XI .] GEOLOGICAL REFORM. 215 



But there is a long step from the demonstration of a tendency 

 to the estimation of the practical value of that tendency, which 

 is all with which we are at present concerned. The facts bearing 

 on this point appear to stand as follows : 



It is a matter of observation that the moon s mean motion is 

 (and has for the last 3,000 years been) undergoing an accelera 

 tion, relatively to the rotation of the earth. Of course this may 

 result from one of two causes : the moon may really have been 

 moving more swiftly in its orbit ; or the earth may have been 

 rotating more slowly on its axis. 



Laplace believed he had accounted for this phaenomenon by 

 the fact that the eccentricity of the earth s orbit has been 

 diminishing throughout these 3,000 years. This would produce 

 a diminution of the mean attraction of the sun on the moon ; 

 or, in other words, an increase in the attraction of the earth 

 on the moon ; and, consequently, an increase in the rapidity 

 of the orbital motion of the latter body. Laplace, therefore, 

 laid the responsibility of the acceleration upon the moon, and 

 if his views were correct, the tidal retardation must either 

 be insignificant in amount, or be counteracted by some other 

 agency. 



Our great astronomer, Adams, however, appears to have found 

 a flaw in Laplace s calculation, and to have shown that only half 

 the observed retardation could be accounted for in the way he 

 had suggested. There remains, therefore, the other half to be 

 accounted for ; and here, in the absence of all positive knowledge, 

 three sets of hypotheses have been suggested. 



(a.) M. Delaunay suggests that the earth is at fault, in conse 

 quence of the tidal retardation. Messrs. Adams, Thomson, and 

 Tait work out this suggestion, and, &quot; on a certain assumption as 

 to the proportion of retardations due to the sun and moon,&quot; find 

 the earth may lose twenty-two seconds of time in a century from 

 this cause. 1 



(&.) But M. Dufour suggests that the retardation of the earth 



1 Sir W. Thomson, loc. cit., p. 14. 



