THE WORK OF GROUND-WATER. 217 



or excavations of any sort have been made to such depths as to indi- 

 cate that its Hmit was being approached, though some of them are 

 a mile or more deep. There is a popular belief that water sinks until 

 it reaches a temperature sufficient to convert it into steam, but except 

 for special localities where hot lava lies near the surface, this behef is 

 not well founded. In the first place, it is not known at what temper- 

 ature water below the surface would be converted into steam. While 

 water boils at sea-level at a temperature of 212° (Falir.) a higher tem- 

 perature would be necessary below that level. 



Assuming the temperature of water sinking beneath the surface 

 to be 50° Fahr., its temperature must be raised 162° to bring it to the 

 temperature at which it would boil at sea-level. On the above assump- 

 tion of initial temperature, the following table shows the depths at 

 which water would reach a temperature of 212° Fahr. under various 

 assumptions as to the rate of increase of temperature. It shows also 

 the pressure in atmospheres which would exist at these several depths 

 if the overlying rock were full of water. 



Depth at which Equivalent 



Rate of Increase of Temperature. Temperature of 212° Pressure in 



would be reached. Atmospheres. 



1° for 50 feet 8,100 feet 238 (approximately) 



1° for 60 '' 9,720 *' 285 



1° for 70 '' 11,340 " 333 '' 



With an initial temperature of 80°, corresponding to that of the 

 warmer parts of the earth's surface, instead of 50°, the table would 

 be as follows: 



1° for 50 feet 6,600 feet 194 (approximately) 



1° for 60 ''... 7,920 '' 214 



1° for 70 " 9,240 '' 272 '^ 



The temperature at which water boils increases with the pressure. 

 A pressure of about 200 atmospheres is the critical pressure for water: 

 that is, the pressure which, if increased, vAW prevent boiling altogether. 

 The depth at which a pressure of 200 atmospheres would be reached, 

 supposing the upper rock to be full of water, is about 6800 feet. The 

 temperature of the water at this depth, under various assumptions 

 as to initial temperature and rate of increase of heat, is shown in the 

 following table; 



