CH. xviii. ' SIR ISAAC NEWTON. 145 



but it was soon clear that he was of no use as a farmer, for 

 though he tried hard to do his work, his mind was not in it, 

 and he was only happy when he could settle down under a 

 hedge with his book to study some difficult problem. At 

 last one of his uncles, seeing how bent the boy was upon 

 study, persuaded his mother to send him back to school and 

 to college, where he soon passed all his companions in 

 mathematics, and became a Fellow of Trinity College, 

 Cambridge, in 1667. But even before this, in the year 1 666, 

 his busy mind had already begun to work out the three 

 greatest discoveries of his life. In that year he discovered 

 the remarkable mathematical process called the * Method of 

 Fluxions] which is almost the same as that now called the 

 * Differential Calculus] worked out about the same time by 

 Leibnitz, a great German mathematician. In that year he 

 also made the discoveries about Light and Colour, which we 

 shall speak of by and by ; and again in that year he first 

 thought out the great Theory of Gravitation, which we must 

 now consider. 



Theory of Gravitation, 1666. In the course of his 

 astronomical studies, Newton had come across a problem 

 which he could not solve. The problem was this. Why 

 does the moon always move round the earth, and the planets 

 round the sun? The natural thing is for a body to go 

 straight on. If you roll a marble along the floor it moves 

 on in a straight line, and if it were not stopped by the air 

 and the floor, it would roll on for ever. Why, then, should 

 the bodies in the sky go round and round, and not straight 

 forward ? 



While Newton was still pondering over this question, the 

 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 



