252 Dr. J. Croll on some Controverted Points 
in summer would be as rare a phenomenon in those regions 
as it would be in the south of England. 
Second. — ' The rays which fall on snow and ice are to a 
great extent reflected back into space. But those that are 
not reflected, but absorbed, do not raise the temperature, for 
they disappear in the mechanical work of melting the ice/ 
This reason is also regarded as absurd. The heat of the 
sun during the perihelion summer would, he says, suffice to 
melt the whole accumulation of winter snow in three or four 
days. " The reader/' he continues, " can easily make a com- 
putation of the incredible reflecting power of the snow and of 
the unexampled 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 
melting-power of a tropical sun. But he misapprehends my 
reasoning 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 into space. The aqueous vapour of the air, on the other 
hand, as the vibrations 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 consideration 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 vapour. 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 favourable for the production of snow. 
Third. — ' Snow and ice lower the temperature by chilling 
the air and condensing the vapour into thick fogs. The great 
strength of the sun's rays during summer, due to his nearness 
at that season, would, in the first place, tend to produce an 
increased amount of evaporation. But the presence of snow- 
clad mountains and an icy sea would chill the atmosphere and 
condense the vapour into thick fogs. The thick fogs and cloudy 
