with each other, have often only the common resemblances which exist among trees, hills, and lakes, taken generally.

There are three simple and distinct modifications, which are thus named and defined.

1. Cirrus. Def. Nubes cirriformis tenuissima, quæ un- Cirrus. dique crescat.

The Cirrus. A cloud resembling a lock of hair, or a fea ther. Parallel flexuous, or diverging fibres, unlimited in the direction of their increase.

2. Cumulus. Def. Nubes densa cumulata, sursum cres- Cumulus

cens,

The Cumulus. A cloud which increases from above in dense, convex, or conical heaps.

3. Stratus. Def. Nubes strata, aquæ modo expansa, de❤ Stratus. ersum crescens.

The Stratus. An extended, continuous, level sheet of

cloud, increasing from beneath.

There are two modifications, which appear to be of an intermediate nature; these are:

4. Cirro-cumulus. Def. Nubecule subrotundæ connexæ Cirro-cumu

vel ordinatè positæ.

The Cirro-Cumulus. A connected system of small round

ish clouds, placed in close order, or contact.

lus.

5. Cirro-stratus. Def. Nubes extenuata, sub-concava Cirro-stratus.

vel undulata. Nubeculæ hujusmodi appositæ.

The Cirro-stratus. A horizontal or slightly inclined sheet,

attenuated at its circumference, concave downward, or undulated. Groups or patches having these characters.

Lastly, there are two modifications, which exhibit a compound structure, viz.

tus.

6. Cumulo-stratus. Def. Nubes densa, quæ basi cu- Cumulo-stra. muli structuram patentem cirro-strati, vel cirro-cumuli superdat.

The 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. Def. Nubes densa, supra patens et cirrifor, Nimbus, is, infra in pluviam abiens.

The

Cirrus de scribed.

The Nimbus. A dense cloud, spreading out into a crówn of cirrus, and passing beneath into a shower.

Of the Cirrus.

This is always the least dense, and commonly the most elevated modification. It is sometimes spread horizontally through a vast extent of atmosphere; the whole breadth of the sky being insufficient to show where it terminates. In this case, its parallel bars appear, by an optical deception, to converge in opposite points of the horizon. At others, it is exhibited in unconnected perpendicular bundles, of the most minute size. Between these extremes, it may be traced in Its formation. every degree of extent and inclination to the horizon. In a serene sky the cirrus is first indicated by a few threads, pencilled in white, on the azure ground. Its increase takes. place in various ways, and may be compared sometimes to vegetation, more often to crystallization. Thus, 1. Parallel threads are added to each other horizontally, and occasionally other strata of the same, crossing the first at right or oblique angles, until a delicate transparent veil is formed. 2. Parallel threads are collected into distinct groups, lying at various angles with the horizon. 3. Flexuous and diverging fibres are extended from the original stem, forming the resemblance of crests of feathers, locks of hair, &c. 4. The first formed threads, become, as it were, the supports from which others bliquely ascend or descend into the atmosphere. Lastly, A dense nucleus is sometimes formed, and short fibres shoot out from it in all directions."'. The great elevation of the cirrus has been ascer ained by geometrical observations. "The small white streaks of condensed vapour, which appear on the face of the sky, İ have found," says Dalton, "by several careful observations, to be from three to five miles above the Earth's surface."

éry lofty.

Viewed from the summits of the highest mountains, they appear as distant as from the plains. A more easy and not less convincing proof of their elevation may be deduced from their continuing to be tiged by the sun's rays in the evening twilight with the more vivid colours of the prism, while the denser clouds, having already passed through the same gradation, are in the deepest shade,

The

The duration of this cloud varies according to its staton Its duration, in the atmosphere, and the presence or absence of other clouds: it is long, extending sometimes to thirty-six hours, when it appears alone, and at its greatest elevation; but shorter, or even very transient, when formed lower, aud in the vicinity of the cumulus.

By an inexperienced observer the cirrus would be pro- Its motion. nounced absolutely motionless. On comparison with a fixed

object, however, it is sometimes found to have a consider

able progressive motion. The propagation of the cirrus, Its connexion and the variable directions of its flexures, merit attentive with the wind. observation, as being intimately connected with the varia

tions of the wind, although undoubtedly not produced by the mere motion of the air,

The general principles, which the imperfect notice his

therto bestowed on it seems to point out, are the following:

1. Its appearance is a general indication of wind; and Indications it is most conspicuous and abundant before storms.

2. It is often a leeward cloud; or, when a group of cirri appears on the horizon, it seems to invite a current towards it: and the wind very often shifts into that quarter towards which the points are directed.

