150 SEVENTEENTH CENTURY. FT. ill. 



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 if it was 

 true that the earth pulled or attracted the moon. To 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 strongest at the 

 centre of the earth, and then as gradually decreasing till it 

 reached the moon. Now the size of the earth was not accu- 

 rately known, so Newton had to use the best measurement 

 he could get, and to his great disappointment his calcula- 

 tions 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 error. He still believed his theory 

 to be true, but he had no right to assume that it was, unless 

 he could work it out correctly. 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 anyone 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 sup- 

 posed. 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. 



