the temperature of the air was higher, and above the dew-point. When, therefore, the wind carried the cloud over the sea it vanished, its moisture being re-dissolved by the atmosphere; but when the body of air in which the cloud had previously existed, arrived at the opposite shore, a second precipitation of moisture took place, and the cloud reappeared.

CLASSIFICATION.

234. Clouds have been divided into seven kinds; three original, viz. the cirrus, the cumulus, the stratus; and four formed by combination, viz. the cirro-cumulus, the cirro-stratus, the cumulo-stratus, and the nimbus.

235. CIRRUS OR CURL CLOUD. This cloud is so called, from the Latin word cirrus or curl, because it usually resembles a distended lock of hair. It is dis tinguished from the other kinds by its fibrous structure, the lightness of its appearance, and the variety of figures it is capable of assuming. After a period of fine weather, slender filaments of the cirrus are frequently seen, stretching like white lines across the azure sky. Sometimes these threads of clouds are arranged in parallel bands, which in the northern hemisphere, (wherever observations have been taken,) are either directed from south to north, or from south-west to north-east; at other times they separate, resembling the tail of a horse; a form which is known in Germany by the name of windtrees. These filaments are also not unfrequently seen crossing each other, and investing the sky with a delicate net-work of gauze-like vapor. One of the most beautiful forms of the cirrus occurs, when the fibres curl from each side of a band of light cloud, and the whole appears like the feathered grain of a rich piece of mahogany, (figure 13, a.)

236. The white color of the cirrus renders it difficult, in all cases, to detect its peculiar structure; for the eye

Into how many classes are clouds divided?

What are they?

Describe the cirrus.

is dazzled by its excessive light. The cloud may, however, be viewed at leisure, by reflection from a blackened mirror, which diminishes the brightness.

237. The cirrus soars the highest of all clouds. Its altitude, at Halle, in Germany, has frequently been estimated, by Kaemtz to be not less than 21,300 feet; and, from the observations of ten years, this distinguished meteorologist has been led to believe, that it is entirely composed of snow-flakes. Indeed, the temperature of the elevated regions in which it floats, must be often far below the freezing point.

238. CUMULUS. This kind of cloud acquires its name from the Latin word cumulus or heap; the vapor seeming to be piled or heaped together. It is usually seen in the form of a hemisphere, resting upon a horizontal base; but at times detached masses gather into one vast cloud upon the horizon; their radiant summits gleaming like the snowy peaks of distant mountains, (figure 13, b.)

239. The cumulus is the cloud of day, and is produced by the ascending currents of warm air, caused by the solar heat. During the fine days of summer, its peculiar figure is most perfect, and its formation and decline occur in the following manner. Although the sun may have arisen in a cloudless sky, a few solitary specks of vapor may be seen towards eight or nine o'clock; these, as the day advances, enlarge trom within, become thicker, and accumulate in rounded masses, which continue to increase in number and size, till the hottest part of the day. After this time they gradually lessen, and often entirely vanish, leaving the sky at sunset again perfectly

serene.

240. The cumulus floats low in the morning; but its

How may its peculiar structure be best discerned?

How far above the general surface of the earth does the cirrus rise ↑
Of what does it consist according to Kaemtz?

Describe the cumulus.

How does it originate?

Describe the mode of its formation and the changes it undergoes 2

altitude increases with that of the ascending currents, which attain their highest elevation soon after mid-day; towards evening the currents subside, and the cloud descends. This circumstance has often been remarked by meteorologists, when stationed on elevated moun tains. In the morning, the cumulus has been seen beneath them; it enveloped them towards noon; then soared above them for several hours, and descended to the vale at the close of day.

What is said of its height in the morning, at mid-day, and in the evening?

241. It is not difficult to account for the facts just detailed. The cumulus begins to be formed, when the warm currents, in their upward progress, arrive at a temperature so low that they become over-saturated with moisture; and the excess is then condensed into a cloud.

The higher the currents rise, the colder is the atmosphere, generally speaking, and the cloud must necessarily enlarge; but when in the afternoon the strength of the currents abates, the clouds which are buoyed up by their force, sink down into warmer regions of the atmosphere, and are either partially or completely dis solved.

242. The rounded figure of the cumulus is attributed by Saussure to the mode of its formation; for when one fluid flows through another at rest, the outline of the figure assumed by the first will be composed of curved lines. This may be seen, by suffering a drop of milk, or ink, to fall into a glass of water; but the shape of a cloud of steam, as it issues from the boiler of a locomotive, presents a far better illustration.

243. STRATUS. This cloud derives its name from the Latin word stratus, or covering; it forms about sunset, increases in density during the night, and disappears at sunrise. It is caused by the vapors which have been exhaled during the heat of the day, but return again to the earth towards the evening, when the temperature has declined, and are then condensed into a sheet of clouds, which stretch along and rest upon the horizon (figure 13., c). This class likewise includes those light and spreading mists, which gather in meadows and vales in the evening of a warm summer's day, floating like a veil over the surface of the ground, and extending but a short distance above it.

244. CIRRO-STRATUS. This cloud is so called, be

What causes the rounded figure of the cumulus?

Describe the stratus.

When does it form, increase and vanish?

How does it originate?

cause it partakes of the characteristics of the cirrus and stratus; originating usually in the cirrus. It is remarkable for its great length, in proportion to its thickness; but though preserving in the main this peculiarity, it assumes many varieties of form.

245. At one time it consists of a number of parallel bars of vapor, in close proximity, blended together at the middle, but separated at the edges (figure 14., b), or it may appear as a streak of vapor, broadest at the middle, and tapering towards either end (c). A third variety consists of small rows of clouds, parallel to one another; each successive row becoming shorter, from the widest part of the cloud to the extremities, (d.) The name of cirro-stratus is also given to that thin, gauze-like cloud, which sometimes overspreads the whole sky, and through which the sun and moon are dimly visible.

246. CIRRO-CUMULUS. It not unfrequently happens, that the heavens appear as if sown with little round masses of clouds, lying near to each other, but perfectly separated by intervals of sky (figure 14., a). This cloud is the cirro-cumulus, and often arises from a change in the cirrus and cirro-stratus; the bars of the latter being divided across the direction of their length, and the different parts rounding into the cirro-cumulus. Sometimes the reverse occurs, and the cirro-cumulus is seen changing into the cirrus and cirro-stratus.

247. The structure of the cirro-cumulus is not always the same at one time the masses are very dense and well-rounded; at another their form is irregular, and the sky often presents a curdled appearance, when covered with this cloud. Sometimes the cirro-cumulus is so light and fleecy, that the rays of the sun, as they traverse it, are scarcely dimmed. Humboldt found them

Describe the cirro-stratus.

How is it produced?

What are some of its varieties?

Describe the cirro-cumulus.

Whence does it arise ?

State some of the peculiarities of this cloud.