THE ATMOSPHERIS 85 



tlie volume expressed by the transporting power of the south- 

 east trade-winds at latitude 25^ for 03 days (209 - 14G6 = 3). In 

 like manner we obtain, in similar terms, an expression for th© 

 volume which these winds bring out at the large or equatorial 

 end, and find it to be as much air as the south-east trade-winds 

 can transport across the parallel of o'' S. in 214 days (28 — 672 = 

 214). Again: 



220. South-east trade-winds stronger near the equatorial limits. — 

 The south-east trade-winds, as they cross the parallel of 5° and 

 come out of this belt, appear to be stronger * than they are when 

 they enter it. But assuming the velocity at each parallel to be 

 the same, we have (§ 219) just 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 south-east trade-wind belt near its polar borders, 

 v/ere to come out at its equatorial edge as south-east trade-winds, 

 there would not be enough air to feed the south-east trade-winds 

 between these two parallels of 5° and 10° S : the annual defi- 

 ciency of air here would be the volume required to supply the 

 trades for 151 days (214-63 = 151). 



221. Speed of vessels through the trade-winds. — The average speed 

 which vessels make in sailing through the trade- winds in different 

 parts of the world has been laboriously investigated at the 

 National Observatory.f 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°, G.l knots an hour, and 5.7 

 between 25° and 30°. 



222. The question, Whence are the soidh-east trade-winds supplied 

 with air? answered. — All these facts being weighed, they indi- 

 cate that the volume of air which investigations show that the 

 south-east trade-winds of the world annually waft across the 

 parallels of 10°-5° S. in 285+ days— for that is their average 

 duration for all oceans taken together — is at least twice as great 



* The force of tlie trade-winds, as determined by the average speed of 2235 

 vessels sailing through them, is greater between 5^ and 10^ S. than it is between 

 "J^^ and 30^ ^.—Maury's Sailing Directions, 1859. 



t See " Average Force of the Ti-ade -winds," p. 857, vol. ii., 8th ed., Maiu-yV 

 Sailing Directions, 1859. 



X Nautical Monographs, Plate I., No. 1, " The Winds at Sea." 



