[Footnote 1: Probably most of our readers know that the thermopile consists of a number of little bars of two different metals, connected in pairs, and having the ends joined in a conducting circuit with a galvanometer. If, now, one set of the junctures is heated more than the other set, a current of electricity will be generated, which will affect the galvanometer. The bars are usually made of bismuth and antimony though iron and German silver answer pretty well. They are commonly about half or three-quarters of an inch long, and about half as large as an ordinary match. The "pile" is made of from fifty to a hundred such bars packed closely, but insulated by thin strips of mica, except just at the soldered junctions. With an instrument of this kind and a very delicate galvanometer, Professor Henry found that the heat from a person's face could be perceived at a distance of several hundred feet. There is however, some doubt whether he was not mistaken in respect to this extreme sensitiveness.] Others repeated the experiment several times between this time and 1856, with more or less success; but, so far as I know, the first quantitative result was that obtained in 1856 by Piazzi Smyth during his Teneriffe expedition. On the top of the mountain, at an elevation of ten thousand feet, he found that the moon's rays affected his thermopile to the same extent as a standard candle ten feet away. Marie Davy has since shown that this corresponds to a heating effect of about 1/1300 of a Centigrade degree. The subject was resumed in 1868 by Lord Rosse in Ireland; and a long series of observations, running through several years, was made by the aid of his three-foot reflector (not the great _six_-foot instrument, which is too unwieldy for such work). The results of his work have, until very recently, been accepted as authoritative. It should be mentioned that, at about the same time, observations were also made at Paris by Marie Davy and Martin; but they are generally looked upon merely as corroborative of Rosse's work, which was more elaborate and extensive. Rosse considered that his results show that the heat from the moon is mainly _obscure, radiated_ heat; the _reflected_ heat, according to him, being much less in amount. A moment's thought will show that the moon's heat must consist of two portions. First, there will be _reflected solar heat_. The amount and character of this will depend in no way upon the temperature of the moon's surface, but solely upon its reflecting power. And it is to be noted that moon-_light_ is only a part of this reflected radiant energy, differing from the invisible portion of the same merely in having such a wave-length and vibration period as to bring it within the range of perception of the human eye.