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.

even so delicate that he was able to discern through them the spots on the moon. The last two classes of clouds, like the cirrus. float at a very lofty height.

CIRRO-STRATUS (b, c, d), CIRRO-CUMULUS (a), NIMBUS (e), CUMULO-STRATUS (f).

. 248. CUMULO-STRATUS. The variety of cloud to which this name is given, combines the characteristics of the cumulus and stratus. Its base consists of a horizontal stratum or layer of vapor, from which rise large, overhanging masses of cumulus (figure 14., f). Some

What is said respecting the height of the cirro-stratus and cirro-cumulus ? Describe the cumulo-stratus.

Of what does it consist?

times contiguous cumulus clouds unite, and passing into the state of cumulo-stratus, form groups of immense size. This cloud is seen in perfection upon the approach of a thunder-storm, when the cumulus clouds, driven together by the wind, are piled upon each other, and assume those peculiar forms commonly known by the name of thunderheads.

249. This modification also frequently arises, when the cumulus is pierced by the cirro-stratus; and it is by no means unusual to see four or five parallel bars of the cirro-stratus, one above the other, passing through the same pile of clouds, which then present successive tiers of the cumulo-stratus.

250. NIMBUS, OR RAIN-CLOUD. This cloud is so called from the Latin word, nimbus, a rainy dark cloud; it possesses no peculiarity of form, but is distinguished by its uniform gray tint and fringed. edges (figure 14., e). It is usually composed of some of the preceding classes of clouds, so blended together that they cannot be distinguished, and is produced by a change in their structure, the result of an increase in density.

251. The nimbus often originates in the cumulo-stratus, which, as it increases in thickness, frequently assumes a black or bluish tint. In a short time this hue changes to gray, a circumstance which indicates that the nimbus is formed and rain descending.

When is this cloud most perfectly formed?

Under what other circumstances is the cumulo-stratus seen?
Describe the nimbus.

How is it distinguished?

Of what does it consist?

How is it caused?

In what cloud does it often originate?

What does a gray tint indicate?

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