Page:Popular Science Monthly Volume 47.djvu/316

304 look to yet another star as the pole. The wabbling of the earth's axis in the heavens thus indicated is due to the attraction of the sun and the moon on the mass of the earth, and we can obtain, from its observed amount and from the forces known to be producing it, some idea as to how the mass of the earth must be distributed. Still, we can not, even with this help, be absolutely sure as to the law of the density, but we may rule out the idea of a hollow earth, and accept, as agreeing well with all the facts, the suggestion of Laplace that the condensing effect of pressure decreases as the density produced becomes greater. This increase of density with pressure is, of course, in part to be accounted for by the pressure of the outer layers of the earth on those beneath, which increases until it is something enormous, and, of course, tends to squeeze together the interior and thus render it more dense.

There are, nevertheless, limits to this squeezing effect; and there is another thing that we know about the earth's interior—namely, that it is hot. Hence, as the effect of heat is to expand, the increase of heat would tend to counteract the condensing effect of the increase of pressure. That the earth is really hotter within, and that thus the literally infernal regions are actually hot whatever may be said of the metaphorical inferno, is shown by various lines of reasoning.

In the first place, the astronomers tell us (although they are not quite so sure now that the earth may not be a lump of coagulated meteorites) that this world has cooled from a fluid mass. If so, of course it must be hotter inside. Further, although we have pierced but so little a way into the earth, yet everywhere we meet an increasing temperature. The rate of increase varies very much, however. In the deep copper mines of Lake Superior, for example, at a depth of three thousand feet the temperature has risen from a surface temperature of 40º F. only up to about 70º F., which is still quite a comfortable working temperature. This gives an increase of only one degree Fahrenheit per hundred feet. Beneath the peninsula of Lower Michigan there are brines and sheets of mineral water lying in basin form, and very rich in salt, bromides, etc., and of great medical and commercial value. They have been reached by numerous wells which run down to about three thousand feet near the center of the basin, as at Alma and Bay City. The water comes up from the bottom of these wells hot (over 90º), showing a decidedly more rapid increase in temperature than in the copper mines. But the famous Comstock lode, where fabulous wealth lured the miners on, showed perhaps the most rapid increase in temperature that man has ever dared to face. It was, however, doubtless due to the action of hot waters rising from still greater depths—probably the same waters