Assuming the planets and satellites of the solar system to be formed in some such manner as Laplace suggested in his "Nebular Hypothesis," the moon, as an orb travelling round the earth, must be regarded as very much older than she is, even in years. Even if we accept the theory of accretion which has been recently suggested as better according with known facts, it would still follow that probably the moon had existence, as a globe of matter nearly of her present size, long before the earth had gathered in the major portion of her substance. Necessarily, therefore, if we assume as far more probable than either theory that the earth and moon attained their present condition by combined processes of condensation and accretion, we should infer that the moon is far the older of the two bodies in years. But if we even suppose that the earth and moon began their career as companion planets at about the same epoch, we should still have reason to believe that these planets, equal though they were in age so far as mere years are concerned, must be very unequally advanced so far as development is concerned, and must therefore in that respect be of very unequal age. It was, I believe, Sir Isaac Newton who first called attention to the circumstance that the larger a planet is, the longer will be the various stages of its existence. He used the same reasoning which was afterwards urged by Buffon, and suggested an experiment which Buffon was the first to carry out. If two globes of iron, of unequal size, be heated to the same degree, and then left to cool side by side, it will be found that the larger glows with a ruddy light after the smaller has become quite dark, and that the larger remains intensely hot long after the smaller has become cool enough to be handled. The reason of the difference is very readily recognised. Indeed, Newton perceived that there would be such a difference before the matter had been experimentally tested. The quantity of heat in the unequal globes is proportional to the volume, the substance of each being the same. The heat is emitted from the surface, and at a rate depending on the extent of surface. But the volume of the larger exceeds that of the smaller in greater degree than the surface of the larger exceeds the surface of the other. Suppose, for instance, the larger has a diameter twice as great as that of the smaller, its surface is four times as great as that of the smaller, its volume eight times as great. Having, then, eight times as much heat as the smaller at the beginning, and parting with that heat only four times as fast as the smaller, the supply necessarily lasts twice as long; or, more exactly, each stage in the cooling of the larger lasts twice as long as the corresponding stage in the cooling of the smaller. We see that the duration of the heat is greater for the larger in the same degree that the diameter is greater. And we should have obtained the same result whatever diameters we had considered.