156 HISTORY OF PHYSICAL ASTRONOMY. 



ton's discoveries in physical astronomy. Rightly 

 to propose a problem is no inconsiderable step to its 

 solution; and it was undoubtedly a great advance 

 towards the true theory of the universe to consider 

 the motion of the planets round the sun as a 

 mechanical question, to be solved by a reference to 

 the laws of motion, and by the use of mathematics. 

 So far the English philosophers appear to have 

 gone before the time of Newton. Hooke, indeed, 

 when the doctrine of gravitation was published, 

 asserted that he had discovered it previously to 

 Newton; and though this pretension could not be 

 maintained, he certainly had perceived that the 

 thing to be done was, to determine the effect of 

 a central force in producing curvilinear motion ; 

 which effect, as we have already seen, he illustrated 

 by experiment as early as 1666. Hooke had also 

 spoken more clearly on this subject in An Attempt 

 to prove the Motion of the Earth from Observations, 

 published in 1674. In this, he distinctly states that 

 the planets would move in straight lines, if they 

 were not deflected by central forces; and that the 

 central attractive power increases in approaching 

 the center in certain degrees, dependent on the dis- 

 tance. " Now what these degrees are," he adds, " 1 

 have not yet experimentally verified ;" but he ven- 

 tures to promise to any one who succeeds in this 

 undertaking, a discovery of the cause of the hea- 

 venly motions. He asserted, in conversation, to 

 Halley and Wren, that he had solved this problem, 



