Page:The evolution of worlds - Lowell.djvu/229

Rh sojourn where it is just too cold for comfort. The factor in question is the blanketing character of our air. Now a blanket may keep heat out as well as keep it in. Our air acts in both capacities. It is by no means simply a storer of heat, as many people seem to suppose; it is a heat-stopper as well. What it really is is a temporizer, a buffer to ease the shocks of sudden change like those comfortable, phlegmatic souls who reduce all emotion to a level. For the heating power of the Sun, even at the Earth's distance away, is much greater than appears. Knowledge of this we owe most to Langley, and then to Very, who continued his results to yet a finer determination, the best we have to-day. In consequence we have learnt that the amount of heat we should receive from the Sun, could we get above our air,—the solar constant, as it is called,—would be over three times what it is on the average in our latitude at the surface, and is rising still, so to speak. For as man has gone higher he has found his inferences rising too, and the limit would seem to be not yet. We see then that the air to which we thought ourselves so much indebted, actually begins its kindly offices by shutting off two-thirds of what was coming to us. As it plays, however, something of the same trick to what tries to escape, we are really somewhat beholden to it after all.

But not so much as has been thought. We used to be told that the Moon's temperature even at midday hardly