4 LIGHT SCIENCE FOR LEISURE HOURS. 



appears, the magnetic needle is often seen to move 

 in a few instants over several degrees.' * During an 

 aurora,' he adds, ' one often sees in the northern 

 region of the heavens luminous streamers of different 

 colours shoot from all points of the horizon. The 

 point in the sky to which these streamers converge 

 is precisely the point to which a magnetised needle 

 suspended by its centre of gravity directs itself. . . . 

 It has, moreover, been shown that the concentric 

 circular segments, almost similar in form to the rain- 

 bow, which are usually seen previous to the appearance 

 of the luminous streamers, have their two extremities 

 resting on two parts of the horizon which are equally 

 distant from the direction towards which the needle 

 turns ; and the summit of each arc lies exactly in that 

 direction. From all this, it appears, incontestably, that 

 there is an intimate connection bet^veen the causes of 

 auroras and those of terrestrial magnetism.' 



This strange hypothesis was, at first, much opposed 

 by scientific men. Amongst others, the late Sir David 

 Brewster pointed out a variety of objections, some of 

 which appeared at first sight of great force. Thus, 

 he remarked that magnetic disturbances of the most 

 remarkable character have often been observed when 

 no aurora has been visible ; and he noticed certain 

 peculiarities in the auroras observed near the polar 

 regions, which did not seem to accord with Arago's 

 view. 



But gradually it was found that physicists had mis- 

 taken the character of the auroral display. It appeared 



