from it, precisely similar to the movement of the air, heated by contact with the land, and perhaps the sloping sides of mountains. The most remarkable ærial currents, and of the greatest importance in navigation, are the trade winds, which, within the tropics, blow continually from the east, though with variable force, and declining north and south, according to the latitude, and season of the year. The primary cause of the trade winds is analagous to that of the land and sea breezes, though much more extensive, and maintaining a constant direction, and likewise united with the influence of another cause, viz: the rotation of the earth. We have already had occasion to remark that within the two tropics lies a belt, over some part of which the sun is vertical at noon, at all seasons of the year, hence this equatorial belt, in the torrid zone, is continually heated by the sun, and a large body of air warmed by contact with the heated ground, rises constantly to the upper regions. Its place is supplied by colder air moving along the surface of the earth, from the colder northern and southern climates. The effect of this would be to produce a north wind north of the equator, and a south wind south of the equator. The portion of air thus transferred from the higher latitudes to the equator has a slower diurnal motion than at the equatorial regions. Perhaps a diagram will make this more plain.

B

E

Let P be one of the poles of the earth, A B a parallel of latitude, and E E, the equator. Now it is evident that the diurnal movement of a body attached to the parallel A B, is slower than one at the equator E E, since the equatorial belt or circle, is of much greater circumference than the circle of latitude, and both are

moved over in the same time. If therefore, a body, still retaining the quantity of motion it had while upon A B, should suddenly be placed upon E E, it is evident that the excess of motion at E E would leave it behind. This is actually the case with the colder air rushing from the higher latitudes to fill the space vacated by the ascent of heated air within the tropics. The air thus transferred, does not acquire at once the motion of the equatorial regions, and consequently lags behind, or the earth moves under it, and thus since the earth moves from west to east upon its axis, it gives the appearance of a wind coming from the opposite quarter, i. e. from the eastward. But, as we have seen before, these winds had a direction from the north, at all places north of the equator, and from the south, at all places south of the equator, combining the two therefore, we will have a constant north-east wind one side of the equator, and south-east the other; these two winds, meeting at the equator, will flow constantly eastward, or destroy each other and produce a calm. Such is the character of that general wind, which encircles the globe, flowing with slight deviations, constantly from the east, and spreading over a zone of more than 500 in breadth. It sweeps the Atlantic ocean from the coast of Africa to Brazil, and the Pacific from Panama to the Phillipine islands, and New Holland; and again over the Indian sea partially, from Summatra to Zanguebar; here however its direction is curiously varied, owing to the peculiar locality of this

ocean.

The course of the trade winds is changed or interrupted by high lands, thus calms and variable winds prevail at the Cape Verd islands, being under the the lee of the African shore, and an eddy, or counter current of air from the south-west, is generated under the coast of Guinea. The lofty barrier of the Andes shelters the sea on the Peruvian shores from the trade winds, which are not felt until a ship has sailed eighty leagues westward, but the intervening space is occupied by a wind from the south. The heated air of the tropics after becoming somewhat cooler in its passage towards the temperate regions, descends to the earth still retaining in a great measure, its equatorial velocity, consequently, it sweeps over the surface of the earth in the same di

rection in which it is turning, but somewhat faster; this gives the appearance of a westerly wind of considerable force and regularity. According to Robbins, a westerly wind almost constantly prevails in about latitude 60° S. in the Pacific ocean. In Hudson's Bay, westerly winds prevail three-fourths of the year, as also in Kamschatka. Still farther north, as at Melville island, the north, and north-west winds prevail. On account of these winds, the Atlantic may be crossed eastward in about half the time of returning westward. The existence of the upper current of the trade winds was shown in a striking manner at the eruptions of the volcano in the island of St. Vincent in 1812. The trade winds blow with great force from Barbadoes to St. Vincent, but ashes erupted by the volcano fell in profusion from a great height upon Barbadoes, which is about 150 miles westward. Since the westerly winds which prevail in the higher latitudes are caused by bodies of air from the torrid zone, which often descend to the earth before the heat is quite gone, such winds are generally warm, and on the same principle winds which blow directly from the arctic pole, are intensely cold, and, as they must appear from the rotation of the earth, to come from the north-east, our easterly and north-easterly winds are always severely cold.

