Id6 SEVENTEENTH CENTURY. PT. IIL 



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 fain 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 further. 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. The ball does not come to you, 

 although the string pulls it, because the sideway pull of the 

 string cannot check its motion onwards, but only alters its 

 direction. This it does at every moment, causing it to 

 move in a circle round you. In the same way the moon 

 does not come to the earth, but goes 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 

 round the earth, and the planets round the sun. But a mere 

 guess is not enough in science, so he set to work to prove 

 by very difficult calculations what the effect ought to be it it 

 was true that the earth pulled or attracted the moon. To 



