150 THE PHYSICAL GEOGRAPHY OF THE SEA. 



ed in giving the trade- winds their easting, or that the effect of the 

 upper and counter current, when drawn down and turned back 

 (§ 223), is such as to counteract their unequal turning in obedience 

 to the varying forces of diurnal rotation. No apology is needed 

 for applying the tests of actual observation to this part of the Hal- 

 leyan theory, notwithstanding the general concurrence of opinion 

 as to its sufficiency. With equal favor that feature of it also was 

 received which ascribes the rising up in the belt of equatorial 

 calms to the direct influence of the solar ray. But the advance- 

 ment which has been made in our knowledge of physical laws 

 since Halley expounded his trade-wind theory suggested a review 

 of that feature, and it was found that, though the direct heat of 

 the sun is one of the agents which assists the air to rise there, it is 

 not the sole agent ; the latent heat which is set free by condensing 

 vapor for the equatorial cloud-ring and its rains is now also (§ 245) 

 recognized as an agent of no feeble power in this calm belt. 



345. Where shall those who are disposed to search look for this 

 Faraday's discovery othcr agcut that is supposcd to be conccmed with 



of magnetism in the , ° .,. ,. . ot 



air. the trade- wmds m their eastmg ; 1 can not say 



where it is to be found, but, considering the recent discoveries in 

 terrestrial magnetism — considering the close relations between 

 many of its phenomena and those both of heat and electricity, the 

 question may be asked whether some power capable of guiding 

 " the wind in his circuits" may not lurk there. Oxygen comprises 

 more than one fifth part (two ninths) of the atmosphere, and Far- 

 aday has discovered that oxygen is para-magnetic. If a bar of 

 iron be suspended between the poles of a magnet, it will arrange 

 itself axially, and point toward them ; but if, instead of iron, a bar 

 of bismuth be used, it will arrange itself equatorially, and point in 

 a direction perpendicular to that of the iron. To distinguish these 

 two kinds of forces, Dr. Faraday has said iron is para-magnetic, 

 bismuth dia-magnetic. Oxygen and iron belong to the same class, 

 and all substances in nature belong to one or the other of the two 

 classes of which iron and bismuth are the types. 



346. This eminent philosopher has also shown that if you place 

 Lines of magnetic ^ magnetized bar of iron on a smooth surface, and 

 ^°"'®" sift fine iron filings down upon it, these filings will 

 arrange themselves in curved lines, as in Fig. 1 ; or, if the bar be 

 broken, they will arrange themselves as in Fig. 2. The earth it- 



