154 TERRESTRIAL MAGNETISM. 



more recent observations have in general been favour- 

 able to the views there expressed, in which " the aurora 

 was not regarded as the cause of the disturbance in the 

 equilibrium of the distribution of the earth's magnetism, 

 but rather as the result of a state of telluric activity 

 excited to the point of the production of a luminous 

 phenomenon ; an activity manifesting itself on the one 

 hand by the fluctuations of the needle, and on the other 

 by the appearance of the brilliant auroral light." In, 

 this view the polar light might appear as a kind of 

 silent discharge, as the termination of a magnetic 

 storm. In electric storms the equilibrium of the dis- 

 turbed electricity is also restored by a development of 

 luminosity, by the flashing lightning, accompanied by 

 the noise of thunder. In a phenomenon so complex and 

 mysterious, the repeated bringing forward of a deter- 

 minate hypothesis ( 204 ) has at least the advantage of 

 inviting a more sustained and careful observation of all 

 those details by which it may be confuted or corroborated. 

 In dwelling on the purely objective description of 

 these phenomena, and availing myself principally of 

 the fine uninterrupted eight-months series of examina- 

 tions for which we are indebted to the sojourn of several 

 distinguished physicists ( 205 ) in the extreme north of 

 Scandinavia in 1838 1839, I would direct the atten- 

 tion in the first place to the dark misty wall which rises 

 gradually on the horizon, and is called the "black 

 segment of the aurora borealis." ( 206 ) The blackness, as 

 Argelander remarks, is not a result of contrast, for it is 

 sometimes seen before the luminous arch begins to 

 bound it: it is a process going forward in apart of the 

 atmosphere, for hitherto we have had no evidence of 



