THE NEWTONIAN PHILOSOPHY. 52-, 



nets/which, though he had fuppofed to be the immediate operation of 

 Mind, as his followers do, yet, having no idea, fuch as his friend Dr 

 Clarke had, of the manner in which Mind moves body, he fuppofed it 

 to be by impulfe, in the way that body moves body : And of the 

 fame kind is his motion of gravitation ; and which, therefore, he en- 

 deavoured to account for by the impulfe of a fluid. 



In order to fhow how, from thefe two r^dillneal motions, the circu- 

 lar motion may refult, he has made ufe of his dodrine of prime and 

 ultimate ratios ; and, by the means of (ides of polygons, evanefcent^ as 

 he calls them, that is, growing infinitely lefs and lefs, as they increafe 

 in number, he accounts for the circular motion b(^ing produced by 

 projection and gravitation, though both operating in ilraight lines *, 



Now, 



* tiens, dum alterum alterl refiftit ; alloqui corpus motum poflet, fine motus proprii 



* detrimento, aliud quodcunque movere.' This, furely, cannot be underftood of 

 motion by Mind. And, in the 4th definition, where Sir Ifaac fpeaks of the 7'/V im- 

 preffui he fays it is * Diverfarum originum, ut ex i£lu, ex preffione, ex vi centripeta.' 

 Now, the iElus and the prejjio can certainly be underftood only of the aftion of body 

 upon body. And, as to the vis centripetal in the following definition he compares 

 it to that force by which a ftone in a fling is kept from flying oiF, and which is cer- 

 tainly, as he explains it, by the a6lion of the hand retaining the ftone, that would 

 otherwife fly off. Of this kind, he tells us, is the motion of the planets, which, by 

 the fame means, are drawn from the redlilineal motion, and obliged to revolve in 

 curve lines ; and he adds, ' et par eft ratio corporum omnium quae in gyrum aguu' 



* tur.' And his laws of motion alfo relate to the fame kind of motion j particularly 

 his third Jaw, concerning aElion and re-adlion, which he cxprefl'es in the following 

 words : * A6lioni contrariam femper et aequalem efle rc-a£lionem, five corporum 



* duorum afliones in fe niutiio fcmpcr efle acquales, et in partes contr.rias dirigi.' 

 Here the rule is exprefsly applied to two bodies a£ting upon one another ; and the ex- 

 plication he gives of the rule will only ap^^ly to fach a cafe: * Q^iicquid premit vcl 



* trahit alterum, tantundem ab eo premitur vcl trahitur •, fi quis Japuiem digito pre- 



* mit, premitur et hujus digitus a lapidc.' Ihis it is impoinble to apply to ths adion 

 of an incorporeal fubftance upon body. 



* This do6lrine of prime and ultimate ratios belongs to what we call JJuxionSj and 

 the French call the geometry of the infinicmsnt petlts- I have been told that fome geo- 

 meters, and particularly the late Ur 8imfon ol Giafgow, had a great deal of doubt of 

 the dodrine of prime and ultimate ratios, and wrote foraething againft it, which, how" 



ever. 



