Page:Physical Geography of the Sea and its Meteorology.djvu/180

154 Between the equator and 5° north, the annual average duration of the trades is 67 days for the north-east, and 199 fur the south-east, with a mean direction for the latter—which are the prevailing winds between those parallels—of S. 47° 30' E. According to the Halleyan theory these should be south-west winds.

343. Velocities of the trade-winds.—In the Atlantic the average velocity of the south-east is greater than the average velocity of the north-east trades. I estimate one to be from 14 to 18, the other from about 25 to 30 miles an hour. Assuming their velocity to be 14 and 25 respectively the following departures show the miles of easting which the trade-winds average per hour through each of the above-named belts:

344. Difference between observation and theory.—That diurnal rotation does impart easting to these winds there is no doubt; but the path suggested by the table does not conform to that which, according to any reasonable hypothesis, the trade-winds would follow if left to obey the forces of diurnal rotation alone, as they would do were diurnal rotation the sole cause of their easting. As these winds approach the equator, the effect of diurnal rotation becomes more and more feeble. But the table shows no such diminution of effect. They have as much easting between 5° and 0° as they have between 30° and 25°. Nay, the south-east trades between the equator and 5° N.—where, by the Halleyan theory, they should have westing—have as much easting (§ 342) as they have between 30° and 25° south. We cannot tell how much the air is checked in its easterly tendency by resisting agents, by friction, etc., but we know that that tendency is about ten times stronger between 30° and 25° than it is between 5° and 0°, and jet actual observations show no difference in their course. This table reminds us that diurnal rotation should not,