292 M. A. tie la Rive on the Diurnal Variations of 



by conducting the electric discharges, gives rise to the lumi- 

 nous 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 develope 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 phaenomenon ? Here is 

 my answer. Place the pole of a powerful electro-magnet be- 

 • neath a large surface of mercury ; let this surface communi- 

 cate 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 cur- 

 rents 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 pecu- 

 liar effect disappears without the voltaic light being inter- 

 rupted when the electro-magnet ceases to be magnetized. 

 With a continuous current of ordinary electricity arriving at 

 the pole of a powerful electro-magnet in rarefied and moist 

 air, luminous effects, still more similar in appearance to those 

 of the aurora borealis, are obtained. 



These phaenomena result from the action of magnets on 

 currents; now the same should apply to the action of the 

 magnetic pole of the earth ; the neutralization of the two elec- 

 tricities 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 perspective appear to us 

 to form the corona of the aurora. The sulphurous odour, 

 and the noise which is said sometimes to accompany the ap- 

 pearance of the aurora, would not be inexplicable ; for the 

 odour would be due, like that which accompanies lightning, 

 to that modification 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 shown, the voltaic arc produces when 

 it is under the influence of a very near magnet. If it seldom 

 occurs in the case of the aurora, 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 sus- 



