Firta stratus described Indications. are also certain forms of it, more deep and dense than or dinary, and arranged on a curved base, which enter into the peculiar features of thunder-storm. It is usually found to accord with a rising barometer. Of the Cirro-stratus. This is a multiform cloud, and can only be detected in its various appearances by an attention to its distinctive characters. It is always an attenuated sheet, or patch, floating on the air, in a position nearly or quite horizontal. As we bave compared the cirrus to dry flax, we may here consider it as drenched in water, and having its spreading fibres reduced to a closer and recumbent form. Viewed over head, it is remarkable for its uniform hazy continuity, and in the horizon for its great appearance of density, the consequence of its being seen edgewise. In this situation, also, it sometimes cuts the sun's or moon's disk across with a dark line; of which Virgil, "Ille ubi nascentem maculis variaverit ortum Or should his rising orb distorted shine Through spots, or fast behind a cloud's dark line For rainy torrents: a tempestuous air, Swift from the southern deep, comes fraught with ill, The cirro-stratus is the natural indication of depression of temperature, wind, and rain. In order to make a proper use of it in this respect, it is necessary to attend to the time of its appearance, to its continuance, and its accompaniments, This cloud sometimes alternates with the cirro-cumulus, either at different intervals of the day, or in the same sky, or eyeu in the same stratum, which may consequently be seen successively in each modification, and at intervais, partly in one, partly in the other. In this case the prognostic is doubtful, doubtful, and regard is to be had to that which ultimately prevails. Again, there is a transient appearance of the cirro-stratus; which often accompanies the production of dew in the even ing, and denotes an atmosphere but lightly surcharged with vapour. Not so when it appears earlier in the day, or at sun-rise (according to the preceding quotation), and attended with the rudiments of the cumulus. In general, the weather may be suspected of a strong tendency to wind and rain, as often as the sky is both hazy, and deformed with numerous small patches of cloud, in which the extenuated character predominates; and these appearances, together with an abundance of cirro-cumulus, indicate thunder. Before storms of wind, there is in particular a feature of cirrostratus, often very slightly expressed, and in one quarter only, which resembles the architectural cy ma. But the most formidable appearance of the cirro-stratus is that of extensive sheets, descending from the highest regions of the atmosphere, and scarcely discernible for a time, but by the prismatic colours which they assume in the vicinity of the sun's or moon's place. These are the skreens on which are described the immense circles of haloes, form- Haloes, par ing, by their occasional intersectious, parhelia, and parasehelia, &c. lenia, mock suns and moons, which sometimes vie in splen dour with the luminaries themselves. It is easy for those who are acquainted with the principles of optics, to conceive how these intersecting circles are produced by light passing through sheets of cloud placed at different heights and angles. Consistent with this is the prognostic of foul weather commonly deduced from the appearance of the halo. After a solar halo in spring, or the early part of summer, a series of wet and cold weather may be expected, although it should not commence for some days; during which, nevertheless, the same state of the atinosphere subsists, as is often manifest from the repetition of the halo. Those which surround the moon in clear nights indicate rain or snow, according to the season of the year. In mountainous and even hilly countries, the cirro-stratus is frequently seen adhering to the more elevated points of land. Cumulo-stra mation de scribed. land. In winter it also visits the plains, in the form of a very wet and durable mist, the drops of which are nevertheless too small to be visible, aud which, unlike the stratus, is more dense on rising grounds than in the valleys. The cirro-stratus usually accords with a sinking state of the barometer. Of the Cumulo-stratus. The formation of the cirro-cumulus, or cirro-stratus, by tus and its for condensed vapour, descending from the higher atmosphere, does not prevent the cumulus from being produced out of the water, which, in the mean time, evaporates from the Earth, and ascends to the middle region. In this case, the two modifications after a while come into contact, and present to the attentive observer a succession of curious appear ances. While the cumulus is rapidly increasing upward, a delicate fleece, of a structure visibly different, sometimes attaches itself to its summit, where it reposes as on a mountain. This fleece is a cirro-stratus; and the materials of which it is formed are brought by a superior current overtaking or meeting the cumulus. Frequently, the cumulus in its increase breaks through the cirro-stratus, and appears again above it, but