BOOK II. OF THE ATMOSPHERE. 259 



the earth. This moisture is precipitated by the cold of the body on which it appears, and 

 will be more or less abundant, not in proportion to the coldness of that body, but in pro- 

 portion to the existing state of the air in regard to moisture. It is commonly supposed 

 that the formation of dew produces cold, but like every other precipitation of water from 

 the atmosphere, it must evidently produce heat. 



1244. Phenomena of dew. Aristotle justly remarked, that dew appears only on calm and clear nights. 

 Dr. Wells shows, that very little is ever deposited in opposite circumstances ; and that little only when the 

 clouds are very high. It is never seen on nights both cloudy and windy ; and if in the course of the night 

 the weather, from being serene, should become dark and stormy, dew which had been deposited will disap- 

 pear. In calm weather, if the sky be partially covered with clouds, more dew will appear than if it were en- 

 tirely uncovered. Dew probably begins in the country to appear upon grass in places shaded from the sun, 

 during clear and calm weather, soon after the heat of the atmosphere has declined, and continues to be depo- 

 sited through the whole night, and for a little after sunrise. Its quantity will depend in some measure on 

 the proportion of moisture in the atmosphere, and is consequently greater after rain than after a long tract of 

 dry weather ; and in Europe, with southerly and westerly winds, than with those which blow from the 

 north and the east. The direction of the sea determines this relation of the winds to dew. For hi Egypt, 

 dew is scarcely ever observed except while the northerly or Etesian winds prevail. Hence also, dew is 

 generally more abundant Ln spring and autumn, than in summer. And it is always very copious on those 

 clear nights which are followed by misty mornings, which show the air to be loaded with moisture. And 

 a clear morning, following a cloudy night, determines a plentiful deposition of the retained vapor. When 

 warmth of atmosphere is compatible with clearness, as is the case in southern latitudes, though seldom in 

 our country, the dew becomes much more copious, because the air then contains more moisture. Dew 

 continues to form with increased copiousness as the night advances, from the increased refrigeration of 

 the ground. 



1245. 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 Pro- 

 fessor 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, 

 28 minutes before sunset, previously weighed parcels of wool and swandown, upon a smooth, unpainted, 

 and perfectly dry fir table, 5 feet long, 3 broad, and nearly 3 in height, which had been placed an hour 

 before, in the sunshine, in a large level grass field. The wool, 12 minutes after sunset, was found to be 

 14 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 13 colder than the air, and was also without any ad- 

 ditional weight. In 20 minutes more the swandown was 14 colder than the neighboring air, and was 

 still without any increase of its weight. At the same time the grass was 15 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 neighboring 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 pro- 

 jection of heat into free space. Now the researches of Professor Leslie and Count Rumford have de- 

 monstrated, 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 calorific emanations which would be dissipated in a clear sky, we shall 

 find a solution of the most mysterious phenomena of dew. 



1246. Rain. Luke Howard, who may be considered as our most accurate scientific 

 meteorologist, is inclined to think, that rain is in almost every instance the result of the 

 electrical action of clouds upon each other. 



1247. Phenomena of rain. Kain never descends till the transparency of the air ceases, and the invisible 

 vapors become vaseuhr, when clouds form, and at length the drops fall : clouds, instead of forming 

 gradually at once throughout all parts of the horiaon, generate in a particular spot, and imperceptibly 

 increase till the whole expanse is obscured. 



1248. Tlie cause of rain is thus accounted for by Dalton. If two masses of air of 

 unequal temperatures, by the ordinary currents of the winds, are intermixed, when 

 saturated with vapor, a precipitation ensues. If the masses are under saturation, then 

 less precipitation takes place, or none at all, according to the degree. Also the warmer 

 the air, the greater is the quantity of vapor precipitated in like circumstances. Hence the 

 reason why rains are heavier in summer than in winter, and in warm countries than in 

 cold. 



1249. Tlie quantity of rain, taken at an annual mean, is the greatest at the equator, and 

 it lessens gradually to the poles ; but there are fewer days of rain there, the number of 

 which increase in proportion to the distance from it. From north latitude 12 to 43 the 

 mean number of rainy days is 78 ; from 43 to 46 the mean number is 103 ; from 46 

 to 50, 134 ; and from 51 to 60, 161. Winter often produces a greater number of rainy 

 days than summer, though the quantity of rain is more considerable in the latter than in 

 the former season j at Petersburgh rain and snow falls on an average 84 days of the 

 winter, and the quantity amounts to about five inches ; on the contrary the summer pro- 

 duces eleven inches in about the same number of days. Mountainous districts are sub- 

 ject to great falls of rain ; among the Andes particularly it rains almost incessantly, while 

 the flat country of Egypt is consumed by endless drought. Dalton estimates the quantity 

 of rain falling in England at 31 inches. The mean annual quantity of tain for the whole 

 globe is 34 inches. 



1250. The cause why less ram falls in the first six months of the year than in the last sir 

 months is thus explained. The whole quantity of water in the atmosphere in January 

 is usually about three inches, as appears from the dew point, which is then about 32. 



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