54 LIGHT SCIENCE FOR LEISURE HOURS. 



with it. Contented with Newton's fame, they had 

 suffered their Continental rivals to shoot far ahead in 

 the course he had pointed out. But the best Conti- 

 nental mathematicians were defeated. In papers of 

 acknowledged merit, adorned by a variety of new pro- 

 cesses, and showing a deep insight into the question at 

 issue, they yet arrived, one and all, at the same con- 

 clusion failure. 



Ninety years elapsed before the true explanation 

 was offered by the great mathematician Laplace. A 

 full exposition of his view r s would be out of place in 

 such a paper as the present, but briefly, they amount 

 to this : 



The moon travels in her orbit, swayed chiefly by 

 the earth's attraction. But the sun, though greatly 

 more distant, yet, owing to the immensity of his mass, 

 plays an important part in guiding our satellite. His 

 influence tends to relieve the moon, in part, from the 

 earth's sway. Thus she travels in a wider orbit, and 

 with a slower motion, than she would have but for the 

 sun's influence. Now, the earth is not at all times 

 equally distant from the sun, and his influence upon 

 the moon is accordingly variable. In winter, when the 

 earth is nearest to the sun, his influence is greatest. 

 The lunar month, accordingly (as any one may see by 

 referring to an almanac), is longer in winter than in 

 summer. This variation had long been recognized as 

 the moon's " annual equation ; " but Laplace was the 



