358 Dr. James Croll—Aqueous Vapour and Perpetual Snow. 
direct sunshine and one shaded diminishes instead of increases as we 
ascend in the atmosphere. This led me to an important conclusion 
in regard to the influence of aqueous vapour on the melting-point ot 
snow; but recent objections to some of my views convince me that I 
have not given to that conclusion the prominence it deserves. I 
shall now state in a few words the conclusion to which I refer. 
The reason why snow at great elevations does not melt, but 
remains permanent, is owing to the fact that the heat received from 
the sun is thrown off into stellar space so rapidly by radiation and 
reflection that the sun fails to raise the temperature of the snow to 
the melting point: the snow evaporates, but it does not melt. The 
summits of the Himalayas, for example, must receive more than ten 
_times the amount of heat necessary to melt all the snow that falls 
on them, notwithstanding which, the snow is not melted. And in 
spite of the strength of the sun and the dryness of the air at those 
altitudes, evaporation is insufficient to remove the snow. At low 
elevations, where the snow-fall is probably greater, and the amount 
of heat received even less than at the summits, the snow melts and 
disappears. ‘This, I believe, we must attribute to the influence of 
aqueous vapour. At high elevations the air is dry, and allows the 
heat radiated from the snow to pass into space; but at low elevations 
a very considerable amount of the heat radiated from the snow is 
absorbed by the aqueous vapour which it encounters in passing 
through the atmosphere. A considerable portion of the heat thus 
absorbed by the vapour is radiated back on the snow; but the heat 
thus radiated being of the same quality as that which the snow itself 
radiates is on this account absorbed by the snow. Little or none 
of it is reflected like that received from the sun. The consequence 
is that the heat thus absorbed accumulates in the snow till melting 
takes place. Were the amount of aqueous vapour possessed by the 
atmosphere sufficiently diminished, perpetual snow would cover our 
globe down to the sea-shore. It is true that the air is warmer at 
the lower than at the higher levels, and by contact with the snow 
must tend to melt it more at the former than at the latter position. 
But we must remember that the air is warmer mainly in consequence 
of the influence of aqueous vapour, and that were the quantity of 
vapour reduced to the amount in question, the difference of tempera- 
ture at the two positions would not be great. 
But it may be urged as a further objection to the foregoing con- 
clusion, that, as a matter of fact, on great mountain chains the snow- 
line reaches to a lower level on the side where the air is moist than 
on the opposite side where it is dry and arid. As, for example, on 
the southern side of the Himalayas, and on the eastern side of the 
Andes, where the snow-line descends some 2000 or 3000 feet below 
that of the opposite or dry side. But this is owing to the fact that 
it is on the moist side that by far the greatest amount of snow is 
precipitated. The moist winds of the S.W. monsoon deposit their 
snow almost wholly on the southern side of the Himalayas, and the 
S.H. trades on the east side of the Andes. Were the conditions in 
every respect the same on both sides of these mountain ranges, with 
