450 THE POPULAR SCIENCE MONTHLY. 



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 re- 

 mains 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 un- 

 derground life, bike that of the Modoc Indians (we commend this idea 

 specially to the more ardent advocates of Brewsterian ideas respect- 

 ing other worlds than ours). And yet there must be a singularly 

 active mechanical process at work in yonder orb. The moon's sub- 

 stance 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 investiga- 

 tion. It is gratifying to us terrestrials to regard her as a mere satel- 

 lite 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 



