of our earth at the corresponding stage of her existence. On this account, then, we must consider that the moon started in planetary existence in a condition as to heat which our earth did not attain till many millions, probably hundreds of millions of years after the epoch of her first formation as a planet. As regards the moon's substance, we have no means of forming a satisfactory opinion. But we shall be safe in regarding quantity of matter in the moon as a safer basis of calculation than volume, in comparing the duration of her various stages of development with those of our own earth. When, in the August number of this Magazine, I adopted a relation derived from the latter and less correct method, it was because the more correct method gave the result most favourable to the argument I was then considering. The same is indeed the case now. Yet it will be better to adopt the more exact method, because the consideration relates no longer to a mere side issue, but belongs to the very essence of my reasoning. The moon has a mass equal to about one eighty-first part of the earth's. Her diameter being less than the earth's, about as two to seven, the duration of each stage of her cooling would be in this degree less than the corresponding duration for the earth, if her density were the same as the earth's, in which case her mass would be only one forty-ninth part of the earth's. But her mass being so much less, we must assume that her amount of heat at any given stage of cooling was less in similar degree than it would have been had her density been the same as the earth's. We may, in fact, assume that the moon's total supply of heat would be only one eighty-first of the earth's if the two bodies were at the same temperature throughout.[63] But the surface of the moon is between one-thirteenth and one-fourteenth of the earth's. Since, then, the earth at any given stage of cooling parted with her heat between thirteen and fourteen times as fast as the moon, but had about eighty-one times as much heat to part with (for that stage), it follows that she would take about six times as long (six times thirteen and a-half is equal to eighty-one) to cool through that particular stage as the moon would. If we take this relation as the basis of our estimate of the moon's age, we shall find that, even if the moon's existence as a planet began simultaneously with the earth's instead of many millions of years earlier, even if the moon was then as hot as the earth instead of being so much cooler that many millions of years would be required for the earth to cool to the same temperature--making, I say, these assumptions, which probably correspond to the omission of hundreds of millions of years in our estimate of the moon's age, we shall still find the moon to be hundreds of millions of years older than the earth.