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AQUEOUS VAPOUR-CLOUDS AND MISTS,

· RAIN, DEW, SNOW, HAIL,

When water is exposed to the air, it is gradually converted into vapour, which, on account of its specific levity, ascends into the atmosphere. This vapour presents itself in various forms. When the air holds it in solution, it is invisible, just as salt dissolved in water is invisible ; but where the air is saturated, the watery particles become visible, either in the form of clouds and mists suspended in the atmosphere, or in that of rain, dew, snow, and hạil, falling to the ground,

Clouds and Mists differ only in this, that the former float in the air, whereas the latter extend along the ground. They are understood to consist of a collection of small vesicles or hollow spheres, and to occupy a sort of intermediate state between water and invisible vapour. The causes which produce these vesicles, are not well understood, though change of temperature and electricity have probably the principal share in the transfor. mation. The height of clouds is very various. In ascending to the summits of mountains, the traveller frequently passes through a zone of clouds, and beholds the vesicular vapours of which it is composed, stretched under his feet like a vast plain covered with snow; and even on Chimborazo, the loftiest peak of the Andes, there are always to be seen, at an immense height, certain whitish clouds resembling flakes of wool. These clouds, which are perhaps many miles from the surface of the earth, are supposed to owe their elevation to negative electricity repelling them from the ground, in the same way as mists are supposed to owe their depression to positive electricity attracting them towards it.

Rain falls from the clouds, when the vesicular vapour, of which they are composed, unites into drops. The fall of the drops of rain, after they are formed, is easily accounted for from the attraction of gravity ; but the

cause of the conversion of vesicular vapour into raindrops is not better understood, than the cause of the conversion of vapour into vesicles; though it is highly probable, that electricity is an agent in the one case, as well as in the other. If the change be owing to the diminution of this fluid, we have a ready explanation of the well-known fact, that mountainous are the most rainy countries ; mountains constituting so many points for drawing off the electric fluid. This supposition is further rendered very probable by the fact, that no rain falls in those regions where thunder is unknown, as in the environs of Lima, and on the coast of Peru. The quantity of rain that falls in different regions of the globe, is very different. It is most abundant within the torrid zone, and decreases in proportion to the distance from the equator. The annual fall at Grenada, in 12° N. lat. is 126 inches; at Calcutta, in 22° N. lat. it is 81 inches ; at Rome, in 41° 54", it is 39 inches; in England, 32 inches; and at Petersburg, in lat. 59° 16", it is only 16 inches. Even in different places in the same country, the quantity that falls is different. But the most curious fact of all, in the natural history of rain, is the difference of quantity, which is collected at different heights at the same place. In one year, a rain-gauge on the top of Westminster Abbey, received 12 inches ; another on the top of a house in the vicinity received 18 inches; and a third on the surface of the ground received 22 inches.

Dew, or the moisture insensibly deposited from the atmosphere on the surface of the ground, is a wellknown phenomenon. It was long supposed, that its precipitation was owing to the cooling of the atmosphere towards evening, which prevented it from retaining so great a quantity of watery vapour in solution, as during the heat of the day. But it has been recently proved, that the deposition of dew is produced by the cooling of the surface of the earth, which takes place previously to the cooling of the atmosphere. The earth is an excellent radiator of caloric, whilst the atmosphere does not possess that property in any sensible degree. Towards evening, therefore, when the solar heat declines,

