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2. Cumulus.--This is a convex aggregate of watery particles increasing upwards from a horizontal base. It is commonly of a dense structure, formed in the lower atmosphere, and moving along in the current of wind which is next to the earth. Its first appearance is generally a small irregular spot, which becomes the nucleus on which it forms. This in. creases in size, preserves a flat horizontal base, and . assuines somewhat of a conical figure. Cumuli vary a little in shape and dimensions, according to pecu! liarities in the operation of the causes which produce them. Sometimes they are pretty well 'defined hemispherical masses ; at others, they rise into mountains, ranged in one plane, their silvery summits presenting a beautiful appearance.
3. Stratus,—The stratus is the lowest of clouds; its under surface usually rests on the earth or water. It may properly be called the cloud of night, as it frequently makes its appearance about sunset, and disappears. soon after sunrise. When ascending in the atmosphere, it often seems to take the form of cumulus. It comprehends what we usually call fogs and mists, which in fine suminer evenings are seen to ascend in spreading sheets from valleys, lakes, and fields. In autumn and winter it sometimes continues throughout the day. It must be remembered, however, that all fogs are not strati ; some appear to be of the modification of cirrostratus,
4. Cirrocumulus.—After she cirrus has ceased to conduct the electric fluid, it probably either disappears by dispersion or evaporation, or it changes into the cirrocumulus or cirrostratus. Its change to the cirrocumulus is frequently marked by the following circumstances : ii loses its cirriform and fibrous structure, 'descends lower in the atmosphere, and assumes the form of a number of well defined and roundish little clouds, lying in close horizontal arrangement : the change is more or less rapid on different occasions, and sometimes takes place in part
of the cloud, while the other part remains cirriform, or approaches to the nature of cirrostratus. The cirrocumulus is frequent in summer, and often forms very beautiful skies : at all times of the year it may be seen, in the intervals of showers, and before an increase of temperature, of which its prevalence is a pretty certain prognostic. Extensive beds of cirrocumuli, floating gently along in different altitudes, must have attracted almost every body's notice; the beautiful appearance of these clouds, with a moonlight evening, has been aptly described by Bloomfield:
Far yet above these wafted clouds are seen,
The Farmer's Boy ;-Winter. 5. Cirrostratus.-In speaking of the cirrus, it has been observed that that cloud frequently changed into some other. Its change is generally into either the cirrocumulus or cirrostratus : when it passes to the latter, it descends lower in the atmosphere, its fibres become denser and more regularly horizontal, and it generally appears subsiding, or altering its forms. The figure of the cirrostratus, like that of the cirrus, is very various : sometimes it consists in dense longitudinal streaks; at others it looks like shoals of fish: sometimes the whole sky is so mottled with it, as to give the idea of the back of the mackerel ; and hence called the mackerel-back sky. It frequently appears like the grains of polished wood, or is composed of fine fibres, disposed after the manner of the fibres of muscles, which often intersect each other.
6. Cumulostratus.—The change of the cumulus into the cumulostratus is effected in the following manner : The cumulus, losing its bemispherical
figure, increases irregularly upward, grows more dense, and overhangs its base in uneven or rugged folds: a pre-existing cirrus, cirrocumulus, or cirrostratus, or one perhaps immediately formed for the occasion, alights on its summit, and inosculates. The cumulostratus varies in appearance: sometimes it overhangs a perpendicular stem, and looks like a great mushroom; frequently a long range of cumulostrati appear together, which have the appearance of a chain of mountains with silvery tops. Before thunder-storms it seems frequently reddish, which some have imagined to arise from its being highly charged with the electric fluid.
