THE PRIME RESULTANT 111 



This acceleration of the moon, as it was called, amounted to 

 nearly ten seconds in a century, and various hypotheses were 

 framed to account for it. The most plausible of these was, that 

 all space was filled with an ethereal medium which opposed such 

 a resistance to the motions of the planets, that the force which 

 kept them in their orbit would gradually overpower their di- 

 minished velocity, and thus shorten their period round the cen- 

 tral body. This hypothesis was supported by Euler, and by the 

 abettors of the undulatory theory, who required the existence of 

 a medium for the propagation of light, and it was adopted with 

 equal eagerness by another class of theorists, who saw in the 

 acceleration of the celestial motions the process by which the 

 Almighty was to destroy the solar system, by precipitating the 

 secondary planets upon their primaries, and the primary planets 

 upon the sun. Laplace admitted the sufficiency of the hypothe- 

 sis, but as he saw no reason for admitting the existence of a re- 

 sisting medium, he did not consider himself warranted in adopt- 

 ing such an hypothesis till it was found that gravitation was in- 

 capable of accounting for the fact (Brewster, M. of N., p. 307). 



The Academy of Science proposed the moon's acceleration 

 as the subject of their prize for 1770. Euler gained it, but came 

 to the conclusion that it was not produced by the force of gravity. 

 The same subject was again proposed in 1772, and the prize was 

 divided between Euler and Lagrange. Euler ascribed the accel- 

 eration to a resisting medium, and Lagrange evaded the diffi- 

 culty. The prize was again offered in 1774, and was gained by 

 Lagrange, and he now doubted the existence of the inequality. 

 It was under these circumstances that Laplace took up the sub- 

 ject and found the acceleration to be ten seconds during a cen- 

 tury, and to be caused by the eccentricity of the earth's orbit. 



The curious subject of the moon's acceleration has recently 

 excited much interest in consequence of Prof. Adams having 

 recently proposed an important correction upon the theory of 

 Laplace, by which the secular acceleration was reduced to 6.11". 

 * * * rphe diff erence between the results of Mr. Adams 

 (confirmed by Delaunay), if correct, and those obtained from 

 ancient eclipses, is very remarkable, and indicates the operation 

 of some cause which remains to be discovered (ibid., p. 310). 



Mr. Sterner and I in our investigation of the theory of the 

 moon's motion, have probably occupied altogether about 8 or 9000 

 hours. There were about 13,000 multiplications of series made, 

 containing some 400,000 separate products ; the whole of the work 

 required the writing of between 4 and 5 million of digits and plus 

 and minus signs. Although the problem now completed con- 

 stitutes by far the longer part of the whole, much remains to be 

 done before it is advisable to proceed to the construction of the 

 tables. (Prof. Ernest W. Brown). 



