312 APPENDIX. Chap. I. 



But, fay the Newtonians, you proceed upon hypothefis, particu- 

 larly upon the hypothefis of the Planetary Motion being compound- 

 ed, which you acknowledge is a falfe hypothefis, for you fay the Mo- 

 tion is fimple. Now, we think Sir Ifaac's demonftrations would (land 

 much firmer upon real fads ; and therefore we hy that projedion 

 and gravitation do really exift. 



But to this, I think, I have given a fufficient anfwer, when I have 

 faid *, that, if the hypothefis I make were abfurd or irapoiTible, the 

 objedion w^ould no doubt be well founded. But this the Newto- 

 nians will not fay, who affirm that in fadt it exifts ; and I have 

 fhewn that there are compounded Motions of the fame kind on 

 earth f . All, therefore, they can fay is, that I fuppofe the Planetary 

 Motion produced by a caufe which does not produce it, though it 

 might produce it. But we ought to difllnguifh betwixt the Motion 

 and the Caufe of the Motion ; for the Motion may be the fame 

 though the Caufe of that Motion may be different. Thus, a Body 

 may be moved in a ftraight line, either by Mind, by the dired im- 

 pulfe of one Body, or by the lateral impulfe of two Bodies ; and 

 yet the Motion is the fame, in which ever of the ways it is produ- 

 ced ; and, therefore, if I can demonftrate, by the propofition con- 

 cerning the compofition of Motion, that, upon the fuppofition of 

 the Body being moved in the lafl of the three ways I- mentioned, 

 its moving force will be fuch or fuch, the fame will be true in which 

 ever of the other ways the Body is moved J. 



This hypothefis of the Motion in a ftraight line being produced 

 by two powers, when in fadl it may be produced only by one, is 

 much ufed by Sir Ifaac in his Principia ; and it is truly the very 



fame 



* Vol. ii. p. 39T. 

 f Vol. ii. p. 426, 

 j. Vol. ii. p. 425. 42<5. 427. 



