304 THE POPULAR SCIENCE MONTHLY. 



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 pro- 

 ducing it, some idea as to how the mass of the earth must be dis- 

 tributed. 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 de- 

 creases 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 40 F. only up to about 70 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 90), showing a decidedly more rapid increase in tem- 

 perature 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 



