wetting the part round the hole, the fpark, by con-
verting this into vapour, may be conveyed to a greater
diftance, with an increafe of rapidity, attended with a'
louder noife than common. Mr. Morgan, by at-
tending to thefe and fome other circumltances, has
melted wires, &c. by the means of fmall bottles.

If the charge of a ftrong battery is pafled through
two- or three inches of fmall wire, the latter will fome-
times appear red hor, firft at the politive fide j and
the rednefs will proceed towards the other end.

If a battery i* difcharged through a frnall fteel nee-
dle,

Experiments with Ekfirical Battety. [Book IVV

die, it will, if the charge is ftrong, communicate mag-
ncdfm to the needle.

If the difcharge of a battery pafies through a fmall
magnetic needle, it will deftroy the polarity of the nee-
dle, and fometimes invert the poles j but it is often
necefiary to repeat this feveral times.

Dr. Priefriey could melt nine inches of fmall iron
wire at the diitance of fifteen feet, but at the diftance
of twenty feet he could only make fix inches of it red
hot, fo that we may infer from this, that notwitMtand-
ing their conducting power, ftill metals refift in fome
degree the .paSage of the electric fluid, and therefore
in eftimating the conducting powers of different fub-
ftances, their length muft not be forgotten.

If a (lender wire is inclofed in a gla(s tube, and a
battery difcharged through this wire, it will be thrown
into globules of different fizes, which may be collected
from the inner ftirface of the tube ; they are often
hollow, and little more than the fcoria or drofs of the
metal.

Dr. Watfbn and fome other gentlemen made feveral
curious experiments to afcertain the diftance to which
the electric {hock might be conveyed, and the velo-
city of its motion, which were briefly mentioned in the
firft chapter. In the firft experiment, the fhock v/as
given, and fpirits fired by the electric matter which had
been conveyed through the river Thames. In another
experiment, the electric fluid was made to pafs through
a circuit of two miles, eroding the New River twice,
going over feveral gravel pits and a large field, and
afterwards conveyed through a circuit of four miles,
This motion was fo inftantaneoufly performed by the
electric fluid, that an obferver, in the middle of a cir-
cuit of two miles, felt himfelf fhocked at the fame in-
ftant that he faw the phial difcharged.

Notwith-

Chap. 4.] Ekftrual Bells. 33$

Notwithftanding this furprifing velocity, it is cer-
tain, hov/cver, that both fides of a charged phial may
be touched fo quickly, even by the beft conductors,
that all the electric fluid has not time to make the
circuit, and the phial will remain but half difcharged ;
and there are feveral inftances where the motion ap-
pears flow, and not eafiiy reconcileable with the amaz'-
ing velocity we have obferved in the inftance above >
indeed it is certain, that this fluid is refilled in fome
degree in its paflage through or over every fubftance. -

There is another part of an electrical apparatus,
originally of German invention, which, before the
concluflon of what may be called the mechanical
part of electricity, it will be proper to notice. It is
chiefly illuftrative of the e lectricd attraction. This
apparatus confifts of three fmail bells, fufpended from
a narrow plate of metal ; the two outermoft by chains,
and that in. the middle, from which a chain pafles to
the floor, by a fiiken firing. Two fmall knobs of
brafs are alfo hung, by fiiken firings, on each fide of
the bell in the middle, which ferve for clappers.
When this apparatus is . connected with an electrified
conductor, the outermoft bells, fufpended by the
chains, will be charged, attract the clappers, and be
ilruck by them. The ' clappers, becoming electrified
likewife, will be repelled by thefe bells, ana attracted
by the middle bell, and will difcharge themfelves upon
it by means of the chain extending to the floor. After
this they Mill be again attracted by the outermoft
bells, and thus, by (Inking the bells alternately, occa-
fion a ringing, which may be continued at pleafure.
Flalhes of light will alfo be feen in the dark between
the bells and the clappers -, and if the electricity is
ftrong, the difcharge will be made without actual con-
tact, and the ringing will ceafc.

If

Aftion of Ekftrophorus explained. [Book IV.

If an Apparatus of this kind is joined to one of thofe
conducting rods, erected to protect buildings from
the effects of lightning, it will ferve to give notice of
the approach and paflfage of an electrical cloud.

It is remarkable, that in certain cafes bodies elec-
trified will retain their electric power for almoft any
length of time, and on this principle a very ingenious
inftrument has been conftructed, called an electro-
phorus. This machine confifts merely of a mafs of
refinous matter, contained in a box for the conveni-
ence of carriage, and a plate of metal fitted to com-
municate with it, which is lifted by a handle of glafs^
or fome non-conducting fubftance. The refinous
mafs being rubbed with a flannel, or even with the
hand, and the plate of metal being applied to it, the
metal will become charged, and give out fparks very
freely to any conducting body ; and this property of
communicating electricity the refinous mafs will retain
for a length of time, without any frefh application
whatever.

To explain thefe phenomena it will be again necef-
fary to recur to what has been faid concerning nega-
tive and pofitive electricity ; it will be necefiary alfo
to recollect, that the negative electricity was originally
termed the reftnous, beeaufe it was firft thought to be
peculiar to thofe fubftances. In the electrophorus,
therefore, the lower plate, or refinous mafs,. being ne-
gatively electrified, the matter is taken from the metal
plate, and this becoming alfo negatively electrified,
the fluid is attracted from any body which is pre-
fented to it.

Several inftruments have been invented for meafur-
ing the quantity of electricity contained in any body.
Thefe generally are formed upon the principle of the
electric attraction, and conflft of a fmall pith ball, or

other

Chap. 4.3 'Electrometer. 337

other light body, fufpended on a moveable arm, with
a kind of femi-dial to mark the degrees. Mr. Adams
recommends Mr. Henley's quadrant electrometer for
this purpofe, which he defcribes as follows : " It con-
fifts (Fig. 3. Plate XXVI.) of a perpendicular ftem
formed at top like a ball, and furnifhed at its lower
end with a brafs ferrule and pin, by which it may be
fixed in one of the holes of the conductor, as at Fig. 4.
or at the top of a Leyden bottle. To the upper part
of the ftem, a graduated ivory femicircle is fixed, about
the middle of which is a brafs arm or cock, to fupport
the axis of the index. The index confifts of a very
flender flick, which reaches from the center of the
graduated arch to the brafs ferrule; 'and to its lower
extremity is faftened a fmall pith ball nicely turned in
the lathe. When this electrometer is in a perpendi-
cular pofition, and not electrified, the index hangs pa-
rallel to the pillar ; but when it is electrified, the in-
dex recedes more or lefs, according to the quantity of
electricity."

VOL. I.

[ 33* ] [Book IV.

CHAP. V.

ELECTRICAL PHENOMENA.

f-.f K.rwna, of Attraction and Repul/ton. — Eleclrical Atmofpkerej— Dif-
ferent Effefls on different SubJlances.-—EU£lrical Cobejton.—'Experi-
'»i Silk Stockings. — On the Evaporation of fluids. — Vegeta-
tion.— Animal Perfpiration. — Inflammation of Spirits. — Animals
killed by Elt&ricitj. — Curious Phenomena in vacua.— Recapitulation

. of Principles.

H E various phenomena of electricity may, for
the fake of perfpicuity, be divided into two
clafles -, the firft of which may be included under the
general head of attraction and repulfion ; and under
the fecond may be ranged all thofe phenomena which
are accompanied with the luminous appearances, and
that effect on the animal frame which is termed the
electrical ihock. Though fome of thefe may appear
at firft to have very little analogy with the former clafs,
y€t we fhall have frequent opportunities of iaferring,
that they are only neceffary effects from one common
caufe, and rather differ in their circumflances than in
their nature. The atmofpherical phenomena will de-
mand a diftinct chapter.

It follows, from what has been already ftated, that
every body electrified, whether by friction, or by com-
munication ; whether by the means of glals, or any
refinous fubfhvnce ; is furrounded by a kind of atmo-
fphere of that fluid which is called the eleclrical mat-
ter.

The

fehap. 5.] Elettrical Fluid. 339

The proof on which this hypothefis refts is, that
light bodies are actually lifted up, and earned to or
from the electrified body, which, the advocates of this
theory alledge, could only be effected by their being
enveloped in fome fluid medium. Thus, when the
hairs on the head of a perfon electrified (land erect, or
the fibres of a foft feather fpread out, as if to meet or
recede from the conductor, according to circumftances,
every particular hair or fibre is fuppofed to be fur-
rounded with an electrical atmofphere.

It is demonstrated by Earl Stanhope, in his inge-
nious treatife on electricity, that the denfity of the elec-
trical atmofphere diminifhes exactly in proportion to
the fquares of its diftance from the center of the elec-
trified body.

Thofe attractions and repulfions, which we have feen
take place when light bodies are brought near to elec-
trified fubftances, are, agreeably to the notion of fome
philofophersj'caufed by two currents of the electric
fluid ; the current which departs from the bodies
brought near to the electrified fubftance caufes thofe
bodies to appear to be attracted, and the current which
departs from the electrified fubftance repels them : as
thefe two effects take place in the fame inftant, it may-
be inferred that thefe two currents are fimultaneous.

A body repelled by an electrified fubftance, will be
attracted by this fubftance as loon as it has touched
any non -electric body.

An electrified, fubftance, if it is left free to move, is
attracted by a non electric body not electrified. Thus
a fmall thin "plate of metal, electrified and fufpendeii
by a thread of filk, is attracted either by a man's hand,
a piece of green wood, or by a metal rod prcfentcd
to it.

Z a All

34° Experiment on Silk Stockings. [Book IV.

All fubftances are not attracted with equal force by
electrified bodies ; in general thofe, which are in their
texture the denfeft and mod compact, are more readily
attracted and repelled, and are fubject to the influence
of electricity at a greater diftance than thofe which are
looter and more porous in their confidence. A rib-
band or thread, when waxed or gummed, becomes
more fubject to this attraction and repulfio'n than it
was in its original ftate. Of all fubftances, gold leaf
appears the moft eafily affected by electrical attrac-
tion.

Electrified bodies adhere fo clofely together, that
the circumftance has given occafion to a new term in
philofophy, and it has been called the eleftrical cohefion.
This fact was pleafantly illuftrated by fome experi-
ments on filk ftockings, communicated to the Royal
Society by Mr. Robert Symner a few years ago. Two
filk ftockings, the one black and the other white, had
been for fome time upon one leg, and were then rub-
bed with the hand, and both pulled off together j it
appears that in this cafe the two ftockings will adhere
together in fuch a manner as to require a confiderable
force to feparate them. M. Briifon, who repeated the
experiment, obferves, that after he had feparated the
white from the black flocking another phenomenon
occurred j for while he held them, one in each hand,
fufpended in the air, they fwelled and puffed up as
wide as if the leg had remained in them j when they
were brought within terror twelve inches of each other,
they rufhed precipitately upon one another, and ad-
hered forcibly together ; but this' adhefion was not fo
great as that which took place while the ftockings were
one within the other. Mr. Symner fuppofed, that the
iuccefs of this experiment depended upon the contrail
between the black and white colour ; but M. Briflbn
i , proves

Chap. 5«] Evaporation increafed ly EleBricity. 341

proves this hypothecs to be without foundation, hav-
ing made the experiment by fubftituting for the black
. ftocking another of a different colour, and even a white
one ; but he confefles, that when the experiment was
made with two white filk {lockings the effects were
weak.

Electrical attraction appears not to be fo ftrong
in vacuo as in the open air. From feveral experi-
ments of Beccaria's we learn, that if the air is tho-
roughly exhaufted out of a glafs receiver, the attrac-
tion and repulfion of electrified" light bodies within
the receiver becomes languid, and .foon ceafes altoge-
ther.

Electricity augments the natural evaporation of fluids,
and efpecially of thofe fluids which are moft fubject to
evaporation of themfelvesj and it has alfo a great ef-»
feet on fluids, when the vefiels containing them are
non-electrics. If a humid body, a fponge for inftance,
is placed upon a conductor pofitively electrified, the
evaporation will proceed much more rapidly, and it
will be much fooner dry, than a fimilar body differently
circumftanced.

Dr. Prieftley alfo gives us reafon to fuppofe, that
plants, when electrified, vegetate earlier and more vi-
goroufly than thofe which have not been fubjected to
this influence.

That electricity increafes the infenfible perfpiration,
of animals, may be inferred from the circumftance that
electrified animals are always lighter than thofe which
are not.

The ftream of electrical fluid has no fenfible heat,
but even appears cold to the touch ; yet we have feen
that the more inflammable bodies, and particularly
fpirit of wine, may be ignited by it. This experiment
piay be eafily made by a fpark from a common ma-
Z chine.

34- Animals killed fry Eleflricity. [Book IV,

chine. Let a perfon infulated, and communicating
with a charged conductor, hold in his hand a quantity
of rectified fpirit in a metal fpoon. If the fpirit has
been a Iktle warmed over a candle previoufly, the ex-
periment will be more certain to fucceed. Let ano-
ther perfon then, not infulated, but communicating
with the floor, prcfent his finger to the fpirit, a fpark
wi'l immediately pafs between the fpoon and the fin-
ger, and the fpirit will be inflamed. This phenomena
might pafs for an exertion of magic in an ignorant
country, or an ignorant age.

By a fmart mock- of electricity from a charged phial,
pr a battery, fmall animals may be killed i but I have
not underftood that human art has yet been able to
conftruct a battery large enough to kill an animal
above the fize of a fheep or a dog. The immediate
or proximate caufe of the death of animals by electri-
city, or by lightning, which is natural electricity, has
not yet been afcertained. Ic was once fuppofed that
the living principle was extinguifhed by the burfting
of fome blood vcflel, from the violence of the fhock ;
but a dog, which was killed by lightning, was carefully
difiected, and none of the velTels found in the lead in-
jured. Beccaria recovered fome perfons apparently
ftruck dead by lightning; and when queftioned with
refpect to the pain or fuffering which they endured,
they only complained of an unufual numbnefs or weari-
nefs in their limbs. The flefh of animals killed by
electricity is rendered extremely tender, and is recom-
mended by Dr. Franklin as an article of luxury. It
will alfo putrify in a much fhorter time than the flefh
of thofe which are killed in any ordinary way.

The luminous effects of electricity are not precifely
the fime in vacuo as in the open air; and indeed a

very

Chap. 5.] Recapitulation of Principles. 343

very curious phenomenon has been produced by in-
jecting the electric light in a vacuum.

If a wire with a round end is included in an exhauft-
cd receiver, and prefented to a conductor of an elec-
trical machine, every fpark will pafs through the va-
cuum in a broad ftream of light, vifible the whole
length of the receiver, moving with regularity (unlefs
it is turned back by fome non-electric) and then di-
viding itfelf into a number of beautiful rivulets, which
are continually feparating and uniting in a pleafing
mariner. When the veflel is grafped by the hand, a
pulfation is perceived like that of an artery, and the fire
inclines towards the hand. A fmall quantity of air is,
however, neceflary to occafion the greateft luminous
effect.

The following is a recapitulation of what M. Brif-
fon confiders as fundamental principles, confirmed, he
fays, by his own experiments/ feconded by thofe of
other philofophers *.

The electric fluid is the fame in efience with that of
light and heat, but combined with a fubftance which
affects the organs of fcent.

When bodies are electrified by glafs, they furnifh
tufts or pencils of light ; but if electrified by fulphur.or
refmous fubflances, they only produce points or fparks
of light ; bodies prefented to thofe electrified by glafs
produce only luminous points, while thofe which are
prefented to bodies which are electrified by fulphur,
produce beautiful pencils or tufts of light.

To electrify bodies by communication, it is necef-
fary to infulate them j the fubftances the mod proper
for this purpofe are thofe which electrify the beft by
friction.

Traitc Element, de Phylique, Tom. iii. p. 435
Z 4

3 44 Recapitulation of Doctrines and Fafts. [ Book I V,

Glafs, though it electrifies very well by friction,
electrifies alfo by communication, even without any
preliminary preparation ; yet it is very proper to infu-
late.

Electrical phenomena are not produced entirely from
the bodies upon which the electrifying machine acts ;
the adjacent bodies or fubftances contribute towards
their production.

The energy of the electric virtue is augmented, ii»
conductors, more by an increafe of furface, than by an
augmentation of the mafs.

Electrified bodies adhere one to another, fo that they
cannot be feparated without a confiderable effort, as
was, exemplified in the cafe of two filk ftockings of va-
rious colours.

Electricity accej/srates the evaporation of liquors, and
the perfpiration of animals.

The pencils or tufts of light, whicfi are feen at the
.extremities or angles of electrified bodies, are always
compofed of divergent rays when they pafs in the air;
but if a non-eleffrif or conducting body is prefented to
them, they lofe a great deal of their divergency j their
rays fometimes become even convergent, in order that
they may approach towards that body which is more
permeable than the air ; and if they are made to pafs into
a vacuum, they will afliime the form of a large branch
of light nearly cylindrical, or in the form of a fpindle.

The fpark which fhines between two bodies is ca-
pable of fetting combuftible matters on fire.

Chap, 6.] [ 345 3

CHAP. VI.

OF THUNDER AND LIGHTNING, METEORS,
WATER-SPOUTS, &c.

fheory of Lighning.—~T>efcription of a '•Thunder Storm. — Obfcr--vat'ioxs
relative to the Eleflricity of the Atmofphere.— •'Melting of Metals by
the Cold Fufeon a vulgar Error.— Conductors of Lightning.— -How
to be fafe in a thunder Storm.— -Application of Eleftricity to other
atmofpherical Phenomena.— Rain. — //«//.-— S»0<;v.— Meteors.— *Wa-
ter-fpouts,

IT no longer remains .a doubt among philofophers,
that the caufe which produces the effects of thun-
der is the fame with that which produce's the ordinary
phenomena of electricity j the refemblance between
them is indeed ib great, that we cannot believe thun-
der itfelf to be any other than a grander fpecies of
electricity, naturally excited without the feeble efforts
of human art. This fluid, probably, is diffufed through
the whole atmofphere at all times, either in a fmallcr
or greater degree, and is occafionally perceptible to our
fenfes, according to the concurrence of natural circum-
Itances.

The cloud which produces the thunder and light-
ning may be confidered as a great electrified body ;
but how has the cloud acquired its electric virtue ? is
the reasonable demand of an inquifitive mind : and
to fatisfy this inquiry it will be neceffary to refer to
what has been before obferved, that this power is pro-
duced in two modes, by friction, and by commu-
pication. Bodies electrified by friction communicate

their

346 How Clouds hcwne eleStrifed. [Book, IV.

their virtue to other bodies which are fufceptible of it,
being infulated, and at a convenient diftance. As air,
therefore, is an idio-electric body, it is not unphilofo-
phical to fuppofe, that in ilormy weather efpecialiy,
when it is common to obferve the clouds and the wind
take contrary courfes, a part of the atmofphere3 rufh-
ing by the other, may caufe the air to be electrified by
the friction of its own particles, or by rubbing againtl
terreflriai objects -which it meets in its parTage, or
perhaps againll the clouds themfelves. It is pro-
bable alfo, that the inflammable fubftances, which
arife and accumulate in the cloudy regions, contribute
to increafe the effects, not only of themfelves, but,
perhaps, ftill more, by the electric matter which they
carry along with them. Another circumftance, which
further inclines me to make this inference is, that
thunder florms are more frequent and tremendous
in thofe times and places, when and where we have
realbn to conclude that thefe exhalations are in the
greateft abundance in the atmofphere, as in warm fea-
fons and climates, as well as in thofe places where
the earth is filled wirji fubftances capable of furnifhing
a large quantity of thefe exhalations, and in particular
in the neighbourhood of volcanoes.

A cloud in a thunder florin may be confidered as a
great conductor, actually infulated and electrified ; and
it may be fuppofed to have the fame effect upon thofe
non- electrics which it meets with in its courfe, as our
common conductors have upon thofe which are pre--
fented to them. If a cloud of this kind meets with
another which is not electrified, or lefs ib than itfelf,
the electric matter flies off from all parts towards this
cloud ;' hence proceed flafhes of lightning, and the for-
midable report of thunder.

f Thunder

Chap. 6.] "Dffcrfytion of a Thunder Storm. 347

* Thunder ftorms/ fays Beccaria, ' generally hap-
pen when there is little or no wind, and their firft ap-
pearance is marked by one denfe cloud, or more, in-
creafing very faft in fize, and riling into the higher
regions of the air; the lower furface black, and nearly
level, but the upper finely arched, and well defined.
Many of thefe clouds feem frequently piled one upon
another, all arched in the fame manner j but they keep
continually uniting, fwelling, and extending their
arches.

' At the time of the rifing of this cloud, the at-
mofphere is generally full of a great number of fepa-
rate clouds, motionlefs, and of odd and whimfical
fhapes. All thefe, upon the appearance of the thun-
der cloud, draw towards it, and become more uniform
in their fhapes as they approach, till coming very near
the thunder cloud, their limbs mutually ftretch towards
ene another; they immediately coalefce, and together
make one uniform mafs. But fometimes the thunder
cloud will fwell, and increafe very faft, without the con-
junction of any of thefe adfcititious clouds, the vapours
of the atmofphere forming themfelves into clouds
wherever it pafles. Some of the adfcititious clouds ap-
pear like white fringes at the fkirts of the thunder
cloud, but thefe keep continually growing darker and
darker as they approach or unite with it.

f When the thunder cloud is grown to a great fize,
its lower furface is often ragged, particular parts being
detached towards the earth, but ftill connected with the
reft. Sometimes the lower furface fwells into various
large protuberances, bending uniformly towards the
earth. When the eye is under the thunder cloud,
after it is grown larger, and well formed, it is feen to
fink lower, and to darken prodigioufly, at the fame
time that a number of adfcititious clouds (the origin of

which

348 Phenomena of Thunder [Book IV.

which can never he perceived) are feen in a rapid mo-
tion, driving about in very uncertain directions under
it. While thefe clouds are agitated with the moft ra-
pid" motions, the rain generally falls in the greateft
plenty, and if the agitation is exceedingly great, it
commonly hails.

' While the thunder cloud is fwelling^ and extend-
ing its branches over a large tract of country, the
Jightning is feen to dart from one part of it to another,
and often to illuminate its whole mafs. When the
cloud has acquired a fufficient extent, the lightning
flrikes, between the cloud and the earth, in two oppo-
fite places, the path of the lightning lying through the
whole body of the cloud and its branches. The longer
this lightning continues, the rarer the cloud grows, and
the lefs dark is its appearance, till at length it breaks
in different places, and difplays a clear fky.'

It is the opinion of the fame author, that the clouds
ferve as conductors to convey the electric fluid from
thofe places of the earth which are overloaded with it,
to thofe which are exhaufted of it.

To prove that the earth is often positively charged
with refpect to the clouds, in one part while it is nega-
tive in another, he adverts to the fall of great quantities
of fand, and other light fubftances, which are often
carried into the air, and fcattered uniformly over a
large tract of country, when there was no wind to
effect this phenomenon, and even when there was,
they have been carried againft the wind ; he therefore
fuppofes, that thefe light bodies are raifed by a large
quantity of electrical matter hTuing out of the earth.

This comparatively rare phenomenon, he thinks,
exhibits both a perfect image and demonftratlon of
the manner in which the vapours of the atmofphere
are raifed to form thunder clouds. The lame electric

rnatter4

Chap. 6.3 Clouds explained.

matter, wherever it iflues, attracts to it, and carries
into the higher regions of the air, the watery particles
difperfed in the atmofphere. The electric matter
afcends, being folicited by the lefs refiftance it finds
there than in the common mafs of the earth, which at
thofe times is generally very dry, and confequently
highly electric. The uniformity with which thunder
clouds fpread themfelves, and fwell into arches, muft
be owing to their being affected by fome caufe, which,
like the electric matter, diffufes itfelf uniformly where-
ever it acts, and to the refiftance they meet with in
afcending through die air.

The fame caufe, which firft railed a cloud from va-
pours difperfed in the atmofphere, draws to it thofe al-
ready formed, and continues to form new ones, till the
whole collected mafs extends fo far as to reach a part
of the earth where there is a deficiency of electric fluid ;
thither alio they will be attracted, and thus the mafs
ferves as a conductor. When the clouds are attracted
in their paffage by thofe parts of the earth, where there
is a deficiency of the fluid, thofe detached fragments
are formed, and alfo thofe uniform depending protu-
berances, which are probably the caufe of water-
fpouts.

fA wind always blows from the place whence a
thunder cloud proceeds, and the wind is more or lef*
violent in proportion to the fudden appearance of the
thunder cloud, the rapidity of its expanfion, and the
velocity with which the adicititious clouds join it. The
fudden condenfation of fuch a prodigious quantity of
vapour muft difplace the air, and agitate it on all
iides.

In three dates of the air, fays' the author above
quoted, I could find no electricity in it. nt. In windy
weather. . 2d. When the fky was covered with diftinct

and

Melting of Metals by Cold Ftifion. [Book IV;

and black clouds, which had a flow motion. 3d. In
molft weather not actually raining.

In rainy weather, without lightning, his apparatus
was always electrified a little time before the rain fell,
and during the time of rain, but ceafed a little before
the rain was over.

The higher his rods reached, or his kites flew, the
fcronger figns they gave of being electrified.

It has been intimated that the clouds are fometimes
pofitively, and fometimes negatively electrified. In the
latter cafe the lightning is fuppofed, upon the Ffankli-
nian theory, to proceed from the earth to the cloud.
The general effects of lightning are precifely the fame
with thofe of the electric fhock, only greatly magnified.
It may not be improper in this place to notice an old
error, namely, the melting of metals by what has been
called the coldfufion. The error is found to reft upon
certain ill attefted relations of fwords being melted in
the fcabbard by lightning, and money in the bag,
without injuring the fcabbard or the bag. A variety
of experiments have been accurately made to determine
the fact; the refulrs of which have been, that the thin
edge of the fword, or of the money, might have been
inftantaneoufly melted, and yet fo inftantaneoufly cool-
ed, as neither to affect the fcabbard nor the bag. A
very fmall wire will inftantly melt and inftandy cool in
the flame of a common candle.

Mr. Kinnefley inclofed a fmall wire in a goofe quill
filled with loofe grains of gunpowder, which took fire
as readily as if they had been touched by a red hot
poker ; tinder was kindled when tied to a piece of the
fame wire ; but no fuch effects could be produced with
a wire twice as large. Hence it appears, as Mr. Kin-
nefley remarks, that though the electrical matter has
no fenfible heat when in a ftate of reft, it will, in pafling

through

Chap. 6.] Conducing Rod*. 35*

through bodies, produce heat iti them, provided they
are proportionally fmall. Thus, in pafllng through
the fmall wire, the particles are confined to a narrower
paflage, and, crowding clofe together, act with a more
condenied force, and produce fenfible heat *»

The difcovery of Dr. Franklin, which eftablifhed
the identity of lightning with the electrical fluid, fug-
gefted an invention, for which we are indebted to the
fame philofopher, for fecuring buildings from this moft
formidable enemy. The reader will perceive that I
allude to that of metallic conductors.

Suppofe Fig. 4. Plate XXVI. to reprefentthe gable
end of a houfe, fixed vertically on the horizontal board
F G ; a fquare hole is made in the gable end at b i9 into
which a piece of wood is fixed ; a wire is inferted in
the diagonal of this little piece j two wires are alfo
fitted to the gable end ; the lower end of one wire
terminating at the upper corner of the fquare hole,
the top of the other wire is fixed to its lower corner;
the brafs ball on the wire may be taken off, in order
that the pointed end may be occafionally expofed to
receive the explofion.

Experiment. — Place a jar with its knob in contact
with the conductor, connect the bottom of the jar
with the hook H, then charge the jar, and bring the
ball under the conductor, and the jar will be dif-
charged by an explofion from the conductor to the
ball of the houfe. The wires and chain being all in
connection, the fire will be conveyed to the outfide of
the jar without affecting the houfe ; but if the fquare
piece of wood Is pLiced (b that the wires are not con-
nected, but the communication cut off, the electric
fluid, in pafllng to the outfide of the bottle, will throw

* Pneftley^ Hilt, of Elecl. p. 394.

out

3 5 2 Conductors fo preferring Buildings. [Book IV.

out the little piece of wood to a confiderable diftance,
by the natural force of the explofion.

Unfcrew the ball, and let the point which is under-
neath be prefented to the conductor, and then you will
not be able to charge the jarj for the fharp point
draws the fire filendy from the conductor, and conveys
it to the coating on the outfide of the jar.

The prime conductor in this experiment is fup-
pofed to reprefent a thunder cloud difcharging its con-
tents on a weather-cock, or any other metal, at the
top of a building; and it may be inferred from this
experiment, that if there is a connection of metal to
conduct the electric fluid down to the earth, the
building will receive no damage j but where the con-
nection is imperfect, it will ftrike from one part to
another, and thus endanger the whole building.

Elevated conductors, applied to -buildings to fecure
them from lightning, will in this, manner difcharge the
electricity from a cloud that pafles over them, and a
greater quantity of the difcharge will pafs through a
pointed conductor, than through one which termi-
nates with a ball ; but whether the difcharge will be
made by a gradual current, or by explofion, will de-
pend upon the fuddennefs of the difcharge, on the
nearnefs and motion of the cloud, and the quantity of
the electricity contained in it. If a fmall cloud hangs
fufpended under a large one loaded with electric mat-
ter, pointed conductors on a building underneath will
receive the difcharge by explofion, in preference to
thofe terminated by balls, the fmall cloud forming an
interruption, which allows only an inftant of time for
the difcharge *.

Vifcount Mahon (now Earl Stanhope) has com-

* Mr. Adams's Effay on Eleft. p. 1 86.

municated

Chap. 6.] tfbtjafeft Situation in a thunder Storm. 353

municated to the public, in a treatife on this fubject *,
fome effentials to be obferved in the erection of con-
ductors to buildings : he advifes, that the uyper fif-
teen or twenty inches of the -rod fhould be compofed
of copper> and not of iron, as the latter, being expofed
to the weather, will ruft, and ruft does not conduct
electricity j that the iron part of the rod fhould Jbe
painted, but not the upper part of it, becaufe paint is
no conductor He further advifes, that the upper ex-
tremity of a conducting rod Ihould not only be accu-
rately pointed and finely tapered, but that it .fhould
be extremely prominent, about ten or fifteen feet above
all the parts of the building which are the neareft it.
It may be added, that a conductor Ihould always be
carried in the earth fome feet beyond the foundation
of the building, and Ihould, if poffible, terminate in
waten

The fafeft fituation during a thunder ftorm is the
Cellar j for when a perfon is below the furface of the
earth, the lightning muft ftrike it before it can reach
him, and will of courfe, in all probability, be expend-
ed on it. Dr. Franklin advifes perfons apprehenfive
of lightning to fit in the middle of a. room, not un-
der a metal luftre, or any other conductor, and to lay
their feet up upon another chair. It will be ftill fafer,
he adds, to lay two or three beds or mattrefles, in the
middle of the room, and folding them double> to place
the chairs upon them. A hammock fufpended by
filk cords would be an improvement upon this appa-
ratus. Perfons in fields fhould prefer the open parts
to the vicinity of trees, &c. The diftance of a thun-
der ftorm, and confequently the danger, is not difficult
to be eflimated. As light travels at the rate of

* Principles of Eleftricity, p. 205.

VOL. I. A a 72,420

3 54 Cautions againjl tie Fear of Lightning. [Book IV.

75,420 leagues in a fecond of time, its effects may be
confidered as inftantaneous within any moderate dif-
tance. Sound, on the contrary, is tranfmitted only at
the rate of 1,142 feet, or about 380 yards in a fecond.
By accurately obferving therefore the time which in-
tervenes between the flafh and the noife of thunder
which follows it, a very near calculation may be made
of its diftance, and I know no better means of remov-
ing unneceffary apprehenfions.

The fuccefs of Dr. Franklin, in afcertaining the
caufe of thunder and lightning, induced fucceeding
philofophers to apply the fame theory to the expla-
nation of the other atmofpherical phenomena. From
a number of obfervations, the indefatigable Beccaria
endeavours to account for the rifing of vapours and
the fall of rain, upon electrical principles ; and, ic
mud be confefied, that if it is not a primary agent in
thefe effects, it would be rafhnefs entirely to deny its
influence. This philofopher fuppofes, that previous
to rain a quantity of electric matter efcapes from the
earth, and in its afcent to the higher regions of the
air collects and conducts into its path a great quan-
tity of vapours. The fame caufe that collects will
condenfe them more and more, till in the places of the
neareft intervals they come almoft into contact, fo as
to form fmall drops, which, uniting with others as
they fall, come down in rain. The rain he fuppofes
to fall heavier in proportion as the electricity is more
vigorous.

Hail, he fuppofes to be formed in the higher regions
of air, where the cold is intenie, and where the elec-
tric matter is very copious. In thefe ' circumflances,
a great number of particles of water are brought near
together, where they are frozen, and in their defcent
collect other particles j fo that the denfity of the fub-
ftancc of the hail-ftone grows lefs and lefs from the
9 center,

Chap. 6.] Aurora Bortalls. 35$

center, this being formed firft in the higher Re-
gions, and the furface being collected in the lower.
Agreeably to this, it is obferved, that on mountains,
hail-ftones as well as drops of rain are very fmall,
there being but a fmall fpate through which they
can fall.

Clouds of fnbw differ in nothing from clouds of
tain, but iti the circumftance of the cold which freezes
trrem. Both the regular diffufion of fnow, and the
regularity of the parts of which it confifts, fiiew the
clouds of fnow to be actuated by fome uniform caufe
like electricity *.

Confident with this theory is the fact, that vapours
never rife to a great height without producing me-
teors. Almoft all volcanic eruptions are accompa-
nied with lightning. The column of vapour, which
proceeds from the bowels of a volcano, is continually
traverfed by lightning, which fometimes feems to pro-
ceed from the higher regions, fometimes from the co-
lumn itfelf. Thefe lightnings were obferved by the
younger Pliny, in the eruption which killed his uncle ;
and Sir William Hamilton has obferved them feveral
times. The aurora borealis is alfo generally fuppofed
to be electrical -, its light feems to be produced by the
electric fluid, while it is condenfed in palling in the
columns of elevated vapour f.

It

* Prieftiey's Hift. Elert. vol. 1.

•}• Mr. Adams's defcription of this meteor, in his Lectures, Is
as follows : ' The appearances of the aurora come under four dif-
ferent descriptions. ift, A horizontal light, like the morning
aurora, or break of day. 2dly, Fine, {lender, luminous beams,
Well denned, and of denfe light. Thefe often continue a quarter,
an half, or a whole minute, apparently at reft, but oftener with a
quick lateral motion. 3dly, Flaihes pointing upward, or in the
fame direction as the- beams, which they always fucceed. Thefe
are only momentary, and have no lateral motion ; but they are
A a 2 generally

356 -Water-fronts. [Book IV.

It was inti mated that water- fpouts were among the
phenomena, which fome philosophers have attempted
to explain on electrical principles. A water-fpout is a

generally repeated many times in a minute. They appear much
broader, more diffofe, and of a weaker light than the beams : they
grow gradually fainter till they difappear ; and fometrmes con-
tinue for hours, flafliing at intervals. 4thly, Arches, nearly in the
form of a rainbow : thefe, whu>n complete, go quite acrofs the
heavens, from one point of the horizon to the oppofhe point.

• When an aurora happens, thefe appearances fcem to fucceed
each other in the following order : i. the faint rainbow-lik.fi
arches : 2. the beams ; and, 3. the fiafhes. As for the northern
horizontal light, it appears to confilt of an abundance of fiafhes, or
beams, blended together by the fuuation of the obferver.

' The beams of the aurora borea'is appear at all places to be
arches of great circles of the fphere, with the eye in the center ;
and thefe arches, if prolonged upwards, would all meet in one
point.

' The rainbow-like arches all crofs the magnetic meridian at
right-angles. When two or more appear at once, they are con-
••entric, and trnd to the call and weft ; alfo the broad arch cf the
• horizontal light tends to the magnetic eaft and wed, and is bi-
.fecled by the magnetic meridian ; and when the aurora extends
«ver any part of the hemifphere, whether great or final!, the line
fcparating the illuminr.ted part of the herqifphere from the clear
part, is half the circumference, of a great circle crofting the mag-
netic meridian at right-angles, and terminating in the eaft ar.d
weft : moreover, the beams, perpendicular to the hori/.on, are only
. thofe on the magnetic meridian.

