38 LIGHT SCIENCE FOE LEISURE HOURS. 



maticians. One after another was foiled by it. Halley,. 

 an English mathematician, had detected the difficulty, 

 but no English mathematician was able to grapple 

 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 Con- 

 tinental 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 views 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 (though the difference is 

 very slight), is longer in winter than in summer. 

 This variation had long been recognised as the moon's 



