INTERNAL CONDITION OF THE EARTH. 313 



underground temperature, the length OA being i 

 F. per 5 1 feet. Thus at P' the rate of augmentation 

 is between six- and seven-tenths of iF. per 51 feet, 

 and you must notice how rapidly the falling away 

 takes place. At the point R, which is about 600,000 

 feet below the surface, the rate of augmentation is 

 only one-tenth of a degree per 51 feet, and at 

 S, about 800,000 feet below the surface, it is 

 sensibly zero. Beyond this point underground 

 temperature does not sensibly increase. 



Fig. 2 shows the excess of temperature above the 

 surface temperature at various depths, assuming the 

 rate of augmentation of temperature used in Fig. I. 

 The depths are measured along OX, and the tem- 

 perature at each depth is measured outward from 

 the line OX to the curve. The length OA repre- 

 sents 7900 F. Thus you will see that the excess 

 of temperature at P', 600,000 feet below the sur- 

 face, is not so much as three times as high as at 

 P, which is, 100,000 feet below the surface, while at 

 S, less than 1,000,000 feet below the surface, a 

 maximum temperature of 7000 F. is reached. 



Now, I do not presume to fix within any limits, 



