86 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



as the voliimo 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 ru*h 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 circum- 

 ference, 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 cany 

 polarward to be croiuded 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 f — 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 diyest 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 /' 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 ti-ade- 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 ^Vind 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 bo thus stated for every band or belt of 

 5° of latitude in breadth, from 30° to the equator. For the band 

 between the jparallels of — 



