104 J. Croll—Aqueous Vapor in Relation to Perpetual Snow. 
ation 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 these altitudes, evaporation is 
insufficient to remove the snow. t 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 
vapor. 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 vapor which it encounters in pass- 
ing through the atmosphere. <A considerable portion of the 
heat thus absorbed by the vapor 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 
or 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 level 
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 
the southern side of the Himalayas, and the S. E. trades, 
the snow on the east side of the Andes. Were the conditions 
