Without an atmosphere to temper the sun's heat as ours does — not, indeed, by impeding the passage of the solar rays, but by bearing aloft the cloud-veil which the sun raises from our oceans — the moon's surface must become intensely hot long before the middle of the lunar day. Undoubtedly the want of an atmosphere causes the moon's heat to be rapidly radiated away into space. It is our atmosphere which causes a steady heat to prevail on our earth. And at the summits of lofty mountains, where the atmosphere is rare, although the mid-day heat is intense, yet so rapidly does the heat pass away that snow crowns forever the mountain-heights. Yet, although the moon's heat must pass away even more rapidly, this does not prevent the heating of the moon's actual surface, any more than the rarity of the air prevents the Alpine traveller from feeling the action of the sun's direct heat even when the air in shadow is icily cold. Accordingly, Sir John Herschel long since pointed out that the moon's surface must be heated at lunar mid-day — or rather, at the time of lunar mid-heat, corresponding to about two o'clock in our afternoon — to a degree probably surpassing the heat of boiling water.
Such, in point of fact, has now been proved to be the case. The Earl of Rosse has shown, by experiments which need not here be described, that the moon not only reflects heat to the earth (which, of course, must be the case), but that she gives out heat by which she has been herself warmed. The distinction may not perhaps appear clear at first sight to every reader, but it may easily be explained and illustrated. If, on a bright summer's day, we take a piece of smooth, but not too well polished, metal, and by means of it reflect the sun's light upon the face, a sensation of heat will be experienced; this is reflected sun-heat: but if we wait while so holding the metal until the plate has become quite hot under the solar rays, we shall recognize a sensation of heat from the mere proximity of the plate to the face, even when the plate is so held as not to reflect sun-heat. We can in succession try — first, reflected heat alone, before the metal has grown hot; next, the heat which the metal gives out of itself when warmed by the sun's rays; and, lastly, the two kinds of heat together, when the metal is caused to reflect sun-heat, and also (being held near the face) to give out a sensible quantity of its own warmth. What Lord Rosse has done has been to show that the full moon sends earthward both kinds of heat; she reflects solar heat just as she reflects solar light, and she also gives out the heat by which her own surface has been warmed.
It may perhaps occur to the reader to inquire how much heat we actually obtain from the full moon. There is a simple way of viewing the matter. If the full moon were exactly as hot as boiling water, we should receive from her just as much heat (leaving the effect of our atmosphere out of account) as we should receive from a small globe as hot as boiling water, and at such a distance as to look just as large