tinued production of fresh vapour that the cloud fubfifts and increases, though continually evaporating. When they wholly disappear, it is because the evaporation is not repaired by the formation of fresh vapour. These phenomena are independent of beat and cold, for clouds are fometimes formed fuddenly in the midft of a bot day, and after they have poured down their water, all is clear again. Sometimes they evaporate after fun-fet, gradually vanishing in the calmeft weather, without change of place. The appearances, on the whole, are fuch as would be produced by a large mass of water in violent ebullition, fufpended invifibly in the atmosphere; and the fimilarity in the effect naturally points out an analogy in the caufe, that is, a fource of vapour in the atmosphere.

When it rains, the fource which furnishes vapours produces them in fuch abundance, that the veficles which are formed are driven against each other even in the bofom of the cloud; and not having time either to difperfe or evaporate, they are united; and the water falling to the lowest part, as in foap-bubbles, they are foon burft, and fall as rain. It is to thefe furcharged veficles we must attribute the pendent fringes which are fometimes feen under the clouds towards the horizon, Experience has fhewn, that it rains under thofe clouds; not that these fringes are rain itself, but the veficles which fall by the augmentation of their weight. As drops of rain are formed, the veficles are destroyed.

Lafting rains proceed from ftrata of clouds which cover the whole heavens; and it is thefe that have the greatest connection with the fall of the mercury in the barometer. "The fource of vapours comprehending a ftratum of confiderable extent, the barometer, after it has announced thefe rains, generally rifes, and continues to rife, as long as they

laft.

laft.

This is a fact often obferved, but to us inexplicable. It is no doubt connected in fome way with the primitive caufe of rain, but with that caufe we are unacquainted. The relation of rain with the barometer, is a fubject as obfcure as the caufe of rain itself.

In the midft fometimes of the finest days, and while ordinary fymptoms indicate that the air is dry, and this as well in the vallies as on the mountains, bright heavy clouds appear on an azure ground, announcing fudden rains. Sometimes thefe clouds increase enormously, and defcend; other clouds form about, and unite to them; the air is darkened, as if a curtain was drawn between heaven and earth. From the tops of high mountains, these clouds may be often feen to accumulate rapidly over the plains; while from thefe the azure ground of the heavens difappears, the wind often rifes, and blows from different quarters in a kind of whirlwind, and laftly it pours with rain. As foon as the rain ceafes, the curtain is withdrawn, the calm is reftored, the fun re-appears, and no other veftiges remain of this grand phenomenon but the water that is on the ground.

When the air is difpofed to produce this phenomenon, you will often fee the clouds rifing from the horizon; fometimes from the fide where the wind proceeds, fometimes from other quarters. Often thefe heavy fhowers are partial: fometimes they re-commence at intervals, accompanied with heavy fqualls. Sometimes thefe heavy intermitting fhowers are a prelude of more lafting rains; in which cafe the clouds unite, the wind goes down, and you have one or more fucceffive days of rain.

Of

OF HAIL.

Sudden ftorms, accompanied with hail and thunder, are among the number of phenomena which fhew how ignorant we are of the causes of those that we obferve in the atmosphere. Hail is a fign of a great degree of cold; but what is the immediate caufe thereof? Whence a fubstance, that must require fo intenfe a cold for it's formation, in feafons fo warm as thofe in which hail is chiefly formed? It is fuppofed, in general, that hailftones are drops of rain, which, falling through a colder region of air, are congealed in their pas fage into a rarified fort of ice. Dr. Halley gives an account of hailftones that weighed five ounces each, and fays, it is very extraordinary that fuch fort of vapours fhould continue undifperfed fo long a tract as fixty miles together; and in all the way of it's paffage, occafion fo extraordinary a coa gulation and congelation in the watery clouds, as to increase the halftones to fo vaft a bulk in fo fhort a fpace as that of their fall.

OF THUNDER.

All the phenomena of ftormy clouds are obfcure, and I am afraid there is very little probability of explaining them independent of each

other. Thofe that are
may find enough at the
ducts himself by the "
will find little to depen
thunder and lightning: we.
the immenfe quantities of et
For the rumbling n

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cquired

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It is thus with
her account for

ifcharged by other.

by being capacity for

for the electric fluid, and that thus electricity was continually conveyed to the atmosphere by evaporation; and this he deduced from an experiment, in which water being evaporated from a body, left that body negatively electrified. This, however, is by no means fatisfactory; for, not to insist on the fallacy of the terms pofitive and negative, as both electricities may be produced by evaporation, if the electric fluid paffed from the earth to the atmosphere by evaporation, and it's return was occafioned by the reduction of vapour into water, there would always be more or lefs lightning when there was violent and fudden rain, for in this cafe it would be rapidly difengaged; but there is much oftener violent and fudden rain without than with lightning. In this cafe lightning alfo fhould always be preceded by rain, whereas there is often lightning among the clouds without any rain. Further, if we are unable to explain rain by the vapours which exifted in the air before the formation of the clouds, the fource of electricity exifting in clouds ought not to be fought for in vapour. Indeed, on this fuppofition, as foon as there was a violent rain the lightning would ccafe, and the fluid would pafs off by the drops, illuminating the air by it's paffage from drop to drop.

There feems to be no other mode of confidering lightning, than as an explosion, that is, as a fudden production of a great quantity of the electric fluid; the fluid which is then manifefted not exifting as fuch but juft before we perceive it's effects; just as the vapour, of which the clouds are formed, do not exift as vapour in the air until the moment of their appearance: the air, as yet tranfparent, contained neither the vapour of which the cloud is formed, nor the electric fluids, but the ingredients proper to give birth to both of them. By fome caufe, of which we are ignorant, clouds of

a certain

a certain kind are formed. During the progrefs of their formation, and by fits, the electric fluid is produced in great abundance, and explodes every time it is thus produced. Obfervations made among mountains where clouds are formed, point out this to be the refult of the phe

nomena.

In a ftorm obferved by Mr. de Luc on the Buet, he had an opportunity of observing this phenomenon with all it's modifications. The air of the strata where he was fituated was perfectly transparent and dry; the thermometer at 6 of Reaumur. Notwithstanding this, clouds formed here and there by degrees they augmented, then became united, embracing the fummit of the Buet, and fupporting themselves against Mont Blanc, and the fummits of the neighbouring mountains. Mr. de Luc and his companions were inundated with rain: though the clouds and the rain formed a complete conductor, communicating with the ground, yet there was a continuance of thunder for a confiderable time, and often very violent. Other inftances may be found in the works of Mr. de Sauffure, of thunder-ftorms where the clouds. formed a conducting communication with the ground, and yet where the thunder fucceeded without interruption.

The rumbling noife of thunder has been explained by a fuppofed analogy between the paffage of lightning and the electric fpark through the air. This explanation might have been admitted as plaufible, if the rumbling noife of thunder had grown weaker and weaker, as being a fucceffion of founds proceeding fucceffively from points more and more diftant; whereas the found of thunder often increafes, and gives us a diftinct perception of it's proceeding from points which are nearer to us than thofe from which it fet out.. It is fomeVOL. IV. M m times