82 PHYSICAL GEOGRAPHY OP THE SEA, AND ITS METEOROLOGY. 



10^ S : the annual deficiency of air liere would be tlie volume re- 

 quired to supply the trades for 151 days (214 — 63 = 151). 



221. The average speed which vessels make in sailing through 

 specd^of vessels ^ the tradc-wiuds in difierent parts of the ^Y0^1d has 

 winds" ''^ ' been laboriously investigated at the National Ob- 

 servatory,* By this it appears that their average speed through 

 the south-east trade-winds of the Atlantic is, between the parallels 

 of 5° and 10^, 6.1 knots an horn*, and 5.7 between 25" and 30^. 



222. All these facts being weighed, they indicate that the 

 The question, Whence volumc of air whicli investigations show that the 

 trad2wi5?ds suppfied south-oast trado-winds of the w^orld annually v\^aft 

 with air? answered, ^cross the parallels of 10^-5'' S. in 285t days— for 

 that is their average duration for all oceans taken together — is at 

 least twice as great 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 ex- 

 cess ?" the reply is, it can only come from above, and m this way, 

 viz. : the south-east trade-winds, as they rush fi'om 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 circumfe- 

 rence, these winds draw down and tm^n back from the counter current 

 above air enough to supply imhidum to larger and larger, and to 

 stronger and stronger currents of sm-face-wind. 



223. The air which the trade-winds pour into the equatorial 

 Whither it goes, 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 m below. They bring it from towards tlie poles 

 — back, therefore, towards the poles the upper cmTents must carry 

 it. On their jom-ney they cross parallel after parallel, each smaller 

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

 tendency vdth the air that these upper currents carry polarward 

 to be crowded out, so to speak — to slough off and turn back. 

 Thus the upper cmTcnt 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 

 }fow is it drawn (§ 514) with vapouT foT its clouds, and with moisture 

 down from above. £qj. ]^ rains, flows off pokrward as comparatively 



* See "Average Force of the Trade -winds," p. 857, vol. ii., Sth ed., Maury '3 

 Sailing^ Directions, 1859. 



t Nautical Monographs, Plate I., No. 1, "The winds at sea." 



