I 



Chap. r. AN TIE NT xM E T A P H Y S I C S. 319 



the fame time, maintained principles that I think dangerous to the 

 Syftem of Theifm, though, I am perfuaded, without intending it, it 

 certainly belonged to a work of this kind, the chief purpofe of which 

 is to maintain that SyRem, to take notice of any error that Sir Ifaac 

 may have fallen into in this refpedl. 



Sir Ifaac's Syftem of the Heavens is, as I underftand it, fhortly 

 this : The Planets are, by an impulfe, or vis imprejfa, as Sir Ifaac 

 calls it, fet in Motion ; which Motion continues in a ftraight line, by 

 virtue of that Power which Sir Ifaac csilh vis in/tta ; and by this 

 Power it will continue forever to be moved in a ftraight line, unlefs 

 its Motion be flopped by fome obftacle, or unlefs it be adled upon 

 by fome other Power. In this way, the planets would have gone 

 on forever in a redlilineal couife, as our author hasfaid in his intro- 

 dudlion to that abridgment of his philofophy, which he has given 

 under the title of ' The Syftem of the World: But, in order to pro- 

 duce their Elliptical Motion, he fays that another Power is employed, 

 which he calls the Vis Centripeta, by which the Planet is carried out 

 of the Redlilineal Diredlion, towards a certain point, as its Centre. 

 How this Power ads upon the Body, whether by Pulfion or Tru- 

 fion, by propelling or by drawing it. Sir Ifaac has not explained in 

 the Definition he has given of it ; which is in thefe words, * Vis 



* ccntripeta eft, qua corpora verfus pundlum aliquod, tanquam ad 



* centrum, undique trahuntur, impelluntur, vel utcunque tendunt.' 

 But, in the demonftration he has given of the effeds of this Centri- 

 petal Force in the firft propofition of the fecond fedion of the tlrft 

 book of his Principia, he fuppofes it to ad, not by drawing, but by 

 impulfe; for his exprefTion is, ' Agat vis centripcta impulfounicofed 

 ' raagno, efiiciatque ut corpus de reda declinet.' And his dodrinc 

 of Prime and Ultimate Ratios, which he has explained in the firft fec- 

 tion of his fecond book, is chiefly intended for the purpofe of ihowing 

 how a Circle or Ellipfi? may be analyfed into a Polygon of an infi- 

 nite 



