Page:Physical Geography of the Sea and its Meteorology.djvu/112

86 as the volume which they annually sweep across the parallel of 25° in 139 days, which is their like average here. Hence in answer to the question (§ 218), "Whence comes the excess?" the reply is, it can only come from above, and in this way, viz.: the south-east trade-winds, as they rush from 25° S. towards the equator, act upon the upper air like an under-tow. Crossing, as they approach the equator, parallels of larger and larger circumference, these winds draw down and turn back from the counter current above air enough to supply pabulum to larger and larger, and to stronger and stronger currents of surface-wind.

223. Whither it goes.—The air which the trade-winds pour into the equatorial calm belt (§ 213) rises up, and has to flow off as an upper current, to make room for that which the trade-winds are continually pouring in below. They bring it from towards the poles—back, therefore, towards the poles the upper currents must carry it. On their journey they cross parallel after parallel, each smaller than the other in circumference. There is, therefore, a constant tendency with the air that these upper currents carry polarward to be crowded out, so to speak—to slough off and turn back. Thus the upper current is ever ready to supply the trade-winds, as they approach the equator, with air exactly at the right place, and in quantities just sufficient to satisfy the demand.

224. How is it drawn down from above?—This upper air, having supplied the equatorial cloud-ring (§ 514) with vapour for its clouds, and with moisture for its rains, flows off polarward as comparatively dry air. The dryest air is the heaviest. This dry and heavy air is therefore the air most likely to be turned back with the trade-winds, imparting to them that elasticity, freshness, and vigour for which they are so famous, and which help to make them so grateful to man and beast in tropical climates. The curved arrows, f g and f' g', r s and r' s', are intended to represent, in the "diagram of the winds" (Plate I.), this sloughing off and turning back of air from the upper currents to the trade-winds below.

225. Velocity of south-east shown to be greater than north-east trade-winds.—According to investigations which are stated at length in Maury's Sailing Directions, on his Wind and Current Charts, and in the Monographs of the Washington Observatory, the average strength and annual duration of the south-east trade-winds of the Atlantic may be thus stated for every band or belt of 5° of latitude in breadth, from 30° to the equator. For the band between the parallels of—