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the surface of the earth. We must no doubt consider that the altitude given above, comprises only two-thirds of the practical height of our atmosphere; the remaining third may, however, without any risk of error, be considered, from the lowness of its. temperature, as totally free from water. Indeed, at 30,000 feet, the quantity is scarcely appreciable. Therefore the mean pressure, diffused through 45000 feet, would be reduced to about one-fifth of an inch, which would raise the barometer only from 30 inches to 30.2 in the atmosphere surrounding a sphere of the temperature of 77°, by a change from absolute dryness to perfect moisture.

The transition into cloud of the invisible aqueous vapour diffused in the air, occurs whenever the atmospheric temperature coincides with the dew point.. This point may be most elegantly and precisely determined by Mr. Daniell's hygrometer. It appears from experiments with this instrument, that the elastic force of aqueous vapour does not diminish progressively with the progressive diminution of the temperature of the air, in its upper regions, but that the elasticity of the vapour continues nearly unimpaired through a great range of elevation, after which it suddenly encounters the dew point, and consequently a cloud is formed. If the vesicular water constituting this cloud, descends by gravity a very little way, it comes once more within the influence of a temperature higher than the dew point, and necessarily resumes the invisible condition. In this way, the under surface of the nebulous stratum becomes nearly concentric

These must likewise We can thus readily

with the surface of the subjacent plane. Above this first range of clouds, the dew point stands much lower, (the air being now stripped of much of its moisture,) and hence another considerable body of air must be passed through, before aemperature sufficiently low, be arrived at, to cause a second deposition of clouds. be slighter than the first. understand the causes of the alternate strata of clouds and clear air, which often diversify the sky in serene weather. We may also comprehend how in stormy weather, a solitary cloud sometimes stands stationary over a mountain top, while myriads of other clouds drift past it with the gale. observer on the summit, feels the vesicular particles of the seemingly fixed cloud, sweeping past him with great velocity, and discovers the stationary aspect which it exhibited below, to be illusory. The inferior beds of air, are relatively warmer, and moister. They dash against the sloping side of the mountain, are reflected up into the plane of condensation, where they give out their excess of water in the vesicular form. Above the cooling influence of the mountain top, the air is not depressed to the dew point of its constituent vapour, and hence it continues clear.

An

The dew point, however, often suddenly shifts, from a higher to a lower degree, and vice versá, from causes ill-understood; most probably connected with electrical changes.

Had not a firmament of permanently elastic fluid been called into being, the aqueous vapours

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rising from the sub-tropical seas would have flowed in constant torrents of fog towards the circumpolar regions, by a rapid distillation; the transfer being effected by a successive displacement of the intermediate rows of vaporous particles. Thus evaporation at the equator, communicating pneumatic pressure to that aqueous atmosphere, would induce speedy deposition of rain, over the arctic regions. By the creation of the air, however, a self-adjusting regulator has been introduced, to distribute the atmospheric waters in graduated quantities. The vapour thereby forced to wind its way amid the aerial particles, has to overcome considerable obstruction; and its progress of ascent is further checked. by the coldness belonging to rarefied air. Hence, only partial nebulosities can take place in certain planes, with clear intervals; instead of a lurid canopy of cloud. Every person indeed must have remarked, that clouds are not always confined to one atmospheric level.

The aqueous particles are not suspended in the atmosphere by any power analogous to that of che mical solution. There is merely a mechanical mixture of particles in juxta-position, a state which most probably represents the mixture of oxygen, azote, and carbonic acid in the permanent atmosphere. Mr. Daniell has the following just remarks on this curious subject.

The constancy of the proportions, in which these gases are found to be combined in every situation, notwithstanding perpetual causes of disturbance, is the never failing theme of wonder. If we suppose a consumption of the oxygen to take place, by the

I

decomposition of the atmosphere, and carbonic acid added, as in the process of combustion, at any given spot, in what way is chemical affinity to act, so as to restore the uniformity of the compound? No new evolution of oxygen takes place, and it cannot be supplied by the contiguous portions; for we can never suppose the affinity of azote for oxygen to be satisfied, by the decomposition of an adjoining mass of azote and oxygen, held together by the same affinity. But, if the oxygen and azote be two distinct elastic atmospheres, as Mr. Dalton originally suggested, mutually permeating one another's interstices, the particles of each pressing only upon their fellows, and offering slight obstacles to the motions of the other sort, then a partial consumption of oxygen would be instantly supplied by a rush of this elastic fluid towards the spot where the equality of pressure had been disturbed. In fact, no sooner does a particle of oxygen quit the azote, and enter into a new combination, than the rows of particles by which it was pressed all around, speedily supply its place. The same reasoning may be applied to the carbonic acid, so profusely generated in combustion and respiration; for if not rapidly dispersed, a city would be uninhabitable in still weather.

The tension of vapour, which rises in evaporation, is determined, not by the temperature of the evaporating surface, but by the elastic force of the preexisting aqueous atmosphere. This proposition has been experimentally proved by Mr. Daniell, and Captain Sabine. While the sea breeze was blowing upon the coast of Africa, the hygrometer denoted the dew-point to be about 60°; but when the wind

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blew strong from the land, it approached in its characters to a Harmattan; and the point of precipitation fell to the extraordinary depression of 37.5°; the temperature of the air being 66°. Hence we see, that notwithstanding the heat of the evaporating surfaces in the interior of that continent, the sands of its deserts yield so little vapour, that the air becomes parched.

As the quantity of vapour increases, the barometer generally falls, and again rises with its decrease; in the daily fluctuations of the weather. This fact is irreconcilable to the hypothesis which ascribes the rise and fall of the mercurial column to the greater or less amount of aqueous particles, and is in unison with that which attributes them to the unequal expansion of balancing currents of air, the main source of which is the fluctuation of the elastic vapour itself.

The dew-point, of which so much has been said, may be found for many purposes, by filling a tumbler with cold water, and noting its temperature at the instant, when in the progress of heating, it ceases to condense dew on its surface. To mark this instant with precision, requires close watching and frequent wiping with a towel; inconveniences from which Mr. Daniell's hygrometer is free. The higher the temperature at which dew continues to appear, the more moisture exists in the atmosphere; or the less the water needs to be cooled below the temperature of the air, in order to cause the dewy deposit, the greater chance there is of rain. When the dew appears on a glass, very slightly refrigerated, it is evident that the aqueous vapour in the