' That point in the heavens to which the beams of the aurora
appear to converge, at any place, is the fame as that to which the
fouth pole of the dipping needle points at that place.

' The beams appear to rife above each other in fucceffion ; fo
..that of any two beams, that Which has the higher bafe. has alfo the
higher fummir.

• Every beam appears broadcft at or near the bafe, and to grow
narrower as i» afcends j fa that the continuation of the bounding
lines would meet in the common center to which the beam tends.

' The height of the rainbow-like arches of the aurora are ef!i-
mated by Mr. Dahon to be above the earth's furfacc about 150
milis.' ^(iarm's Le3ttr,«s, vol. iv. p. 542.

Chap. 6".] Ajcending and defending Water-Jpouts. 357

moft formidable phenomenon, and is indeed capable
of canfing great ravages. It commonly begins by a
cloud, which appears very fmall, and which mariners
call the fquall, which augments in a little time into an
enormous cloud of a cylindrical form, pr that of a
reverfed cone, and produces a noifc like an agitated
fea, fometimes emitting thunder and lightning, and
alfo large quantities of rain or hail, furBcient to in-
undate large veffels, ovcrfet trees and houfes, and
every thing which oppofes its violent impetuofity.

Thefe water-fpouts are more frequent at fea than
by land, and failors are fo convinced of their dan-
gerous confequences, that when they perceive their
approach, they frequently endeavour to break them by
firing a cannon before they approach too near the
fhip. They have alfo been known to have committed
great devaluations by land : though, where there is no
water near, they generally aflame the harmlefs form
of a whirlwind.

In accounting for thefe phenomena upon electrical
principles, it is obferved, that the effluent matter pro-
ceeds from a body actually electrified towards one
which is not fo ; and the affluent matter proceeds from
a body not electrified towards one which is actually
fo. Thefe two currents occafion two motions anala-
gous to the electrical attraction and repulfion. If the
current of the effluent matter is more powerful than
the affluent matter, which in this cafe is compofed of
particles exhaled from the earth, the particles of va-
pours, which compofe the cloud, are attracted by this
effluent matter, and form the cylindrical column, called
the defcending water Jpcut ; if, on the contrary, the
affluent matter is the ftrongeft, it attracts a fufficient
quantity of aqueous particles to form gradually into
A a 3 a cloud,

35* Dffcription ,of [Book IV.

a cloud, and this is commonly termed the afcending
•water-Jpout.

A different explanation of thcfe phenomena hat
been, however, given by other philofophers, and to this
it will be proper to advert in the fucceeding book>
which treats of the nature and properties of air *.

* Mr. Nicholfon, who has given both theories, has the follow-
ing obfervations, which greatly ftrengthen the hypothecs which.
afcribes thefe phenomena to electricity :-— * It was obferved of
water-fpouts, that the convergence of winds, and their confequeut
whirling motion, was a principal caufe in producing that effeft;
but there are appearances, which caji hardly be foJved by fuppof-
ing that to be the only caufe. They often vanifh, and prefently
appear again in the fame place ; whitifh ox yellowifh flames have
fometimes been feen moving with prodigious fwiftnefs about them*
and whirlwinds are obferved to eleftrify the apparatus very ftrongly.
The time of their appearance is generally thofe months which are
peculiarly fubjeft to thunder-ftorms, and they are commonly pre-
ceded, accompanied, or followed by lightning, the previous itate of
the air being alike in both cafes. And the long eftablifhed cuf-
tom, which the failors have, of prefenting (harp fwords to difperfe
them, is no inconfiderablecircumftance in favour of the fuppofition
of their being electrical phenomena. Perhaps the afcending mo-
tion of the air, by which the whirling is produced, may be the
current known to iflue from ele&rified points, as the form of the
protuberance in the fea is fomewhat pointed; and the elcftrified
drop of water may afford confiderable light in explaining this ap-
'i vol. ii. p. 361.

Chap.;.] [ 359

CHAP. VU.
MEDICAL ELECTRICITY.

Declaration of the Abbe Nollet on this Subjett. —Mr. Adams an Ad-
'vacate for Medical Electricity. — M-bde of Application, — Difiafes
to 'which it may be applied.— Apparatut mojl proper for Medical
Purpofes.

THE declaration of Abbe Nollet, that he received
more pleafure from difcovering that the motion
of fluids in capillary tubes, and the infenfible perfpira-
tion of animated bodies, were augmented by electricity,
than from any other difcovery he had made, reflects
the higheft honour upon his character as a friend of
mankind.

Mr. Adams, who was not inferior in humanity and
philanthropy to the French philofopher, ftrongly con-
tends for the medicinal effects of electricity, and brings
to aid his arguments the acknowledged property in
the electric fluid, to accelerate the vegetation of plants.
We may indeed be convinced, by a variety of experi-
ments, that the electric fluid is materially connected
with the human frame, and is continually exerting its
influence upon it. As the natural equilibrium of this
fluid is eafily deftroyed in the human body, we may
fafely infer, that any alteration in the quantity or in-
tenfity of the action of this powerful fluid, will produce
correfponding changes In the habit or health of the
body. The following experiment proves the effect of
this fluid upon organized bodies.

A a 4 Let

360 Medical [Book IV.

Let the charge of a large jar or battery pafs from
the head to the back of a moufej if the mock is fuf-
ficiently ftrong, it will kill the animal. If the dif-
charge is made in the fame manner after its death, the
fluid will pafs vifibly over the body, and not through
it; from which circumftance we may infer, that the
power or medium which tranfmitted the fhock through
the animal is loft with its life.

The late Dr. Cullen was of opinion, that electricity,
when properly applied, is one of the mod powerful
flimulants that can be employed to act upon the aer-
vous fyftem of animals.

Mr. Adams, therefore, infers, from various experi-
ments, that electricity is applicable to palfies, rheuma-
tifms, intermittents j to fpafm, obftruction, and inflam-
mation. In forgery alfo, he adds, it has considerable
effect. The gout, the fcrophuk) or king's evil, are
ranked among thofe difeafes to which this remedy is
applicable j and there is reafon to fuppofe, that in the
beginning of thefe difeafes its application has been oc-
cafionally fuccefsful.

Modern electricians have contrived various modes
for applying the electric fluid to the remedy of difeafes.

The ftream of this fluid may, without a fhock, be
made to pafs through any part of the body j it may
alfo be thrown, upon, or extracted from, any part j and
its action in each cafe may be varied, by caufing the
fluid to pafs through materials which refift its palfage
in different degrees j it may be applied to the naked
integuments, or to the (kin covered with different re-
fifling fubftancesj and its power may be rarified or
condenfed, confined to one fpot, or more diffufed, as
the difcretion of the operator may direct him.

The apparatus the mod proper for thefe medicinal
operations is, an electrical machine with an infulated

cufhion,

Chap. 7.] Eleftricitj. 361

cufhion, properly conftructed to afford a continued and
ftrong ftream of the electrical fluid.

The total want of experimental knowledge upon this
fubject, difables me from deciding upon the efficacy
of this remedy. Electricity is certainly a powerful
agent in nature, but its effects are tranfient, and the
cafe with which the fluid is tranfmitted through the hu-
man body will probably operate againft its producing
a permanent effect. Thus far, however, may with
truth be advanced, that it is a fafe and, eafy remedy,
and therefore mould never be omitted where there is a
chance of doing good. Medical men are, however,
the only proper judges when it ought to be applied ;
^nd it ihould be a maxim, that the fafeft and mofl in-
noxious medicines may have the moft fatal confe-
(juences in unikilful hands.

[BookV.

BOOK V.

OP AIR.
CHAP. I.

HISTORY OF DISCOVERIES RELATIVE TO
AIR.

Vague Notions ?f the early Chemifis.—Van Hetment.—Choak and Fire
Damp.— Mr. £oy!e.—Di/coveries of Dr. Hales.— Of Dr. Black. —
Of Dr. PrieJtley.—Of Mr. CavendiJh.—Of Lavoi/ter. — Vital or
dtphlogijiicated Air dif covered by Dr. Priejlley.— Composition of
Water and ofNitrout Air discovered by Mr. Cavend'ijh.

THOSE aerial fluids, which in their nature and
effects are different from the air of our atmo-
fphere, did not efcape the notice of the early chemifts;
but they paid little attention to the nature of them,
contenting themfelves with giving them a name which
meant nothing, denominating them, in general, Jpiritus
Jyheftris.

Van Helmont diftinguilhed them by the name of
gas, which he defined to be a fpirit or incoercible va-
pour, as the word gas, or rather ghoaft, in the Dutch
language, fignifies. He fuppofes the gas to have been
retained by the fubftances from which it is extracted,
in a fixed or concrete form. He afitrts, that fixty-
two pounds of charcoal contain fixty-one of gas, and
only one of earth, and attributes the fatal effects which

workmen

Chap, i.] Cboak and Fire Damp* 363

workmen experience occafionally in mines to the eman-
cipation of this fpirit. On the fame principle he ac-
counts for the eructations from the ftomach and
bowels, and for the floating of drowned bodies j and
he concludes by determining, that this gas is a fluid of
a nature quite different from that of our common air.

The exigence of two different kinds of vapour, or
elaftic fluids, had been previoufly obferved in mines
and coal-works : the one was obferved to affect ani-
mals with a fenfe of fuffocation, and to extinguifh life,
and it therefore obtained the name of the cboak-dampi
the other, from the dangerous property of catching fire
when a candle or any ignited body was brought in
contact with it, was termed the fire-damp.

A fpecimen of the tire-damp, or inflammable air,
was collected from a coal-mine of Sir James Lowther,
in Cumberland, and brought up in bladders to be ex-
hibited to the Royal Society at London, in the year
1733; and in the year 1736 Mr. John Maud pro-
cured, from the folution of iron in oil of vitriol, a
quantity of the very fame fpecies of inflammable air,
and demonftrated that the fame might be procured
from moil of the metals in certain circumftances.

The experiments of Van Helmont were greatly im-
proved upon by the fagacious Boyle. He changed
the name of gas to that of artificial air; he demon-
ftrated, that this artificial air was not always the fame;
for inftance, that the air produced by fermentation is
efientially different from that which is formed from
the explofion of gunpowder. He was, I believe, the
firft who perceived that the volume of air was di-
minifhed by the eombuftion of certain fubftances.

This laft obfervation of Mr. Boyle feems particu-
larly to have attrafted the attention of the indefatigable
Dr. Hales, and he invented inftruments for determin-
ing

364 Dr. Hales. [Book V.

ing the quantities both of the air, which was on fomc
occafions produced, and on other occafions abforbed,
by different fubftances. Thefe experiments deferve
the attention of every philofopher, and for accuracy or
ingenuity have never been exceeded *.

Among other circumftances, which were particu-
larly remarked by Dr. Hales, was the great quantity of
air contained in the acidulated mineral waters, and to
this air he fufpected they were indebted for their fpark-
]ing and brightnefs, and fome other of their peculiar
qualities. In obferving the abforption of air by bodies
in combuftion, he faw that this abforption had its li-
mits : he remarked alfo, in fome cafes, the alternate
production and abforption of air, as for inftance in re-
fpect to the air which he produced from the burning
of nitre, which air, he obferved, was very foon di-
minifhed in bulk, though he did not perceive that
the abforption was owing to the water, which he
always ufed in his experiments. The production
of an air capable of inflammation from the diftilla-
tion of certain fubftances did not efcape his obferva-
tionj and he has advanced, that the augmentation
of weight in the metallic calces was in fome de-
gree owing to the air which they imbibed. That the
phofphorus of Homberg diminifhes the air in which
it is burned i that nitre cannot explode in vacuo ; and
that air is in general neceflary to the cryftallization of
falts, are among the fads which are noticed by this
philofopher.

From the uncertainty, however, of Dr. Hales and
his predeceftbrs, with rrgard to fcveral material cir-
cumftances, of which they appear to have had fome
cafual glimpfes, and from their total ignorance of others*

* See Hales's Vegetable Statics, pajjim.

the

Chap, i.] Dr. Black, Mr. Cavendi/b, fcfr.

the doctrine of the aerial fluids was but in a ftate of
infancy, till the decifive experiments of Dr. Black,
Mr. Cavendifh, and Dr. Prieftley, furnifhed us with a
new fyftem in this important department of natural
hiftory.

The firfl of rhefe philofophers obferved, that lime
and magnefia, in their mild ftate, confift of an union
of a certain aerial fluid with the earthy bafe j that this
aerial matter is actually extracted by the operation of
burning, which reduces ordinary calcareous earth to
the ftate of quick-lime; and th.;t it is afterwards re*
abforbed by the quick-lime when expofed to the air.
On this principle he was able, not only to account for
the lofs of weight by rhe burning of lime-ftone, but
to eftimate to the greateft nicety the additional weight
which it could acquire from the atmofphere. He ex-
tracted the gas, to which he gave the name of fixed
or fixable air, alfo by another procefs, namely, by dif-
folving the calcareous earth in acids; he found thaj
the caufticity of lime depended upon its violently at-
tracting from, vegetable ,and animal matter a portion of
that air of which it hacl been deprived, and that upon,
this principle he was enabled to render cauflic the al-
kaline falts.

To Mr. Cavendifh the fecond place in the order of
this hiftory belongs. He purfued th-s experiments of
Dr. Black, and afcertained the quantity of fixed air
which could be retained by the fixed and volatile al-
kalis. He accounted for the nature of acidulated
•waters, by the fixable air which they contained. He
procured a fpecies of inflammable air from folutions
of iron and zinc in vitriolic acid j and he was the firft
who remarked, that a folution of copper in fpirit of
fait, inltead of yielding inflammable air, lilie that of

iron

366 Dr. Prufty. [Book ^

iron or zinc, afforded a particular fpecies of air, which
loft its clafticity by coming in contact with water.

Dr. Prieftley commenced his philofophical career
byjbfne experiments upon fixable air; and the firft of
his communications to the public related to the im-
pregnating of water with this air, by means of chalk
and oil of vitriol, a method firft hinted by Dr. Brown-
riggof Whitehaven, and now commonly practifed irt
the imitations of the acidulated mineral waters* The
Doctor tried the power of fixnble air upon animal and
vegetable life, and found it fatal to both; and he made
fever.il other valuable experiments, the fubftance of
which will be related in the chapter on fixed air*

The indefatigable mind of Dr. Prieftley was hot>
however, to be fatisfied with the inveftigation of a fmglc
object. He next turned his attention to the nature of
atmofpheric air. He obfcrved, after Dr. Hales, its
diminution by different procefTes, as. by combuftion,
&c. but differed as to the caufe. Dr. Hales fuppofed
the fpecific gravity of the air to be increafed ; but Dr*
Prieftley judged, that the denfer part of the air is pre*
cipitated, and that the remainder is actually made
lighter. The difcovery that the atmofpheric air is
purified by vegetation is alfo Dr. Prieftlcy's.

On purfuing the experiments of Mr. Cavendifli on
inflammable air, the Doctor found that it was not only
producible from iron and zinc, but from every inflam-
mable iubftancc whatever.

Dr. Prieftley difcovered the caufe that air, which
has been refpired, is fatal to animal life, to be, that it
becomes impregnated with fomething ftimulating to
the lungs, for they are affected in the fame manner ai
when expofcd to any other kind of noxious air. His
experiments on the means of reftoring falubrity to air
are highly interefting and entertaining, and afford a

pleafing

Chap, i.] M. Lavoijier.

pleafing inftance of well-directed afliduity. But one
of the mod (Inking difcoveries of this phiJofbpher is,
that the nitrous air, which he procured from the fo-
lution of certain metals in the nitrous acid, had the pro-
perty of diminifhing a quantity of the pureft part of the
common air, the remainder being by this procefs ren-
dered noxious and unfit for combuftion j and upon
this principle nitrous air was for a long time received
as a teftofthe purity of the atmofphere, though it will
afterwards appear that this tell is imperfect. Dr.
Prieftley alfo purfued the laft mentioned experiment
of Mr. Cavendifh, and found that a fimple acid, or
alkali, might be made to affume the form of a per-
manently elaftic fluid; and thefe fluids he diftinguifhed
by the title of acid and alkaline airs. But to fpecify
all Dr. Prieftley's difcoveries, even in this very con-
cife manner, would greatly exceed my limits ; I muft
therefore be content with only curforily mentioning the
moft remarkable.

The publication of thefe experiments of the Englifh
philofophers excired the attention of feveral ingenious
foreigners; but the only difcoveries worthy of notice
in this place are thofe of M. Lavoifier. The experi-
ments of this philofopher, afcertaining the precife quan-
tity of water and elaftic fluid, which are contained in
flaked lime and mild alkali, alfo thofe upon the burn-
ing of.pholphorus, are the neateft and mod complete
that have ever been -publifhed. The only new <3if-
covery of any note, which we can attribute to him,
was, demonftrating that the calcination of metals is
owing to the abforption of a certain elaftic fluid ; but
he did not at firft perceive that this fluid was in any
refpect different from the fixable air produced by effer-
vefcing mixtures. In a memoir, however, which he
read after the publication of his elTays, before the

French.

368 M.LavofJter. [Book V*

French Academy, he was of opinion, that the air which
is abforbed by the calcination of metals is common air,
but that it is of the very pureil kind, and more com-
buftible and refpirable than that in which we exift.

This opinion verges fo clofely upon the dephlogif-
ticated, vital, or empyreal air of Dr. Prieftley, that
were we not informed by good authority, that M. La-
voifier firft received from cur Englifh philofopher *
the hint of extracting air from mercurius calcinatus,
the circumftance would in fome meafure affcc~l the
priority of his claim to that great difcovery. Dr*
Prieftley confeffes, that accident, rather than a pre-
concerted plan, was his guide upon this occafion. He
had been employed in extracting air from differenc
fubftances, and in particular in the converfion of the
different acids into fluids permanency elaftic. Among
the fubftances from which he endeavoured to extract
air was calcined mercury, which afforded it in confi-
derable quantities ; and upon applying the different tefts,
he found this air of a purer nature than the common
atmofpheric air. The air which was produced from,
red precipitate was equally pure with that which was
afforded by the mercurius calcinatus per Jet A fimilar
product was procured from red and white lead, from a
variety of fubftances moiftened with fpirit of nitre ;
laftly, from common nitre itfelf, from fedative fait, and
Roman vitriol. I omit noticing a number of erroneous
opinions, which were ftarted in the infancy cf the
fcience, as my prefent bufinefs is only to trace- the flepa
by which our knowledge has been gradually improved
in this department of nature.

Dr. Prieftley continued his experiments on inflam-
mable air, and found that all the metals which yield ic

* PricRIcy on Air, voh ii. p. 36, and 320.

when

Chap, i.] Great Difcevery of Mr. Cavendijh.

when diflblved in acids, yielded it by means of- heac
alone i his mode of extracting it was by fubjecting the
filings of the different metals in vacuo to the action
of a burning glafs.

The next remarkable, and perhaps the moft impor-
tant difcoyery, was that of Mr. Cavendifh, which has
explained to us the nature, and compofition of water.
Mr. Cavendifh was led to this great difcovery by the
experiment of Mr. Warltire, related by Dr. Pricftley,
in which it was found, that on firing a mixture of com-
mon and inflammable air by the electric fpark, a lofs
of weight always enfued, and that the infide of the vef-
fel in which it was fired became always moid or dewy,
though ever fo carefully dried before. On repeating
the experiment, Mr. Cavendifh did not perceive the
diminution of weight which Mr. Warkire fuppofed to
take place, but the latter effect was completely exem-
plified. In profecuting the experiment, it appeared,
that it was only the pure or empyreal part, that is about
one-fourth, of the common air which was confumed,
and the water produced was perfectly taftelefs and
pure ; on mixing empyreal with inflammable air in a
due proportion, and pafllng through them an electric
fpark, the whole portion loft its elafticity, and was con-
denfed into water.

Mr. Cavendifh fmrfued his experiments with re-
markable fuccefs, to afcertain the conflituent principles
of phlogifticated air, or that which conftitures the im-
pure and unrefpirable portion of the atmofpheric air,
and by pafiing the electric fpark through common air,
and through a certain mixture of empyreal and phlo-
gifticated airs, he was able totally to condenfe the lat-
ter, and to afcertain its constituent principle to be the
fame with that of nitrous acid, with (as he then thought)
a fmall portion of inflammable matter. In this latter

VOL. I. E b opinion,

376 Cavehdijh and Lavoificf. [Book V.

opinion, however, he has fmce been corrected by La-
voifier, and other modern chemifts, who have proved
that azotic, or phlogifticated air (as it is called by the
Englifli chemifts) is no other than the bafis of the
nitrous acid *.

On thefe experiments and difcoveries the whole of
the modern fyftem of chemiftry and phyfiology is
founded ; but their importance will be more completely
proved, in treating more at large of the different fpe-
cies of air, and of the fucceeding fubjech, in thefe vo-
lumes.

* In Mr. Cavendifh's Experiment, as he probably ufed air
which had been rendered impure by combuftion, fome fmall por-
tion of charcoal or other inflammable matter might be contained
in the air.

Ghap. 1.] [ 371 ]

CHAP. II.

OF OXYGEN GAS, OR PURE, VITAL, EM-
PYREAL, OR DEPHLOGISTICATED AIR.

Explanation of Terms.— Reafons for the different Names ajfigned to
this Fluid. — Ho-w procured. — From Calces of Metals,— By Pege-
iation.-. — From Water. — Properties of Oxygen Gas. — A powerful
Agent in the Syjlem of Nature. — - Ho-iv ejfential to Flame and Life.—-
Various Modes provided by Nature for furnijbing a Supply of thit
Fluid.

IT has been already intimated that£&r, fignifying
fpirit or ebullition, was a term employed by Van
Helmont, and other Dutch and German chemifts, to
clefcribe thofe elaftic fluids, which appeared in their
nature different from common or atmofpheric air.
From the preceding hiftory of fire or caloric, the
reader will be at ho lofs to underftand, that every
aeriform fluid confifls of a bafts, or matter peculiar to
itfelf combined with the matter of heat, which is in-
deed the real efficient caufe of all fluidity whatever.
The word gas has therefore been employed by the
French chemifts to denote an aeriform Auid compofed
of a certain bafis, which gives it its peculiar character,
combined with the matter of heat or fire. It will be
alfo proper to remember, that of thofe fluids which
are termed elaftic, fome are permanently elaftic, as the
aeriform fluids, others, fuch as common vapour from
water, are condenfible by coldj and that it is only
of the former kind that we have now to treat.

The fluid under our immediate confidcration was

originally termed dephlogifticated air, a name given it

Bb 2 by

37 2 Different Namts ajpgned to this Fluid. "[Book V.

by Dr. Prieflley from fuppofing it free from phlogifton
or inflammable matter ; when it was found effential to
animal life, it obtained the name of pure or vital air ;
and when it was found to contribute efientially to ig-
nition, and the other phenomena of fire, it was termed
empyreal air ; but the French chemifts, having difco-
vered that it is the fubftance which imparts the acid
character to all the mineral and vegetable acids, have
cliftinguifhed it by the name of oxygen * gas.

Oxygen, or the bafis of oxygen gas, is naturally or
artificially combined with a great variety of fubftances.
From fome of theie it may be detached by the fimple
application of heat, fince it has a remarkable attrac-
tion for the matter of fire, with which, when it unites,
-it becomes expanded, and aflumes the form of gas
or air.

The fubflances from which it may be moft eafily
f xtracted, by means of heat, are red lead, calcined
mercury, nitre, and manganefe. Dr. Prieftley ex-
.pofed a quantity of red lead in the focus of a burning
glafs twelve inches in diameter. A quantity of fixed
air, or carbonic acid gas, as it is now called, was al-
ways produced at firft ; but after that was fcparated,
the remainder was found to fupport flame, and to fuf-
tain animal life much more vigoroufly than common
air, and to have all the characters of dephlogifticated
air, or oxygen gas.

By fucceeding experiments of Dr. Prieftley and
others it however appears, that dephlogifticated or
oxygen air, may be obtained not only by means of
heat, but alto by the action of the vitriolic and nitrous

* From o|v; (oxus) " fharp or acid," and yt\i'.py.\ (ginomai)
" to beget or produce." — Oxygen is then literally tke principk
«'r fubftance producing acids.

acids

VOL.1 .p. 3 7.3-

Chap. 2.] Hbw Oxygen Gas may be obtained. 373

acids upon a variety of mineral and metallic fub-
ftances.

In a fmall phial AB (Plate XXIX. Fig. i.) to
the mouth of which is fixed a bent tube C D, put an
ounce of the oxid of mercury, or hydrargyrus calci-
natm ; put it to heat over the chaffing difli R; and
after the atmofpheric air, which rilled the phial, is etf-
haufted, place the extremity D of the bent tube under
a long narrow glafs verTd (Fig. 2.) filled with the-
fluid in. the pneumatic apparatus or tub (Fig. 3.) and
place this w-Tel upon the board E F above the aper-
ture c or d *.

As the mercury revives and becomes liquid, a com*
preflible, elaftic, tranfparent fluid may be obferved
to difengage itfclf, and pafs into that narrow glafs vef-
fel ; this is air of the pureft kind that we are able to
procure, namely, vital air or oxygen gas.

This kind of air may alfo be obtained by the fame
procefs, from the native oxid or calx of manganefe, or
from minium or red lead, which, it is well known, is an
oxid of lead, or lead united with oxygen.

The better to underftand thefe effects it mud be
recollected, as was obferved in the beginning of this
chapter, that this fluid is not found in thefe fubftances
in an entire Hate; they only contain the bafis of ira
which is the oxygen \ for metals neither calcine nor
burn but in confequence of their combination with
oxygen, which by that means becomes folid, and joins
its weight with theirs. This oxygen is then expelled
by the heat or caloric, which, combining with it, caufes
it to pafs into the ftate of an elaftic fluid » during this
procefs, the metal, lofmg the oxygen which had re-
duced it to the ftate of an oxid or calx, aflurfies its

« Traite Elem. de Phyf. torn, ii. p. 2 j.,

B b 3 metallic

374 Means of obtaining pure y&>. [Book V,

metallic properties, and lofes the weight which it had
acquired in becoming oxidated *.

There is, however, a method by which oxygen gas
may be obtained with lefs heat and greater facility,
and it is as follows ; put fome red lead into a bottle,
together with fome good ftrong oil of vitriol, but with-
out any water. Let the red lead fill about a quarter
of the bottle, and the vitriolic acid be about the fame
quantity, or very little lefs ; then apply the bent tube
to the bottle by inferting it through a cork, and hav-
ing inverted another bottle filled with water in a bafon
about half- filled alfo with water, direct the other enc|
of the crooked tube into the bottle inverted in the
water. In this ftage of the procefs we mud obfervc,
that without heat this mixture of red lead and vitriolic
acid will not afford any oxygen air, or a very incon-
fiderable quantity ; it is necefTary, therefore, to apply
the flame of a candle or wax taper to the bottle con-
taining the ingredients, while the crooked tube opens
a communication between this bottle and that inverted
in the water. In this manner the red lead will yield
a quantity of elaftic fluid, which will pafs through the
crooked tube into the inverted bottle, and as the quan-
tity of dephlogifticated air increales in the inverted
bottle, the water in it will be feen to fubfide j this air
will not be ail pure, becaufe a confiderable quantity of
fixed air enters with it. In order to feparate the fixeci
from the pure air, the inverted bottle, when filled with
the compound pf both, muft be agitated in a bafon of
lime water, by which means the lime water will abforbj
the whole of the fixed air, and leave the dephlo?
gifticated air or oxygen gas by itfelf.

.Oxygen gas may alfo be obtained in confiderabls

* Briflbn, torn. ii. p. 25.

Chap. 2.] Purt Air by Means of Vegetatim. 375
quantities from water, efpecially from pump water,
which, when cxpofed to the fun, emits air flowly ;
but after it has remained fb for a considerable time, a
green matter adheres to the bottom and fides of the
glafs veffel in which it remained -y afterwards it emits
pure air in great quantities, and continues to do fo for
a long time after the green matter has exhibited fymp-
toms of decay by turning yellow.

Dr. Ingenhoufz rightly fuppofed this green matter
to belong to the vegetable- kingdom, and procured
pure air by putting the leaves of plants into water, and
expofing them to the fun. He obferves, that of land
vegetables the fitteft for this pnrpofe are the poifonous
plants, fuch as hyofcyamus, lauro»cerafus, night-fhade,
&c. But he extracted the pureft air from fome aqua^-
tic vegetables, and from turpentine trees, but efpecially
from the green matter he collected from a ftone
trough, which had been kept filled with water from 4
fpring near the high road.

While Dr. PrieflLey was engaged in a feries of ex-
periments to enable him to purify contaminated air,
he difcovered that vegetables anfwered this purpofe
moft effectually. The experiment by which he illuf-
trates his afTertion was this -} having rendered a quantity
of air very noxious, by mice breathing and dying in it,
he divided it into two receivers inverted in water, intro •
ducing a fprig of mint into one of them, and keeping
the other receiver with the contaminated air in it alone.
He found, in about eight or nine days after, that the air
of the receiver, info which he had introduced the fprig
of mint, had become refpirable ; for a moufe lived
very well in this, but died immediately upon being
introduced into the other receiver, containing the con-,
taminated air alone.

B b 4 It

376 Count RumforcFs Experiments. [Book V.

It has fincc been obferved, that feveral animal lub-
ftances, as well as vegetables, have a power of fepa-
rating ck'phlogifticated air, or oxygen gas, from water,
when expofed to the action of the fun for a confider-
able time.

The ingenious Count Rumford obferved, that raw filk
has a remarkable power of producing pure air from wa-
ter. He found, that by introducing thu tv grains of this
fubftance, firft wafhed in water, into a thin glals globe
four inches and a half in diameter, having a cylindrical
neck three- fourths of an inch wide and twelve inches
long, inverting the globe in a jar filled with the fame
kind of water, and expofmg it to the action of the fun
in the window, in lefs than ten minutes the filk be-
came covered with an infinite number of air bubbles,
gradually increafmg in fize, till at the end of two
hours the filk was buoyed up, by their means, to the
top of the water. They feparated themfdves by de-
grees, and formed a collection of air in the upper part
of the globe, which, when examined by the eftablifhed
teft, appeared to be very pure. In three days he col-
lected three an,d three-fourths of a cubic inch of pure air,
into which a wax taper being introduced, that had juft
before been blown out, the wick only remaining red, it
inftantly took fire, and burned with a bright and en-
larged flame. The water in the globe had acquired
the fmell of raw filk, it loft fomt thing of its tranlpa-
rency, and aflfumed a faint grcvhifh caft.

It \vas obferved, that when this experiment was
made in the dark, only a few inconfiderab^e bubbles
were formed, which remained attached to the filk -, nor
\vas it otherwife when the glafs globe was removed
into a German (love. In the latter cafe, indeed, fome
(ingle bubbles had detached themfelves from the filk^

and

Chap. 2.] Pure Air from Silk, &V. 377

and afcended to the top, but the air was in too fmali
3 quantity to be either meafured or proved.

In thefe experiments it is probable that the oxygen
or pure air was extracted by an a6tual deccmpofition
of a part of the water, by means of a capillary attrac-
tion, aided by the folar influence ; and in effect the
fame philofopher was enabled to extract it, though in
a fmaller quantity, by means of a number of very
minute glafs tubes immerfed in water and expofed to
the^fun.-

The properties or functions of this fluid are fome
of the moft important in nature ; nor, except caloric
o- heat, is there any natural agent more univerfal or
more active.

i ft. It is elTential to combuftion ; nor do we know
of any procefs by which flame can be fupported with-
out a fupply of oxygen gas, or empyreal air.

idly. In certain proportions it is abfolutely necefiary
to fuftain animal life j fo that the whole animal creation
may be faid to depend upon this fluid for their exift-
ence.

3dly. Its bafis oxygen gives the acid character to all
mineral and vegetable falts, the bafes of which are found
to be entirely infipid till combined with oxygen.

4thly. The calcination of metals is altogether effect-
ed by their union with oxygen. Thus for moft of the
mineral pigments, and a very numerous clafs of medi-
cines, we are indebted to this ufeful element.

5thly. It forms a conftituent part of that necefFary
fluid, water, which confifts of 85 parts of oxygen, and
15 of hydrogen, or the bafis of inflammable air.

Oxygen gas, or air, is more elaftic than common
air ; it exceeds it alfo in fpecific gravity, for the pro-
portion between pure and common air is as 160 to
152.

On,

3y3 Properties of Oxygen Gas. [BookV.

On introducing a lighted candle into pure or dephlo-
gifticated air, the flame becomes larger and brighter ;
and whenever the air is very pure, the candle burns
with a crackling noife, as if the air contained fome
combuftible matter, while the tallow or wax wades,
or is conformed, incredibly raft. When after this pro-
cefs the candle is extinguifhed, it will be found that
two-thirds of the bulk of air employed will be con-
verted into fixed air. When the fixed air is taken up
by lime water or cauftic alkali, the fmall remainder
will be as pure as before.

In common procefles, not more than one-tenth of
the air employed is converted into fixed air. It is
probable, that in thefe experiments fome diminution in,
the volume of air muft take place, from the fuperior
gravity of fixed air, and the confequent condenfation
of the other,

If live coals are introduced into a veflel filled with
dephlogifticated air, it will be found to be diminifhed
one-fourth of its quantity. When this experiment is
repeated with fulphur, the flame will become larger
and more vivid than in common air, and three-fourths
of the quantity will be loft. If a piece of phofphorus,
is put into a feven ounce meafure of this kind of air,
the mouth of the bottle being corked, and the phof-
phorus being fet on fire within it, the phial will break
in pieces, as fopn as the flame is extinguiihed., by the
prerTure of the external air.

The purity of vital air is afcertained by its degree
of diminution with nitrous air, or gas obtained from
nitrous acid, and this proccfs is to be confidered as a.
jpecies of combuftion, efpecially as a cpnfiderable de-
gree of heat is generated by it. Very great differences,
however, are perceived in this refpecl } and according
to the quantity of diminution, the air is faid to be two,

three,

Chap. 2.] Why purs Air is eflentlal to Life. 379

three, or four times better than common air. Dr.
Prieftley mentions Tome extracted from red lead five
times as pure as common air.

Pure or oxygen air is not abibrbed by water, nor
foluble in it j but it may, as was juft intimated, be al-
moft entirely condenfed by nitrous gas, with which it
combines, as will be proved when treating of that
fluid i and this combination is foluble in water,
and forms nitrous acid} for this acicj is compofed of
the bafis of nitrous gas combined with oxygen, the,
whole being diffolved in water.

The reafon that pure air is the moft eifcntial of all
the fluids to the fupport of life is, probably, becaufe a
great quantity of heat is neceffary for this purpofe, and
becaufe this fluid contains it in great quantity, and
parts with it very freely when it meets with any fub-
jftance for which it has itfelf a (Irong attraction. But as
its bafis (oxygen) combines itfelf very eafily with the
)bafis of coal which is found in the blood and lungs, and,
during this combination, lofespart of its caloric or heat,
which goes to the fupport of life, the remainder of
the caloric and oxygen, combined with the coal, form
t,he carbonic acid gas or fixable air, which is always
found to exift in a larger quantity in air which has
been refpired, than in atmofpherical air which has
not been fubfervient to that function. Of this a very
eafy experiment affords fufficient proof j it is founded
on the property which the carbonic gas has of render-
ing lime-water turbid. A crooked tube open at both
ends is partly filled with lime-water j a perfon applies
his mouth to one end of the tube, and infpires, by
drawing the air through the lime-water contained in
it. By this the tranfparency of the lime-water is not
affe&ed ; but it becomes turbid as foon as the perfon
£xpires, which is owing to the carbonic acid formed

in

3So Vfe of Oxygen Gas in Reftlration. [Book V.

in the lungs. It is therefore the great attraction which
exifts between the matter of coal and the bails of pure
air which renders this fluid fo proper for breathing.
The pure air which we breathe performs two functions
equally necefritry to our prefervation ; it carries off
from the blood that matter of coal, the fuperabundancc
of which would be pernicious, and the heat which this
combination depofits in the lungs repairs the continual
lofs of heat which we experience from the attraction
of furrounding bodies. According to Dr. Prieftley
and others, the bafis of oxygen gas is alfo abforbed by
the blood.

