AURORA BOREALIR. 189 



festccl cause of this disturbance, but rather as a result of 

 telluric activity, manifested on the one side by the appear- 

 ance of the light, and on the other by the vibrations of the 

 magnetic needle." The splendid appearance of coloured polar 

 light is the act of discharge, the termination of a magnetic 

 atorm, as in an electrical storm a development of light the 

 flash of lightning indicates the restoration of the disturbed 

 equilibrium in the distribution of the electricity. An electric 

 storm is generally confined to a small space, beyond the limits 

 of which the condition of the atmospheric electricity remains 

 unchanged. A magnetic storm, on the other hand, shows its 

 influence on the course of the needle over large portions of 

 continents, and, as Arago first discovered, far from the spot 

 where the evolution of light was visible. It is not im- 

 probable that as heavily charged threatening clouds, owing 

 to frequent transitions of the atmospheric electricity to an 

 opposite condition, are not always discharged, accompanied by 

 lightning; so likewise magnetic storms may occasion far- 

 extending disturbances in the horary course of the needle, 

 without there being any positive necessity that the equilibrium 

 of the distribution should be restored by explosion or by the 

 passage of luminous effusions from one of the poles to the 

 equator, or from pole to pole. 



In collecting all the individual features of the phenomenon 

 in one general picture, we must not omit to describe the 

 origin and course of a perfectly developed aurora borealis. 

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

 heavens which is intersected by the magnetic meridian, 

 the sky which w r as previously clear is at once overcast. A 

 dense wall or bank of cloud seems to rise gradually 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 cloudy stratum, as when a 

 dense smoke darkens the sky. A broad brightly luminous 

 arch, first white, then yellow, encircles the dark segment; 

 but as the brilliant arch appears subsequently to the smoky 

 gray segment, we cannot agree with Argelander in ascribing 

 the latter to the effect of mere contrast with the bright 

 luminous margin.* The highest point of the arch of light is, 



* Argelander, in the important observations on the northern light 

 embodied in the Vortrdgen gehalten in der physikcdisch-OJconomisclien 

 Qestellschaft zu Konigsberg, bd. i. 1834, s. 257-264. 



