1243. Shelter, whether by walls, hedges, strips of plantation, or trees scattered over the surface, may be considered, generally, as increasing or preserving heat, and lessening evaporation from the soil. But if the current of air should be of a higher temperature than the earth, screens against wind will prevent the earth from being so soon heated; and from the increased evaporation arising from so great a multiplication of vegetable surfaces by the trees, more cold will be produced after rains, and the atmosphere kept in a more moist state, than in grounds perfectly naked. When the temperature of a current of air is lower than that of the earth, screens will prevent its carrying off so much heat; but more especially scattered trees, the tops of which will be chiefly cooled whilst the under surfaces of their lower branches reflect back the rays of heat as they radiate from the surface of the soil. Heat, in its transmission from one body to another, follows the same laws as light; and, therefore, the temperature of the surface in a forest will, in winter, be considerably higher than that of a similarly constituted soil exposed to the full influence of the weather. The early flowering of plants, in woods and hedges, is a proof of this: but as such soils cannot be so easily heated in summer, and are cooled like others after the sinking in of rains, or the melting of snows, the effect of the reflection as to the whole year is nearly neutralised, and the average temperature of the year of such soils and situations will probably be found not greater than that of open lands.
