CLOUD, a collection of vapours suspended in the atmosphere.

That the clouds are formed from the aqueous vapours, which before were so closely united with the atmosphere as to be invisible, is universally allowed; but it is no easy matter to account for the long continuance of some very opaque clouds without dissolving, or to give a reason why the vapours, when they have once begun to condense, do not continue to do so till they at last fall to the ground in the form of rain, snow, &c. The general cause of the formation of clouds, it has been supposed, is a separation of the latent heat from the water of which the vapour is composed. The consequence of this separation must be the condensation of that vapour, in some degree at least. In such case it will first appear as a smoke, mist, or fog, which, if interposed betwixt the sun and earth, will form a cloud; and the same causes continuing to act, the cloud will produce rain or snow. But though the separation of this latent heat in a certain degree is the immediate cause of the formation of clouds, the remote cause, or the changes produced in the atmosphere, by which such a separation may be induced, are much more difficult to be discovered. In common observation, we see that vapour is most powerfully condensed by cold substances, such as metals, water, and the like. But cold alone cannot in all cases cause the condensation of the atmospherical vapours, otherwise the nights behoved to be always foggy or cloudy, owing to the vapours raised during the day by the heat of the sun being condensed by the superior coldness of the night. Great rains may happen in very warm weather, when the union of the vapours with the atmosphere ought rather to be promoted than dissolved, if cold were the only agent in their condensation. The serenity of the atmosphere, also, in the most severe frosts, abundantly shows that some other cause besides mere heat or cold is concerned in the formation of clouds and the condensation of the atmospherical vapours.

The electric fluid is now so generally admitted as an agent in all the great operations of nature, that it is no wonder to find the formation of clouds attributed to it. This has accordingly been stated by Beccaria as the cause of the formation of all clouds whatsoever, whether of thunder, rain, hail, or snow. The first, he thinks, are produced by a very great power of electricity, and the others by one more moderate. But though it is certain that all clouds, or even fogs and rain, are electrified in some degree, it still remains a question whether the clouds arc formed in consequence of the vapour of which they are composed being first electrified, or whether they become electrified in consequence of its being first separated from the atmosphere, and in some measure condensed. This has not yet, as far as we know, been ascertained by the experiments of Beccaria, or of any other person; and indeed, notwithstanding the multitude of electrical discoveries which have been made, there seems to be little or no foundation for ascertaining it. Electricity is known to be in many cases a promoter of evaporation; but no experiments have as yet been brought to prove that electrified air parts with its moisture more readily than air that is not electrified: and hence, until the properties of electrified air are further investigated, it is impossible to lay down any rational theory of the formation of clouds upon this principle.

But whether the clouds are produced, or, in other words, the invisible vapours floating in the atmosphere condensed so as to become visible, by means of electricity, or not, it is certain that they do contain the electric fluid in inconceivable quantities, and that many terrible and destructive phenomena have been occasioned by clouds very highly electrified. Perhaps the most extraordinary instance of this kind on record happened in the island of Java in August 1772. On the 11th of that month, at midnight, a bright cloud was observed covering a mountain in the district called Cheribon, and at the same time several reports like those of a gun were heard. The people who dwelt upon the upper parts of the mountain not being able to fly fast enough, a great part of the cloud, almost three leagues in circumference, detached itself under them, and was seen at a distance rising and falling like the waves of the sea, and emitting globes of fire so luminous that the night became as clear as day. The effects of this phenomenon were astonishing. Every thing was destroyed for seven leagues round; the houses were demolished, plantations were buried in the earth, and 2140 people lost their lives, besides 1500 head of cattle, and a vast number of horses, goats, and other animals.

Another instance of a very destructive cloud, the electric quality of which will at present scarcely be doubted, is related by Mr Brydone, in his Tour through Malta. It appeared on the 29th of October 1757. About three quarters of an hour after midnight, there was seen to the southwest of the city of Melita, a great black cloud, which, as it approached, changed its colour, till at last it became like a flame of fire mixed with black smoke. On its approach a dreadful noise was heard, which alarmed the whole city. It passed over the port, and came first on an English ship, which in an instant was torn in pieces, and nothing left but the hulk; part of the masts, sails, and cordage, were carried to a considerable distance along with the cloud.
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The small boats and feluccas that fell in its way. were all broken to pieces and sunk. The noise increased and became more frightful. A sentinel, terrified at its approach, ran into his box; but both he and it were lifted up and carried into the sea, where lie perished. It then traversed a considerable part of the city, and laid in ruins almost every thing that stood in its way. Several houses were levelled with the ground, and it did not leave a steeple standing in its passage. The bells of some of them, together with the spires, were carried to a considerable distance; the roofs of the churches were demolished or beat down. It went off at the north-east point of the city, and demolishing the light-house, is said to have mounted up into the air with a frightful noise, and passed over the sea to Sicily, where it tore up some trees, and did other damage, but nothing considerable, as its fury had been mostly spent at Malta. The number of killed and wounded amounted to nearly two hundred; and the loss of shipping, &c. was very considerable.

The effects of thunder-storms, and the vast quantity of electricity collected in the clouds which produce these storms, are so well known, that it is superfluous to mention them. It appears, however, that even the clouds are not so highly electrified as to produce fatal effects on those who are immersed in them. It is only the discharge of part of their electricity upon such bodies as are either not electrified at all, or not so highly electrified as the cloud, that does all the mischief. We have, however, only the following instance on record, of a person immersed in the body of a thunder-cloud. Professor Saussure and young Mr Jalabert, when travelling over one of the high Alps, were caught among clouds of this kind; and, to their astonishment, found their bodies so full of electrical fire, that spontaneous flashes darted from their fingers with a crackling noise, and the same kind of sensation as when strongly electrified by art.