Since, therefore, a great quantity of heat is difen-
gaged from pure air in refpiration, it follows, that this
fluid muft be very pernicious to animals who breathe
this air alone for a confiderable time; and this is con-
fonant with the obfervations of phyficians, who have
attempted to cure pthifis by the refpiration of vital
air.

The bafis of this empyreal or pure air, or oxygen,
as the French chemifts term it, is one of the confti-
tuent parts of water. It has been mentioned, that it-
is alfo the matter which gives the acid character ta.
all the acids ; fulphur, for inftance, is a very innoxious,
infipid body, till by burning, that is by abforbing oxy-
gen, it becomes vitriolic acid. "Whether the bafis of
this empyreal air is a fimple or compound fubftance,
•we are unable to determine j in the prefent ftate,
however, of philofophical knowledge, we are juftifiecl
in tonfidering it as a fimple elementary body, for it
has never yet been decompofed.

If the limits of this work permitted, or if the re-

fearches of philofophers had furniihed us with fufficient

materials, it would be a mofl pleafing fpeculation to

trace the wifdom of Providence in the very ample

8 meajis

Chap. 2.] Natural Proceflesfor its Production. 381

means which he has provided for fupplying us with
this necefiary fluid. It is evident, that immenfe quan-
tities of it are, by the various procefTes of combuftion,
deftroyed, or, to fpeak more philofophically, con-
denfed, and by its union with inflammable matter
formed into water. This water is again raifed into
the atmofphere in the form of vapour $ it falls in dew
or rain upon the leaves of plants, and there, by the
genial action of the folar rays, a new dccompofition
again takes place, and every branch, every leaf, every
blade of grafs, is occupied in the beneficial function of
again impregnating the atmofphere with this Talutary
fluid. The quantities too, which are abforbed by the
calces of metals, mufl be immenfe ; but by the va-
rious procefles for the fmehing and reduction of
thefe metals, the oxygen 'is again fet free, and a frefh
fupply is produced. Even the air, which is injured
by refpiration, is doubtlefs again, by a variety of
modes, the greater part concealed from our view, pu-
rified, and rendered once more fit for ufe, fince fixed
air, in a difengaged ftate, is, comparatively fpeaking,
but a rare fubftance in nature, and fince there is rea-
fon to fuppofe that many of the carbonic bodies may
be recruited alfo by its decompofition. Ignorance of
nature is proverbially the fole fource of atheifm ; and
who can contemplate this aftonHhing revolution, this
circulation of benefits, and not fmile at the extreme
. folly of the man, who can fuppofe thefe appointments
cftablifhed without intelligence or defign.

t 382 ] tBookV,

CHAP. III.

AZOTIC « GAS, OR PHLOGISTICATED AIR.

Azotic Gas is the unrefplralle Part of the Atmofphere.— HO--W pr+-
cured. — Air Bladders of Fijhes filled <witb it. '—Its Properties. —
Azote tie Ba/is of nitrous Aridt

TH E azotic gas was at firft called by Lavoifier mo-
fete, and by Dr. Prieftley phlogifticated air. It
conftitutes about three-fourths of the atmofphere, but
is not refpirable by itfelf \ whence it derives the name of
azote, as being unfit for the fupport of animal life.
Philofophers have proved, that this fluid is completely
formed in the atmofphere, and that it may be pro-
cured by merely abforbing or deflroying the pure air, or
oxygen gas, with which it is united in atmofpheric air.
Azotic gas, therefore, is always found to remain
after a quantity of common air has undergone the re-
fpiration of animals, the combuftion of bodies, or pu-
trefaction j becaufe in alt chefe cafes the pure air is
abforbed or condenfed. Azotic gas is equally invifible
with common air, and fomething more elaftic. Mr.
Kirwan procured fome air, by means of a mixture of
iron-filings and fulphur, fo perfectly free from vital air,
that it was not in the leaft diminilhed by the teft of
nitrous air. When this kind of air is fo produced, and
dried by introducing dry filtering paper under the jar

* Or '* air which takes away life," from the Greek privative
particle a. and fa (zoe) " life." Dr. Prieftley called it phlogiiti-
•ated air, from fuppofing it chiefly compoietl of an imaginary fub-
flance, which Scheele and his followers termed phlogilton (or the
food of fire) ; this appellation is now found to be erroneous.

fat

Chap. 3.] Air Bladders of Fijh fitted toitb Azotic Gas. 383
that contains it, its weight will be found to be to tha?
of common air as 985 to j ooo, the barometer ftandino-
at 30.46, and the thermometer at 60".

Various fubilances alfo are productive of this air;
and M. Fourcroy has difcovered, that the air bladders
of n"fhes, and particularly of the carp, are full of it ;
and that it may be collected by breaking them under
glafs vefTels inverted in water. The air, however,
which is contained in the bladders of marine plants, is
found to be confiderably purer than atmofpheric air.

In fpeaking of the properties of this fluid it is necef-
fary to remark, ift. That azotic gas affords no fign of
acidity, not being capable of turning the blue colours
of vegetables red *.

2d. It does not precipitate lime diffolved in water;
for if a fmall quantity of lime-water is put into a tube
filled with this gas, it will remain clear and limpid ;
there will be neither lime precipitated nor chalk formed,
which evinces that it is radically different from fixed
or carbonic acid air.

jdly. Another property of this gas is that of fuddenly
extinguiming fubftances on fire, and not fuftaining the
vital principle in animals which arc plunged into itf.
This may be proved by introducing an animal or a
burning candle, into a veffel full of this gas ; the animal
will be fuddenly fuffocated, and the candle inilantly
extinguifhed.

* The common teft of che mills, to prove whether any flu54 con-
tains an acid. They ccrnmonly drop a fmall portion of the fluid
into a clear phial containing fyrup of violets ; but the rhofl delicate
teft is, a paper ftained with tinclure of Uirjifole or litmus, to which
the acid is applied by a feather.

f Animal life, however, does not appear to be deftroyed by any
noxious quality in this air, as is the cafe in breathing fixed air;
but the animal dies for want of the necelfory pabulum of oxygen
gas.

4thly.

384 Nature and Properties of Azotic Gas. [Book V.

4thly. Azotic gas is rendered refpirable by vegeta-
bles, which, in certain circumftances, furnifh vital air.
This property is probably owing to their retaining the
hydrogen of the water which they abforb, while they
part with the oxygen. There is no doubt that azotic
gas is really a conftituent principle of the atmofphere;
for if feventy-thrre parts of it are mixed with twenty-
feven of pure air, an air will be produced refembling
that of the atmofphere, and refpirable as that is *.

5thly. It is now a well eftablifhed fact, that the
azote, or bafis of phlogiilicated air, is literally the bafis
of the nitrous acid f j for being mixed in proper pro-
portions with oxygen or pure air, which is neceflary to
give to thefe bafes the acid character, and fet on fire
by palling through them an electric fpark, nitrous acid
is uniformly produced, as is evident from the experi-
ments of Mr. Cavendifh.

6thly. Late difcoveries have alfo evinced, that the
volatile alkali is formed from a union of this gas with
hydrogen, or inflammable air.

As this air conftitut.es fo large a proportion of the
common air which we breathe, its further ufes and
properties will be better explained when I come to
treat of the atmofphere.

* Briflbn, torn. ii. p. 3 5.

f It may perhaps be neceflary to inform the young reader, that
nitrous acid is that fubftance commonly fold in the Ihops under the
name of aquafortis. It may feem furpriiing to be tcdd, that the air
which we are commonly breathing is effentially aqua fortis— But
though that extremely corrofive and deadly fluid is really com-*
pofed of azote and oxygen, yet they are then in a ftate of cofr*
, whereas in aynofpheric air they are only mix-:d.

Chap. 4.]

CHAP. IV.

OF CARBONIC ACIB GAS; FIXED OR FIXABLE AIR.

The Bajis of this Air is the elementary Matter of Charcoal — Combined
with Oxygen.-:— Modes of producing it. — Fermentation.— Quantity
contained in different Kinds of Wines.— -Choak Damp.— Properties of
Fixed Air. — Great fpecifo Gravity.— May lie poured out of a VeJJel
like Water Refijls Putrefaction.

IN enumerating the elementary principles of bodies,
it will be recollected, that coal, or carbon (accord-
ing to the French chemifts) was confidered as a fimple
elementary fubftance. Carbonic acid gas, however, is
by no means entitled to this character. It receives
the name of carbonic, becaufe its actual bafis is the
matter of coal, or, more properly, charcoal. It is
called acid, becaufe a quantity of oxygen enters into its
compofition. It is denominated a gas, from the mat-
ter of fire which gives it the character of a perma-
nently elaftic or aeriform fluid.

As this was the firft of the gaffes which excited the
attention of the philofbphers of Europe, it has been
diftinguifhed by various appellations. It was at firft
only known under the general and uncharacteriftic
name of mephitic or (foul) air j when it was found
exifting in large quantities in a fixed or folid flate, as
in lime, chalk, or alkaline falts, from which it might
be expelled by the action of heat, it obtained the name
of fixed or fixable airj when by the ufual tefts, and by
otherinfallible marks, it was found to pofiefs the cha-
racters of an acid, it was called by the learned Bergman
• VOL. I. C c the

386 Different Msdes in which [Book V.

the aerial acid, becaufe it could be obtained only in a
pure or uncombined ftate in an aerial form ; and from
its exifdng in confiderable quantity in chalk, &c. it
\yas denominated by other chemifts the cretaceous or
chalky acid. I have preferred the name which has
been adopted by the judicious Lavoifier, as more im-
mediately expreilive of its characleriftic, and peculiar
bafis, which is undoubtedly the matter of coal.

The proportion of the materials which enter into
this kind of air is about eighteen parts of oxygen and
feven parts of that matter which the French philofo-
phers denominate carbon, or tcoal. If, for example,
charcoal is burnt in a clofe veflel with oxygen gas, the
air which remains after combuftion is carbonic acid
gas. By the experiments of Lavoifier and De la Place
it appeared, that one ounce of charcoal required for
its combultion three ounces and one-third of vital air,
and produced three ounces and an half of fixable air.

There are feveral methods of procuring fixed or
ftxable air, or carbonic acid gas •„ firft, by the /ermen-
tation of liquors, in which operation its formation is
owing to the combination of the carbon of the facharine
matter with the o>:yr:n of the water.

It is evident that a great -quantity of fixed air is pro-
duced, when vegetable or animal fubilances (efpecially
the former) are in a ilate of vinous fermentation. In
breweries there is always a ftratum of fixed air on the
furface of the fermenting liquor, reaching as high as
the edge of the vats; and it is owing to the produc-
tion and ekfticity of fixed air, that fermenting liquors,
•when put into clofe veffels, often are known to burft
them with great violence.

DF. Prieftley, in order te determine the quantity of

fixed air contained in feveral fpecies of wine, took a

glafi phial (fitted with a ground ftopple and tube) ca-

6 pable

Chap. 4.] Fixed Air may fa procured. 387

pable of containing an ounce and half meafure. This
he filled with wine, plunging it into a vefiH of water.
The whole was then put over a fire, and the water in
which the phial was plunged fuffered to boil. The end
of the tube in the itopple being placed under the
mouth of an inverted receiver, filled with quickfilver,
the heat expelled the fixed air from the wine, which;
entering into the receiver, afcended in bubbles through
the quickfilver to the top, removing in its pafTage a
part of the metal, and afiuming its place in the receiver.
The refult of the Doctor's experiment may be inte-
refting to fome readers, and to others it may at leaft
afford entertainment.

1 £- oz. of Madeira produced Ti- of an oz. meaf. of fked air.
Port 6 years old - ?'T
Hock of 5 years - -^
Barrelled Claret - TL
Tockay of 16 years ^
Champagne of z years - 2 oz. meafures.
Bottled Cyder of 12 years 3 f

"Fixed air may be eafily obtained by mixing together
equal parts of brown fugar and good yeafl of beer, and
adding about twice the quantity of water. This mix-
ture being put into a phial, to which a bent tube with
a cork or ftopple may be adapted* will immediately
ferment, and yield a confiderable quantity of fixed air,
which may be received into a phial filled with quick-
filver or water.

adly. Fixed air is produced by the refpiration of
animals ; in which cafe the oxygen of the air infpired
furni fives part of its heat to the fupport of life, and
combines with the carbonaceous or coaly matter,
which is difengaged from the blood in the lungs.

3dly. From what has been previoufly ftated it is evi-
dent, that fixed air may be produced by thfc combuf-
|ion of any carbonaceous or coally matter. •

C c 2 4tbly.

3$ 8 Fixed Air from calcareous Earth. [Book V.

4thly. Fixed air is alfo extricated in large quantities
by the action of acids on calcareous earth.

Fill a phial or a glafs receiver with water, and in-
vert it (in the lame manner as clefcribed in the chapter
on dephlogifticated air) in a bafon half filled v.'ith wa-
ter. Then put fome chalk or marble grofsly pow-
dered into another bottle, fo as to fill about a fourth
or £fch part of it, and pour water upon it until the
chalk is covered, then add fome vitriolic acid to
it, in quantity about the fourth or fifth part of
the water, and apply a cork with a tube as before
to the bottle, fo chat the extremity of the tube may
pafs through the water of the bafon into the neck of
the other bottle which is inverted in the water. The
mixture of chalk and oil of vitriol will then begin to
cffervefce, and heat is produced, which may be felt by
applying the hand to the outfide of the veffel. Fixed
air is copioufly emitted from this mixture, and, pafTmg
through the bent tube, will proceed into the bottle in-
verted in the water, and afcend to the top of it. By
thefe means the inverted bottle may be filled with fixed
air, and being corked under water, may be removed
from the baton and kept for ufe.

5thly. Fixed air is alfo expelled in large quantities,
by the application of heat only, from lime, chalk, mag-
nefia, or alkaline bodies, in what is called their mild
ftate, oppofed to cauftic; and by the experiments
of Dr. Black it was .found that this fubftance con-
ftituted nearly one-third of the weight of thofe bodies.
The alkalies and calcareous earths have confequently
a very powerful attraction for this fluid in their cauftic
ftate ; and it is therefore eafily condenfed by agita*
tion with lime water, as has been already intimated.

This gas was long known to miners by the name of
choakdamp, fo called from its fatal fuffocating effects;

and

Chap. 4,] Cboak Damp. ^89

and its properties may be enumerated in few words,
ift. It extinguifhes flame, id. It is fatal to animal
life. jd. It is heavier than common air. 4th. From
its acid character it refifts putrefaclion. ^th! It ren-
ders alkalies, &c. mild. 6th. Water, under the com-
mon prefiure of the atmofphere, and at a low tempe-
rature, abforbs fomewhat more than its bulk of this
gas, and in that ftate conftitutes a weak acid rather
agreeable to the tafte, whence fixed air is a conftituent
principle in moft mineral waters ; indeed the water of
Iprings and rivers is feldom free from it. yth. It is
alfo a conftituent principle of all fermented liquors,

If a lighted wax taper is let down into a bottle filled
with fixed air, the flame will be inftantly extinguimed,
and an animal inclofed in a veflel which contains it will
immediately expire.

This fixed air will be found to be much heavier than
common air ; its fpecific gravity being to that of com-
mon air as 151 is to 100.

From the greater .weight of this gas it always falls
to the bottom of the veflel in which it is contained.
An animal (as was before obferved) introduced into a
ftratum of this air immediately expires; and it is ow-
ing to the prefence of this fluid that the Grotto del
Cani in Italy is fatal to animals whole organs of refpi-
ration are placed below the level of the mouth of that
cavern. This gas may be poured out of one veflel
into another like water, or may be poured on a can-
dle, which it will extinguifh as effectually as that fluid.
Among the moft ufeful properties of fixed air, it
has been remarked, that water 'impregnated with it
becomes a powerful antifeptic. Moft of the famous
mineral waters may be imitated by impregnating wa-
ter with fixed air, and then adding that quantity of fait
or metal, chiefly iron, which thofe mineral waters, by
C c 3 analyfis,

390 Properties of Fixed Air. [Book V.

^nalyfis, arc known to con^in. It is from this pro-
perty of preventing put:cfa6licn, that fixed air and vege-
tables, fugar, and other fubfbnces, v hich abound with
that prircipic, are fuppofed to be powerful r-.nr.lies
in putrid difrafes.

Fixable air not only preferves fruit, but m^at alfo,
from pttotefa&fGfk, and that for a very coniidcrablc
time, and even in the hotteft weather.

Chap. 5.] [ 39* ]

CHAP. V.

INFLAMMABLE AIR OR HYDROGEN GAS*.

This Gas forms the Ba/is of Water. — Prc port ion of Hydrogen and Oxy-
gen 'which enter into the Compojiticn of Water. — Modes of procuring
inflammable Air.—Ignes Fatui. — Fire Damp in Mines. — Lightejl of
all Fluids. — Remarkable Prefer ties. —Life in Air Balloons, — 'Curious
wial Fireworks.

TO that fluid, which we term inflammable air, the
French chemifts have given the name of hydro-
gen gas, becaufe its bafis is the peculiar conftituent part
of water j but what this bafis may be in its nature,
whether fimple or compound, is at prefent unknown,
. becaufe it cannot be feparated from the heat or caloric
which gives it the aerial form, without fixing it in
another fubftance.

According to M. Lavoifier, water is compofed
of eighty-five parts of oxygen and fifteen parti of
hydrogen. This philofopher has inftructed us in
the following method of obtaining this gas by heat
only f«

Let water pafs drop by drop thrbugh the barrel of
a gun, while it remains red hpt amidft burning coals j
let a crooked tube placed at the end of this iron, and

* '"f$up (hydor) " water" and ysi*o««* (geinomai) " to pro-
duce."

f Briffon, torn. ii. p. 73.

C c 4 bent

395 How inflammable Air may be procured. [Book V.

bent fo that it may be patted into a glafs vefiel full of
water inverted in the pneumatic apparatus. There
will then pafs into the glafs vefiel an aeriform fluid,
which is inflammable air or hydrogen gas. In this
' procefs the water fuffvrs a decompofition, and while
the hydrogen pafles into the glafs receiver, the oxygen
unites with the fubftance of the gun barrel, and oxy-
dates or rufts its internal furf ice.

By means of acids, however, inflammable air may
be obtained in greater abundance, and with more fa-
cility. When iron, zinc, or tin, are acted upon by
diluted vitriolic, or marine acid, confiderable quanti-
ties of this gas are extricated. In this cafe alfo the
water is decompofed, as is plain from the concen-
trated vitriolic acid not anfwering the fame ends as
the diluted, either in furnifhing the air or diflblving
the iron, &c.

The apparatus for procuring this gas is the fame as
that which has been defcribed for producing fixed air,
only employing inftead of chalk, iron filings, fmall nails,
fmall pieces of iron wire, or grofsly powdered zinc. To
thefe materials fome oil of vitriol and water muft be
added, in the fame proportion as in the procefs for pro-
duping fixed air.

The electric fpark, taken in any fpecies of oil, pro-
duces hydrogen or inflammable air, this fubftance being
a coniliiuent part of all the oils. The fame may be
iaid of ether, and alcohol or fpirits of wine, which con-
tain a great proportion of hydrogen.

Mr. Cavallo informs us, that he has procured this
kind of air from the ponds about London, in the fol-
ing manner. Fill a wide--mouthed bottle with pond-
water, and keep it inverted in it; then with a ftick
ftir the mud at the bottom of the pond juft under the '

inverted

Chap. 5-1 Ignes Fatul and Fire .T)amp* 393

inverted bottle, fo J»s to permit the bubbles of air
which rife to be received in the inverted bottle ; and
this air will be found Co be inflammable.

The ignes fatui are fuppofed to proceed from the
inflammable air which abounds in marfhy grounds,
and to be fet on fire by eledbric fparks.

This gas, as well as fixed air, was long known to
miners before it was noticed by philofophers ; and
among the colliers and other workmen of that clafs, it
obtained the name of the fire damp. It is, however,
feldom found pure in mines or coal works, but is gene-
rally combined with fulphureous matter, or what is
called hepatic gas, or with carbonic acid air j and this
admixture varies its fpecific gravity, and in general
renders it fomething heavier than pure inflammable
air. The fire damp generally forms a whitifh cloud
in the upper part of the mine, and appears in fome-
thing of a globular form ; from its levity it will not
mix with the atmofpheric air, unlefs fome agitation
takes place ; and it is difpofed to lodge in any little
cavity in the fuperior part or roof of the mine.
When it appears in this form, the miners generally
fet fire to it with a candk, lying at the fame time
flat on their faces to efcape the violence of the
fhock. It will not, however, take fire unlefs in
contact with atmofpheric air, for the obvious rea-
fon, that a mixture of oxygen ga$ is necefiary to
its inflammation. The danger arifes entirely from
its inflammability on the approach of any ignited body,
for when the fire damp confifts of pure inflamma-
ble air, the explofion is like that of gunpowder;
but when it is mixed with carbonic acid, it burns
with a lambent flame. The eaficft and fafeft me-
thod, therefore, of clearing the mine from this for-
midable fluid is by leading a long pipe through the

fhaft

394 General Properties of [Book V.

fhaft of the mine to the afh-pit of a furnace, when
the infiammablc vapour will be conftantly attracted to
feed the fire.

Dr. Prieftley has fufftciently proved by experi-
ments, that there is no acid contained in inflammable
air. He alfo afierts that charcoal, by the heat of a
burning lens, may be almoft totally converted into
this .kind of air, but that fomc moifture is neceflary
in the procefs. The neceflity of moifture, how-
ever, to the fuccefs of this experiment, iufficiently
evinces the fallacy of the conclufion which has been
drawn from it. Perfectly pure charcoal, abftradted
from every other body, is indeftrudtible by heat.
Where, however, there is moifture there is water.
In this cafe the oxygen of the water is attracted by
the carbon, forming with it carbonic acid, and the
hydrogen, the other constituent part of water, rifes
to the top of the receiver. Pure hydrogen gas
is the lighted of all elailic fluids, its fpecific gravity
is to that of common air as 8,04 is to 100,00*.

The moft remarkable properties of this gas are,
ift. Its great inflammability, which ariles from its
propenfity to unite with oxygen and form water,
adly. Its extraordinary levity, as already noticed.
3clly. Metals are very eafily revived or reduced from
a calx or oxyd to the metalic ftate when heated in
a receiver filled with this air. This alfo arifes from
its attraction for oxygen, which in this cafe is ex-
pelled from the calx, and, uniting with the hydrogen
in the receiver, leaves the metal pure, and in its na-
tural ftate. 4thly. Plants vegetate in this fluid with-
out impairing its inflammability, fthly. Water will
imbibe about one-thirteenth of its bulk of this gas,,

* See Briffor, torn, ii, p. 77.

which

Chap. 5.3 Inflammable ATT, *y$

which may be again expelled by heat, and will then
be equally inflammable as before. 6thly. Hydrogen
gas, cr inflammable 'air, is fatal to animal life; in
proof of which Mr. Cavallo relates, that the Abbe
Fontana, having filled in his prefence a large bladder
with inflammable air, began to breathe it, after having
fnade a, violent expiration. The firft infpiration pro-
duced a painful oppreffion on his lungs ; the fecond
caufed him to look pale ; and the third was fcarcely
accomplifhed, when he fell on his knees through
weaknefs. Small animals are alfo killed by a very
few infpirations of this noxious fluid. 7thly. This
gas is faid to have a fmaller fhare of refractive power
than common air.

It is on account of its lightnefs that hydrogen gas
has been moft frequently employed in aeroftation.
The method of filling a balloon is only enkrging
the procefs which has been defcribed for producing
inflammable air on a fmall fcale.

Very pleafing fireworks may be made from this
gas, by filling bladders with it, and fixing brafs
cocks to them, by means of which the gas may be
difperfed into any number of glafs tubes bent in va-
rious fhapes, and with fmall . holes in various parts
of them ; then by preffing the bladders more or lefs,
as occafion may require, the gas will pafs into the
tubes, and ifTue out of the fmall holes, to which a
lighted taper may be applied; by thefe means the
air will take fire, and will continue to burn until the
courfe of it is Hopped by Ihutting the cock at the
neck of the bladder. Thefe aerial fireworks may be
made to reprefent different figures, either movable
or immovable, and may be ornamented with different
colours. The white coloured flame is produced by
hydrogen gas procured from common coal; again,

by

396 Pbilofopbtial Fireworks. [Book V.

by mixing an equal quantity of this air with atmofpheric
air, a flame of a blue colour will be produced ; the
pure hydrogen from metals furnifties a red flame ;
and if by breathing, fome carbonic acid gas or fixable
air is added, the flame will appear beautifully tinged

with purple *.

•
* See BriUbn, torn. ii. p. 81.

Chap. 6.] [ 397 ]

CHAP. VI.
NITROUS AIR OR GAS,

Nature of this Fluid. — How produced. — Its Properties.— Rejijis Putrf-
fatlion. — Abforls and condenfes pure Air. — The Eudiometer.

NITROUS gas ought properly to be confidered
as an intermediate ftate of that elementary fub-
flance which is the bafis both of azotic gas and nitrous
acid. Azote, perfectly faturated with oxygen, forms
pale nitrous acid ; with a fmaller portion, it conftitutes
the ordinary orange-coloured and fuming nitrous acid;
with ftill lefs, it becomes nitrous gas j and when wholly
uncombined with oxygen, is denominated azotic gas.
In the ftate of azotic gas it is infoluble j but in pro-
portion to the quantity of oxygen with which it is
combined, its difpofition to afTume an aeriform ftate is
diminifhed, and its attra&ipn for water increafed.
* In order to produce nitrous air, put copper, brafs, or
mercury, firft into the bottle (with the fame apparatus
as for the other airs) fo as to fill about one-third of it,
then pour a quantity of water into it, fo as juft to cover
the metal filings ; and, laftly, add the nitrous acid, in
quantity about one half or one-third, according to the
ftrength which is required. Nitrous air contains, in
100 grains, 68 of oxygen, and 32 of azote.

On its relation to the nitrous acid the diftinguifliing
properties of this gas will be found to depend.

i ft. Nitrous air is as invifible and tranfparent as com-
mon air j in its fmell it refembles nitrous acid. Though

this

398 General Properties [Book V.

this kind of air extinguishes flame, it may, by certain
procefies, be brought to fuch a ftate. that a candle will
burn in it with an enlarged flame, and it then becomes
\vhat Dr. Prieftley calls drpblogifticated nitrous air. Its
fupporting flame in this inftance evidently depends on
the large quantity of oxygen which enters into its com-
pofition.

id. When oxygen or empyreal air is added to ni-
trous air, it hnparts to it the acid character, and it be-
comes true nitrous acid. Mr. Cavendifh impregnated
fifty ounces of diftilled water with fifty-two ounce mea-
fures of nitrous air, mixed with as much common air
as was neceffary to decompound it. The water thus
impregnated was fenfibly acid, and being diftilled, the
firft runnings were very acid, and fraelt pungent: what
came next had no tafte or fmell > but the laft runnings
were very weak nitrous acid *.

3d. Of all the different fpecies of air, this feems the
moft noxious to animal life. Infects, which can bear
azotic and inflammable air, will die immediately upon
their being immerfed in this. Even fiihes will not
live in water impregnated with it.

It may fccm extraordinary that nitrous gas, which is
of fo deleterious a nature, and fo oppofite in its qualified
to common air, fhould yet fubftantially confift of the
fame principles, differing, however, in the proportions.
To remove the difficulty, it will be necefiary to recol-
lect what has been more than once intimated concern-
ing the difference between mixtxre and combination. In
fimple mixture the two bodies dill retain their own-
diftinct properties •> but in chemical combination a third
fubftance is formed from the two, entirely different
from both in its nature and properties. Thus, from

• Phil. Tranf. for 1784*

marine

Chap. 6.] of Nitrous Air.

marine acid or fpirit of fait, and cauftic alkali, both ex-
tremely corrofive, is formed that innocent and whole-
fome fubftance, common fait; and from two fubftances
innoxious to the human frame, fulphur and oxygen,
vitriolic acid, or fpirit of vitriol, is formed. In com-
mon air, azote and oxygen are indeed in a ftate of mix-
ture, but they are not combined; for to make them
enter into a ftate of combination, the operation of a
flrong agent, fuch as fire from the electric fpark, is
neceffcry, and without this, azote appears to have little
or no attraction for oxygen. In the ordinary procefs
of refpiration the mixed fubftances are inhaled; and
it is probable that they are foon again feparated in this
procefs, and each differently difpofed of. In nitrous
gas, the azote and oxygen are in a ftate of chemical
combination, and it is a third fubftance different in
qualities from both ; it is, indeed, an imperfect nitrous
acid in an aerial form ; though azote, therefore, in its
fimple uncombined ftate, has no attraction for oxygen,
it is different when by combination it becomes an acid ;
it has then a ftrong attraction for that fubftance which
is ncceflary to give it the true acid character, and it will
abforb it till it arrives at what the chemifts call the
point of faturation, that is, till it is made a perfect acid -9
and this is the reafon that it fo rapidly attracts and con-
denfes the pure air of the atmofphere.

4th. Nitrous air pofTcjfTes the property of preferving
animal fubftances from putrefaction, and of reftoring
thofe already .putrid, in a ftill greater degree than
fixed air, and on this the antifeptic power of nitre, may,
perhaps, chiefly depend. On putting two mice, the
one juft killed, the other putrid and foft, into a jar of
nitrous air, and letting them continue in it twenty-five
days, in the months of July and Auguft, there was little
or no change in the quantity of. air; both mice were

perfectly

400 Eudiometer. < [Book V.

perfectly fwect; the firft quite firm, the flefh of the
fecond flill foft, but not in the leaft putrid. From thefe
experiments Dr. Prieftley recommends nitrous air as
an antifeptic. Unfortunately, however, though ani-
mal fubftances may be preferved from putrefaction for
feveral months by nitrous gas, yet they become dry,
diftorted, and ofTenfive to the palate, fo as to render
the difcovery of little public utility.

5th. The fpecific gravity of nitrous air is to that of
the atmofphereas 1195^0 1000.

6th. One of the moft remarkable properties of this
air is, that it condenfes or dlminifhcs in bulk with oxy-
gen or dephlogifticated air, by which means it becomes
a teft with rcfpect to the quantity of that pure element
contained in the atmofphere. With pure dephlogifti-
cated air the diminution is almoft to nothing, at the
fame time that nitrous acid in fome quantity is repro-
duced by the condenfation of the nitrous air j but as the
air of our atmofphere is always mixed with a confider-
able quantity of azotic or phlogifticated air, on which
nitrous air has no effect, the diminution in this cafe is
never fo confiderable. Upon this principle the eudio-
meter for meafuring the purity of air is formed.

To underfland the nature of this inflrument, let a
glafs tube (Fig. 4.) of about nine inches long, clofed at
One end, and of about three-fourths of an inch diameter,
be filled with and inverted in water; then take a phial
of about half an ounce meafure, filled with common air>
and plunging it under the water contained in the fame
bafon with the inverted tube, let that quantity of air
enter into the tube j it will then rife to the top of the
tube while the water fublldes. Let a mark be made
on the tube at the height of the water in it, to (how
how much of the tube is filled by that meafure of air.
In the fame manner inject four or five me^fures of com-
mon.

Chap. 6.]' Eudiometer. 401

mon air, marking the height of the water at every one
refpeclively. After this procefs, if three meafures of
either nitrous or common air are introduced into the
tube, they will caufe the water to fubfide to the third
mark ; but if two meafures of common air and one
meafure of nitrous air, or one meafure of the common
and two of the nitrous air, are put into the tube, they
will fill a fpace much fhort of the third mark. When
thele two kinds of air come firft in contact, a reddifh
appearance is perceived, which foon vanifhes, and the
water, which at firft nearly reached the third mark,
rifes gradually into the tube, and becomes nearly fta-^
tionary after about two or three minutes, by which it
appears, that the diminution takes place in a gradual
way.

Nitrous air is neither foluble in water nor pofTefles
any figns of acidity ; for it has not the power of chang-
ing the blue colour of vegetables red, unlefs it is mixed
with common or dephlogifticated air, by which it ac-
quires the true acid character.

VOL. I. D d

[ 402 ] [Book V.

CHAP. VII.

Of HEPATIC GAS.

Mature of this Gas. — Means of producing it. — Its Properties. — A chief
Conftituent of Sulphureous Mineral Waters.— Turns Metals black.—
Hoiv decompofea1, &c.

MGengembre, who has made an analyfis of this
• kind of air, regards it as a combination of pure
hydrogen and fulphur. The moft proper method of
obtaining it is by pouring marine acid on liver of
fulphur *, which extricates it in confiderable quantities.
It is equally produced from all livers of fulphur, whe-
ther they are made with alkalis or earths. By various
experiments, however, it now feems to be afcertained,
that as hepatic gas is compofed of fulphur and hydro-
gen in certain proportions, it cannot be produced ex-
cept water is prefent, the decompofition of which af-
fords the hydrogen. Thus, if marine acid air is ap-
plied to very dry liver of fulphur, fcarcely any hepatic
gas is produced, from the defecl of humidity. Liver
of fulphur, when heated, affords hepatic gas with the
addition of mere water without acid. In this cafe alfo
the water is decompofed -, its hydrogen unites with part
of the fulphur to form hepatic gas, while the oxygen
of the water uniting with another part, produces vi-
triolic acid, and this with the alkali forms a neutral fait
which will be defcribed in treating of vitriolic falts.

» A fubltance ufually formed frojn fixed alkali, or fait of tartar,
ind fulphur, combined by heat. •.

Ift. He-

Chap. 7.] General Properties cfHepctic Gas. 403

i ft. Hepatic gas is very foluble in water, which ic
converts into a ftate perfectly refembling that of ful-
phureous mineral waters, ad. It detonates with vital
air when fet on fire. 3d. It is <flot clearly afcertained
in what manner fulphur is fufpended in hepatic gas.
Sulphur melted by a burning glafi, in inflammable air
over mercury, produces a fluid which has the proper-
ties of hepatic gas ; and if inflammable air is paflM
through fulphur in fufion it is converted into hepatic
gas. 4th. The fmellof this air is very unpleafant, and
its vapour has a very difagreeable effect upon many
metallic fubftances, particularly filver, lead, copper,
&c. deftroying their colour, and rendering them al-
moft black. 5th. It is extremely pernicious in refpi-
ration. 6th. It may be decompofed by vitriolic and
nitrous air, by vital air, and by the contaft of atmo-
fpheric air, in which cafe it depofits fome fulphur. Its
great attraction for fome of the metals and their calces,
makes it the bafis of fome fympathetic inks.

The volatile alkali, and moft of the acids, may be
made to afiume an aerial form, and have been diftin-
guifhed under the appellation of alkaline and acid airs;
it is unnecefiary, however, to introduce the fubjedl in
this place, and it will be better underftood when the
acids and alkalies are treated of, as they will be in the
fucceedins; book.

Dd 2

[ 404 ] [BookV.

CH*P. VIIL

OF ATMOSPHERIC AIR.

Atmbfpbtrt compofed chiefly of Two Kinds of Air : — Contains alfo Fixed

• Air, and occajionally ether Subflances» — Effefls of this Mixiurc on

Metals and Purple Dyes. — Means of purifying the Atmofphere from

fixed Air, and putrid Vapours. —Effects of Moijiure contained in

Air. — The Hydrometer. — Cold in the higher Regions cf the Atmt*

fphere. ~-Caufe.

WHATEVER has been hitherto dated relative
to the different fpecies of claflic fluids is chiefly
important, becaufe the knowledge of thcfe fluids is
neceflary to enable us to comprehend the nature of
that atmofphere in which we exift, and which is in-
deed of itfelf one of the principal agents of our exift-
cnce.

In treating feparately of the different kinds of air, it
was neceflary in fome meafure to anticipate the. prefer^
fubjecl:, and to intimate that the air of our atmofphere
is not, as was formerly fuppofed, a fimple homogenous
fluid, but that in reality it is compofed of two different
fluids, which have been defcribed under the appellations
of azotic and oxygen gas, or phlogifticated and vital air.

In one hundred parts of atmofpheric air there are
contained about feventy-two parts of azotic gas to
twenty-feven of oxygen, befides one part of carbonic
acid gas1 or fixed air, which is generally found united
with them, or to fpeak in round numbers, in order to
.be better underftood, we may fay that the air of our at-
mofphere contains rather better than one-fourth of pure

or

Chap. 8.] Compcfition of Atmofpberic Air. 405

or refpirable air, and that the remaining three-fourths
are unfit for refpiration, and equally unfit for combu£-
tion, fincethe fame fluid which iupports flame is found
equally to contribute to the fupport of animal life.

