flask, open it into the receiver, when a momentary mistiness will be perceived in the flask, which is moisture, condensed into a cloud by the cold caused by rarefaction.

The cloud is here formed under peculiar disadvantages, being every where surrounded, at so short a distance, by al warmer vitreous surface throwing in heat upon it. But a cloud which is visible in so small a quantity, would be pretty dense on the large scale. I have never tried this experiment without succeeding; but I believe it may fail, when the air contains little moisture, if the receiver be not large compared with the flask, or if the connecting stopcock have a very narrow bore. By means of an additional orifice and stopcock to the flask, the air might be renewed in it, and the experiment repeated many times, with one exhaustion of a large receiver. I tried a flask with a minute perforation in its bottom, to which I applied my finger, and repeated the experiment more than twenty times with one exhaustion; but this depends on the state of the air. The heat of the hand, however, is unfavourable to the experiment*.

Enough, I presume, has now been said to warrant us to conclude, that when air ascends sufficiently in the atmosphere, it must (being cooled by dilating) constitute a cloud, or, if. moist enough, produce rain, &c. For example, if a current of air traverse the ocean till it become very moist; and then, if this current, on arriving at the shore, have to rise higher as: the land rises; we have at once the reason why rain usually commences nearer the sea, and extends thenceforward with the wind-why more elevated situations are generally more liable to rain, and also why a wind from the land is more rarely attended with rain on its approaching the shore.

But there is reason to think, that a stream of air, which has been gradually elevated by traversing a rising ground, does

* I have sometimes been conjecturing whether a hygrometer could be formed on these principles; or, by injecting some additional air into a flask, and then producing a cloud by the cold attending the opening of the flask. The latter method readily produces a dense cloud with a large flask-I have never tried it with a small one. Could the least degree of condensation, which would produce a cloud on opening the flask, be readily ascertained, it might afford the means of estimating the moisture in the air.

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not always descend again where the surface does so; but, on the contrary, may, from the upward force it has acquired, even rise higher. In this manner, it may deposit rain whilst rolling far above the tranquil plain, as well as when contending with the asperities of the more elevated surface. If the air over which the raining current passes, be not saturated with moisture, it may absorb or redissolve a part or the whole of the rain descending in it. This is, no doubt, the way in which clouds seem suspended in the air, though in fact they may be falling or floating on with the wind beneath; whilst their place is continually supplied by the successive condensation of other vapour, which, in its turn, is redissolved or swept away by the wind. Much in the same way, clouds are apparently stationed over elevated peaks, or even over large portions of hills; while the fact is, that their particles are moving onward.

The circumstance of clouds frequenting hills, or apparently moving towards them, will admit of a similar explanation, without the aid of an imaginary force residing in hills for attracting clouds and rain. The notion that mountain-caps, or clouds hovering about the tops of hills, are produced by the cooling influence of the summit, seems to me extremely improbable, because such phenomena frequently occur when the air is considerably colder than the surface of the hill; though it is clear they cannot long continue of very different tempe ratures. But were the colder temperature of the hill really the cause, the cloud would not only touch, but be densest next the surface, wetting it profusely; whereas, the cloud is often observed to be several feet or many yards clear of the hill, and the surface as dry, at least, as that of the surrounding country. A more natural explanation, I presume, is, that the cloud is formed in that part of a current of moist air which is sufficiently cooled by the rarefaction attending its sudden increase of elevation in ascending and rolling over the summit; and that, according as this current afterwards passes on to where it absorbs heat or recovers its temperature by increase of pressure, will it regain its transparency. The reason why

* Since such a cloud may apparently remain at rest in the wind, it is manifest that the motion of clouds affords a very doubtful measure of the velocity of the wind.

the cloud is frequently clear of the summit, especially during a brisk wind, is, that the centrifugal force due to the curvature of its course over the hill, carries it clear of the summit; and the intervening arched space is left to be occupied by comparatively still air, into which the air cooled by recent rarefaction scarcely enters.

The origin of the cloud called the cumulus may be traced, by means of its horizontal base, to the rarefaction of air. Such a cloud may be situated partly in an ascending portion of a current of moist air, and partly in a descending portion; or it may sometimes occupy the most elevated part of an arched-like sweep of the current. Whenever the air reaches the requisite elevation, it will become opaque, but will regain its transparency so soon as it descends again sufficiently. The opacity will terminate underneath, nearly all at the same level, or in a horizontal plane, if the heat and moisture have been uniformly or proportionally distributed through the air of the current. When the distribution has been unequal, the base will of course be irregular, or may deviate from a horizontal plane. Some other modifications might, perhaps, be accounted for on similar principles.

