a large store of energy, but still it is of limited quantity, and it is also on the wane. This heat is occasionally copiously liberated by volcanoes, but ordinarily the transit of heat from the interior to the surface and its discharge from thence by radiation is a slow process. It is, however, sufficient for our present purpose to observe that slow though the escape may be, it is incessantly going on. There is only a definite number of units of heat contained in the interior of the earth at this moment, and as they are gradually diminishing, and as there is no source from which the loss can be replenished, there is here no supply of warmth that can be relied on permanently. It must also be mentioned that there exists another store of energy which under certain conditions admits of being transformed into heat. I allude to the energy which the earth possesses in virtue of its rapid rotation on its axis.
In this respect we may liken our globe to a mighty flywheel which contains a certain quantity of energy that must be poured forth as its speed is reduced. It is the action of the tides which enables this form of earth energy to be transformed into heat. The tides check the speed with which the earth rotates. The energy thus lost must in part at least be transformed into heat, which is then again lost by radiation into space. Of course the quantity of energy which the earth possesses by reason of its rotation is of limited amount, and it is steadily being dissipated, just as the internal heat is being lost and just as the potential heat that exists in consequence of unsatisfied chemical attraction is also declining. It seems that whenever the tides shall have so checked the earth that it only rotates at half its present speed, the quantity