By the gradual introduction of nitrous air into a clofe
veffel filled with atmospheric air, it will be found that
about a fourth part of the whole bulk of the air will
disappear; the fame quantity is in effedt deftroyed by
the combn tion of any inflammable fubftance, and the
combuilic-i gradually ceafes in proportion as that fluid
is dinr.-in-ird which is necefTary to its fupport.

The f?.rne quantity is deftroyed by the procefs of re-
ipirarion. SPutrefacUon alfo feparaues the. pure airj
and the power of (epamting, and alfo of reuniting the
two fluids, which 'aft m?.y be done, when both are pro-
duced by artificial means, very fj.fl' iently proves them
4diflin6t in their nature and properties, and alfo that
they are united in the air of our atmofphere.

Azotic gas being fpedfieally lighter than oxygen, it
might naturally be fuppofed, that fince they only exift
in the atmofphere in a mixed ftate, and not in a (late of
chemical combination, a fpontaneous feparation would
take place, and that the azote would occupy the higher
regions of the atmofphere; whereas it is found by ex-
periments with the eudiometer, that the upper regions
of the air actually contain a greater proportion of oxy-
gen than thofe nearer the furface of the earth. Whether
this is to be attributed to the attraction which azote
may have for the earth, or to fome unknown property
in the oxygen, we cannot now determine, and can
only take the fact as it (lands, without attempting its
explanation *.

* A mixture of empyreal and inflammable airs (the latter of
which is much lighter than the former) remaining all night, was
found the next morning in the molt perfeft ftate of mixture,
and the eleftric fpark puffed through them with the ufual effeft.
eriments , vol. vi. p. 2.7.

D d 3 From

406 More Vital Air m Summer than Winter. [Book V.

From the great confumption of oxygen by various
natural and artificial procef es, it might be expected
that a deficiency of this fluid in the atmofphere might
fometimes occur ; but the wifdom of Providence is
evident in this, as well as in every other inftance ; for
we have already feen that the proceiTes in nature which
deftroy this air are nearly balanced by thofe which
produce it. A feries of experiments were made at
Stockholm by the indefatigable Scheele, to afcertain
the goodnefs of the air during every day in the courfe
of a year. He found that the diminution by the eudio-
meter never exceeded one-third, nor was lefs than eight
thirty-thirds. The quantity of vital air was lead in
March, November, and December, and in general lefs
in the winter than in the fummer months, which may
be attributed to the redundant fupply of this matter
by the copious vegetation which takes place at that
period. The air at fea is generally found in a purer
ftate than at inland places.

Extraordinary as this mixture of fluids in the at-
mofphere may appear, it is eiTential to' our health, and
even our exiftence, and demonftrates no lefs the wif-
dom and goodnefs of Providence, than all his other
beneficial appointments. This pure vital air, fays
Briton, fo wholefome, fo necelTary in a moderare quan-
tity, like fpirituous liquors, or falutary medicines, muft
be ufed with precaution, and would be fatal in the ex-
cefs. If we weve indeed to breath pure or oxygen
air without any mixture or alloy, we mould infallibly
perifli by the unnatural and fatal accumulation of heat
in our bodies •, ii, again, the whole atmofphere was
compoied only of vital air, combuftion would not pro-
9eed in that gradual and moderate manner which is
neceilary to the purpofes of life and of focietyj and
even iron, and the metals themfelves, would blaze with

a rapidity

Chap. 8.] Het erogenous Matters in Air. 407

a rapidity which would carry deftruction through the
whole expanfe of nature.

The air of our atmofphere is, however, notfo fimple
a fubftance as to be formed only of two ingred/ents.
Befides the fmall portion of carbonic gas or fixed air
which it contains, equal to one hundredth part, as was
intimated in the beginning of this chapter, it is alfo
well known that a large portion of water is ufually held
in the atmofphere, fometimes in a ftate of perfect fo-
lution, or entirely invifible, and fometimes vifible in
the form of mifts and clouds. The atmofphere is alfo
the general recipient of all thofe fubftances which are
fubject to evaporation, and which preferve their aeri-
form ftate under its ordinary heat and preffure.

From the mixed nature of the mafs, and particu-
larly from the mixture of carbonic acid gas or fixed air,
feveral effects are produced, and fome of them it may
be proper to notice. Fixed air being in reality an im-
perfect acid, contributes to ruft metals, and to change
the colour of fuch purple dyes as are produced from
vegetable fubftances. This is an effect which moft
perlons have noticed, though the caufe has not been
underftood \ and the delicate nature of thefe colours
has been almoft an invariable objection -a»ainft their
ufe.

The air of the atmofphere is moft generally injured
by the deft ruction of the pure part, and the generation
of carbonic acid gas, as in moft of the procefles of
combuftion, and in that of refpiration. When it is ne-
ceffary to purify the air from the carbonic acid, which
may be too abundant in it, any contrivance for bringing
it into contact with lime-water will fufficiently anlwer
this purpofe. A cloth dipped in that liquor, and fuf-
pended near the floor, will generally purify the air of
a room from any contamination of'fixed air.

D d 4 Combuftion

40$ Burial Places in great "Towns. [Book V.

Combuftion or refpiration are, however, not the only
means by which atmofpheric air is injured. Phofphorus
of every kind, liver of fulphur, oil of turpentine,
cements of wax, oils of mint, cinnamon, &c. nitrous
acid, and even nitrous aether, at once diminifh and de-
prave it.

The air is alfo rendered unwholefome . by the ab-
forption of putrid * or inflammable vapours, the ex-
plofion of gunpowders by oil paints, by the volatile
fpirit of fal ammoniac, by fpirit of wine, by every
kincl of perfumery or artificial fcents, by the vapour of
new plaiftered walls, by 'all putrid fubftances, and ef-
pecially by ftagnate water j thcfe fubftances all diffufe
a quantity of mephitic air or vapour through the
furrounding atrnofphere, and fome of them confume
the pure or vital part. Even the vapour of pure wa-
ter in confiderable quantities is pernicious to animal
lifej Mufchenbroek obierved, that it threw a bird
into great anxiety; that the vapour of vinegar had a
fimilar effect ; that the vapour of fpirit of wine killed
a b}rd j and that feveral others were fatal to life f.

From thefe fads it is manifefl that the burying of
the dead in populous towns is a wretched and danger,
rous mockery of police. I know a certain great town
where, in burial places in the very middle of the town,
the dead are buried not fix inches below the furface j
and in London, notwithstanding the ad: of parliament,

* A quantity of corrupted fifti were once thought to ha\'e oc-
cafioned a violent epidemic fever at Venice. The fame effeft was
produced at Delft, by the corruption of vegetables. The Arabs,
when defiroas of injuring the Turks at 6a flora, break down the
banks of the river near that city, fo as to permit it to overflow a
great traft of land ; a violent fever is generally the cor.icquer.ee
of the putrid mud, &c. which is left behind after the water is
evaporated.— Cav. on Air, 457.

•j- £avallo on Air, p. 447,

\vhac

Chap. 8.] Means of purifying Air. 409

what with the prefent evafion of that act, the depofit-
ing in vaults, and the frequent breaking up of the
ground, and removing putrid bodies, the cafe is not
much better ; and indeed much might yet be done to
render the air of London more falubrious than it is.

I have taken no notice of the accounts which fome
ingenious men have afforded us of the falubrity of the
air in different places, convinced that we are not as
yet pofleffed of a complete teft of the falubrity of air;
and till this can be procured our only guide muft be
experience.

By agitating putrid and inflammable air in diftilled
water, or water from which the air has been expelled
by boiling, a considerable diminution will take place,
fometimes above a third of the bulk, and the air will
be conficlerably purified. Thus the agitation of the
fea, and of large lakes, has probably the happieft effect
in purifying the atmofphere.

.Dr. Hales found that air might be breathed much
longer, when in the act of refpiration it was made to
pafs through feveral folds of cloth dipped in vinegar,
a folution of fea fait, or oil of tartar, than when no
fuch contrivance was ufed * j the reafon of which is
briefly, that thefe fubftances abforb the fixable air which
comes from the lungs.

Putrid air was alfo reftored by a mixture of car-
bonic acid air. The experiment was, however, in
fome rrieafure rendered doubtful by the air having been
palled through a veflel of water in order to its admix-
ture. If, however, the fact is well founded; lime kilns
in the vicinity of populous cities may poflibly not be
fo unwholefome as is generally imagined, as in thole
places the putrid air and vapours abound more than
jhe carbonic acid.

» Kale's Eff. p. 266,

The

4io Why mo'ift Air gives the So/alien of Cold. [Book V.

The eudiometer is a good teft of air as far as re-
gards the diminution of the oxygenous part} but it is
on the whole an imperfect inftrtiment, as it affords no
means of diftinguilhing the deleterious vapours with
which the atmofphere may occafionally be charged.

Befides their deleterious properties, the mixture of
watery particles and vapour in air has alfo confiderable
tffect with refpeft to its power of conducting heat
from our bodies. The rarity or denfity of air feems
to have little effect with refpect to its conducting
power, which indeed appears entirely to depend on
the quantity of moifture it contains. A moift air con-
ducts heat with much greater rapidity than a dry air.
Whence (fays the ingenious Count Rumford) c I can-
not help obferving with what infinite wifdom and
goodnefs, Divine Providence appears to have guarded
us againft the evil effects of exceftive heat and cold in
the atmofphere j for were it pofTible for the air to be
equally damp during the fevere cold of the winter
months, as it fometimes is in fummer, its conducting
power, and confequently its apparent coldnefs, when
applied to our bodies, would be fo much increafed by
fuch an additional degree of moidure, that it would be-
come quite intolerable i but, happily for us, its power
to hold water in folution is diminifhed, and with it its
power to rob us of our animal heat, in proportion as its
coldnefs is increafed. Every body knows how very
difagreeable a very moderate degree of cold is when
k is very damp; and hence ic appears, why the ther-
mometer is not always a juft meafure of the apparent
or ienfible heat of the atmofphere. If colds or ca-
tarrhs are occafioned by our bodies being robbed of
our animal heat, the reafgn is plain why thofe diforders
prevail moft during the cold autumnal rains, and upon
the breaking up of froft in the Ipring. It is like wife
5 plain,

Chap. 8.] Reafon of Autumnal Colds, &c. 411

plain, whence it is that fleeping in damp beds and in-
habiting damp houfes is fo very dangerous, and why
the evening air isfo pernicious in fummer and autumn,
and why it is not fo in the hard frofts of winter. It
has puzzled many to account for the manner in which
fuch an extraordinary degree, or rather quantity of
heat is generated, which an animal body is fuppofed to
lofe if expofed to the cold of winter, which it com-
municates to the furrounding atmofphere in warm
fummer weather; but is it not more than probable,
that the difference of the quantities of heat actually
loft or confumed, is infinitely lefs than what they
have imagined*?'

Various inflruments have been invented under the
general name of hygrometers f, for afcertaining the
quantity of moifture contained in the atmofphere.
Mod bodies attract moifture, and are expanded by it.
Wood and other folid bodies are fwelled by the moif-
ture infmuating itfelf between the fibres, and confe-
quently a piece of wood cut tranfverfely, will be ex-
tended in length by the abforption of damp or wet.
Cord, catgut, &c. the fibres of which extend longitu-
dinally, will increafe in thicknefs, but will contract in
length on the application of moifture. On this laft
principle the common weather-houfe is conftructed,
which is no bad hygrometer for general purpofes ; the
contraction of the itring by wet forces the man out of
the door, and when by the return of fine weather the
ftring or catgut is difpofed to re fume its natural length,
an elaftic wire acts upon it, and the worn. in appears.
The firft regular and graduated hygrometer that de-
ferves to be mentioned, as made in this country, was

* Thompfon's Experiments. Phil. Tranf. Vol. Ixxvi.
f 'Yyoj, (hygros) "moifture;" and Mfyor, (metron) "amea-
frre."

that

=r. [Book V.

th.ic of the late ingenious Mr. Smeaton. He em-
ployed 5n its conftrucYion a flaxen firing of three
omrr.or.ly ufed in making nets, which, in or-
der so make it attract the 'moifture readily, he fteepecl
in- fait and water. This he extended along a board
properly </ and to one part contrived to at-

tach an index, which ferved to fhew its variations.
M. Sauffure employed a hair for the fame purpofe,
which he fufpexided by a weight of three grains, and
contrived it to a^l upon an index pointing to a gra-
duated fcale ; and this was found to be a very delicate
and accurate inftrumenr.

M. de Luc, however, conceived that a folid body
would afford the rooff: accurate and fteady meafure of
damp or drynefs, and was'lefs likely to be out of or-
der than fibrous and twifted fubftances. He fuccef-
fively employed ivory, box- wood, and whalebone; but
after feveral trials preferred the latter, becanfe of its
great power of expanfion, which fometimes exceeded1
one-eighth of its length, and becanfe of its ' fteadincfs,
in always coming to the fame point in extreme moif-
ture. His hygrometer therefore eonfifts of a very thin
flip of whalebone, about 11 inches in length, and a line
in breadth, cut tranfverfely to the direction of the fibres.
This he extends by a fmall fpring, and the -variations
may be either meafured by a mark on the whalebone
qr by an index. This is at prefent the mod perfect
hygrometer, and that which is in mo-ft general ufe.

In the higher regions of the atmofphere the cold is
found to be intenfr, and yet the moitture is generally
faid to be lefs abundant, than in thole nearer the fur-
face of the earth. This was experienced by all the
adventurers in air balloons, as well as by thofe tra-
vellers who have afcended to the tops of high moun-
tains. It is well known that the fummits of the alps

and

,Chap. 8.] Cold in toe higbsr Rgglons of Air. 413

and other great elevations are ufually covered with,
ihow. There is indeed always a certain height of the
atmofphere where water will be found at the freezing
point, and this has been called by philofophers the
line of perpetual fhow. The line, however, varies
according to -climate and circumftances. On the peak
of Tenerif it commences at the height of about two
miles and a half, and in England it is generally found
at the height of a mile, or a mile and a half. Some
botanifts have afferted, that the variation of climate in
afcending mountains, was difcernible from- the vege-
tables found upon them, the plants which required a
mild temperature being commonly found near the
bottoms, and the hardier and more northern vegetables
towards the fummit.

Different opinions have been entertained concerning
the cold in elevated filiations. It was for a confider-
able time imagined, that it depended altogether on the
rarity of the atmofphere in thofe regions, which is very
confiderable ; but it has been remarked, on the autho-
rity of Count Rumford, that the rarity or denfity of the
air appears to have little effect on its conducting power.
Some have fuppofcd, that as the air is (b much rarer
in the upper regions, lefs fire or caloric is required to
keep it in a ftate of fttii'.uty, and confequently that there
is a real deficiency of that element. The hypothefls
of M. Bouguer *, however, comes recommended by
its fimpficity, and by its agreement with mod of the
other phenomena of heat, and I mail therefore adopt
it with only ibrne fli'ght variations. t

Without entering into the controverfy concerning
the identity of fire and light, it is only neceffary to
aflume as a principle the well known circumftance,

• Reafons for the cold on the top of the Andes.

that

414 Cold in the higher [Book V.

that the a<5tion of light is capable of producing heat in
bodies ; and the equally well eftablimed fact, that the
action of light upon a tranfparent medium, through
which it is eafily tranfmitted, is extremely feeble.
This may be proved by the eafieft of all poffible ex-
periments. If highly rectified fpirit is inclofed in a
pure glafs phial, or any perfectly tranfparent veffel, the
rays of light concentrated by the moft powerful burn-
ing glafs, will not inflame it ; if, however, the fpirit
is placed in a fpoon, or if that part of the tranfpa-
rent veffel which is not oppofed to the burning glafs,
is coated with paint or any fubftance, which intercepts
die rays of light, imbibes or condenfes them, the fpi-
rit will be inftandy fet in a blaze. The earth is there-
fore the great receptacle of heat, where it is abfdrbed
and kept as in aftore-houfe; but the furface of earth
•which is expofed to the fun on the tops of mountains
is but very fmall, and cannot imbibe much of the fun's
heat. The rays from the fun can indeed only ftrike
the different fides of the mountain for a fhort period
in every day, and in fome days and in fome parts, not
at all. " A horizontal plain alfo when the day is clear,
is expofed at mid-day to the perpendicular and undi-
minimed action of the fun's rays, while they fall ob-
liquely on a plain which is much inclined, or on a pile
of rocks." In theie elevated fituations, therefore, the
majority of the fun's rays pafs through a tranfparent
medium, as through the fpirit inclofed in a clear glafs
veflel j and there is a mafs of opake matter to collect or
condenfe the heat. The atmofphere, therefore, in
thofe regions is neceflarily colder than in thofe which
approach nearer the furface of the earth.

The cold in thefe higher regions of the atmofphere,
may be one caufe why vapours are "not collected fo
plentifully there as nearer the earth. The clouds are

feldom

Chap. 8.] Regions of the Atmofpherc. 415

feldom more than a mile in height, and they do not
often attain that degree of elevation. From the fum-
mit of a high mountain, therefore, the profpeft is in-
expreflibly grand. The clouds roll beneath the fpec-
tator's feet like the vaft waves of a troubled ocean;
and the forked lightnings play between thofe immenfe
mafies in various directions ; while the great body of
air in the vallies beneath (clear and tranfparent as it
appears to thofe who inhale it, but in reality charged
with vapours) appears like the water of a ftagnant
lake, involving mod of the objects in total darknefs,
and partially revealing others, which feem as if intend-
ed to adorn the margin of the flood, and ferve to en-
liven and diverfify the fcene.

416 ] [BookV.

CHAP. IX.

OF THE WEIGHT, ELASTICITY, AND OTHER
GENERAL PROPERTIES OF THE AIR.

General Properties of Air. — Caitfe of its Elajlicity, — (Opinions of the
Antients, — Torricellian Experiment. — Barometer.— The Air Pump.
— Weight and fpedjic Gravity of Air. — Immenfe Preffure of the
Attmfphere.—ComprfJpbility of Air.— Cupping GlaJJei. — Ejj'eds of
the Air's Elajlicity. -^- Air Guns, fcffc.- — Motion cf Particles in £o~
dies.-— Nature of the Atmofphere. — Its probable Height*

THE air, confidered as a fluid, without any re*
fpect to its component principles, has alfo fome
properties which are of the utmoft importance in the
lyftem of nature ; and the confideration of thefe pro-
perties will ferve to illuftrate and explain the nature of
all other elaftic fluids.

Atmofpheric air, confidered in itfelf, is a ponderous,
compreffible, elaftic, tranfparent body, without colour>
invifible, and incondenfable by any degree of cold that
can be produced in the temperature of this earth. It
never becomes the conftituent part of any body;
though it bafes, that is, oxygen and azote, enter into
the compofition of many.

The FLUIDITY of the air is caufed by the matter of
fire or heat, which produces in it a degree of elafticity
that always tends to dilate the mafs, and preferves
the motion of its parts. If the air was not elaftic,
it might be formed into a hard body, like fnow, when
its particles are prefled forcibly together.

It is eafy to prove that air adheres, with a confider-
able degree of force, to the furface of bodies ; for when

water

Chap. 9-] Vague Notions of the Ancients. 417

water is put into a vefTel and heated, the ftratum of
air which adheres to the fides of the vefTel, and which
occupies a fituation between the water and the fides,
foon becomes perceptible there in the form of bubbles,
in confequence of the rarefaction which is caufed by
the heat. It becomes .fenfible in the fame manner in
a vacuum, in confequence of the dilation occalioned by
the preiTure being removed *.

The ancients knew air to be a fluid, but their im-
perfect knowledge of thofe fubftances in general, ap-
pears to have difabled them from ufmg thofe means
which the moderns have employed for drawing off
and expelling this fluid from a certain fpace. They
were, indeed,, utterly unacquainted with the fact, that
air is a ponderous fluid. They admitted that there
were two kinds of bodies in nature; heavy bodies,
fiich as {tones, metals, and in general all bodies which,
being left to themfelves, had a propensity to defcend ;
and light bodies, fuch as air, flame, vapours, &c. be-
caufc thefe bodies appeared to them to afcend fponta-
neoufly into the upper regions of the atmofphere.
They fuppofed, therefore, agreeably to this fentiment,
that air was endued with abfolute levity; and that all
the effects which the moderns attribute to the principle
of gravitation, were to be afcribed to the horror which
nature had, according to them, for a 'vacuum. It was,
therefore, a long prevailing opinion, that air was defti-
t'.ite of weight : and it is not above a hundred and
fifcy years fince phjlofophers have been convinced of
this error. The engineers of the Count de Medici,
Great Duke of Florence, having received orders to
raife fome water fifty or fixty feet by means of a com-
mon -pump, perceived, when they made the attempt,

* Brifibn, Tom. ii, p. 93.
VOL. I. E e that

4i B Erroneous Nations concerning a Vacuum. [Book V.

that water would mount only to a certain height, after
which in appeared to them, by the void fpace which
they found, that nature was reconciled to a vacuum,
or at leaft fuffered this defect without thofe terrible
effects which ancient writers had predicted from it.
This -apparent caprice, on the part of nature, was com-
municated by the engineers to Galileo, who paid fome
attention to itj though, previous to this accident, he,
as well as all others, had fatisfied himfelf with the com-
mon opinion of the horror which nature was fuppofed
to entertain for a vacuum. He was at length con-
vinced, by reiterated experiments, that water would
rife only to about thirty- two feet perpendicular in
pumps, and that the remainder of the pipe or tube, if
it was longer, would be empty. He could then no
longer retain the opinion refpecting the horror of a
vacuum, but began to conceive that this horror had
its limits, and that thefe phenomena might proceed
from a phyfical caufe very different from that to which
they had hitherto been attributed. What he had fuf-
pected, Torricelli, his difciple, proved by direct ex-
periment. He firft made it appear in the year 1645,
that a column of air, as it exilts in the atmofphere,
may be placed in equipoife with a column of another
fluid, which has the fame bafe ; at length, to avoid the
inconvenience of a long pipe, inftead of water he made
ufe of mercury. He took a glafs tube (Fig. 5.) of
about three feet in length and two or three lines diame-
ter, hermetically fealed * at one end, and open at the
other; he filled it with pure mercury, and having
(topped the orifice with his finger, he reverted the tube,
and placed the open end in a veflel full of rhe fame
tfiercury. He had no fooner removed his finger, than
the column of mercury, which was about thirty-fix
* Clofed by melting the glals, and confolidating it.

inches

Chap. 9.} PafM's and Perrier* s Experiments. 419

inches long, was reduced to the length of about twenty-
eight inches. Now, if we compare the experiment of
Galileo with that of Torricelli, we {hall find that fluids
a<5t in counterpoife to each other, exactly in proportion
to their refpective denfities ;' and that the fame caufe
(the prefiure of the air) Which elevates water to the
height of thirty- two feet, cannot fuftain a column of
mercury above the height of twenty-eight or thirty
Inches.

Pafchal added confiderably to the proofs of this
doctrine which Torricelli had afforded, and he rea-
foned in this manner: — If, faid he, the air is the caufe
of this phenomenon, it is becaufe it has ponderance
and fluidity ; it muft prefs, therefore, in the fame man-
ner as liquids, and its preflure mud be greater or lefs
according to its heights and every column , of what-
ever fluid is placed in counterpoife with it, will always
be longer or fhorter in proportion to its denfity.
Hence he proceeded to prove, that a column of air
muft produce a preflure greater or lefs, and was ca-
pable of fuftaining a column of any fluid higher or
lower in proportion to its own height, and confequently
that a column of v/ater or mercury, at the bottom of
a mountain,. would rife higher in the Torricellian va-
cuum than at the fummit. M. Pafchal next pre-
vailed upon his brother-in-law, M. Perrier, who was
at Clermont in Auvergne, to make the following ex-
periment at the bafe and fummit of the mountain
known by the name of Puy de Dome.

M. Perrier fixed a tube of Torricelli's upon a perpen-
dicular plank (fee Plate XXIX. Fig. 5.) graduated into
inches and lines ; and having obferved te» what height the
mercury was raifed in the tube at the foot of the moun-
tain, he found that it fell gradually in proportion as he
E e 2 afcended

420 ffo Barometer. [Book V,

afcendcd towards the fummit j and alfo, on the contrary,
that it role again in the fame proportion as he defcend-
ed : the difference was found to be three inches and one
line between the height of the mercury at the fummit
and the bafe. This experiment, fuggeited by Pafchal,
and repeated feveral times, always produced the fame
refult; whence it was concluded, that mercury was
fuftained above its level in the Torricellian tube, by
the prefTure of the atmofphere upon the rdtrvoir,
fince the mercury in the tube was obferved to fall, when
the column- of air which had the refervoir for its bafe
was diminilhed in height. Thefe experiments, in
proving incontrovertibly the weight of air, have au-
thentically reftored to this fluid a great number of na-
tural properties and effects, which were before attri-
buted to a caufe merely chimerical.

M. Pafchal afterwards repeated the lame experi-
ment with water, wine, oil, &c. and the heights of the
columns of thefe liquors were always found to be
proportional to their denfities ; an evident proof that
they were counterpoifed by a weight, which could in
thofe cafes be no other than the preffure of the air.

Many philofophers afterwards, having procured
'Torricellian tubes, placed them according to the man-
ner of M. Perrier, upon a fcale graduated into inches
and lines, and by frequent obfervations they perceived,
that the height of the mercury in the tube often varied.
They concluded, therefore, that the preffure of the
air, which was the caufe of the iufpenfion of the co-
lumn of mercury, was fome times greater and fome-
times leis, and confequently that it acted more or Jefs
forcibly upon the human frame. From thefe caufcs
and effects the idea was fuggefted, of making from 'the
Torricellian tube a new meteorological inftrumcnt, the

, fame

Chap. 9.] The Barometer. 4*1

fame which is now commonly known by the name of
a barometer *.

Air acls upon barometers in two modes, by its
weight and by its elafticity. The variation, therefore,
of the preffure upon the refervoir is produced by two
caufes, by the variation in the weight of the incumbent
air, and by that of its elafticity. The weight of air
varies according to its denfity, and its intermixture
with other fubftances which are foluble by it j its elaf-
ticity varies according to its denfity, and the quantity
of heat with which it is charged. The greater pare
of foreign fubftances which intermix with air only,

* '' To fill a barometer tube, (fays Mr. Adams) I take a clean
gjafs tube about thirty-three inches long, and pour quickfilver into
it by means of a fmall paper funnel; you obfcrve, that as the
quickfilver rifes in the tube, there are bubbles of air left behind in
ieveral parts : I continue pouring the quicklilver till it fills the tube
within about half an ipch of the top. I then apply my finger hard
and clofe upon the top of the tube, and invert it; by which means
the air that was on the top, now rifing through all the quickfilver,
gathers every bubble in its way. I revert the tube or turn it up
again, and the babble of air re-afcends, and if there are any fmall
bubbles left, carries them away; if, however, any remain, the opera-
tion muft be repealed. I now fill the tube to the top, and placing-
my finger on the open end of the tube, plunge that end into this
bafon of quickfilver; when the end of the tube is perfectly fub-
rnerged in the qaickfilver, I take my finger away, and you fee the
quickfilver remains fufpended in the tube, leaving a vacuum at top.
The column of quickfilver is about thirty inches in height; now
you will obferve that there can be no air in the fpace between the
quickfilver and the top of t>2 tube, for till the finger that clofed
the orifice in the bafon was taken away, that fpace was filled with
quickfiiver, and the quickfilver, which was thirty-three inches high,
funk in the tube, and left that fpace free from air, for no air could
get into the tube, unlefs it could force its way through the quick-
lilver in the bafon, and the thirty inches in the tube; or penetrate
through the fealed end of the tube : but as neither of thofe can be
done, it follows, that in the part of the tube which the quickfilver
leaves, there muft be a vacuum." Adams' '; Lefiures. Vol. i, p. 32.

E e 3 under

Okfervations on Barometers. [Book V.

under the form of elaftic fluids, diminish the weight of
the column of air, becaufe they are lighter than it ; but
thofe fubftances which are foluble in air augment its
denfny, and confequently its weight, in the fame man-
ner that fah diflblved in water increafes its weight and
den!lty.

Tiie barometer has, therefore, another property, not
lefs ufeful to philofophers than that which has been
already mentioned. It points out the changes of the
weather, efpecially when they are likely to be confi-
derable.

From the numerous obfervations and experiments,
which have been made from time to time upon baro-
meters, the feveri following propofitions have been efta-
bli/hed by M. BriiTon. " Firft, That the mean height
of mercury in France is twenty-feven French inches
and an half. Secondly, That the variations from this
height feldom exceed three inches, that is, that its leaft
elevation is twenty- fix inches, and its greateft twenty-
nine. Thirdly, That thefe variations become lefs
towards the equator, and greater in the northern cli-
mates. Fourthly, That when the mercury falls in the
barometer it announces rain or wind, or in general what
is called bad weather. Fifthly, On the contrary, when
the mercury rifes it announces fine weather. Sixthly,
That thefe predictions fail fometimes, efpecially if the
variations in the height of the mercury are very flow
and inconfiderable. Seventhly, That the predictions
are almoft infallible, when the mercury afcendsor de-
fcends confiderably in a Ihort time j as for example,
about one-third of an inch (or three or four lines) ia
the courfe of a few hours *."

Thus in relating the difcovery of the barometer, we
have feen that philoiophers were convinced that an

* Brifibu. Vol. i.

aftual

Chap. 9.] Invention of the Air Pump. 423

actual vacuum might be formed. The air-pump,
however, was not difcovered till 1654. ' For the firft
invention of this, the world is indebted to Otto Gueric,
a German ; but it was our countryman Boyle who
converted it to real ufes ; it was he who improved it,
and applied it to philofophical purpofes. In the hands
of Gueric it was a mechanical inftrument j in thofe of
Boyle it was a truly philofophical machine. By this
machine we can with cafe empty a glafs veffcl of its
air, and put what bodies into it we think fit. Thus
comparing the changes wrought upon bodies by being
kept from air, with the fames bodies when expofed to
air, we require a knowledge of the effects of that fluid
upon bodies in general.

As the air-pump is a machine very generally known,
I mall not attempt a new defcription of itj but for the
lake of thofe readers who are but little acquainted with
philofophical apparatus, take the defcription of it from
one of the moft popular writers on thefe fubjects, in
order that its conftruction may be the more eafily un-
derftood.

Having put a wet leather on the plate L L (fee
Plate XXX. Fig. i.) place the large glafs veflel or re-
ceiver M with its mouth downwards upon the leather,
fo that the hole / in the plate may be within the glafs.
Then turning the handle F backward and forward, the
air will be pumped out of the receiver by the ac-
tion of the mechanifm below. As the handle F,
reprefented more at large (Fig. 2.) is turned back-
wards, it raifes the fucker or pifton de in the hollow
barrel B K by means of the toothed wheel E engrain-
ing in the toothed rack D d: and as the pifton or
fucker is leathered fo tight as to fit the barrel exactly,
no air can get between this pifton and the barrel, and
therefore all the air above d in the barrel is lifted up
E e ^. towards

4H Ctnftruftion cf the Air Pump. [Book V.

towards B, and a vacuum made in the barrel from
e to b :• upon which part of the air in the glafs M
(Fig. i.) by its fpring rufhes through the hole i in the
brafs plate L L along the pipe G G, which communi-
cates with both barrels by the hollow trunk I H K,
and pufhing up the valve b (a valve is a bit of leather
that covers a hole as the flapper of a bellows, admitting
the air in, but fuffering none to go back) the air then
raifing the valve enters into the vacuity b e of the bar-
rel B K. For the air will naturally prefs into thofe
places where it is leaft refifted. All this is done by
drawing the handle towards D. — Next turning the
handle forward the contrary way towards C> the pifton
de is depreffed in the barrel, and as the air which had
got into the barrel cannot be pulhed back through the
valve b, for the valve clofes like the flapper of a bel-
lows, and will not let the air back the way it came,
the air muft therefore afcend through an hole in the
piftonj and efcapes through a valve at d; and is hin-
dered by that valve from returning into the barrel
when the pifton is again raifed. At the next raifing
of the pifton, a vacuum is again made in the-fame
manner as before, between b and £, upon which more
of the air which was left in the glafs receiver M gets
cut thence> and runs into the more empty barrel B K
through the valve b. The fame thing is effected with
regard to the other barrel A I, and as the handle F is
turned backwards and forwards, ir alternately raifes
and deprefles the piftons in their barrels, always raifing
one while it depreffes the other. And as there is a
vacuum nude in each barrel when its pifton is raifed,
every particle of air irr the receiver M pufiies out ano-
ther through the hole i and pipe G G into the barrels,
until at Jail the air in the glafs receiver comes to bu
fo much rarefied tha: it can no longer get through

the

Chap. 9.] The Air Pump. 425

the valves, and then no more can be taken our of the
receiver. Hence it appears, that there is no fuch
thing as making a perfe<5t vacuum in the receiver ; for
the air that leaves the receiver is driven out by that
which remains behind, and there muft therefore fome
portion remain behind at laft.

Such is the conftruclion and nature of the air-pump.
Some inftruments at firft contrived only for explain-
ing fcience, become at laft, by freqtrent ufe, a part of
the fcience icfelf, and demand an equal explanation.
Such is the cafe with this; and the reader muft pardon
fome prolixity in the defcription. There is a cock k
below the plate LL, which being turned lets air into
the receiver again. There is a glafs tube Imn open
at both ends, and about thirty-four inches long, the
upper end communicating with a hole in the pump
plate, and the lower end immerled in quickfilver at n in
the veffe,! N. To this tube is fitted a wooden ruler
m m, divided into inches and parts of an inch from the
bottom at #, where it is upon a level with the furface of
the quickfilver, and continued up to the top, a little
below /, to thirty or thirty-one inches. Now the
quickfilver in this tube rifes as the air is exhaufted in
the receiver, for it opens into the receiver through the
plate L L. And the more the air is exhaufted, the
more will the quickfilver rife, fo that by this means
the quantity of air pumped out of the receiver may be
very exactly meafured *.

From all the preceding facts, and efpecially from
the experiment of Torricelli, it appears, that air is a
PONDEROUS fluid ; in other words, that it pofiefTes
gravity, and its weight may be eafily afcertained.

From a large phial (or rather from a flafk, or any
glafs veflel of a globular form, for reafons that will

* Goldfnjith's Philofophy, vol. ii. p, 56.

afterwards

426 Weight of Air. [Book V.

afterwards appear) to the neck of which is annexed a
itop-cock, the air may be exhaufted either by means
of the air-pump, or by filling the flafk with mer-
cury, and emptying it gradually into a veflel containing
utity of that Quid, and turning the cock before
the neck is entirely extricated, which produces a more
perfect vacuum than that made by the air-pump.
The veflel thus emptied of its air may be weighed by
a nice balance •, and this done, re-admit the air by
turning the cock, when it will rufh in with confider-
able violence j and though the fiafk was balanced be-
fore, it will now become heavier, and preponderate.
The air contained in a quart flafk will by this experi-
ment be found to weigh about fourteen grains and a
halt

To find the fpecific gravity of the air, the flafk muft
be Mired with pure water, and again weighed. The
weight of a cubic foot of pure diitilled water is about
ijOoo ounces avoirdupois, and of a cubic inch 253
grains and not quite one-fifth *. Dividing the weight
of the water contained in the flafk, therefore, by this
number of grains, will give the number of its cubic
inches ; and as this furnifhes us with the number of
cubic inches of air as well as of water, their relative
gravity is eaiiiy known. By feveral very accurate
experiments, Mr. Haukfbee fixed the fpecific gravity
of r.ir to that of water to be in proportion as i to 885.

By means of its gravity, the atmofphere preffes
with great force upon all bodies, according to the ex-
tent. of their furface. According to M. Pafchal, the
quantity of this pre fibre is not Ids than 2,232 pounds
upon every fquare foot of furface, or upwards of fif-

* 253-18 grains. Decimal arithmetic .fhould always be em-
ployed in philoibphical calculations, for the fuke of accuracy.

teen

Chap. 9.] Preffure of Air en tie Human Body. 427

teen pounds upon every fquare inch. Computing,
therefore, the furface of a man's body at 15 iquare
feet, the whole preflure, which each pcrfon fuftains,
will be nearly equal to 3 3,480 pounds. By this enor-
mous preflure we fliould undoubtedly be crufhed in a
moment, if every part of our body was not filled with
air, or fome other elaftic fluid, the fpring of which is
fufficient to counteract the preflure. — <c We are fear-
" fully and wonderfully made!"

