EASTING OF THE TKADE- WINDS, ETC. 149 



shall better satisfy the theory than those half a million and more 

 now do, he regarded as the sole cause of the easterly dhection of 

 the trade-winds. It suggests either that other agents are concerned 

 in gi^'ing the trade-winds theh easting, or that the effect of the 

 upper and counter cm-rent, when drawn down and tm^ned hack 

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

 to the vaiying forces of dimiial rotation. No apology is needed 

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

 leyan theor)^, notwithstanding the general concmTence of opinion 

 as to its sufficiency. With equal favom- that featm-e 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 smce Hal- 

 ley expounded his trade-wind theory suggested a review of that 

 featm-e, and it was fomid 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 vapom^ 

 for the equatorial cloud-ring and its rains is now also (§ 252) re- 

 ■cognized 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 other agcut that is supposcd to be conccmed with the 

 ^f magnetism in t e ^^^^-j^.^^.^-^^jg jj^ ^]^g^, castiug ? I canuot Say wliore 

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

 trial magnetism — considering the close relations between many of 

 its phenomena and those both of heat and electricity — the question 

 may be asked whether some poAver capable of guiding " the wind 

 in his circuits " may not Im'k there ? Oxygen comprises more than 

 one fifth part (two ninths) of the atmosphere, and Faraday has dis- 

 covered that oxygen is para-magnetic. If a bar of iron be sus- 

 pended between the poles of a magnet, it will arrange itself axially, 

 and point toAvards 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 natm-e belong to one or the other 

 of the two classes of which iron and bismuth are the types. 



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

 Lines of masnetic ^ maguctizcd bar of irou ou a smooth smface, and 

 force. " g-f^ £j^g ^,Qj^ filings doAvn upon it, these filings will 

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



