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 formation of clouds 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 or snow, &c. Under the article BAROMETER, No. 23, we have hinted at the general cause of the formation of clouds, namely, a separation of the latent heat from the water whereof the vapour is composed. The consequence of this separation, as is undeniably proved by Dr Black, 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 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, whereby 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, &c. But cold alone cannot in all cases cause the condensation of the atmospheric vapours, otherwise the nights behoved to be always foggy or cloudy, owing to the vapours, raised throughout the day by the heat of the sun, being condensed by the superior coldness of the night. Great rains will happen in very warm weather, when the union of the vapours with the atmosphere ought rather to be promoted than dissolved, if cold was the only agent in their condensation. The ferocity of the atmosphere, also, in the most severe frosts, abundantly shews that some other cause besides mere heat or cold is concerned in the formation of clouds, and condensation of the atmospheric 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 hath accordingly been given by S. 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 are formed in consequence of the vapour whereof 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 hath not yet, as far as we know, been ascertained by the experiments of Beccaria, or any other person, and indeed, notwithstanding the multitude of electrical discoveries that have lately 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 yet been brought to prove, that electrified air parts with its moisture more readily than such as is not electrified; so that, till 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, i.e., 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 prodigious and inconceivable quantities, and many very terrible and destructive phenomena have been occasioned by clouds very highly electrified. The most extraordinary instance of this kind, perhaps, on record, happened in the island of Java in the East Indies 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 were heard like those of a gun. The people who dwelt on 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 it 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, &c.

Another instance of a very destructive cloud, the electric qualities 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 south-west 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. A dreadful noise was heard on its approach, 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. 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 he perished. It then traversed a considerable part of the city, and laid in ruins almost everything that stood in its way. Several houses were laid level with the ground, and it did not leave one steeple in its passage. The bells of some of them, together with the spires, were carried to a considerable distance; the roofs of the churches demolished and beat down, &c. 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 near 200; and the loss of shipping, &c., was very considerable. See Hurricane, and Whirlwind.

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 these clouds are not so highly electrified as to produce their 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 any person's being immersed in two people involved in the body of a thunder-cloud. Professor Saussure, and young Mr Jalbert, 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 height of summits of very high mountains being commonly quite the clouds. Cloud free from them, as Mr Brydon experienced in his journey up mount Etna: but those which are most highly electrified descend lowest, their height being often not above seven or eight hundred yards above the ground; nay, sometimes thunder-clouds appear actually to touch the ground with one of their edges; but the generality of clouds are suspended at the height of a mile, or little more, above the earth. Some, however, have imagined them to rise to a most incredible and extravagant height. Maignan de Tholoule, in his treatise of Perspective, p. 93, gives an account of an exceeding bright little cloud that appeared at midnight in the month of August, which spread itself almost as far as the zenith. He says that the same thing was also observed at Rome; and from thence concludes, that the cloud was a collection of vapours raised beyond the projection of the earth's shadow, and of consequence illuminated by the beams of the sun. This, however, can by no means be credited; and it is much more probable that this cloud owed its splendor to electricity, than to the reflection of the solar beams.

In the evenings after sun-set, and mornings before sun-rise, we often observe the clouds tinged with beautiful colours. They are mostly red; sometimes orange, yellow, or purple; more rarely bluish; and seldom or ever green. The reason of this variety of colours, according to Sir Isaac Newton, is the different size of the globules into which the vapours are condensed. 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. This 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 usually of those colours. In this opinion he was greatly confirmed by observing, when he was in Switzerland, that the snowy summits of the Alps turned more and more reddish after sun-set, in the same manner as the clouds; and he imagines that the semi-transparency of the clouds, and the obliquity of their situation, tend to make the colours in them much more rich and copious than those on the tops of snowy mountains.

CLOVES, in botany. See Caryophyllus.

CLOVE-TREE, in botany. See Caryophyllus.