a sheltered surface of water would, at the mean dryness of winter, lower it 0.018 in., and at the mean of summer 0.048 in. And he gives the following instance of its use: Suppose a pool for the supply of a navigable canal exposed a surface equal to 10 English acres, and that the atmometer sunk 80 parts during the lapse of twenty-four 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 vapour, 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 vapour are formed, which, floating in the sky, or drifting through it with the wind, at different elevations, with every variety of colour 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 vapour is present; but in those where the latter condition may be wanting, it is evident that the development 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 vapour 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 vapour, 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 vapour 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 beau tiful 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 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

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

parallel to the curve of perpetual congelation in the polar regions, but bends nearer to it in approaching the equatorial parts.

Infinitely diversified as the forms of clouds may appear to be, correct observers have stated that they may all be comprised in seven modifications. Names and definitions have been given to these by Mr. Howard and Mr. Forster. By this classification and nomenclature their appearances may be noted down and transmitted to contemporary and future observers, for the purposes of comparison and record. A great advance has consequently been made in the perspicuous description which has succeeded to the vague and unintelligible generalities of preceding ages. Mr. Howard's names are in Latin; to them we annex Mr. Forster's English nomenclature.

These following modifications are arranged in the order of their ordinary elevation, but which is very frequently deranged:

In the annexed engraving are representations of the more usual forms of these genera, and we subjoin a few remarks on each to render their classification still more easy. In doing this, we shall depart from the above order, for the purpose of taking the simpler forms first.

CIRRUS-Curlcloud. Fig. 1.

The curling and flexuous forms of this cloud constitutei ts mest obvious external character, and from these it derives its name. It may be distinguished from all others by the lightness of its appearance, its fibrous texture, and the great and perpetually changing variety of figures which it presents to the eye. It is generally the most elevated, occupying the highest regions of the atmosphere.

The comoid cirrus cloud, vulgarly called the mare's tail, is the proper cirrus. It has, as represented in the engraving, somewhat the appearance of a distended lock of white hair, or of a bunch of wool pulled out into fine pointed ends. (a*.)

In variable and warm weather in summer, when there are light breezes, long and obliquely descending bands of cirrus are often observed, and seem sometimes to unite distinct masses of clouds together. Frequently, by means of the interposition of these cirri, between a cumulus and some other cloud (as, for example, cirrostratus), the cumulostratus, and ultimately the nimbus, is formed.

Upon a minute examination of the cirrus, every particle is found to be in motion, while the whole mass scarcely changes its place. Sometimes the fibres which compose it, gently wave backwards and forwards, to and from each other.

After a continuance of clear, fine weather, the cirrus is often observed as a fine whitish line of cloud, at a great elevation, like a

* See Indications of Weather, p. 84.

white thread stretched across the sky; the ends of which seem lost in each horizon (b *).

To this line of cirrus others are frequently added laterally; and sometimes becoming denser by degrees, and descending lower in the atmosphere, inosculate with others from below, and produce rain. To this kind the name of linear cirrus has been given. Sometimes on the sides of the first line of a cirrus, clouds of the same kind are propagated, and sent off in an oblique or transverse direction, so that the whole phenomenon has the appearance of net-work; this has been denominated reticular cirrus.

Though the above-mentioned varieties of the cirrus are all composed of straight lines of cloud, either parallel, or crossing each other in different directions; they are ranged under the head of cirrus, or curlcloud, from their analogy of texture to the substance from which this cloud is named.

CUMULUS-Stackencloud. Fig. 8..

This cloud is easily known by its irregular hemispherical or heaped superstructure, hence its name cumulus, a heap or pile. It has usually a flattened base. The mode of its formation is by the gathering together of detached clouds, which then appear stacked into one large and elevated mass, or stackencloud. The best time for viewing its progressive formation is in fine settled weather. About sunrise small thinly-scattered specks of clouds may be observed. As the sun rises, these enlarge, those near each other coalesce, and at length the cumulus is completed. It may be called the cloud of day, as it usually exists only during that period, dissolving in the evening, in a manner, the exact counterpart of its formation in the morning. Cumuli, which are of a more regular hemispherical form, whitish coloured, and which reflect a strong silvery light when opposed to the sun, appear to be connected with electrical phenomena. Those seen in the intervals of showers are more variable in form, and more fleecy with irregular protuberances. When this kind of cloud increases so as to obscure the sky, its parts generally inosculate, and begin to assume that density of appearance which characterizes the cumulostratus.

STRATUS-Fall-cloud. Fig. 11

This kind of cloud rests upon the surface of the globe. It is of variable extent and thickness, and is called stratus, a bed or covering. It is generally formed by the subsidence of vapour in the atmosphere, and has therefore been denominated fallcloud. This genus includes all fogs, and those creeping mists, which in summer evenings fill the valleys, remain during the night, and disappear in the morning. The best time for observing its formation is on a fine evening, after a hot summer's day: as the cumuli which have prevailed through the day decrease, a white mist forms by degrees close to the ground, or extends only for a short distance above it. This cloud arises at its density about

See Indications, p. 84.

Inosculation is a union by the conjunction of the extremities.