Page:Astronomy for Everybody.djvu/134

The first question to suggest itself is, how far toward the earth's centre does this increase of temperature extend? The most that we can say is that it cannot be merely superficial, because, in that case, the exterior portions would have cooled off long ago, so that we should have no considerable increase of heat as we went down. The fact that the heat has been kept up during the whole of the earth's existence shows that it must still be very intense toward the centre, and that the rate of increase near the surface must go on for many miles into the interior.

At this rate the material of the earth would be red hot at a depth of ten or fifteen miles, while at one or two hundred miles the heat would be sufficient to melt all the substances which form the earth's crust. This fact suggested to geologists the idea that our globe is really a molten mass, like a mass of melted iron, covered by a cool crust a few miles thick, on which we dwell. The existence of volcanoes and the occurrence of earthquakes gave additional weight to this view, as did also other geological evidence, showing changes in the earth's surface which appeared to be the result of a liquid interior.

But in recent years the astronomer and physicist have collected evidence, which is as conclusive as such evidence can be, that the earth is solid from centre to surface, and even more rigid than a similar mass of steel. The subject was first developed most fully by Lord Kelvin, who showed that, if the earth were a fluid, surrounded by a crust, the action of the moon would not cause tides in the