SIR ISAAC NEWTON 



centre of the earth with an ever-increasing swiftness, 

 and which acted without apparent diminution, on the 

 top of the highest towers or mountains, he thought, 

 why may not this power extend even to the moon, 

 and then what more would be needed to hold it 

 in its course around the earth ? The next step then 

 would be to see if the same power would retain the 

 planets in their orbits around the sun. These were but 

 conjectures; but he soon proceeded to verify them by 

 the appropriate calculations. To arrive at the effect of 

 gravity between the earth and the moon it was neces- 

 sary to use the radius of the Earth for the first factor. 

 This radius, was at that time calculated from the 

 length of a degree on the earth's curvature being 

 equal to sixty miles. From this datura, which was 

 thought to be correct, Newton found that the law that 

 " gravitation was directly as the mass and inversely 

 as the square of the distance between the centres" 

 would not account for the moon's motion around the 

 earth. He therefore laid aside this hypothesis for 

 many years as incapable of verification, until later 

 correct measurements proved the length of a degree 

 to be sixty-nine and one-tenth miles, and not sixty 

 miles, as he had been led to believe. 



On the cessation of the Plague, at the end of 1666, 

 he returned to Cambridge. In 1669 he was appointed 

 in the place of Barrow, who had resigned in his favor, 



109 



