On the Aurora Borealis. 261 



wire in the observatory, when distinct separation of the gold 

 leaves took place. At 6 p.m. an Aurora was visible from E. to 

 W. and N.W. across the zenith ; it was in the form of bands or 

 streamers. I again tried the electrometer, and again perceived 

 distinct divergence of the gold leaves. This Aurora disap- 

 peared about 7 p.m. Again, at 8"30, there was an Aurora, 

 stretching from S.S.W. to S.S.E., in the form of a bent arch or 

 horseshoe, the key being in S.S.E. Again the electrometer 

 was connected, and a still greater divergence of the gold leaves 

 than before was noticed. This may be from the greater lumi- 

 nosity of the Aurora. I tried paper saturated with iodide of 

 potassium, interposed between two platinum wires, connected 

 with the chain and the water, but no decomposition took place 

 and no spot was obtained. 12 p.m. : this Aurora is still visible, 

 but with no particular shape; it extends from S.S.W. by S. to 

 1ST., and not only horizontally but vertically scintillations appear. 

 It is most luminous towards the S. where occasionally a wave 

 appears, not like a pulse, as was the case the last two nights, 

 but as if the cloudy appearance had been connected in the 

 S.S.E. with an electric machine which, when turned, caused a 

 flash of light to proceed from S.S.E. to S. Thin streamers 

 passing towards the zenith ; the body of the light decidedly 

 obscures the stars of all magnitude behind it. Temperature 

 — 23°-5, bar. 29 3 82/' 



So much for my own observations. Before, however, de- 

 ducing thence any theory, I will condense a few of the latest 

 and most plausible. M. Biot's is in substance as follows : — 

 That the luminous clouds of which the Aurora consists are 

 composed of metallic particles, reduced to an extremely minute 

 and subtle form. Such metallic clouds — -if the expression may 

 be permitted — will be conductors of electricity, more or less 

 perfect, according to the greater or less proximity of their con- 

 stituent particles. When such clouds arrange themselves in 

 columnar forms, and connect strata of the atmosphere at dif- 

 ferent elevations ; if such strata be unequally charged with 

 electricity, the electrical equilibrium will be re-established 

 through the intervention of the metallic columns, and light and 

 sound will be evolved in proportion to the imperfect conducti- 

 bility of the metallic clouds, arising from the extremely rarefied 

 state of the fine dust or vapour of which they are composed. 

 If the metallic cloud possess the conducting power in a high 

 degree, the electric current may pass through it without the 

 evolution of light or sound ; and thus the magnetic needle may 

 be affected as it would be by an Aurora, though none be visible. 

 If any cause alter the conductibility of those columnar clouds, 

 suddenly or gradually, a sudden or gradual change would follow 

 in the splendour of the Aurora. 



