260 J. Croll—Geological Climatology. 
ampled transparency of the air required to keep the snow 
unmelted for three or four months.” Incredible as it may 
appear to Professor Newcomb, I shall shortly show that a less 
amount of snow than the equivalent of the two feet of ice 
which he assumes does actually, in some places, defy the melt- 
ing power of a tropical sun. But he misapprehends my reas- 
oning here also, by overlooking the more important factor in 
the affair, namely, the keeping of the air in the summer below 
the freezing point. The direct effect that this has in preventing 
the sun from melting the snow and ice will be discussed shortly, 
but the point to which I wish at present to direct special 
attention is the fact that if the air is kept below, or even at 
the freezing point, snow will fall and not rain. Snow is a good 
reflector of heat, consequently a large portion of the sun’s rays 
falling on the snow and icy surface is reflected back to space. 
The aqueous vapor of the air, on the other hand, as the vibra- 
tions of its molecules agree in period with those of the snow 
and ice, cuts off a large portion of the heat radiated by the 
snow surface ; but here in the case of reflection under consid- 
eration the rays are not cut off; for the reflected rays are of the 
same character as the incident rays which pass so freely 
through the aqueous vapor. And in respect to the remaining 
rays which are not reflected but absorbed by the snow, they do 
not manage to raise the temperature of the snow above the 
freezing point. Consequently the air is kept in the condition 
most favorable for the production of snow. 
evaporation. But ce of snow-clad mountains and an 
icy sea would chill the atmosphere and condense the vapor into 
thick fog e thick fogs and cloudy sky would effectually pre- 
On this Professor Newcomb’s criticism is as follows: ‘‘ Here 
he, Mr. Croll, says nothing about the latent heat set free by the 
condensation, nor does he say where the heat goes to which the 
air must lose in order to be chilled. The task of arguing with 
a disputant who in one breath maintains that the transparency 
of the air is such that the rays reflected from the snow pass 
freely into space, and in the next breath that thick fogs effect- 
ually prevent the rays ever reaching the snow at all, is not free 
from embarrassment.” : 2 
If he really supposes my meaning to be that the air 15 80 
transparent as to allow the incident and reflected rays of the 