There is good reason for supposing that air, which has just been suddenly elevated and dilated, is thereby reduced to a much lower temperature than what obtains in air which has remained at that elevation for some considerable time, receiving heat from below, from the sun, or other sources*. Because, the entire fall of temperature due to such dilatation far exceeds a reduction of 1° F. for every 100 yards of ascent. Indeed, the experiment of the cloud in the flask shows that, if an equal weight of air in the upper regions did not contain far more heat than in the lower, the sky would be perpetually obscured with cloudst. Aqueous vapour, by its superior elas

*Moisture which has ascended in the form of transparent vapour, and descended again as rain, &c., must have left its latent heat above. But much heat no doubt moves upward, from its propensity to render the atmosphere of one temperature throughout its whole height, and from the tendency which warmer air has to rise above the colder.

When the atmosphere is much agitated, and intermixed to a great height, it becomes obscured above, probably from getting colder there, while it becomes warmer and more transparent beneath. In such cases, the clouds often present a very deep blue colour,

ticity, would always shoot up through the air to where it would form a cloud, did the temperature decrease as fast as some suppose it to do. It is true, that transparency might still be aided by rapidly descending currents; and it may be partly owing to these, that portions of the sky become visible when the air is loaded with moisture. I am also aware that, in all this, allowance should be made for the different and perpetually varying conditions of the several strata composing the atmosphere.

A very probable reason why the temperature at the tops of mountains is generally found to be lower than that of air at the same height over the plains, is, that mountains are cooled by, and enveloped in, recently dilated ascending currents of air rolling over them. Snow-clad mountains are, besides, cooled, particularly in dry weather, by the evaporation from the snow. And we may remark, by-the-bye, that, at the same temperature, dry air is, for the above reason, less efficacious in melting snow than moist. Because, the moist air, in place of spending its heat to form vapour, does, in consequence of, its touching a colder body, part with a portion of the latent heat of the vapour it already contained, which greatly aids in liquefying the snow. The melting of snow does not, therefore, depend solely on the temperature of the wind, but likewise upon its being previously charged with moisture. When dry air passes over snow on a high mountain, the evaporation, and consequently the reduction of temperature will be greatly promoted by the diminished pressure which obtains at such heights. This may help to explain why snow winds, as they are called, should be found so intensely cold. The mere circumstance of wind having passed over a cold mountain, is not a sufficient reason why it should be cold after its descent.

The rise of water, in the water-spout, has been long accounted for in a very rational way. The collision of currents of air from different quarters produce a whirlwind. The air near the axis of rotation is rarefied by the centrifugal force. The pressure on the spot under this attenuated air is necessarily diminished; and, of course, when a whirlwind occurs on the sea, &c., the water rises in the axis, on the same principle as in the common pump. But the rarefied air itself ascends, in virtue of its

levity. Its place is supplied by the concourse of the heavier adjacent air, which, in its turn, is rarefied; and, in this manner, an upward current of air is produced, which aids the ascent of the water. Thus far, the explanation is very satisfactory. The other principal part of the phenomenon, the apparent descent of a dense stem from the clouds, nearly over the spot where the water rises, has been ascribed to electricity. But this, in my humble opinion, is rather an evasion than an explanation, and seems nearly allied to the notion that hills attract clouds.

The column which apparently descends from the clouds (for any descent is altogether illusory till water actually fall) may be accounted for in the same way as the mountain-cap, viz. that it is moisture condensed by the cold due to the rarefaction which is occasioned both by the whirling motion of the air and by its rapid ascent. The more rapid the rotation, the greater will be the rarefaction and cold; and, of course, the lower down will the condensation take place. The sound and flashes of light seem to be thunder in miniature.

In attending to this and other atmospherical phenomena, most people are embarrassed with a preconception which is not easily overcome-that clouds are solids, or something much more substantial than the air in which they are formed.

Volta supposed that bodies, while passing into the gaseous form, absorb electricity, which they emit again on being condensed. Objections have been made to this theory; but as they rest, in a great measure, on the assumption that we possess perfect electrometers, they do not convince me that the theory is unfounded. Thunder is unknown in the Polar regions, and rarely takes place any where in cold weather. From which it appears, that thunder does not occur in air incapable of containing much moisture; as is further confirmed by its being ordinarily produced in a dense cloud. The conjecture which I have to throw out is, that when a large mass of warm damp air is suddenly moved upward, it dilates, is cooled, and deposits a considerable share of its moisture, which, in laying aside the gaseous form, parts with electricity and emits lightning. The sound may be partly a tremor, which the air sustains at the moment the pressure is relaxed by the vapour