EASTING OF THE TEADE-WINDS, ETC. 155 



until more numerous and accurate observations shall better 

 satisfy the theory than those half a million and more now do, be 

 regarded as the sole cause of the easterly direction of the trade- 

 winds. It suggests either that other agents are concerned in 

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

 iipper and counter current, when drawn down and turned back 

 (§ 232), is such as to counteract their unequal turning in obe- 

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

 needed for applying the tests of actual observation to this part of 

 the Halleyan theory, notwithstanding the general concurrence of 

 opinion as to its sufficiency. With equal favour 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 advancement 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 vapour for the equatorial cloud-ring and 

 its rains is now also (§ 252) recognized as an agent of no feeble 

 power in this calm belt. 



345. Faraday's discovery of magnetism in the air. — Where shall 

 those who are disposed to search, look for this other agent that 

 is supposed to be concerned with the trade-winds in their east- 

 ing? I cannot 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 Faraday 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 

 towards 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 in which the iron pointed. 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. 



