It may be easily imagined, that as the interstices of the gaseous fluid can contain but a certain quantity of elastic vapor, there would naturally be a limit to evaporation. This is actually the case. It very often happens that the interstices are found to be full, and can hold no more, and that then evaporation ceases; sometimes, indeed, they may be said to run over, and it is then we see the excess in the shape of steam, or mist, or cloud. The capacity of these interstices of the gaseous fluid becomes larger or smaller in proportion to the temperature of their particles, and the effect of their contraction or expansion is precisely similar to the grasp or relaxation of the hand on a piece of imbibing sponge. At a low temperature, or when the grasp is tightest, a certain quantity can only enter. On the contrary, at a high temperature, or when the sponge is permitted to expand to the utmost, its capacity is increased, and a large volume may be contained.

The total quantity of aqueous elastic vapor which can enter between the interstices of the gaseous fluid, or which the latter can hold suspended, depends upon temperature, but this quantity is invariably at the same temperature. A volume of air may contain less than this quantity, but never more. When it has this exact quantity, it will remain transparent, and is said to be saturated, or at its point of saturation. It is then as damp as it can be; any attempt to insert more vapor will fail, and the rejected vapor will become visible in the form of steam. If we lower the temperature, the aërial interstices will contract, and some of the contained vapor will be squeezed out in the same form. We may increase the temperature to any extent without any visible change, but we render the air drier in proportion to the degree to which we ascend, and in the same degree capable of receiving and supporting an additional quantity of humidity. Atmospheric pressure also affects the amount of the quantity suspended, by opposing the diffusion and retarding the formation of the vapor. From the aqueous fluid being so abundantly spread over the face of the earth, there can be no doubt that the permanently elastic or gaseous atmosphere would very speedily be saturated with its vapor, did not some cause prevent its universal diffusion. This never-failing cause is inequality of temperature, which excites, or diminishes, or suspends, in the way we have described, the process of evaporation.

The absolute quantity of moisture that air is capable of containing, may be conceived from the following statement of Mr. Leslie:-" Air, at the freezing point, is capable of holding a portion of moisture equal to the 160th part of its own weight; at the temperature of 59°, the 80th part; at that of 86°, the 40th part; at 113°, the 20th part; and at that of 140°, the 10th part; so that the air has its dryness doubled at each rise of temperature, answering to 27° of Fahrenheit. While the temperature, therefore, advances uniformly in arithmetical progression, the dissolving power, which

this communicates to the air, mounts with the accelerating rapidity of a geometrical series."

By the improved instruments and accurate observations of this gentleman and others, the total quantity of moisture which could be suspended at one time in the air can be correctly estimated. It has been stated by him, that, at 68° Fahrenheit, a cubic mass of air, measuring 40 inches every way, can retain 252 grains of water. But if a larger scale be preferred, the same numbers will express in pounds troy the quantity of water required to saturate a perfectly dry mass of air constituting a cube of twenty yards in dimension. If the greatest amount possible of the aqueous element were to be suspended in the atmosphere, and this were to pass from a state of absolute dampness into that of extreme dryness, and discharge the whole of its watery store, it would form a sheet of somewhat less than five inches in depth. To furnish the usual supply of rain, the air must, therefore, undergo very frequent changes, equal to that of from dryness to humidity in the course of the year.

The average amount of evaporation in the neighbourhood of London per annum, calculated by Mr. Daniell's hygrometer, a most elegant and perfect instrument for ascertaining the humidity of the atmosphere, is 23,974 inches. The average weight of the quantity of water raised by this process, from a circular surface of six inches diameter, 0.31 gr. per minute. The results of actual measurement by Mr. Howard accord most satisfactorily with this method of estimating the amount of evaporation, and prove most incontestably the accuracy of the calculations upon which it is founded.

The rate at which this process proceeds near London, during the several months of the year, is estimated by Mr. Daniell, and recorded in the British Almanac, as follows:

The smallest quantity of water is, therefore, lifted into the atmosphere during the month of January, and the greatest in June. The mean quantity held in solution in a cubic foot of air, is 3.789 gr.

The rate of exhalation from the surface of the ground is scarcely of less consequence than the fall of rain, and a knowledge of it might often direct the most important operations. Mr. Leslie invented an instrument for measuring the quantity of moisture exhaled from a humid surface in a given time. This he called the Atmometer, and he has estimated that the daily exhalation from a sheltered surface of water would, at the mean dryness of winter, lower it 0.018 inches and at the mean of summer 0.048 inches.

And he gives the following instance of its use: Suppose a pool for the supply of a navigable canal exposed a surface equal to ten English acres, and that the atmometer sunk 80 parts during the lapse of 24 hours, the quantity exhaled in that time would be 2904 cubic feet, or about 81 tons, equal to 1700 imp. gall. per acre.

