moist atmosphere, the character of the upper part often ap proaches more nearly to the cirro-stratus than the cirrits, The more perfectly distinct and local the shower, and the clearer the rest of the air from other clouds, the more pera fect the crown of cirrus, which, indeed, sometimes assumes an almost geometrical precision in its form and internat structure; the threads of the cirrus tending from all sides directly towards the top of the column. cumuli. The pure nimbus commonly moves with the wind, and Increased by from the rapidity of its passage affords but little to the raingauge. But it often happens, that it is formed in the thidst of cumuli, which have already arrived at a great size. In this case the latter may be seen to enter successively into the focus at the top of the column, whence they never emerge; being visibly converted to the purpose of supplying materials for the irrigation, which thus becomes more abundant; and the shower is also occasionally thus propa gated in a direction opposite to the wind. The nimbus, moreover, does not always originate in a Changes, cirrus. The cumulus, and more often the cumulo-stratus, may be seen to expand at their summit into a cirrose sheet; while the lower part is resolved into rain. On the contrary; the rain suddenly ceasing, and the nimbus remaining entire; the sharp extremities of the crown often retire into it; the sides assume the swelling folds, and the character is exchanged for that of cumulo-strátus. When the shower has expanded itself, and the sheets break, the superior portions usually turn to the cirro-cumulus or cirro-stratus, and the lower to the cumulus. When a total evaporation of the re- Indications: maining cloud follows a shower, it is a very favourable prog hostic. A nimbus is frequently accompanied by a cirrostratus or two lying near it, and on a level with the densest part of the cloud. The nimbus of thunder-storms has many of these, as before observed of the cumulo-stratus, arranged at différent heights; which, with the grotesque form of each cloud, and the hazy state of the medium, are sufficiently characteristic of the high electric state of the air at such times, and want only an attentive perusal (in nature) to en= able the observer to ascertain it on future occasions. It ap pears, that the cumulo-stratus passes to the nimbus by a sudden Origin of clouds. sudden change in its electricity: for in tracing the progress of a thunder-storm, through a long range of these clouds in the horizon, we have been satished, that the clouds, which had ceased to afford explosive discharges, had undergone this change in their superior part, and were pouring down rain; while others, among which the lightning stili played, or which were situate beyond it, retained their swelling and rounded forms some time longer. Of the Origin, Suspension, and Destruction of Clouds. These aggregates consist of water, raised by evaporation, and become visible by condensation in the atmosphere. Evaporation. Respecting evaporation, and the state in which vapour subsists, there has been much diversity of opinion: and, of the several theories proposed, there is not one comprehensive enough to merit exclusive adoption. A number of general principles, however, have been established; which we shall employ, with the aid of those of electricity (hitherto not enough considered in its silent and gradual effects), to explain, though in an imperfect manner, the principal phenomena of clouds. Vapour. The air has no on it. Laws of the natural pro cass. Evaporation consists in the union of water with caloric, and the escape of the compound as an invisible fluid, which we shall exclusively denominate vapour. The solvent action of the air, to which this effect has been solvent action attributed by chemical philosophers in general, has been proved by comparative experiments on the force of vapour in air, and with air excluded, to have no perceptible share in it. The laws which govern the natural process, for these alone here interest us, may be thus briefly stated. The force by which water is converted into vapour is directly as its temperature, other things being equal: but this force has to overcome an opposing one, of the same nature, inherent in the vapour which already exists in the atmosphere. For such vapour, by its elastic property, tends to exclude from the space it occupies every additional portion; and consequently to prevent the escape from the water of new vapour. Hence the temperatures being equal, the quantity. of vapour produced will be less, the greater the quantity already diffused in the air. But evaporation. But, though the chemical action of air is imperceptible, Mechanical of Its mechanical effect is great. A moving atmosphere may fect of air on double or triple the rate of evaporation, according to its velocity. For not only is the surface, from which only the vapour escapes, thus enlarged and changed; but the nat cent vapour itself, which would otherwise hover a while upon it, to the obstruction of the process, is immediately brushed away and diffused. nas mens. By applying these principles, we may explain to ourselves Explanation of various natural phenomena: as for instance; why the wind, various photos after rain, becomes colder than even the rain which fell; being robbed of its caloric by the evaporation of the floating and deposited water, with which it is in contact: why snow sometimes totally disappears without melting, and the surface of ice becomes sensibly wasted and chanuelled; for these are warm, compared with the dry and frosty air which blows at such times, and consequently evaporate freely. Ta what manner, again, a strong westerly wind in summer or autumn