1855.] 



ON THE CAUSE OP THE AURORA BOREALIS. 



125 



This last statement indicates that I regard the cause as elec- 

 trical. Thi-5 view has often been presented before, and was 

 brough forward bj Arago at the time of Oersted's diseoveiy. 

 Yet no one, to uiy knowledge, has explained the mode of 

 action and production of the electricity, or the attendant phe- 

 nomena resulting from this cause. 



Without going into any historical details, I will briefly des- 

 cribe the aurora borealis itself and its effects, and then pass 

 to my own theory, the accordance of which with facts I shall 

 endeavour to point out. 



1. Description of the Aurora and its accom- 

 panying EFFECTS. 



I cite the following, details principally from the Cosmos. 

 They are derived mostly from the descriptions of Hansteen, 

 Bravais, Lottin, and other travellers who have been in favorable 

 places for observing the aurora. The learned author of Cos- 

 mos has grouped the facts with great skill, presenting in an 

 admirable manner the prominent points, and seems with scien- 

 tific tact to reach towards the true theory of the phenomena 

 which he describes. 



An aurora borealis is always preceded by the formation in 

 the horizon of a kind of nebulous veil, which ri-ses slowly to a 

 height of four to six or eight, or even ten degrees about the 

 mignetic meridian ; the sky though before pure, becomes 

 darkened, and over this obscure segment, whose color varies 

 from brown to violet, the stars are seen as through a thick 

 haze. An arc of light, first white, and afterwards yellow, bor- 

 ders the dark segment. Sometimes this luminous arc is 

 agitated for hours by a sort of effervescence, and a constant 

 change of form, before it rises into the rays or columns of 

 light which mount to the zenith. The more intense the 

 emission of the polar light, the brighter are the colors that 

 appear, which from violet and bluish white pass by interme- 

 diate shades to green and purplish red — just as electric sparks 

 are coloured only when the tension is strong, and the explosion 

 violent. Sometimes the columns of light proceeding from the 

 luminous arc are mixed with blackish or smoky columns; 

 sometimes they rise simultaneously from ditfercnt points of the 

 horizon ; or they may unite in a sea of flames of indescribable 

 magnificence, the form and brilliancy of which are in incessant 

 change. The motion gives greater visibility to the phenome- 

 non. Around the spot in the heavens towards which the 

 dipping needle points, the rays appear to cluster and form a 

 corona. Rarely the aurora continues till the corona is on all 

 sides complete, and when this happens, it announces that the 

 end of the exhibition is near at hand. The rays then become 

 feebler, shorter, and less bright in their colors. Soon, only 

 large nebulous motionless spots, of a pale or ashy tint, are 

 seen over the celestial vault ; and finally, traces of the dark 

 segment in the northern horizon, where the appearances began, 

 alone remain. 



The connection between the polar light and a certain kind 

 of cloud is recognized by all observers, who affirm, that the 

 polar light sends fitrth its bn'f/htest roliimns whrn the upper 

 reijionsdf tlie air contain masses o/ cirro-stratus rluuds of great 

 tenuity, leliieli tend to form a corona around tin- light. Some- 

 times the clouds are grouped and arranged like the auroral 

 columns ; and in this case they appear to disturb the magnetic 

 needle. After a brilliant aurora, the trains of clouds in the 

 morning have sometimes been fimnd to indicate the positions 

 of as many luniimms columns during the nijjht. 



The absolute height of the aurora has been variously esti- 

 mated. For a long time it was supposed that it might be 



ascertained by the observations of distant observers on the 

 corona : but it is now well known that the corona is only an 

 effect of perspective, due to the apparent convergence of rays 

 which are parallel to the dipping needle ; so that each sees his 

 own corona, as each his own rainbow. Moreover the aspect of 

 the phenomenon depends on the position of the observer. 

 The scat of the aurora is in the upper regions of the atmos- 

 phere; but .sometimes it appears to be produced within less 

 elevated regions, where clouds are formed. Suth ob.servations 

 as those of Capt. Franklin appear to establish the latter conclu- 

 sion, who saw an aurora which lighted up the under surface of 

 the clouds, whilst Mr. Kendall, two or three miles distant, saw 

 no light whatever, although awake and constantly observing 

 the sky. Captain I'arry also asserts his seeing an aurora 

 depicted on the flank of a mountain : and it is said that a 

 luminous arc has been see« on the surface even of the sea, 

 around the magnetic pole. 



Mairan and Dalton believed the aurora borealis to be cosnii- 

 cal, and not atmospheric. But Biot, who had an opportunity 

 of observing the aurora at the Shetland Isles in 1817, proved 

 it to be an atmospheric phenomenon, from finding that it did 

 not partake of the movement of the stars from west to east, 

 and consequently moved with the earth's rotation. Since then, 

 nearly all observers have come to the same conclusion ; and in 

 particular MM. Lottin and Bravais, who have observed more 

 than 143 auroras, and given detailed description of them. 



It is therefore quite certain that the aurora is not extra- 

 atmospheric. To the evidence from its appearances, we may 

 also add the crackling noise sometimes affirmed to be heard by 

 the inhabitants in the far north, and the sulphurous odor 

 which also has been observed. And, in fine, if the phenomenon 

 is wholly beyond our planet, why should it be located about 

 the polar regions ? M. de Tessan, in the voyage of the Venus 

 around the world, saw a fine aurora australis, which he des- 

 cribes with care. It was 14° in height, and the centre of the 

 arc was in the magnetic meridian. He heard no sounds con- 

 nected with it, which he attributes to its distance : but he men- 

 tions that M. Verdier, a French naval officer, on the night of 

 Oct. 13th, 1819, while on the coast of New Holland, heard 

 distinctly a kind of crepitation, during a brilliant aurora. All 

 the details mentioned by 31. de Tessan prove the exactness of 

 the observations. 



As concomitant effects of the aurora, we have mentioned 

 the crackling sound, and the sulphurous odor. M. Matteucci 

 has also observed during the appearance of a late aurora, 

 satisfactory evidence of positive electricity in the air. But of 

 all the phenomena, those which arc of most invariable occur- 

 rence are the magnetic. The magnetic needle undergoes per- 

 turbations, either to the west or cast, and u ually the latter. 

 These disturbances vary in intensity, but never fail of taking 

 place ; and they are at times manifested in places where no 

 aurora is seen. This coincidence of magnetic disturbance with 

 the aurora, shown by Arago to be without exception, fiom 

 many years of observations, enabled this philosopher to tell, 

 while in the basement of the Faris Observatory, when there 

 was an aurora in our hemisphere. M. Matteucci has observed 

 this magnetic influence under a new form. iJuring the aurora 

 of Nov. 17, 184S, the armatures of soft iron used with the 

 Electric Telegraph between Florence and I'isa remained 

 attached to their electro-magnets as if strongly magncti.-cd, 

 although the apparatus was not in action, and the batteries cut 

 of use. 



M. de Tessan cites an oliscrvation made in 1818, by M. 

 Baral, imother French naval oiScer, on the same coasts of New 



