158 HISTORY OF PHYSICAL ASTRONOMY. 



of the sun as an emanation, which must become 

 more feeble in proportion to the increased spherical 

 surface over which it is diffused, and therefore in 

 the inverse proportion of the square of the dis- 

 tances 24 . In this view of the matter, however, the 

 difficulty was to determine what would be the mo- 

 tion of a body acted on by such a force, when the 

 orbit is not circular but oblong. The investiga- 

 tion of this case was a problem which, we can 

 easily conceive, must have appeared of very formid- 

 able 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 meeting 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 Trans- 

 actions for January, 1686, professedly inserted as 

 a preparation for Newton's work, contains some 

 arguments against the Cartesian hypothesis of gra- 

 vity, which seem to imply that Cartesian opinions 

 had some footing among English philosophers ; and 

 we are told by Whiston, Newton's successor in 

 his professorship at Cambridge, that Cartesianism 



24 Bullialdus, in 1645, had asserted that the force by which 

 the sun "prehendit et harpagat," takes hold of and grapples 

 the planets, must be as the inverse square of the distance. 



