PHYSICAL ASTRONOMY. 



derial revolution of the Moon, 3.2'.32".39; and 

 in a year 40.40'.2(). 



MAYER'S Tables mate the latter 40.41'.33". Dr 



/ 



STEWART'S Theorem gives 3.1'.20", supposing 



= ^7-, the value assigned to it, 250. 



090 



The result of these investigations, therefore, agrees 

 nearly with observation ; but it cannot be denied 

 that the principle on which they are founded is lia- 

 ble to some objections, so that if it were not for 

 the information derived from the direct solution 

 of the problem of the three bodies, it might still be 

 doubted, whether the principle of gravity account- 

 ed exactly for the motion of the Moon's apsides. 

 CLAIRAUT, who first compared the result of that 

 solution with observation, met with the same diffi- 

 culty that NEWTON had done, and found that his 

 formula gave only half the true motion. He there- 

 fore imagined that gravity is not inversely as the 

 squares of the distances, but follows a more compli- 

 cated law, such as can only be expressed by a for- 

 mula of two terms. In seeking for the co-efficient 

 of the second term, he was obliged to carry his 

 approximation farther than he had done before; in 

 consequence of which, the co-efficient he sought for 

 came out equal to 0, and the motion of the apsides 

 was found to be completely explained by the sup- 

 position. 



