Proceedings of the Polytechnic Association. 927 



that it may instantly congeal under some mechanical agitation. 

 Now the water at the upper surface of a cloud may be in such a 

 state. We can, therefore, see how aggregation in the form of sheet 

 falling from a superposed stratum may produce the necessary agita- 

 tion, and thus become immediately coated with a laminated covering 

 of ice, forming hailstones. But I will not dwell farther on this 

 complex subject. 



AUEOKA BOREALIS 



Is a phenomenon which I take to be as fully due to atmospheric 

 electricity as are any of those before considered. 



According to the testimony of all observers, this phenomenon is 

 always accompanied by a peculiar /mse or veil, which, although 

 allowing the light of the stars to pass, gives the sky a sombre aspect, 

 Tliis haze is known to be composed of fine transparent needles of ice. 



M. M. Bixio and Baral, who, being raised in a balloon to a great 

 height, found themselves, on a sudden, although the sky was serene 

 and the atmosphere cloudless, in the midst of a perfectly transparent 

 veil, formed of a multitude of little icy needles, so fine that they 

 were scarcely visible. Dr. Richardson, in a temperature of fifty- 

 seven degrees below freezing point, having seen an auro9'a, the arch 

 of which was near the zenith, remarked that, although the sky 

 appeared perfectly serene during the display, a fine snow was falling, 

 scarcely perceptible to the nalced eye. At another time he witnessed 

 a similar fact under a brilliant sun, the rays of which permitted him 

 to distinctly see the transparent icy needles. Gisler says, that in 

 Sweden, upon the high mountains, the traveler is sometimes suddenly 

 enveloped in a transparent fog of a whitish grey color, passing into 

 green, which is transformed into an aurora horealis. When such 

 haze or particles of ice are precipitated from vapor highly electrized, 

 the electricity becomes free and luminous, as was the case in the 

 snow cloud described by Beccaria. We are led to the conclusion, 

 therefore, that the aurora arises from electric discharges, which take 

 place between the luminous icy particles suspended in the air, and 

 which, in infinite numbers, communicate with the earth or moist air 

 below. 



The arch from which the aurorial streamers are seen to radiate, is 

 the boundary between the cold region occupied by tlie icy needles, 

 and the milder region of moist air in which the discharges cease to 

 be luminous. We may suppose thut tlie production of atirora in the 

 arctic and antarctic regions should be the normal state, and of daily 



