The Evolution of the Sciences 



tions made in the shallow layer of the earth, 

 accessible to our investigation. The ideal con- 

 dition would be presented by a layer of ground 

 sufficiently homogeneous to be of constant 

 calorific conductivity throughout, and thick 

 enough to permit the precise measurement of 

 what is called the geothermic degree, that is to 

 say, the thickness of ground which corresponds 

 to an increase of temperature equal to i centi- 

 grade. 



This condition was very nearly realised in a 

 boring sunk at Sperenberg, south of Berlin. 

 Out of a total depth of 1380 yards the drill 

 remained for nearly noo yards in a bed of rock 

 salt; at the bottom the temperature reached 

 48 centigrade, corresponding to a mean geother- 

 mic degree of thirty-six yards. However, in 

 spite of the remarkable homogeneousness of the 

 formation the thermometer readings taken at dif- 

 ferent depths were far from showing a regular rise 

 of temperature. In particular the deepest section, 

 between 1170 and 1340 yards down, gave a 

 geothermic degree of 145 yards. 



These results were at first interpreted un- 

 favourably to the central heat theory. Carl 



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