1385. Cause of dew. Dew, according to Aristotle, is a species of rain formed in the lower atmosphere, in consequence of its moisture being condensed by the cold of the night into minute drops. Opinions of this kind, says Dr. Wells, are still entertained by many persons, among whom is the very ingenious Professor Leslie. (Relat. of Heat and Moisture, p. 37. and 132.) A fact, however, first taken notice of by Garstin, who published his Treatise on Dew in 1773, proves them to be erroneous; for he found that bodies, a little elevated in the air, often become moist with dew; while similar bodies, lying on the ground, remain dry; though necessarily, from their position, as liable to be wetted, by whatever falls from the heavens, as the former. The above notion is perfectly refuted by the fact, that metallic surfaces, exposed to the air in a horizontal position, remain dry, while every thing around them is covered with dew. After a long period of drought, when the air was very still, and the sky serene, Dr. Wells exposed to the sky, twenty-eight minutes before sunset, previously weighed parcels of wool and swandown, upon a smooth, unpainted and perfectly dry fir table, five feet long, three broad, and nearly three in height, which had been placed, an hour before, in the sunshine, in a large level grass field. The wool, twelve minutes after sunset, was found to be 14ﾦ colder than the air, and to have acquired no weight. The swandown, the quantity of which was much greater than that of the wool, was, at the same time, 13ﾦ colder than the air, and was also without any additional weight. In twenty minutes more, the swandown was 14.5ﾦ colder than the neighbouring air, and was still without any increase of its weight. At the same time the grass was 15ﾦ colder than the air, four feet above the ground. Dr. Wells, by a copious induction of facts, derived from observation and experiment, establishes the proposition, that bodies become colder than the neighbouring air, before they are dewed. The cold, therefore, which Dr. Wilson and M. Six conjectured to be the effect of dew, now appears to be its cause. But what makes the terrestrial surface colder than the atmosphere ? The radiation or projection of heat into free space. Now, the researches of Professor Leslie and Count Rumford have demonstrated that different bodies project heat with very different degrees of force. In the operation of this principle, therefore, conjoined with the power of a concave mirror of cloud, or any other awning, to reflect or throw down again those caloric emanations which would be dissipated in a clear sky, we shall find a solution of the most mysterious phenomena of dew.