The whole quantity of preflure upon the earth muft
thus be immenfe, and has been computed equal to
that of a globe of lead of fixty miles in diameter.

It is the gravity of the air which caufes that ftrong
preflure upon the hand, when it is placed upon the
mouth of a receiver open at the top, in which a va-
cuum has been made by an air-pump ; for as foon as
the air in the receiver has been* rarefied by the action
of the machine, it is no longer capable of fuftaining
the exterior preflure of the, air, as it would have been
if its denfity.had not been altered. It is, therefore,
the weight of the exterior air, which prefles the hand
with fuch force to the edges of the receiver; and this
preffure is according to the fize of the aperture of the
receiver, becaufe the column of air is enlarged in pro*
portion to the diameter of the aperture.

Our furprife is excited by obferving, that notwith-
ftanding this great prefiure upon a glafs receiver, when
a vacuum is obtained, the glafs is not darned to pieces
as might be expected. Its prefervation is in a great
meafure owing to the rotundity of its figure, and to
the excels of the exterior furface over the interior; for
the fubflance which compofes the body of the veflfel
refembles, in this cafe, the fubftance which compofes
an arch in a bridge. We may be convinced of the
trutlj of this obfervation by taking a receiver of ano-
ther

4:3 Benefits from tie Gravity of Air. [Bock V.

ther form, that is open at both ends, and covered with
a bladder at the top, and beginning to exhauft the air,
when the bladder will infallibly be burrt by the pief-
fure of the exteiior atmofpheric air.

This gravity of the atmofphere accomplices many
vft'ful purpofes in nature, fuch as preventing the blood
veffels of animals, and the fap veffels of plants, from
being too much diflended by the expanfive power,
which has a perpetual tendency to fwell them out.
On this account we fee, that in the operation of cup-
ping, where the preffure of the air is taken off from
a particular part, the expanfive force inftantly acts,
and fwells out the veiTcls to a great degree. This is
alfo the rcafon why the bodies of animals fwell when
they are put into an air-pump. It is owing to the
gravity ,of air that fubftances remain liquid, which
would become aeriform in vacuo. Salts and oils re-
main united in air, but feparate as foon as that fluid
is extracted. When hot water is put under an ex-
haufted receiver, it boils violently j becaufe the pref-
fure of the air being now taken off, there is nothing
to prevent it from affuming the (late of vapour.

4 This preflure of the armofphere,' fays Lavoifier,
f caufes- water to remain in a liquid (late till it is
raifed to 212° of Fahrenheit's thermometer, the quan-
tity of heat which it receives in a lower temperature
being inefficient to overcome the preffure of the at-
mofphere. Whence it appears, that without this pref-
fure we fhould not have any permanent liquid, ancj
fiiouki only be able to fee bodies in that (late of exif-
tence in the very inflant of melting, as the (mailed
additional heat would inftantly feparate their particles,
and diflipate them through the furrounding medium.
Befides, without this atmofpheric preffure, we mould
not even have any aeriform fluids, (Iridly (peaking,

becaufe

Chap. 9-] Compreffibiiity of Air. 429

becaufe the moment the force of attraction is over-
come by the repulfive power of the heat, the particles
would feparate themfelves indefinitely, having nothing
to limit their expanfion, unlefs their own gravity might
collect them together, fo as to form an atmolphere *.

Air, being an elaftic fluid, is confequently COM-
PRESSIBLE, as the very word implies; the weight,
therefore, of the atmofphsre compreflcs its lower
parts, for in low vallies it is more comprefTed, and
has more denfity than upon high mountains ; but this
is not the cafe with water, which not being elaflic in
its ordinary ftate, is hardly compreffible at all; fo that
the different portions of the fame mafs of water have
nearly the fame denfity throughout its whole depth.

IV! . Amontons contends, that there is no fixing any
bounds to the condenfation of air. Dr. Hal ley has
afierted, in the Philofophical Tranfactions, that, from
the experiments made at London and Florence, it
might be fafejy concluded, that no force whatever is
able to reduce air into 8co times lefs fpace than that
which it naturally poffefles on the furface of our
earth.

It has been proved by various experiments, that a
column of cotnprefied air is diminifhed in proportion
to the augmentation of the preflure by which it is
condenfcd.

The fimplcfb of thefe experiments is, to pour a
quantity of 'quickfilver into the 'tube ABC (Plate
XXIX. Fig. 6.) clcfed at A, and open at C. When
the tube is filled with quickfilver to E, then the air.
inclofed in the leg A B, will prevent its rifmg higher
than D, and the column D B will be in equilibrium with
FB ; confequentiy the quickfilver contained between
FD will not at all prefs on the air between A and D ;

* 'Lavoificr's Elem. of Chemift. p. 8.

but

43 o Nofuch Thing as SuSlion. [Book V*

but the column E F, acting with its whole weight on
the quickfilver between F and D, caufcs it to prefs on
the air at D, and condenfe it. By increafing the
quantity of quickfilver the condenfation is increafed }
and it is found, that the {paces into which the air is
condenfed by different weights, bear a regular pro-
portion to thofe weights, and its denfity is confequently
increafed in proportion to the degree of prefTure exerted
upon it.

It is, however, very probable, after all, that this
comprefllon has its limits, for we know of no body
which can be comprefled ad infinitum.

From all that has been ftated, and particularly from
the experiment of Torricelli, it will be evident, that
the common notion of faction is a vulgar error ; and
that when a fluid rufhes fpontaneoufly into a given
fpace, it is in confequence of the air being expelled, or
made thinner in that fpace, than in that which is con-
tiguous to it, when the preflure of the atmofphere acts
upon the fluid, and forces it to occupy the fpace from
which the air is either entirely or partially expelled.
This is the principle on which the common pump is
conftructed ; for a vacuum being made in the tube by
the rifing of the pifton, the weight of the atmofphere
prefies upon the circumadjacent water, and forces it
up into the body of the pump : but this engine will
be more particularly defcribed in treating of hy-
draulics, r

The elafticity of the air is now generally allowed to
depend upon the latent caloric or fire, which retains it
in its fluid form. If we take a bladder well clofed at
the neck, and containing but a fmall quantity of air;
while this bladder is expofed to the prefiure of the at-
mofphere it will remain in its primitive ftate, as when
the fmall quantity of air was admitted -t but if it is
i placed

Chap. 9-] Llaflidiy of Air. 43 1

placed under the receiver of an air-pump, and rhe ma-
chine fet in rnocion to exhault the air furrounuing the
bladder, it will begin to open and fwell, and tint in
proportion to the diminution of the deniity of the air
in the receiver *.

Philosophers have doubted whether this elaftic power
of the air is capable of being deftroyed or diminiflied.
Mr. Boyle endeavoured to difcover how long air
would retain its fpring after having aflumed the greareft
degree of expanfion his air-pump could give it ; but he
never obferved any fenfible diminution. Defaguliers
fays, that air, which had been enclofed half a year iii
a wind-gun, had loft none of its elafticity ; and Rober-
val afferts, that he has preferved air in the fame man-
ner for fixteen years j and that after that period he ob-
ferved, that its expanfive projectile force was the fame
as if it had been newly condenfed. Dr. Hales and
Mr. Haukfoee on the contrary conclude, from other
experiments, that the elafticity of the air is capable of
being impaired and diminiflied by a variety of caufes.
M. Lavoifier, however, has folved thefe difficulties,
by proving, that the elafticity of all gaffes or elaftic
fluids depends upon that of caloric, which feems to
be the moft eminently elaftic body in nature. No-
thing, fays he, is more readily conceived, than that one
body fliould become elaftic by entering into combina-
tion with another body porTefied of that quality. £ We
muft allow that this is only an explanation of elafti-
city, by an afTumption of elafticity; and that we thus
only remove the difficulty one ftep farther j and that
the nature of elafticity, and the reafon for caloric being
elaftic, remain ftill unexplained f.'

On the elafticity and compreflibility of air depend
the ftructure and ufes of the air-gun. In thefe inftru-

* Briflbn, torn. ii. p. 103. f Elem. of Chem. p. 22.

MCflO

43 2 2"£* Ar Can. [Book V.

ments a quantity of air is condenfed by various con-
trivances, in fuch a manner that the condcnfmg force
being removed, a bullet will be fent to a confiderable
dillance with little or no noife, but with great force.
' The common air-gun is made of brafs, and has two
barrels. The middle barrel K A (fee Plate XXX.
Fig. 3.) from which the bullets are {hot, and the larger
outfide barrel, clofed up at the end C D, and in this
the air is driven and kept condenfed, by means of a
iyringe M, which drives the air in, but fuffers none to
go back. This fyringe having been worked for fome
time, the air is accumulated in great quantities in the
external barrel, and this air may be made to ftrike
upon the ball K by means of the trigger O, which
pulls back the fpiral R, and this ipiral opens a valve
behind the ball. When the valve is open, the air
condenfed in the outward barrel rufhes in behind the
ball, and drives it out with great violence, fo great,
that at twenty-fix yards diilance it would drive through
an oak board half an inch thick. If the valve behind
K is iliut fuddenly,-one charge of condenfed air may
make feveral difcharges of bullets. The little pellet
guns, in the hands of children, Ihew alfo the force
and fpring of the air; for one pellet ftopping the
mouth of the gun at one end, and another being driven
in at the oppofire end, the air contained in the bore
of the gun between each pellet is continually condenf-
i:ig, as the hinder pellet is driven towards the foremofl,
till at lad the fpring becomes fo great as to drive the
foremoft pellet forward with fome noife and violence.
In the large air-gun, however, the noife is by no
means fo great : upon its difcharge nothing is heard
but a fort of a rulhing wind ; and it is very poflible,
that what we are vulgarly told of fome men killing
others, by loading their piftols with dumb powder,

might

Chap; 9.] ?he Air Gun. 433

might have proceeded from the Client effects of the
air-gun *.'

The air gun defcribed by the author from whom
the above is quoted has been in a great meafure
fuperfeded by one of a more fimple conftruction, ori-
ginally invented by the late ingenious Benjamin Mar-
tin. It is formed like a common gun, with a fingle
barrel, and the condenfed air is contained in a brafs
ball, which fcrews on below the lock. The ball is
charged with a ftrong fyringe, and is furniflied with a
ftop cock, and fcrews on the end of the fyringe to be
charged, and then, when the cock is turned, it may
be fcrewed on to the gun. The bullet is made to fit
the barrel very exactly, and is rammed in as the ball
of a mufket. Each gun is generally furnifhed with
two brafs balls, which will contain fufficient air for
about twenty discharges ; and that which is not in pre-
fent ufe may be carried in the pocket. The gun is
charged by turning the cock, which fills a fmall cham-
ber at the butt end of the barrel with condenfed air,
when the cock may be turned again to fave the reflc
for further difcharges. The pulling of the trigger
opens a valve, and the fpring of the air forces out the
bullet, as in the inftrument already defcribed.

The elafticity of the air produces alfo confiderable
effects in the natural world j for by mfmuating itfelf
into the pores of bodies, and pofleffing this power
of expanding, which is fo eafily excited, it muft necef-
farily put the particles of bodies into which it infinuates
itfelf into a ftate of almoft perpetual ofcillation. The
truth of this obfervation is evinced particularly in the
air veffels of plants, which perform the office of lungs ;
for the contained air, expanding and .contracting al-

* Goldfinith's Plulofophy, Vol. II. p. 96. * '

VOL. I. F f ternately,

434 Suppofed Height [Book V.

tcrnatdy, according to the increafe or decreafe of the
heat, preffes the vefiels, and eafes them again alter-
nately, thus keeping up a continual circulation in the
fluids. Even entire columns of marble have been
known to cleave, from the increafed elafticity of feme
fmall bubbles of air contained in them.

Putrefaction and fermentation are procefles de-
pending entirely on the action of the air ; for we
know by numerous experiments, that neither of thefe
changes will take place in vacuo, even in fubjects the
mod favourably difpofed to them.

In fpeaking of the terrefhial atmofphere it has been
intimated, that it is found to be nearly the fame as to
tfompofition in all climates and in all places, as well
upon the tops of high mountains as in the vallies be-
low, but that it is confiderably lei's denfe in proportion
to the height. The whole globe of the earth is entirely
enveloped with it ; the whole atmofphere is carried
along with the terreftrial orb, both in its diurnal and
annual motion, and is a principal operator in the me-
chanifm of nature.

Various means have been devifed for afcertaining
the height of the atmofphere. c Thefe attempts,'
ftys Mr. Adams, c commenced foon after it was dil-
covered, by means of the Torricellian tube, that air
is a gravitating fubftance. Thus it alfo became known
that a column of air, whofe bafe is a fquare inch, and
the height that of the whole atmofphere, weighs fif-
teen pounds j and that the weight of air is to that of
mercury, as I to 10,800 : whence it follows, that if the
weight of the atmofphere is fufBcient to raife a column
of mercury to the height of thirty inches, the height
of the aerial column muft be ten thoufand eight hun-
dred rimes as much, and confequently a little more
than five miles high.

•Jt

Chap. 9«] i>f the Atmofyhere* 43$

' It was not however at any time fuppofed, that this
calculation could be juft; for as the air is an elaf-
tic fluid, the upper parts muft expand to an immenfe
bulk, and thus render the calculation above related ex-
ceedingly erroneous. By experiments made in diffe-
rent countries, it has been found that the fpaces, which
any portion of air takes up, are reciprocally propor-
tional to the weight with which it is compreffed. Al-
lowances were therefore to be made in calculating the
height of the atmofphere. If we fuppofe the heighc
of the whole divided into innumerable equal parts, the
denfity of each of which is as its quantity, and the
weight of the whole incumbent atmofphere being alfo
as its quantity, it is evident, that the weight of the in-
cumbent air is every where as the quantity contained
in the fubjacent part, which makes a difference be-
tween the weights of each tv/o contiguous parts of air.
By a theorem in geometry, where the differences of
magnitudes are geometrically proportional to the mag-
nitudes themlelves, it appears that thefe magnitudes
are in continual arithmetical proportion j therefore, i£
according to the ftippofition, the altitudes of the air,
by the addition of new parts into which it is divided,
do continually increafe in arithmetical proportion, its
denfity will be diminimed, or (which is the fame
thing) its gravity decreafed in continual geometrical
proportion.

* It is now eafy, from ilrch a feries, by making two

or three barometrical obfervations, and determining

the denfity of the atmofphere at two or three different

frations, to determine its abfolute height, or its rarity

at any afiignable height. Calculations accordingly

'were made upon this plan j but it having been found

that the barometrical obfervaticns by no means cor-

relponded with the denfity which, by other experi-

F f 2 ments,

436 Computed Height [Book V,

merits, the air ought to have had, it was fufpected that
the upper parts of the atmofpherical regions were not
fubject to the fame laws with the lower ones. Philo-
fophers, therefore, had recourfe to another method for
determining the altitude of the atmofphere, viz. by a
calculation of the height from which the light of the
fun is refracted, fo as to become vifible to us before
he himfelf is feen in the heavens. By this method
it was determined, that at the height of forty-five
miles the atmofphere had no power of refraction ; and
confequently beyond that diftance was either a mere
vacuum, or the next thing to it, and not to be re-
garded.

f This theory foon became very generally received;
and the height of the atmofphere was fpoken of as fa-
miliarly as the height of a mountain, and reckoned to
be as well afcertained, if not more fo, than the heights
of mod mountains are. Very great objections, how-
ever, which have never yet been removed, arife from
the appearances of fome meteors, like large globes of
fire, not unfrequently to be feen at vaft heights above
the earth. A very remarkable one of this kind was
obferved by Dr. Halley in the month of March 1719,
whofe altitude he computed to have been between
fixty-nine and feventy-three and a half Englifti miles ;
its diameter two thoufand fight hundred yards, or up-
wards of a mile, and a half, and its velocity about
three hundred and fifty miles in a minute. Others,
apparently of the fame kind, but whofe altitude and
velocity were ftill greater, have been obferved, parti-
cularly .that very remarkable one, Auguft i8th, 1783,
whofe diftance from the earth could not be lefs than
ninety miles; and its diameter not lefs than the former,
at the fame time that its velocity was certainly not lefs
than one thoufand miles in a minute. Fire-balls, in

appearance

Chap. 9-] of the Atmofphere. 437

appearance fimilar to thefe, though vaflly inferior in
fize, have been fometimes obferved at the furface of
the earth. Of this kind, one was feen on board the
Montague, 4th November, 1749, which appeared
as big as a large millftonej it broke with a violent
explofion.

f From analogical reafoning, it feems very probable
that the meteors, which appear at fuch great heights
in the air, are not efTentially different from thofe which,
like the fire-ball juft mentioned, are met with on the
furface of the earth. The perplexing circumflances
with regard to the former are, that at the great heights
above-mentioned, the atmofphere ought not to have
any denfity Jufficient to Jupport flame, or to -propagate
found; yet thefe meteors are commonly fucceeded by
one or more explofions, nay, are fometimes faid to be
accompanied with a hifllng noife as they pafs over
pur heads. The meteor of 1719 was not only very
bright, infomuch that for a fhort fpace it turned night
into day, but was attended with an explofio'n, heard
over all the ifland of Britain, occafioning a violent con-
cuffion in the atmoiphere, and feeming to (hake the
earth itfclf. That of 1783 alfo, though much higher
than the former, was fucceeded by explofions ; and,
according to the teftimony of feveral people, a hifling
noife was heard as it paffed. Dr. Halley acknow-
kdged, that he was unable to reconcile thefe circum-
ftances with the received theory of the height of the
atmofphere; as, in the regions in which this meteor
moved, the air ought to have been three hundred
thoufand times more rare than what we breathe, and
the next thing to a perfect vacuum.

f In the meteor of 1783, the difficulty is ftill
greater,' as it appears to have been twenty miles far-
ther up in the air. Dr. Halley offers a conjecture,
F f 3 indeed,

438 Suppcfed Height [Book V.

indeed, that the vaft magnitude of fuch bodies might
compenfate for the thinnefs of the medium in which
they moved j whether or not this was the cafe, cannot
indeed be afcertained, as we have fo few data to go
upon; but the greateft difficulty is to account for the
brightnefs of the light. Appearances of this kind are,
indeed, with great probability, attributed to electricity,
but the difficulty is not thus removed; though the
electrical fire pervades with great eafe the vacuum of
a common air-pump, yet it does not in that cafe ap-
pear in bright well defined fparks as in the open air,
but rather in long ftreams refembling rhe aurora bo-
realis. From forne late experiments, Mr. Morgan
concludes that the electrical fluid cannot penetrate a
perfect vacuum. If this mould be the cafe, it fhews
that the regions we fpeak of are not fuch a perfect
vacuum as can be artificially made ; but whether they
are or not, the extreme brightnefs of the light fhews
that a fluid was prefent in thofe regions, capable of
confining and condenfing the electric matter as much
as the air does at the furface of the ground -, for the
brightnefs of thefe meteors, considering their diflance,
cannot be fuppofed inferior to that of the brighteft
flames of lightning.

' It appears, therefore, that the abfolute height of
the atmofphere is not yet determined. The begin-
ning and ending of twilight, indeed, mew, that the
height at which the atrrofphere begins to refract the
fun's light is about forty- four or forty-five Englifh
miles. But this may, not improbably, be only the
height to which the aqueous vapours are carried ; for
it cannot be thought any unreafonable fuppofition, that
. light is refracted only by means of the aqueous va-
pour contained in the atmoiphere: and where this
ceafes, it is dill capable of fupporting the electric fire

at

Chap. 9.] cf the Atmofplert. 439

at lead as bright and ftrong as at the furface. That
it does extend much higher, is evident from the
meteors already mentioned j for all theie are un-
doubtedly carried along with the atmofphere i other-
wife that of 1783, which was feen for about a minute,
muft have been left one thoufand miles to the weft-
ward, by the earth flying out below it in it's annual
fourfe round the fun V

* Adams's keftures, vol. i, p. 52.

[ 440 ] [Bode V.

CHAP. X.

OF SOUND.

Sound cenfidered in Three Points ofView.-—Caufed by a Vibration in
the Parti of Bodies. — Propagated by an undulatory Motion of the
Air. — Thij proved by Experiment.— GlaJJes broken by an Effort of
the Voice.-*- -Elafilc Fluids 'not the only Means of tranfmitting Sound.—
Water or folid Bodies convey it. — Velocity of Sound.— Experiments
on this Subjefi. — Echoes."- Whifpering Gallery.

THERE is another property of air, which could
not fo conveniently be introduced into the pre-
ceding chapter j I mean the power of tranfmitting
founds.

Sound is produced by a vibrating motion, excited
in a fonorous body by a blow or a fhock from another
body, and the fame motion is communicated by this
fonorous body to the fluid which furrounds it, and
tranfmitted by this fluid to the ear, which is an organ
admirably adapted to receive its impreflion.

From this definition it follows, that found fhould be
confidered in three different views -3 firft, with refpect
to the fonorous body which produces it; fecondly, as
to the medium which tranfmits it j and, thirdly, as to
the organ which receives the impreflion.

Thofe bodies are properly calltdfonorous which af-
ford a found diftinct, and of fome duration, fuch as
bells, the firings of a violin, &c. and not thofe which
caufe only a confufed noife, fuch as a ftone produces
when it falls upon a pavement. When bodies are,
ftri&ly fpeaking, fonorousj they are neceflarily elaftic,

as

VbL.l . /'. in

Chap. 10.] Vibrations ofjonorous Bodies. 441

as will be afterwards proved •> and their found, as to
its force and duration, is proportionate to their vibra-
tions.

Suppofe, for example, the bell of a clock to be ftruck
by any folid body, a kind of undulating or tremulous
motion is imparted to the minute particles ; and this
motion may be even perceived by the hand or fingers
when applied to the bell.

To underftan4 this more completely, let us conceive
that a bell is compofed of .a feries of circular zones,
decreafing in diameter all the way to its top, each of
which may be confidered as a flat ring, compofed of
as many concentric circles as ics thicknefs will admit
of. If this ring is ftruck at the point a (Plate XXX.
Fig. 4.) the part fo ftruck tends towards g, and at the
feme time the parts b and d tend towards i and m, and
this action in thefe parts neceflarily caufes the point c
to approach towards e\ by their elaftic power, how-
ever, thefe parts prefently regain the pofition in which
they were before the bell was ftruck ; but as they re-
turn with an accelerated force, they generally go be- e
yond the point where they ought to reft. The part
a, therefore, after having returned from g to a, tends
towards /, the part c towards h, and the parts b and
d towards k and / j whence it happens that the bell, at
firft of a circular form, really becomes alternately oval
in two different directions j it follows then, that in
thofe parts where the curvature is the greateft, their
exterior points depart from each other.

The fame circumftance happens to the mufical cord
of a harp, or other ftringed inftrument, when it is
touched, for, in order to become angular, as B C D
or B E D (Fig. 5.) it is necefiary that the firing mould
!>e ftretched or lengthened, and confequently its par-
ticles

442 Cattfe of acute and grave Sbwids. [Book V.

tides be in fome meafure removed from the point of
contact.

There are then two vibrations which take place in
fbnorous bodies j the general vibration, which changes
the form of the body, and the particular vibration,
-which affects the- minute particles, in confequence of
the former. M. de la Hire has proved *, that the
found is not owing to the general vibration, but rather
to the vibration of the particles \ for whenever the two
vibrations can be feparated, it is found that the for-
mer produces no found; but when the general vibra-
tion is accompanied with a, vibration of the particles,
the latter it is that regulates the duration, the force,
and th° modulation of the found: if, on the contrary,
thefc vibrations are flopped or interrupted by touch-
ing the fonorous body, the found immediately ceafes.
On this account clock-makers attach to the hammer*
which (rrikes the bell of the clock, a (hull fpring*
which elevates it again the moment it has ft ruck, ard
prevents it from remaining upon the bell, which would
conficlerably deaden or deilroy the found.

Acute founds are produced, when the vibrations of
the founding body are more frequent; grave or deep
founds, when they are lefs fo: no medium between
acqte and grave founds can be found. Sonorous bo-
dies arc faid to be in unifon when they vibrate with
the fame frequency; when one vibrates twice as fait
as the other, they differ by an octave; and other ratios*
with refpect to the quicknefs of vibration, are diflin-
guilhed by other names. Cords, which are fhort and
tightly ftr etched, produce acute founds; thofe which
are long and lax, grave founds.

The motion or vibration of bodies at a diftance
from us would not affect our fenfe of hearing without

• Mem. de 1'Acad. 1716, p. 264.

• the

Chap. 10.] How Air transmits Sound. 443

the medium of fome other body, which receives an
impulfe from this motion, and communicates the vi-
brations to our organs. Thus a hard blow upon an
anvil or upon a bell could not be heard by us, even at
a very fmall diftance, if there was not a medium be-
tween thofe objects and us capable of tranfmitting the
vibrations to our auditory nerves. Elaftic fluids are the
moft effective mediums for this purpofe, and confe-
quently the air is the moft common vehicle of found,
which is very eafily proved by ringing a bell under the
receiver of an air-pump, the found it affords being
found gradually to diminim as the air becomes ex-
haufted, till at length it ceafes to be heard at all.
That the air is capable of being agitated with great
force appears from the violent concufiions produced by
explofions of gunpowder, as well as from the power,
which fome perfons are known to poffefs, of breaking
drinking glaffes, by means of their voice, when found-
ed in unifon with the note which the glafs would have
produced when ftruck. The tremulous motion ex-
cited in the air by founding bodies has been fuppofed
analogous to the fucceffive rings which are produced
by disturbing the furface of water. This hypothe-
fis, however, was difproved by the obfervation that
founds, whether weak or loud, always travel with the
fame velocity, which does not hold true with refpedt
to the rings on the furface of water, fince thefe move
fafter or flower according to the force of the caufe
which excited them.

Every found is rendered ftronger or weaker, and may
be heard at a greater or lefs diftance, according to the
denfity* or rarity of that elaftic fluid, by which it is

* That fome degree of denfity is necefiary in a fluid, to enable,
it to convey founds, is evident from this faft, that light, which is
a fluid extremely rare, is totally d?fli:ute of this power. — Tralte
$km. de Pkjfiiue, torn. ii. p. rfo.

propagated.

444 Experiments on the Tranfmijfion of found. [Book V.

propagated. According to Mr. Haukfbee, who has
*made deep refearches into this branch of philofophy,
when air has acquired twice its common denfity it
tranfrnits found twice as far as common air ; whence
he reafonably concludes, that found increafes, not
only in direct proportion to the denfity of the air, but
in proportion to the fquare of this denfity.

If found was propagated in an elaftic fluid more
denfe than the air, it ^would be carried proportionably
farther. I have proved this, fays M. Briflbn *, by
putting a fonorous body into carbonic acid gas or
fixable air, the denfity of which is about one-third
more than that of atmofpherical air; the confequence
was, that at that time, and in that fituation, the found
was very confiderably increafed. For the fame rea-
fon, the drynefs of the air, which increafes its denfity,
has a confiderable effe6b in rendering found louder
and more audible. Sound is alfo much increafed by
the reverberation of the pulfes of the air from thofe
furrounding bodies againft which they ftrike, whence
it happens that mufic is fo much louder in a clofe
apartment than in the open air.

Elaftic fluids are, however, not the only medium
through which found may be tranfmittedj for it may
be propagated by means of water and other liquors,
which may be proved by immerfmg a fonorous body
in water; but it muft be obferved, that in this
cafe the found will be lefs perceptible, and will not
extend to fo great a diftancej the caufe of this dimi-
nution is, becaufe mediums for the tranfmiflion of
found fhould be elaftic, and that is a property which
water and other liquors poffefs only in a very re-i
ftrided degree.

* Elem. de Phyfique, torn, ii. p. i6|.

Sound

Chap. 10.] Velocity of Sound. 44 <j

Sound is alfo tranfmitted by folid bodies, provided
they pofTefs a fufficient degree of elafticity to produce
this effea.

Light, we have already fecn, is projected or re-
flected with incredible velocity; but found is tranf-
mitted much more flowly, and its progrefiion is very
perceptible to our fenfes. The fiafh from a cannon,
or even a mufket, may be feen fome feconds before
the found reaches our ears. * As the motion of light,
therefore, is inftantaneous with refpedl to any mode-
rate diftance, this has been the .common means em-
ployed for afcertaining the progrefs of found. Sir
Ifaac Newton obferves < that " all founding bodies
propagate their motions on all fides by fucceflive con-
denfations and relaxations} that is, by an alternate
progrefiion and return of the particles;" and thefe
vibrations, when communicated to the air* are termed
pulfes of found.

All pulfes move equally faft. This is proved by
experiment; and it is found that they pafs about
one thoufand one hundred and forty-two feet in a
fecond, whether the found is loud or low, grave or
acute *.

Some

* That we labour under a deception with regard to tones, and
that they become higher as they come from a greater diftance, may
be inferred from mufical compofition. The greateft matters in this
art, when they would imitate a diftant echo, generally take the
founds an odlave higher. A few years ago, a fellow exhibited in
Weftminfter the art of imitating founds at any diftance whatever.
I remarked, that whenever he defigned to imitate a voice coming
from a great diftance, he not pnly made the found more low and
indiftindt, but raifed the tone feveral pitches higher than that ufed
in his nearer imitations. A few obfervations fmce made upon
founds, indues me to believe, that they become higher as they
come from a diftance more remote ; while, op the contrary, that
they deepen the more the vibrations approach the labyrinth of

the

446 Experiments on the Velocity of Sound. [Book V.

Some curious experiments were made, relative to
the propagation of found, by Meflieurs de Thury,
Maraldi, and de la Caille, upon a line fourteen thou-
fand fix hundred and thirty- fix fathoms in length,
having the tower of Mount Lheri at one end, and the
pyramid of Montmartre at the other extremity of that
diftance : their obfervatory was placed between thofe
two objects. The refult of their obfervations were
thefe, i ft. That found moves one hundred and fe-
venty-three fathoms French in a fecond, when the air
is calm. ad. That-found moves with the fame degree
of fwiftnefs whether it is ftrong or weak ; for thefe
gentlemen obferved, that the dilcharge of a box of
half a pound of gunpowder exploded at Mont-
martre was heard at Mount Lheri in the fame fpace
of time as the report of a great gun charged with
nearly fix pounds of powder. 3d. That the motion
of found is uniform j that its velocity neither acce-
lerates nor diminimes through the whole courfe of its

the ear. The following eafy and common experiment, I think,
will prove it. Take any thing whatever, capable of giving a
found ; let it be a common poker far inftance, and tying on a
garter at top, fo as that both ends of the garter are left at liber-
ty; thefe ends muft be rolled round the firfl finger of each hand,
and then with thefe fingers flopping the ears clofe, ftrike the poker
thus fufpended againft any body whatfoever. The depth of the
tone which this new mufical inftrument returns will be amazing.
The deepeft and largeft bell will not equal it. Whence is thisv
nnlefs from the clofe approach of the founding body, whofe vi-
bration* are immediately communicated to the internal parts of
the ear. I am fenfible that many objections may be made to tlri»
laft opinion; fucceeding experience muft, however, determine
whether it be juft or not; but fuch as make them muft be particu-
larly careful not to let their former experience correct their im-
mediate fenfations. This alteration of tone, with diftance, how-
ever, muft diminim but by great intervals. GoMfmith's Pbilofopky,
vol. u. p. 195, it,6.

jprogrefs.

Chap. 10.] Experiments on the Velocity of Sound. 447

progrefs. 4th. That the velocity of found is the
fame, whether a cannon 'is placed towards the pcr-
fon who hears its report, or turned a contrary ways
in other words, a great gun fired from the Tower of
London eaftward, would be heard at Weflminfter in
the fame interval of t]me as if it was difcharged to-
wards the latter place. And if the gun was dif-
charged in a direction perpendicular to the horizon,
it would be heard as foon as if difcharged in a right
line towards the hearer. By other experiments, how-
ever, the progrefs of found appears to be impeded bf
a (Irong wind, fo that it travels at the rate of about
One mile flower in a minute againft a ftrong wind thaa
with it.

A knowledge of the progrefTion of found is not an
article cf mere fterile curiofity, but in feveral in-
fiances ufeful j for by this we are enabled to deter-
mine the diftance of mips or other moving bodies.
Suppofe, for example, a veiTel fires a gun, the found
of which 4s heard five feconds after the flafh is feenj
as found moves 1 142 Englifh feet in one fecond, this
number multiplied by 5 gives the diftance of 57 10
feet. The fame principle has been already mentioned
as applicable in ftorms of lightning and thunder.

The waves or pulfes of found being reflexib!^ in
their courfe when they meet with an extended folid
body of a regular furface, an ear placed in the paf-
fage of thefe reflected waves will perceive a found
fimilar to the original found, but which will feem to
proceed from a body fituated in a fimilar pofitioa
and diftance behind the plane of reflection, as the real
(bunding body is before it. This reflected found is
commonly called an ECHO, which, however, cannot
take place at lefs than fifty-five feet; becaufe it is
2 nrcefTary

44* Echoes. [Book V,

neceffary that the diftance Ihould be fuch, and the
reverberated or reflected found fo long in arriving,
that the ear may diftinguifh clearly between that and
the original found *.

Reflected

* " It is in general known, that caverns, grottoes, mountains,
and ruined buildings return this image of found. Image we may
call it, for in every refpeft it refembles the image of a vifible ob-
jeft reflected from a poliflied furface. Our figures are often re-
prefented in a mirror without feeing them ourfelves, while thofe
flanding on one fide are alone fenfible of the reflection. To be
capable of feeing the refle&ed image of ourfelves, we muft be di-
re£Uy in a line with the image. Juft fo is it in an echo ; we muft
ftand in the line in which the found is reflected, or the repetition
\v-ill be loft to us, while it may, at the fame time, be diftindlly
heard by others who itand at a fmall diftance to one fide of us.
I remember a very extraordinary echo, at a ruined fbrtrefs near
Louvain, in Flanders. If a perfon fung, he only heard his own
Voice, without any repetition ; on the contrary, thofe who flood at
Ibme diftance, heard the echo but not the voice ; but then they
heard it with furprifmg variations, fometimes louder, fometimes
fofter, now more near, then more dlftant. There is an account,
in the memoirs of the French academy, of a fimilar echo near
Rouen. The building which returns it is a femicircular court-
yard; yet all buildings of the fame form do not produce the fame
effects. We find fome mufic halls excellently adapted for founds,
while others, built upon the fame plan, in a different place, are
found to mix the tones, inftead of enlarging them, in a very dif-
agreeable manner.

" As we know the diftance of places by the length of time a
found takes to travel from them, fo we may judge of the diftance
of an echo, by the length of the interval between our voice and
its repetition. The moft deliberate echoes, as they are called,
are ever the moft diftant; while, on the contrary, thofe that are
very near, return their founds fo very quick as to have the inter-
val almoft imperceptible ; when this i.s the cafe, and the echo is
fo very near, the voice is faid to be increafed and not echoed ;
however, in faft, the increafe is only made by the fwiftly purfuing
repetition. Our theatres and concert rooms are beft fitted for mu-
fic or fpeajcing, when they enlarge the found to the greateft pitch
at the fmalleft interval : for a repetition which does not begin the

word

Chap. 10.] Whijpsring Gallery. 449

Reflected found may be magnified by much the
fame contrivances as are ufed in optics refpecting light :
hence it follows, that founds utered in one focus of an
elliptical cavity are heard much magnified in the other
focus. The whifpering gallery at St. Paul's cathedral
in London is of this defcription ; a whifper uttered at
one fide of the dome is reflected to the other, and may
be very diftinclly heard. The fpeaking and ear
trumpets are conftructed on this principle. The belt
form for thefe inftruments is a hollow parabolic conoid,
with a fmall orifice at the top or apex, to which the
month is applied when the found is to be magnified,
or the ear when the hearing is to be facilitated.

