shall not probably err very greatly in taking the interval as at least two hundred millions of years. But I could point out that in reality it is a matter of small importance, so far as my present argument is concerned, whether we adopt Bischoff's period or a period differing greatly from it. For if instead of about three hundred millions the earth required only thirty millions of years to cool from a surface temperature of two thousand degrees Centigrade to a temperature of two hundred degrees, we must assume that the rate of cooling is ten times greater than Bischoff supposed. And we must of course extend the same assumption to the moon. Now, since the sole question before us is to what degree the moon has cooled, it matters nothing whether we suppose the moon has been cooling very slowly during many millions of years since she was in the same condition as the earth at present, or that the moon has been cooling ten times as quickly during a tenth part of the time, or a hundred times as quickly during one-hundredth part of the time. We may, therefore, continue to use the numbers resulting from Bischoff's calculation, even though we admit the probability that they differ widely from the true values of the periods we are considering. Setting the moon, then, as about two hundred and fifty millions of years in advance of the earth in development, even when we overlook all the eras preceding that considered by Bischoff, and the entire sequent interval (which must be long, for the earth has no longer a surface one hundred degrees Centigrade hotter than boiling water), let us consider what is suggested by this enormous time-difference. In the first place, it corresponds to a much greater interval in our earth's history. During the two hundred and fifty millions of years the moon has been cooling at her rate, not at the earth's. According to the conclusion we deduced from the moon's relative mass and surface, she has aged as much during those two hundred and fifty million years as the earth will during the next fifteen hundred million years.