Immagini della pagina
PDF
ePub

exuberance over the higher belts, effectually shield the earth from the fierce rays of the sun, and check the pro gress of evaporation. The soil, thus shaded, is always moist, and the air warm and humid; and from the causes already stated, such results are here produced as we should readily infer-excessive rains in the lower forests (190), and clouds of mist upon the more elevated ranges.

CHAPTER III.

OF CLOUDS.

218. The name of clouds is given to those collections of vapor, that float at a lofty altitude above the earth.

219. Though differing from fogs in situation, they originate in precisely the same causes; being formed, in the higher regions of the atmosphere, by the union of warm and cold air, when the combining volumes are over saturated. The excess of humidity, when slight, then appearing in the atmosphere in the form of clouds.

220. During the daily process of evaporation, warm, humid currents of air are continually ascending from the earth; the higher they ascend, the colder is the atmosphere into which they enter; and, as they continue. to rise, a point at length will be attained, where, in union with the colder air, their original humidity can no longer be retained; a cloud will then appear, which increases in bulk with the upward progress of the current into colder regions.

If the cloud however, in its ascent, either meets with a warmer stratum of air, or descends towards the earth into a region of a higher temperature, a portion of the

What is the subject of chapter third ?

Define clouds.

How do they originate?

In what manner do ascending currents produce clouds?

minute globules of water which compose it, perhaps all, will be re-dissolved, and the cloud will either contract in size, or completely vanish, according to the increase of heat to which it is subjected.

221. The entire atmosphere, to the altitude of many thousand feet, is constantly traversed by numerous horizontal currents of air, flowing in different directions, and at different heights. Combinations of vast volumes of air, varying in temperature, must therefore at times inevitably occur, as well in the higher as in the lower regions of the atmosphere; and when the excess of moisture resulting from this union is but small, clouds, with their ever-changing forms, obscure the serenity of the sky.

222. When Clayton, on the 31st of July, 1837, ascended in a balloon from Louisville, Ky., the direction of his course was altered no less than five times, in the space of fourteen hours. Once, when at a very great height, he beheld, a mile below him, a snow-white cloud of a mountain shape drifting in an opposite direction to that in which he was traveling.

223. The upward impulse given to the warm atmosphere near the earth, when driven by the wind against the sloping sides of mountains, is also a fruitful source of clouds. (230.)

224. STRATA OF CLOUDS. When an extended range of clouds settles down towards the earth, its under surface often copies the outline of the landscape immediately beneath, assuming a horizontal direction. This is owing to the high temperature of the air below the cloud, in consequence of which the latter would cease to be visible, were it to descend lower; for the globules of vapor would then be dissolved by the warm atmosphere.

225. Above the first range of clouds, the temperature is often much higher than in the region of vapors be

What is said as to the existence of horizontal currents, and their effects? Relate the illustration.

What is the influence of mountains?

What is said as regards the figure of the under surface of clouds?

How is the existence of successive ranges of clouds explained?

[ocr errors]

neath. Here the air will be clear, and a tract of considerable thickness frequently intervenes before we arrive at a second range of clouds; to this may succeed another body of pure air, and still higher a third range of clouds, and so on, alternately.

226. The following account, given by Jolliffe, of his aërial voyage, which took place in England, in 1826, is instructive in this connection.

"Our progress, during the first quarter of a mile, was so gradual, as to be nearly imperceptible; but on discharging a portion of the ballast, the balloon ascended with a rapidity, which, in a few minutes, buried us in the vapors of a dense mass of clouds. The temperature was here cold and raw; such as I have felt on a mountain-top, when enveloped in fog. We loitered here for some time; but at length rose with uncontrollable velo city, and burst, almost suddenly, out of this dark barrier into realms of light and glory. The stratum of clouds from which we had emerged, seemed depressed to a vast distance below us, involved in radiant folds, which completely shut out all view of the earth.”

227. THICKNESS. The thickness of clouds is sometimes immense. On the 29th of Sept. 1826, Peytier and Hossard, two French engineers, were upon the Pyrenees, and so stationed, that they beheld, at the same time, the upper and lower surfaces of the same cloud. As the altitude of each station was known, the thickness of the cloud was readily determined, and found to be 1,476 feet. On the succeeding day, the thickness of the clouds was 2,788 feet; or more than half a mile.

228. HEIGHT. The height of clouds has been variously estimated. According to observations given by Dalton, two-fifths of all the clouds observed in England for the space of five years, were more than 3,150 feet above the earth. By noting when the upper and lower surfaces of the clouds touched the peaks of the Pyre

Relate the account given by Jolliffe.

What is said respecting the thickness of clouds?
What of their height?

nees, which had previously been measured, Peytier and Hossard obtained no less than forty-eight altitudes. It was thus found, that the lower surfaces here varied in height from 1,476 feet to 8,200, and the upper from 2,952 feet to 9,840.

229. The computations of many distinguished observers have been collected by Kaemtz; and from these it appears, that clouds range, in height, from 1,300 feet to 21,320.

The extreme elevation here given is, however, not sufficiently great; for clouds are sometimes seen floating above the summit of Chimborazo, which rises 21,480 feet above the sea-level; and when Gay Lussac, in the month of September, 1804, ascended in a balloon to the altitude of 23,000 feet, he beheld clouds still soaring above him, apparently at a great height.

30. CLOUDS ON MOUNTAINS. When a mountain range is viewed from a distance, the various peaks are frequently seen capped with a cloud; while the atmosphere between them is perfectly clear. This appearance sometimes continues for hours, and even entire days; and was often noticed amid the Alps by the celebrated Saussure. It is caused by the wind impelling up the sides of the peaks the warm, humid air of the vales, which, in its ascent, gradually sinks in capacity, until it is over-saturated, when the excess of moisture becomes visible, and appears as a cloud.

231. This phenomenon is illustrated by figure 12. Let A B C represent the outline of a mountain peak, up the sides of which a warm current flows, in the direction of the arrows. Above the line D E, the temperature is below the dew-point of the current, and its humidity is condensed into a cloud at B. As the wind sweeps over the summit, the cloud B is carried below the line D E, on the opposite sile, and re-dissolved in the warm atmosphere beneath; but its place, meanwhile, is occupied by a fresh cloud, caused by the ascent

What is the appearance sometimes presented by distant mountains 7 How is this accounted for?

D

Fig. 12.

A

A CLOUD UPON A MOUNTAIN PEAK.

of the warm air on the side A B. It thus occurs, that though the cloud upon the mountain is stationary for hours together, yet the particles which compose it are continually changing.

232. The appearances just described are finely displayed upon the St. Gothard, a mountain in Switzerland, about 6000 feet high. Dark, heavy clouds that have formed on one side of the mountain, are frequently seen, passing rapidly over its summit, and descending in dense masses into the vale of Tremola, on the opposite side; but, instead of filling the plains beneath with thick vapor, the clouds are dissolved by the warm air into which they are precipitated.

233. A singular instance of the alternate appearance and disappearance of a cloud occurred, not long since, upon the coast of England. A cloud was seen, borne along by the wind, apparently passing from one side of an arm of the sea to the other, but not extending across the water. It was visible over the land, on each shore, but the sky above the water was perfectly serene. This phenomenon may be thus explained. Over the land, in the region of the cloud, the air was below the dew-point; but over the water, the sea being warmer than the land,

Explain from the figure.
Give the illustrations.

« IndietroContinua »