398 HISTOEY OF PHYSICAL ASTRONOMY. 



easily conceive, must have appeared of very formidable complexity 

 while it was unsolved, and the first of its kind. Accordingly Halley, 

 as his biographer says, " finding himself unable to make it out in any 

 geometrical way, first applied to Mr. Hooke and Sir Christopher Wren, 

 and meetinof with no assistance from either of them, he went to Cam- 

 bridge in August (1684), to Mr. Newton, who supplied him fully with 

 what he had so ardently sought." 



A paper of Halley 's in the Philosophical Transactions for January, 

 1686, professedly inserted as a preparation for Newton's work, contains 

 some arguments against the Cartesian hypothesis of gravity, which 

 seem to imply that Cartesian opinions had some footing among Eng- 

 lish philosophers ; and we are told by Whiston, Newton's successor in 

 his professorship at Cambridge, that Cartesianism formed a part of the 

 studies of that place. Indeed, Rohault's Physics was used as a class- 

 book at that University long after the time of which we are speaking ; 

 but the peculiar Cartesian doctrines which it contained were soon 

 superseded by others. 



With regard, then, to this part of the discovery, that the force of 

 the sun follows the inverse duplicate proportion of the distances, we 

 see that several other persons were on the verge of it at the same time 

 with Newton ; though he alone possessed that combination of distinct- 

 ness of thought and power of mathematical invention, which enabled 

 him to force his way across the barrier. But another, and so far as 

 we know, an earlier train of thought, led by a different path to the 

 same result ; and it was the convergence of these two lines of reason- 

 ing that brought the conclusion to men's minds with irresistible force. 

 I speak now of the identification of the force which retains the moon 

 in her orbit with the force of gravity by which bodies fall at the earth's 

 surface. In this comparison Newton had, so far as I am aware, no 

 forerunner. We are now, therefore, arrived at the point at which the 

 history of Newton's great discovery properly begins. 



