VAPOUR, in meteorology, a thin hu mid matter, which, being rarefied to a .certain degree by the action of heat, ascends to a particular height in the at mosphere, where it is suspended, until it returns in the form of dew, rain, snow, &c. On this subject we refer our readers to the articles EVAPORATHIN and M ETF.011, (moor, and shall make a few additional observations on dew, which is a pheno menon proper to clear weather. It be gins to be deposited about sun-set, is most constant in vallics, and on plains near rivers, and other collections of wa ters, and abounds on those parts of surface which are clothed with vegeta tion. It is often suspended when rain is approaching, as likewise in windy wea the•; and before thunder storms ; an un usually copious deposition however some times precedes rain. The following is said to he the usual appearance in the valley through which the Thames passes. After a clear warm day there is gradually formed on the horizon a continuous haze, rising sometimes to a considerable height, and often tinged by the setting sun with a fine gradation of red and violet shades. This is the precipitated water become faintly visible in its descent. Dew is al ways to be found on grass by the time that this haze has become conspi cuous, and its abundance is proportioned to the density and permanence of the latter. The following facts are deserving of notice.
In this country the dew is observed more copiously in the mornings of spring and summer than at other times in the year. Sometimes, however, in autumn and winter, an abundant dew is deposited in the night. In countries nearer the equator, the dews are generally observed in the morning throughout the whole year ; and in some places they are so very copious as in a great measure tp supply the deficiency of rain, which sel dom falls in those places. The conden sation of the vapour which forms the dew mostly takes place while the sun is below the horizon ; the greatest deposi tion taking place soon after the setting of the sun. In cloudy weather there is little or no dew deposited : the most considerable quantity is observed in a morning, subsequent to a clear, still, and cool night, which has followed a pretty warm day. The lower parts of bodies
that are exposed to the ambient air are first covered with dew. The most singu lar circumstance is, that dew is not depo sited upon all kinds of substances indis criminately: it falls upon certain bodies much more abundantly than on others, and upon some even not at all. The drops of dew attach themselves to glass, crystals, and porcelain, much more rea dily than to other bodies ; next to these come the leaves of vegetables, wood, especially when varnished, and common earthen ware; but the dew adheres least of all to all sorts of metallic bodies. We may now notice Mr. Dalton's observa tions, which are the result of a variety of well conducted and very accurate experi ments on this subject. 1. That aqueous vapour is an elastic vapour 871i generic, diffusible in the atmosphere, but forming no chemical combination with it. 2. That temperature alone limits the maximum of vapour in the atmosphere. 3. That there exists at all times, and in all places, a quantity of aqueous vapour in the atmo sphere, variable according to circum stances. 4. That whatever quantity of aqueous vapour may exist in the atmo sphere at any time, a certain tempera ture may be found, below which a por tion of that vapour would unavoidably fall, or be deposited, in the form of rain or dew, but above which no such dimi nution could take place with chemical agency. This point may be called the extreme temperature of vapour of that density ; and 5. That whenever any body colder than the extreme temperature of the existing vapour, is situated in the at mosphere, dew is deposited upon it, the quantity of which varies as the surface of the body, and the degree of cold be low the extreme temperature. The reader may be referred to an excellent and elaborate article on this subject in Dr. Rees's New Cyclopedia, a work, of which it may be fairly and honourably said, that as it advances in its progress, it increases in merit and reputation.