524 DISSERTATION ON 



in that ftralght line forever : And, further, that it has fo violent a 

 tendency the other way, that it would be carried to the center by a 

 force which increafes, as the fquares of the diftances, inverfely ; for 

 all this muft neceflarily follow, if projection and gravitation are laid 

 afide, and the centripetal and centrifugal forces afcribed only to the 

 nature of the movement. 



From thefe confiderations, it appears to me altogether unaccount- 

 able, how the circular or elliptical motion fhould be, by its naturer ne- 

 ceflarily combined, and not as fimple as the line in which it is mo- 

 ved. There are, however, fome of Sir Ifaac's followers, who pretend 

 that he has demonftrated, by lines and figures, and geometrical necef- 

 fity, that it is fo combined ; and, as I know that geometry has its pa- 

 radoxes, as well as other fciences, I was at firft difpofed to believe that 

 it might be fo ; but, upon looking into the Principiat I was convinced 

 that Sir Ifaac there only treats of one of the two kinds of motion 

 mentioned above, viz. of motion produced by the Impulfe of body up- 

 on body ; nor does he appear to me to have had any idea of the mo- 

 tion of body by Mind, at leafl:, of the manner of that motion. And, 

 accordingly, it is evident, both from his definitions, and from his 

 laws of motion, that he fuppofed the motion of the celeftial bo- 

 dies to be compofed of different motions in ftraight lines, that 

 being the only motion which can be produced by body ading 

 upon body *. Of this kind is the projedile motion of the pla- 

 ne ts> 



* That Sir Ifaac, In his Pritjcipiay fpeaks only of this kind of motion, is evident, as 

 1 have faid, both from his definitions and his laws of motion; for, in his third defi- 

 nition, he lays it duwn, that theie is rcCflence in every body which changes its flate, 

 cither of reft or of motion. Now, this cannot be conceived, unlcfs it be impelled by 

 anothtr bocJy ; for it c.n refifl nothing that is not folid and extended like itfelf, and 

 vhich therefore rtfiflb a? much as it is refifled. And, accordingly, the Commentary 

 upon this definition plainly (uppofes that it applies only to the adlion of body upon 

 body; foi he fays, • Ettnim, nulla potefl tfle a6lio corporis in corpus, quin iudlatio 

 * quaedam, ut loquitur Clar. h'crvmnuSx in Phoroncmiaf fiat inter ccrpuo agens ct pa» 



* tieus. 



