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or is about to leave the dock at fuch lime. Sc? Docks, 

 and Canai,. 



GRAVISKOI, in Geography, a fortiefs of Ruffia, ia 

 the, government of Kolivan, on the Irtifch ; 240 miles 

 S. S. W. of Kolivan. N. lat. 50' 15'. E long. 75' 14'. 



GRAVITAS, BafUTn;, in the Anc'unt Mufic, was ufed 

 to fignify a found produced by the remiflion or falling of 

 the voice. Gravitas differs from remiflion, as the efltft from 

 the caufe. 



Gravitas is alfo ufed to denote the ftate of a woman 

 going with child. See Puegnancy. 



GRAVITATION, the exercife of gravity, or the 

 aftion which a body exerts on another body by the power of 

 gravity. It is fometimes diftinguiflied from gravity. Thus, 

 Maupertuis takes gravity for that force by which a body 

 would fall to the eartli fuppofed at reil ; and gravitation 

 for the fame but diminilhed by the centrifugal force. See 

 figure of the earth deterra. p. 206 of Mr. Murdoch's 

 tranflation. 



It is only gravitation, or gravity thus blended with the 

 centrifugal force, that we can meafnre by our experiments. 

 However, methods liave been found to dillinguilh what 

 remains of primitive gravity, and what has been dellroyed 

 by the centrifugal force. 



It is one of the laws of nature, difcovered by fir Ifaac 

 Newton, and now received by mod philofophers, that every 

 particle of matter in nature gravitates towards every other 

 particle ; wliich law is the hinge on which the whole New- 

 tonian philofophy turns. See Newtonian Philofophy. 



What we call gravilalton, with refpcft to the gravitating 

 body, is properly called atiradlon with refpedl to the body 

 towards which the other gravitates. 



The planets, both primary and fecondary, and alfo the 

 comets, do all gravitate towards the fun, and towards each 

 other ; and the fuu towards the.m : and that in proportion 

 to the quantity of matter in each. 



The Peripatetics, &c. hold, that bodies only gravitate 

 when out of their natural places, and that gravitation ccafcs 

 when they are reftorcd to the fame, the intention of nature 

 being then fultiUed. The final caufe of this faculty, as 

 they maintain, is only to bring elementary bodies to their 

 proper place, where they may reft. But the moderns 

 Ihew, that bodies exercife gravity even when at rell and in 

 their proper places. 



This is particularly fhewn of fluids ; and it is one of the 

 laws of hydrollatics demonftrated by Mr. Boyle and others, 

 that fluids gravitate m/iro/i;v5 /(7fO, the upper parts prcfling 

 on the lower, &c. 



For the laws of gravitation of bodies in fluids fpecifically 

 bghter, or heavier than themfelves, fee Specific Ghavity, 

 Fluid, &c. 



Gravitation, Theory of Univerfal. Newton is very 

 juftly confidered as the author of this great hypothefis, 

 though feveral preceding and coteraporary writers fecm to 

 have formed conjectures on this fubjeft that did not differ 

 very widely from the truth. From the time of Kepler it 

 had every day become more and more evident, that fome 

 mechanical caufe had a very material influence on the laws 

 of the phnetary motions. The problems which the difco- 

 veries of that great aftronomer had left fur future ages to 

 folve were ihefe : Why do the planets and fatclHtes delcribe 

 eUiptic orbits, the former round the fun, the latter roond 

 their primary planet ? Why is the centre of motion in the 

 focus rather than in the centre of the ellipfe ? Why are 

 equal areas defcribed in equal times about the centre ; and 

 what can be the caufe of that remarkable law that is fowid 



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cnnftantly to fubfill between the diflancei of the planeta 

 from the fun and the times of their revolution ? 



Des Cartes firft attempted a folution of thefe difficulties, 

 by fuggeftiiig a fimple mechanical caufe. He fuppofed the 

 cxiflence of a fubtile fluid, which, though imperceptible to 

 the fcnfes, was in conflant motion round the fun, and that 

 it involved the planets in its powerful agency. This theory, 

 on its firll enunciation, docs not feem deticient in plauli- 

 bility, but upon more careful examination, it is found inca- 

 pable of explaining any one aflronomical phenomt-non cor- 

 rectly, and indeed the aftion of fuch a fluid wou'd produce 

 effefts in many cafes exaftly the revcrfe of what it was 

 intended to explain The fpirit of rational enquiry that 

 row began to be exerted on all philofophical fubjc£ls, ren- 

 dered this theory of fliort duration. 



Kepler at a much earlier period had formed more cor- 

 reft opinions on this lubjccl than Des Cartes : indetd he 

 feems to have had a very diflincl idea of the exiftence of 

 gravitation, though he never could have been aware of its 

 :>gency being fo extenfive, or that it would ever ex lain in 

 fo fatisfaftory a mann.-r as it has done hio own great difco- 

 veries. Gravity, he fays, iu his " Commentary on Mars," 

 is only a mutual and corporeal affe£lion between fimilar 

 bodies. Heavy bodies do not tend to the centre of the 

 world, but to that of the round body of which they form 

 a p.ut ; and if the earth were not fpherical, lieavy bodies 

 would not fall towards its centre, but towards different 

 points. This conjecture has been fully verified by theory 

 and obfervation ; the _ earth appears not to be a perfeft 

 fplure, and accordingly a plumb-line docs i:ot tend to the 

 cxaft centre of gravity of the whole earth, but to a point 

 confiderably remote from it, as has been explained under 

 Deokek, Y-Aiivu, Jigurt of, &c. S:c. 



Kepler likewife llrongly fufpecled that the attraction of 

 the moon was the caufe of the tides, and that the lunar 

 irregularities arofe from the action of the earth and fun. 

 Perhaps, from tiie earliell times, fome indiftinct conciptions 

 are to be occafionally tr.aced in diffen-nt autliors concerning 

 the exiilence of this univerfal principle. 



Fermat, who preceded Kepler, aflinred, that the weight 

 of a body was the fum of the tendencies of all its particles 

 to all the particles of die earth ; and Kepler was of opinion 

 that two bodies left alone in free fpace would approach 

 each other with velocities inverfely proportional to their 

 malTes or quantities of matter. 



A fliort time previous to the great difcovery of Newton, 

 Dr. Hooke made a iliU nearer approach to the truth. At a 

 meeting of the Royal Society, May 3, 1668, he cxprcfltd 

 himfelf in the following manner : " I will explain a fytlem 

 of tlie world very different from any yet received, and it i( 

 founded on the three follo«-ing pofitions : 



1. Tiiat all the heavenly bodies have not only a gravitatioB 

 of their parts to their own proper centres, but that they 

 alfo mutually attract each other within their fphercs of 

 aClion. 



2. That all bodies having a fimple motion will continue to 

 move in a ftraight line, unlets continually deflected from it by 

 fome extraneous force, caufing them to delcribe a circle, an 

 ellipfe, or fome other- curve. 



3. Tiiat this attraiSlion is fo mucli the greater as the 

 bodies are nearer. As to the proportion in which ihofc 

 forces diminifli by an increafe of dillance, I own I have not 

 yet difcovered it, although I have made fome experiments to 

 this purpofe. I leave this to others who ha^-c time and 

 knowledge fufficient for the taflc." 



This is a very precife enunciation of a proi»cr philofophical 



theory. The phenomenon, the change of motion, is con. 



4 P 2 fidcred 



