PHYSICAL SCIENCE 27 



circle instead of in Galileo's straight course, perhaps 

 the familiar force of gravity, by which an apple falls 

 to the ground, might be the cause also of the moon's 

 continual fall towards the earth. 



It was probable that any force directed towards a 

 central point, and operative throughout surrounding 

 space, would vary inversely as the square of the dis- 

 tance. And, on this hypothesis, Newton calculated 

 what the effect of gravity would be at the distance 

 of the moon. Misled at first by an inaccurate value 

 of this distance, he found that the actual fall of the 

 moon towards the earth was less than that calculated. 

 But, six years later, a redetermination of the size of 

 the earth gave a different value for the moon's 

 distance, and enabled Newton to show that gravity, 

 which is operative on the earth's surface, is the effec- 

 tive cause of the moon's circular motion. 



Newton's next great achievement was the geo- 

 metrical proof that the inverse square law would 

 explain the motion of the planets in ellipses, and that 

 no other law would do so. Then, on the assumption 

 that each particle of matter produced its own effect, 

 he showed that a sphere attracted as though all 

 its mass were concentrated at a central point, and 

 was hence led to the final form of his theory, that 

 every particle of matter in the universe attracts 

 every other particle with a force that varies inversely 

 as the square of the distance between them. 



For two years Newton was absorbed in the task 



