156 TERRESTRIAL MAGNETISM. 



intensity is feeble. As the tone of colour becomes more 

 vivid it passes into yellow, the middle of the broad beam 

 becoming full yellow, and detached red and green 

 appearing at either margin. When the beams are long 

 and narrow, the red is above and the green below. 

 When the motion is lateral from left to right, or vice 

 versa, the red always appears on the side towards which 

 the movement is directed, and the green remains behind." 

 Of these complementary colours, green and red, it is 

 very rare to see one without the other. Blue is not seen 

 at all ; and a dark red, resembling the reflection of a 

 fire, is so rare in the North, that Siljestrom only saw it 

 once.( 210 ) The illuminating power of the aurora borealis 

 never quite equals, even in Finmarken, that of the full 

 moon. 



The connection so long maintained by me as probable, 

 of the polar light with the formation of the " smallest 

 and most attenuated cirrus, or light fleecy clouds, the 

 parallel equidistant lines of which most often follow 

 the direction of the magnetic meridian," has indeed 

 found in more recent times many defenders ; but whether, 

 as the northern traveller Thienemann and Admiral 

 Wrangel think, these lines of cirri form the substratum 

 of the aurora, or whether they are not rather, as has 

 been the opinion of Franklin, Kichardson, and myself, 

 the effects of a meteorological process accompanying and 

 produced by the magnetic storm, still remains unde- 

 cided. ( 211 ) 



Besides the general conformity of the direction of the 

 magnetic declination with that of the regularly arranged 

 and very delicate cirrus ("bandes polaires"), my atten- 

 tion was strongly attracted, both in Mexico in 1803 