with a visible change in the aggregation, which now becomes rocky, perpendicular, and, finally, overhanging. If the cirro-stratus fhould itself increase too fast to be swallowed up by the cumulus, the latter after a while extends its protuberances laterally, and attaches itself by them to the superior mass of cloud. When the cirro-cumulus, in like manner, occupies the superior place, a cumulus rising beneath it is susceptible of the same union by mutual attraction; the result of which, as in the former case, is a large, lofty, and dense cloud, which often subsists through the day; and in the evening undergoes the usual evaporation. It is not, however, absolutely necessary to the production of this cloud, that either of the superior modifications should be previously forined. In a favourable state of the atmos phere, the cumulus itself, after having arrived at a certain n.agnitude, suddenly begins to overgrow its base, and pro duces duces a cloud, which, in regard to both its form and its rapid growth, may be compared to a mushroom." The cumulo-stratus usually prevails in the completely overcast sky. In this it presents appearances not easy to be described, but which may be classed by a due attention to the theory of this cloud. At present it is intended to comprehend under it every mode of union between different ftrata, which is not productive of rain. Future investigation may point out distinctions, which at present we are not prepared to make. This modification is most frequent during a mean elevation Indications. of the barometer, or that which is denominated changeable, when the wind blows from the west, with occasional deviations towards the north and south. In respect to temperature, it has a wide range, and may usher in a fall of snow, as well as a thunder-storm. Of the latter, indeed, it is among the regular harbingers, but with peculiar appearances. During the suffocating calm which prevails before the first discharge of the atmospheric electricity, it may be seen in different points of the horizon, rapidly swelling to a stupen dous magnitude, most curiously wreathed and curled, "fretted and embossed" in its substance, and flanked at different heights by the delicate opake streaks of the cirro-stratus. The whole presents a spectacle of peculiar magnifi cence, in contemplating which one may imagine an invisible agent collecting in this immense laboratory the energies of the storm, and arranging innumerable batteries for the subsequent explosions. It will appear by what we have already stated, that the cumulo-stratus affords in general a doubtful prognostic. When it is formed in the morning, the day often proves fair, though overcast; and if the cirro-stratus has contributed to its formation, there will probably ensue heavy showers on the second or third day. When it subsists a long time, the character of its superior spreading part may be consulted, which, if it be decidedly either that of the cirrostratus, or cirro-cumulus, the usual result of their appearance may be expected. Of the Nimbus. To have a correct notion of this cloud, the reader has only Nimbus de to scribed. to take the opportunity of examining a shower in profile as it approaches from the horizon. He will see the dense gloom, which experience teaches him to regard as a mass of descending rain, losing itself above in a cloud, which commonly spreads in one continuous sheet to a great distance all around the shower; insomuch that while the latter is on the horizon at several miles distance, the edge of the cloud has frequently arrived in the zenith. He will perceive, that this spreading crown of the shower advances regularly before it, and that, whether viewed from a distance or over-head, it exhibits in a greater or less degree the fibrous structure of the cirrus. After the shower has passed over, he will com monly observe the same appearances in the part of the cloud which follows it; and in squally weather he will sometimes be able to repeat these observations on many different showers appearing successively; or at the same time, in different quarters. The term nimbus is intended strictly to de pote no more than this inverted cone of cloud, from which a sudden or dense local shower, whether of rain, snow, or hail, for the difference is not essential in either case, is seen to descend. As it rises to a great height in the atmosphere, it may be seen from a distance of many miles; and so constant is the result of a shower arriving with it, that though, in a few instances, perhaps from the small quantity of the rain, we have not been able to discover the usual obscurity be peath it, while at a distance, we believe it may be laid down as a general tule, on as good grounds as in most other cases, that rain, snow, or hail, is falling on the tract over which it is spread. "Qualis ubi ad terras abrupto sidere nimbus It mare per medium, miseris heu prescia longé So while far off at sea the storm-cloud lowers, There is a great difference, at different times, in the proportion which the inverted cone of cloud bears to the column of rain, &c., in which it terminates; and in a very turbid and |
