X GEOLOGICAL REFORM 831 



Laplace believed he had accounted for this phe- 

 nomenon by the fact that the eccentricity of the 

 earth's orbit has been diminishing throughout 

 these 3,000 years. This would produce a diminu- 

 tion 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, con- 

 sequently, 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 re- 

 tardation 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 consequence of the tidal retardation. 

 Messrs. Adams, Thomson, and Tait work out this 

 suggestion, and, "on a certain assumption as to 

 the proportion of retardations due to the sun and 

 moon," find the earth may lose twenty -two seconds 

 of time in a century from this cause.^ 



(h.) But M. Dufour suggests that the retardation 



^ Sir W. Thomson, loe. cit. p. 14. 



