THE AURORA. 193 



see only the upper part of its southern half, corre- 

 sponding to the aurora shown in Fig. 58. 



In the aurora shown in Fig. 61, seen from the Vega, 

 in latitude 65 N., we have an arch formation without 

 streamers. A series of concentric arch segments, more 

 or less perfect, is seen ; the outer one less than a semi- 

 circle, and the most perfect of the inner ones greater 

 than a semicircle ; the central one, a double arch, with 

 the nucleus of a second double arch above the junc- 

 tion. From an inspection of the figure, it is evident, 

 that the perfect arches would appear as complete circu- 

 lar belts to an observer under the central point near 

 the horizon. 



Difference of longitude, as well as latitude, must also 

 modify the appearance : as that portion of the arch 

 which appears to one observer as its summit, appears 

 to another, at a distant east or west point in the same 

 latitude, as its east or west base. And, supposing the 

 first observer placed in the magnetic meridian which 

 coincides with the center of the aurora, the effect of 

 perspective would cause it to assume a different appear- 

 ance to him, from that seen by the other observer, 

 viewing it from a different angle. A streamer, seen 

 from one position, would appear foreshortened; while 

 at a different angle it would appear elongated: to one 

 observer it might appear as a narrow ray, to another as 

 a broad band. 



Hence, we may infer, that we see in the arch, rising 

 from the horizon, the outer edge of a circular belt 

 of electric light, with its varied phenomena of arches, 

 streamers, rays, and coronae, covering a large area, 

 parallel to the earth's surface, and extending, as it 

 increases in size, from a region surrounding the pole, 



