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tlie fun, you attribute to an attraiSlive energy of that body, 

 ytt I mull entreat, that, in the publication of my obfcrva! 

 tioiis, you would explain all the ceie'l al motions by my 

 hypothcfis, rather than by that of Copernicus, which I 

 k'iow you would otherv.ifc incline to follow." (Life of 

 Tycho Brahe.) 



Ktpler, however, in his own works, conftantly maintains 

 the doitrine of attraction, and even carries it farther than 

 Copeniicu's had ever done. Thus, he calls gravity " a cor- 

 p.ireal and mutual affctiion between fimilar bodies, in order 

 to their union." He aifo remarks, with Coper.iicus, atjainft 

 the Peripatetics, that " tlic heavenly bodies do not ter.d to 

 the centre of the univerfe, but to the centre of thofe L-.rgcr 

 round bodies, of which they make a part ; fo that if tlie 

 earth were not fpherical, things would not fall from all 

 p:)iuts towards its centre. If a Hone, for inilance, were to 

 be placed at a diftance from another ftonc, in any part of 

 the univerfe, without the fphere of a&ion of a third body, 

 like two magnets, they would come together in fome ir.ttr- 

 nudiate point ; each advancing, in fpace, in the invcrfe pro- 

 portion of their quantities of matter. Hence, if the" moon 

 and the earth were not kept afunder by fome power, in 

 their refpective orbits, they would move towards each other ; 

 the moon paffing over fifty-three parts of the way while 

 the earth paiTed over one, fnppofing their denaties equal." 

 {Aftron. Nov. in Introdu^t.) 



From the fame principle, Ktpler alfo accounted for the 

 general motion of the tides ; viz. by tiie attraction of the 

 moon, and exprefsly calls it ■virtus Iracioria qua In hna efl ; 

 adding, that if the earth did not exert an attraftive power 

 over its own waters, they would rife and rulh t ; the moon. 

 We alfo find him fnfpecling that certain irregularities in 

 the«motion of the moon are owing to the combination of 

 the earth and fun upon this body. (Ibid.) 



Thefe and other refleflions concerning the univerfality of 

 attrattion, he accompanies with an ingenious anticipation of 

 a law of nature, from conjefture only, but which was after- 

 wards verified by experiment. The Ichools hud taught that 

 fome bodies were by their nature heavy, and fo fell to the 

 ground ; and that others were naturally light, and for that 

 reafon afcended. But Kepler pronounced, that no bodies 

 whatever are abfolutely light, but only relatively fo ; and 

 confcquently that all matter is fubjecled to the law of gra- 

 vitation. So far the genius of Kepler was fortunate, in 

 tracing out the great principle which prevents the planets 

 from flying off from the fun ; buthTS fagacity failed him, when 

 he endeavoured to (liew by what means they were kept from 

 falling into that immenfe body, and what power perpetuated 

 their motion in their orbits : a general inveftigation of the 

 laws of motion was yet wanting ; the difcovery of which, 

 as A-ell as many other things, being referved, as he himfeif 

 prophefies at the end of his work, " for the fucceeding age, 

 when the Author of nature would be pleafed to reveal thefe 

 mylleries." 



The firlt perfon in this country, who embraced the doc- 

 trine of attraftion, was. Dr. Gilbert, a native of Colcheller, 

 and a phyfician at London, i?i a work publilhcd in the year 

 1600, intitled, " De Magnete Magneticifque Corporibus;" 

 which contains a number of curious things ; but he d.d not 

 properly diltinguiih between attraftion and magnetifm. The 

 next after him was lord Bacon, who, thougii not a convert 

 to the Copernican fyllem, yet acknowledged an attradive 

 power in matter (Nov. Organ, lib. ii. aphor. 36.45. &4S.); 

 and in the dawn of philofophy, in which he lived, he con- 

 flantly recommends an inquiry into the phyfical caulcs and 

 reaions of things ; obferving, « tliat he who Ihall duly at- 

 tend to the appetences and general affcifiions of matter (which 



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both in the earth and heavens are exceedingly powtrful, ard 

 indeed pervade the univerfe) will receive, from what he Uti 

 palling on the earth, clear information concerning tho nature 

 of the cckftial bodies ; and, contrariwife, from n otioiis 

 which he fliall difcovtr in the heavens, will learn many par- 

 ticulars relating to the things below, which now lie con- 

 cealcd from us." (De Digu. Sc Augm. Scieut. lib. iii. 

