§ 221-223. THE ATMOSPHERE. g| 



Southeast trade- ^^^ wheii tlicj enter it. But assuming the velocity 

 ;t°'e\urrfa[ lim- ^t each parallel to be the same, we have (§ 219) 

 ^^^' j ust three times as much air with southing in it coming 



out of this belt on the equatorial side as with southing in it we find 

 entering (§ 218) on the polar side. From this it is made plain 

 that if all the air, whether from the southward and eastward, or 

 from the southward and westward, which enters the southeast trade- 

 wind belt near its polar borders, were to come out at its equatorial 

 edge as southeast trade-winds, there would not be enough, by the 

 volume required to supply them for 151 days (214—63 = 151), to 

 keep up such a constant current of air as we find the southeast 

 trade-winds actually conveying across this parallel of 5° S. 



221. The average speed which vessels make in sailing through 

 Speed of vessels the tradc-wiuds in different parts of the world has 



through the trade- ii-i' • i itvt-t r\^ 



winds. been laboriously nivestigated at the JNational Ob- 



servatory.'-^ By this it appears that their average speed through 

 the southeast trade-winds of the Atlantic is, between the parallels 

 of 5° and 10°, 6.1 knots an hour, and 5.7 between 25° and 30°. 



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

 The question, Whence ^^^c of air which investigations show that the 

 trade-winda 'supplied southcast tradc-wluds of the world annually waft 

 with air? answered. ^^^^^^ ^^^ parallels of 10°-5° S. in 285t days—for 

 that is their average duration — is at least twice as great as the vol- 

 ume which they annually sweep across the parallel of 25° in 139 

 days, which is their average here. Hence the answer to the ques- 

 tion (§ 218), Whence comes the excess? is, it can only come from 

 above, and in this way, viz. : the southeast trade-winds, as they 

 rush from 25° S. toward the equator, act upon the upper air like 

 an under-tow. Crossing, as they approach the equator, parallels 

 of larger and larger circumference, they draw dawn and turn back 

 from the counter current above air enough to supply jiohulum to 

 larger and larger, and to stronger a.nd stronger currents of wind. 



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

 Whither it goes, calm belt (§ 213) riscs up, and has to flow off as an 



sailing through them, is greater between 5° and 10° S. than it is between 25° and 

 30° S. — Maury's Sailing Directions, 1859. 



* See "Average Force of the Trade-winds," p. 857, vol. ii., 8th ed., Maury's 

 Sailing Directions, 1859. 



f Nanticiil Monographs, Plate I., No. 1, "The winds at sea." 



F 



