40 LIGHT SCIENCE FOR LEISURE HOURS. 



more slowly, which causes the observed discrepancy. 

 Now it resulted from Laplace's labours as he was the 

 first to announce that the period of the earth's rotation 

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

 the last two thousand years. 



The question thus satisfactorily settled, as was sup- 

 posed, was shelved for more than a quarter of a century. 

 The result, also, which seemed to flow from the discus- 

 sion the constancy of the earth's rotation-movement 



was accepted ; 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 labours 

 of Professor Adams co-discoverer with Leverrier 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 labours were 

 thrown away. The explanation offered by him is 

 indeed a just one. But it is insufficient. Properly esti- 

 mated it removes only half the difficulty which had 

 perplexed mathematicians. It would be quite impos- 

 sible to present in brief space, and in form suited to 

 these pages, the views propounded by Adams. What, 

 for instance, would most of our readers learn if we 

 were to tell them that, ' when the variability of the 

 eccentricity is taken into acccount, in integrating the 



