SOME UNSOLVED SCIENTIFIC PROBLEMS 



old, and it can hardly be said that the attempted com- 

 putations have added much of definiteness to that propo- 

 sition. They have, indeed, proved that the period of 

 time to be drawn upon is not infinite; but the nebular 

 hypothesis, to say nothing of common-sense, carried us 

 as far as that long ago. 



If the computations in question have failed of their 

 direct purpose, however, they have been by no means 

 lacking in important collateral results. To mention but 

 one of these, Lord Kelvin was led by this controversy 

 over the earth's age to make his famous computation 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 circumference, has 

 gained pretty general acceptance. It still remains an 

 open question, however, as to what proportion the lacunas 

 of molten matter bear at the present day to the solidified 

 portions, and therefore to what extent the earth will be 

 subject to further shrinkage and attendant surface 

 contortions. That some such lacuna? do exist is demon- 

 strated daily by the phenomena of volcanoes. So, after 

 all, the crust theory has been supplanted by a compro- 

 mise theory rather than completely overthrown, and 



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