Chap. 5.] Motion of the Heavenly Bodies. 33 



in its motion as to enable it to pafs in one hour the 

 length of one thoufand yards, would give to another 

 body, half as great as the former, twice the degree of 

 velocity, and would enable it to pafs in the fame time 

 the length of two thoufand yards *. Hence the quan- 

 tity of motion is always eftimated by the fwiftnefs of 

 the motion, and the quantity of matter which is moved. 

 If A and B, bodies of equal fize, move with equal ve- 

 locity, their quantity of motion is equal; but if A 

 contains twice as much folid matter as B, and moves 

 equally fwift, it pofielTes a double quantity of mo- 

 tion f. 



It follows evidently from the fame law, that if a 

 new force is imprefTed upon a body in motion, in the 

 direction in which it moves, its motion will always be 

 increafed proportionably to the acceffion offeree, how- 

 ever frequently repeated. 



It follows alfo, and may be proved by a very eafy 

 experiment, that if a new force is imprefTed upon a 

 body not in 4 the direction in which it moves, but in an 

 oblique direction, the body will take a direction nei- 

 ther exactly the fame as that in which it was proceed- 

 ing, nor yet in the direction of the new force which is 

 imprerTcd upon it, but a direction between both. On 

 this is grounded the commonly received opinion con- 

 cerning the motion of the heavenly bodies. The cen- 

 trifugal force is that which is fuppofed to have been 

 imprefied upon them at their firft formation, and 

 which would carry them forward in a direct line ; this 

 is counteracted by the force of gravitation (or centri- 

 petal force) which always inclines them to that body 

 round which they revolve ; the confequence of thefe 

 two forces acting in different directions is, that the 



* Pemberton's View, p. 29, 36. 

 f Elements of Nat. Phil, by Mr, Locke, chap, i. 

 VOL. I, D bodies 



