ELECTRICITY OF THE ATMOSPHERE AND AURORA BOREALIS.233 



it is difficult to believe, with Hansteen andBravais, that the position of 

 this bow is determined solely by the magnetic pole. 



The apex ot tbe polar bow is rarely in the exact direction of the needle 

 of declination. Of two hundred and twenty-six observations of the posi- 

 tion of the azimuth of the luminous polar bow, 30 out of 100 placed it 30° 

 more to the west, 32, 10° to 20°, 7, 0° to 10°, and 4, 0° to 20° to the east ; 

 from which it is evident that the position of the bow varies over a space 

 01 from 25 to 30^ and more. These variations are very great to be 

 explained by accidental perturbations in terrestrial magnetism, particu- 

 larly as the greatest deviation in magnetic declination — that is, from 6° 

 to about 7° — observed occasionally in polar regions is due to polar light 

 itself. 



We may then consider it certain that terrestrial magnetism relatively 

 plays but a secondary part in the ijhenomenon of polar liglit; that tbis 

 part essentially consists in a directive action upon the rays of this 

 light, and in a rotary movement imparted to these rays — facts i)ositively 

 demonstrated by the experiments of M. de la Rive. 



The formation of the auroral crown, which takes place when polar 

 light is very intense and its rays are united around the magnetic zenith, 

 is generally supposed to be an effect of perspective. When a certain 

 number of polar rays, jiarallel to the direction of the needle of inclina- 

 tion, are projected to a considerable height, they ought to appear to 

 unite around the magnetic zenith ; but the aspect they present should 

 be rather that of a lengthened point or a funnel, according as the ob- 

 server is placed on one side or in the middle of the phenomenon. In 

 polar regions it often happens that the polar rays start from all parts 

 of the horizon, which is the case when the observer is within the ring. 

 If then the crown was a phenomenon of perspective, the rays should 

 appear to unite at a rather sharp angle. Now, this is by no means the 

 case, for they form a vault, resembling very much the cupola of a churcli. 

 Although my experience is not sufficient for me to make a positive 

 assertion, I am strongly inclined to believe that, under the influence of 

 terrestrial magnetism, and perhaps also through the effect of the con- 

 ducting power of the medium, the rays of light undergo a flexion, the 

 result of which is to unite thera really, not merely in appearance, in the 

 upper parts of the atmosphere. In proof I may cite the polar light I 

 observed on the 18th of October, under the 17th degree of latitude. 

 The rays starting from all parts of the horizon formed an immense 

 ring, and united around the magnetic zenith, where the crown was 

 formed in a perfectly regular manner, i^resenting the appearance of a 

 flattened cupola. How^ever, the experiments of M. de la Rive, which 

 have demonstrated the influence of magnetism upon electric light, un- 

 der circumstances almost identical with those presented by polar light, 

 furnish no proof that the rays of this light are really united under this 

 influence. 



