1331. Heat is radiated by the sun to the earth, and if suffered to accumulate, Dr. Wells observes, would quickly destroy the constitution of our globe. This evil is prevented by evaporation and the radiation of heat from the earth to the heavens, during the night, when it receives little or no heat in return. But through the wise economy of means, which is witnessed in all the operations of nature, the prevention of this evil is made the source of great positive good; for the surface of the earth, having thus become colder than the neighbouring air, condenses a part of the watery vapour of the atmosphere into dew, the utility of which is too manifest to require elucidation. This fluid appears chiefly where it is most wanted, on herbage and low plants, avoiding, in a great measure, rocks, bare earth, and considerable mosses of water. Its production, too, tends to prevent the injury that might arise from its own cause; since the precipitation of water, upon the tender parts of plants, must in them lessen the cold which occasions it. The prevention, either wholly or in part, of cold, from radiation, in substances on the ground, by the interposition of any solid body between them and the sky, arises in the following manner: the lower body radiates its heat upwards, as if no other intervened between it and the sky; but the loss, which it hence suffers, is more or less compensated by what is radiated to it, from the body above, the under surface of which possesses always the same, or very nearly the same temperature as the air. The manner in which clouds prevent, or greatly diminish, the appearance of cold at night, upon the surface of the earth, is by radiating heat to the earth, in return for that which they intercept in its progress from the earth towards the heavens. For although, upon the sky becoming suddenly cloudy during a calm night, a naked thermometer, suspended in the air, commonly rises 2 or 3 degrees, little of this rise is to be attributed to the heat evolved by the condensation of watery vapour in the atmosphere; for the heat so extricated must soon be dissipated, whereas the effect of greatly lessening, or preventing altogether, the appearance of a superior cold on the earth to that of the air, will be produced by a cloudy sky, during the whole of a long night.
