96 



THE AURORA BOREALIS. 



serrations at the same instant of time, and the total impossibility of the two ob- 

 servers being certain of directing their observations to precisely the same point of 

 the aurora. To such causes must be ascribed the widely-varying estimates of 

 the height of auroras ; obtained in this manner estimates which vary from 

 fifty to three hundred miles from the surface of the earth. Meanwhile, what- 

 ever be their height, it is evidently subject to continual variation, even in the 

 same aurora, as is rendered apparent by the sudden changes which the phe- 

 nomenon undergoes, and by the progressive motion of its arcs. 



Great differences have existed among meteorologists respecting the sounds 

 which are said to proceed from auroras. The inhabitants of the northern re- 

 gions, where these appearances most prevail, are unanimous in declaring that 

 they are frequently accompanied by hissing and cracking noises in the air, 

 like those produced by artificial fireworks. Persons engaged in the whale- 

 fisheries make the same statements. M. Biot found the inhabitants of the 

 Shetland islands unanimous on the question ; and M. Lottin found the same 

 impression among the far-distant inhabitants of Siberia. On the other hand, 

 during the sojourn of M. Biot in the Shetland isles, he witnessed several great 

 auroras, but heard no sound. During M. Lottin's expedition, he witnessed 

 one hundred and forty-three auroras, in not one of which was he sensible of 

 any sound. The only strictly scientific observer who appears to have person- 

 ally experienced such sounds is Cavallo, who states that he has distinctly 

 heard them on several occasions, but limits his testimony to this general form, 

 assigning neither time nor place. Such discordancy of evidence can oaly be 

 reconciled by the supposition that such sounds are audible on rare occasions, 

 when the region in which the aurora is developed is within a very limited dis- 

 tance of the observer ; and if the existence of such sounds be thus admitted, 

 it must be also admitted that the height of the aurora is, at least in such cases, 

 infinitely less than is commonly estimated ; and if, in particular cases, its 

 height be so small, it is probably in all others proportionally under the highest 

 estimates which have been made of it. 



From a comparison of all the observed effects, it may then be assumed as 

 nearly, if not conclusively, proved, that the aurora borealis is composed of real 

 clouds, proceeding generally from the north, and formed of extremely attenuated 

 and luminous matter floating in the atmosphere, which frequently arrange them- 

 selves in series of lines or columns parallel to the dipping needle. What tho 

 nature of the matter is composing such clouds must, in the present state of 

 science, rest upon mere conjecture. The following is the substance of the 

 theory of M. Biot on this subject already referred to : 



Among material substances, certain metals alone are susceptible of magnet- 

 ism. Since, then, the luminous matter composiag the aurora obeys the magnetic 

 influence of the earth, it is very probable thai the luminous clouds of which it 

 consists are composed of metallic particles reduced to an extremely minute and 

 subtile form. This being admitted, another consequence will immediately ensue. 

 Such metallic clouds, if the expression be allowed, will be conductors of elec- 

 tricity, more or less perfect, according to the degree of proximity of their con- 

 stituent particles. When such clouds arrange tliemselves in columnar forms, 

 and connect strata of the atmosphere at different elevations, if such strata be 

 unequally charged with electricity, the electrical equilibrium will be re-estab- 

 lished through the intervention of the metallic columns, and light and sound 

 will be evolved in proportion to the imperfect conductability of the metallic 

 clouds arising from the extremely rarefied state of the metallic vapor, or fine 

 dust, of which they are constituted. All the results of electrical experiment* 

 countenance these suppositions, when the phenomena are produced in tk 

 more elevated regions, where the air is highly rarefied, little resistance being 