 C.4.) 



In France, alfo, we find Fermat and Robcnal, mathtma- 

 ticsans of great tmintuce, maintaining the fame opinion. 

 The latter, ip particular, made it the fundameiilal principle 

 of bis fyllem of phylxal allron<imy, vhich he publ:ni;.d in 

 1644, ""tltr the title of " Arift. Samii de Mui.di Syliera." 

 In this work, Roberval attributes to all the parts of' matter 

 of which the univerfe is compofed, the property of ha\iiig 

 a tendency towards er.clr cth.er ; ubferving, that this is the 

 reafon why they airange themfelvis in fpherical figures, not 

 by-viitue of a centre, but by their mutual attra£tioas, and 

 fo that t)ne may be placed in an equilibrium with another. 

 GaliJco, in Italy, had hkewife cor.ccived ihisidea ; but with 

 far lels prccifion and cxtenCon than we find it in his cootcm- 

 porarics Bacon and Kepler. 



But no one, before Newton, had entertained fuch juft and 

 clear notions of the dodrine of univerfal gravitation, or had 

 approached fo near to the making a general application of it 

 to the laws of nature, as the celebrated Dr. Hooke. The 

 philofophers before mentioned had feized, fom.c one biancl^ 

 and fome another ; but Hooke, in his work, called " An At- 

 tempt to prove the Motion of the Earth," 1674, 410., apjji-ars 

 to have embraced it in nearly the whole of its generality. He 

 there obferves, that thehypothtfis upon whicii he explains the 

 fyilem of the world, is, in many refpccis different from all 

 others; and which is founded upon tlicthree following princi- 

 pies: l.Thacall the celellial bodies have not only an attraction 

 or gravitation towards their proper centres, but that they mu- 

 tually attrati each other within their fphere of activity. 2. 

 That all bodies which have a fimple and direft motion, con- 

 tinue to move in a right une, if fome force, which operates 

 without ceafing, does not conftrain them to defcribe a circle, 

 an ellipfe, or fome othei" more co.mplicaicd curve. 3. Tiiat 

 attraAion is fo much the more powerful, as the attraftinT 

 bodies are nearer to each other. 



He alfo made feveral experiments with a view to ftrengthen 

 the preceding conjectures. For this purpofe, he fufpcnded 

 a bullet to the end of a long ftring, and after it had been 

 made to ofcillate, he imprciTed upon it a fmall lateral mo- 

 tion ; and remarked, that the bullet defcribcd an ellipfe, or 

 a curve of that form, round the vertical line. He then at- 

 tached to the ftring of the firft bullet, another, which carried 

 afmaller one ; and after ha\ing given to tiie latter a circalar 

 motion round the vertical line, he put the other in motion, 

 as in the former experiment ; when it was found, that neither 

 one or the other defcribed an ellipfe, but moved rou:id a 

 point at a mean dillance between them, which appeared to 

 be their centre of gravity. (Life of Dr. Hooke, prefixed 10 

 his poUluimous works.) 



Tliis was certainly very ingenious; but Hooke did not 

 confiJer that the centre of force rcfides in one oi the foci 

 of the ellipfe, and not in its centre ; and though tlic obfer- 

 vation was fuggefted to him, and he was even excited by the 

 promiie of a very confiderable reward, to dctetmine the law 

 of attraction, which would occafion a body to defcribe an 

 ellipfe round another quiefctnt body, placed in one of its 

 foci, he was unable to accomphlh the undertaking. The 

 problem, which belongs to the iiigher geometry, "was too 

 (liiHciilt for that time ; this admirable difcovery, which dees 

 the highcll hoiiour to the human tiiiad, being rcfer>tJ for 



tl-,e 