3. Horizontal sheets of the cirrus, more particularly those which carry streamers pointing upward, are among the indications of rain approaching, while the fringe-like depending ones are found to precede fair weather.

Of the Cumulus.

from it.

Clouds in this modification are commonly of dense Cumulus destructure. They are formed in the lower atmosphere; and scribed. move with the wind, or more properly with that current which flows next the Earth. The phenomena of the cumulus are usually these: In the latter part of a clear morning, Its formation. a small irregular spot appears suddenly at a moderate elevation. This is the nucleus, or commencement of the cloud, the upper part of which soon becomes convex and well defined, while the lower continues irregularly plane. On the convex surface the increase visibly takes place, one heap or protuberance succeeding another, and again losing itself in a subsequent one, until a pile of cloud of an irregular hemispherical

Decrease,

Indication.

Stratus described.

Indication.

hemispherical form is raised; which floats along, presenting its apex to the zenith, while the base, or rather, the lower surface of the baseless fabric, continues parallel to the horizon.

When these clouds are of considerable magnitude, they remain at proportionably great distances. When smaller, they croud the sky by a nearer approach to each other. In each case the bases range in the same plane; and the increase of each keeps pace with that of its neighbour, the intervening space remaining clear.

The cumulus often arrives at its greatest magnitude early in the afternoon, when the temperature of the day is at its maximum. As the sun declines, it gradually decreases, retaining its character till towards sun-set, when it is more or less hastily broken up, and evaporates, leaving the sky clear, as in the early part of the morning. Its tints are often vivid, and pass through the most pleasing gradation during this last hour of its existence.

The preceding phenomena form the history of the pure cumulus, as it may be termed, when no other modification appears along with it. They are both the accompaniments and prognostics of the fairest weather.

Of the Stratus.

The stratus has a moderate degree of density. It is the lowest of the modifications, being formed in contact with the earth or water. It comprehends those level creeping mists, which, in calm evenings, spread like an inundation from the valleys, lakes, and rivers, to the higher ground.

Unlike the cumulus, which belongs to the day, and rarely survives the setting sun, this cloud accompanies the shades of night, and componly vanishes before the ascending luminary. The evaporation commences from below. At the moment of the separation of the stratus from the Earth, its character is changed, and it puts on the appearance of the nascent cumulus.

The nocturnal visits of the stratus have been always held a presage of fair weather. Thus Virgil:

"At nebula magis ima petunt, campoque recumbunt."

Then mists the hills forsake and shroud the plain.

The

The meteorological axioms of this great poet were probably selected from the popular ones of his age, as confirmed by his own experience. Hence they ever agree with that of his readers. There are few days in the whole year more calm and serene than those the morning of which break out through the stratus. They are the halcyon days of our autumn: an interval of repose between the equinoctial gales and the storms of winter.

Of the Cirro-cumulus.

may be seen

The intermediate nature of this cloud may be ascertained Cirro-cumulu describedby tracing its origin, as well as inferred from its structure. The cirrus, in its slow descent through the air, to pass into this and the next modification; although its previous appearance does not seem absolutely necessary to the production of either.

Most of our readers will recollect the appearance of the icy efflorescences on the panes of windows, gradually melting into an assemblage of drops, which adhere to the glass, retaining somewhat of the same figure, deprived of its right lines and angles. Such is the change of form which the cirrus undergoes, in passing to the state of the cirro-cumulus. And, as the water on the windows is occasionally converted again into spicule of ice, so these small rounded masses sometimes suddenly resume the forms of the cirrus. In the oblique denser tufts of the latter, the change to the spheroidal form often begins at one extremity, and proceeds gradually to the other, during which the cloud resembles a ball of flax, with an end left unwound and flying out. All the cirri in the same group, and frequently all those in view, observe the same law in these changes.

The cirro-cumulus forms a very beautiful sky. Numerous distinct beds are sometimes seen floating at different altitudes, which appear to consist of smaller and still smaller clouds, as the eye traces them into the blue expanse. It is Indications; most frequent in summer; is the natural harbinger of increased temperature; and, consequently, one of the best indications of fair weather, when permanent or frequently repeated. A more transient display of it is, however, frequent in the interval of warm showers, and in winter. There

are