XL] GEOLOGICAL REFORM. 247 



more than a century old, 1 but deduced from it some of 

 its more important consequences, such as the constant 

 turning of one face of the moon towards the earth. 



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 follow : — 



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

 motion is (and has for the last 3,000 years been) under- 

 going an acceleration, 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 pheno- 

 menon by the fact that the eccentricity of the earth's 

 orb it has beendiminishing 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, 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 retardation could be 

 accounted for in the way he had suggested. There 



1 "Untersuchuno; dcr Frage ob die Erde in ihrcr Umdrelnmg urn die 

 Achse, wodruch sic die Abwechselung des Tages mid dcr Nacht hervorbringt, 

 einige Veranderung scit den ersten Zeiten ihres Ursprunges erlittepr kabc\ 

 &c." — Kant's Sammtliche Wtrke, Ld. i. p. 178. y^Sc\^'^i 



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