1340. Heat from a covering of snow. 'The covering of snow,' the same author observes, 'which countries in high latitudes enjoy during the winter, has been very commonly thought to be beneficial to vegetable substances on the surface of the earth, as far as their temperature is concerned, solely by protecting them from the cold of the atmosphere. But, were this supposition just, the advantage of the covering would be greatly circumscribed; since the upper parts of trees and of tall shrubs are still exposed to the influence of the air. Another reason, however, is furnished for its usefulness, by what has been said above; which is, that it prevents the occurrence of the cold, which bodies on the earth acquire, in addition to that of the atmosphere, by the radiation of their heat to the heavens during still and clear nights. The cause, indeed, of this additional cold, does not constantly operate; but its presence, during only a few hours, might effectually destroy plants which now pass unhurt through the winter. Again, as things are, while low vegetable productions are prevented, by their covering of snow, from becoming colder than the atmosphere, in consequence of their own radiation, the parts of trees and tall shrubs which rise above the snow are little affected by cold from this cause; for their uttermost twigs, now that they are destitute of leaves, are much smaller than the thermometers suspended by me in the air, which, in this situation, very seldom became more than 2ﾦ colder than the atmosphere. The larger branches, too, which, if fully exposed to the sky, would become colder than the extreme parts, are in a great degree sheltered by them; and, in the last place, the trunks are sheltered both by the smaller and larger parts; not to mention that the trunks must derive heat by conduction through the roots from the earth kept warm by the snow. In a similar way is partly to be explained the manner in which a layer of earth or straw preserves vegetable matters from the injurious effects of cold in winter.' (Essay on Dew.)
