3SASTING OF THE TRADE-WINDS, ETC. 



153 



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. 



§ 341-368. — THE EASTING OF THE TRADE-WINDS, THE CROSSING AT 

 THE CALM BELTS, AND THE MAGNETISM OF THE ATMOSPHERE. 



341. Motley's ihewy not fully confirmed hy 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 experhnentum crucis to this part of the 

 famous theor}'. The abstract logs, as the observing-books are 

 <:;alled, have placed within my reach no less than 632,460 obser- 

 vations — 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 wdnds. 



342. Observed course of tJie trade-winds. — From these observa- 

 tions 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. 



30^ and 25^ 

 25° aud 20^ 

 20° and 15° 

 15° and 10= 

 10° and 5° 

 5° and 0° 



Mean 



Course. 



K 51° 

 51° 30' 

 53° 30' 

 52° 30' 

 53° 30' 

 54° 30' 



E. 



.. N. 52°45'E. 



No. of Obs. 



G8,777 

 44,527 

 33,103 

 30,339 

 30,841 

 67,829 



S.E. Trades. 



Course. 



S. 46° E. 

 49° 20' 

 52° 



49° 40' 

 51° 40' 

 48° 40' 



S. 40° 33' E. 



No. of Obs. 



6G,G35 

 60,395 

 46,604 

 43,817 

 54,648 

 72,945 



