258 Dr. J. Croll on some Controverted Points 
consequently, we have only the evaporation of a solid, which, 
of course, is necessarily small. 
It may here be observed that at low elevations, where the 
snowfall is probably greater, and the amount of heat received 
even less, than at the summits, the snow melts and disappears. 
Here, again, the influence of that potent agent, aqueous 
vapour, comes into play. 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 temperature at the two positions would not be great. 
But it may be urged, as a further objection to the foregoing 
conclusion, 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 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 south-west monsoon deposit their snow 
almost wholly on the southern side of the Himalayas, and the 
south-east trades on the east side of the Andes. Were the 
conditions in even- respect the same on both sides of these 
mountain-ranges, with the exception only that the air on one 
side was perfectly dry, allowing radiation from the snow to 
pass without interruption into stellar space, while on the other 
side the air was moist and full of aqueous vapour absorbing 
the heat radiated from the snow, the snow-line would in this 
