Page:Astronomy for Everybody.djvu/135

ocean, but would merely tend to stretch out the entire earth in the direction of the moon, leaving the relative positions of the crust and the water unchanged.

Equally conclusive is the curious phenomenon which we shall describe presently of the variation of latitudes on the earth's surface. Not only a globe of which the interior is soft, but even a globe no more rigid than steel could not rotate as the earth does.

How, then, are we to reconcile the enormous temperature and the solidity? There seems to be only one solution possible. The matter of the interior of the earth is kept solid by the enormous pressure. It is found experimentally that when masses of matter like the rocks of the earth are raised to the melting point, and then subjected to heavy pressure, the effect of the pressure is to make them solid again. Thus, as we increase the temperature we have only to increase the pressure also to keep the material of the earth solid. And thus it is that, as we descend into the earth, the increase of pressure more than keeps pace with the rise of temperature, and thus keeps the whole mass solid.

Another interesting question connected with the earth is that of its density, or specific gravity. We all know that a lump of lead is heavier than an equal lump of iron, and the latter heavier than an equal lump of wood. Is there any way of determining what a cubic foot of earth would weigh if taken out from a great depth of its vast interior? If there is, then we can determine what the