NEWTON AND THE LAW OF GRAVITATION 



any one until the seventeenth century's awakening 

 of science. Then the consideration of Kepler's Third 

 Law of planetary motion suggested to many minds 

 perhaps independently the probability that the force 

 hitherto mentioned merely as centripetal, through the 

 operation of which the planets are held in their orbits, 

 is a force varying inversely as the square of the dis- 

 tance from the sun. This idea had come to Robert 

 Hooke, to Wren, and perhaps to Halley, as well as to 

 Newton ; but as yet no one had conceived a method by 

 which the validity of the suggestion might be tested. 

 It was claimed later on by Hooke that he had discov- 

 ered a method demonstrating the truth of the theory 

 of inverse squares, and after the full announcement of 

 Newton's discovery a heated controversy was precip- 

 itated in which Hooke put forward his claims with 

 accustomed acrimony. Hooke, however, never pro- 

 duced his demonstration, and it may well be doubted 

 whether he had found a method which did more 

 than vaguely suggest the law which the observations 

 of Kepler had partially revealed. Newton's great 

 merit lay not so much in conceiving the law of in- 

 verse squares as in the demonstration of the law. He 

 was led to this demonstration through considering the 

 orbital motion of the moon. According to the familiar 

 story, which has become one of the classic myths of 

 science, Newton was led to take up the problem through 

 observing the fall of an apple. Voltaire is responsible 

 for the story, which serves as well as another; its 

 truth or falsity need not in the least concern us. Suf- 

 fice it that through pondering on the familiar fact of 

 terrestrial gravitation, Newton was led to question 



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