1893-94-] The Interior of the Eartli. 99 



increase varies in different places — in great part, no doubt, owing 

 to differences in the conducting capacity for heat of the differ- 

 ent kinds of rocks. This fact may account for variations in 

 the increase of temperature in the same boring. 



In the mines of Cornwall and Devon, Mr Henwood made a 

 number of observations of temperature at various depths. They 

 were among the first experiments of the kind. The average 

 result in 200 of these mines gives the following figures: — 



Depth. Temperature. 



200 feet .... About 57° Fahr. 



600 „ . . . . „ 62° „ 



900 i, . . . . n 68° i, 



1200 „ m 78° 11 



The average increase below 300 feet being about 1° F. for 

 every 43 feet of descent. 



In Eose Bridge Colliery, near Wigan, where a shaft was carried 

 down to a depth of 2445 feet, Mr Brigham, the manager, made 

 a careful series of temperature observations during the process of 

 sinking. The heat at the lowest point was found to be 94° F. 

 These observations showed an average increment of 1° F. for 

 every 55 feet of depth. The actual heats obtained were as 

 follows : — 



Depth. 



564 feet .... 



1674 „ .... 



2013 n .... 



2445 ,, .... 



While, then, there appears to be some variation in the rate of 

 this increase at different points, it is an undoubted fact that 

 the temperature of the rocks does always increase after passing 

 below the first 5 feet ; and it seems that the average rate of 

 this increase of temperature may be taken at about 1° F for 

 every 51 feet of depth. 



The heat of water in deep wells and borings at the bottom 

 is in agreement with the above conclusion. The water which 

 flows from the artesian well of Grenelle, near Paris, and which 

 rises from a depth of 1800 feet, has a constant temperature 

 of 82° F., which is about 31° hotter than the mean tempera- 

 ture of the ground beneath Paris. This indicates a rate of 



