HEAT FROM THE MOON.
ALONG-VEXED question—one which astronomers and phy- sicists have laboured and puzzled and even quarrelled over for two centuries at least—has at length been set at rest. Whether the Moon really sends us any appreciable amount of warmth has long been a moot point. The most delicate experiments had been tried to determine the matter. De Saussure thought he had succeeded in obtaining heat from the moon, but it was shown that he had been gathering heat from his own instruments. Melloui tried the experiment, and fell into a similar error. Piazzi Smyth, in his famous Teneriffe expedition, tried the effect of seeking for lunar heat above those lower and more moisture-laden atmospheric strata which are known to cut off the obscure heat-rays so effec- tually. Yet he also failed. Professor Tyndall, in his now classical " Lectures on Heat," says that all such experiments must inevitably fail, since the heat rays from the moon must be of such a character that the glass converging-lens used by the experi- menters would cut off the whole of the lunar heat. He himself tried the experiment with metallic mirrors, but the thick London air prevented his succeeding.
The hint was not lost, however. It was decided that mirrors, and not lenses, were the proper weapons for carrying on the attack. Now, there is one mirror in existence which excels all others in light-gathering, and therefore necessarily in heat-gathering, power. The gigantic mirror of the Bosse telescope has long been engaged in gathering the faint rays from those distant stellar cloucUeta which are strewn over the celestial vault. The strange clusters with long out-reaching arms, the spiral nebulas with mystic convolutions around their blazing nuclei, the wild and fantastic figures of the irregular nebulas, all these forms of matter had been forced to reveal their secret under the searching eye of the great Parsonstown reflector. But vast as are the powers of this giant telescope, and interesting as the revelations it had already made, there was one defect which paralyzed half its powers. It was an inert mass well poised ;—indeed, so that the merest infant could sway it, but possessing no power of self-motion. The telescopes in our great observatories follow persistently the motions of the stars upon the celestial vault, but their giant brother pos- sessed no such power. And when we remember the enormous volume of the Bosse Telescope, its tube—fifty feet in length—down which a tall man can walk upright, and its vast metallic speculum weighing several tons, the task of applying clock-motion to so cum- brous and seemingly unwieldy a mass might well seem hopeless? Yet without this it was debarred from taking its part in a multitude of processes of research to which its powers were wonderfully adapted. Spectroscopic analysis, as applied to the stars, for example, requires the most perfect uniformity of clock-motion, so that the light from a star, once received on the jaws of the slit which forms the entrance into the spectroscope, may not move off them even by a hair's breadth. And the determination of the moon's heat required an equally exact adaptation of the telescope's motion to the apparent movement of the celestial sphere. For so delicate is the inquiry, that the mere heat generated in turning the telescope upon the moon by the ordinary arrangement would have served to mask the result.
At enormous cost, and after many difficulties had been encountered, the Bosse reflector has at length had its powers more than doubled, by the addition of the long-wanted power of self- motion. And among the first-fruits of the labour thus bestowed upon it, is the solution of the famous problem of determining the moon's heat.
The delicate heat-measurer, known as the thermopile, was used in this work, as in Mr. Huggius's experiments for estimating the heat we receive from the stars. The moon's heat, concentrated by the great mirror, was suffered to fall upon the face of the thermopile, and the indications of the needle were carefully watched. A small but obvious deflection in the direction signify- ing heat was at once observed, and when the observation had been repeated several times with the same result no doubt could remain. We actually receive an appreciable proportion of our warmth-supply from "the chaste beams of the wat'ry moon." The view which Sir John Herschel had long since formed on the behaviour of the fleecy clouds of a summer night under the moon's influence was shown to be as correct as almost all the guesses have been which the two Herschels have ever made.
And one of the most interesting of the results which have fol- lowed from the inquiry confirms in an equally striking manner another guess which Sir John Herschel had made. By comparing the heat received from the moon with that obtained from several terrestrial sources, Lord Bosse has been led to the conclusion that at the time of full moon the surface of our satellite ih raised to a temperature exceeding by more than 280° (Fahrenheit) that of boiling water. Sir John Herschel long since asserted that this must be so. During the long lunar day, lasting some 300 of our hours, the sun's rays are poured without intermission upon the lunar surface. No clouds temper the heat, no atmosphere even serves to interpose any resistance to the continual down-pour of the fierce solar rays. And for about the space of three of our days
the sun hangs suspended close to the zenith of the lunar sky, so that if there were inhabitants on our unfortunate satellite, they would be scorched for more than seventy consecutive hours by an almost vertical sun.
There is only one point in Lord Rosse's inquiry which seems doubtful. That we receive heat from the moon he has shown con- clusively, and there can be no doubt that a large portion of this heat is radiated from the moon. But there is another mode by which the heat may be sent to us from the moon, and it might be worth while to inquire a little more closely than has yet been done whether the larger share of the heat rendered sensible by the great mirror may not have come in this way. We refer to the moon's power of reflect/7w heat. It need hardly be said that the reflection and the radiation of heat are very different matters. Let any one hold a burnished metal plate in such a way that the sun's light is reflected towards his face, and he will feel that with the light a considerable amount of heat is reflected. Let him leave the same metal in the sun until it is well warmed, and he will find that the metal is capable of imparting heat to him when it is removed from the sun's rays. This is radiation, and cannot happen unless the metal has been warmed, whereas heat can be reflected from an ice- cold plate. There has been nothing in the experiments conducted by Lord Rosse to show by which of these two processes the moon's heat is principally sent to us ; nor do we know enough of the con- stitution of the moon's surface to estimate for ourselves the relative proportions of the heat she reflects and radiates towards us.
We do not mention this point from any desire to cavil at the results of one of the most interesting experiments which has recently been carried out. But the recent researches of Milner upon the light from the planets, has shown how largely the surfaces of the celestial bodies differ as respects their capacity for reflecting and absorbing light, and there is every reason to infer that similar peculiarities characterize the planets' power of absorb- ing and reflecting heat. The whole question of the heat to which the moon's surface is actually raised by the sun's heat depends upon the nature of that surface, and the proportion between its power of absorbing heat or reflecting it away into space.
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