42 STUDIES, SCIENTIFIC AND SOCIAL chap. 



general substratum of molten matter between the crust 

 and a possibly solid interior. The first two causes are now 

 generally admitted to be inadequate, and our choice is 

 practically limited to one of the latter. 



There are also very important evidences of internal heat 

 derived from the universal phenomenon of a fairly uniform 

 increase of temperature in all deep wells, mines, borings, 

 or tunnels. This increase has been usually reckoned as 

 1° F. for each 60 feet of descent, but a recent very careful 

 estimate, by Professor Prestwich, derived from the whole 

 of the available data, gives 1° F. for every 47*5 feet of 

 descent. It is a curious indication of the universality of 

 this increase that, even in the coldest parts of Siberia, 

 where the soil is frozen to a depth of 620 feet, there is a 

 steady increase in the temperature of this frozen soil from 

 the surface downwards. Much has been made by some 

 writers of the local differences of the rate of increase, vary- 

 ing from 1° in 28 feet to 1° in 95 ; and also of the fact 

 that in some places the rate of increase diminishes as the 

 depth becomes greater. But when we consider that 

 springs often bring up heated water to the surface in 

 countries far removed from any seat of volcanic action, and 

 the extent to which water permeates the rocks at all depths 

 reached by man, such divergences are exactly what we 

 might expect. Now this average rate of increase, if con- 

 tinued downwards, would imply a temperature capable of 

 melting rock at about twenty miles deep, or less, and we 

 shall see presently that there are other considerations 

 which lead to the conclusion that this is not far from the 

 average thickness of the solid crust. 



Before going further it will be well to consider certain 

 objections to this conclusion, which for a long time were 

 considered insuperable, but which have now been shown 

 to be either altogether erroneous or quite inconclusive. In 

 Sir Charles Lyell's Frincijdes of Geology, Mr. Hopkins is 

 quoted as having shown that the phenomenon of the ipre- 

 cession of the equinoxes, due to the attraction of the sun 

 and moon on the equatorial protuberance, requires the 



1 In a recent deep boring at Wheeling, Virginia, the rate of increase 

 was found to be greater as the depth increased. 



