TRADE-1VINDS blow continuously from one direction, and aro so called because of the facil ities which they afford to trade by sea. They differ from monsoons, which blow one-half of the year from one direction and the other half from an opposite or nearly opposite direction. There are two trade-winds, tho north-east on the north of the equator, and the south-east to the south. Like all winds, these are put in motion by the heat of the sun, and are directed by the daily rotation of the earth. The belt or zone of the S.E. trades is broader than that of the N.E. trades. Its current even cro.ssea tho equator, and invades the belt of the N.E. trades, and discharges itself into the region of equatorial calms. Ships sailing on the ocean calculate on meeting the trade-wnds and monsoons in certain parallels of latitude, and sail for weeks with their ropes and sails unaltered. When the N.E. and S.E. trades meet, the equa torial calms are produced, in which constant min prevails ; it is the condensed vapour of the ocean. Trade-winds, in the Pacific Ocean, blow from the N.E. between lat. 9° and 27° N., and from the S.E. between lat. 3° and 25° S. But there is on the polar side of the north-east trade winds an immense area of arid plains for the heat of the solar ray to beat down upon, also an area of immense precipitation. These two sources of heat hold back the north-east trade-winds, as it were, and when the two are united, as they are in India, they are sufficient not only to hold back the north-east trade-wind, but to reverse it, causing the south-west monsoon to blow for half the year instead of the north-east trade.
The south-east trade-wind seldom blows be yond lat. 10° S. between October and March. In the other months its influence is felt more to the north, but seldom up to the equator, and then so much altered in character that it may be said the zone between 10° S. and the equator is the region of ceculian winds and calms. While the sun is south of the equator in January, February, and March, the space does not lie on a parallel, but occupies a diagonal belt from Sumatra to the Mauritius. In April, the winds,
as a rule, are very light over all the Indian Ocean, northward of 10° S. This is the period when the great change of season occurs, and the many currents are each striving for mastery.
The Mauritius lies in the S.E. trade-winds, and there the influence of the sun during the day is to double the velocity of the wind, and to impress upon it a more truly easterly direc tion.
In the trade-wind regions at sea, evaporation is generally in excess of precipitation while in the extra-tropical regions the reverse is 'the case, that is, the clouds let down more water there than the winds take up again ; and these are the regions in which the Gulf Stream enters the Atlantic. Along the shores of India, where experiments have been carefully made, the evaporation from the sea amounts to three-fourths of an inch daily. The effect of diurnal rotation upon the currents of the sea is admitted by all—the trade-winds derive their easting from it'; it must therefore extend to all the matter which these currents bear with them, to the largest iceberg as well as to the merest spire of grass that floats upon the waters, or the minutest organism that the most powerful microscope can detect among the im palpable particles of sea-dust. Investigations show that in the Atlantic Ocean the south-east trade-wind region is much larger than the north east, that the south-east trades are the fresher, and that they often push themselves up to 10° or 50° of north latitude ; whereas the north-east trade-wind seldom gets south of the equator. The peculiar clouds 'of the trade-winds axe formed between the upper and lower currents of air. The zone of the north-east trades extends on an average from about 29° to 7° N. And if we examine the globe, to see how much of this zone is land and how rnuch water, we shall find, com mencing with China, and corning over Aka, the broad part of Africa and so on, across the con tinent of America to 'the Pacific, lanthenough to fill up, as nearly as may be, just one-third of it.— Manry's Physical Geography.