The ftru6ture of the ear is one of the moft compli-*
cated and difficult fubjetfts of phyfiology, and it could
fcarcely be comprehended without fome previous
knowledge of the conftituent parts of the animal
frame ; for this reafon it will be neceffary to defer the
confideration of the manner in which we receive ideas
of found, till I come to treat of that part of the animal
economy which refpects the fenfe of hearing.

word till the fpeaker has finiftied it, throws all the founds into
confufion. Thus the theatre at the Hay-market enlarges the
found very much ; but then at a long interval after the finger or
fpeaker. The theatre at Drury-lane, before it was altered, en-
larged the found but in a fmall degree; but then the repetition was
extremely quick in its purfuit, and the founds, when heard, were
therefore heard diftindlly. Dergolife, the great; mufical compofer,
ufed to fay, that an echo was the belt fchool-miftrefs; for let a
man's ownmufic be ever \o good, by playing to an echo me would
teach him to improve 'iX."—Goldfmithys Pbilofophy, .vol. ii. p. 201
—204.

VOL. I. G g

[ 453 1 [BookV.

CHAP. XI.

WINDS.

Different Opinions concerning the general Caufe of i'T'inds. — Of General
or Trade Winds. — Of Monfoons.—Of Sea and Land Breezes. —
Caiifes of thefe. — Variable Winds. — Storms. — Hurricanes.— Tbf
Harmattan. — The Sirocco Wind. — The Samiel. — Moving Pillars of
Sand.'— The Simoom* — Wbifkuoinds, — Waierfpcuts. — Tornadoes.

TH E opinions of philofophers have varied much
refpecling the caufe of winds, and many of their
theories are little more than mere conjectures ; but it
muft be confefied, that electricity and a chemical
knowledge of air have latterly in ibrne degree im-
• proved our imperfect acquaintance with" thefe aerial
currents.

It has been already obferved, that air is expanded
by heat, and its fpring confequently increafed ; and it
is well known alfo that its elafticity is weakened by-
cold or freezing mixtures. From experiments which
have been made for the illustration of thefe properties
of air, we are enabled to point out the cauies of many
phenomena that occur in the atmofphere.

When a firej is made in the open air, the rarefied
part of that fluid will afcend in a current, and the
cooler and denfer air will rulh in on all fides, in confe-
quence of which a 'wind is generated, and blows
constantly towards the fire. The wind produced in
this manner will be too inconfiderable to be perceived
at any great diftance; but the rarefactions which arife
from natural caufes may be fuch as to agitate pur at-
mofphere fufficiently to produce thofe torrents of air

which

Chap, ii.] General Winds. 451

which have always a powerful effect in nature, and
which fomerimes overwhelm and deftroy the faireft
productions of human art.

M. BrifTon is inclined to believe, that electricity is
the firfl and general caufe of all variable winds : f Thun-
der and water- fpouts *,' fay,> he, c are now acknow-
ledged to be electrical phenomena, and thefe are fre-
quently accompanied with formidable winds. Why
may not the caufe which produces thefe phendmeria
be alfo that of the winds which accompany them ? .If
electricity is the caufe of thefe winds, why may it not
be the caufe of the others j- :'

Winds are commonly divided into three clafles, viz.
general, periodical, and variable winds.

General or permanent winds blow always nearly in
the fame direction. In the Atlantic and Pacific Oceans,
under the equator, the wind is almoft always eafterly ;
it blows, indeed, in this direction, on both fides of the
equator to the latitude of 28°. More to the north-
ward of the equator, the wind generally blows be-
tween the north and eaft, and the farther north we
proceed, we find the wind to blow in a more northern
direction ; more to the fouthward of the equator it
blows between the fbuth and eaft, and the farther to
the fouth, the more it comes in that dire6tion.

Between the parallels of 28° and 40° fouth lat. in,
that tract which extends from 30° weft to 100° eaft'
longitude from London^ the wind is variable, but it
moft. frequently blows from between the N. W.
and S. W. fo that the outward bound Eaft India fhips

* With refpeft to the latter I entertain many doubts, at leaff
as to electricity being the proximate or efficient caufe. See the
latter part of this chapter.

f Briflbn, Truitc Elem. de Phyfique, torn, ii, p. 180.

G g 2 generally

452 frade Winds. [BookV.

-generally run down their eafting on the parallel of
36' fouth *.

Navigators have given the appellation of tradc-
winds to thefe general winds.

Thofe winds, which blow in a certain direction for
a time, and at certain dated fcafons change and blow
for an equal fpace ef time from the oppofite point of
the compafs, are called monfoons. During the months
of April, May, June, July, Auguft, and September,
the wind blows from the (buthwara over the whole
length of the Indian Oevanj viz. between the parallels
of 28° N. and 28° S. lac. and between the eaftern coaft
of Africa and the meridian which pafles through the
weftern part of Japan •, but in the other months, Octo-
ber, November, December, January, February, and
March, the winds in all the northern parts of the
Indian Ocean fhift round, and blow directly contrary
to the courle they held in the former fix months.
For fome days before and after the change, there are
calms, variable winds, and tremendous dorms, with
thunder, &c.

Philofophers differ in their opinions refpecting the
caufe of thefe periodical winds ; but a moil probable
theory of the general trade-winds is, that they are oc-
-cafioned by the heat of the fun in the regions about the
equator, where the air is heated to a greater degree, and
confequently rarefied more than in thofe parts of the
globe which are nearer the poles. From this expanfion
of the air in thefe tropical regions, the denfcr air, in
higher latitudes, rufhes violently towards the equator
from both fides of the globe. By this conflux of the
denfer air, without any other circumftances intervening,
a direct northerly wind would be produced in the noi th-

* See Nicholfon's Phil. vol. ii. p. 56.

crn

-Chap, ii.] Land and Sea Breezes. 453

crn tropic, and a fouthern one in the other tropic ;. but
as the earth's diurnal motion varies thedireft influence
of the fun over the furface of the earth, and as by that
motion this influence is communicated from eaft to
weft, an eafterly wjnd would be produced, if this in-
fluence alone prevailed. On account of the co-optra-
tion of thefe two caufes at the fame time, the trade -'
winds* blow naturally from the N. E. on the north, and
from the S. E. on the fouth of the line, throughout the
whole year; but as the fun approaches nearer the'tropic
of Cancer in our ftimrner feafon, the point towards
which thefe winds are directed will not be invariably
the fame, but they will incline more towards the north
in that feafon, and more towards the fouth in our
winter.

The Ian4 zndfea breezes in the tropical climates may
be confidcred as partial interruptions of the general
trade winds, and the caufe of thefe it is not very diffi-
cult to explain. From water being a better conductor
of heat than earth, the water is always of a more even
temperature. During the day, therefore, the land be-
comes confiderably heated, the air rarefied, and con--
fequently in the afternoon a breeze lets in from the
fea, which is lefs heated at that time than the land.
On the other hand, during the night, the earth lofes its
furplus heat, while the fea continues more even in its
temperature. Towards morning, therefore, a breeze
regularly proceeds from the land towards the ocean,
where the air is warmer, and confequently more rarefied
than on ihore.

The caufe of the mcnfoons is not (b well underflood
as that of the general trade winds j but what has been
jufl remarked, fuggefts, at lead, a probable theory
on the fubject. It is well known, that at die equator the
changes of heat and cold are occafioned by the diurnal
Gg 3 motion

454 Moyfoons tr [Book V.

motion of the earth, and that the difference between
the heat of the day and the night is almoft all that is
perceived in thole tropical regions j whereas in the
polar regions the great vipifiitudes of heat and cold
are occafioned by the annual motion of the globe,
which produces the fenfible changes of winter andfum-
tner j confequently, if the heat of the fun was the only
caufe of the variation of the winds, the changes, if
any, that would be produced by thofe means in equa-
torial regions, ought to be diurnal only, but the changes
about the pole fhould be experienced only once in fix
rnonths. As the effecls arifing from tV> heat of the
fun upon the air muft be greater at the equator than at
the poles,1 the changes of the wind arifing from the ex-
panfion of the air by the fun's rays muft be more fteady
in equatorial than in polar regions. The incontrover-
tible evidence of navigators proves this truth, that
winds are more variable towards the poles, and more
conftant towards the equator. But in fummer, the
continual heat, even in high latitudes, comes to be fen-
fibly felt, and produces changes on the wind, which are
diftinctly perceptible. In our own cold region, the
effects of the fqn on the wind are felt during the fum-
mer months ; for while the weather in that feafon of
the year is fine, the wind generally becomes ftronger as
the time of the day advances, and dies away towards the
evening, and affumes that pleafing fcrenity fo delight-
ful to our feelings. Such are the diurnal changes of
the wind in northern climates. The annual revolution
of the fun produces ftill more fenfible effects. The
prevalence of the weftern winds during fummcr4 we may
attribure to this caufe, -which is ftill more perceptible
in France and Spain j becaufe the continent of land to
the eaftward, being heated more than the waters of the
Atlantic Ocean, the air is drawn, during that feafon,

towards

Chap, li.] Periodical trade Winds. 455

towards the eaft, arid confequently produces a weftern
wind.

But thefe effects are much more perceptible in
countries near the tropics than with us. For when the
fun approaches the tropic of Cancer, the foil of-Perfia,
Bengal, China, and the adjoining countries, becomes
fo much more heated than the fea to the fouthward of
thofe countries, that the current of the general trade
wind is interrupted, fo as to blow, at that fcafon, from
the fouth to the north, contrary to what it wouid do if
no land was there. But as the high mountains of
Africa, during all the year, are extremely cold, the low
countries of India, to the eaftward of it, become hotter
than Africa in iutnmer, and the air is naturajjy drawn
thence to the eaftward. From the fame caufe it fol-
lows, that the trade wind, in the Indian Ocean, from
April till October, blows in a north- eaft direction,
contrary to that of the general trade wind, in open feas,
in the fame latitude j but whenxhe fun retires towards
the tropic of Capricorn, thefe northern parts' become
cooler, and the general trade wind afllimes its natural
direction.

Having given the mod obvicus caufes of the pe-
riodical monfoons in the Indian feas, it is necefiary to
obfervr, that no monfoon takes place to the fouth-
ward of the equator, except in that part of the ocean
adjoining to New Holland. There the fame caufes
concur to produce a monfoon as in the northern tro-
pic, and fimilar appearances take place. From Octo-
ber till April the monfoon fets in from the N. W. to
S. E. oppofite to the general courfe of the trade wind
on the other fide of the line j and here aifo the general
trade wind re fumes its ufual courfe during the other
months, which conftitute the winter fcafon in thefe
regions. It may not be improper to conclude this
G g 4 account

456 Variable Winds. [Book V,

account of the tropical winds, by enumerating fome of
the principal inflexions of the monfoons.

Between the months of April and October the
wind blows conftantly from W. S. W. in all that part
of the Indian ocean which lies between Madagafcar
and Cape Commorin, and in the contrary direction
from October till April, with fome fmail variation in
different places ; but in the bay of Bengal thefe v.inds
are neither fo ftrong nor fo conftant as in the In 'ian
ocean. It muft alfo be remarked, that the S. W. winds
in thofe feas are more foutherly on the African fide,
and more wefterly on the fide of India ; but thefe va-
riations are not fo great as to be repugnant to the ge-
neral theory. The caufe of this variation is, as \vas
before intimated, that the mountainous lands of Africa
are colder than the flatter regions of Arabia and India,
confequently the wind naturally blows from thefe cold
mountains, in the fummer feafon, towards the warmer
lands of Afm, which occafions thofe inflections of the
wind to the eaftward during the fummer months.
The peninfula of India, lying fo much farther to the
fouth than the kingdoms of Arabia and Perfia, adds
greatly to this effect, becaufe the wind naturally draws
towards them, and produces that eaftcrly variation of
the monfoon which takes place in this, part of the
ocean, while the fandy deferts of Arabia draw the winds
more directly northward, near the African coaft. A
fimilar chain of reafoning will ferve to explain any
other inflexions or variations that may occur in the
perufal of books of travels, &c.

The variable winds, .which take place in thefe cli-

- rnates, depend upon different caufes ; but I am in-
clined to agree with M. Briffon in attributing them

• chiefly to electricity. It is to be remembered, that

whatever deftroys the equilibrium of the air, in other

6 words.

Chap, n.] theory of Winds. § 457

\vords_, any caufe which produces a fudden rarefadion
in any part of the atmoCphere, produces a current of
wind towards the part where the rarefaction takes
place >• winds are, therefore, not only produced by the
earth being heated in a particular part, but by thunder
ftorms or other electrical phenomena. The rays of
the"- fun are alfo fometimes obftruded by clouds or
mifts in particular places, and one part of the world,
or even of a particular country, will confequently be
lefs heated than another ; in that cafe there will always
be a current of air from the cold to the warm region.
Befides this, the falling of rain, or other circumftances,
produce occafional alterations in the temperature ; and
whenever thefe take place in any country, they muft
be attended with wind. The great Bacon was the firft
who attempted a theory of the wind ; and it is to be
lamented that his plan has not been purfued by fuc-
ceeding philofophers. The following is a (ketch of
his general principles, with a few additions by modern .
obfervers.

1 At fea the winds are more regular than at land ;
for there nothing oppofes their progrefs, or alters the
fun's influence.

'The air at fea is more equable, as well as more
conftant : at land it blows in fits of force and intermif-
fion j but at fea the current is ftrong, fteady, and even.

' In general, at fea, on this fide the equator, the eaft
and north winds are moft violent and boifterous : on
the contrary, at land, the weft and Couth winds moft
frequently produce hurricanes and tempefts.

' The air is often feen to move in two contrary cur-
rents, and this almoft ever previous to thunder. The
clouds, in fuch a cafe, are feen to move one way, while
;he weathercock points another.

'The

45$ Theory of 'Winds. [BookV.

cThe winds are more violent at certain heights than
upon the plain, and the higher we afcend Jolty moun-
tains, the greater is the force of the wind, till we get
above the ordinary heights of the clouds. Above this
the fky is ufually fcrene and 'clear. The reafon is,
that the wind, at the lurface of the earth, is continually
interrupted by hiils and rifings : fo that, on the plnin,
between any two of thele, the inhabitants are in a kind
of fhelter; but when once the interpofition of fmall
hills no longer ftops the wind's courfe, it then .becomes
itrbnger, as the interruptions it meets with are fewer.
At the tops of the higher mountains its interruptions
are leaft of all ; but it does not blow with violence
there ; for its denfity is fo much diminiihed by the
height, that its force is fcarcely perceptible, and the
fform falls mid;vay below. What is commonly called
a high vvind moves at the rate of about thirty-five
miles an hour.

' A current of air al-ways augments in force in pro-
portion as the pafihge through which it runs is dimi-
nilhed. The law of this augmentation is, that the air's
force is compounded of , its fwiftnefs and denfity, and
as thefe are increafed, fo will the force-of the wind. If
any quantity of wind moves with twice the fwiftnefs of
a fimilar quantity, it will have twice its force ; but if,
at the fame time that it is twice as fwift, it moves
through twice a fmaller tube, and the fides of the canal
give no refiftanceto its motion, it will have four times
the force. This, however, is not entirely the cafe ;
for the fides of the tube give a refiftance, ancl retard
its motion, in a proportion that is not eafily calculated.
From this increafe of the wind's denfity in blowing
through narrow pafiages, it is that we fee the dorms fo
very violent that fometimes blow between two neigh-
bouring hills. It is from this, that when caugbt in

Chap, ii.] Hurricanes. 4 59

long arcades opening at one end, the wind blows with
great force along them. From this increafed dehfity
it is, that we meet with fdch cold blafts at the corners
of frreets. In fhcrt, whatever diminimes its bulk,
without taking entirely away from its motion, increafes
the vehemence of the wind. This alfo is the reafon
why the air reflected back from the fide of a mountain
is often more violent than the air which firft ftruck its
fide i for it is by this means condenfed, and its force
augmented. The countrymen and farmers have a
diftinftion which is not without its foundation ; for
they make a difference between a fwift and an heavy
florin : the fwift ftorm is loud, boifterous, and inoffen-
five; the heavy ftorm more bpiftercus and alfo more
dangerous. This fhewr- the infurEciency of thofe in-
ftrnments made for meafuring winds, by meafuring the
rapidity only with which they move*.'

It would be happy indeed for fcience and for man-
kind if thefe refearches could have been carried further.
To predict an eclipfe, fays a late writer, is an object
merely of curiofity ; to predict an approaching ftorm
would be of inconceivable benefit. What is (till un-
accomplimed with refpect to our own climate, has how-
ever been attempted with refpecl to thofe alarming
itorms which happen in the Weil-Indies, and which
are commonly denominated hurricanes.

Thefe dreadful convulfions of nature, Dr. Perkins^
of Bofton, in America, fuppofes to be caufed by fome
occasional obftruftion in the uiual and natural progrefs
of the equatorial trade winds. The reafon he aiTigns
for this conjecture is, the more than ufual calm which
commonly precedes them. Iti the natural courfe of
the trade winds, the air rifes up in the line, and pafles

*Goldfmith's Plxilofophy, vol. ii. p. 143.

off

460 . Defcription of [Book V.

off towards the poles, and, in the more contracted de-
grees of the higher latitudes, takes the courfe of the
weft trade winds, fo that could their afcent be pre-
vented through the whole circle of the zone, there
would be no more weft winds in thofe latitudes than in
any other. Very violent rains and cold, however, tend
to check the afcent of air out of this circle, rather
caufing it to defcend. dear clouds and vapour ge-
nerate cold and wet, while rain beats down the air j
and as -hefe prevenrthe rifing of the air out of the line,
ib they hinder its ufual progrefs from the tropics on both
fides ; hence the calms which ufually precede hur-
ricanes. Calms, in thefe tropical regions,. are caufcd
by the afcent of the air into the higher part of the at-
mofphere, inftead of its remaining near the line : the
accumulation of air above then becomes heavier by
the cold which it meets in thofe regions, and defcends
into the more rarefied region below. Thefe heavy
gales, therefore, will continue to defcend till the upper
regions are entirely exonerated *".

In Mr. Beckford's hiftory of Jamaica there is a very
detailed and ftriking account of the dreadful hurricane
which defolated the iflands in the year 1780, but it is
too long for infertion as an extract, and in an abridged
ftate .the defcription would lofe its force. ' It is in the
rainy feafon (fays Mr. Adams) principally in the month
of Auguft, that they are afiaulted by hurricanes, which
deftroy at a ftroke the labours of many years, and
proftrate the mod exalted hopes of the planter, and
that, often when he thinks himfclf out of the reach of
fortune. It is a fudden and violent ftorm of wind,
rain, thunder, and lightning, attended with a furious
fwelling of the feas, and fomecimes with an earthquake;

f Arr.cricap Phil. Tranf. vol. i.

in

Chap, ii.] an Hurricane. 461

in fhort, with every circumftance which the elements
can affemble, that is terrible and deftruclive. Firft,
they fee, as a prelude to the enfuing havock, whole
fields of fugar canes whirled into the air, and fcattered
over the face of the country. The ftrongeft trees of
the foreft are torn up by the roots, and driven about
like ilubble ; their wind-mills are fwept away in a
moment j their works, the fixtures, the ponderous
copper boilers, and ftills of feveral hundred weight,
are wrenched from the ground and battered to pieces ;
their houfes are no protection ; their roofs are torn off
at one blaft, whilft the rain rulhes in upon them with
irrefiftible violence.

f There are figns by which the Indians of thefe iflands
taught our planters to prognofticate the approach of
an hurricane. The hurricane comes on either in the
quarter or at the full change of the moon. If it comes
on at the full, then, at the preceding change, the fky is
troubled, the fun more red than ufual •, there is a dead
calm below, and the mountain tops are free from thofe
mifts which ufually hover about them. In the caverns
of the earth, and in wells, you hear a hollow rumbling
found, like the ruihing of a great wind. At night the
flars feem much larger than ufual, and furrounded with,
a fort of burs ; the north- weft Iky has a black and me-
nacing appearance ; the fca emits a ftrong fmell, and
rifes into vail waves often without any wind. The
wind itfelf now forfakes its ufual fteady eafterly ftream,.
and fhifts about to the weft ; whenctr it fometimes,
with intermifilons, blows violently and irregularly
about two hours at a time. You have the fame figns
at the full moon : the moon herfelf is furrounded. with
a great bur, and fometimes the fun has the fame ap-
pearance*.'

* Adams's Leftures, vol. iv. p. 540.

The

4#2 Tbt liar mat tan. [Book V.

The lunmttan is a very fingular wind, which blows
periodically from the interior parts of Africa towards
die Atlantic Ocean. The feafon in which it prevails
is during the months of December, January, and Fe-
bruary.; it comes on indifcrimmately at any hour of
th~ day, at any time of the tide, or at any period of
the moon, and continues fornetimes only a day or two,
fumetimes five or fix days, and it has been known to
laft fifteen and fixteen days. There are generally
three or four returns of it every feafon. It blows with
a moderate force, but not quite fo ftrong as the fea
breeze.

A fog or haze is one of the peculiarities which
always accompany the harmattan. The Englifh,
French, and Portuguefe forts at Whydah, are not quite
, a quarter of a mile afunder, .yet are frequently quite
invifible to each other ; the fun, concealed the greatelt
part of the day, appears only about a few hours at
noon, and then of a mild red, exciting no painful fen-
fation on the eye. The particles which conftitute this
fog are depofited on the leaves of trees, on the fkins of
the negroes, Sec. and make them appear whitifh.

Extreme drynefs makes another extraordinary pro-
perty of this wind \ no dew falls during its continu-
ance ; vegetables are withered, and the grafs becomes
dry like hay. The natives take this opportunity to
clear the land, by fetting fire to the trees and plants
while in that dry and exhaufted (late. The drynefs is
fo extreme, that the covers of books, even clofely fhut
up in a trunk, are bent as if expofed to die fire, llouf-
hold furniture is much damaged ; the pannels of wain-
fcots fplit, and fineerecl work flies to pieces. The
joints of a well-laid floor of feafoned wood open fufE-
ciently to admit die breadth of a finger between them ;
but become as clofe as before on the ceafuig of the

harmattan.

Chap, ii.] f be Sirocco. 463

harmattan. The human body does not efcape the
parching effects of this wind j the eyes, noftrils, lips,
and palate, are rendered dry and uneafy ; the lips and
nofe become fore, and though the air is cool, there is
a troublefome fenfation of pricking heat on the fkin.
If the harmattan continues four or five days, the fcarf-
fkin peels off, firfc from the hands and face, artd after-
wards from the reft of the body.

Though this wind is fo fatal to vegetable life, and
occafions thefe troublefome -effects to the human fpe-
cies, it is nevertheiefs highly conducive to health -, it
flops the progrefs of epidemics, and relieves the patients
labouring under fluxes and intermittent fevers. In-
fection is not eafy at that time to be communicated,
even by inoculation. It is alfo remarkable for the
cure of ulcers and cutaneous difeafes *.

The firocco (fo called by the Italians becaufe it is
fuppofed to blow from Syria, and in the South of
France, the Levant wind) refembles in fome of its
effects the harmattan, but it differs from it in being ex-
tremely ir.falubrious. It fometimes blows for feveral
days together, to the great annoyance of the whole
vegetable and animal creation ; its medium heat is
calculated at 1 1 2 degrees j it is fatd to vegetation and
deftructive to mankind, and efpecially to flrangers ; it
depreffes the fpirits in an unufual degree ; it fufpends
the powers of digeflion, fo that rhofe who venture to
eat a heavy fupper, while this wind prevails, are com-
monly found dead in their beds the next morning, t of
what is called an indigeftion. The fick, at that afflict-
ing period, commonly fink under the prelTure of their
difeafes 3 and it is cuilomary in the morning, after this

*Dobf. Account, Phil. Trar.f. vol. Ixxi. part i.

wind

464 tte Sirocco. [Book V.

xvind has continued a whole night, to inquire who is
dead ».

Whether

* * The evil mod to be dreaded in travelling thefe regions is,

^s, the firocc, or lout') wind, which it is imagined blows
from the burning deferts of Africa, and is fcmctimes productive of
dangerous confequences to thofe who are expofed to its fury.
During the con linuance of this wind all nature appears to lan-
guifh, veg-station withers and dies, the bcafts of the n>ld droop,
the animal fpirits feem too much exhauiled to admit of the leaft
bodily exertion, and the fpring ar.d elafticity of the air appear to
be loll. The heat exceeds that of the moft fervid weather in Spain
or Malta, and is felt with peculiar violence in the city and neigh-
bourhood of Palermo.

* The fenfation occafioned by the firocc wind is very ftriking-
and wonderful. In a moment the air becomes heated to an ex-
ceffive degree, and the whole atmofphere feels as if it was in-
flamed, the pores of the body feem at once opened, and all the
fibres relaxed. During its continuance the inhabitants of Pa-
lermo (hut their doors and windows to exclude the air, and where
there are no window (hutters, wet blankets are hung on the infide
of the window, and the fervants are kept continually employed in
fprinkling the apartments with water. No creature, whofe ne-
ceflities do not compel him to the exertion, is to be feen while this
tremendous wind continues to blow, and the ftreets and avenues
of the city appear to be nearly deferted.

' The iirocc generally continues fo fhort a time in Sicily, that it
feldom produces thofe complaints which are the confequcnce of the
duration of its fcorching heats in feveral parts of Italy, though its
'violence in thofe countries is much inferior to what is felt in this
ifland. Here it feldom endures longer than thirty* fix or forty hours,
a time not fufficicnt to heat the ground, or the walls of the houfes,
in a very intenfe continued degree. It is commonly fucceeded by
the tramontane, or north wind, which in a fhort time reltores the
exhaufted powers of animal and vegetable life, and nature foon
aflumes her former appearance. The caufe of the firocc wind has
been frequently attempted to be explained, but the different hypc-
thefes are perhaps more to be admired for their ingenuity and fancy
than for being very fatisfadlorily explained. The fuperior intenfe-
nefs of this fcorching wind at Palermo, may probably be accounted
for from the fituation of that city, which is almoft furrounded by

lofty

Chap, ii.] FbeSamiel. 46$

Whether the fatal effects of the firocco depend en-
tirely upon the degree of fever, which is produced by
the extreme heat which accompanies it, or whether it
is really charged with any mephitic gas, I have never
been fufficiendy informed ; but I wiih that any intel-
ligent traveller would examine the ftate of the air by
the eudiometer, and by other tefts, during the pre-
valence of this wind. Should it be found loaded with
carbonic gas, its ill effects might eafily be obviated by
fufpending, in the different apartments, cloths dipped
in lime water; but from the prefent ftate of the evi-
dence I am difpofed to think that all its evil confe-
quences depend upon the fudden increafe of the tem-
perature only.

f An extraordinary blafting wind is felt occafionally
at Falklands Iflands. Happily its duration is fhort :
it feldom continues above twenty-four hours. It cuts
the herbage down as if fires had been made under
them j the leaves are parched up, and crumble into
duft. Fowls are feized with* cramps fo as never to
recover. Men are oppreffed with a ftopped perfpi-
ration, heavinefs at the breaft, and fore throat; but
ufually recover with care.

* But beyond all others in its dreadful effects, is the
famiel, or mortifying wind, of the deferts near Bagdad.
The camels, either by inftinct or experience, have

lofty mountains, the ravines and valleys of which are parched and
almoil burnt up in fummer. The numbeilefs fprings of warm
water muft alfo greatly increafe the heat of the air, and the practice
of burning brufh wood and heath on the neighbouring mountains,
during the warm feafon, mult undoubtedly tend to increafe the heat
of the wind in paffing over the country of Sicily, though it had
previoufly .been difarmed of part of its violence by travelling over
the fea which divides Sicily from Africa." Prefent State of Sicily
and Malta, p. i$o,.

VOL. I. Hh notice

466 We Samlel [Book V.

notice of its approach, and are fo well aware of it, that
they are faid to make an unufual noife, and cover up
their nofes in the fand. To efcape its effects, travellers
throw themftlves as clofe as poflible to the ground,
and wait till it has pafled by, which is commonly in a
few minutes. As foon as they who have life dare to
rile again, they examine how it fares with their com-
panions, by plucking at their arms or legs ; for if they
are deftroyed by the wind, their limbs are abfolutely
mortified, and will come afunder. It is faid of this
wind, that if it happens to meet with a fhower of rain
in its courfe, and blows acrofs it, it is at once deprived
of its noxious quality, and becomes mild and innocent.
It is alfo faid, that it was never known to pafs the walls
of a city V

This account of the famiel is extracted from the
travels of Mr. Ives over land to the Eaft Indies. Irs
fatal effects, if the ftatement is perfectly correct, evi-
dently proceed from a certain portion of extremely
putrid vapours with which it is charged, and I fufpect
it only happens when a ftrong wind chances to blow
over fome very putrid and ftagnant lake, which is not
far diftant ; travellers, however, are on fuch occafions
commonly in a ftate of too much alarm to note cir-
cumftances with accuracy, and too much of their ac-
counts is collected upon hear-fay evidence. This
wind, after all, may only confift of a mephitic vapour
which deftroys life "when inhaled ; and the putridity,
which is faid fo rapidly to take place, may depend
more upon the climate than the nature of the wind.

A wind or haze was obferved by Mr. Bruce, in the
courfe of his travels to difcover the fources of the
Mile, refembling the preceding in fome of its effects,

* See Adams's Le£ures, vol. iv. p. 541.

though

Chap, ii.] Moving Pillars of Sand. 467

though in others it may be thought more analogous
to the firocco. It is called by Mr. Bruce the fimoom,
and from its effects-upon the lungs, I can entertain
but little doubt, that it confifts chiefly of carbonic acid
gas in a very denfe ftate, and perhaps mixed with fome
other noxious exhalations.

In the fame defert Mr. Bruce obferved the afto-
nifhing phenomenon of moving pillars of fand *,
which are probably the effects of a number of
whirlwinds in thofe torrid regions. As the defcrip-
tion of thefe pillars is in fome degree blended with
that of the fimoom, I fhall extract the whole pafiage.
In relating the particulars of his journey acrofs a
certain part of the dcferts of Africa, Mr. Bruce ob-
ferves, c We were here at once furprifed and terri-
fied by a fight furely one oft the moft magnificent in
the world. In that vaft expanfe of defert, from weft
and to the north weft of us, we faw a number of pro-
digious pillars of fand at different diftances, at times
moving with great celerity, and at others (talking ori
with a majeftic flownefs j at intervals we thought they
were coming in a very few minutes to overwhelm us ;
and fmall quantities of fand did actually more than
once reach us. Again they would retreat fo as to be
almoft out of. fight, their tops reaching to the very
clouds. There the tops often feparated from the'
bodies j and thefe, once disjoined, difperfed in the air,
and did not appear more. Sometimes they were
broken near the middle, as if (truck with a large can-

* " So where our wide Numidian waftes extend,
Sudden th' impetuous hurricanes defcend,
Wheel through the air, in circling eddies play,
Tear up the fands, and {Veep whole plains away ,
Th'. affrighted traveller, with wild furprife,
Sees the dry defert all around him rife,
And, fmother'd in the dufty 'whirlwind, dies."

Addifon's Cato.
H h 2 non

468 Moving Pillars of Sand. [Book V.

non fhot. About noon they began to advance with
confiderable fwiftnefs upon us, the wind being very
ftrong at north. Eleven of them ranged alongfide of
us about the diftance of three miles. The greatefl
diameter of the larged appeared to me at that diftance,
as if it would meafure ten feet. They retired from us
with a wind at fouth eaft j leaving an impreflion upon
my mind to which I can give no name, though furely
one ingredient in it was fear, with a confiderable deal
of wonder and aftonilhment. It was in vain to think
of flying ; the fwifteft horfe, or faded failing (hip,
could be of no ufe to carry us out of this danger ; and
the full perfuafion of this ri vetted me as if to the fpot
where I dood, and let the camels gain on me fo much
in my date of lamenefs, that it was with fome difficulty
I could overtake them/

The fame phenomena again occurred in the courfc
of a few days. c The fame appearance of moving pil-
lars of land prefented themfelves to us this day, in form
and difpofition like thofe we had feen at Waadi Hal-
boub, only they feemed to be more in number, and lefs
in fize. They came feveral times in a direction clofe
upon us j that is, I believe, within lefs than two miles.
They began immediately after fun-rife, like a thick
wood, and almod darkened the fun : his rays fhining
through them for near an hour, gave them an appear-
ance of pillars of fire. Our people now became defpe-
rate : the Greeks (hrieked out, and faid it was the day
of judgment. Ifmael pronounced it to be hell, and the
Tucorories, that the world was on fire. I afked Idris
if ever he had before feen fuch a fight ? he faid he had
often feen them as terrible, though never worfe ; but
what he feared mod was the extreme rednefs of the air,
which was a fure prefage of the coming of the fimoom.
I begged and intreated IdrLs that he would not^ ay one

word

Chap, ii.] ^he Simoom. 469

word of that in the hearing of the people, for they had
already felt it at Imhanfara, in their way from Ras el
Feel to Teawa, and again at the Acaba of Gcrri,
before we came to Chendi, and they were already nearly
diftracted at the appre'henfion of rinding it here.

c At half pad four o'clock in the afternoon, we left
Waadi Del Aned, our courfe a little more to the weft-
ward than the direction of Syene. The fands which
had difappeared yefterday fcarcely (hewed themfelves
at all this day, and at a great diftance from the hori-
zon. This was, however, a comfort but of fhort du-
ration. I obferved Idris took no part in it, but only
warned me, and the fervants, that, upon the coming of
the fimoom, we mould fall upon our faces, with our
mouths upon the earth, fo as not to partake of the
outward air as . long as we could hold our breath.
We alighted at fix o'clock at a fmall rock in the
fandy ground, without trees or herbage, fo that our
camels faded all that night. This place is called
Ras el Sheah, or, by the Bimareen, El Mout, which
fignifies death, a name of bad omen.

f On the 1 6th, at half paft ten in the forenoon, we
left El Mout, {landing in the direction clofe upon
Syene. Our men, if not gay, were, however, in better
fpirits than I had feen them fince we left Gooz. One
of our Barbarins had even attempted a fong; but
Hagi Ifmael very gravely reproved him, by telling
him, that finging in fuch a fituation was a tempting
of Providence. There is, indeed, nothing more dif-
ferent than aftive and paffive courage. Hagi Ifmael
would fight, but he had not ftrength of mind to fuffer.
At eleven o'clock, while we contemplated with great
pleafure the rugged top of Chiggre, to which we
were fad approaching, and where we were to folace
oiirfelves with plenty of good water, Idris cried out,
H h 3 with

470 1'bs Simoom. [Book V.

with a loud voice, Fail upon your faces, for here is
the fimoom. I faw from the fouth-eaft a haze come,, in
colour like the purple part of the rainbow, but not fo
c.ompreffed or thick. Ic did not occupy twenty yards
in breadth, and was about twelve feet high from the
ground. It was a kind of bluih upon the air, and it
moved very rapidly, for I fcarce could turn u fall
upon the ground with my head to the northward,
when J felt the heat of its current plainly upon my
face. We all lay flat on the ground, as if dead, till
Idris told us it was blown over. The meteor, or
purple haze, which I faw, was indeed patted, but the
light air that (till blew was of heat to threaten fuffo-
cation. For my part, I found diftinctly in my breaft,
that I had imbibed a part of it ; nor was I free of an
afthmatic fenfation till I had been fome months in
Italy, at the baths of Poretta, near two years after-
wards *.' *

Whirlwinds and water-fpouts have by many philo-
fophers been conlidered as entirely electrical pheno-
mena, while others have attributed them to a different
eaufe, and accounted for them upon the principles of
hydroftatics. It is poflible, however, that there may
really be two kinds of water-fpouts, the one the effect
ef the electrical attraction as defcribed in Book iv.
c. 6. and the other caufed by a vacuum, or extreme
and fudden rarefaction of the air. The whirlwinds
which I have obferved in this country, were, I
am perfuaded, of the latter kind; at leaft whatever
was the original caufe, the circumagitation or fpiral
motion of the air muft have continued long after every
electrical power had ceafed to act.

It is well known that even a common fire produces
a kind of circulation of the air in a room, but in a

* Brace's Travels, vol. iv. p. 553, 555.

different

Chap, n.] Whirlwinds. 47 \

different form. It is therefore not difficult to con-
ceive, that when any part of the column of air upon
the furface of the earth or water is fuddenly rarefied,
either by electricity or any other caufe, a vacuum, at
lead comparatively to the reft of the air, will imme-
diately take place, and the circumambient air rufhing
in at once from every quarter to fill the void, a con-
flict of winds enfues, and confequently a circular mo-
tion, by which light bodies will be taken up and turned
round with confiderable velocity; this1 violent rufhing
of the air on all fides into the vacuum then forms
what is commonly called at land a whirlwind.

