148 



THE PHYSICAL GEOGRAPHY OF THE SEA- 



CHAPTER YII. 



§ 341-369. — THE EASTING OF THE TRADE- WINDS, THE CROSSlJTG 

 AT THE CALM BELTS, AND THE MAGNETISM OF THS ATMOS- 

 PHERE. 



341. Halley's theory of the trade-winds, especially so much 

 iiaiiey'8 theory not of it as ascribcs their easterly direction to the effect 



fully confirmed by-.^. . r>i i i 



observations. 01 the diumal 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 com- 

 mon system of research concerning the physics of the sea, and oc- 

 cupied it with observers, that we have been enabled to apply the 

 experimentum cruets to this part of that famous theory. The ab- 

 stract 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 be- 

 ing regarded as the sole cause does not entirely account for the east- 

 ing of these winds. 



342. From these observations the following table has been com- 

 observed course of pil^d. It shows the mcau annual direction of the 

 the trade-winds. trade- wiuds 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 de- 

 rived : 



Between the equator and 5° north, the annual average duration 

 of the trades is 67 days for the northeast, and 199 for the south- 

 east, with a mean direction for the latter — which are the prevail- 



