GROUND-WATER 197 



the summit of the hill than elsewhere, though farther from the 

 surface. In regions of even moderate precipitation the water- 

 surface beneath the hills rarely sinks to the level of that in the low- 

 lands about them, before it is raised by further rains. 



The water-surface beneath the lowlands also sinks. Some of 

 it finds its way into valleys, some of it sinks to greater depths, and 

 some of it evaporates; but since the water-surface beneath the 

 elevations sinks more rapidly than that beneath the lowlands, the 

 two approach a common level. Their difference will be least at 

 the end of a drought, and greatest just after heavy rains. 



Depth to which ground- water sinks. The depth to which 

 ground-water penetrates has not been determined by actual 

 observation. The deepest borings or excavations of any sort are 

 little more than a mile deep, and at this depth there is nothing to 

 indicate that the limit of water is being approached. There is a 

 popular belief that water sinks until it reaches a temperature 

 sufficient to convert it into steam; but except for special local- 

 ities where hot lava lies near the surface, this belief does not 

 appear to be well founded. 



Assuming the temperature of water sinking beneath the surface 

 to be 50 F., its temperature must be raised 162 to bring it to 

 the temperature at which it would boil at sea-level. With this 

 initial temperature, the following table shows the depths at which 

 water would reach a temperature of 212 F. under various assump- 

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

 pressure in atmospheres which would exist at these several depths 

 n the overlying rock were full of water. 



Depth at which 



Rate of Increase of Temperature Temperature of Equivalent Pressure in 



Initial temperature 50 F. 212 would be Atmospheres 



reached 



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



1 for 75 " 12,150 " 357 



1 for 100 " 16,200 " 478 



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, will prevent boiling 

 altogether. The depth at which a pressure of 200 atmospheres 



