722 NATURAL SCIENCE. 



DEC, 



the action of the tidal couple upon the substance of that interior. 

 "The whole heat generated, from first to last," says Professor Darwin, 

 " gives a supply of heat, at the present rate of loss, for 3,560 millions 

 of years." 



Re-investigating, mathematically, '* the effects of the tidal yielding 

 of the earth, on a tide of short period, according to the canal theory," 

 Mr. Fisher reaches the conclusion that on an earth with a liquid 

 interior and a solid crust, ocean tides would exist, of the same type 

 as those upon a solid earth. The amount of tide in the former case 

 would be about two-fifths of the amount of what it would be in the latter. 

 This he considers to be " a sufficient answer to the objection brought 

 against the theory of internal liquidity, that in such a case there 

 could be no measurable ocean tides." 



He points out that the conclusion, that in a solid globe nothing 

 short of a high degree of rigidity could sustain the weight of 

 continents and mountains, disappears wholly in the light of Airy's 

 hypothesis that the crust is supported in a state of approximate 

 hydrostatic equilibrium on a yielding nucleus. Hence, all conclusions 

 drawn from the supposed necessity of a high degree of viscosity, 

 vanish with it ; so that all numerical estimates founded upon this 

 must be wholly inapplicable in the case of a liquid interior. 



Mr. Fisher next goes on to show that on the tidal theory the 

 whole estimated amount of heat generated in the earth remains the 

 same, whether the question of time be considered or not. He holds 

 that if we take this total amount of heat, as given above, it is difficult 

 to understand how it can possibl}' have been dissipated. Under 

 present conditions only a very small portion of the heat generated- in 

 this manner could be taken up by the ocean waters and radiated 

 into space. " Unless ocean tides have been in operation for a length 

 of time exceeding any estimate hitherto suggested,- it does not appear 

 probable that any considerable portion of the heat generated, 

 according to the hypothesis of the moon being shed from the earth, 

 can be got rid of by this means." Unless this heat is accumulating, 

 it must have been got rid of by (i) " Conduction through the 

 solidified crust ; (2) by transference of internal heat to the surface 

 by volcanic action ; or (3) by conversion of heat into work against 

 gravity, &c." 



He then calculates that the amount of heat carried off by 

 conduction through the earth's crust, even in 100 millions of years, 

 must be quite inconsiderable compared to the full amount presumably 

 generated in the interior. Next, as an extreme case of volcanic, 

 action, he shows that presuming the ocean to be four miles deep, 

 and covering the entire globe, and that the whole of its waters were 

 originally in solution with the magma of the earth, and are now 

 cooled down to the freezing point, the total reduction of heat by this 

 process would only use up 8^ F. of the 3,000° F. to which the interior 

 is estimated to have been heated ; and, lastly, supposing heat has 



