302 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



at great depths, finds a passage to the surface. This water, having 

 found its way from the surface into those depths, is generally 

 subject to a very strong hydrostatic pressure, and possesses the 

 temperature of the depth from which it is liberated. The 

 bore-holes, by means of which these subterranean reservoirs are 

 tapped, are especially fitted for experiments as to the tempera- 

 tures of various depths, since their depth, while being bored 

 is accurately known, and since they are always filled with water. 

 Such experiments have repeatedly been made, and have led to the 

 complete and incontrovertible confirmation of the fact that the 

 temperature of every constant depth, beneath the influences of 

 the variations of temperature on the surface is invariable, and 

 that the temperature increases continually with the depth. The 

 following tables contain some of the most remarkable observa- 

 tions of this nature. 



Artesian well at Rudersdorf, near Berlin : — 



Depth. Temperature. 



380 feet 17.12° Centigrade. 



500 " 17.750 « 



655 " 19.75° " 



880 « 23.50° " 



Artesian well of Grenelle, in Paris : — 



Depth. Temperature. 



917 feet 22.20° Centigrade. 



1231 ".........*. 23.75* " 



1555 " 26.43° " 



1684" 27.70° . " 



Artesian well of Neusalzwerk, Westphalia : — 



Depth. Temperature. 



580 feet 19.7° C entigrade. 



1285 « 27.5° " 



1935 " 31.4° " 



2144 " 33.6° " 



In the artesian well at Mondorff, in the Grand Duchy of 

 Luxemburg, at a depth of 2066 feet, a temperature of 34 ° Cen- 

 tigrade was even observed. We have already seen that the results 

 of the experiments in mines, as to the depth corresponding to one 

 degree of increase, varied considerably. The results obtained in 

 artesian wells as to this point were much more satisfactory. The 

 distance corresponding to an increase of 1 9 was found to be: 



