210 IMPORTANCE OF SNOW. 
adds so much to the winter supply of water received from the 
snow by the ground. This quantity, in all probability, much 
exceeds the loss by evaporation, for during the period when 
the ground is covered with snow, the proportion of clear dry 
weather favorable to evaporation is less than that of humid 
days with an atmosphere in a condition to yield up its moisture 
to any bibulous substance cold enough to condense it.* 
In our Northern States, irregular as is the climate, the first 
autumnal snows pretty constantly fall before the ground is 
frozen at all, or when the frost extends at most to the depth 
of only a few inches.t| In the woods, especially those situated 
upon the elevated ridges which supply the natural irrigation 
of the soil and feed the perennial fountains and streams, the 
ground remains covered with snow during the winter; for the 
trees protect the snow from blowing from the general surface 
into the depressions, and new accessions are received before the 
covering deposited by the first fall is melted. Snow is ofa 
color unfavorable for radiation, but, even when it is of con- 
siderable thickness, it is not wholly impervious to the rays of 
the sun, and for this reason, as well as from the warmth of 
lower strata, the frozen crust of the soil, if one has been formed, 
is soon thawed, and does not again fall below the freezing- 
point during the winter. | 
* The hard snow-crust, which in the early spring is a source of such keen 
enjoyment to the children and youth of the North—and to many older persons 
in whom the love of nature has kept awake a relish for the simple pleasures 
of rural life—is doubtless due to the congelation of the vapor condensed by 
the snow rather than to the thawing and freezing of the superficial stratum ; 
for when the surface is melted by the sun, the water is taken up by the ab- 
sorbent mass beneath before the temperature falls low enough to freeze it. 
+ The hard autumnal frosts are usually preceded by heavy rains which 
thoroughly moisten the soil, and it is a common saying in the North that ‘‘ the 
ground will not freeze till the swamps are full.” 
{ Dr. Williams, of Vermont, made some observations on the comparative 
temperature of the soil in open and in wooded ground in the years 1789 and 
1791, but they generally belonged to the warmer months, and I do not know 
that any extensive series of comparisons between the temperature of the 
ground in the woods and in the fields has been attempted in America. Dr, 