The height of clouds in general is not great; the summits of very high mountains being commonly quite free from them, as Mr Brydone experienced in his journey up Mount Aetna. But those which are most highly electrified descend lowest, their height being often not more than seven or eight hundred yards above the ground; nay, sometimes thunder-clouds appear actually to touch the ground with one of their edges; yet the generality of clouds are suspended at the height of a mile, or little more, above the earth. Some, however, have imagined that clouds rise to a most incredible and extravagant height. Maignan of Toulouse, in his Treatise of Perspective, gives an account of an exceedingly bright little cloud that appeared at midnight in the month of August, and spread itself almost as high as the zenith. He says that the same thing was also observed at Rome; and thence concludes that the cloud was a collection of vapours raised beyond the projection of the earth’s shadow, and consequently illuminated by means of the sea. This, however, can by no means be credited; and it is much more probable that this cloud owed its splendour to electricity, than to the reflection of the solar beams.

In the evenings after sunset, and the mornings before sunrise, we often observe the clouds tinged with beautiful colours. They are mostly red, sometimes orange, yellow, or'purple, more rarely bluish, and seldom or never green. The reason of this variety of colours, according to Sir Isaac Newton, is the different sizeof the globules into which the vapours are condensed. But this is controverted by Mr Melville, who thinks that the clouds reflect the sun’s light precisely as it is transmitted to them through the atmosphere; which reflects the most refrangible rays in the greatest quantity, and therefore ought to transmit the least refrangible ones, red, orange, and yellow, to the clouds, which accordingly appear most commonly of those colours. In this opinion he was greatly confirmed by observing, while he was in Switzerland, that the snowy summits of the Alps turned more and more reddish after sunset, in the same manner as the clouds; and he imagines that the semitransparency of the clouds, and the obliquity of their situation, tend to make the colours in them much richer and more copious than those on the tops of snowy mountains.

The motions of the clouds, though sometimes directed by the wind, are not always so, especially when thunder is about to ensue. In this case they seem to move very slowly, and often to be absolutely stationary for some time. The reason of this most probably is, that they are impelled by two opposite streams of air nearly of equal strength, by which means their velocity is greatly retarded. In such cases both the aerial currents seem to ascend to a very considerable height; for Messrs Charles and Roberts, when endeavouring to avoid a thunder-cloud in one of their aerial voyages, could find no alteration in the course of the current, although they ascended to the height of four thousand feet from the surface of the earth. In some cases the motions of the clouds evidently depend on their electricity, independently of any current of air whatsoever. Thus, in a calm and warm day, we often sec small clouds meeting each other in opposite directions, and setting out from such short distances, that we cannot suppose any opposite winds to be the cause of such motions. These clouds, when they meet, instead of forming a larger one, become much less, and sometimes vanish altogether; a circumstance undoubtedly owing to the discharge of opposite electricities into each other. This serves also to throw some light on the true cause of the formation of clouds; for if two clouds electrified, the one positively and the other negatively, destroy each other in contact, it follows, that any quantity of vapour suspended in the atmosphere, while it retains its natural quantity of electricity, remains invisible, but becomes a cloud when electrified either plus or minus. A difficulty, however, still occurs, namely, in ’ what manner a small quantity of vapour surrounded by an immense ocean of the same kind of matter can acquire either more or less electricity than that which surrounds it; and this indeed we seem not as yet to have any data to solve in a satisfactory manner.

The shapes of the clouds are likewise undoubtedly owing to their electricity; for in those seasons in which a great commotion has been excited in the atmospherical electricity, we shall perceive the clouds assuming strange and whimsical shapes, which vary almost every moment. This, as well as the meeting of small clouds in the air, and then vanishing upon contact, is an almost infallible sign of thunder.

Besides the phenomena of thunder and rain, the clouds are intimately connected with those of wind, and always assume a particular shape when a strong continued wind is about to ensue; though it is remarkable, that in the strongest winds we often observe them stationary. Sometimes also, on the approach of a cloud, we find a sudden and violent gust of wind arise; while at others, the wind, though violent before, ceases on the approach of a cloud, and recovers its strength as soon as the cloud is past. This connection of the clouds with wind is most remarkable in mountainous countries, where the peaks are sufficiently high to have their tops involved in clouds. There is a very remarkable mountain of this kind at the Cape of Good Hope, from the clouds on whose top, according to the relations of travellers, the winds issue forth as if they had been confined in a bag; and something similar has been observed of mountains in other parts of the world.

The uses of the clouds are evident, as from them proceeds the rain which refreshes the earth, and without which, according to the present system of nature, the whole surface of the earth must be a mere desert. They [7:7:4] are likewise of great use as a screen interposed between the earth and the scorching rays of the sun, which are often so powerful as to destroy the grass and other tender vegetables. In the more secret operations of nature, also, where the electrical fluid is concerned, the clouds bear a principal share, and serve especially as a medium for conveying that fluid from the atmosphere into the earth, and from the earth into the atmosphere; but in doing this, when electrified to a great degree, they sometimes produce terrible effects. This subject will be further treated under the article Meteorology.