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moisture, the compound, which will have the temperature of 40°, will not be capable of retaining all the moisture which was contained in the two before their union; and, consequently, the quantity which they are unable to retain, is condensed or precipitated. This, from the rapidity with which the capacity for moisture diminishes with the diminution of temperature, may take place, though one, or even both of the currents of air, be not completely saturated; and thus clouds may be formed, and rain may fall even although the hygrometer may indicate a certain degree of dryness in the air.

When different portions of the atmosphere are intermixed, so as to produce a deposition of moisture, the consequence will be, the formation of a cloud. This cloud, from its increased specific gravity, will have a tendency to sink downwards; and were the lower strata of the atmosphere of the same temperature with the cloud, and saturated with moisture, it would continue to descend till it reached the surface of the earth in the form of rain, or, what is commonly called mist. In general, however, the cloud in its descent passes through a warmer region, where the condensed moisture again passes into vapour, and, consequently, ascends till it reach a temperature sufficiently low to recondense it, when it will begin again to sink. This oscillation will continue till the cloud settles at the point where the temperature and humidity are such, as that the condensed moisture begins to be dissipated, and which is found, on an average, to be between two or three miles above the surface of the earth.

When the condensation of moisture is rapid and copious, there appears to be no reason to doubt that rain will be immediately produced. Some philosophers, indeed, have maintained that such a condensation, however rapid, never can produce any thing but a cloud, and that the production of rain is the consequence of certain electric processes that afterwards take place among the minute particles of which the cloud is composed. That electricity is a frequent and powerful agent in the formation of rain is extremely probable; but that it is, in all cases, essential to that phenomenon, appears to be a gratuitous assumption, which, however ingeniously supported, is still destitute of satisfactory proof. The well-known fact, that the rain which accompanies a thunder-storm, is more copious than in any other circumstances, is evidence sufficient that it is frequently modified or increased by the influence of electricity; but this is all that is certainly known upon the subject, and we deem it unnecessary, therefore, to enter on any particular examination of the hypothesis, that the particles of moisture in a cloud, are held at a distance from each other by certain electrical atmospheres; and that it is only by the removal of these that the particles unite, and form drops of rain.

The atmospherical currents, whether those produced by general motions, by the sea and land-breezes, or by any other cause, have a constant tendency to the formation of clouds, and it not unfrequently happens that the ARTS, No. 4.

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moisture which falls is actually derived from a current moving in a direc tion opposite to the wind which blows on the surface of the earth, and carries the cloud after its formation. Thus, the west wind is, in the British islands, the one which is most generally saturated with moisture, and yet rain most frequently falls through the action of a cold, and comparatively dry, east wind, blowing along the earth's surface.

It is but recently that any attempt has been made to arrange the clouds into classes, and give them descriptive names. This has been done by Mr. Howard, who distinguishes them into the different classes of Cirrus, or curl-cloud; Cirro-cumulus, or sonder-cloud; Cirro-stratus, or wave-cloud; Cumulus, or staken-cloud; Cumulo-stratus, or twain-cloud; Stratus, or fall-cloud; and Nimbus, or rain-cloud.

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The light portions, No. 1, toward the top of this cut, are representations of Cirrus: it is a light fleecy cloud which floats at a great height in the atmosphere, and has often the appearance of locks of hair. No. 2, in the same cut, is a representation of Cirro-strata: this cloud is remarkable for its flatness, the great length to which it extends, and the constant changes of its form. No. 3, toward the bottom of the cut, are specimens of Cumulus, which, from its being piled up of great masses, gets the name of stacking-cloud, as it were a cloud stacked or built up of many portions.

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Figure 1, in this cut, is a representation of Cirrus passing into Cumulus'; and 2, in the same, is a representation of Cumulus, in that agitated form which prevails during thunder, passing into Nimbus, a rain cloud, and discharging forked lightning.

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Fig. 1, in this cut, is a specimen of Cumulo-stratus, against which is a light Nimbus, with rain-bows. No. 2, is a representation of Stratus, or fall-cloud.

Cirrus is generally the most elevated of clouds; it forms in the higher

regions of the atmosphere in threads, or like net-work, and is always light and white in its appearance. The formation of it is often the first indication of a change to foul weather, more especially when it takes that form which is called the mares'-tail cloud.

