Chap. VI. ANTIENT METAPHYSICS. 423 



Having thus, by feparating the Defcent of the Planet from 

 the reft of its Motion, and confidering the Defcent abftradcdlv, 

 difcovered the Law of it, I proceed to what remains to be dif- 

 covered, and which is the principal thing fought, — By what Force 

 the Planet is moved in its Orbit. For "this purpofe, I make ano- 

 ther Hypothefis, or, rather, it is no new Hypothefis, but a necef- 

 fary confequence of the former ; for, if the Body be moved by any 

 Power towards the Centre, it is evident it muft be moved by fome 

 other Power in the diredion of the Tangent from which it falls, 

 otherwife it never could defcribe an Ellipfis, but muft fall into the 

 Centre : And thus we have the two Motions of Projection and Gra- 

 vitation, and the two moving Powers, of which the Planetary Motion 

 is fuppofed to be combined, the Centripetal Force and the Projedlile. 



But how are we to difcover the Moving Power In the Line of 

 Projection ? For, unlefs we can difcover that, we have done nothing j 

 becaufe it is evident that, by th«r Moving Force in the Line of Pro- 

 jection, the Planet is carried on in its Orbit, the Centripetal Force 

 ferving no other purpofe, except to direCt the Motion in the Prc- 

 jeCtile Line, and bend it into a Curve, inftead of going on, as it 

 would otherwife do, in a ftraight line* 



But, though the Centripetal Force ferve no other purpofe but to ■ 

 direCt the Motion, it is evident that it muft have a neceflary con- 

 nection with the Projectile Motion in the Tangent,, or, which is 

 the fame thing,, the Motion of tlie Planet in its Orbit : For, as that 

 Motion is quicker or flower, the Defcent from the Tangent muft be 

 greater or lefs in the fame time ; and, as the Curvature of the Or- 

 bit is greater or lefs, the Velocity, with which the Phmet performs - 

 its revolution in the time given, is greater or lefs ; and, with the 

 Curvature of the Orbit, as well as with the Velocity of the Mo- 

 tion in it, the Defcent from the Tangent is alfo neceflarily con- 

 neded. 



F.urther, 



