326 GEOLOGY. 



of that of the others, Stromboli being asleep, while 

 JEtna is raging. The fire is probably seated at some 

 considerable distance under the surface, but the 

 erupted matter does not appear to come from a very 

 great depth. The source of this fire remains unknown, 

 notwithstanding many plausible conjectures. Beds of 

 coal and pyrites do not account for it, neither do the 

 pure metallic basis of potass and soda. 



Temperature of the Earth at various Depths. 



At 10 fathoms . . . 50' 18 



20 to 30 . . . . 60-98 



90 to 100. . . . 69-08 



150 to 160. . . . 75-02 



150 to 200. . . . 75-92 



230 .... 78-44- 



240 .... 82-04 



At the bottom of the mine at Dolcoath, in Corn- 

 wall, at 240 fathoms depth, there issues from a vein 

 a very abundant jet of water, the constant tempera- 

 ture of which is 82. 



Hot Springs and Central Heat. 



In the coal-mine of Killing worth, the deepest in 

 Britain,* the annual temperature at the surface is 

 48 ; at 300 yards below the surface, 70 ; and at 

 400 yards, 74 Fahrenheit. 



In the British mines generally, the increment of 

 temperature is about one degree for every 15 yards 

 of descent; in Saxony, for every 22 yards of de- 

 scent. 



Taking 20 yards as a mean, if the increase follow 

 the same arithmetical ratio to a considerable depth, 

 we ought to find the temperature of the Bath waters 



* The deepest excavation near London, is the well sunk by the 

 present Earl Spencer, at Wimbledon, which is 563 feet deep. 