and after sunset, when it entirely ceases, the earth rapidly cools by : radiating heat towards the skies ; whilst the air has no means of parting with its heat, but by coming in contact with the cooled surface of the earth, to which it communicates its caloric. Its solvent power being thus reduced, it is unable to retain so large a portion of watery vapour, and deposits those pearly drops called dew. This view of the matter explains the reason why dew falls more copiously in calm than in stormy weather, and in a clear than in a cloudy atmosphere. Accumulations of moisture in the atmosphere not only prevent the free radiation of the earth towards the upper regions, but themselves radiate towards the earth; whereas, in clear nights, the radiation of the earth passes without obstacle through the atmosphere to the distant regions of space, whence it receives no caloric in exchange. The same principle enables us to explain the reason, why a bottle of wine taken fresh from the cellar, (in summer particularly,) will soon be covered with dew. The bottle, being colder than the surrounding air, absorbs caloric from it; the moisture therefore, which that air contained, becomes visible, and forms the dew, which is deposited on the bottle. In like manner, in a warm room, or in a close carriage, the inside of the windows is covered with vapour, because the windows being colder than the breath, deprive it of part of its caloric, and by this means convert it into watery vapour. Bodies attract dew in proportion as they are good radiators of caloric; as it is this quality which reduces their temperature below that of the atmosphere. Hence we find, that little or no dew is deposited on rocks, sands, or water; while grass and living vegetables, to which it is so highly beneficial, attract it in abundance; a remarkable instance of the wise and bountiful dispensations of Providence. The same benevolent design we may observe, also, in the abundance of dew in summer and in hot climates, in which its cooling effects are so much required. The more caloric the earth receives during the day, the more it will radiate afterwards; and consequently, the more rapidly its temperature will be reduced in the

evening, in comparison with that of the atmosphere. In the West Indies, accordingly, where the intense heat of the day is strongly contrasted with the coolness of the evening, the dew is prodigiously abundant. When dew is frozen the moment it falls, it gets the name of hoar-frost.

Snow is another of the forms which the vapours of the atmosphere assume. It consists of aqueous vapour, congealed either while falling, or when in the air previous to falling. The first crystals, produced at a great height in the atmosphere, determine, as they descend, the crystallization of aqueous particles, which, without their presence, the surrounding air would retain in a state of solution. The result is the formation of hexagonal darts, or stars of six rays, when the weather is sufficiently calm, and the temperature not too high to deform the crystals by melting off their angles; but when the atmosphere is agitated, and the snow falls from a great height, the crystals clash together, unite in groups, and form irregular flakes.

Hail, according to all appearance, is a species of snow, or of snowy rain, which has undergone a variety of congelations and superficial meltings, in its passage through different zones of the atmosphere, of different temperatures. Its formation evidently depends on electricity. It is by an electrical apparatus, that we can produce artificial hail; and it is well known, that volcanic eruptions are often followed by the fall of hailstones of enormous size.

Such are the principal circumstances which are supposed to concur in the formation of aqueous meteors. Their beneficial influence upon the earth is a point more easy to determine. We observe all nature languish, when the atmosphere retains, for too long a time, the moisture arising from the earth. Plants fade, and droop; animals feel their strength failing them; man himself, breathing nothing but dust, can with difficulty procure shelter from the sultry heat, by which his frame is parched and overpowered. But scarcely have the waters of heaven descended from the clouds, when all living beings begin to revive; the fields resume their

green attire; the flowers their lively tints, animals the sportive freedom of their motions, and the elements of the air their healthful equilibrium. Snow itself, whose very name alarms the natives of the tropics, is productive of real advantages in the economy of nature : it secures the roots of plants against the effects of intense cold; it serves to moisten gently those lands, from which, owing to their local situation, the rain is too soon carried off; and it paves for the inhabitant of the north, commodious and agreeable roads, along which he gaily skims in his light and nimble sledge. Hail alone, of all the aqueous meteors, never appears but as a harbinger of distress. Birds and quadrupeds instinctively conceal themselves, as soon as they have any presentiment of its coming. Man can neither foresee its approach, nor arrest its ravages; he has been able to ward off the thunderbolts of the sky, but he sees the hail destroy his corn, break his fruit trees, and shatter the very house where he dwells, without being able to prevent it.

M‘Culloch's Course of Reading.

ON THE DELUGE.

It stands on record in Scripture, that this globe was twice enveloped in water; once, when God by his work of six days, described in the first chapter of the book of Genesis, raised it up from what is usually called its chaotic state ; and a second time in the days of Noah. Now, the effects of these two immersions of the earth in water are distinctly marked in the present form of it.

In regard to the first, it is a vulgar error, to which the Scripture gives no countenance, that the earth was first brought into existence when God commenced his six days' work. A more careful reading of the narrative will convince you, that this work was merely

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