Cumulus is called the cloud of day, from the fact of its very frequently forming about sun-rise, and clearing away in the evening. When the cumuli are regularly formed in the morning, firm and permanent in their masses, and melt away after sun-set, the electric state of the atmosphere is generally the same as that of the earth, and there is no tendency to change of weather; but when the cumuli are constantly altering their shapes, and especially when they are veering in opposite directions, showers and varying gusts of wind are the result. The cumuli appear to be produced by the ascent of the lower strata of the atmosphere into a colder region by the daily action of the sun; and their disappearance at night seems to be owing to the descent of the colder stratum of air.

Stratus, or fall-cloud, gets its name from its resting upon the surface of the earth; it consists of those fogs and mists which fill the valleys and cover the surfaces of lakes and damp places in the evening, more especially in summer, remain during the night, and melt away again in the morning. Stratus is called the cloud of night. In autumn, and the early part of winter, when the difference of temperature in the air over the earth's surface is very great between the night and the day, stratus appears in great abundance, and in low and moist situations it often continues during the whole day, or for days or weeks together. The fogs of London are a very striking instance of stratus, and serve to explain its cause-the surface of the earth being colder than the atmosphere over it; for the smoke and other sources of artificial heat communicate a considerable degree of temperature to the atmosphere, which, as it comes in contact with the cold earth, deposits its humidity in very minute particles. In mountainous countries, cirro-stratus often puts on nearly the same appearance as fall-cloud; a belt of cloud is often seen crossing a mountain near its summit, which, when one approaches it, proves to be a dense mist. There is this difference, however, between it and stratus, that the cirro stratus always precipitates moisture, and wets one as certainly as a shower of rain, while the stratus is often dry. When in this state, the stratus always indicates an excess of electricity, as compared with the atmosphere; and for this reason, no doubt, the particles, of which it consists, are repelled by the earth. Indeed, this may be one cause of its formation, inasmuch as almost the same circumstances which under one electric state produce stratus, produce dew under another.

Cirro-cumulus is composed of a number of small clouds united together. It occurs before storms, more especially before thunder-storms; and, between one part of it and another, lightning is often seen playing in a very beautiful manner, and without any thunder, or hazard, or, indeed, con

nexion with the earth. Cirro-cumulus is sometimes formed originally, that is, without the previous appearance of any lighter cloud, and sometimes it is produced from cirrus or cirro-stratus. That fine dappled sky, which is styled mackarel back, and which obtains in fine summer evenings, is occasioned not by cirro-cumulus but by cirro-stratus. Cirro-stratus is a light cloud, and in it those appearances called halos, mock suns, and other distortions or reflections of the celestial bodies, which are pretty certain indications of storms, are generally formed.

Cumulo-stratus is formed by the floating masses of the cumuli being arrested, as it were, in their progress, and uniting at their bases; and the upper part of it is very often mixed with cirrus or cirro-stratus. It thus becomes a much denser species of cloud than any of those formerly mentioned, and when it accumulates in great quantity, it has an exceedingly black colour, unless when the edges are turned toward the sun, and then it is exceedingly white. Electricity appears always to have less or more to do with the collecting of cumulo-stratus. The different cumuli seem to have different states of electricity, and are thus attracted together, and when they are in this way strongly excited, great changes of temperature take place in the mass, various portions of it reel about in a singular manner, and thunder, or large drops of rain or hail is the result. Sometimes cumulo-stratus again dissolves and passes off both with and without thunder; but the most general result is its change into the ulterior form of nimbus, or rain cloud.

Nimbus is the only species of cloud in which the collected moisture has acquired too much density for being suspended in any part of the atmosphere, and when it is once formed, it invariably falls upon the earth in the form of rain, or hail, or snow. The size of the drops of rain appears to increase with the height from which they fall, and the force with which they strike the earth increases with these jointly. Rain usually falls in largest drops during partial showers, and when the weather is comparatively warm. Hail also happens and the hail-stones are large, in proportion to the heat at the time; while the particles of snow are always more minute in proportion as the weather is colder. Thus, in very cold latitudes, there seldom or never falls any hail; in latitudes a little warmer, hail is more frequent in the latter part of spring than during the winter; and as the latitude is warmer, the hail-stones increase in size, so that those which fall in the warmer countries of Europe are often very destructive to vegetation by the force with which they strike.

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The causes of all these circumstances are very obvious: when drops of rain fall for a considerable height through the atmosphere, they unite in their descent; when the cold is very intense, the particles of water are crystallized into minute pieces of ice at the very instant of their formation, and this being the case, they have little or no tendency to stick together, though they touch one another in their descent. If, however, the cold be

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