GRAVITY. 



light ; it feems m if all forces whofe aflion could be per- 

 ceived at fenfiWe diftances obeyed this law. It has 

 lately been obfervcd, that the attractions and repulfions of 

 eletlricliy and magnetifm decrcafe in proportion to the 

 fquares of the diftances. A remarkable property of this 

 law is, that if the dimenfions of all the bodies of the uni- 

 verfe, their mutual diftances and vekcities were to be aug- 

 mented or dimiiiiftied proportionally, they would defcribe 

 curves entirely iimilar to thofo defcribedat prefent, and their 

 appearances would be entirely the fame. For the forces 

 which animate them being entirely the refult of attractions 

 proportional to the mafies divided by the fquares of the 

 diftances, they would be augmented and diminiflied propor- 

 tionally to the dimenfions of this imaginary univerfe. It 

 may be remarked at the fame time that this property can 

 only belong to the law of nature. Thus the apparent mo- 

 tions are independent of its abfolute dimenfions, as they are 

 likewife of the motions it may have in fpace, and we can only 

 obferve and recognize relative phenomena. 



It is thiii law which gives to fpheres the property of 

 attrafting each other mutually, as if their whole maffes 

 were united at their refpeclive centres. It terminates alfo 

 the orbits and tlie figures of the celeftial bodies by lines 

 and furfaces of the fecond order, at leall if we negk-tt 

 their perturbations and fuppofe them fluid. 



We have no method of meafuring the length of time in 

 ■which gravity is propagated, becaufe the aiftion of the fun 

 having once reached the planets, it continues to acl on th:-m 

 as if the attratlive force was communicated inftantaneoufty 

 to the extremities of tlie fyftem. We cannot, therefore, 

 afcertain in how long a time it is tranfmitted to the earth, 

 no more than we could meafure the velocity of light, were it 

 not for tlie aberration, and the eclipfes of Jupiter's fatellites. 

 But it is not the fame with the fmall difference that may 

 exift in the aftion of gravity upon bodies according to the 

 direftion of their velocity. 



M. de la Place has found by analyfis, that an acceleration 

 (hould refult in the mean motions of the planets round the 

 fun, and in the mean motions of the fatelhtes about tlieir 

 planets. 



M. de la Place had affumed this method of explaining the 

 fecular equation of the moon, when he believed, with other 

 geometricians, that it was inexplicable on the principle of 

 univerfal gravitation. He found that if it arofe from this 

 caufe, a velocity muft be fuppofed to exift in the centre of the 

 moon, in order to releafe it entirely from its gravity towards 

 the earth, at leaft fix million times greater than that of 

 light : the true caufe of this equation being now known, 

 we are certain that the aftion of gravity is much greater 

 than this. This force, therefore, a6ls with a velocity which 

 we may confider as infinite ; and we may conclude, tliat the 

 action of the fun is tranfmitted in an indivifible inftant to the 

 extremities of the planetary fyftem. 



Do any other forces act on the heavenly bodies befides 

 their mutual attractions ? 



We are unacquainted with any, and we may affirm that 

 their efieCt is totally infenfible. We may likewife be cer- 

 tain that thefe bodies experience no fenfible refiftance trom 

 the fluids through which they pafs, as light, the tails of 

 comets, or the zodiacal light. 



The attractive force difappears between bodies of an in- 

 confiderable magnitude, and re-appears in their elements 

 under a variety of forms. The foHdity of bodies, their 

 cryftalllzation, the refraction of light, the elevation and de- 

 preflion of fluids in capillary tubes, and all chemical com- 

 binations, generally are the refults of attractive forces, the 



knowledge of which forms the principal obje^ of nattiral 

 philofophy. 



Are thefe forces the fdmc as that of the gravity obfenred 

 in the celeftial regions, and modified on tlie earth by the 

 figures of the integrant 5)artic!es ? To admit this hypothe- 

 fis we muft fuppofe much Tnore fpace ePipty than full in all 

 bodies, fo thit tlie denfity of their particles ir.uft be incom- 

 parably greater than the mean denfity of their whole volume. 

 A fphencal particle of one hundred ihoufandth of a foot in 

 dianiiter fiiould have a denfity at leaft ten thoufand million 

 of times greater than the mean denfity of the earth, to ex- 

 ert at its furface an attraction equal to the terrcftrial gravity. 

 But the attractive forces of bodies greatly furpafs this gra- 

 Tity, fince they infleCt light, whofe direCtion is not changed 

 by the attraCtion of the earth. The denfity of thefe par- 

 ticles ft'.ould, therefore, be fo that of fubilanccs in a r lio 

 which the imagination would fear to admit, if their affinities 

 depended on the law of univerfal gravitation. The ratio 

 oi' the interYals which feparate the parti, les of bodies to 

 their refpeCtive dimenfions, would be of the fame order as 

 in ftars which form a nebula, which in this point of view 

 may be confidered as a great luminous body. There is no 

 reafon, however, which abfolutely forbids us to confider all 

 bodies in this manner. Many phenomena are favourable to 

 the fuppofition, particularly the extreme facility with whicK 

 light penetrates diaphanous fubftances in all direClions. The 

 affinities would then depend on the integrant particles, and 

 we might, bv the variety of thefe forms, explain all the va- 

 riety of attraClive forces, and reduce to one general law all 

 the phenomena of aftronomy and natural philofophy. But 

 the impoffibihty of afcertaining thefe figures, renders thi» 

 inveftigation ufelefs to the advancement ot fcience. 



Some geometricians, to account for thefe affinities, have 

 added to the laws of attraction, inverfely as the fquares of 

 the diftances, new terms which are infenfible at fmall dif- 

 tances ; but thefe terms would be the expreffions of as many 

 different forces, and befides being complicated with the dif- 

 ferent figures of the particles, they would only complicate 

 the explanation of the phenomena. 



Amidll thefe uncertainties the wifeft plan feems to be, to 

 endeavour to determine by numerous experiments the laws 

 of affinities ; and to cffedt this, the m.oft fin-.ple method ap- 

 pears to be, by comparing thefe forces with liie repuHwe 

 force of heat, which may be itfelf compared with that of 

 gravity. Some experiments already made with this view, af- 

 ford lis reafon to hope, that one day thefe laws will be per- 

 fectly known, and that then, by the application of analyfis, 

 the philofophy of terreftrial bodies may be brought to the 

 fame degree of perfection, which the difcovery of univerfal 

 gravitation has procured for aftronomy. 



Ok.wity, in HydrojiaUcs. The laws of bodies granuting 

 in fluids, make the bufiuefs of hydrottatics. 



Gravity is here divided into alfolute w\di J^;af.c. 



Gk.wity, abfolute or Inn, is the whole force vfliercwitli 

 the body tends downward. 



Gu.wiTY, Sp.'ci/ic, called alfo rt/jlivr, comparative, and 

 apfarfnt gravity, is the excefs of gravity in any body, above 

 tliiit of an equal quantity or bulk of another ; and denote* 

 that trravity, or weight, peculiar to each fpccies, or kind 

 of natural body ; and whereby it is diftinguiihed from ulJ 

 other kinds. 



In this fenfe, a body is faid to be fp:cifcj!lj btav'er than 

 another, when under the fame bulk it contains a greater 

 weight than tiiat other ; and that other is faid to be ^.-.•- 

 fciilly lighter than the firft. Thus, if there be two equal 

 fpheres, each a foot in diameter ; the one wood, ti»e otner 

 lead, fmve the leaden one is found heavier thao the wooden 

 4 R « oar. 