We will now consider the causes which result in changing the direction of the trade winds in the Indian ocean, producing what are termed the monsoons. When it is summer in the northern hemisphere the great body of land above the Indian ocean becoming more heated than the sea, a breeze sets towards the land, which, modified by the rotation of the earth, gives a strong southeast wind. On the contrary, when it is winter in the northern latitudes, the great body of water, as also the vast island of New Holland, becoming more heated than the continent farther north, a wind sweeps over the Indian and southern oceans, whose general direction is south-west; this wind not being opposed in direction to the rotation of the earth, is more powerful than the other. The interval which separates the monsoons is variable, but occurs generally near the equinoxes, during this interval, violent gales occur called Typhoons.

According to Mr. Redfield, who has most ably and successfully

investigated the phenomena of the great storms which traverse the Atlantic coast, they may be traced to the N. E. of the West India islands, and are from thence, drifted to the westward on a track which inclines generally to the northward and eastward. The rate of progression varies from 12 to 30 miles per hour, and the storm whirls or blows from right to left, in a horizontal circuit, on a vertical and some what inclined axis of rotation which is carried forward by the storm. Mr. Espy has proposed a theory which to us appears at variance with the facts derived from general observation, and independent of any hypothesis. According to this doctrine, it is alleged that during hurricanes, the wind, instead of blowing in a circle, rushes directly from the exterior towards the centre, to supply a vertical current under influence of the suction, or vacuum power caused by the rarefaction and ascent of air, consequent from the extrication of the latent heat of vapor during condensation. This theory has been ably defended by Dr. Hare. The rotary theory of storms is now pretty generally acknowledged, and has been advocated by Mr. Redfield, Sir John Herschel, Lieut. Col. Reid, Prof. Dove of Prussia, and Mr. Alex. Thom. The latter gentleman has published a book on the nature and course of storms, in which, he traces the origin, and describes the phenomena, of the great hurricanes annually occurring in the Indian ocean, showing them to be vast circuits of wind, revolving in a particular direction with unerring regularity; that they move from about latitude 10° S. by a southwesterly track, curving towards the tropic. Their diurnal rate of progression diminishes as they recede from the equator. These "hurricanes are formed by the westerly monsoons, and the S. E. trade winds; the following diagram is extracted from Mr. Thoms' work, and will illustrate the manner in which these storms are generated.

When the hurricanes are first noticed they are from 400 to 500 and even 600 miles in extent, or diameter, and consequently, as the space between the monsoons and the trades is seldom more than 100 miles, the outer portions of the circle must be involved in the two winds for the space of two hundred miles on each side. Each wind therefore communicates a part, if not the

whole of its motion, in a tangential direction, and by their reverse directions produce a uniform circular motion. If the south-east

S. E. Trades.

от

Westerly Monsoons.

trade impinges upon one side at the rate of 30 miles per hour, and the monsoon at the same velocity on the other, the amount, converted into a rotary motion, would be equal to 60 miles at the exterior. The stormy revolution appears to extend to a great height, passing over the lofty mountains of the Isle of Bourbon and the Mauritias without being destroyed, though these arrest the trade winds. The same cause which produces the gyratory motion carries the storm forward.

The awful phenomena developed during a hurricane, depend upon the sudden reductions of the masses of air involved in it, of different temperatures, to the same standard, producing the most terrific exhibitions of thunder and lightning, attended with rain, hail &c. The trade winds are cool, but the monsoons are hot and humid. In most hurricanes, the fall of the mercurial column is equal to 1 inch, or even 2 inches, and the velocity of the wind just entering the focus as great as 150 miles an hour. In the focus of a rotary storm the horizontal direction of the wind suddenly ceases, and often, the storm appears, to one ignorant of its nature, to have passsed off, when suddenly it commences again with the utmost fury, and at a point of the compass opposite to where it left off. This fact is strikingly noticeable even in the small hurricanes which in the autumn and spring, blow over our western lakes, and we have several times witnessed a violent hail and thunder storm, during which the wind raged with the

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