7. Nimbus.—Clouds of any one of the abovementioned modifications, at the same degree of elevation, may increase so much as completely to obscure the sky: two or more different modifications may also do the same thing in different elevations, and the effect of this obscuration may be such as would induce an inattentive observer to expect the speedy fall of rain. It appears, however, from attentive observation, that no cloud effuses rain until it has previously'undergone a change sufficiently remarkable to constitute it a distinct modification, to which the term nimbus has not inaptly been applied. · The best time for viewing the progress of nimbification is by stormy weather; cumuli may then be seen rising into mountains and becoming cumulostrati, while long strata of cirrostratus permeate their summits; and the whole phenomenon has the appearance of a range of mountains, transfixed by the mighty shafts of giants. After having existed some while in this form, they become large and irregular, and they get darker by intensity, till all seem concentrated in a dense black mass, with a cirrose crown extending from the top, and ragged cumuli entering from below, and eventually the whole resolves itself into rain. · According to Mr. Howard's theory, the origin of clouds is from the surface of the earth and waters.
That the vapour upraised by the accession of the diurnal temperature, in the manner described, is condensed into a visible cloud, either by cold, or by the ait, from other causes ; losing its power of holding so much water in solution as before; or by the joint influence of these causes. That cumuli are the immediate result of this process; and that in the evening, when the heat is diminished, the air deposits its vapour again in the form of dew, which gravitates to the ground, becoming more dense as it approaches the earth, because the lower atmosphere is now the coolest; and finally lodges on the surface of the herbage, or of the ground, where it awaits the reascending sun to be again evaporated. Cumuli also are represented to be dispersed, and their constituent particles to come to the ground in the same manner. According to the same theory, it appears that the other modifications are also the consequence of vapour carried up into the atmosphere, while their peculiarities are more immediately effected by the agency of the electric fluid.
We shall conclude with a brief review of the modifications ascending from the Stratus, formed by the condensation of vapour on its escape from the surface to the Cumulus, collecting its water in the se. cond stage of its ascent, both probably existing by virtue of a positive eletricity. From these proceeding through the partially conducting Cumulostratus to the Cirrostratus and Cirrocumulus; the latter positively charged, and considerably retentive of its charge; the former less perfectly insulated, and, perhaps, conducting horizontally; we arrive thus at the region where the Cirrus, light and elevated, obeys every impulse or invitation of that fluid, which, while it finds a conductor, ever operates in silence; but which, embodied and insulated in a denser collection of watery atoms; sooner or later bursts its barrier, leaps down in lightning, and glides through the Nimbus from its elevated station to the earth.
For further information on this interesting subject, we refer the inquisitive student to The Philosophical Magazine, vol. xvi ; Ree's Cyclopedia, art. Cloud; and particularly to Mr. Forster's very ingenious Researches about Atmospheric Phenomena, just published ; to which last work we are indebted for some of our preceding observations. II. Rules FOR PREDICTING CHANGES IN THE
WEATHER. § i. Prognostics from the Atmosphere, Winds, 8c.
1. A south wind or great heat in summer portends a whirlwind, Job xxxvii, 9. - .. .
2. Cold or fair weather is indicated by the north wind, which drives away rain, Job xxxvii, 9, 22.
3. A red sky in the evening foretels fair weather; in the morning, foul, Matthew xvi, 2.
The evening red and morning grey,.
Is a sign of a fair day. A red evening and a grey morning sets the pilgrim a walking. - 4. A cloud rising out of the west indicates rain, Luke xii, 54.
5. A south wind is a sign of heat, Luke xii, 54.
6. If clouds appear white, and drive to the northwest, it is a sign of several days' fair weather; and thus Pliny, If the rising sun be encompassed with an iris, or circle of white clouds, and they disperse equally, this is a sign of fair weather. .“. If woolly fleeces spread the heavenly way,' .
Be sure no rain disturbs the summer day." 7. If in the evening a white mist be spread over a meadow contiguous to a river, it will be evaporated by the Sun's rays on the following morning, and is an indication of fine weather throughout the day; so in the morning, if a mist, which is impending over low lands, draw off towards those which are more elevated, it announces a fine day,