152 Diurnal Variations of the Magnetic Needle. 



are not always exhibited in the same manner, nor at all seasons of 

 the year. 



It is now well proved that the aurora borealis is an atmospheric 

 phenomenon, as we long ago suspected. The name of magnetic 

 storm, by which Von Humboldt designates it in his Cosmos, implies 

 the same idea, which is moreover confirmed by the interesting de- 

 tails which he gives of this meteor. The observations of Parry, 

 Franklin, and especially those of MM. Bravais and Lottin, so nu- 

 merous 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 northern hemisphere, no longer heats so much our hemi- 

 sphere, the aqueous vapours which have accumulated during the sum- 

 mer 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 por- 

 tions 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 semi-transparent mist. Now these half- frozen fogs con- 

 duct 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, extend 

 in an almost uninterrupted manner from the polar regions to some- 

 what southern latitudes, which must be of rare occurrence. These 

 same clouds, when they are partial, which is frequently the case, 

 produce the halos. 



Now the analogy pointed out by nearly all observers between the 

 mists which accompany the aurora borealis and those which produce 

 the halos, is a somewhat remarkable circumstance. It is easy to 

 verify by direct experiment the identity which exists between the 

 light of the aurora borealis, and that obtained by passing a series 

 of electric discharges into rarified air containing a large quantity 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 ascer- 

 tained that highly rarefied, but perfectly dry air, gives but a very 

 faint light, and that in the experiment of the vacuum tube it is es-^j 



