There are various theories with regard to the nature of the earth's interior. One of these already referred to, but now antiquated, supposes our planet to consist of a thin, solid crust lying on a molten interior, so that the world would be something like an egg with its thin shell and liquid, or semi-liquid, interior. Now, there are grave reasons for refusing to accept this idea. In the first place, a certain slow movement of the earth known as "precession," because it causes the precession of the equinoctial points on the earth's orbit, could not possibly take place as it does if the earth's interior were in this loose and molten condition. That is a matter decided by mathematical calculation, on which we will not dwell further. Secondly, we obtain some very valuable evidence on this abstruse subject from the well-known daily phenomenon of the tides, caused, as the reader is probably aware, by the attractions of the sun and moon; but much more by the moon, because she is nearer, and so exerts a greater pull on the ocean as each part of the world is brought directly under her by the earth's daily rotation on its axis. The waters of our oceans rise up twice each day as they get in a line with the moon, and then begin to fall again. Thus we get that daily ebb and flow seen on our shores. Now, it has been clearly proved by Sir William Thomson, and others, that if any considerable portion of the interior of the earth were in a fluid condition, it too would rise and fall every day as the ocean does. So we should in that case have a tide _below_ the earth as well as on its surface, and the one would tend to neutralise the other, and the ocean tide ought to appear less than it actually is. Even if the earth's crust were made of solid steel, and several hundreds of miles thick, it would yield so much to the enormous pulls exerted by both the sun and moon that it would simply carry the waters of the ocean up and down with it, and we should therefore see no appreciable rise and fall of the water relatively to the land. As a matter of fact, there _is_ a very slight tide in the solid earth below our feet, but so slight that it does not practically affect the tide which we see every day in the ocean. But we wish to show that were the interior of the earth in anything approaching, to a fluid or molten condition, the phenomena of the tides would be very different from what they actually are. All geologists are therefore agreed that we must consider our earth as a more or less solid body, and not as being something like an india-rubber ball filled with water.