So marked is this steady increase of temperature as we go downwards, that it has been seriously proposed to make very deep borings in order to obtain supplies of warm water for heating our towns. Arago and Walferdin suggested this method for warming the Jardin des Plantes at Paris; and now that such important improvements have been devised in carrying borings to enormous depths, the time may not be far distant when we shall draw extensively upon these supplies of subterranean heat. At the present time the city of Buda-Pesth is extensively supplied with hot-water from an underground source. Should our coal-supply ever fail it may be well to remember that we have these inexhaustible supplies of heat everywhere beneath our feet. But although we may conclude that at the moderate depths we have indicated such high temperatures exist, it would not be safe to infer, as some have done, that at a distance of only 40 or 50 miles from the surface the materials composing our globe are in a state of actual fusion. Both theory and experiment indicate that under increased pressure the fusing point of solid bodies is raised; and just as in a Papin's digester we may have water retained by high pressure in a liquid condition at a temperature far above 212° F., so in the interior of the earth, masses of rock may exist in a solid state, at a temperature far above that at which they would fuse at the earth's surface. We may speak of such rock-masses, retained in a solid condition by intense pressure, at a temperature far above their fusing point at the earth's surface, as being in a 'potentially liquid condition.' Upon any relief of pressure such masses would at once assume the liquid state, just as the superheated water in a Papin's digester immediately flashes into steam upon the fracture of the strong vessel by which it is confined. We have already seen how the action at volcanic vents often appears to indicate just such a manifestation of elastic forces, as would be exhibited by the relief of superheated masses from a state of confinement by pressure. In reasoning upon questions of this kind, however, we must always be upon our guard against giving undue extension to principles and laws which seem to be clearly established by experiment at the earth's surface. It is well to remember how exceedingly limited is our command of extreme pressures and high temperatures, when compared with those which may exist within a body of the dimensions of our globe. [Sidenote: EFFECT OF PRESSURE ON FUSION-POINT.]