THE HAMILTON ASSOCIATION. 103 
at different times, has been giving special study to the Moon. 
He believes he has discovered the presence of snow or hoar 
frost on the Moon. Let us examine his arguments. If we 
agree that volcanic action on the Moon has not yet ceased, it 1s 
‘evident that gaseous pressure of some kind must exist to pro- 
duce the effect. On earth, water-vapor or steam is responsible 
for the explosive energy, and possibly is so on the Moon. This 
steam rising up into the Moon’s atmosphere becomes cooled 
down and denosited as hoar frost or snow. Surrounding some 
craters, notably Abulfedae, Censorinus and Linne, are white 
patches or halos, which are largest-at sunrise and gradually di- 
minish towards sunset, only to reappear of their original size 
at the next sunrise. Then again some of the lunar neaks ex- 
hibit a dazzling brightness as though crowned with eternal 
snows, while equally illuminated surfaces are darker. (See 
Appennines.) The lunar poles exhibit an unusual whiteness. 
The same unusual brightness distinguishes a large area sur- 
rounding Tycho, and many lunar craterlets are lined with some 
substance which is dazzlingly bright under suitable illumina- 
tion. Polar brightness may, of course, be due to snow caps, 
the linings of craters caused by congealed vapors from their 
vents, and the mountain caps stolen from the scanty moisture 
of the lunar atmosphere. But to me the evidence is not con- 
clusive. It does not seem to be asserted or proven that the 
polar whiteness waxes and wanes in size, as one would expect, 
if it represented caps of snow or ice. The well known bright 
rays are also attributed to snow by Prof. Pickering, though no 
proof is forthcoming. All these appearances might just ds 
well be explained by other hypothesis, except the waxing and 
waning of the bright spots already referred to about Linne, 
Censorinus and Abulfedae. Perhaps there, if we could trans- 
port ourselves on etherial waves, we might behold dazzling 
fields of snow, unless closer scrutiny showed the illusion to 
have been caused by peculiarities of illumination of the rough 
uneven surface. Plato has also been carefully scrutinized by 
Pickering, who finds that the relative nrominence of the numer- 
ous little craters on its central plain is very variable, which he 
