IMPORTANCE OF SNOW. 209 
mate as a natural apparatus for accumulating the water that 
falls upon ‘the surface and transmitting it to the subjacent 
strata, we must compare the condition and properties of its soil 
with those of cleared and cultivated earth, and examine the 
consequently different action of these soils at different seasons 
of the year. The disparity between them is greatest in climates 
where, as in the Northern American States and in the extreme 
North of Europe, the open ground freezes and remains imper- 
vious to water during a considerable part of the winter ; though, 
even in climates where the earth does not freeze at all, the 
woods have still an important influence of the same character. 
The difference is yet greater in countries which have regular 
wet and dry seasons, rain being very frequent in the former 
period, while, in the latter, it scarcely occurs at all. These 
countries lie chiefly in or near the tropics, but they are not 
wanting in higher latitudes; for a large part of Asiatic and 
even of European Turkey is almost wholly deprived of summer 
rains. In the principal regions occupied by European cultiva- 
tion, and where alone the questions discussed in this volume 
are recognized as having, at present, any practical importance, 
more or less rain falls at all seasons, and it is to these regions 
that, on this point as well as others, I chiefly confine my atten- 
tion. 
Importance of Snow. 
Recent observations in Switzerland give a new importance 
to the hygrometrical functions of snow, and of course to the 
forest as its accumulator and protector. I refer to statements 
of the condensation of atmospheric vapor by the snows and 
glaciers of the Rhone basin, where it is estimated to be nearly 
equal to the entire precipitation of the valley. Whenever the 
humidity of the atmosphere in contact with snow is above the 
point of saturation at the temperature to which the air is cooled 
by such contact, the superfluous moisture is absorbed by the 
snow or condensed and frozen upon its surface, and of course 
14 
