where is the power that guides that from the north over to the south, and that from the south up to the north? The conjectures in the next chapter as to "the relation between magnetism and the circulation of the atmosphere" may perhaps throw some light upon the answer to this question.
CHAPTER VII.
THE CALM BELTS,
AND THE MAGNETISM OF THE ATMOSPHERE.
341. Halley's theory not fully confirmed by observations.—Halley's theory of the trade-winds, especially so much of it as ascribes their easterly direction to the effect of the diurnal rotation of the earth, seems to have been generally received as entirely correct. But it is only now, since all the maritime nations of the world have united in a common system of research concerning the physics of the sea, and occupied it with observers, that we have been enabled to apply the experimentum crucis to this part of the famous theory. The abstract logs, as the observing-books are called, have placed within my reach no less than 632,460 observations—each one itself being the mean of many separate ones—upon the force and direction of the trade-winds. It appears from these that diurnal rotation being regarded as the sole cause, does not entirely account for the easting of these winds.
342. Observed course of the trade-winds.—From these observations the following table has been compiled. It shows the mean annual direction of the trade-winds in each of the six belts, north and south, between the parallels of 30° and the equator, together with the number of observations from which the mean for the belt is derived:—
| Between. | N.E. Trades. | S.E. Trades. | ||||||
| Course. | No. of Obs | Course. | No. of Obs | |||||
| 30° and 25° | N | 51° | E. | 68,777 | S. | 46° | E. | 66,635 |
| 25° and 20° | 51° 30' | 44,527 | 49° 20′ | 66,395 | ||||
| 20° and 15° | 53° 30' | 33,103 | 52° | 46,604 | ||||
| 15° and 10° | 52° 30' | 30,339 | 49° 40′ | 43,817 | ||||
| 10° and 5° | 53° 30' | 30,841 | 51° 40′ | 54,648 | ||||
| 5° and 0° | 54° 30' | 67,829 | 48° 40′ | 72,945 | ||||
| Mean | N. | 52°45' | E. | S. | 40° 33′ | E. | ||
