CONSTANT WINDS. The the surface heated is, roughly speak ing, a whole zone, as in the case of the tropics, a surface-wind will set in toward the heated tropical zone from both sides: and uniting will ascend, and then separating, flow as upper currents in opposite directions. Hence, a surface-current will flow from the higher latitudes toward the equator, and an upper current toward the poles. If, then, the earth were at rest, a n. wind would prevail in the northern half of the globe, and a s. wind in the southern half. But these directions are modified by the rotation of the earth on its axis from w. to east.. In virtue of this rotation, objects on the earth's sur face at the equator are carried round toward the c., at the rate of 17 m. a minute. But as we recede from the equator, this velocity is continually diminished; at let. 60°, it is only 8+ m. a minute, or half of the velocity at the equator; and at the poles it is nothing. A wind, therefore, blowing c!mg the earth's surface to the equator, is constantly arriv ing at places which have a greater velocity than itself. Hence, the wind will lag behind, that is, will come up against places toward which it blows, or become.= east wind. Since, then, the wind n. of the equator is under the influence of two forces—one draw ing it s., the other drawing it w.—it will, by the law of the composition of forces, flow in an intermediate direction, that is, from n.e. to s.west. Similarly, in the south ern tropic, the wind will blow from s.e. to n.west. All observation confirms this
reasoning. From the great service these winds render to navigation, they have been called the trade-winds. It is only in the Pacific and Atlantic oceans that the trade winds have their full scope. In other parts of 'the trades' zone, such as southern Asia and intertropical Africa and America, they are more or less diverted from their course by the unequal distribution of land and sea (on which see MoNsooN). It is generally stated that in the Atlantic the north trades prevail between lat. 9° and 30°, and in the Pacific, betweeft lat. 9° and 26°- and the south trades, in the Atlantic, between let. 4° n. and 22° s., and in the Pacific, between lat. 4° n. and 231° south. These limits, however, are not stationary, but follow the sun, advancing northward from January to June, and southward from July to December.
Region of Calms.—This Is a belt, e or 5* broad, stretching across the Atlantic and Pacific, parallel to the equator. It marks the meeting-line of the n. and s. trades where they mutually neutralize each other. Here also occur heavy rains, and thunder- almost ost daily. This belt varies its position with the trades, reaching its most northern limit in July, and its most southern in January. When the belt of calms nears the African coast, in the gulf of Guinea, the copious rainfall gives rise to the , strong steady-blowing gales of that coast, called tornadoes.