But suppose a tube (how it is made will be explained later), large or small, regular or irregular, to extend far into the earth, near or through any great source of heat resulting from condensation, combustion, chemical action, or central fire.  Now suppose this tube to be filled with water from surface or subterranean sources.  Heat converts water, under the pressure of one atmosphere, or fifteen pounds to the square inch, into steam at a temperature of two hundred and twelve degrees.  But under greater pressure more heat is required to make steam.  The water never leaps and bubbles in an engine boiler.  The awful pressure compels it to be quiet.  A cubic inch of water will make a cubic foot—­one thousand seven hundred and twenty-eight times as much—­of steam under the pressure of one atmosphere.  But under the pressure of a column of water one thousand feet high, giving a pressure of four hundred and thirty-two pounds to the square inch at the bottom, water becomes steam, if at all, only by great heat.  Every engineer knows that the pressure exerted by steam increases by great geometrical ratios as the heat increases by small arithmetical ratios.  Steam made by two hundred and twelve degrees exerts a pressure, as we have said, of fifteen pounds.

To simply double the two hundred and twelve degrees of heat increases the steam pressure twenty-three times.

Now suppose the subterranean tube or lake of Old Faithful to be freshly filled with its million gallons of water.  Sufficient heat makes steam under any pressure.  It rises up the tube and is condensed to water again by the colder water above.  Hence no commotion.  But the whole volume of water grows hotter for an hour.  When it is too hot to absorb the steam, and the tube is too narrow to let the amount made bubble up through the water, it lifts the whole mass with a sudden jerk.  The instant the pressure of the water is taken off in any degree, the water below, that was kept water by the pressure, breaks into steam most voluminously, and the measureless power floods the earth and sky with water and steam.

It is also known that superheated steam suddenly takes on such great power that no boiler can hold it.  Once let the water in a boiler get very low and no boiler can hold the force of the resultant superheated steam.  The same heat that, applied to water, gives perfect safety, applied to steam gives utter destruction.  Hence the amazing force of the vast jets of the geyser that follow the first spurts.

As soon as the steam is blown off the subterranean waterworks fill the tube and the process is repeated.

This modus operandi was first proposed as a theory by Bunsen in 1846, and later was demonstrated by the artificial geyser of Professor J. H. J. Muller, of Freiburg.

[Illustration:  Pulpit Terrace and Bunsen Peak.]

MOUNDS OF MINERAL DEPOSITS