84 



IGNEOUS AGENCIES. 



Increasing Temperature of the Interior of the Earth.— Beneath the 



invariable stratum the temperature of the earth everywhere increases, 

 for all depths to which it has been penetrated, at an average rate of about 



Fig. 74. 



Fig. 75. 



1° for every 53 feet. This very important fact has been determined by 

 numerous observations on the temperature of mines and of Artesian 

 wells in almost every part of the earth. All the facts thus far stated 

 are graphically illustrated in the accompanying figure (Fig. 74), in 

 which the line a b represents depth below the surface, and the diverging 

 line c d the increasing heat ; m the invariable stratum ; n the line of no 

 daily variation ; the curves p e, c e, o e, the temperatures in summer, 

 autumn, and winter, respectively ; the space ceo the annual swing of 

 temperature ; and the smaller curves meeting on the line n 9 the daily 

 variation or swing of temperature. 



We have given the rate of increase as about 1° in 53 feet. It 

 varies, however, in different places, from 1° in 30 feet to 1° in 90 feet. 

 Except in the vicinity of volcanic action, this difference is probably 

 due to varying conductivity of the rocks. The lines, or rather surfaces, 

 which join places in the interior of the earth, having equal tempera- 

 tures, may be called isogeotherms. If the rate of increase were every- 

 where the same, the isogeotherms would be regularly concentric ; but, 

 as this is not the case, they are irregular surfaces (Fig. 75), rising 

 nearer the earth-surface and closing upon one another where the con- 

 ductivity is poor, and sinking deeper and separating where the con- 

 ductivity is greater. 



Constitution of the Earth's Interior.— From the facts given above 

 it is probable that the temperature of the interior of the earth is very 

 great. A rate of increase of 1° for every 53 feet would give us, at the 

 depth of twenty-five or thirty miles, a temperature, sufficient to fuse most 



