80 THE PHYSICAL GEOGRAPHY OF THE SEA. 



average annual duration of the southeast trade-wind in all seas, 

 we may, for the sake of illustration, liken this belt of winds which 

 encircles the earth, say between the parallels of 5° and 25° S., to 

 the frustum of a hollow cone, with its base toward the equator. 



219. Now, dividing the winds into only two classes, as winds 

 Winds with north- with noftlimfj and winds with southing in them, act- 

 goathfng'^^n'^%hem ^^^ obscrvatious show, taking the world around, 

 contrasted. ^|^^^ wiuds haviug southiug in them blow into the 

 southern or smaller end of this cone for 209 days annually, and 

 out of the northern and larger end for 286 days.* They appear 

 (§ 221) to come out of the larger end with greater velocity than 

 they enter the smaller end. But we assume the velocity at going 

 in and at coming out to be the same, merely for illustration. 

 During the rest of the year, either winds with northing in them 

 are blowing in at the big end, or out at the little end of the imag- 

 inary cone, or no wind is blowing at all : that is, it is calm. Now, 

 if we suppose, merely for the sake of assisting farther in the il- 

 lustration, that these winds with northing and these winds with 

 southing move equal volumes of air in equal times, we may sub- 

 tract the days of the one from the days of the other, and thus 

 ascertain how much more air comes out at one end than goes in 

 at the other of our frustum. "Winds with northing in them blow 

 in at the big end for 72 days, and out at the little end for 146 days 

 annually. Now, if we subtract the whole number of winds (146) 

 with northing in them that blow out at the south or small end, 

 from the whole number (209) with southing in them that blow 

 m, we shall have for the quantity that is to pass through, or go 

 from the parallel of 25° to 5°, the value expressed bj^ the trans- 

 porting power of the southeast trade-winds at latitude 25° for 63 

 days (209 — 146 = 63). In like manner we obtain, in similar 

 terms, an expression for the volume which these winds bring out 

 at the large or equatorial end, and find it to be as much air as the 

 southeast trade-winds can transport across the parallel of 5° S. in 

 214 days (286-72 = 214). Again, 



220. The southeast trade-winds, as they cross the parallel of 

 5° and come out of this belt, appear to be strongerf than they 



* Nautical Monographs, No. 1, "The Winds of the Sea,"Observatoiy, Washing- 

 ton, 1859. 



t The force of the trade-winds, as determined by the average speed of 2235 vessels 



