46 ELEMENTARY GENERAL SCIENCE CHAP. 



as it is sometimes expressed, the earth attracts all bodies to 

 itself. Experiments and observations made by Newton led him 

 to the conclusion that it was the rule of nature for every body to 

 attract every other body, and that this force of attraction is 

 proportional to the body's mass, a large mass exerting a greater 

 force of attraction than a small mass. But the farther these 

 bodies are apart the less will be the attraction between them, 

 though it is not less in the proportion of this distance, but in 

 that of the square of the distance. This diminution of a force 

 according to the inverse proportion of the square of the distance 

 applies to so many cases that it ought to be clearly understood 

 before going further. To give an example : two bodies of equal 

 mass are one foot away from one another and attract each other 

 with a certain force, call it a unit force. One body is now 

 moved until its distance is two feet away from the second body, 

 what will be the force of attraction between them ? The square 

 of 2 is 2 x 2 = 4 and the inverse of 4 is j, therefore the force 

 of attraction is one quarter of the unit force. In the same way, 

 if the bodies were three feet apart, the force of attraction would 

 be ^ of the unit force. Putting Newton's law together it stands 

 thus: Everybody in nature attracts every other body with 

 a force directly proportional to the product of their masses 

 and inversely proportional to the square of the distance 

 between them ; and the direction of the force is in the line 

 joining the centres of the bodies. 



Returning to the case of the falling body, think 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 

 the ball's ; and for practical purposes this is the same as saying 

 that only the ball moves and that the earth remains still. 

 Were our methods of measurement sufficiently refined we 

 should, of course, be able to measure the small amount of the 

 earth's movement. 



This force of attraction between all material bodies is called 

 the force of gravity, and we must point out that this is 

 only a name. Calling this force "gravity," and the rule accord- 

 ing to which it acts the "law of gravity," does not teach us 

 anything about the nature of the force itself. There is, however, 



