206 ESSAYS BIOGRAPHICAL AND CHEMICAL 



menon ; the oscillations of the compass may even exceed 

 ten minutes of arc, as observed by Mr. James Glaisher l in 

 1847. And many hypotheses have been brought forward 

 to account for the connection between the two simul- 

 taneous phenomena. The last few years have seen the 

 equipment of expeditions to Iceland, Finland, and Northern 

 America, which have had for their principal object the 

 observation of the earth's magnetic disturbances and the 

 corresponding auroral displays. Many theories have been 

 advanced, and it will be my task to try to bring them 

 before you, and to supplement them where they appear to 

 be wanting. 



Let us first, however, listen to an eloquent description 

 of the Northern Lights from the pen of the celebrated 

 Alexander von Humboldt : 2 



' Low down in the distant horizon, about the part of the 

 heavens which is intersected by the magnetic meridian 

 (i.e. the point to which the compass-needle is directed), 

 the sky, which was previously clear, is at once overcast. 

 A dense wall or bank of cloud seems to rise higher and 

 higher, until it attains an elevation of 8 or 10 degrees. 

 The colour of the dark segment passes into brown or 

 violet, and stars are visible through the smoky stratum, 

 as when a dense smoke darkens the sky. A broad, 

 brightly luminous arch, first white, then yellow, encircles 

 the dark segment. . . . The luminous arch remains some- 

 times for hours together, flashing and kindling in ever- 

 varying undulations before rays and streamers emanate 

 from it and shoot up to the zenith. The more intense 

 the discharge of the northern light, the more bright is the 

 play of colours, through all the varying gradations from 

 violet and bluish-white to green and crimson. The mag- 

 netic columns of flame rise either singly from the luminous 



1 Philosophical Transactions, Iviii. 



2 Cosmos (Bohn's edit.), vol. i. p. 189. 



