68 THE OCEAN. [Book HI. 



will, sooner or later, according to circumstances, 

 bring the body to a state of rest. 



In the case of the ball just mentioned, the ball is 

 taken from a state of rest ; a certain amount of force 

 is requisite to take it from that state of rest, and that 

 force is spent to overcome the vis-inertige of the ball ; 

 still more force is requisite to hurl the ball into the 

 air or along the ground. Here it may be said that, 

 but for the friction of the air or ground, the ball 

 would move continuously. This is an error. But 

 even supposing it were only this friction (unassisted 

 at all by the vis-inertiae of the ball) which stops its 

 motion, even then, since this friction is the com- 

 munication of motion to surrounding particles whose 

 vis -inertias resists the motion, it is the vis-inerti^e of 

 matter which brings the ball to rest. 



Let us, however, consider this case on its merits, 

 apart from any previous arguments. The ball is 

 thrown through the air. And the reason why the 

 air tends to resist and check the motion of the ball 

 is because the ball has to displace particles of the 

 air in its passage ; and as motion from the ball is 

 communicated to those particles, less in proportion 

 remains in the ball. This is the same if the ball be 

 rolled along the ground. The particles it strikes .or 

 touches, it tends, more or less, to move ; but the 

 vis-inertia3 of those particles tends to keep them 

 at rest, and therefore resists the effort of the ball 

 to move them ; and thus the ball and the particles 



