148 SEVENTEENTH CENTURY. PT. in. 



water clock, a mill turned by a mouse, a carriage moved 

 by the person who sat in it, and many other ingenious contri- 

 vances. When he was fifteen his mother sent for him home 

 to manage the farm which belonged to their estate ; 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, per- 

 suaded his mother to send him back to school and to 

 college, where he soon passed all his companions in mathe- 

 matics, and became a Fellow of Trinity College, Cambridge, 

 in 1667. But even before this, in the year 1666, his busy 

 mind had already begun to work out the three greatest dis- 

 coveries of his life. In that year he invented the remarkable 

 mathematical process called the l Method of Fluxions] which 

 is almost the same as that now called the ' Differential 

 Calculus,' invented about the same time by Leibnitz, a great 

 German mathematician. In that year he also made the dis- 

 coveries 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, w r hich 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 \vas 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 



