Nay, we may even take a position still less favourable to my argument. Let us overlook the long ages during which the two orbs were in the vaporous state, and suppose the earth and moon to be simultaneously in that stage of planetary existence when the surface has a temperature of two thousand degrees Centigrade. From Bischoff's experiments on the cooling of rocks, it appears to follow that some three hundred and twenty millions of years must have elapsed between the time when the earth's surface was at this temperature and the time when the surface temperature was reduced to two hundred degrees Centigrade, or one hundred and eighty degrees Fahrenheit above the boiling point. The earth was for that enormous period a mass (in the main) of molten rock. In the moon's case this period lasted only one-sixth of three hundred and twenty million years, or about fifty-three million years, leaving two hundred and sixty-seven million years' interval between the time when the moon's surface had cooled down to two hundred degrees Centigrade and the later epoch when the earth's surface had attained that temperature. I would not, however, insist on these numerical details. It has always seemed to me unsafe to base calculations respecting suns and planets on experiments conducted in the laboratory. The circumstances under which the heavenly bodies exist, regarding these bodies as wholes, are utterly unlike any which can be produced in the laboratory, no matter on what scale the experimenter may carry on his researches. I have often been amused to see even mathematicians of repute employing a formula based on terrestrial experiments, physical, optical, and otherwise, as though the formula were an eternal omnipresent reality, without noting that, if similarly applied to obtain other determinations, the most stupendously absurd results would be deduced. It is as though, having found that a child grows three inches in the fifth year of his age, one should infer not only that that person but every other person in every age and in every planet, nay, in the whole universe, would be thirty inches taller at the age of fifteen than at the age of five, without noticing that the same method of computation would show everyone to be more than fifteen feet taller at the age of sixty-five. It may well be that, instead of three hundred and twenty millions of years, the era considered by Bischoff lasted less than a hundred millions of years. Or quite as probably it may have lasted five or six hundred millions of years. And again, instead of the corresponding era of the moon's past history having lasted one sixth of the time required to produce the same change in the earth's condition, it may have lasted a quarter, or a third, or even half that time, though quite as probably it may have lasted much less than a sixth. But in any case we cannot reasonably doubt that the moon reached the stage of cooling through which the earth is now passing many millions of years ago. We