SIX ISAAC NEWTON 



In 1679 he wrote to Newton on the nature of the 

 course of projectiles, presenting as a certain fact that 

 an eccentric ellipse would be the consequence of re- 

 ciprocal gravity in the ratio of the squares of the 

 distances from the centre of the earth. Newton was 

 still unwilling to give expression to his own opinions, 

 since he could not reconcile his calculations of the 

 distance of the moon based upon the diameter of the 

 earth as it was then given. In 1682 he learned that 

 the measure of a terrestrial degree had been lately 

 made with extreme care by Picard. Obtaining 

 the length of a degree thus calculated, Newton re- 

 turned home, and, taking up his calculations made 

 in 1665, he revised it with the change therein made 

 by the new length of a degree. As he advanced 

 and saw the result it would have on his theories, he 

 became so excited that he could not continue his cal- 

 culations, and was obliged to ask a friend to com- 

 plete them for him. This time, the accordance of 

 his theory with the observations was perfect. The 

 effect of weight at the surface of the earth, as drawn 

 from his experiments with the fall of bodies, when 

 applied to the moon diminishing as the square of the 

 distance between the centres of the respective bodies 

 was found to be identically equal to the centrifugal 

 force of the moon, and conclusive as to the rapidity 

 of its course and of the observed distances. He who 

 8 113 



