THE FOREST IN SUMMER AND WINTER. 
147 
sistence, when the temperature of the air a few yards above 
has not been brought down to the dew point, still less to 32°, 
the degree of cold required to congeal dew to frost.* 
b. Trees as Conductors of Heat. 
We are also to take into account the action of the forest as 
a conductor of heat between the atmosphere and the earth. 
In the most important countries of America and Europe, and 
especially in those which have suffered most from the destruc¬ 
tion of the woods, the superficial strata of the earth are colder 
in winter, and warmer in summer than those a few inches 
lower, and their shifting temperature approximates to the 
atmospheric mean of the respective seasons. The roots of 
large trees penetrate beneath the superficial strata, and reach 
earth of a nearly constant temperature, corresponding to the 
mean for the entire year. As conductors, they convey the 
heat of the atmosphere to the earth when the earth is colder 
than the air, and transmit it in the contrary direction when 
the temperature of the earth is higher than that of the atmo¬ 
sphere. Of course, then, as conductors, they tend to equalize 
the temperature of the earth and the air. 
c. Trees in Summer and Winter. 
In countries where the questions I am considering have 
the greatest practical importance, a very large proportion, if 
not a majority, of the trees are of deciduous foliage, and their 
radiating as well as their shading surface is very much greater 
in summer than in winter. In the latter season, they little 
obstruct the reception of heat by the ground or the radiation 
from it; whereas, in the former, they often interpose a complete 
* The radiating and refrigerating power of objects by no means depends 
on their form alone. Melloni cut sheets of metal into the shape of leaves 
and grasses, and found that they produced little cooling effect, and w ere 
not moistened under atmospheric conditions which determined a plentiful 
deposit of dew on the leaves of vegetables. 
