170 ABSORBING AND EMITTING SURFACE. 
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 es- 
pecially in those which have suffered most from the destruction 
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 pene- 
trate 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 atmosphere. Of course, then, as con- 
ductors, they tend to equalize the temperature of the earth and 
the air. 
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 radi- 
ating 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 ; 
to the dew deposited on them and thus this dew be converted into frost when 
globules of watery fluid floating in the atmosphere near them, in the condition 
of fog or vapor, do not become congealed. 
It has long been known that vegetables can be protected against frost by 
diffusing smoke through the atmosphere above them. This method has been 
lately practised in France on a large scale: vineyards of forty or fifty acres have 
been protected by placing one or two rows of pots of burning coal-tar, or of 
naphtha, along the north side of the vineyard, and thus keeping up a cloud of 
smoke for two or three hours before and after sunrise. The expense is said 
to be small, and probably it might be reduced by mixing some less combusti- 
ble substance, as earth, with the fluid, and thus checking its too rapid burning. 
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 were not mois- 
tened under atmospheric conditions which determined a plentiful deposit of 
dew on the leaves of vegetables. 