The dissipation of moisture is much accelerated by the agency of sweeping winds, the effect being sometimes augmented 5 or even 10 times. In general, this augmentation is proportional to the swiftness of the wind, the action of still air itself being reckoned equal to that produced by a celerity of eight miles each hour.

CLOUDS, FOGS, and Mists.

The presence of the ocean of vapor, which we have described as constantly ascending from the earth, and constituting part of the atmosphere, is, as has also been observed, not always evident to the sight; in its elastic state it is always invisible, and, therefore, it is only in some of its changes that the eye can detect it. By one of the most remarkable of these, those masses of visible aqueous vapor are formed, which, floating in the sky, or drifting through it with the wind, at different elevations, with every variety of color and form, are called clouds; or which, recumbent on the surface of the land or of the water, and spread over greater or smaller portions of them, are denominated fogs, or mists, according to their intensity. In all cases, their composition is similar, and consists of the moisture deposited by a body of air, in minute globules.

Their formation, in every position, is a consequence of decrease of temperatures in some parts of the atmosphere where a certain proportion of aqueous elastic vapor is present; but in those where the latter condition may be wanting, it is evident that the developement of cloud will not follow the decrement of temperature. Nothing is more common than the fact of the necessary conditions existing in some of the atmospheric strata, and at the same time being absent in others; and thus we can understand the causes of the alternate beds of clouds and clear air, which often diversify the sky in serene weather. We can hence also comprehend how, in stormy weather, a solitary cloud sometimes appears to stand stationary over a mountain-top, while myriads of other clouds drift past it on the gale. An observer on the summit feels the multitudinous dew-drops of the seemingly fixed cloud sweeping by with great velocity, and discovers the stationary aspect which it exhibited below to be altogether an illusion. The fact is, the inferior invisible beds of air are relatively warmer and more moist. They dash against the sloping side of the mountain, and are reflected up to the plane of condensation in the atmosphere, where they give out their excess of water in the form of clouds. Above the cooling influence of the mountain-top the temperature of the air may not be depressed to the same point, and hence it continues clear.

If the globules of water which constitute a cloud, descend, in consequence of their weight, and come once more within the influence of an elevated temperature, the aqueous vapor necessarily becomes again invisible. In this way, the under surface of a stratum of clouds becomes nearly parallel, or rather concentric, with the surface of the sub-adjacent landscape over which it floats. Above this first range of clouds the temperature may still be considerably higher, and hence another large body of air must be passed through, before a temperature sufficiently low be arrived at, to cause a second deposition of clouds.

M. Fresnel ingeniously supposes that the air contained between the minute globules of vapor, or the very fine crystals of snow, which form a mass of clouds, is always of a higher temperature than the surrounding clear air. He supports this opinion on the well known facts, already alluded to, that the rays of the sun will pass through the air without heating it, unless the air be in contact with water, land, or some other reflecting object. The cloud accordingly forms such a body as will stop the sun's rays, and force them to warm, not only the air in external contact with it, but all the air in its interstices. It follows, therefore, that though the mass of waters in a cloud be heavier than the surrounding air, the warmer air in the interior of the cloud buoys it up, and causes it to float.*

M. Gay Lussac, on the other hand, refers the mounting of clouds in the air to the impulsion of the ascending currents, which result from the difference of temperature between the surface of the earth and the air in elevated regions.

The formation of clouds may be observed with most advantage in Alpine countries, as they are there so frequently produced under the eye, upon the sides or the summits of mountains, by the condensation of the vapor in the sheet of air immediately over them. A mountain cloud is at first of but small extent, but it enlarges insensibly, and is swept by the winds into the bosom of the air, where it either meets and unites with others, or various tufts of these are scattered over the sky. These aërial groups appear, while drifting through the sky, to avoid dashing themselves upon the mountain peaks in their course, and, as if endowed with instinctive repulsion, they bound over the crest of a mountain in a concentric curve, and slide down into the valley on the other side. The French naturalists, with much plausibility, ascribe this beautiful phenomenon to electricity. M. Bory de St. Vincent thinks, that, when small tufts of cloud are carried towards the sides or the summit of a mountain, they move with less rapidity than the force (wind) which moves them, and this force consequently arriving sooner at the obstacle, is reflected, and meets and checks the cloud in its progress.

The mean height of the clouds may be conceived by the following extract from Mr. Leslie. "We shall not err much, if we estimate the position

Annales de Chim. et de Phys. xxi. 260.

of extreme humidity at the height of two miles at the pole, and four miles and a half under the equator, or a mile and a half beyond the limit of congelation. This range is nearly parallel to the curve of perpetual congelation in the polar regions, but bends nearer to it in approaching the equatorial parts.