INERTIA. FORCE. GRAVITATION 25 



and inversely proportional to the square of the distance between the 

 bodies ; and the direction of the force is in the line joining the centres of 

 the bodies. 



Consider the case of a cricket ball on the top of a house. The earth 

 attracts the ball, and, by Newton's law, the ball attracts the earth. 

 The ball, if free to move, falls to the earth ; to be correct, however, we 

 must think of the ball and the earth moving to meet one another along 

 the line joining their centres. But the ball moves as much farther 

 than the earth as the earth's mass is greater than that of the ball ; and 

 for practical purposes this is the same as saying that only the ball moves 

 and that the earth remains still. 



This force of attraction between all material bodies is called the force 

 of gravitation, but we must point out that this is only a name. Calling 

 this force " gravitation," and the rule according to which it acts the 

 " law of gravitation," does not teach anything about the nature of 

 the force itself. 



