That the heat of the earth increases as we penetrate its surface has been learned from mining, tunneling, and the boring of wells. Yet on testing the wells they do not seem to show any uniform temperature. The deepest Artesian well is in St. Louis and has a depth of 3,843½ feet, while its water is found to be of a temperature of 105°.[14] This would seem to indicate that the earth is hotter as we descend into it; still, there may be reasonable causes therefore without its heat extending after all to any great depth. When we remember that the earth is 4,000 miles from surface to centre we find that this deep well is not even 1/5000 part of the distance, and what may be in the interior of the earth is yet quite uncertain. It may possess elements, that from earth’s swift revolutions on its axis, and far swifter flight through space, would supply any loss of heat over and above what is received from the sun. Heat may also penetrate earth’s surface more easily than it escapes, for earth is surrounded by an atmosphere that receives the sunlight readily, but not so readily lets it go; and prevents the outside cold of 200° below zero from falling upon it nightly. Further it is said, with seeming reasonableness; “No rock has the requisite rigidity to resist the crushing weight of a mountain twenty miles high.” Whatever movements may take place in the earth’s crust, involve masses so great and forces so enormous that the resistances of ordinary matter are inconsiderable. The most solid rocks are essentially fluid or viscid. Now, such movements must necessarily result from two causes: First, a slow shrinkage of the earth through loss of heat; secondly, the attraction of the sun and moon, which cause tidal protuberances on the surface of the earth, however rigid it may be; and these, continually shifting their positions, as the oceanic tides do, result in daily motions adequate to develop a large amount of frictional heat.[15]