62 THE FIGURE OF THE EARTH 



which have been passed through in borings, rock-salt is 

 distinguished as possessing the highest conductivity, and 

 consequently the borings which have been made through 

 it should afford a lower temperature gradient than the 

 average. If now we turn to the results actually obtained 

 in the deep Sperenberg boring,* which passed chiefly 

 through rock-salt, we find a temperature increase of 1 for 

 51J feet, and the British Association Committee remarks 

 upon it, " This result is entitled to special weight, not 

 only on account of the great depth (of the boring), but 

 also on account of the powerful means employed to ex- 

 clude convection." The mean result for observations 

 made from a large number of bore holes in the most 

 various kinds of other rock is given as 1 for 60 feet. 



It will be seen, then, that the gradient for the Speren- 

 berg bore, so far from being below the mean, as it should 

 have been, owing to the high conductivity of the material 

 passed through, actually rises not inconsiderably above 

 it ; and thus the connexion between gradient and con- 

 ductivity does not afford a sufficient explanation of the 

 divergence of extremes from the mean. 



Again, the Sperenberg boring shows that the escape of 

 heat from the earth does not proceed at a uniform rate ; 

 for while the mean rate of escape is 51'4 gram degrees, 

 the rate at Sperenberg is 176*2 gram degrees per year per 

 square centimetre. 



Besides the cause already suggested in explanation of 

 local irregularities in the temperature gradient, there is 

 another which evidently plays a very important part ; 

 this is the chemical action, which takes place in under- 

 ground mines where metallic sulphides are present. This 

 has been specially emphasised by Messrs. Haldane and 

 Meachem.f 



* " Eeport Brit. Assoc.," 1882, p. 88. 



f Proc. Soc. Mining Engineers, 1889, Haldane and Meachem. 



