the Magnetic Needle and on Aurorce BorealeS. 291 



his Cosmos, implies the same idea, which is moreover con- 

 firmed by the interesting details which he gives of this meteor. 

 The observations of Parry, Franklin, and especially those of 

 MM. Bravais and Lottin, so numerous and carefully made, 

 are likewise quite favourable to this opinion, which followed 

 equally from the observations of M. Biot at the Shetland 

 Isles. 



Admitting this point, I explain the production of the aurora 

 borealis in the following manner: — When the sun, having 

 passed into the southern hemisphere, no longer heats so much 

 our hemisphere, the aqueous vapours which have accumulated 

 during the summer in this part of the atmosphere begin to 

 condense, the kind of humid cap enveloping the polar regions 

 extends more and more, and facilitates the passage of the 

 electricity accumulated in the upper portions of the air. But 

 in these elevated regions, and especially at this period of the 

 year, the aqueous vapours must most frequently pass into the 

 state of minute particles of ice or snow floating in the air, 

 similar to those which give rise to the halos ; they form, as it 

 were, a kind of semitransparent mist. Now these half-frozen 

 fogs conduct the electricity to the surface of the earth near 

 the pole, and are at the same time illumined by these currents 

 or electric discharges. In fact, all observers agree in asserting 

 that the aurora borealis is constantly preceded by a mist 

 which rises from the pole, and the margins of which, less 

 dense than the remainder, are coloured the first; and indeed 

 it is very frequent near the pole in the, winter months, and 

 especially in those where there is abundance of vapour in the 

 air. For it to be visible at great distances from the pole, it is 

 necessary that these clouds, composed of frozen particles, ex- 

 tend in an almost uninterrupted manner from the polar regions 

 to somewhat southern latitudes, which must be of rare occur- 

 rence. These same clouds, when they are partial, which is 

 frequently the case, produce the halos. 



Now the analogy pointed out by nearly all observers be- 

 tween the mists which accompany, the aurora borealis and 

 those wiiich produce the halos, is a somewhat remarkable cir- 

 cumstance. It is easy to verify by direct experiment the iden- 

 tity which exists between the light of the aurora borealis and 

 that obtained by passing a series of electric discharges into 

 rarefied air containing a large quantiiy of aqueous vapour, and 

 especially through a very thin layer of snow or a slight layer 

 of hoar frost deposited on the glass. I have ascertained that 

 highly rarefied but perfecdy dry air gives but a very faint light, 

 and that in the experiment of the vacuum-tube it is essentially 

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



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