Trade-winds

(Meteor.) Easterly winds which constantly prevail, with slight variations, in certain regions within the tropics. The T.-W., in the Atlantic and Pacific oceans, extend to about 280 of latitude each side of the equator; so that a ship, after pa ing 30°, may every day expect to meet them. But on first entering them, they will be found to blow from the east, or even a little southerly; and, as the ve el advances, to draw round gradually to north-east. In the East Indies the T.-W. are periodical, and are called monsoons. The T.-W. are caused chiefly by the comparatively high temperature of the torrid zone, and the rotation of the earth from west to east. The heated air ascends, and is replaced by air from the neighborhood of the poles. This would have a direction from north to south, or vice versâ, according to the pole whence it comes; but, as the velocity of rotation of the airs near the poles is le than at the equator, the air from the poles, when approaching the equator, is left behind during the earth's rotation; it has, therefore, two motions, one from the poles to the equator, the other in opposition to the earth's motion, or from east to west; and, therefore, in accordance with the laws of motion, its direction is compounded of both; that is, its direction is towards the north-west or southwest; and the resulting winds are north-easterly or south-easterly. The same causes modify the tidal current. See TIDES. [s. 903]