CH. xviii. NEWTON S STUDIES. 149 



plague broke out in Cambridge in the year 1665, and he was 

 forced to go back to Woolsthorpe. Here he was sitting one 

 day in the garden, meditating as usual, when an apple from 

 the tree before him snapped from its stalk and fell to the 

 ground. This attracted Newton's attention ; he asked 

 himself, Why does the apple fall ? and the answer he found 

 was, Because the earth pulls it. This was not quite a new 

 thought, for many clever men before Newton had imagined 

 that things were held down to the earth by a kind of force, 

 but they had never made any use of the idea. Newton, on 

 the contrary, seized upon it at once, and began to reason 

 farther. If the earth pulls the apple, said he, and not only 

 the apple but things very high up in the air, why should it 

 not pull the moon, and so keep it going round and round 

 the earth instead of moving on in a straight line ? And if 

 the earth pulls the moon, may not the sun in the same way 

 pull the earth and the planets, and so keep them going 

 round and round with the sun as their centre, just as if they 

 were all held to it by invisible strings ? 



You can understand this idea of Newton^s by taking a 

 ball with a piece of string fastened to it, and swinging it 

 round. If you were to let the string go, the ball would fly 

 off in a straight line, but as long as you hold it, it will go 

 round and round you. Thus Newton imagined that every- 

 thing near the earth is pulled towards it by an invisible force, 

 as you would pull the ball by the string ; but the ball does 

 not come to you, although the string pulls it, because of the 

 other force which is carrying it onwards j and in the same 

 way the moon would not come to the earth, but would go on 

 revolving round it. 



Newton felt convinced that this guess was right, and that 

 the force of gravitation, as he called it, kept the moon going 



