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press equally in all directions applies to elastic fluids as well as to liquids ; therefore, every square inch of our bodies sustains a pressure of fifteen pounds, and the weight of the whole atmosphere may be computed by calculating the number of the square inches on the surface of the earth, and multiply them by fifteen.

The weight of a small quantity of air may be ascertained by exhausting the air from a bottle, and weighing the bottle thus emptied. Suppose that a bottle, six cubic inches in dimension, weighs two ounces; if the air be then introduced and the bottle re-weighed, it will be found heavier by two grains, showing that six cubic inches of air (at a moderate temperature) weigh about two grains. In estimating the weight of air, the temperature must always be considered, because heat, by rarefying air renders it lighter. The same principle, indeed, applies almost without exception, to all bodies. In order to ascertain the specific gravity of air, the same bottle may be filled with water, and the weight of six cubic inches of water will be 1515 grains ; so that the weight of water to that of air is about eight hundred to one.

Thus, within the limits of from twenty-eight to thirty degrees on each side of the equator, the movements of the atmosphere are carried on with great regularity: but beyond these limits, the winds are extremely variable and uncertain, and the observations made have not yet led to any satisfactory theory by which to explain them. It appears, however, that beyond the region of the trade winds, the most frequent movements of the atmosphere are from the south-west, in the north temperate zone. This remark must be limited to winds blowing over the ocean, and in maritime countries; because those in the interior of continents are influenced by a variety of circumstances, among which the height and position of chains of mountains are not the least important. These south-west and north-west winds of the temperate zone, are most likely occasioned in the following manner :- In the torrid zone there is a continual ascent of air, which, after rising, must spread itself to the north and south in an opposite direction to the trade winds below : these upper currents, becoming cooled above, at last descend, and mix themselves with the lower air; part of them may perhaps fall again into the trade-winds, and the remainder, pursuing their course towards the poles, may occasion the north-west and south-west winds, of which we have been speaking. It has also been conjectured that these winds may frequently be caused by a decomposition of the atmosphere towards the poles, from part of the air being at times converted into water.

Hurricanes have been supposed to be of electric origin. A large vacuum is suddenly created in the atmosphere, into which the surrounding air rushes with immense rapidity, sometimes from opposite points of the compass, spreading the most frightful devastation along its track, rooting up trees, and levelling houses with the ground. They are seldom experienced beyond the tropics, or nearer the equator than the 9th or 10th parallels of latitude; and they rage with the greatest fury near the tropics, in the vicinity of land or islands, while far out in the open ocean they rarely occur. They are most common among the West India islands, near the east coast of Madagascar, in the Islands of Mauritius and Bourbon, in the Bay of Bengal, at the changing of the monsoons, and on the coasts of China. Whirlwinds sometimes arise from winds blowing among lofty and precipitous mountains, the form of which influences their direction, and occasions gusts to descend with a spiral or whirling motion. They are frequently, however caused by two winds meeting each other at an angle, and then turning upon a centre. When two winds thus encounter one another, any cloud which happens to be between them, is of course condensed and turned rapidly round ; and all substances sufficiently light are carried up into the air by the whirling motion which ensues. The action of a whirlwind at sea, occasions the curious phenomenon called a waterspout.

LESSON X.

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 atmospheres, or in that of rain, dew, snow, and hail, 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 transformation. 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 vapour, 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

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 time. In one year, a rain guage 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. 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 lieight, 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 hail-stones of enormous size.

LESSON XI.

THE OCEAN. The vast body of water which surrounds the continents, and is the common receptacle of their running waters, is indispensably necessary to the support of animal and vegetable existence upon the earth. Its perpetual agitations purify the air; and the vapours which the atmosphere draws from its surface being condensed and dispersed through the upper regions, form clouds, which are the source of a constant supply of rain and moisture to the land. The ocean also, by the facilities for communication which it offers, is the means of uniting the most distant nations; while it enables them to interchange, with mutual advantage, the productions of their several climates.

The bottom of the sea appears to have inequalities similar to those on the surface of the continents; the depth of the water is, therefore, extremely various. There are vast spaces where no bottom has been found; but this does not prove that the sea is bottomless,

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