J. Croll—Geological Climatology. 265 
only ten inches notwithstanding the fact that it has the power 
to melt sixteen feet. 
In short, there is not a place on the face of the globe where 
the amount of heat received from the sun is not far more than 
sufficient to melt all the snow which falls upon it. If it were 
true, as the objection assumes, that the amount of snow melted 
is proportional to the amount of heat received by the snow 
then there could be no such thing as perpetual snow. 
The reason why the amount of snow and ice melted is not 
necessarily proportional to the amount of heat received is not 
far to seek, Before snow or ice will melt its temperature must 
be raised to the melting point. No amount of heat, however - 
great, will induce melting to begin unless the intensity of the 
heat be sufficient to raise the temperature to the melting 
point. Keep the temperature of the snow below that. point 
and though the sun may shine upon it for countless ages it 
Will still remain unmelted. It is easy to understand how the 
Snow on the lofty summits of the Himalayas and the Andes 
hever melts. According to the observations made at Mount 
Whitney, to which reference has already been made, the heat 
of even a vertical sun would not be sufficient at these altitudes 
to raise the temperature of the snow to near the melting point; 
and thus melting, under these conditions, is impossible. The 
‘Snow will evaporate but it cannot melt. But owing to the 
frozen condition of the snow even evaporation will take place 
with extreme difficulty. If the sun could manage to soften 
the snow crystals and bring them into a semi-fluid condition, 
evaporation would, no doubt, go on rapidly; but this the rays 
of the sun are unable to do. Consequently, we have only the 
evaporation of a solid which, of course, is necessarily small. 
t may here be observed that 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. 
Here, again, the influence of that potent agent, aqueous vapor, 
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 
tom the snow is absorbed by the aqueous vapor which it 
encounters in passing through the atmosphere, A  consider- 
able 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 
piace, Were the amount of aqueous vapor pr by the 
Am. Jour, a Series, Vou. XXVI, No. 154.—Ocr., 1883. 
