THE AURORA. 191 



Some of the great auroras have been seen ft^r several 

 nights in succession ; their occurrence during the inter- 

 vening days also being highly probable. 



AURORAL ARCHES, CORONA, AND STREAMERS. 

 It first appears, usually, as a low arch of light, in the 

 direction of the pole, resembling the dawn of day; 

 whence its name, aurora, the morning. This arch is 

 often accompanied by a low bank of clouds, lying 

 under it, next the horizon. As the arch slowly rises 

 streamers of light, differing in color, size, and brilliancy, 

 dart up through it ; extending from the horizon to 

 a considerable height above the arch; their color 

 varying from a pale white to a light red; though yel- 

 low, green, and blue tints have also been observed; 

 the prevailing tints differing more or less in different 

 localities. 



These streamers appear to radiate from a central 

 region below the horizon, cutting the arch vertically, 

 at right angles, as shown in Fig. 58. The streamers 

 sometimes appear to rise from widely separated points 

 in the horizon; and, as the aurora increases in size and 

 brilliancy, they culminate at the zenith, as shown in 

 Fig. 59, forming a corona of more or less prominence ; 

 one of the most prominent being shown in Fig. 60. 



By comparing the three cuts, it will be seen, that if 

 the center of the corona shown in Fig. 59, or Fig. 60, 

 were below the horizon, the appearance would be the 

 same as in Fig. 58. So that, supposing the observer 

 placed below the horizon, under the center from which 

 the streamers seem to emanate, he would see the 

 corona above him, as in Figs. 59 and 60. And, con- 

 versely, an observer in the latitude of Paris, looking at 

 the corona, observed in latitude 70 N., Fig. 60, would 