brings up clouds, which on its cessation descend in rain: for it promotes evaporation by its mechanical effect, and the vapour escapes into an atmosphere already too moist to carry it off to any great distance. This will be evident by recurring to the principle before stated, that the vapour escapes by the force of the temperature of the water out of which it is formed; and, consequently, into a colder atinosphere it will still escape, though continually decomposed thereby. Vapour is decomposed by air, in consequence of the su- Decomposi perior affinity of the latter to caloric. This happens in two tion of vapout, ways. 1. When vapour escapes or is propelled into air colder than itself; the result being a local dense cloud, 2. When a mixture of air and vapour is cooled; in which case there ensues a general turbiness, which we shall exclusively denominate haze. It is occasioned by minute floating pár- Hase. ticles of water; the caloric which, united to these, formed transparent vapour, having passed into the air. Out of this haze clouds may be afterwards formed, by imple aggregation, or by electrical attraction. It abounds in the atmosphere during the most part of the year, occupying sometimes the higher, sometimes the lower, part thereof. VOL. XXX-SEPT. 1911. E The Nature of the Clouds not so The quantity in which it exists may be judged of, at some periods, by the appearance of distant objects seen horizontally at others, by the degree of intensity of the blue colour of the sky, which becomes paler by it, if indeed the blueness is not wholly due to this part of the medium. Of the Nature of the Stratus. This cloud is an example of the decomposition of vapour thrown into air of a lower temperature. The earth or water on which it reposes is always warmer than the cloud, as is also the clear air above. Thus, in a stratus, formed over a field with ponds, the temperature of the earth just below the turf was 57°; of the water, 59°; of the air, at an elevation of thirty feet, 55°; while that of the cloud, at four feet from the ground, was 49.5°. Hence this cloud preserves a level surface; and hence it uniformly vanishes; or begins to be driven upward, as soon as its temperature becomes equal to that of the earth. It is consequently due to the decomposition (in a small portion of the atmosphere) of the vapour which the earth and water continue to emit, after sunset, by the force of a temperature previously acquired. But the change in the lower air, which gives occasion to this local decomposition, is not so easily to be explained: for it appears that very often, in the evening of a clear day, the decrease of temperature in the atmosphere takes place in the same order in which the increase did in the morning: viz. beginning from the surface of the earth and proceeding upward. If the air never became colder, on these occasions, than the contiguous soil, the effect might very well be ascribed to the absorption of a quantity of caloric by the latter. But we see that, in the present instance, it became colder by seven degrees, though vapour was still decomposing: and this in a perfect calm, which, in a great degree, forbids another supposition, of the exchange of a quantity of heated air below, for as much cold air from the higher atmosphere, otherwise this would seem a sufficient account of the matter. The electric charge of the stratus, which is always posigood conductive, and sometimes highly so, notwithstanding the contact of its lower surface with the earth, seems to prove, that a cloud tors as supé posed, cloud is not even so good a conductor as has been supposed, and that the fluid, in certain cases, may be very gradually transmitted through it. Positive electricity being that pro per to the atmosphere in fair weather, we should naturally expect to find it in this cloud. It might be worth while to examine the air above, with a view to discover whether there exists in the latter a negative counter-charge. It will appear, from a consideration of the principles before stated, why this cloud is almost peculiar to the autumn. The gradual decline of the sun, at this season, keeps the atmosphere constantly surcharged with vapour, which is ultimately disposed of in rain; and hence follow gales of wind. The stratus, therefore, though an immedi- Indications, ate indication and accompaniment of fair weather, affords an unfavourable prognostic in the early part of summer; as it shows that a tendency has already begun to extensive precipitation, at a time when the usual predominant feature is increasing dryness. Of the Nature of the Cumulus. cumulus. The heating effect of the sun's rays on the atmosphere is Nature of the greatest near the surface of the Earth, and diminishes gradually in ascending. The diminution proceeds in fair weather at the rate of about one degree for each hundred yards, as appears by observations with the thermometer on stations of known difference in altitude. This inequality appears to give rise to the cumulus, on the same principles as those of the stratus, but the effects are more complicated. Vapour is generated, as before, at the surface of the Earth, but it is thrown into an atmosphere heated by the sun. Here it maintains its elastic state, and, in proportion to the supply from below, the whole quantity existing in the atmosphere is compelled to rise. In doing this, it changes its climate, and arrives among air of a lower temperature, where a portion is continually decomposed, filling the middle region with haze. Of this, small aggregates begin to be formed, the increase of which is at first determined by no particular law. But the aggregate is not in equilibrium with the air. It tends to subside, and in the |