TRADE

S<'<? Commerce. TRADE WINDS, are winds which in the torrid zone, and often a little beyond it, blow generally from the same quarter, varying, according to circumstances, from N.E. to S.E. The cause of this wind is to be ascribed principally to the high comparative temperature of the torrid zone, combined with the rotation of the earth from W. to E. The heated air at the surface ascending into the higher regions of the atmosphere, its place is supplied by the colder air rushing from the poles; which also becoming rarified, ascends in its turn, and is carried in the upper regions towards the poles to supply the stream of the under current: these under polar currents moving in progre towards the equator from the zones where the earth's motion is slower to others where it is more rapid, acquire an apparent relative motion in a westerly direction. The currents from the two hemispheres meeting near the equator, their meridional motions are there destroyed, and they therefore advance together with the remaining motion from the eastward round the globe. The regularity of the trade winds is disturbed in some places by local causes, and chiefly by the superior rarefaction of the air over land heated by the sun's rays. They extend farther to the northward or southward according as the sun's declination is north or south; and in some places they become periodical, blo\xing one half of the year in one "direction, and the other half in the opposite'one. (See Monsoon.) In the great Pacific ocean, however, the trade wind blows with a uniform and gentle breeze all the year round. The name of t7-ade winds was given them from their important influence in nav i. gation. See Winds. TRADl' [s. 1261]