=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. Depth at which Equivalent Rate of Increase Temperature of 212° Pressure in of Temperature. 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, 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: Rate of Increase Temperature at a Initial Temperature. of Temperature. Depth of 6,800 Feet. 50° 1° for 50 feet 186° 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° „