Diurnal Variations of the Magnetic Needle. 153 



sentially the moisture adhering to the inner sides of the tube, which, 

 by conducting the electric discharges, gives rise to the luminous 

 effects. It will be conceived that the electric discharges transmitted 

 by this kind of network of ice must, on becoming concentrated near 

 the pole, produce there a far more brilliant light than they develop 

 when they are distributed over a much greater extent. 



But why does the magnetic pole, and not the terrestrial pole, 

 appear to be the cause of the phenomenon ? Here is my answer. 

 Place the pole of a powerful electro-magnet beneath a large surface 

 of mercury ; let this surface communicate with the negative pole of a 

 powerful battery ; bring near to it the point of a piece of charcoal 

 communicating with the positive pole of the battery ; immediately the 

 voltaic arc is formed, and the mercury is seen to become agitated 

 above the electro-magnet; and wherever this is placed, luminous 

 currents are observed to rotate around this pole, and throw out from 

 time to time some very brilliant rays. There is always, as in the 

 case of the aurora borealis, a dark portion in the form of a circular 

 point over the pole of the magnet : this peculiar effect disappears 

 without the voltaic light being interrupted when the electro-magnet 

 ceases to be magnetised. With a continuous current of ordinary 

 electricity arriving at the pole of a powerful electro-magnet in rare- 

 fied and moist air, luminous effects, still more similar in appearance 

 to those of the aurora borealis, are obtained. 



These phenomena result from the action of magnets on currents : 

 now the same should apply to the action of the magnetic pole of the 

 earth ; the neutralisation of the two electricities probably takes place 

 over a somewhat large extent of the polar regions ; but the action 

 of the magnetic pole causes the conducting mists to rotate around 

 it, sending forth those brilliant rays which, by an effect of perspec- 

 tive, appear to us to form the corona of the aurora. The sulphu- 

 reous odour, and the noise which is said sometimes to accompany the 

 appearance of the aurora, would not be inexplicable ; for the odour 

 would be due, like that which accompanies lightning, to that modifi- 

 cation which the passage of electric discharges produces upon the 

 oxygen of the air, which M. Schonbein has called ozone ; while, as 

 regards the noise, it would be analogous to that which, as I have 

 shewn, the voltaic arc produces when it is under the influence of 

 a very near magnet. If it seldom occurs in the case of the au- 

 rora, it is owing to its being very rare that the luminous arch is 

 sufficiently near the earth, and consequently to the pole. However, 

 the description which has been given of this noise by those who have 

 heard it, is perfectly identical with that which I have given, without 

 suspecting the analogy, of the noise which the voltaic arc produces in 

 the action of the magnetism. 



The magnetic disturbances which always accompany the appear- 

 ance of an aurora borealis are now easily explained. This accidental 

 union of a greater proportion of the accumulated electricities must 



