THE HEAT OF THE MOON. 
3 
dark or invisible, and luminous or visible — the former consisting 
of rays which have been absorbed by the lunar surface, and then 
radiated from it ; the latter of rays coming as part and parcel of 
the reflected sunlight ; and it is generally assumed that the 
former quality preponderates. Of course we can know nothing 
of the capacity of the lunar surface matter for receiving and 
radiating dark heat, but upon the premise that bad reflectors 
are good radiators, we may base a conjecture that this capacity 
is rather great, for the general surface of the moon can hardly 
be called a good reflecting one. Some of the craters and spots, 
it is true, shine under high illumination with what appears like 
metallic lustre ; but these form an insignificant part of the 
whole area of the visible hemisphere, and between them and the 
general surface there is a vast difference of brightness. When 
we observe the moon telescopically, and screen the eye with a 
smoke-tinted glass, these bright spots glow like heated phos- 
phorus, while the general surface of the moon presents the 
dusky appearance of brown paper. Assume the former to have 
even a metallic polish, and the reflective power of the latter 
comes out very low. If the one has less than this assumed 
lustre — and we can hardly expect the craters to be mirrors — the 
other must be dull indeed ; and if so dull to the eye, it has 
doubtless a strong appetite for heat. Sir John Herschel inferred 
the temperature of the lunar crust, after its 300 hour-long day, 
to be far hotter than the boiling point of water on the earth.* 
The German physicist Althous from calculation deduced that it 
becomes heated to several hundred degrees of our thermometer’s 
scale ; f and Lord Rosse, as one of the results of his observations 
to be presently described, considered that the absorbed solar 
heat raised the moon’s surface material to a temperature of 
about 500° Fahrenheit. :J 
The invisible rays of heat being wholly or in part intercepted 
by transparent media, it is obvious that those of this qualitv 
which come from the moon do not reach the earth’s surface, but 
are absorbed in the higher regions of the atmosphere. In this 
case they must be effective in evaporating high clouds, dis- 
persing such as are light, raising and thinning those that are 
dense. Herschel ascribed to this influence the tendency of the 
full moon to clear the sky, which proverbs assert, and sailors 
and peasants believe in ; and although an astronomical ob- 
server, Mr. Ellis of Greenwich, § has refuted the grounds for 
such a belief, so far as ordinary cloud registers supply means of 
examination, it has been confirmed by Professor Piazzi Smyth’s 
observations upon the high clouds over the Peak of Teneritfe. 
* Herschel, “ Outlines,” sect. 432. f Poggendorf, u Annalen,” xc. 544. 
X u Proc. of Roy. Soc./’ xvii. 436. § “ Phil. Mag.” 4th ser. xxxiv. 61. 
