THE ATMOSPHEEE. 83 



dry air. The dry est air is the heaviest. This dry and heavy air is 

 therefore the air most hkely 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 cm^ved arrows, 

 f g and/' g , r s and r s, are intended to represent, in the " dia- 

 gram of the wTuds " (Plate I.), this sloughing off and tmning back 

 of ah^ from the upper currents to the trade-mnds below. 



225. According to investigations which are stated at length in 

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

 s?^L?r than north- TBut Charts, and in the Monographs of the Washing- 

 east trade-wind^. j^Qj^ 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, fr-om 30° 

 to the equator. For the band between the parallels of — 



Ann. duration. Force. No. ofobs. 



30^ and 25^ S 124 davs. 5-6 miles.* 19,817 



25^ and 20^ 157 „' 5.7 „ 20,762 



20^ and 15^ 244 „ 5.9 „ 17,844 



15^ and 103 295 „ 6.3 „ 14,422 



lO^andS^ 329 „ 6.1 „ 13,714 



S^andO^ 314 „ 6.0 „ 15,463 



It thus appears that the south-east 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 

 The air sloughed off (^ 204 [7]), a different process of slou2;hinoj off and 



irom the counter 2" . K -JV . . "•■ tt n • t ii 



trades, moist air. tummg bacK IS gouig ou. ±1016 the wmcis are blow- 

 ing toivards the poles ; they are going from parallels of large to 

 parallels of smaller circumference, while the upper retm^n current is 

 doing the reverse ; it is widening out with the increasing circumfe- 

 rence of parallels, and creating room for more air, while the narrow- 

 ing cun-ent below is crowding out and sloughing off air for its winds. 



227. In the other case (§ 224), it was the heavy dry air that 

 The air sloughed off ^yas sloughcd off to joiu the winds below. In this 

 current dry. ^ ' casc it is the moist v.iid. lightest air that is crowded 

 out to join the current above. 



228. This is particularly the case in the southern hemisphere. 

 The meteorological where, entirely around the globe between the paral- 

 ingSnms oSs't lels of 40^ and G0° or 65^, all, or nearly aU, is water. 

 '*^^- In this great austral band the winds are in contact 



* Distance per hour that vesseb average while sailing through it, 



g2 



