LUNAR TEMPERATURES. 449 



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 pre- 

 vents 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, correspond- 

 ing to about two o'clock in our afternoon to a degree probably sur- 

 passing 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 de- 

 scribed, 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 

 vol. in. 29 



