THE AUROEA BOREALIS. 741 



charge already formed, but cannot contribute to its formation, which 

 must depend only on the conditions in which the different layers 

 of air are found. Although terrestrial magnetism has an influence 

 upon the position of the luminous arc of the Aurora, it is difficult 

 to admit with Hansteen and Bravais that the position of this arc 

 should be determined only by the magnetic pole. Thus, the summit 

 of the polar arc is rarely in the exact direction of the declination 

 needle. From 226 observations made upon the position of the 

 azimuth of the luminous polar arc we find 36 per cent., which give 

 for this position 30° more to the west ; 32, which give 10° to 20°, 

 7, 0° to 10°, and 4, 0° to 26° to the east ; from which it appears that 

 the position of the arc varies from 25° to 30° and more. These 

 variations are too great to be explained by accidental perturbations 

 in the terrestrial magnetism, as much as about 6° to 7° more than 

 the greatest deviations in the magnetic declination. 



We therefore think that terrestrial magnetism plays only a re- 

 latively secondary part in the phenomena of the polar light ; that 

 this part consists essentially in a direct action upon the rays of this 

 light, and in a movement of rotation exercised upon the rays, cir- 

 cumstances demonstrated positively by the experiences of M. de la 

 Eive. 



(p. 160.) The experiments of M. de la Eive, which have shown 

 the influence of magnetism on the electric light in circumstances 

 nearly identical with those which the polar light presents, do not 

 at all furnish the proof that the rays of this light are really unite! 

 under this influence. The polar light considered as an electric 

 discharge gives the following results : — 



(1.) An electric current arising from the discharge itself, which 

 takes place slowly. 



(2.) Eays of light consisting of an infinite number of sparks, each 

 spark giving rise to two induction currents going in opposite direc- 

 tions. 



(3.) A galvanic current going in an opposite direction to that of 

 the discharge, and having its origin in the electro-motive force dis- 

 covered by M. Edland in the electric spark. 



To be developed, these currents require a closed circuit ; it is true 

 that in the phenomena of the Aurora, strictly speaking, it does 

 not exist, but it is not necessary, seeing that in this case the earth 

 and the rarefied air of the upper regions are immense reservoirs of 

 electricity, which produce the same effect as if the circuit were 

 closed. 



According to the theory of M. de la Rive, the positive discharge 

 of electricity from the air to the earth produces a current which I 

 shall call the principal current. This is counterbalanced in part by 

 the current due to the electro-motive force of the spark. 



We see, by observations made with telegraph wires during the 

 presence of the Aurora, that sometimes one and sometimes the 

 other prevails, the first being in general predominant, the current 

 given by telegraph wires being more frequently from north to 

 south than from south to north. 



(p. 163.) The cause of the clouds of the higher regions being 



