1892.] liquid condition of the Earth's interior. 311 



any great depth, it ought to be so still if it is solid. How 

 then, it may be asked, could this enormous amount of heat be 

 perpetually being communicated to the central parts, and they 

 still remain solid ? It seems that they must have become heated 

 far above the temperature of fusion appropriate to the pressure, 

 and must now be liquid ; as nearly all geologists believe. 



I think I have proved in the Physics of the Earth's Crust* 

 that, if the crust is as thin as geologists suppose, and if the age 

 of the world is anything approaching to what geological pheno- 

 mena appear to indicate, then there must exist convection currents 

 in the interior, which prevent the crust from growing thicker 

 by melting off the bottom of it nearly as fast as it solidifies. 

 But I made no suggestion to account for such currents being 

 maintained. Here however we appear to find the explanation. 

 This centrically generated heat would be amply sufficient to 

 support fusion, and to keep the currents in action. Indeed the 

 difficulty is rather to see what would become of it all. Darwin's 

 result, regarding the localization of the heat generated, does not 

 depend upon the viscosity, for the coefficient (v) which is intro- 

 duced into the calculation does not appear in the final result - }-; 

 but it applies only to the heat generated within the earth by 

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

 The distribution of heat within the earth caused by the tidal 

 couple will still follow the same law if only a portion of it is 

 generated within the earth, and the rest within the water of the 

 ocean. Suppose for instance that the earth was either perfectly 

 rigid or perfectly fluid. In either such case no heat would be 

 generated within the earth. But without doubt the friction of 

 the oceanic tidal flow would, in a sufficiently long time, reduce the 

 speed of the rotation |. The heat in that case would be generated 

 only in the water, and be radiated into space. But besides friction 

 there seems reason to believe that some amount of heat may be 

 generated in the ocean owing to the fact that the speed of the 

 forced tide wave differs from that of the free wave with which a 

 disturbance, would travel round the earth under the influence of 

 gravity alone. The question is an interesting one, and the following 

 attempt is made to solve it. 



We have cf> the west longitude of P the place of observation, 

 cot the moon's angular distance west of the prime meridian. Then 

 the moon is cf) — cot east of P. 



* 2nd Ed. pp. 77 and 349. 



t " Problems connected with tides of a viscous spheroid," p. 558, equation (28), viz. 



where H is the average loss of heat throughout the earth. 



J Sir W. Thomson on "Geological Time." Trans. Geol. Soc. of Glasgow, 

 vol. in. pt. i., 1868, p. 6. 



