But the most fertile source of prognostics of the first class, is to be found in the various and ever changing appearance of the clouds. As the proximate cause of rain or snow, they have in all ages been regarded as affording the surest and most direct intimation of ap proaching changes ; and there are few perhaps who are not conscious of having frequently looked, as it were instinctively, to the appearance of the clouds, in order to form some opinion or conjecture respecting the fu ture state of the weather. At the same time, there are few subjects perhaps on which there exists so great a diversity and vagueness of opinion. Indications drawn from the appearance of the clouds themselves are exceedingly indistinct, unless when accompanied with other circumstances which render them more de finite, such as the colour of the sky at sunrise or sun set, the settling of clouds on the summit of hills, the appearance of mist or fogs at particular periods of the moon's age, &c. and though there arc no doubt certain forms and modifications of clouds, which nine perhaps out of ten weather-wise persons would, without hesita tion, pronounce indications of rain or snow ; yet, if they were required to assign a specific reason for their opinion, scarcely two of them would be found exactly to agree. This uncertainty arises necessarily from the rapid changes which the clouds undergo, as well as the endless variety of forms which they assume, circum stances which seem to preclude the possibility of any thing like classification, or accurate description. An attempt, however, to supply this desideratum, was made a few years ago by Alr. Howard, a philosopher to whose ingenuity and researches the science of me teorology is not a little indebted ; and though his sys tem has been pronounced fanciful, which is perhaps to a certain extent true, it is even in its present state highly deserving the attention of meteorologists, and will in all probability sooner or later become the stand ard nomenclature of the clouds. As this part of our subject is designed to be strictly popular, we shall ab stain at present from saying any thing on the origin and formation of clouds, and simply state the charac ters by which they are distinguished and described in the system referred to.
Mr. Howard defines a cloud, a visible aggregate of minute drops of water suspended in the atmosphere, and includes in this definition every such aggregate. whatever be its position relatively to the observer. Of course he comprehends under the term cloud, what are commonly called mists or fogs, because at a greater distance or elevation they are found to assume all the appearance, as they possess, according to his definition, all the qualities of clouds. He then proceeds to define the various forms and modifications of clouds, which he arranges under the seven following classes.
1. Cirrus.—A cloud resembling a lock of hair, or a feather. Parallel flexuous, or diverging fibi es, mited in the direction of their increase.
2. Cumulus.—A cloud which increases from above in dense, convex, or conical heaps.
3. Stratus.—An extended, continuous, level sheet of cloud, increasing from beneath.
These three, Mr. Howard denominates simple and distinct modifications, constituting, as will immediately appear, the elements of every other variety. The two next are of what he calls an intermediate nature.
4. Ci•ro-cumulus.—A connected system of small roundish clouds, placed in close order, or contact.
5. Cirro-stratus.—A horizontal, or slightly inclined sheet, attenuated at its circumference, concave down ward, or undulated. Groups or patches having these characters.
Lastly, says Mr. Howard, there are two modifications, which exhibit a compound structure, viz.
6. Cumulo-stratus.—A cloud in which the structure of the cumulus is mixed with that of the cirro-stratus, or cirrocumulus. The cumulus flattened at top, and overhanging its base.
7. Nimbus.—A dense cloud, spreading out into a crown of cirrus, and passing beneath into a shower.
We have given these definitions in Mr. Howard's own language, but our limits will afford room only for a very condensed abridgment of his illustrations, and we refer our readers to the treatise itself, published in Tilloch's Philosophical Ilfagazinc, or Nicholson's Journal, vol. xxx. p. 35.
The Cirrus, he observes, is always the least dense, and generally the most elevated modification of clouds, sometimes covering the whole face of the sky with a thin transparent veil, and at other times forming itself into distinct groups of parallel threads, or flexuous fi bres. Its height, according to Mr. Dalton, is from three to five miles above the earth's surface. It is ge nerally found to be an indication of wind. When formed into horizontal sheets, with streamers pointing upwards, it indicates approaching rain ; with depending ing fringe-like fibres, it is found to precede fair wea ther.
The cumulus is generally of dense structure, appear ing after a clear morning, increasing from above, where its surface is convex, and forming, at its greatest mag nitude, -a pile of irregular hemispherical cloud. This takes place early in the afternoon, about the time of the greatest heat, and gradually diminishes towards evening, when it disappears. In this case it is an indication of fine weather.
The stratus is of moderate density, and compre hends those creeping mists which rise from valleys and lakes in calm evenings. It frequently disappears in the morning, and is then an indication of the finest wea ther.
The cirrocumulus appears to be formed by the de scent of the cirrus, the oblique denser tufts of the lat ter changing into the spheroidal form, when the cloud assumes the appearance of a ball of flax with one end left unwound or flying out. The cirrocumulus some times consists of distinct beds, floating at different al titudcs, the clouds appearing smaller and smaller, till they are lost in the blue expanse. It is most frequent in summer, and, when permanent, affords one of the surest indications of increasing temperature, and fine weather.
The cirro stratus assumes various appearances, from its being frequently connected with other modifica tions. By itself it is always an attenuated sheet, or patch, of an uniform hazy continuity when viewed over head, and of great apparent density towards the horizon. In this state it gives rise to the phenomena of halos, mock suns, &c. and indicates depression of temperature, wind, and rain. When it alternates with cirro-cumulus, the prognostic is doubtful. It is fre quently seen resting on the summit of high' hills, and in this state has been long regarded as foreboding raipy weather.