Chap. V.] GRAVITATION AND INERTION. 69 



of air or earth set in motion by the ball are at length 

 brought to a state of rest by the vis-inertise of matter. 

 Now there is certainly no more reason for suppos- 

 ing the vis-mertite which brings the whole to a state 

 of rest to reside in the particles of air and earth 

 set in motion by the ball than in the ball itself set 

 in motion by the hand. In fact, a certain amount 

 of force is exerted to overcome the vis-inertise of 

 matter, and an amount of motion in the ball and 

 particles of air and earth which it sets in motion is 

 caused in proportion to the motive force exerted ; 

 but, as the motive force is not continuously exerted, 

 it must at length be exhausted, and the bodies set in 

 motion be brought to a state of rest by the vis-inertia3 

 of matter. 



It thus appears that if in any body whatsoever 

 there be not some motive force acting continuously 

 to keep it in motion, its own inherent property of 

 vis-inertiae must (even if no other cause arise), in the 

 course of time, bring it to a state of rest. 



Let us, however, proceed to consider on what 

 grounds and by what arguments the motion of the 

 earth round the sun has been asserted to be main- 

 tained by an innate property of vis-inertise, in con- 

 sequence of which it tends to move uniformly for- 

 wards in a straight line. The force which keeps it 

 in motion is said to be its innate property of vis- 

 inertiag ; and the force which resists that of solar 

 attraction, and thus keeps it at its mean distance 



