Fig. 201.—Diagram to illustrate the relations of ground-water to the surface.

The water-level 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 elevation sinks more rapidly than that beneath the lowland, 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 empirically. No borings or excavations of any sort have been made to such depths as to indicate that its limit 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 belief is not well founded. In the first place, it is not known at what temperature water below the surface would be converted into steam. While water boils at sea-level at a temperature of 212° (Fahr.) a higher temperature 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 assumption 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.

Rate of Increase
of Temperature.		 Depth at which
Temperature of 212°
would be reached.		 Equivalent Pressure
in 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, will 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:

Initial Temperature.Rate of Increase
of Temperature.		Temperature at a
Depth of 6,800 Feet.
50°1° for 50 feet186° Fahr.
50°1° for 60 “ 163° “
50°1° for 70 “ 147° “
80°1° for 50 “ 216° “
80°1° for 60 “ 193° “
80°1° for 70 “ 177° “

Only one of these temperatures reaches the boiling-point of water at sea-level. It is therefore clear that at this depth water has not even closely approached the boiling temperature for this depth, and since this is the depth of the critical pressure, it is clear that it cannot boil at any greater depth. The descent of water is therefore not stopped, under normal conditions of crustal temperature, because it reaches its boiling-point. Locally, as in the vicinity of active or recently extinct volcanoes, the case may be different.

It is conceivable that water may descend until it reaches its critical temperature (somewhere between 610° and 635° Fahr.). The depth at which the critical temperature would be reached, under various assumptions, is shown in the following table:

Initial Temperature.Rate of Increase
of Temperature.		Depth of Critical Temperature.
50°1° for 50 feet28,000 to 29,250 feet
50°1° for 60 “ 33,600 to 35,100 “
50°1° for 70 “ 39,200 to 40,950 “
80°1° for 50 “ 26,500 to 27,750 “
80°1° for 60 “ 31,800 to 33,300 “
80°1° for 70 “ 37,100 to 38,850 “