THE AURORA BOREALIS. 479 



the middle of the bottle. The iron was covered with an insulating 

 material, except at the end, and over that was a copper ring, connected 

 with an electrical machine. The copper was then charged with posi- 

 tive electricity, and the iron, having been put in communication with 

 the soil, was negatively electrified by induction. The two electricities 

 combined in the rarefied atmosphere of the bottle, forming a luminous 

 sheaf, like that of the lights in the Geissler tubes ; but, when the iron 

 was magnetized, a corona or concentric aureole, whence radiated brill- 

 iant jets, was formed around its free end. As a little reflection will 

 show, the iron represented the earth and the terrestrial magnet ; the 

 copper, the upper strata of air ; and the free end of the magnetized 

 rod, the polar regions. 



The fact mentioned by Mairan, that auroras are most frequently 

 seen during the equinoctial months, March and September, is easily 

 explained on M. de La Rive's theory. March corresponds with a 

 period of increasing heat in the tropical part of the northern hemi- 

 sphere, while September coincides with the time when fogs are con- 

 densed from vapors near the pole. In the one case, an excess of 

 electricity is developed ; in the other, a more ready combination of the 

 two fluids. Perhaps the supposed eleven-years period, corresponding 

 with the sun-spot period, may be explained in a similar way. There 

 may also be secular variations in the prevalence of the phenomenon, 

 but too little time has passed since careful observations have been 

 made for their law to be as yet apprehended. 



In the last months of 1878, M. Nordenskjold, who was wintering 

 in Berhing Strait, remarked on clear nights, when the moonlight was 

 not too strong, the presence of a feebly luminous arc, with its crest 

 toward the north-northeast. Regular in form and curvature, this 

 arc rested on a segment of a circle which was itself limited by the 

 •horizon, and covered about 90'', or a quarter of the horizon. Its 

 lower limit was quite clearly marked on the dark segment, probably 

 by contrast, but its outer outline was less distinct, and it was hard to 

 measure its thickness exactly ; but that was estimated at about five 

 degrees. The light of the arc was calm and uniform, without any 

 appearance of rays, but dull enough, as we have said, and displayed 

 nothing comparable to the draperies, the brilliant flashes, and the 

 streaks of the Scandinavian auroras. M. Nordenskjold observed it from 

 day to day, taking notice of all the special features he could remark, 

 and came to the following conclusions : Above the surface of the 

 earth, at a distance of about four hundred kilometres, is situated a 

 permanent, or nearly permanent, luminous corona, which encircles the 

 entire globe without its direction coinciding with that of the parallels, 

 for its center does not correspond with the north pole, but with the 

 magnetic pole. 



So our globe has, by this theory, a ring like Saturn's, but with 

 some differences. The ring of the latter planet is around its equator. 



