clouds are sometimes warmer than the surrounding air; and if the mere coldness of the atmosphere produced a condensation of vapour, then the night should always be attended with fogs or clouds, as the result of the condensation of the vapour raised during the day.,

An extremely high temperature is sometimes continued for a long time without the formation of clouds. It is stated that in January, 1785, the mean temperature during the month was 6610, a temperature that must have greatly aided evaporation, and yet not a drop of rain fell, and the moisture of the air diminished so rapidly that it at last almost disappeared. Such a phenomenon cannot be easily explained.

It is generally admitted that clouds are combinations of aqueous drops, and it is almost universally acknowledged that their parts have a vesicular structure. Saussure states that, when passing over the Alps, he saw a multitude of small globules, like soap-bubbles, the component parts of a cloud, floating before him, being generally about the size of a pea, and apparently covered with an inconceivably thin coating. These particles, being charged with electricity of the same name, repel each other, and they are from this cause prevented from taking a liquid state and falling as rain. M. Pouillet, for whose talents we entertain the highest respect, as one of the best teachers and most discriminating observers in Europe, has stated that electricity is not given off during evaporation without chymical change; but we have made experiments of so decided a character, as to enable us to state that this result is not to be depended on, for we can prove that in every case of evaporation electricity is developed. We shall take this result as though it were proved, leaving the enumeration of our experiments, and the statement of the means by which the fact was ascertained, to another occasion. Now, if electricity is given off during evaporation, and the vapour be formed into vesicles, they must all be similarly electrified, and consequently repel each other; but if there be no truth in the principle we have stated, there are ample means of accounting for the accumulation of electricity from the action of other causes.

CLASSIFICATION of clouds.

In order that meteorologists may compare their observations and results, a system has been adopted by which they are

量

able to class the several kinds or modifications of clouds. There are seven modifications; three of which are simple, two intermediate, and two compound. The following table will form the basis of the remarks we shall make upon the several kinds :

Simple Modifications.

1. CIRRUS; parallel, flexuous, or diverging fibres, extensible by increase in any or in all directions.

2. CUMULUS; convex or conical heaps, increasing upward from a horizontal base.

3. STRATUS; a widely-extended, continuous, horizontal sheet, increasing from below.

Intermediate Modifications.

1. CIRRO-CUMULUS; small, well-defined, roundish masses, in close horizontal arrangement.

2. CIRRO-STRATUS; horizontal, or slightly inclined masses, attenuated towards a part or the whole of their circumference, bent downward or undulated, separate, or in groups consisting of small clouds having these characters.

Compound Modifications.

1. CUMULO-STRATUS; the cirro-stratus blended with the cumulus, and either appearing intermixed with the heaps of the latter, or superadding a wide-spread structure to its base.

2. CUMULO-CIRRO-STRATUS, or NIMBUS, the rain-cloud. A cloud, or system of clouds, from which rain is falling. It is a horizontal sheet, above which the cirrus spreads, while the cumulus enters it laterally and from beneath.

For this classification of clouds, which is sufficient y accurate for the present state of the science of meteorology, we are indebted to Mr. Howard, who has rendered more than ordinary services to this interesting branch of physics. The eye of the casual observer may rest upon the broad expanse of heaven a thousand times, year by year, and in it he may find nothing in form to "stir the soul to ecstasy," much less to induce a philosophical discrimination of causes. But although there may be little to interest the man whose highest aim is the personal satisfaction of his grosser nature, or him who traverses the earth without knowing more of its condi

tion than it knows of itself, and never for an instant imagined the sense of the phrase, "the poetry of nature," yet there is a language in the most fickle of all things, the morning cloud. As the face of man portrays the passions by which the man is governed, and their intensity, so the clouds of heaven give evidence of the causes by which the atmosphere is held in control, and a philosophical knowledge of the influence of those causes enables an individual to prognosticate future results from present appearances.

THE CIRRUS.

This modification of clouds has a less density, and generally a greater elevation, than any other.

Sometimes it

may be seen stretching over the half of the hemisphere, and at other times it may appear as small thin streaks, here and there pencilled upon the clear blue sky. Its duration is as variable as its extent; for, although it will frequently retain the same form for many hours, it does occasionally change in appearance so rapidly as not to be recognised, after a few minutes, as the cloud which was first observed. Its direction is not less various. From the primitive threads which are first wove, others are thrown, some laterally, others pward or downward, some or all becoming in time the

branches of new shoots; while, under some circumstances, transverse lines are formed, which, intersecting the lateral threads, produce a reticulated structure. There is, in fact, no modification that is so various in its extent, duration, and form, as the cirrus; but we think it will be found more constant in all these particulars when formed at great heights, than when at small elevation.

The cirrus has been considered the sign of wind; but it is ever desirable to remember, that when we prognosticate atmospheric changes from the presence or appearance of clouds, we must be careful to consider the circumstances under which the cloud was formed. Horizontal sheets of cirri frequently attend wet weather, ever changing here and there into cirrostratus; and small groups of the same cloud are generally distributed over the sky during fine weather. When the cirrus precedes a storm, it is lower and denser than under any other circumstance; and generally rises in a direction opposite that in which the storm advances.

It is now generally supposed by meteorologists, that the cirrus acts the part of an electric conductor from cloud to cloud, or from one mass of air to another. This supposition is warranted by the form of the cloud; and it is worthy remark, that phenomena usually attributed to electricity frequently attend the long-continued presence of this cloud, when accompanied with dry easterly winds.

The cirrus has been, not inappropriately, called the Proteus of the sky. The variety of form it assumes may possibly at first confuse the student, but a little perseverance, and an attentive examination, will soon enable him to detect it under all the varied forms it presents. It might be desirable to arrange the cirri in different classes, for we are convinced that the several kinds cannot be traced to precisely the same causes, and are not the harbingers of the same results. Dr. Forster appears to have made an arrangement of this kind, and we may be permitted to quote his judicious remarks on the subject. "Of late, by way of distinction, I have used certain specific names for the various forms of each modification. I have called this net-like feature the reticular cirrus. Those which are local and detached, and which ramify in many directions, giving the idea of a distended lock of hair, may be denominated comoid cirri. Sometimes numerous little filaments appear, like bundles of thread,

which I have called filiform cirri. In fair dry weather, with light gales, obliquely descending bands of fibrous texture are often seen, and frequently move slowly along from the leeward in a supervening current. I by no means intend by the above account to infer that the appearances of the different kinds of cirri, or indeed of any cloud, are ever quite uniform; on the contrary, scarcely two occur exactly alike; and there are many features so various and so mixed, that a particular description of each can scarcely be attempted. In some kinds of weather, the numberless and ever-changing figures which this cloud is continually presenting to the eye baffle all attempts at description. Observation affords the only means of becoming acquainted with them." Some such arrangement as that adopted by Dr. Forster is desirable; but it should be entirely founded upon observation, for in such a one all meteorologists might agree.

THE CUMULUS.

The cumulus is a dense hemispherical lump of cloud, rising from a horizontal base, and is generally formed in the lowest regions of the atmosphere. Its first appearance is as a small

irregular cloud, but previous to rain it rapidly increases in size, mass rolls upon mass, often presenting the appearance