Winds. 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 southeast. 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 northeast and south-east 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 suiter 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 north-west 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 ail 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 011 the coast of Malabar than anywhere; 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 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 posit on 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 northwest and south-west winds of which we have been speaking. 1 his interchange between the heated air of the tropics, and the cold air of the polar regions, greatly tends to moderate the climate of each. Besides, the air from the tropics being richer in oxygen, on account of the more luxuriant vegetation decomposing a larger quantify of carbonic acid, is well calculated to supply any deficiency in the amount of this most important substance, which might occur from the barrenness of a less favoured climate.
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.