154 MEMOIR OF AUGUSTE BRAVAIS. 



Independently of tlie detailed journal of observations of the auroras, 

 printed in the work already referred to, and the splendid plates of the 

 physical atlas, which represent the most remarkable appearances ob- 

 served by the four physicists, M. Bravais has inserted in the same jiub- 

 licatiou a Mcmolre snr les aurores hormles, cited by competent judges iis 

 more precise than anything heretofore written on the subject. The fol- 

 lovring rapid summary of the contents of this essay may not be without 

 interest. 



When the first doubtful gleams of an aurora begin to diffuse them- 

 selves in the sky, there is first perceived at the horizon, a little to the 

 west of north, a dark segment, which, according to the very probable 

 conjectures of M. Bravais, is nothing else than the compact mass of 

 fogs with which the temperate waters of the Polar Sea are almost con- 

 stantly covered. Above the dark segment gleams of light like those of 

 a conthigration soon make their appearance, simi)ly resulting, i)erhaps, 

 from the still distant glow of the aurora retlected on the surface of the 

 marine vapors. Some time afterward a luminous arc is traced above 

 the segment, its two extremities resting on the horizon, and its culmi- 

 nating ])oint, which divides it into two e<iual and symmetrical parts, 

 being situated most frequently in the neighborhood of the magnetic 

 meridian. On an average it falls a little to the west of that meridian, 

 from wliich it i)rogressively diverges as it becomes more remote Irom 

 the northern edge of the horizon, especially when, having passed the 

 zenith, it apx)roaclies the southern horizon, from which in certain cases 

 it is distant but a few degrees. Sometimes several different arcs show 

 themselves at the same time ; very often there are two, more rarely three, 

 but as many as nine have been counted at one time. Their breadth, 

 which at a mean is from seven to eight degrees, occasionally exceeds 

 twenty-five degrees, particularly in the culminant part wdien it passes near 

 the zenith. Through a combination of nu^asurements this last reniark 

 has led to the conclusion that the arcs of the aurora borealis are tlat- 

 teued parallel to the surface of the earth, and thus one of the means 

 proper for furnishing the measure of the height at which these arcs are 

 situated above the surface was suggested to M. Bravais. 



The height in question had long before occupied attention, and it had 

 with reason been thought that it might be calculated from the parallax 

 resulting from two observations of the same arc, made simultaneously 

 by two observers placed at a known distance. With a view to this 

 means of determination M. Bravais passed thirteen days of January 

 1838 at Jupvig, situated fifteen kilometres to the north of Bossekop, in 

 order to observe the auroras from that point, while his colleagues ob- 

 served them at the same instants of time from their usual station. The 

 forms of a great number of arcs, and especially those of the most regu- 

 lar arcs, were taken with much care by the commission, and M. Bravais, 

 by discussing them, through means of elegant geometric constructions 

 and trigonometrical formulas skillfully reduced to the greatest simpli- 

 city, has shown that all these arcs, conformably with the hypothesis of 

 our distinguished correspondent, M. Hansteen, of Christiania, ma^^ be 

 considered as the perspectives of circular rings, having their center on 

 the terrestrial radius directed toward the magnetic pole, and their plane 

 perpendicular to that radius. His formulas have given him, for each 

 case, the elevation of the ring above the surface of the earth, and this 

 means of measurement, combined with the two others already indicated, 

 have led him to the conclusion that the arcs of the aurora borealis are 

 situated at an altitude of one hundred to two hundred kilometres, in 

 the region where the shooting stars and bolides become incandescent 



