INFLUENCE OF WOODS ON PRECIPITATION. 
181 
off, or perhaps half thawed, several times during the winter. 
The water from the melting snow runs into the depressions, 
and when, after a day or two of warm sunshine or tepid rain, 
the cold returns, it is consolidated to ice, and the hared ridges 
and swells of earth are deeply frozen.* It requires many days 
of mild weather to raise the temperature of soil in this condi¬ 
tion, and of the air in contact with it, to that of the earth in 
the forests of the same climatic region. Flora is already plait¬ 
ing her sylvan wreath before the corn flowers which are to 
deck the garland of Ceres have waked from their winter’s 
sleep ; and it is not a popular error to believe that, where 
man has substituted his artificial crops for the spontaneous 
harvest of nature, spring delays her coming. 
In many cases, the apparent change in the period of the 
seasons is a purely local phenomenon, which is probably com¬ 
pensated by a higher temperature in other months, without 
any real disturbance of the average thermometrical equilib¬ 
rium. We may easily suppose that there are analogous par¬ 
tial deviations from the general law of precipitation; and, 
without insisting that the removal of the forest has diminished 
the sum total of snow and rain, we may well admit that it has 
lessened the quantity which annually falls within particular 
limits. Yarious theoretical considerations make this probable, 
the most obvious argument, perhaps, being that drawn from 
the generally admitted fact, that the summer and even the 
mean temperature of the forest is below that of the open coun¬ 
try in the same latitude. If the air in a wood is cooler than 
that around it, it must reduce the temperature of the atmos¬ 
pheric stratum immediately above it, and, of course, whenever 
a saturated current sweeps over it, it must produce precipita¬ 
tion which would fall upon or near it. 
But the subject is so exceedingly complex and difficult, 
* I have seen, in Northern New England, the surface of the open 
ground frozen to the depth of twenty-two inches, in the month of Novem¬ 
ber, when in the forest earth no frost was discoverable; and later in the 
winter, I have known an exposed sand knoll to remain frozen six feet 
deep, after the ground in the woods was completely thawed. 
