§ 217, 218. THE ATMOSPHERE. 79 



continually, and the wind retiirneth again according to his cir- 

 cuits." — EccL, i., 6. 



217. Of course, as the surface winds, H, J, K and T, U, V, ap- 

 sioughing off from P^oach the polcs, there must be a sloughing off, if 1 

 the counter trades, ^-^^y. ^q allowcd the cxprcssion, of air from them, in 

 consequence of their approaching the poles. For as they near the 

 poles, the parallels become smaller and smaller, and the surface 

 current must either extend much higher up, and blow with greater 

 rapidity, or else a part of it must be sloughed off above, and so 

 turn back before reaching the calms about the poles. The latter 

 is probably the case. Such was the conjecture. Subsequent in- 

 vestigations* have, by proving the converse^ established its correct- 

 ness, and in this way : they show that the southeast trade-winds, 

 as in the Atlantic, blow, on the average, during the year, 124: days 

 between the parallels of 25° and 80° S., and that as you approach 

 the equator their average annual duration increases until you 

 reach 5° S. Here between 5° and 10° S. they blow on the aver- 

 age for 829 out of the 865 days. 



218. Now the question may be asked, Where do the supplies 

 The air which sup- which fumish air for these winds for 829 days come 



plies the southeast „ _ in it t • 



trade-wind in the Irom ( The " tradcs could not convey this fresh 



band 5° does not , o* i ^^ ^ n r^ -y • 



eross the band 25°. supply of air across tho parallel of 25^ S. during 

 the time annually ;;lIotted for them to blow in that latitude. 

 They can not for these reasons : (1.) Because the trade-winds in 

 lat. 5° are stronger than they are in lat. 25°, and therefore, in 

 equal times, they waft larger volumes of air across 5° than they 

 do across 25°. (2.) Because the girdle of the earth near the equa- 

 tor is larger than it is farther off, as at 25° ; therefore, admitting 

 equal heights and velocities for the wind at the two parallels, it 

 would, in equal times, bear more air across the one of larger cir- 

 cumference. Much less, therefore, can the air which crosses the 

 parallel of 25° S. annually in the 124 trade-wind days of that 

 latitude be sufficient to supply the trade- winds with air for their 

 829 days in lat. 5°. Whence comes the extra supply for them in 

 this latitude ? (8.) Of all parts of the ocean the trade- winds ob- 

 tain their best development between 5° and 10° S. in the Atlan- 

 tic Ocean, for it is there only that they attain the unequaled an- 

 nual average duration of 829 days. But referring now to the 

 * Nautical Monographs, No. 1, Observatory, Washington, October, 1859. 



