Miscellaneous Intelligence. 143 



of the dark segments lying generally below the auroral arches, or at 

 their base. This segment, according to some, is merely an effect of 

 contrast; according to others it is something material, (or real,) but in- 

 dependent of the aurora, caused perhaps by the polar fogs; and others 

 consider it the generating source of the auroral light. I next show, 

 that the light cannot be an effect of reflection, except in rare cases, and 

 actually exists where it is observed. 



In the second paragraph I consider the forms and portions of arches, 

 their movements, light, and apparent structure. According to Hans- 

 teen, an auroral arch is a luminous ring situated in the upper regions 

 of the atmosphere, sustained in alt its parts at the same height,, above 

 the earth's surface, and whose axis corresponds nearly with the mag- 

 netic axis of the globe. Such a ring ought to appear more or less 

 elevated above the horizon according to the position of the observer, 

 and it ought to be seen to cut the plane of the magnetic meridian at 

 right angles. The hypothesis of Hansteen, which is altogether the 

 roost probable, has been made the basis of our investigations with regard 

 to the orientation, the height, and the amplitude of the arches. I under- 

 stand by amplitude, the angular distance between the east and west 

 sides measured on the plane of the horizon and on the north sides of 

 tne sky. At Bossekop, the summit of the arc is not only eight to ten 



agrees to the left of the magnetic north, but the deviation goes on in- 

 creasing as the arch rises from the north toward the zenith and from 



»e zenith to the south. The amplitude increases quite regularly dur- 

 ln g this movement of the arc. It does not become one hundred and 



^ghty degrees until the arch has passed the zenith to the southern part of 

 the sky. 



u also results from our observations, that the curve of the arch is 

 ve ry similar to that of a small circle of the celestial sphere. This 

 n . Clr cle projected upon the vertical plane which contains the cul- 

 unant point of the arch is a straight line ; and I show that on approach- 

 es the horizon, this ricrht line becomes a hyperbolic curve though 

 scarcely appreciable, and important only in a theoretical poirit of view, 

 r °m Us connection with the theory assumed. 



. * rom the simultaneous variation of the heights and amplitudes, (adopt- 

 ,n P the theory of Hansteen,) I have found the mean elevation above 

 11 earth to be 227 kilometers, (140 miles Eng. statute,) which cor- 

 Ve sponris to the upper limits of our atmosphere, or the region of falling 

 stars, &c. 



The third paragraph is devoted to the rays of the aurora borealis. 



j he rays (streamers) are columns of light suspended in the air ; they 

 undergo rapid movement or changes, and appear to converge toward* 

 wie magnetic zenith, where they thus form what is called the corona. 



J638,1839 



?«39 et 1840, sur U corvette La Recherche commandite par *-*f"**l H?**- 



peao | publics par ordre da Roi sous la direction de M ; Paul Ga unard, I res- 

 df> In r\ ,., •. • r .- - . ^ i.. tvt _j n **it_- ;.,.i...p Rpnorts on Astron- 



ide *>t de la C 



v include Reports on Astr-m- 

 1 vol.; Meteorology, 3 vols 



