PRELUDE TO THE EPOCH OF NEWTON. 157 



but his solution was never produced. The proposi- 

 tion that the attractive force of the sun varies 

 inversely as the square of the distance from the 

 center, had already been divined, if not fully esta- 

 blished. If the orbits of the planets were circles, 

 this proportion of the forces might be deduced in 

 the same manner as the propositions concerning 

 circular motion, which Huyghens published in 1673; 

 yet it does not appear that Huyghens made this 

 application of his principles. Newton, however, 

 had already made this step some years before this 

 time. Accordingly, he says in a letter to Halley, 

 on Hooke's claim to this discovery 23 , " When Huy- 

 genius put out his Horologium Oscittatorium, a 

 copy being presented to me, in my letter of thanks 

 I gave those rules in the end thereof a particular 

 commendation for their usefulness in computing 

 the forces of the moon from the earth, and the 

 earth from the sun." He says, moreover, "I am 

 almost confident by circumstances, that Sir Chris- 

 topher Wren knew the duplicate proportion when 

 I gave him a visit; and then Mr. Hooke, by his 

 book Cometa, will prove the last of us three that 

 knew it." Hooke's Cometa was published in 1678. 

 These inferences were all connected with Kepler's 

 law, that the times are in the sesquiplicate ratio 

 of the major axes of the orbits. But Halley had 

 also been led to the duplicate proportion by another 

 train of reasoning, namely, by considering the force 



23 Biog. Brit., art. Hooke. 