When this vacuum takes place at fea, from the
nature of fluids, the water will rife to a certain height
by the prefiure of the atmofphere, as in a common
pump ; but as the vacuum is not quite perfect, the
water will be divided into drops, and as thefe va-
cuums are generally caufed by heat, it will be rarefied
when it reaches the upper regions of the atmolphere,
and affume the appearance of a cloud.

Mr. Oliver *, whofe theory I have adopted with
little variation, illuftrates the phenomenon by a very
eafy experiment. In a ftiff paper card he made a
hole juft large enough to infert a goofe quill ; after
cutting the quill off fquare at both ends, he laid the
card upon the mouth of a wine glafs, filled with water
to within a fifth or fixth part of an inch from the
lower orifice of the quill ; then applying his mouth to
the upper part, he drew the air out of the quill, and
in one draught of his breath drew in about a fpoon-
ful of water; and this he was able to repeat, the quill
remaining as before. The water, he adds, did not
afcend to his mouth in a dream, as it would have done

*' Philad. Tranf. vol. ii.

H h 4 bad

472 Red and fallow Rain. [Book V.

had the quill reached the water, but broken, and con-
fufedly mixed with the air which afcended with it.
The ufual phenomena of water-fpouts are exactly
agreeable to this theory. They appear at a diftance
like an inverted cone, or the point of a fword, which
is owing to the water rifing in large drops at the firft,
and being expanded as it afcends; and a cloud is ge-
nerally fufpended over the body of the phenomenon.
The water which is taken up is undoubtedly lalt at
the firft, but by the rarefaction in the fuperior regions,
it undergoes a kind of natural diftillation, and lofes all
the heavy faline particles with which it was charged.
Water-fpouts have been obferved at land, of which
two v~ry remarkable inftances are recorded in the
Philofophical T ran factions. Other phenomena have
been remarked, which can be explained upon thefe
principles only. Accounts have been given of red
and yellow rain, of frogs and tadpoles, and even fmall
times having been rained upon the tops of houfes.
The red and yellow rain was, I apprehend, compofed
of the blofibms of vegetables, or of infects, taken up
by one of thefe aerial tubes; and the frogs and fifhes
were probably part of the contents of lome pond, in
which the water-fpout originated, or over which it
might have paiTed in its perambulation.

The point or cone of the water-fpout is generally
oblique, depending on the force and direction of the
wind which drives it along.

Dr. Perkins, whom I had occafion to mention, when
treating of hurricanes, in a paper publiflied in the
fame volume of American Transactions, is difpofed to
adopt a different theory of water-fpouts. Captain
Melling informed him, that in a voyage from the
\Vcft India Iflands to Boflon, a water-fpout came
acrofs the Hern of the veflcl where he then was, a

flood

Chap, ii.] Jfater-fpouts. 473

flood of water fell upon him with fuch violence as al-
moft to beat him down, and die fpout immediately
paffed off with a roaring noife into the lea. The
water from the ipout, he remarked, was perfectly
frefli.

Dr. Perkins adds feveral other inftances on the
teilimony of mariners, who all affirmed, that tltey faw
the water dejcend from the cloud through the water-
fpout into the fca, contrary to the opinion of Mr.
Oliver, that it always afcends.

A whirlwind, therefore, in the opinion of Dr. Per-
kins, cannot be the caufe of a water-fpout; nor
can both of thefe phenomena proceed from the fame
caufe. A whirlwind, he fuppofes to be produced by
the afcentof the heated or rarefied air into or through
the colder regions of the atmofphere above. Now,
Dr. Arbuthnot fays, that the rarefaction of the hotteft
day renders the air but one tenth lighter than it is in
the coldcft.

This roaring noife alfo, as remarked by Captain
Melling, does not agree with the theory of the afcent
of water in the fpout, as it is not very clear why
fuch a noife fnould accompany the fimple afcent of
water *.

To determine the matter, it is to be wiihed, that
future obfervers would be careful to remark, ift. The
incipient ftate of a \vater-fpout, and in particular whe-
ther any cloud is feen hovering over the part in which it
commences j and 2dly, whether the conical part feems
gradually to defcend from the body of the cloud.

A tornado feems to partake much of the nature of
the two preceding phenomena, but is more violent
in its effects. It commences very fuddenly, feveral

* Philad. Tranf. vol. ii.

clouds

474 tornadoes... [Book V.

clouds being previoudy drawn together, when a fpout
of wind, proceeding from them, ftrikes the ground in
a round fpot of a few rods or perches diameter, in the*
courfe of the wind of the day, and proceeds thus half
a mile or a mile. The pronenefs of its defcent
makes it rebound from the earth, throwing fuch
things as are moveable before it, but fome fideways or
in a lateral direction from it. A vapour, rnift, or
rain defcends with it, by which the path of it is marked
with wet.

The gentleman, xvho furniflies the above general
defcription, gives an account of one which happened
a few years fince at Leicefter, about fifty miles from
Bofton, in New England, ' It happened in July, on a
hot day, about four o'clock in the afternoon. A few-
clouds having gathered weftward, and coming over
head, a fudden motion of their running together in a
point being obferved, immediately a fpout of wind
Struck the ground at the weft end of a houfe, and
carried it away with a negro man in it, who was
afterwards found dead in the path of it. Two men
and a woman, by the breach of the floor, fell into
the cellar; and one man was driven forcibly up into
the chimney-corner. Thefe were preferved, though
much bruifed j they were wet with a vapour or milr,
as were the remains of the floor, and the whole path
of the fpout. This wind raifed boards, timbers, &c.
A joift was found on one end, driven near three feet
into the ground. The fpout probably took it in its
elevated ftate, and drove it forcibly down. The
tornado moved with the celerity of a middling wind,
and conftantly declined in ftrengch till it em".-
ceafed.'

Chap. 12.] [ 475 .]

CHAP. Xir.

OF THE HEAT OF THE A T MOS PHERE AND
IGNEOUS VAPOURS.

OljeSls of Meteorology as a Science.'— -Partly anticipated. — Tempera-
ture.— Heat of the Earth. — Effetts of the Sun's Rays en different
Mediums. — Difference ivitb refpeSl to Temperature between Land
and Water. — Effects of Clouds on the Temperature. — Of Evapora-
tion.— U/iufual Cold, ho-iv produced in Summer and Winter.—*.
Aqueous Meteors. — Igneous Meteors. — Fire Balls. — Shooting Stars.
— Ig-nes Fatni.

METEOROLOGY, in its mod extenfive
fenfe, would embrace a large fcope of fcience.
It includes every thing that concerns our atmofphere,
climate, temperature, vapours, fogs, dew, rain, hail,
ihow, the igneous vapours, as proceeding from in-
flammable air, and even thunder and lightning, and
all thofe phenomena which are produced by what is
termed natural electricity.

The arrangement adopted in thefe volumes, which
was the cleared that fuggefted itfelf to my mind, ne-
ceffarily excludes many of thefe fubjedts from the pre-
fent chapter. The electrical phenomena have been
already treated of, and the theory of rain, fnow, &c.
as adopted by the electrical philofophers, has been
briefly explained ; and what remains to be faid on
aqueous meteors will be more properly introduced in
the book which is dedicated to the fubject of water y
and will be better underftood when the properties of
that fluid are more fully explained. .

The

47 5 Temperature of tie [Book V.

The phenomena, which prefent themfelves for our
immediate confederation, will therefore be thofe which
are, ftrictly fpeaking, aerial or atmofpherical. The
temperature of the atmofphere will therefore, with
propriety, be confidered, and the igneous meteors
with which it is occafionally charged, and of which
the air appears not only to be the vehicle but the pa-
bulum.

The variations of temperature which we experience
are chiefly produced in the atmofphere, at no great
diftance from the furface of the earth. This is evi-
dent from a fimple and well known fad, that the
earth, at a certain depth beneath the furface, always
preferves nearly the fame temperature, and the degree
of heat at thofe depths generally approaches the mean
annual heat of the climate. Even where there is a
communication with the external air, the earth, at the
depth of 80 or 90 feet, commonly varies but little in
its temperature; and where there is no fuch commu-
nication the variation muft be ftill more inconfiderable.
Thus the temperature of fprings does not vary with
the feafon; and thus the cave of the obfervatory at
Paris, which is about ninety feet below the pavement,
preferves the conftant temperature of about 53 de-
grees, never varying above half a degree in the coldeft
years. Van Swinden has remarked, that the moil
extreme cold, even exceeding o in Fahrenheit's fcale,
if it endures for only a few days, penetrates no fur-
ther than twenty inches, even when the ground is not
covered with fnow, and not more than ten inches when
there is a coat of fnow on the furface of the earth.

The earth may, therefore, be confidered as the great
repofitory of heat; but when its furface is rapidly
cooled, the interior parts experience a diminution of
their heat in fome meafure proportionable, as the heat

is

Chap. 12.] Earth and Atmofpbere. 477

is in that cafe drawn off towards the furface. Hence in
Switzerland it has been remarked, that the fnow ge-
nerally begins to melt at the bottom ; and if the heat
of the fun- is not ftrong, the fame thing may be ob-
ferved in the progrefs of a thaw in this country.

The furface of the earth is capable of receiving a
great acceffion of heat from the fun's rays. But it has
been before remarked, that light has not the fame ef-
fecl: on a tranfparent medium, for thefe mediums af-
ford a free paffage to the rays of the fun, which ap-
pear to aft only as fire, when accumulated and con-
fined within the minuteft interftices of bodies. Hence
the tops of high mountains are always, even under
the equator, covered with fnow ; and hence at a cer-
tain height, which varies in almoft every latitude, it
freezes during the night in every feafon, as was ftated
in a preceding chapter.

Heat is obferved to diminim as we afcend into 'the
atmofphere, nearly in an arithmetical proportion. In
the vicinity of Paris, lat. 48° 50' the temperature of
the earth being 47°, at the eflimated height of 1 1,084
feet, it was found by M. Charles, the aeroftatical ad-
venturer, to be at 21° or 11° below congelation; near
Dijon, lat. 47% on the 25th of April, the temperature
near the earth was 56°, but at the height of 10,63 i
feet, it was found by M. Morveau to be 26° ; and
Lord Mulgrave, at the bottom of Hacklyt Hill, lat.
80°, found the temperature of the lower air 50° j but
on the fummit of the hill, 1 503 feet, only 42°.

Water refembles air in being little arTe-fled by the
paflage of the fun's rays j but the bottom of every fea
or lake, being opake, the heat is ftill capable of being
excited or collected there. Between water and earth
there is, however, this difference, that land or earth
(particularly if dry) receives heat very readily from the

rays

47 S ' Temperature of the [Book V.

rays of the fun, but conducb it through its own fub-
ftance very flowly to any great depth j whereas water,
from its tranfparency, receives heat from light but
flowly; but the heat is diffufed through the whole
mafs with gitat rapidity. Dr. Hales relates, that in
Auguft, 1724, when the air and the furfaoe of the
earth were both at 88, a thermometer, placed at only
two inches depth in the ground, flood at 85, another
at fixteen inches at 70, and another at twenty-four
inches at 68. The two laft preferved the fame tem-
perature day and night to the end of the month, and
then only fell to 63. On the -26^1 of Odober, a ther-
mometer expofed to the air by the fame philofopher,
(lood at 35° 5, but one funk two inches in the earth
•was heated to 43° 85, another funk fixteen inches
reached 48° 8, and one at twenty-four inches 50°.
He even found, that between the i ft and 21! of No-
vember, when the external air was ac 27% a thermo-
meter at twenty four inches depth ftood at 43° 8; but-
from March to September, the following year,, the ex-
ternal air was much warmer than the earth at fixteen
inches or two feet ; but the fcafon was rainy, and the
evaporation being confiderable, prevented rhe earth
near the furface. from being confiderably warm.'.'d.

From thefc experiments it r*ppears, that the furface
of the earth may be confiderably heated, and yet that
the heat fhall not penetrate to any confiderable depth;
it appears alfo, that the earth parts with its heat with
difficulty to the air, and will retain its natural temp-*-
rature, which is between 40° and 50% at a very fmall
depth beneath the furface, even when the air is below
the freezing point. In water, on the contrary, the
heat is not accumulated in a particular part, but is
equally diffufed through the whole mafs, and the tem-
perature, if the furface is extenfive, will be more in
4 agreement

Chap. 12.] ' Earth and sttmofpbere. 47 £

agreement with that of the atmofphere than with that
of the earth. Near Marfeiiles, Dr. Raymond found
the fand frequently -heated to 160°, but never found
the fea hotter than 77% and even this degree of heat ic
appeared to receive chiefly by its communication with
the'land, for on the ipth of July, 1765, he found that
part of the bay, which was next the land, heated to
74°, while the middle was 72°, and the entrance only
70°. In winter, he obferved the earth cooled down
frequently to 14° or 15°, but the fea never lower than
44° or 45°.

It is by the temperature of the atmofphere that we
always judge when we term the weather cold or hot;
but the atmofphere derives the greater pat t of its heat
from its communication with land or w.iter. The ri-
gours, therefore, of the winter's cold arc tempered by
the heat imparted from the earth itfdf ; yet as the
earth parts but flowly with its heat, and as the furfacc
is found to be extremely cool, while the interior parts
arc heated to the degree of 40 or 50, and as the heat
of water is more equally diffufed, and more readily
parted with, it follows that the portion of air, which is
incumbent on the fea, will be of a warmer tempera-
ture in the extreme cold of winter than that which is
incumbent upon the land.

I (lands are more temperate than continents, becaufe
they participate more of the temperature of the fea.
With refpecl to thofe countries alfo, which border on
the ocean, thofe which lie fouth of the Tea, at leaft in
our hemifphere, will be warmer than thofe which have
the fea to the fouth of them, becaufe the winds which
would cool them in winter, if they blew over-land,
are tempered by pafling over the fea, whereas thofe
which lie north of the fea are ccokd in fummer by the
breezes that proceed from it.

Every

480 Vegetation m ttigb Latitudes. [Book V.

Every habitable latitude muft enjoy a heat of 60°
at leaft for two months in the year, in order to pro-
duce and bring to maturity corn and the other ve-
getable productions. The quicknefs with which ve-
getation proceeds in high latitudes is chiefly owing to
the long duration of the fun above the horizon during
their fummer. Dr. Halley, indeed, has proved, that,
abftracting from the intervention of fogs, mifts, and
mountains of ice, the hotted weather might take place,
even under the poles, the duration of the fun's light
compenfating for the obliquity of its direction.

Among the caufes of the changes of weather in thefe
climates, efpecially with refpect to heat or cold, we
muft account the circumftance of the air being charged
with vapour. The air, when cloudy, is capable of
receiving and retaining more of the fun's heat, than
when clear, for the obvious reafon, that a tranfparent
medium permits thofe rays to pafs through it, which
are intercepted if the medium is thicker and lefs pel-
lucid. Hence a cloudy air is frequently found warmer
than the earth, on which it is incumbent. The air is
alfo warmed by the condenfation of vapour, and hence
the origin of hail, which is rain condenfed by patting
through air which is colder than that which produced it.

A continuance, however, of cloudy or mifty weather
will intercept the fun's rays from reaching the earth,
which will therefore be prevented from receiving its
due portion of heatv The winter of the year 1783-4
was unufually fevere j and it is to be remarked, that
during feveral of the fummer months which preceded
it, where the effect of the fun's rays to heat the earth
fhould have been the greateft, the whole continent of
Europe was covered with a. kind of fog, fuppofed to
proceed from the fmoke of fome volcanoes, near
Mount Hecla, in Iceland. This fog was of a dry kind,

and

Chap, i:.] Caujes of Change in the ' Atmofabere. 481

and confequently the fun's rays were incapable of difTi-
pating it j and they were fo faint, that in pafling
through it, when collected in the focus of a burning
glafs, they would fcarcely kindle brown paper *.

A principal caufe of the varieties and changes of
temperature, and a moft powerful agent in producing
cold, is evaporation. On this fubject it is remarked,-
firft, that in our climates the evaporation is about four
times as great between the vernal and autumnal
equinox as in the reft of the year. idly. Other cir-
cumftances equal, it is increafed in proportion to the
difference between the temperature of the air and the
evaporating furface ; it is confequently leaft when they
are nearly of equal temperature. The former part of
this propofition muft be underflood with fome reftric-
tion; for if the air is more than 15° colder than the
evaporating furface, there is feldom any evaporation at
all, and the air will more frequently, in that cafe, de-
pofit moifture than receive it. jdly. The degree of
cold produced by evaporation is much greater when
the air is warmer than the evaporating furface, than
when the latter is the warmer of the two ; for in the firft
cafe the dilation of the vapour is increafed, and in
fecond, it is checked. The more vapour is dilated,
the more fire or heat it abforbs ; and hence it is coldtft
in an exhaufted receiver, where it abforbs moft.
Hence warm winds, as the harmattan, firocco, &c. arc
more deficcatory than cold winds. 4thly. Evaporation
is always increafed greatly by a current of air flowing
over the evaporating furface. Hence a calm day is
always warmer than one in which there is a ftror.g
wind f.

*See Dr. Franklin's Meteorological Conjectures,
f Kirwan on Climate, c. I .

VOL. I. I i From

4? 2 Statt cf WeMhcr m woody Countries. [Book V.

From thefe fads, and from what is preyioufly re-
marked on the fubjeft of evaporation in the fecond
feoolc, it is plain, that trafts of land which are covered
with trees or luxuriant vegetables are much colder
than thofe where there is a lefs furface of vegetable
nutter, fuch grounds emitting one third more vapour,
according to fome experiments of Mr. Williams, than
the fame fpaee would if actually covered with water*.
Hence too, a reafon will evidently be found for that
amazing change of climate which a country undergoes
by being cleared and cultivated. America is not the
lame country at prefent, either with refpect to tem-
perature or falubrity, as when it was covered witb
woods; and Guiana affords a ftill more remarkable
inftance. Of that country, only a part has been cleared
from wood fmce the beginning of this century ; the
heat in that part is already become exceffive ; whereas,,
in the woody parts of the fame country, the inhabitants
are obliged to light a fire every night.

It is further obferved, that the pureft fprings arc
generally found beneath the friendly fhelter of a
grove j and that in proportion as the woodlands in
any country are cleared, the watercourfes are dimi-
nifhed.

Hence may be inferred the neceflity of preferving
trees about thole places whence water-fprings difcharge
their currents, if it is an object to preferve them j and
alfo of improving fmall fprings, by planting trees
around them, and efpecially oaks.

And hence,- alfo, it is a fair conclufion, that in
this climate, where the cold certainly predominates,
•woody fixations cannot be wholefome ; and that,
adjacent to houfes efpecially, the land Ihould be laid
open.

* Philad. Tranf. vol. ii. p. 150.

From

Chap. 1 2.] Caujes of unufual Cold. 4gj

From the whole of what has been ftated, it will
follow, that a wet fummer will generally be fucceeded
by afevere winter, becaufe the cloudinefs of the feafon
will prevent the earth from receiving a due portion of
heat, and becaufe the increafed evaporation will con-
tribute to leffen the quantity already lodged there.
Much will, however, depend upon other circumftances,
and particularly upon the courfe of the wind.

Unufual cold in fummer is produced — ift. From
the long continuance of eafterly or northerly winds.

adly. From frequent and heavy rains, which are fol-
lowed by a confiderable evaporation.

jdly, JFronva long continuance of cloudy weather,
which prevents the earth from receiving a proper por-
tion of heat from the fun.

Unufual cold in winter commonly happens—

i ft. From unufual cold or wet in the preceding
fummer. In January 1709, the weather was uncom-
monly cold, and it was remarked, that in the preceding
June the thermometer was near the freezing point,
and the ruin confiderable *.

adly. From the immediate effect of heavy rains,
followed by eafterly or northern winds. This ftate
of things produces cold in any feafon from the increafed
evaporation.

jdly. From wefterly or foutherly currents in the
\*pper regions of the atmofphere, while eaft or north
winds prevail nearer the furface of the earth.

4thly. From the arrival of Siberian or North Ame-
rican winds. It has been calculated, that wefterly
winds may arrive in a few days from America ; and if
the ocean has been previoufly cooled by northern gales,
even thefe will feem cold to us. The Siberian winds

» Derham's Phyfic. Theol. L L c. 3.

I i 2 will,

434 '-nc us jMttccr.i . [ Bt •

v.v1l, if they originate from a lower latitude, fcem to
us to come from the fouth-caft ; and it" they originate
in a higher latitude, they will appear north-eaft, be-
'caufe they will be deflected to the ibuth.

5thly. From the deicent of a fuperior flratum of the
Iphcre. This happens when a cold wind HI the
upper regions pafles o\'er a country where the I
ilrata of the atmofphere are Ipecifically lighter. —
Hence a low ftate of the barometer generally pre-
cedes extraordinary cold which is produced- from this
c'aufe *.

On the ftate of the atmofphere with refpeft to hea?
and eold, and ftill more on the degree of evaporation,
all the phenomena of the aqueous meteors of rain>
hail, fnow, &c. will be found to depend ; but thcfe
will be treated of with more propriety in another part
of thefe volumes. The igneous vapours are alto con-
nected with the fame caufcs, and are in a confiderabls
degree the effects of evaporation ; but their materials
are different, as well as their effects, though, from
their evanefcent nature, they are fcarcely at prefent
fufficiently vnderftcod.

As the phenomena which are ftriclly electrical have
been already treated of, the only meteors of the igne-
ous kind, which remain to be conlidered, may be re-
duced to three claffes, viz. fire-balls, falling-ftars, and
ignes fatui.

It has been already fta ted, that the atmofphere is she
general reiervoir of thofe particles which are exhaled
from every body which is volatile, OF fubject to eva-
poration. In fpeaking of the fire damp in mines it
has been fhewny that inflammable air will rife in large
quantities, and to a considerable height in the atrno-

* Kirwan on Climate, c. 15.

fphere.

'Ohap. i :•.] Ntiicns of the Ancient;. 4? 5

inhere. There are alfo ibme phofphoric matters, which
•will aitb occafionally be rendered volatile, and thefe
particles are fupplied in great abundance from all pu-
trefcent jubilances, whether animal or vegetable. It
has been fhewn, that hydrogen or inflammable air rea-
dily combines with fulphur3 and forms what is called
hepatic gas; it will afterwards appear alfo, that it will
combine with phofphorus, and the phofphorated hy-
drogen gas thus formed is remarkable for , the property
of fpontaneoufly inflaming v/hen it comes into contact
with atmofpherical air. Thus we are furnilhed with
fufficient materials f$r the formation of all the different
appearances that ;have juft been enumerated ; and
though the matter of the meteors themfelves has, for 4
the reafon affigned, jiever been chemically analized,
yet from analogy it is not diffipult to judge of their
nature and properties.

Thofe phenomena, which are clafTed together under
the general appellation of fire-balls, were divided by
the antients into feveral fpecies, according to the ex-
ternal form or appearan-ce which they afifumed. They
were alfo regarded by them in a mueh more formidible
light than they are by us, as the certain prognoltics of
great and awful events in the moral and political world.
Even the philofophic Cicero fpeaks of the t: ab oc-
cidente faces," as the certain harbingers or indications
of thole bloody Icenes which in his time ronvulfed and
defolated the Ro;r,an commoiiwealtli.

Under the geoeral name of comets^ Pliny enume-
rates .a number of thefe phenomena. . If the fire corrir
mences at one extremity of the meteor, and burns
by degrees, he terms it, from its form and appearance,
R temp, or torch ; if an extended mafs of fire pafles -lon-
gitudinally through the atmofphere, he calls it a dart j
and if its length and magnitude are confiderable, and it
J i maintains

48 6 Extraordinary Tnftances of [Book V.

maintains its ftation for any fpace of time, it is a beam ;
if the clouds feem to part, and emit a quantity of fire,
he terms it a cbafm * ; but this laft appear? t~ be, ftrictly
{peaking, an electrical phenomenon, indeed only a
ftrong and vivid flafh of lightning.

Several inftance? of thefe meteors are recorded by
the fame author. Durin6 t r .-^p.'-l- of gladiators
exhibited by Germanicus, or ( : ^em patted rapidly
by the faces of the fpedtators at noon- day. A meteor
of that fpecies which he calls a beam, he adds, was
feen when the Lacedemonians were defeated at fea, in
that memorable engagement which loft them the em-
pire of the fea f. He alfo mentions a fanguineous kind
of meteor, a flame as red as blood, which fell from hea-
ven about the 1 07 th Olympiad, when Philip of Mace-
don was concerting his wicked plan for enflaving the
republics of Greece J, He relates, that when he was
himfelf on the watch during the night in the Roman
camp, he was a fpe<ftator of a fimilar appearance — a
number of refplendent lights fixed upon the palifadoes
of the camp, fimilar, he fays, to thofe which manners
fpeak of as attaching themfelves to the mafts and yards
of a fhip|j.

In tropical climates thefe meteors are more common
and more ftupendous than in thefe more temperate re-
gions. * As I was riding in Jamaica,' fays Mr. Barb-
ham, ( one morning from my habitation, fituated about
three miles north-weft from St. Jago de la Vega, I faw
a ball of fire, appearing to me about the bignefs of a
bomb, fwiftly falling down with a great blaze. At
firft I thought it fell into the town j but when I came

* Lampades, faces, bolides, trabes, and chaftna coe'.i. See Ptin.
Nat. Hift. 1. ii, c. 25, 26.

f Plin. Nat. Hift. 1. ii. c. 25, 26.
J Ib. c. 27. U Ib. c. 37.

nearer,

Chap. 12.] Ignecus Meteors. 487

jiearer, I faw many people gathered together, a little to
the fourhward, in the Savannah, to whom I rode up,
to inquire the caufe of their meeting : they were ad-
miring, as I found, the ground's being ftrangely broken
up and ploughed by a ball of fire ; which, as they faid,
fell down there, I obferved there were many holes in the
ground j one in the middle, of the bignefs of a man's
head, and five or fix fmaller roundabout it, of the big-
nefs of one's fift, and fo deep as not to be fathomed by
fuch implements as were at hand. It was obferved,
alfo, that all the green herbage was burnt up near
the holes ; and there continued a ftrong fmell of ful-
phur near the place for fome time after/

Ulloa gives an account of one of a fimilar kind at
Quito *. < About jnine at night/ fays he, f a globe of
fire appeared to rife from the fide of the mountain Pi-
chinca, and fo large, that it fpread a light over all the
part of the city facing that mountain. The houfe where
I lodged looking that way, I was furprifed with an ex-
traordinary light, darting through the crevices of the
window-mutters. On this appearance, and the buftle
of the people in the ftreet, I haftened to the window,
and came time enough to fee it, in the middle of its
career, which continued from weft to fouth, till I loft
fight of it, being intercepted by a mountain that lay
between me and it. Jt was round, and its apparent di-
ameter about a foot. I obferved it to rife from the
fides of Pichinca, although, to judge from its courfe,
it was behind that mountain where this congeries of
inflammable matter was kindled. In the firft half of
its vifible courie it emitted a prodigious effulgence,
then it began gradually to grow dim ; fo that, upon its
difappearing behind the intervening mountain, its light
was very faint.'

* Ulloa, vol. i. p. 41.

I i 4 ' « Meteors

488 Inflances of [Book V.

e Meteors of this kind are very frequently feen be-
tween the tropics ; but they fometimes, alfo, vifit the
more teinperate regions of Europe. We have the de-
fcription of a very extraordinary one, given us by
Montanari, that ferves to Ihew to what great heights,
in our atmofphere, thefe vapours are found to afcend.
In the year 1676, a great globe of fire was feen at Bo-
nonia, in Italy, about three quarters of an hour after
fun-fet. It pafled weftward, with a moft rapid courfe,
and at the rate of not lefs than a hundred and fixty
miles in a minute, which is much fwifcer than the force
of a cannon ball, and at laft flood over the Adriatic
fea. In its courfe it croffed over all Italy; and, by
computation, it could not have been lefs than thirty-
eight miles above the furface of the earth. In the
whole line of its courfe, wherever it approached, the
inhabitants below could diftin&ly hear it, with a hifllng
noife, refembling that of a fire-work. Having patted
away to lea, towards Corfica, it was heard at laft to go
off with a moft violent explofion, much louder than
that of a cannon ; and, immediately after, another noife
was heard like the rattling of a great cart upon a ftony
pavement, which was, probably, nothing more than the
echo of the former found. Its magnitude, when at
Bononia, appeared twice as long as the moon one way,
'and as broad the other j fo that, confidering its height,
it could not have been lefs than a mile long and half a
mile broad *.

Two of thefe meteors appeared in this country in
the year 1783, of which a moft particular and truly
philofophical account, by Dr. Blagden, is publrflied in
the Philofophical Tranfaftions of the following year;
and as his defcription will apply to many phenomena
of the kind, I cannot take any better method to eluci-

* Goldfmith'i Hid. Earth, Vol. I. p. 382.

date

Chap. 1 2.] Extraordinary Metscrs. 489

-date this part of the fubjeft, than by preferring my
readers with a mort abftract of this very curious and
learned memoir.

The firft of the two meteors in queftion was feea
on the 1 8th of Atiguft, and was, in appearance, a lu-
minous ball, which rofe in the N. N. W. nearly round ;
it, however, foon became elliptical, and gradually
affumed a .tail as it afcended, and, in a certain part of
its courfe, feemed to undergo a remarkable change,
compared to buifting; after which it proceeded no
longer as an entire mafs, but was apparently divided
into a clufter of balls of different magnitudes, and all
carrying or leaving a train behind, till, having palled
the eaft, and verging confiderably to the fouth, it gra-
dually defcended, and was loft out of. fight. The time
of its appearance was about fixteen minutes paft nine
in the evening, and it was vifible about half a minute.
It was feen in all parts of Great Britain, at Paris, at
Nuits in Burgundy, and even at Rome, and is fuppofed
to have defcribed a tract of one thoufand miles atleafl
over the furface of the earth. It appears to have burft
and re-united feveral times; and the firft burfting of it
which was noticed feems to have been fome where over
jLincolnfhires perhaps near the commencement of the
fens. This change in the meteor correfponds with
the period in which it iuffered a deviation from its
courie. If, indeed, the explofion was any kind of
effort, we cannot wonder that the body mould be di-
verted by it from its direct line j and, on the other
hand, it feems equally probable, that if it was forced
by any caufe to change its direction, the confequence
would naturally be a leparation of its parts.

The illumination of thefe meteors is often fo great
as totally to obliterate the ftars, to make the moon
look dull, and even to affect the fpectarors like the

fun

49<> Met ears [Book V,

fun itfelf. When this meteor was oblerved at Bruflels,
die moon appeared quite red, but when it was paffed,
recovered its natural light. This effect, the Doctor
remarks, muft have depended on the contrail of colour,
and Ihews how large a proportion of the blue rays
enters into that light which could even make thtfiher
moon appear to have an excefs of red. The body of
the fire-ball, even before it burft, did not appear of an
uniform brightnefs, but corvfifted of lucid and dull
parts, which were conftantly changing their refpective
pofitions, fo that the whole effect was to fome eyes
like an internal agitation or boiling of the matter. By
the beft accounts that could be procured concerning
the height of the meteor, it feems to have varied from
fifty-five to fixty miles. In thefe two laft particulars it
feems to have wonderfully correfponded with fome
other phenomena of the fame kind.

A report was heard fome time after the meteor dif-
appeared, and this report was loudeft in Lineolnfhire
and the adjacent parts, and again in the eaftern parts of
Kent ; the report we may therefore fuppofe to be the
effect of the two explofions of the body, firft over
Lineolnfhire, and afterwards when it entered the con-
tinent ; a hiding found was faid alfo to have accom-
panied the progrefs of the meteor. Judging from the
height of the meteor, its bulk is conjectured to have
been not lefs than half a mile in diameter \ and when
we confider this bulk, its velocity cannot fail to aftonifh
us, which is fuppofed" to be at the rate of more than
forty miles in the feccnd.

The other meteor, which appeared on the 4th of
October, at forty-three minutes pad fix in the evening,
was much fmaller than the former, and of a much
fhorter duration. It was firft perceived to the north-
ward, as a ftream of fire, like die common Ihooting
ftars, but large i but prefcntly burft out into that in-
8 tcnfely

Chap. 12.] eb/eruedin 1783. 491

tenfely bright bluifh flame, which is peculiar to fnch
meteors. It left behind it a dufky red ftreak of fire,
and, except this, had no tail, but was nearly globular.
After moving not lefs than ten degrees in this bright
ftate, it became fuddenly extinct without any explofion.
The height of 4he meteor muft have been between
forty and fifty miles ; and its duration was not more
than three feconds.

The Doctor is of opinion, that the general caufe of
thefe phenomena is electricity, which opinion he
grounds upon the following circumftances : — ill, The
velocity of thefe meteors, in which they correlpond
with no other body in nature but the electrical fluid.
2dly, The electrical phenomt-na attending meteors,
the lambent flames, and the fparks proceeding from
them, which have fometimes damaged fhips and houfes
in the manner of lightning, and, added to thefe, the
hi/Ting found, refembling that of electricity pafllng
from a conductor. As a third argument in favour of
this hypothefis, the Doctor remarks the connection of
meteors with the northern lights. Inftances are re-
corded, where northern lights have been feen to join,
and form luminous balls, darting about with great
velocity, and even leaving a train like fire-balls. The
aurora borealis appears to occupy as high, if not a
higher, region above the furface of the earth, as may
be concluded from the very diftant countries to which
it has been vifible at the fame time. 4thly, The moft
remarkable analogy, the Doctor thinks, is the courfe
of at leaft all the larger meteors, which feems to be
conftantly from or towards the north or north-weft
quarter of the heavens. Of above forty different fire-
balls described in the Philofophical Tran factions, twenty
are fo defcribed, that it is certain their courfe was in
that direction j only three or four feem to have moved
the contrary way ; and with refpect to the remainder,

it

49 2 Theory of Igneous Meteors. [Book V.

jt is left doubtful, from the imperfect ftate of the re-
Jations.

Notwithflanding the Doctor's ingenious arguments,
I cannot, on my own part, fubfcribc to the opinion,
that thefe phenomena are altogether electrical. The
duration of the fire-bal], the unequal confiflency of the
mafs, and feveral other points in the narration, feem
to indicate that its materials were of a lefs rare and
evanefcent nature than the electric fire. The union of
phofphorus and hydrogen in the atmofphere, will fuf-
ciently account for the inflammation of thefe mafies of
volatile matter, and their colour will depend .on the
nature of the composition, as is plain from what has
t>een faid upon the fubject of the fire-worjcs producoj
from inflammable air *.

One inftance more ,of this kind of phenomena I
(hall beg leave to mention, particularly as it differs in
many refpects from the preceding; and from its dura-
tion, and the ftrong fmell which attended the explofion,
in feems not to have been the effect of "electricity.

On board the Montague, under the command of
Admiral Chambers, in lat. 42° 48'. long. 9° 3'. on the
4th of November 1749, about ten minutes before
twelve, as the author, Mr. Chalmers, was taking an
observation, one of the quarter- mailers defired he
would look to the windward. On directing his eye
that way, he obferved a large ball of blue fire about
three miles diftance from them j they immediately
lowered the topfails, but it came fo faft upon them,
that before they could raife the main-tack, they ob-
ferved ''the ball rife almoft perpendicularly, and not
nbove forty or fifty yards from the main chains, when
it went off with an explofion as great as if hundreds of
cannon hr.J been difcharged at the fame time, leaving
behind it a ftrong fulphureous fineil. By this t'xplo-
* i:ec Chap. V.

' f,.jh

Chap, i i.] Fatting Stzrs. 49-5

fion the main-topmaft was (battered in pieces, and
the main mad fcnt quite down to the keel. Five
men were knocked .down, and one of them was greatly
bruifed, and fome other damage of lefs importance was
done to the (hip. Juft before the explofion, the ball
fcemed to be of the fize of a large millftone.

