Page:Physical Geography of the Sea and its Meteorology.djvu/111

Rh the volume expressed by the transporting 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. South-east trade-winds stronger near the equatorial limits.—The south-east trade-winds, as they cross the parallel of 5° and come out of this belt, appear to be stronger than 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 southing 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 10° S: the annual deficiency of air here would be the volume required to supply the trades for 151 days (214-63=151).

221. Speed of vessels through the trade-winds.—The average speed which vessels make in sailing through the trade-winds in different parts of the world has been laboriously investigated at the National Observatory. By this it appears that their average speed through the south-east trade-winds of the Atlantic is, between the parallels of 5° and 10°, 6.l knots an hour, and 5.7 between 25° and 30°.

222. ''The question, Whence are the south-east trade-winds supplied with air? answered.''—All these facts being weighed, they indicate that the volume of air which investigations show that the south-east trade-winds of the world annually waft across the parallels of 10°-5° S. in 285 days—for that is their average duration for all oceans taken together—is at least twice as great