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 wifh y 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.~~Ne r ujtonian Paradox. 
 
 CHAP. IV. page \6. 
 Of Attradion and Repiilfion. 
 
 fiiii Kindt sf Attraction. CokeJtonCombination~-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 ' iit J*"~ Ex t eriments 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'hc f 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~zL'>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.Drftti 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 conJtru8ed t 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 ad 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. 
 
 BOK
 
 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 BaUoons t <-~C.uriou$ 
 aerial Fireivor&s. 
 
 CHAP. VI. page 397, 
 Nitrous Air or Gas. 
 
 Nature of this Fluid. Ho-iv produced. /// Properties. Rejifts Putre? 
 fation.-^-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 ftatura y 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 JncientisNew 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, 
 v are 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. 
 
 T7X1 i ENSIONis 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 tho r e 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 mode 1 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 attraction 7 , 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 in 4 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 plain t 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 i63o / 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. 6o Q 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 I245'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 rr ow 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 b y 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 p f 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
 
 5 S 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 C 3 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 c y 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 pullies 3 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 f or 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> d y 
 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 d t 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 e y 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 b t 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 pq y pr 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 u y 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 c 9 
 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 h t which has nine teeth, 
 and a fcrew which has but one thread, and which, at 
 each round, caufes only one toth 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 b y 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 fubjel. 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 inc s 9 lines - 8o T Vy 
 Rarified air, - - barom. at 6 - 1 1 lines - 8o T \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 ^ -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 T A- - 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 water 3 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 de r 
 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 con r 
 
 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 
 
 3l 
 
 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 cone 1 .?- 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 ther 7 " 
 
 mcmeter 32277 240 
 
 Greateft heat of his final 1 air furnace 21877 J 6 
 
 Caft iron melts 17977 *3 
 
 Greateft heat of a common fmith's forge 1/327 125 
 
 Welding heat of iron, greateft *34 2 7 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 g 2O 
 
 Colours of iron arc burned oft* - _ g oo 
 
 Mercury boih, by fome placed at 600 700 
 
 Polifhed iron takes \f\ill blue ^ oo 
 
 Polifhed iron takes a purple . . 66 O 
 
 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 AQ 
 
 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 ofArijtotle 3 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,rfel 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 refl p clion, 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 fie 
 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. xj 
 
 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.' 
 
 Higgini 1 ! 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 plane 1 -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\ r e 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 b y and it 
 is reflected back exactly in the fame direction j e c is a 
 ray falling obliquely on the furface at c 3 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 d y 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 i t 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^ed y 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, , d y <?, 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, b y , 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 water 3 
 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 a y 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 ef t 
 and dh y (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 dC f 
 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 q y 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 k 3 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 x y 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. 
 
 fi g- 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, hi r . 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 h y 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 
 r r iys 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 E 3 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'rface 1 
 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 q t 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 q y 
 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 q y 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 it a 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 B 3 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 QJ 3 C, 
 Cpq and QJP : PC :: Cp :pq. Now if I is very 
 near to E, Qj 3 , 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 q y 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 q t may be confide, red as converging to p t 
 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/ : fq t 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 the 4 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. 3 A
 
 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'it 3 i-bout the Ti Vj 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/ : E r , 
 
 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 nnJiruied % -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 QJ 3 
 (fig. 44.) is qp y 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 on 7 .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 n y and paffing through the hole P in the large 
 mirror, they would meet fomewhere about r t 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 b t 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 b t 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 E 3 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^Tj 3 , 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 be y 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 Imn 3 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 k at 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 corroborate 1 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 glafll j s, 
 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~ 6 P 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 blue 4 
 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 fpctator, 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 l b ' 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 Difco c ueries. 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 hiffin 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 
 , ufed v 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 bair t 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 
 N lubfrance 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; 
 r343- 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 a 3 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 d y 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-condur.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> d y 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.-EUlrical 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, 
 yt 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. ////.- S0<;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 atmofphere 3 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 
 
 rnatter 4
 
 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. 
 
 f A 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 i 9 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 Je t 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 ir a 
 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, laurocerafus, 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 p r +- 
 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. 
 
 4 thly.
 
 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 t coal. 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 T i- of an oz. meaf. of fked air. 
 Port 6 years old - ? ' T 
 Hock of 5 years - -^ 
 Barrelled Claret - T L 
 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 Js 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 decompofea 1 , &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 gas 1 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 - aainft 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 exceeded 1 
 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- 
 reUy 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."Goldfmith y s 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 fire j 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 fummcr 4 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. 
 
 c The 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 Leures, 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 of t 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; this 1 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 heat v 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. 4 gj 
 
 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.v 1 l, 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 fhewn y 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 fulphur 3 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. Durin 6 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 
 jLincolnfhire s 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
 
 49 s 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. 
 
 4 th. 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 wet y 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.
 
 UNIVERSITY OF CALIFORNIA LIBRARY 
 
 Los Angeles 
 This book is DUE on the last date stamped below. 
 
 DEC 17 1965 ' 
 
 Form L9-50w.7,'54(5990)<U4
 
 I II 
 
 - 1 I II II I . 
 
 3 1158 01023 0422 
 
 157 
 
 G86e 
 
 v.l 
 
 A 000 007 747 9
 
 m