The fhooting or falling ftar is a common pheno-
menon, but though fo frequently obferved, the great
diftance, and the tranfient nature of thefe meteors,
added to the entire confumption of their materials *,
have hitherto fruftrate-d every attempt to afccrtain their
caufe. It is, however, reafonable to fuppofe, thac they
are intrinficaliy the fame with the larger meteors, as in
mo ft of their properties they perfectly correfpond with
them. If the larger meteors are formed from any
mixture or combination of inflammable air with phof-
phorus, or any other fubftance, the fhooting flars are
probably the fame. If, on the contrary, the larger
meteors are electrical, there is equal reafon for fuppof-
ing the fmaller ones to proceed from the fame caufe.
Some philoibphers, indeed, reprefent both as mafies of
electricity, at fo great a diftance that their angular ve-
locity is not fufficient to prevent the eye from ciifcern-
ing their fhape. There are, however, three reafons
which Operate againft this hypothefis. ift, The height
of thefe meteors is frequently above that to which
clouds afcend, and clouds are the common atmofphe-
rical conductors of electricity. idly, They do not
proceed from a cloud, as flames of lightning uniformly
do. And, jdly, There is no noife refembling that
of thunder at their firfc emiffion or appearance j the

* It is a vulgsr notion, that the fmall mafles of white jelly,
which are forneumes found in the fields, are produced from the
falling ftars, and it is called ftar jelly. This jelly, however, i.3
the excrement of the heron, bittern, or fome animal of the crane
kind, which feed on aquatic animals, and have peculiar organs of
digeftion.

ncife

494- W>t Igms [Book V.

noife in the large meteors only takes place when the
mafs feparates or goes off like a fky-rocket, and in
this cafe the effecl: is fimilar to that of gunpowder, or
any difploding matter.

Concerning the nature and compofition of the ignis
fatuus, or will-o'-the wifp, there is lefs difpute ; the
generality of philofophers being agreed that it is caufed
by fome volatile vapour of the phofphoric kind, pro-
bably the phofphoric hydrftgen gas. The light from
putrefcenc fubilances *, particularly putrid fifh, and
thofe fparks emitted from the fea, or fea-water when
agitated in the dark, correfpond in appearance with this
meteor. Sir Ifaac Newton defines the ignis fatuus to
be " a vapour Ihining without heat ;" and it is ufually
vifible in damp places, about dunghills, burying
grounds, and other fituations, which are likely to
abound with phofphoric matter.

A remarkable ignis fatuus was obferved by Mr.
Derham, in fome boggy ground, between two rocky
hills. He was fo fortunate as to be able to approach
it within two or three yards. It moved with a brifk
and defultory motion about a dead thiltle, till a flight
agitation of the air, occafioned, as he fuppofed, by his
near approach to it, caufed it to jump to another place j
"and as he approached, it kept flying before him.
He was near enough to fatisfy himfelf, that it could
not be the mining of glow-worms or other infects—
it was one uniform body of light.

M. Beccaria mentions two of thefe luminous ap-
pearances, which were frequently obferved in the
neighbourhood of Bologna, and which emitted a light
equal to that of an ordinary faggot. Their motions
were unequal, fometimes rifmg, and fometimes fink-
ing towards the earth; fometimes totally difappearing,

* This fubjeft will be more amply treated of in the fucceeding
!?ook, under the title Phofphorus, Book VIII.

though

Chap. 12.] Fatuus.

though in general they continued hovering about fix
feet from the ground. They differed in fize and
figure ; and, indeed, the form of each was fluctuating,
fometimes floating like waves, and dropping fparks of
fire. He was afTured there was not a dark night in the
whole year in which they did not appear ; nor was
their appearance at all affected by the weather, whether
cold or hot, fnow or rain. They have, been known to
change their colour from red to yellow ; and generally
grew fainter as any perfon approached, vanifhing en-
tirely when the obferver came very near to them,
and appearing again at fbme diftance.

Dr. Shaw alfo defcribes a fmgular ignis fatuits?
which he faw in the Holy Land. It was fometimes
globular, or in the form of the flame of a candle j and
immediately afterwards fpread itfclf fo much, as to
involve the whole company in a pale inoffenfive light,
and then was obferved to contract itfelf again, and liid-
denly djfappear. In lefs than a minute, however, it
would become vifible as before, and run along from one
place to another j or would expand itfelf over more
than three acres of the adjacent mountains. The at-
mofphere at this time, he adds, was thick and hazy.

In a fuperftitious age we cannot wonder that thefe
phenomena have all been attributed to fupernatural
agency ; it is one of the nobkft purpofes of philofophy,
to releafe the mind from the bondage of imaginary
terrors i and by explaining the modes in which the
Divine Providence difpofes the different powers of na-
ture, to elevate our thoughts to the ctf^firft caufej to
teach us to fee " God in all, and all in God."

[BockV,

CHAP. XlH.

OF THE PROGNOSTICS OF THE
WEATHER.

Imperfetl Stat-' cfthis "Branch of Science.— Prognofiics of Weather front
the previous Slate of the Sea/an. — From the Undulations of the At-
mofpbere. — From the Barometer. — From Fogs.— From Chuds.—*
From ProfpeBs. — From the DFW. — From the Sfy* — From the Mc.cn.
"—From the Wind.

A METHODICAL arrangement of meteorolo-
gical phenomena, by which more certain prog-
nofticsofthe weather might be procured, is a great
defideratum in the fcale of ufeful knowledge. That
philofophers have already a confiderable acquaintance
with the nature of heat, water, and air, their numerous
and ingenious experiments {efficiently prove; but
when thefe three ingredients of nature are in a com-
pound ftate floating round our globe, and producing
all thcfe various agitations and combinations, known
under the general denomination of weather, then their
knowledge feems to be without fyftem, without cer-
tainty, and, contrary to the very end of true philofophy,
almoft without utility. From the combination of air
and water with heat, from their circulation and their
decompofitlon, ariles all that variety of weather of
which the atmofphere of all countries, and particularly
that of .iflands, is fo fufceptible.

The

Chap. I j.] Prognqfiics of tie Weather, 497

The atmofphere itfelfis influenced and modified by
the variations of its denfity ; by its humidity ; by the
precipitation of the aqueous particles into rain.j by
the wind; by the power of electricity; and by the
agency of heat and cold, as remarked in the preceding
chapter.

Though the fcience of predicting the weather is at
prefent vague and imperfect, becaufe it is but lately
that accurate obfervations have been made on the
changes of the weather, yet from what we may collect
from the works of De Luc, De Saufifure, Marfhallj
and Kirwan, we are authorized to expect fome fuc-
cefs in thofe inquiries. But it can hardly be fuppofed
that their obfervations, in the prefent (late of fcience*
will be fufficient to form a perfect theory, till fe-
conded by thofe of fucceeding times. For this falu-
tary end it will be neceflary to make as many obfer-
vations on the different figns of the weather as pofii-
ble, fince it is only by their combination and concur-
rence, that uncertainty can be removed.-

The principal means of predicting the changes of
weather, and particularly with refpect to rain or
drought, may be reduced to feven, viz. id. From
the preceding ftate of the weather, ad. From the
undulations of the atmofphere. 3d. From the baro-
meter. 4th. From the appearance of the clouds.
5th. .From the colour of the fky. 6th. From the
wind. 7th. From the moon.

I. As the caufes of every change of weather muft
have preceded for fome time the effect, it is in general
by an attention to its previous ftate, that we are
enabled to form the moft accurate judgment of what
weather is to be in future expe&edj from a feries of

Voi. I. K k obfervations

49s Prognoftics of [Book V.

obfervations made from 1677 to 1789, Mr. Kirwan
lays down the following rules or principles.

i ft. When there has been no ftorm before or after
the vernal equinox, the enfuing fummer is generally
dry, at leaft five times in fix.

2d. When a ftorm happens from an eafterly point
cither on the 1 9th, 2Oth, or 2 1 ft of March, the fuc-
ceeding fummer is dry, four times in five.

3d. When a ftorm arifes on the 25th, 26th, or 27th
of March, and not before, in any point, the fucceed-
ing fummer is generally dry, four times in five.

4th. If there mould be a ftorm at S. W. or W. S. W.
on the 1 9th, 2Cth, or 22d, the fucceeding fummer is
generally wety five times in fix.

Mr. Kirwan adds, that it rains lefs in March than
in November, in the proportion of feven to twelve.
It generally rains lefs in April than in October, in
the proportion of one to twoj lefs in May than
September, in the proportion of three to four. When
it rains plentifully in May, it generally rains but little
in September; and the contrary. A week is ac-
counted wet when it contains four wet days, or more;
a month, when it contains three wet weeks ; and a
feafon, or quarter of a year, when it contains two wet
months. He terms that a wet day in which rain falls
to the amount of one pound troy, in the fpace of a
IqOare foot.

In any given year, the probability of a dry fpring
is in the proportion of twenty-two to fix wet, and
thirteen variable. Of a wet fummer it is twenty to
fixteen dry, and five variable. Of a variable autumn,
nineteen to eleven of wet or dry. That is, out of
forty-one years the fpring in twenty-two will be dry,
&c. j and fo in proportion *.

• Mem. Royal Iriih Acad. Vol. v.

II. Among

Chap, i j.] ibe Weather. 499

. II. Among the various means of prognofticating
the weather, remarked by the late Mr. 'Adams f,
one of the moft important, in his opinion, feems to
be that undulating motion, or tumult in the air, which
is excited by the heat of the fun. The humidity
raifed from the earth by the heat of the fun, is fuf-
tained in the atmofphere by its heat, and the agita-
tion of the air. Though this motion is not always
vifible to the naked eye, yet by the help of a good
telefcope it becomes eminently confpicuous; every
object appears to be in violent agitation, and the
boundary line of the fenfible horizon, which would
otherwife be clear and well defined, is waved like a
field of corn agitated by the wind, or the furface of
the fea in a frefh gale. While thefe undulations con-
tinue in the air, the vapours remain there ; but when
the fun departs, and they fubfide, thefe aqueous par-
ticles become condenfed, and defcend to the ground
during the night, and in the morning affume the ap-
pearance of dew.

III. The greateft arquifition, perhaps, that ever was
made to natural philofophy, with refpect to afcer-
taining the changes of the weather, was the difcovery
of the Barometer. The nature and ufes of this inftru-
ment have been previoufly defcribed J. It is evident,
- that when the mercury rifes in the tube, the preffure,
weight, or denfity of the air muft be augmented ; but
the relation that exifts between this preffure and the
change of weather, which does not take place feme-
times till ten or twelve hours afcerwards, ftill remains
to be explained.

The preffure of the air upon the refervoir of the
barometer proceeds in general from its weight, and

f Diflertation on the Barometer.
I See this Book, Chap. IX.

K k 2 fometime*

500 Signs of Weather [Book V.

fometimes from its elafticity. It has been proved,
that thefe two properties of air fometimes vary, and,
confequently, the preflure which they produce. When-
ever the air diflblves a great quantity of water, its
fpecific gravity is increafed ; the column of air which
refts upon the reftrvoir of the barometer becomes hea-
vier, and the mercury riles. If the folution is not per-
fect, the tranfparency of the air will be clifturbed j
hence a kind of milt will be produced, which will ge-
nerally caufe the mercury in the barometer to rife ;
but if the folution is perfect, the tranfparency of the
air will be complete, and fine weather return, as the
mercury in the barometer predicted by its afcent.
While certain caufes determine this water, which is
held in folution, to defcend into the lower region of
the atmofphere, before it is fufficiently condenfed to
be regularly formed into rain, there is another part
of it which will have previoufly arrived at the furface
of the earth. As a proof of this, it is obfervable, that
when the weather is about to change to rain, all bodies
which are impenetrable to water, fuch as bars of iron,
hard ftones, &c. are found to be moift or wet. The
column of air which prefles upon the refervoir of the
barometer, will become, therefore, lighter by the lofs
of that portion of water already arrived at the earth ;
and the barometer will defcend, and predict the rain,
which will come in a fliort time after, being formed by
the remainder of the water, which will then have had
time to be formed into regular drops *.

It muft be confeffed, that there are fome appear-
ances which feem to contradict this explanation. It
fometimes happens, that the barometer riles even
during rain, while the air difcharges itfelf of the water
which it held in folution : it alfo happens frequently,

• Briflbn. Vol. i.

cfpecially

Chap. 13.3 from the "Barometer. 501

efpecially in the winter, that, during whole months,
every time that the mercury rifes in the barometer,
rain continues to fall ; and every time that it defcends,
fine weather returns. Still this may be reconciled to
what has been dated ; for fmce (as has been already
obferved) it is the great quantity of water ditfblved in
the air which augments its weight, if, therefore, during
rain, a new folution of water mould by any means be
effected in greater abundance than the quantity which
falls (and this we know may happen from various
caufes) the barometer will rife. If the water fo dif-
fblved remains in the lower region, this rife of the
barometer will predict a frefh fall of rain, which often
happens in fuch cafes. In fliort, if the air diffolyes 3
great quantity of water, and at the fame time cold, or
fome other caufe, mould impede that water from dif-
folving perfectly, and rifing to a great height, it will
augment the weight of the air in a proportionate de-
gree, and will caufe the barometer to rife j and in the
mean time it will be ready to be collected into drop§,
and formed into rain, which will foon after take place.
"While this rain continues to fall, if there is no new
folution effected, the air will become lighter j the ba-
rometer will fall; and, notwithftanding that, it will
predict fine weather, which, according to this rule,
ought to happen. That kind of relation which appears
to fubfift between the weight of air and the change of
weather, according to circumftances, may be atrcou ited
for, therefore, in this manner. Fine weather may
happen, notwichitanding the diminution of the weight
of air, when fome other elaftic fluid, lighter than it,
becomes intermixed with it, without taking away the
tranfparency. In fhort, the elafticity of air, the force
oi' which may vary from different caufes, will Hill con-
tribute to vary its preffure. This elafticity acts fome-
K k 3 times

co i Signs of Weather [Book V.

times in conjunftion with the weight, fo as to increafe
the effect of it ; at other times it acts in a contrary
way, and may aifo diminifh, or even counterbalance,
the effect of the augmentation of weight. It follows,
then, that fine or bad weather may continue, however
high the mercury may be in the barometer j and ft ill
this does not weaken the explanation which has before
been given of this fact.

Obfervation, however, in thefe cafes is always pre-
ferable to theory ; and from long and attentive ob-
fervation, and from a careful infpection of thole of
other philofophers, Mr. Adams was enabled to lay
down the following -principles in his ufeful treatife on
this inftrument.

1. It generally happens, that, when the mercury in
the tube falls, the air being lighter, it will depofit its
vapour, and produce rain : but when it rifes, the air
being heavier, the vapours will be fupportedj and fine
weather is the ufual confequence.

2. When the mercury falls in frofty weather, either
fnow or a thaw may be expected j but if it riles in
the winter with a north or eaft wind, it generally fore-
bodes a froft.

3. It is neceflary to attend to the progrefs of the
rife and fall 3 thus, if it finks flowly, the rain may be
expected to be of fome continuance. In the fame
manner, when the mercury riles gradually, we may
be inclined to believe, that the fine weather will be
lafting.

4. When the barometer is fluctuating, rifing and
falling fuddenly, the weather may be expected to be
like it, changeable.

5. When it falls very low, there will be much rain.

6. But if its fall is low and fudden, a high wind fre-
quently follows.

7. When

Chap. 13.] from the Barometer. 503

7. When an extraordinary fall of the mercury hap-
pens, without any remarkable change near at hand,
there is fo;ne probability of a ftorrn at a diftance.

8. The barometer will defcend fomecimes as an in-
dication of wind only ; nor is its rife always a certain
fign of fair weather, particularly if the wind is to the
north or the eafb.

9. A north-cad wind generally caufes the barometer
in England to rife, and it is generally lowed with a
fouth-wcd wind.

If the air in foggy weather becomes hotter by the
action of the fun alone, the fog generally diffipates,
and the air remains ferene ; but if the barometer falls,
and the change of temperature is from a fouth or fouth-
wed wind, the fog rifes and forms into clouds, and its
afcent is generally a fign of rain.

" We have," fays Mr. Adams, " at prefent no cer-
tain data from obfervations, whereby certain conclu-
fions may be formed relative to fogs, and their con-
nection with rain.'*

In winter, when the cold decreafes fuddenly, rain
may be expected •, but in fummer, a fudden increafe of
heat forebodes rain.

IV. Several prognodic figns of the weather may be
collected from the various appearances of the clouds ;
when they appear to difiblve fuddenly into air, and be-
come invifible, it may be confidered as a itrong indi-
cation of fair weather ; but, on the contrary, when
they feem to form themfelves^into maficsfrom the fur-
rounding air, and to increafe in denfity and magnitude,
rain may reafunably be predicted.

Upon the approach of heavy rain every cloud rifes

larger than the preceding one, particularly when a

thunder-dorm is near, when f nail fragments of clouds

collect, and in a little time cover .the whole face of the

K.k 4 ' Iky.

504 Prognoftics from the Clouds. [Book V.

iky. Fifhermen, by this rule, frequently prognofticate
a ftorm, from a fmall point of a cloud appearing on the
vifible horizon at fea.

When the clouds appear like fleeces, deep and denfc
towards the middle, and white at the edges, with a
bright blue fky about them, either hafty fhowers of
rain, hail, or fnow, may be expected.

Mr. Jones, in his philofophical difquifitions, fays,
that he predicted a high wind forty hours before it
began, from the complexion of a fingle cloud, with
white edges, and dark diverging lines from it j after
this appearance there was a great dorm, which laded
for two days and two nights.

When the clouds, as they come forward, appear
to diverge from a point in the horizon, a wind may
be predicted, either from that or the oppofite quarter.

When the fky is covered with clouds above, and
there are fmall black fragments of clouds, like Imoke,
fly ing underneath, rain is generally near, and frequently
lading.

The mod certain fign of rain is two different cur-
rents of clouds, efpecially if the lower current flies
fad before the wind j when two fuch currents appear
in hot weather, they forebode a thunder-dorm.

The inhabitants of the Alps, when didant objects
appear didinct and well defined, and when the fky ap-
pears of a deep blue, fuppofe it a decifive fign of rain,
though no other fign of it may appear. The blue
colour of the fky in any country is certainly occafioned
' by a quantity of vapour equally diffufed through the
air at the time.

Mr. Adams obferves of the dew, that, when it ap-
pears plentifully upon the grafs after a fair day, another
fair day may be expected ; but if after fuch a fair day
there is no dew upon the ground, and no wind dirring,
6 it

Chap. 13.] Prognojtics from the Sky. 505

it is a fign that the vapours afcend, and that there will
be an accumulation above, which mud terminate in
rain. When the dew, or hoar froft, abounds at an
unufual feafon, and the barometer is low, it is in ge-
neral a fign of rain.

V. As they#y indicates the ftate of the vapours In
the atmofphere, its colour may be confidered as an index
to the weather.

When the vapours, which appear red in the even-
ing, are difperfed, the fky in the morning in general
becomes clear j but when they continue to float in the
atmofphere, the morning fky becomes red alfo, and
rain frequently follows.

When a lowring rednefs fpreads far upwards from
the horizon, whether in the morning or evening, it;
is fucceeded frequently by either rain or wind, fome-
times by both.

When a rednefs in the fky extends tov/ards the
zenith in the evening, the wind may be expecled to
proceed from the weft, or fouth-wefl, accompanied
with rain in considerable quantity. Perhaps one of the
moft certain figns of fine weather is the loftinefs of
the canopy of the fky.

As the rays of light which pafs from the fun, moon,
or ftars, to the earth, are certainly affedbed in their co-
lour by the ftate of the vapours through which they
pafs, thofe colours may be confidered as indications
of the quantity and nature of thofe vapours.

When the clouds in the eaft, about fun-rife, appear
of a gay orange colour, it is generally, and not impro-
perly, fuppofed to be a fign of rain.

VI. The firft of Roman poets, and not the laft of
natural philofophers, Virgil, obferves, that a pale moon
is a fign of rain ; that a red one forebodes wind ; and

that

5^6 Prognojlifs frcm the Moon. [Book V.

that when me wears her own natural whitenefs, with a
ferene fky, it is a fign of fair weather.

Mr. Jones, in his phyfiological difquifitions, fays,
that when it rains during a moon, the following change
will probably produce clear weather for a few days,
and then a continuation of jrain j but on the contrary;,
•when it has been fair throughout, and it rains at the
change, the fair weather will probably be reftored
about the fourth or fifth day of the moon, and continue
as before. This gentleman adds confiderable weight
to this obfervation, by afferting, that he has rriadc hay
after thefe prognoftics for twenty years, without having
once had the mortification to fee it damaged by rain.
| muft, however, confefs, that the reafon of the
•feet is not clear to my mind j and I therefore give it
iblely upon his authority, and recommend it to fu-
ture obfervers to confute or confirm it by accurate
obfervations.

VII. A whittling, or howling wind has been generally
efceemed almoft an infallible fign of rain.

Though thefe principles have never as yet been re-
duced to a regular fyftem j yet from obfcrving care-
fully the above prognoftics, or rather the combinations
and coincidences of them, very tolerable conjectures
may be formed of the weather which may be expected,
particularly with refpect to drought or moifiure. It
is obfervation only, however, which can enable any
man to form fuch conjectures with tolerable accuracy.
The knowledge of weather is rather a practical than
fpeculative fcience; to " difcern the face of the fky"
was an art poflefied by ruftics at a very remote period
of foeiety ; and, at this time, the judgment of a fliep-
herd or ploughman on this fubject will commonly be
found a more infallible guide than that of a philoibpher.

Chap. -14,] [ 507 ]

CHAP. XIV,

AEROSTATION.

lliftory cf Aercftation.~Difco<very of Air Balkans ly M. M. Mont got-
fier- — Firft Balloon exhibited at Annonay. — Ealloon filled with /«-
flammable Air exhibited at Paris, — Pilatre de Rozier afcends in a,
Balloon. — F irft Balloon exhibited in England. — Afeent of M. Lu-
nar di, — Voyage of M. Blanchard and Dr. Jeffries acrofs the Chan-
nel.— Unfortunate Catajlrophe of M. M. de Rosier and Remain.—.
Mr. Baldwin' i Defer iption of the Profpeci from a Balloon. — Prin-
ciples of Aerojiation. — Modes of filling Balloons. — Ufe to which they
ha-ve been applied.

WHEN the principles of natural philofophy arc
confined to theory only, they may amufe and
inftruct the inquiring few, without exciting either the
curiofity or admiration of the multitude •, but when
thofe theories are reduced to practice, and illuftrated
by experiment, it becomes then more generally in-
terefting, and attracts the attention of the moft unin-
formed minds. Perhaps the principles upon which the
air balloons are conftructed might be among the
amnfing fpeculations of a Boyle or of a Newton, but
the actual exhibition of thofe aerial machines was re-
ferved to awake the curiofity, and excite the aftonifh-
ment, of the prefent'age.

The Hon. Henry Cavendifh, in the year 1766, dif-
cqvered that inflammable air was at leaft feven times
lighter than common air. - Soon after this it occurred to
Dr. Black, that-if a bladder^ futfkiemly light and thin,

was

50$ Speculations of Dr. Black and Mr. Cavallo. [Book V.

was filled with this air, as the mafs would be fpecifi-
cally lighter than the fame bulk of common air, it
would necefiarily rife in that fluid. A few years after-
wards Mr. Cavallo made fome experiments on this
fubjecl, and to him belongs the honour of bringing the
fuggeftion of Dr. Black firft into public notice, in a
paper which was read to the Royal Society on the
2Oth of June 1782. He found that the thinneft blad-
ders were too heavy, and that China paper was
permeable to the inflammable air j he proceeded there-
fore no further than blowing up foap-bubbles with in-
flammable air, which afcended rapidly to the ceiling of
a room, and broke againft it, and theie may be termed
the firft inflammable air balloons which were ever ex-,
hibited.

While the art of aeroftation was thus on the point
of being difcovered in Britain, M. M. Stephen and
Jofeph Montgolfier, paper manufacturers at Annonay,
in France, diftinguifhed themfelves by exhibiting an
aeroftatic machine of confiderable magnitude *.

After various inferior experiments, a grand one
was made at Annonay, on the 5th of June, 1783,
before a great multitude of fpectators. A flaccid bag
was fufpended on a pole thirty-five feet high ; draw
and chopped wool were burnt under the opening at
the bottom ; the vapour, or rather fmoke, foon inflated
the bag fo as to diftend it in all its parts, and this im-
menfe mafs afcended in the air with fuch rapidity, that

* The principle upon which the aerial machines of MefTrs,
Montgolfier were conftrudled was that of air rarefied by heat, by
\vhich it became expanded, and therefore difpofed to afcend in the
common air. As in various other philofophical experiments, fo
in this of the two brothers, accident offered her precarious aid, and
they had the judgment to make a proper application of a cafual
difcovery.

in

Chap. 14.] Diftovery of Montgolfier. 509

in iefs than ten minutes it reached the height of fir
thoufand feet. It was carried in a horizontal di-
rection to the diftance of feven thoufand fix hundred
and fixty-eight feet, and then defcended gently on the
ground.

The true caufe of the afcent of thefe machines is,
the air being rarefied and expanded within them by
the application of heat.

Thefe experiments were no fooner communicated
to the philofophers of Paris, than it occurred to them,
that as the weight of inflammable air was not more
than the eighth or tenth part of that of common air, a
balloon might be inflated with this light air, which
would anfwer all the purpofes of thofe of M. Mont-
golfier, with feveral additional advantages. They
conftructed a globe of luteftring, which was made im-
pervious to the inclofed air by a varnifh of elaftic gum
diffolved in fpirits or eflential oil. On the 2jd of
Auguft, 1783, they began to fill a globe of thirteen
feet diameter with inflammable air ; on the 27 th of the
fame month it was carried to the Champs de Mars, and
being difengaged from the cords, it arofe in two mi-
nutes to the height of three thoufand one hundred and
twenty three feet. When this balloon went up, its
weight was thirty- five pounds Iefs than the fame bulk
of common air.

The firft perfon who afcended into the atmofphere
in one of thefe machines was M. Pilatre de Rozier.
On the I5th of October, 1783, this adventurer went
up from a garden in the Fauxbourg St. Antoine in
Paris, in a balloon of the Monrgolfier kind, or thofc
inflated by heat or rarefied air ; its diameter was about
forty-eight feet, and its height about feventy-four ; he
afcended from amidft an aftonimed multitude to the
height of eighty-four feet from the ground, and there

kept

510 M. Lunar dis AJcent. [Book Vi

kept the machine afloat during four hours and twenty-
five minutes by repeatedly throwing ftraw and wool
upon the fire. It tjien defccnded to the ground, and
the intrepid adventurer allured the fpectators, that he
had not received the leail inconvenience during this
aerial excurfion.

The firft balloon was exhibited in England on the
25th of November, 1783, in the Artillery-ground,
London, by Count Zambeccari, an ingenious Italian.
It was launched from that place at one o'clock in the
afternoon, and at half paft three was taken up near
Petworth, in Suflex, forty-eight miles from London.
It therefore went nearly at the rate of twenty miles an
hour, and its defcent was occafioned by a rent fup-
poled to be the effect of the rarefaction of the inflam-
mable air, when the balloon afcended to the rarer
parts of the atmofphere.

The firft aerial navigator, however, who amufed
the intelligent, and aftonifhed the uninformed of this
country, was Vincent Lunardi, a native of Italy •, his
balloon was about thirty-five feet diameter; the air
for filling it was produced from zinc, by means of a
diluted vitriolic acid. He afcended from the Artil-
lery-ground at two o'clock, on the ifth of September,
1784. His balloon firft took the direction of north
weft by weft, but it foon fell into a current of air which
carried it nearly north. At ten minutes paft four he
defcended on a meadow near Ware, in Hertfordfhire :
during the courfe of his voyage the thermometer was
as low as 29% and the drops of water which adhered
to the balloon were frozen.

I was myfelf a fpectator of the flight of M. Lunardi,
and I muft confefs I never was prefent at a fight fo
interefting and fublime. The beauty of the gradual
afcent, united with a fentiment of terror on account

of

Chap. 1 4.] Aerial Voyage acrcfs the Channel. 511

of the danger of the man, and the novelty and gran-
deur of the whole appearance, are more than words
can exprefs. A delicate woman was fo overcome
with the fpectacle, that me died upon the fpot as the
balloon afcended j feveral ftinted; and the filent ad-
miration of the anxious multitude was beyond any
thing I had ever beheld.

The moft daring of all aerial voyages, however, was
that performed on the yth of January, 1785, by M.
Blanchard and Dr. Jeffries, acrois the Straits of Dover
to France. At about one o'clock, the ballocn was
launched near the high .cliff in that vicinity j the ballidt
was all thrown out except three bags of ten pounds
each; there being but little -wind their progrefs v/ss
very flow ; they defcribed the profpecl; which they had
of the fouthern coaft of England as extremely delight-
ful ; and they were able to count thirty-feven villages.
Perceiving the machine to defcend, they threw out at
feveral times all their ballaft, books, &c. and at about
twenty -five minutes paft two, they had a moft en-
chanting profped of the French coaft. « We threw
away,' fays Dr. Jefferies, c our only bottle, which, in
its defcent, caft out a fteam like fmoke, with a ruming
noife, and when it ftruck the water, we heard and fek
the fhock very perceptibly on our car and on the bal-
loon.' At length they paffed over the high lands
between Cape Blanc and Calais, when the machine
rofe to a greater height than it had reached during
the whole voyage. They defcended in fafety among
fome trees in the foreft of Guiennes. In confequence
of this voyage, the king of France prefented M.
Blanchard with a purfe of 12,000 livres, and granted
him a penfion of 1,200 livres a year.

The art of navigating through the air made fo rapid
9. progrefs, that within two years from its firft difco-

verjr

5ii Pi fa/re de Rozier and Remain. [Book V.

very more than forty different perfons performed the
experiment without any material injury j and it may
be juftly queftioned, lays M. Cavallo, whether the
firft forty perfons who trufted themfelves to the fea in
boats or veflels efcaped fo fafe. We mutr, however,
conclude this account of aerial travellers by a melan-
choly fid, the fate of the gallant Rozier (who had
been the firft aerial navigator) and of his companion,
M. Romain.

This unfortunate experiment was undertaken with
a view cf difcovering a method of raifing or lowering
aeroftadc machines at pleafure. For this purpofe a
imall balloon with rarefied air was attached to the
larger one, which was filled with inflammable air.
The fmall montgolfier was placed at a proper diftance
beneath the larger one, and it was fuppofed, that by
increafing or diminiming the fire in the lower machine,
the abfolute weight of tire whole would be propor-
tionably diminimed or augmented.

On the i4thofjune, 1785, the fe gentlemen afcend-
cd in the machine, prepared as has been related. They
had not been long in the air, when the balloon, filled
with inflammable air, was feen to fwell very confider-
bly, and the aeronauts appeared very anxious to open
the valves, and facilitate their defcent, by letting the
inflammable air efcape. The whole machine was
fhortly after obferved to be on fire, at the height of
about three quarters of a mile from the ground. The
lilk, which compofed the inflammable balloon, was
about a minute after perceived to collapfe, and the ap-
paratus defcended with fuch rapidity that both of the
gentlemen were killed. M. P. de Rozier appeared
quite dead when he reached the ground ; M. Romain
was found with fome ligns of life, but expired almofl
immediately after.

This

Chap. 1 4.] Principles of Aerojlatkn. 513

This dreadful cataftrophe feems to have contributed
to put an end to thefe experiments. Mr. Baldwin,
of Cheder, however, afcended from that city in the
month of September, in the fame year, and has pub-
lilhed a very accurate and curious account of his ob-
fervations during his voyage. In his afcent he ob-
ferved, that the lowed bed of vapour neareft the earth
appeared like pure white clouds in detached pieces,
which feemed to increafe as he rofe. They prefently
coalefced, and formed, as he fays, f a fea of cotton,
tufting here and there by the action of the air in the
imdifturbed part of the clouds.' The whole foon be-
came an extended white floor of cloud j above which,
at great and unequal diftances, he obferved a vaft af-
femblage of thunder clouds, each parcel confifting of
whole acres in the denfeft form j he compares their
form and appearance to the fmoke of cannon, only
denfer, and fomewhat refembling vaft mafles of fnow.
Some clouds had motions in flow and various direc-
tions, forming a fcene upon the whole truly flupendous
and majedic.

The principles on which balloons afcend in the at-
mofphere will, after what has been dated, be eafily
underftood. It is a well known rule in hydrodatics,
that when a body is immerfed in any fluid, if its weight
is lefs than an equal bulk of that fluid, it will rife to
the furface, but if heavier it will fink, and if equal it
will remain in the place where it is fird dationed. On
this principle, fmoke or vapour afcends in the atmo-
fphere, and heated air in that which is colder. That
heated air will afcend is eafily proved, by bringing a
red-hot iron under a fcale of a balance, which will in-
ftantly afcend, becaufe the hot air, being lighter than
that which is colder, afcends, and drikes the bottom,

VOL. I. L 1 and

514 Modes cf inflating Balkons. [Book V.

and impels ic upwards; but as the denfity of the atmo-
fphere decreafes, on account of the diminifhed preffure
of the fuperincumbent air, and the elaftic property
which it poffefTes, at different elevations above the
earth, an air balloon can rife only to a height in which
the furrounding air will be of the fame fpecific gravity
with itfelf. When it is in this fituation, it will either
float, or be carried in a direction with the wind or cur-
rent of air which it may happen to encounter in thofe
upper regions.

The whole theory of aeroftation depends upon this
' principle ; for the fame effect is produced, whether
we make the air lighter, by introducing heat into it,
or inclofing a quantity of gas lighter than the com-
mon airj both will afcend on the fame principle.
Philofophers have found by experiments, that a cubic
foot of air weighs about five hundred and fifty-four
grains, and that it is expanded by every degree of heat
marked on Fahrenheit's thermometer, about one
five-hundredth part of the whole ; by hearing, there -
• fore, a quantity of air to five hundred degrees, we
double its bulk when the thermometer (lands at 54*
in the open air, and consequently its weight will be
ciiminifhed in the fame proportion.

With refpect to the mode of inflating a balloon with
headed air, nothing more is neceffary than the injec-
tion of heat into the machine, by burning combuflibles
under it. The air for filling the inflammable air-
balloons 'may be obtained in feveral ways, but the
bed methods are, by applying acids to certain metals,
or, by expofing a quantity of water with certain mi-
neral fubftances, in a clofe vellel, to a ftrong fire.
M. Lavoifier, for this purpofe, made the fleam of
boiling water pafs through the barrel of a gun kept
5 red-

Chap. 14.3 Ufes of Balloons. 515

red-hot by burning coals. Dr. Prieftley recommends
a tube of red-hot brafs, filled with fmall pieces of
iron. By this method inflammable air is produced,
the fpecific gravity of which is to that of common
air as i to 13.

The beft varnifh for coating the filk of the bal-
loon in order to retain the inflammable air, is that
ufed by M. Blanchard, which confifts of elaftic gum,
or caoutchouc, cut fmall, and boiled in five times its
weight of oil of turpentine, the folution being after-
wards boiled for a few minutes with dryingilinfeed
oil. This varnifh muft be ufed warm. An aperture,
with a valve, to which is attached a cord, muft be left
in the top of the balloon, to prevent its buriling, by
too great inflation, in the higher regions, where the air
is lefs denfe.

The only practical ufe hitherto difcovered for bal-
loons, is that to which the French engineers have ap-
plied them in the prefent war, which is, by raifmg them
to a convenient height, to enable the engineer to re-
connoitre the camp of the enemy, or a fortified place,
fo that he can direct the attack to that part which is
moft eafily afiailed.

That fo extraordinary an invention mould, however,
terminate here, it is not eafy to believe. Thfi curiofity
of the public has for the prefent been fatiated ; and
the few accidents which have happened have dimi-
nifhed the fpirit of adventure. The difficulty, indeed,
of regulating the courfe of thefe aerial machines feems
an almoft infurmountable bar to their general utility.
But who will prefume to fet bounds to the ingenuity
and courage of man ? The firft mortal, who com-
mitted himfelf to the waves on a mifhapen raft, had
probably no fufpicion of even thofe trivial improve-
ments

516 Ufa of Balloons. [Book V.

mcnts which were foon to fucceed; and the art of
navigation was long known, before the mariners com-
pafs enabled the daring but fcientific genius of a Co-
lumbus to traverfe the vaft expanfe of the Atlantic
ocean.

END OF THE FIRST VOLUME.

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