Page:Popular Science Monthly Volume 3.djvu/464

 as the moon does. Or a disk of metal would serve equally well. Now, the experiment may be easily tried. A bronze halfpenny is exactly one inch in diameter, and as the moon's average distance is about 111 times her own diameter, a halfpenny at a distance of 111 inches, or three yards and three inches, looks just as large as the moon. Now let a halfpenny be put in boiling water for a while, so that it becomes as hot as the water; then that coin taken quickly and set three yards from the observer will give out, for the few moments that its heat remains appreciably that of boiling water, as much heat to the observer as he receives from the full moon supposed to be as hot as boiling water. Or a globe of thin metal, one inch in diameter and full of water at boiling-heat, would serve as a more constant artificial moon in respect of heat-supply. It need not be thought remarkable, then, if the heat given out by the full moon is not easily measured, or even recognized. Imagine how little the cold of a winter's day would be relieved by the presence, in a room no otherwise warmed, of a one-inch globe of boiling water, three yards away! And, by-the-way, we are here reminded of an estimate by Prof. C. B. Smyth, resulting from observations made on the moon's heat during his Teneriffe experiments. He found the heat equal to that emitted by the hand at a distance of three feet.

But, after all, the most interesting results flowing from the recent researches are those which relate to the moon herself. We cannot but speculate on the condition of a world so strangely circumstanced that a cold more bitter than that of our arctic nights alternates with a heat exceeding that of boiling water. It is strange to think that the calm-looking moon is exposed to such extraordinary vicissitudes. There can scarcely be life in any part of the moon—unless it be underground life, bike that of the Modoc Indians (we commend this idea specially to the more ardent advocates of Brewsterian ideas respecting other worlds than ours). And yet there must be a singularly active mechanical process at work in yonder orb. The moon's substance must expand and contract marvellously as the alternate waves of heat and cold pass over it. The material of that crater-covered surface must be positively crumbling away under the effects of these expansions and contractions. The most plastic terrestrial substances could not long endure such processes, and it seems altogether unlikely that any part of the moon's crust is at all plastic. Can we wonder if, from time to time, astronomers tell us of apparent changes in the moon—a wall sinking here, or a crater vanishing elsewhere? The wonder rather is, that the steep and lofty lunar mountains have not been shaken long since to their very foundations.

Our moon presents, in fact, a strange problem for our investigation. It is gratifying to us terrestrials to regard her as a mere satellite of the earth, but in reality she deserves rather to be regarded as companion planet. She follows a path round the sun which so nearly