164 



ON THE CAUSE OF THE AL'HOKA BOREALIS. 



[1855. 



On the Cause of the Aurora Borealis. 



BY PROF. A. DE LA RIVE.* 

 (Continued from page 124.) 



Agreement of the theory with the facts. 



We have remarked that all observers agree now in regarding 

 the aurora as an atmospheric phenomenon, and we have cited 

 facts in support of this view. One more fact may be alluded to 

 here which places it beyond doubt ; it is from the observations 

 on the aurora borealis published in the history of the Voyage of 

 Captain Franklin. Lieutenant Hood and Dr. Richardson were 

 55 miles apart for the purpose of making simultaneous ob- 

 servations, in order to ascertain the parallax of the phenomenon 

 and consequently its height. The result from three trials place 

 it alike at a height of 6 to 7 miles. On the 2nd of April, at 

 the most northerly station a brilliant arc was seen 10° above 

 the horizon ; at the other station, it was not visible. The 6th 

 of August the aurora was at the zenith at one station, and 

 9° in height at the other. On the 7th of April it was again 

 in the zenith at the first station, and 9° to 11° in height at 

 the second. 



Again, Hansteen, and after him, MM. Lottin and Bravais, 

 were" led to believe as a consequence of their observations, that 

 the arc of the aurora is a luminous ring whose difierent parts 

 are sensibly equidistant from the earth, and which is centered 

 around the magnetic pole so as to cut at a right angle all the 

 magnetic meridians which converge towards this pole. Such 

 a rino- is the auroral arch and its apparent summit is necessarily 

 in the magnetic meridian of the place. 51. Bravais also ob- 

 serves that the arc seems to have a kind of movement of 

 rotation from the west to the east passing by the south. 

 From this description the phenomenon is quite similar 

 to the result of the experiment described above, and the di- 

 rection of the rotation in the luminous ring is precisely that 

 which ought to take place according to the laws governing 

 the mutual action of currents, if it be the positive electricity 

 which passes from the atmosphere to the surface of the earth, 

 thence to penetrate about the north magnetic pole, reunite with 

 the negative electricity, and thus constitute the cuiTent. 



The diameter of the luminous ring will be greater, as the 

 rnagnetic pole is more distant from the earth's surface, 

 since this pole ought to be foimd in the intersection of the 

 plane of the ring with the axis of the terrestrial globe. 



It hence results that each observer sees the summit of the 

 auroral arc in his own magnetic meridian ; and hence only 

 those on the same magnetic meridian see the same summit, 

 and can take simultaneous observations for ascertaining the 

 height. 



If the summit of the arc pass the zenith of the observer, he 

 is surrounded on all sides by the matter of the aurora, or 

 the auroral influences which proceed from the earth, and then, 

 if at all, the crackling sound which has been alluded to should 

 be heard. If it does not reach the zenith, the observer is 

 then outside of the region ; and the aurora is more or less 

 distant according to its altitude. The noise may be produced 

 by the action of a powerful magnetic pole on luminous electric 

 jets very near this pole, as I have proved by experiment; I 

 have succeeded in producing a similar sound by bringing a 

 piece of iron, strongly magnetised, to the luminous arch 



* Mem. Soc. de Phys. et Hist. Nat. de Geneve, xii,, and Bib. Univ., 

 xxiv, 337, Dec. 1853. 



formed between the poles of a voltaic battery. 



As to the sulphurous odor, it proceeds like that which ac- 

 companies lightning, from the conversion of the oxygen of the 

 air into ozone by electric discharges. 



The light of the aurora is not polarized, as was remarked by 

 Bi.;t in 1817, from his observations at the Shetland Islands. 

 This neg'ative result is confirmed by Mr. Macquorn Eankine, 

 who has shown that this absence of polarisation is not due to 

 the feebleness of the light, since this same light viewed after 

 reflection from water is found to be polarised by this reflection. 

 The most careful study and experiment have found no trace of 

 polarisation in electric light, whether the discharges be made 

 in the air or in a vacuum. This is a new proof of the identity 

 of these two kinds of phenomena. 



Finally, we discover in the resemblance between auroral ap- 

 pearances and certain clouds as well as the disturbances of the 

 magnetic needle, a further important confirmation of our 

 theory. 



The observations of Dr. Piichardson already mentioned, 

 which show that the aurora exists at moderate elevations, also 

 indicate that it is often connected with the formation of difierent 

 kinds o{ cirro-stratus clouds. Lieutenant Hood, in speaking 

 of the luminous bands or columns of the aurora, sajs that he 

 is convinced that they are carried by the wind, because they 

 retain exactly their relative situation, which is not the case 

 when the luminous matter moves in the air by its own direct 

 action. Finally, the coexistence of the aurora with small ice 

 needles in the atmosphere, such as exist in elevated clouds, is 

 shown by Captain Richardson, whohavingseen at a temperature 

 near — 32° C. ( — 35° F.) an aurora whose superior arc was 

 near the zenith, remarked that although the sky appeared per- 

 fectly serene during the phenomenon, there fell a fine snow 

 hardly perceptible to the eye, though easily obserA'ed as it fell 

 on the hand and melted. The same fact had been previously 

 observed in full sunshine, the rays of the sun rendering the 

 floating particles of ice visible. 



Observers are agreed with reg^ard to the existence of a stratum 

 or dark segment, whish rests in the northern horizon, and ap- 

 pears to be the source of the auroral display. The numerous 

 observations of M. Struve at Dorpat, and those of M. 

 Argelander at Abo confirm this apppearance. It is like a veil, 

 which 'although permitting the light to pa£s gives the sky a 

 more sombre aspect ; moreover it is horded by a luminous are. 

 The existence of such a dark segment is confirmed by an obser- 

 vation of Gisler, who says that in Sweden, upon the high 

 mountains, the traveller is sometimes suddenly enveloped in a 

 very transparent mist of a grayish white colour, verging 

 towards green, which rises from the soil, and is changed into 

 the aurora borealis. 



The cirro-cumtifiis and the mists become luminous when 

 they are traversed by electric discharges sufficiently energetic, 

 provided daylight does not efface the feeble light. They 

 may sometimes be detected in the day : thus Arago establishes 

 most incontestably that Dr. H. Usher was not deceived in a 

 notice published in volume II. of the memoirs of the Irish 

 Academy, where he describes an aurora seen at mid-day, on the 

 24th of May, 1788. This observer, during the day after a night 

 in which he had witnessed a brilliant aurc^a, having observed an 

 oscillation of the stars as seen with his lens, perceived in the 

 sky rays of a white quivering light which rose from all points 

 in the horizon towards the pole of the dipping needle, where 

 thev formed a lisht and whitish corona like that which the most 



