and contains a good deal of iron inside. This great ball is undoubtedly very hot in its interior. We have many reasons for knowing this to be the case. The eruptions of volcanoes afford the simplest proof. The smoke, the ashes, and the molten lava which volcanoes pour forth show us that the earth is anything but cool in the lower regions. Other terrestrial phenomena bear similar testimony. Hot springs, for instance, evince the heated condition of the deep-seated rocks. Any miner will tell you that the deeper his mine the hotter he finds his work to be. The gain in heat arises from the fact that the deeper the mine the nearer it lies to the central incandescence. We are not now referring to such heat as is produced by combustion. We are discussing the way in which a body that has once been heated by a fire, or by some other agency, gradually parts with its heat and falls in temperature. There is no means of replenishing to any large extent the heat of the inside of our earth by combustion. The earth's interior temperature must, therefore, on the whole, be simply falling in accordance with the laws of cooling.
The conclusion to which we are led by this reasoning is a remarkable one. We know that our earth has been in existence for an incalculable period of time. We are not even able to estimate how many thousands of years have elapsed since man began his course on our globe, but the human period is merely the latest of all the great periods in world-history. Long before man commenced to live here the earth was the abode of life; countless races of animals, large and small, now generally extinct, roamed through forests of trees, or through vegetation of a kind largely if not wholly different from anything
