Page:Physical Geography of the Sea and its Meteorology.djvu/110

84 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 circumference. 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 lat. 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 encircles 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 northing and winds with southing in them contrasted.—Now, dividing the winds into only two classes, as winds with northing and winds with southing in them, actual observations 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 year, 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. Now, if we suppose, merely for the sake of assisting farther in the illustration, that these winds with northing 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 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°,