CH. xviii. . NEWTON'S STUDIES. 147 



make these calculations it was necessary to know exactly 

 the distance from the centre of the earth to its surface, 

 because the attraction would have to be reckoned as if all 

 the mass of the earth were collected at the centre, and then 

 as decreasing gradually till it reached the moon. Now the 

 size of the earth was not accurately known, so Newton had 

 to use the best measurement he could get, and to his great 

 disappointment his calculations came out wrong. The moon 

 in fact moved more slowly than it ought to do according to 

 his theory. The difference was small, for the pull of the 

 earth was only one-sixth greater than it should have been : 

 but Newton was too cautious to neglect this want of agree- 

 ment. He still believed his theory to be true, but he had 

 no right to assume that it was, unless he could make his 

 calculation agree with observation. So he put away his 

 papers in a drawer and waited till he should find some way 

 out of the difficulty. 



This is one of many examples of the patience men must 

 have who wish to make really great discoveries. Newton 

 waited sixteen years before he solved the problem, or spoke 

 to any one of the great thought in his mind. But more 

 light came at last ; it was in 1666, when he was only twenty- 

 four, that he saw the apple fall; and it was in 1682 that he 

 heard one day at the Royal Society that a Frenchman 

 named Picart had measured the size of the earth very accu- 

 rately, and had found that it was larger than had been 

 supposed. Newton saw at once that this would alter all his 

 calculations. Directly he heard it he went home, took out 

 his papers, and set to work again with the new figures. 

 Imagine his satisfaction when it came out perfectly right ! 

 It is said that he was so agitated when he saw that it was 

 going to succeed, that he was obliged to ask a friend to 

 finish working out the calculation for him. His patience 



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