38 president's address. 



the earth's interior is sensibly affected by variations in the 

 amonnt of heat received from the sun, with consequent mani- 

 festation of earthquake action due to increased shrinkage, 

 when, through any cause, there is a falling off in the amount of 

 sun heat received. This position seems to me equally untenable. 

 It is well known that the heat of the sun does not affect the 

 temperature of the soil more than a few feet from the surface, 

 and in the light of Lord Kelvin's work, it appears certain that no 

 variation in surface temperature within even far wider range than 

 is now experienced, can have any sensible effect on the rate of 

 transmission of heat outwards from the interior. Were it the 

 case that mere change in surface temperature had any such 

 effect, we should surely have distinct manifestations of differential 

 shrinkage every winter, through the unequal loss of heat follow- 

 ing the change of season. To me it seems that unequal loss of 

 heat through winter ruling in one hemisphere while summer 

 was warming the other, would be far more likely to induce earth- 

 quake tremors than even a very considerable increase in the rate 

 of heat loss, uniformly distributed. 



In coming to a conclusion on this matter, it will, perhaps, be 

 helpful to consider what is the actual amount of heat escaping 

 from the interior of the earth. Taking the recognised values for 

 heat conductivity of the crustal rocks and for heat gradient near 

 the surface, Lord Kelvin has shown that the loss of heat amounts 

 to about 92 horse-power per square kilometre.* This may be 

 stated in another way, which will, perhaps, convey a more 

 distinct impression to the mind. A horse-power is equal to 

 the raising of 33,000 lbs. one foot high per minute, and 92 

 horse-power per square kilometre is the equivalent of the 

 evaporation of 10 J lbs. of water per square mile of surface per 

 minute. Even allowing a considerable margin of error for 

 assumed mean conductivity and heat gradient, the result would 

 be of the same order, and as it stands may thus be taken as 



* Trans. Geol. Soc. Glasgow, Vol. ill. Part ii., 1869, p. 234. Popular 

 Lectures and Addresses, Vol. ii., 1894, p.l 16. 



