84 PHYSICAL GEOGRArilY OF THE SEA, AND ITS METEOROLOGY. 



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 most air across the one of larger circum- 

 ference. 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 

 329 days in laf. 5°. Whence comes the extra supply for them 

 in 5° ? (3.) Of all parts of the ocean the trade-winds obtain 

 their best development between 5° and 10° S. in the Atlantic 

 Ocean, for it is there only that they attain the unequalled annual 

 average duration of 329 days. But referring now to the average 

 annual duration of the south-east trade-wind in all seas, we may, 

 for the sake of illustration, liken this belt of winds which en- 

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

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

 219. Winds with nortliing and ivinds with southing in them con- 

 trasted. — Now, dividing the winds into only two classes, as \\dnds 

 with northing and winds with southing in them, actual observa- 

 tions show, taking the world around, that winds having southing 

 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 3'ear, either 

 winds with northing in them are blowing in at the big end, or out 

 at the little end of the imaginary cone, or no wind is blowing at 

 all-: that is, it is calm. Kow, if we suppose, merely for the sake 

 of assisting farther in the illustration, that these winds with 

 nortliing and these winds with southing move equal volumes of air 

 in equal times, we may subtract 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 d^js, and 

 out at the little end for 146 days annuallj'. 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 quan- 

 tity that is to pass through, or go from the parallel of 25" to 5^, 



* Nautical Monographs, No. 1, " The Winds of the Sea,'' Observatory, 

 Washington, 1859. 



