THE ATMOSPHERE. 81 



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 i\\Q imagi- 

 nary 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 illus- 

 tration, 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 oiu* 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 

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

 from the parallel of 25^ to 5°, the volume expressed by the trans- 

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

 days (209-1466 = 3). 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 

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

 214 days (28 - 672 = 214). Again : 



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

 South-east trade- 5^ aud como out of this belt, appear to be strongert 

 Tht^equatomT ^"^^^ thau 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 so^^^Awi^ 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, were 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 



* Naniioal Monographs, No. 1, " The Winds of the Sea," Observatory, Wash- 

 ington, 1859. 



t The force of the 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 

 25' and 30^ ^.—Maury's Sailing Directions, 1S59. 



G 



