Chap. V,] GRAVITATION AND IXERTION, 07 



direction ; ai^d this force, acting in opposition to 

 solar gravitation, is assumed to be an innate tendency 

 in the planets to move in straight lines, from which 

 they are constantly deflected by the gravitation of 

 the sun. This is mere assumption. It is not based 

 on any known phenomena.^ 



It is said that m the simple case of a ball, thrown 

 in the air or rolled on the ground, being set in 

 motion, its vis-inerti£e tends to keep it in motion 

 until it is stopped by the resistance of the air or 

 the ground against which it runs. This is an error. 

 It is not the vis-inertia3 of the ball which tends to 

 keep it in motion. It is vis-inertiae, and vis-inertia3 

 only, which stops, and must in time stop the course 

 of that ball. Vis-inertia3 resists the motion of the 

 ball from first to last. And it is because the force 

 which set the ball in motion is not continuously 

 acting, and because vis-inertiae is continuously act- 

 ing, tending to bring it to rest, that, no matter what 

 the original velocity of the ball may be, its motion 

 must at length be spent. 



If the force which set a body in motion continue 

 to act constantly upon it, the motion will be con- 

 tinued ; but if the motive force be removed, then vis- 

 inertise, so far from tending to keep the body moving 

 onwards in the motion communicated to it, must and 



' The origin of this ' assumption ' is more fully explained in 

 Chaptei-s XII., XVI., and IX. of The Neio Principles of Natwal 

 Philosophi/. 



P 2 



