A HISTORY OF SCIENCE 



but one of these, Lord Kelvin was led by this controver- 

 sy over the earth's age to make his famous computa- 

 tion in which he proved that the telluric structure, as a 

 whole, must have at least the rigidity of steel in order 

 to resist the moon's tidal pull as it does. Hopkins had, 

 indeed, made a somewhat similar estimate as early as 

 1839, proving that the earth's crust must be at least 

 eight hundred or a thousand miles in thickness; but 

 geologists had utterly ignored this computation, and 

 the idea of a thin crust on a fluid interior had continued 

 to be the orthodox geological doctrine. Since Lord 

 Kelvin's estimate was made, his claim that the final 

 crust of the earth could not have formed until the mass 

 was solid throughout, or at least until a honeycomb of 

 solid matter had been bridged up from centre to cir- 

 cumference, has gained pretty general acceptance. It 

 still remains an open question, however, as to what 

 proportion the lacunae of molten matter bear at the 

 present day to the solidified portions, and therefore to 

 what extent the earth will be subject to further shrink- 

 age and attendant surface contortions. That some 

 such lacunas do exist is demonstrated daily by the 

 phenomena of volcanoes. So, after all, the crust theory 

 has been supplanted by a compromise theory rather 

 than completely overthrown, and our knowledge of the 

 condition of the telluric depths is still far from definite. 

 If so much uncertainty attends these fundamental 

 questions as to the earth's past and present, it is not 

 strange that open problems as to her future are still 

 more numerous. We have seen how, according to 

 Professor Darwin's computations, the moon threatens 

 to come back to earth with destructive force some 



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