THE AURORA BOREALIS. 481 



Besides the slow variations of its radius and its thickness, besides the 

 oscillations which displace its center movements, the laws of which are 

 worth studying, the luminous arc rises, falls, and fades away for in- 

 tervals of some hours. Its light, generally uniform, is heightened by 

 " knots of light " that play from one end to the other. Sometimes a 

 second arc is formed parallel to the first ; according to M. Norden- 

 skjuld, this is nearly always concentric with the usual arc and situated 

 in the same plane with it, but farther from the surface. Sometimes, 

 also, the two arcs amalgamate, and a vertically flattened aurora results. 

 Not rarely, supplementary arcs intervene, and frequently luminous 

 rays play between the two arcs and into the undefined exterior space. 

 If, now, we imagine the phenomena growing more complicated and 

 becoming irregular, with the arcs rising above the horizon and the 

 rays multiplying, shooting through the curves in such a way as to 

 illuminate the vacant space, and extending themselves out toward the 

 magnetic south in somewhat oblique directions, we have the common 

 aurora borealis passably explained. Within the projection of the 

 corona, toward the magnetic pole, is a zone where we may observe 

 the auroras in a southerly direction, and, still nearer to that pole, the 

 meteor only rarely illuminates the horizon. A few travelers. Dr. 

 Hayes, for example, noticed this fact some time ago. The zone 

 of no auroras embraces a circle having a radius of about eight de- 

 grees. 



The labors of M. Lenstrom, in Lapland, are of particular interest, 

 because they constitute a direct and definite proof of the electrical 

 nature of the aurora borealis. They go further than those of M. de 

 La Rive, for the Swedish observer, instead of operating in his labora- 

 tory, has succeeded in reproducing the meteor itself in the open air, 

 and has compelled it to manifest itself, as Franklin forced the light- 

 ning to come down from the sky, so that he could examine it scien- 

 tifically. We must not forget, furthermore, that it is a very meri- 

 torious thing to work in a cold of twenty degrees below zero, with a 

 strong wind blowing and the frost all the time clogging the apparatus, 

 having to be constantly on the watch, and enjoying no better shelter 

 than a charcoal-burner's hut. 



Not satisfied with provoking artificial auroras, the Finnish expe- 

 dition, of which M. Lenstrom was a part, has collected a number of 

 important data relative to the free manifestation of the phenomenon. 

 The observations were made at Sodankyla (lat. 67° N., long. 27° E.), 

 and Kultala (lat. 78° 30' N., long. 27° E.), Lapland, in November and 

 December, 1882. In the former place " the polar aurora appeared fre- 

 quently of a very great intensity, but did not exhibit much variation. 

 It generally began with a faint arc in the north, which shortly de- 

 veloped into an arc with rays and sometimes into draperies extending 

 from the east to the west, most frequently a little toward the north. 

 But little change of color took place ; nearly always a pale-yellow 



TOU IXIV. — 81 



