56 LIGHT SCIENCE FOR LEISURE HOURS. 



moon travelling more quickly, but our earth rotating 

 more slowly, which causes the observed discrepancy. 

 Now, it resulted from Laplace's labors as he was the 

 first to announce that the period of the earth's rota- 

 tion has not varied by one-tenth of a second per century 

 in the last two thousand years. The question thus 

 satisfactorily settled, as was supposed, was shelved for 

 more than a quarter of a century. The rasult, also, 

 which seemed to flow from the discussion the con- 

 stancy of the earth's rotation-movement was ac- 

 cepted ; and, as we have seen, our national system of 

 measures was founded upon the assumed constancy of 

 the day's duration. 



But mathematicians were premature in their re- 

 joicings. The question has been brought, by the 

 labors of Professor Adams co-discoverer with Le- 

 verrier of the distant Neptune almost exactly to the 

 point which it occupied a century ago. We are face 

 to face with the very difficulties somewhat modified 

 in extent but not in character which puzzled Halley, 

 Euler, and Lagrange. It would be an injustice to the 

 memory of Laplace to say that his labors were thrown 

 away. The explanation offered by him is indeed a 

 just one, but it is insufficient. Properly estimated, it 

 removes only half the difficulty which had perplexed 

 mathematicians. It would be quite impossible to pre- 

 sent in brief space, and in form suited to these pages, 

 the views propounded by Adams. What, for instance, 



