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it presses with a weight of about 15 pounds on every square inch of the earth's surface; and, therefore, its pressure on the body of a middle-sized inan, is equal to about 32,000 pounds, or 14 tons avoirdupois, a pressure which would be insupportable, and even fatal, were it not equal on every part, and counterbalanced by the spring of the air within us. The pressure of the whole atmosphere jupon the earth is computed to be equivalent to that of a globe of lead, 66 miles in diameter; in other words, the whole mass of the air, which surrounds the globe, compresses the earth with a force or power equal to that of five thousand millions of millions of tons. This amazing pressure is, however, essentially necessary for the preservation of the present constitution of our globe, and of the animated beings which dwell on its surface. It prevents the heat of the sun from converting water, and all other fluids into vapour; and preserves the vessels of all organized beings in due tone and vigour. Were the atmospherical pressure entirely removed, the elastic fluids contained in the finer vessels of men and other animals would inevitably burst them, and life would become extinct; and most of the substances on the face of the earth, particularly liquids, would be dissipated into vapour.
Besides these, the atmosphere possesses a great variety of other admirable properties, of which the following may be mentioned. It is the vehicle of smells, by which we become acquainted with the qualities of the food which is set before us, and learn to avoid those places, which are damp, unwholesome, and dangerous. It is the medium of sounds, by means of which knowledge is conveyed to our minds. Its undulations, like so many couriers, run for ever backwards and forwards, to convey our thoughts to others, and theirs to us, and to bring news of transactions which frequently occur at a considerable distance. · A few strokes on a large bell, through the ministration of the air, will convey signals of distress, or of joy, in a quarter of a minute, to the population of a city containing a hundred thousand inhabitants. It transmits
to our ears all the harmonies of music, and expresses every passion of the soul : it swells the notes of the nightingale, and distributes alike to every ear the pleasures, which arise from the harmonious sounds of a concert. It produces the blue colour of the sky, and is the cause of the morning and evening twilight, by its property of bending the rays of light, and reflecting them in all directions. It forms an essential requisite for carrying on all the processes of the vegetable kingdom, and serves for the production of clouds, rain, and dew, which nourish and fertilize the earth. In short, it would be impossible to enumerate all the advantages we derive from this noble appendage to our world. Were the earth divested of its atmosphere, or were only two or three of its properties changed or destroyed, it would be left altogether unfit for the habitation of sentient beings. Were it divested of its undulating quality, we should be deprived of all the advantages of speech and conversation, of all the melody of the feathered songsters, and of all the pleasures of music ; and, like the deaf and dumb, we could have no power of communicating our thoughts but by visible signs. Were it deprived of its reflective powers, the sun would appear in one part of the sky of a dazzling brightness, while all around would appear as dark as midnight, and the stars would be visible at noon-day. Were it deprived of its refractive powers, instead of the gradual approach of the day and the night, which we now experience, at sun-rise we should be transported, all at once, from midnight darkness to the splendour of noonday; and, at sun-set, should make a sudden transition from the splendours of day to all the horrors of midnight, which would bewilder the traveller in his journey, and strike the creation with amazement. In fine, were the oxygen of the atmosphere completely extracted, destruction would seize on all tribes of the living world, throughout every region of earth, air,
A change in the temperature of a portion of air; an increase or a diminution of the quantity of water, which it holds in a state of vapour; in short, any circumstance which causes it either to contract or expand, destroys the equilibrium among the different parts of the atmosphere, and occasions a rush of air, that is, a wind, towards the spot where the balance has been destroyed. Winds may be divided into three classes : those, which blow constantly in the same direction; those, which are periodical; and those, which are variable. The permanent winds are those which blow constantly between, and a few degrees beyond, the tropics, and are called trade-winds. On the north of the equator, their direction is from the north-east, varying at times a point or two of the compass each way: on the south of the equator, they proceed from the south-east. The origin of them is this :—the powerful heat of the torrid zone rarefies, or makes lighter, the air of that region; the air, in consequence of this rarefaction, rises, and to supply its place, a colder atmosphere from each of the temperate zones moves towards the equator. But these north and south winds pass from regions, where the rotatory motion of the earth's surface is less, to those where it is greater. Unable at once to acquire this new velocity, they are left behind, and instead of being north and south winds, as they would be, if the earth's surface did not turn round, they become north-east and southeast winds.
The monsoons belong to the class of periodical winds. They blow half the year from one quarter, and the other half from the opposite direction: when they shift, variable winds and violent storms prevail for a time, which render it dangerous to put to sea. The monsoons of course suffer partial changes in particular places, owing to the form and position of the lands, and to other circumstances; but it will be sufficient to give
their general directions. From April to October, a south-east wind prevails north of the equator, southward of this a south-east wind ; from October to April, a north-east wind north of the equator, and a northwest between the equator and 10° of south latitude.
The land and sea-breezes, which are common on the coasts and islands situated between the tropics, are another kind of periodical winds. During the day, the air, over the land, is strongly heated by the sun, and a cool breeze sets in from the sea ; but in the night, the atmosphere over the land gets cooled, while the sea, and consequently the air over it, retains, a temperature nearly even at all times ; accordingly, after sunset, a land-breeze blows off the shore. The sea-breeze generally sets in about ten in the forenoon, and lasts till six in the evening; at seven the land breeze begins, and continues till eight in the morning, when it dies away. These alternate breezes are, perhaps, felt more powerfully on the coast of Malabar than any where; their effect there, extends to a distance of twenty leagues from the land.
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 tem. perate 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 zones, 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 its 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 water-spout.
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