The question is one that stands on a very different footing from that it occupied before the days of Galileo and Copernicus. _Then_ the earth was believed to be the centre of the universe, and all the heavens and stars created for it. _Then_ the commencement of the world was the commencement of the universe, its destruction would be the destruction of all. _Now_, thanks to the revolution in feeling that has been accomplished by the progress of astronomy, we have learned our own insignificance, and that amongst the infinite number of stars, each supporting their own system of inhabited planets, our earth occupies an infinitesimally small portion, and the destruction of it would make no difference whatever--still less its becoming uninhabitable. It is an event which must have happened and be happening to other worlds, without affecting the infinite life of the universe in any marked degree. Nevertheless, for ourselves, the question remains as interesting as if we were the all in all, but must be approached in a different manner. Numerous hypotheses have been put forth on the question but they may mostly be dismissed as vain. Buffon calculated that it had taken 74,832 years for the earth to cool down to its present temperature, and that it will take 93,291 years more before it would be too cold for men to live upon it. But Sir William Thomson has shown that the internal heat of the earth, supposed to be due to its cooling from fusion, cannot have seriously modified climate for a long series of years, and that life depends essentially on the heat of the sun. Another hypothesis, the most ancient of all, is that which supposes the earth will be destroyed by fire. It comes down from Zoroaster and the Jews; and on the improbable supposition of the thin crust of the earth over a molten mass, this is thought possible. However, as the tendency in the past has been all the other way, namely, to make the effect of the inner heat of the earth less marked on the surface, we have no reason to expect a reversal. A third theory would make the earth die more gradually and more surely. It is known that by the wearing down of the surface by the rains and rivers, there is a tendency to reduce mountains and all high parts of the earth to a uniform level, a tendency which is only counteracted by some elevating force within the earth. If these elevating forces be supposed to be due to the internal heat--a hypothesis which cannot be proved--then with the cooling of the earth the elevating forces would cease, and, finally, the whole of the continent would be brought beneath the sea and terrestrial life perish.