VAPOUR

, in Meteorology, a watery exhalation raised up either by the heat of the sun, or any other heat, as fire, &c. Vapour is considered as a thin vesicle of water, or other humid matter, filled or inflated with air; which, being rarefied to a certain degree by the action of heat, ascends to some height in the | atmosphere, where it is suspended, till it returns in form of rain, snow, or the like. An assemblage of a number of particles or vesicles of vapour, constitutes what is called a cloud.

Some use the term Vapour indifferently, for all fumes emitted, either from moist bodies, as fluids of any kind; or from dry bodies, as sulphur, &c. But Newton, and other authors, better distinguish between humid and dry fumes, calling the latter exhalations.

For the manner in which Vapours are raised, and again precipitated, see Cloud, Dew, Rain, Barometer, and particularly Evaporation.

It may here be added, with respect to the principles of solution adopted to account for evaporation, and largely illustrated under that article, that Dr. Halley, about the beginning of the present century, seems to have been acquainted with the solvent power of air on water; for he says, that supposing the earth to be covered with water, and the sun to move diurnally round it, the air would of itself imbibe a certain quantity of aqueous Vapours, and retain them like salts dissolved in water; and that the air warmed by the sun would sustain a greater proportion of Vapours, as warm water will hold more dissolved salts; which would be discharged in dews, similar to the precipitation of salts on the cooling of liquors. Philos. Trans. Abr. vol. 2, p. 127.

Mr. Eeles, in 1755, endeavoured to account for the ascent of Vapour and exhalation, and their suspension in the atmosphere, by means of the electric fire. The sun, he acknowledges, is the great agent in detaching Vapour and exhalations from their masses, whether he acts immediately by himself, or by his rendering the electric fire more active in its vibrations: but their subsequent ascent he attributes entirely to their being rendered specifically lighter than the lower air, by their conjunction with electrical fire: each particle of Vapour, with the electrical fluid that surrounds it, occupying a greater space than the same weight of air. Mr. Eeles also endeavours to shew, that the ascent and descent of Vapour, attended by this fire, are the cause of all the winds, and that they furnish a satisfactory solution of the general phenomena of the weather and barometer. Philos. Trans. vol. 49, pa. 124.

Dr. Darwin, in 1757, published remarks on the theory of Mr. Eeles, with a view of confuting it; and attempting to account for the ascent of Vapours, by considering the power of expansion which the constituent parts of some bodies acquire by heat, and also that some bodies have a greater affinity to heat, or acquire it sooner, and retain it longer, than others. On these principles, he thinks, it is easily understood how water, whose parts appear from the æolipile to be capable of immeasurable expansion, should by heat alone become specifically lighter than the common atmosphere. A small degree of heat is sufficient to detach or raise the Vapour of water from the mass to which it belongs; and the rays of the sun communicate heat only to those bodies by which they are refracted, reflected, or obstructed, whence, by their impulse, a motion or vibration is caused in the parts of such bodies. Hence he infers, that the sphericles of Vapour will, by refracting the solar rays, acquire a constant heat, though the surrounding atmosphere remain cold. If it be asked, how clouds are supported in the absence of the sun? It must be remembered, that large masses of Vapour must for a considerable time retain much of the heat they have acquired in the day; at the same time reflecting how small a quantity of heat was necessary to raise them, and that doubtless even a less will be sufficient to support them; as from the diminished pressure of the atmosphere at a given height, a less power may be able to continue them in their present state of rarefaction; and lastly, that clouds of particular shapes will be sustained or elevated by the motion they acquire from winds. Philos. Trans. vol. 50, p. 246.

For the Effect of Vapour in the Eormation of Springs, &c, see Spring, and River.

The quantity of Vapour raised from the sea by the warmth of the sun, must be far greater than is commonly imagined. Dr. Halley has attempted to estimate it. For the result of his calculations, see Evaporation.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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TYR
TYSHAS
VACUUM
VALVE
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* VAPOUR
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