318 PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 



of the handkerchief on its windward side. The thermometer 

 thus situated was, for several nights, compared with another 

 lying on the same grass plat, but on a part of it fully exposed 

 to the sky. On two of these nights, the air being clear and 

 calm, the grass close to the handkerchief was found to be four 

 degrees warmer than the fully exposed grass ; on a third night 

 the difference was six degrees. An analogous fact is men- 

 tioned by Gersten, who says that a horizontal surface is more 

 abundantly dewed than one which is perpendicular to the 

 ground. 



Snow forms an excellent covering, and seems to be a provis- 

 ion of nature for the protection of many tender roots and plants 

 which would otherwise perish. Its usefulness as a plant-pro- 

 tector may be disputed, from the fact of their tops being exposed 

 to the influence of the atmosphere, while their roots and lower 

 parts only are protected. In reply to this, however, we may 

 observe, 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, in 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-growing vegetable 

 productions are prevented, by the covering'of snow, from becom- 

 ing 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 outer- 

 most twigs, now that they are destitute of leaves, are much 

 smaller than the thermometer suspended by us in the air, which, 

 in this situation, seldom became more than two degrees colder 

 than the atmosphere. The large 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 larger and smaller parts, 

 not to speak of the heat they derive by conduction through the 

 roots, from the earth kept warm by the snow. In a similar man- 

 ner is partly to be explained the way in which a layer of straw 



