82 



THE PHYSICAL GEOGRAPHY OF THE SEA. 



upper current, to make room for that which the trade-winds are 

 continually pouring in below. They bring it from toward the poles 

 — back, therefore, toward the poles the upper currents must carry 

 it. On their journey they cross parallel after parallel, each small- 

 er than the other in circumference. There is, therefore, a constant 

 tendency with the air that these upper currents carry 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. This upper air, having supplied the equatorial cloud-ring 

 How it is drawn (§ ^1^) "^^^^ vapor for its clouds, and with moist- 

 down from above, ^^q £qj, ^|.g ^aius, flows off polarward as compara- 

 tively 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 

 vigor for which they are so famous, and which help to make them 

 so grateful to man and beast in tropical climates. The curved 

 arrows,/^/ and/' (/', r s and / 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. According to investigations which are stated at length in 

 veiocityofsoutheast Maury's Sailing Directions, on his Wind and Cur- 

 JhannoSh^asuradT- I'sut Charts, and iu the Monographs of the Observ- 

 ^°^^- atory, the average strength and annual duration of 

 the southeast trade-winds of the Atlantic may be thus stated for 

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

 For the band between 



Ann. duration. 

 30° and 25° S 124 days. 



25° and 20° 157 



20° and 15° 244 



15° and 10° 295 



10° and 5° 329 



5° andO° 314 



It thus appears that the southeast trade-winds of the Atlantic 

 blow with most regularity between 10° and 5°, and with most 

 force between 10° and 15°. 



226. On the polar side of 35°-40°, and in the counter trades 



* Distance per hour that vessels average while sailing through it. 



