Moreover, we find that hot bodies contract faster than those that are merely warm, so that a red-hot poker contracts more during the first few minutes after it is taken out of the fire than it does after it has passed the red-hot stage. Hence it is easy to see that the interior portions of the earth, which are hotter, must be contracting at a greater rate than its external parts, for they evidently have very little heat to lose. This may seem rather puzzling to the reader at first; for it might be argued that the heat from below _must_ pass through the external layers, or crust, as it is often called. But it should be remembered that this is not the only way in which the earth loses heat. Think of the vast amount of heat given out from the earth every year by volcanic eruptions, and you will see at once that much of the cooling takes place in this way, and not as a direct flow of heat from the interior, as in the case of the poker. A single big lava-stream flowing out from a volcano, and cooling on the surface of the earth, represents so much heat lost forever; and so do the clouds of steam emitted during every eruption; so, again, do even the hot springs that are continually bringing up warm water. If, then, the lower portions of the earth are slowly contracting, they must tend to leave the outer portions of the crust unsupported, so that they would be compelled by their own enormous weight to settle down. Now, we know that something like this happens in coal mines; and as long passages are hollowed out below, the ground begins to "creep," or slowly sink. Think what would be the effect of a slow sinking of any portion of the earth down towards the centre; it would inevitably be curved up and down into numerous folds, as it endeavoured to get itself onto a smaller space, much in the same way that a table-cloth, when thrown onto a table in a kind of arch, settles down in a series of waves, or folds. And this, it is thought, is the way in which it happens that the pressure comes, as we said just now, sideways, instead of from below upwards. It is on this theory that many geologists account for the enormous side-pressure to which rocks have in many cases been subjected.