AURORA BOREAL IS. 209 



The corona seldom continues complete longer than one hour. The streamers 

 then become fewer and les^s inteui^ely colored ; the luminous arches break up, 

 while a dark segment is still visible near the northern horizon ; and at la'^t 

 nothing remains but masses of delicate cirro-cumulus clouds. During the ex- 

 hibition of brilliant auroras, delicate fibrous clouds are commonly seen floating 

 in the upper regions of the atmosphere ; and on the morning after a great noc- 

 turnal display, we sometimes recognize the same streaks of cloud which had 

 been luminous during the preceding night. Sometimes during the day these 

 clouds arrange themselves in forms similar to the beams of the aurora, constituting 

 what has been called a day mirora. 



5. Waves or flashes of light. The luminous beams sometimes aitpear to 

 shake with a tremulous motii n ; flashes like waves of light roll up towards the 

 zenith, and sometimes travel along the line of an am-oral arch. Sometimes the 

 beams have a slow lateral m^)tion from east to west, and sometimes from we.'Jt 

 to east. These sudden flashes of auroral light are known by tlie name of merry- 

 dancers, and form an important feature of nearly every splendid aurora. 



The duration of auroras is very variable. Some last only an hour or two ; 

 others last all night ; and occasionally they appear on two successive nights, 

 under circumstances which lead us to believe that, were it not for the light of 

 the sun, an aurora might he seen uninterruptedly for '.i6 or 48 hours. For more 

 than a week, commencing August 28, 1859, in the northern part of the Uuitr^d 

 States, the aurora was seen almost uninterruptedly every clear night. In the 

 neighborhood of Hudson's bay the aurora is seen for months almost without ces- 

 sation. 



Auroras are cnaracterizcd by recurring Jits of brilliancy. After a brilliant 

 aurora has faded away, and almost wholly disappeared, it is common for it to 

 revive, so as to rival and often to surpass its first magnificence. Two such fits 

 are common features of brilliant auroras ; and sometimes three or four occur on 

 the same night. 



The color of the aurora is very variable. If the aurora be faint, its light is 

 usually white or a pale yellow. When the aurora is brilliant, the sky exhibits 

 at the same time a great variety of tints ; some portions of the sky are nearly 

 white, but with a tinge of emerald green ; other portions are of a pale yellow or 

 straw color ; others are tinged v.-ith a rosy hue, while others have a crimson hue 

 which sometimes deepens to a blood red. These colors are ever varying in po- 

 sition and intensity. 



Auroras are sometimes observed simultaneously over large portions of the 

 globe. That of August 28, 1859, was seen over more than 140 degrees of lon- 

 gitude from California to eastern Europe ; and from Jamaica on the south to 

 an unknown distance in British America on the north. The aurora of Septem- 

 ber 2, 1859, was ^eeu at'the Sandwich Islands ; it was seen throughout the Avhole 

 of North America and Europe ; and the magnetic disturbances indicated its 

 presence throughout all northern Asia, although the sky was overcast, so that 

 at many places it could not be seen. An aurora was seen at the same time in 

 South America and Xew Holland. The auroras of September 25, 1841, and 

 November 17, 1848, were almost equally extensive. 



Dark scg7ncnt — In the United States an aurora is uniformly preceded by a 

 hazy or slaty appearance of the sky, particularly in the neighborhood of the 

 northern horizon. When the auroral displa}'- commences, this hazy portion of 

 the sky assumes the form of a dark bank or segment of a circle in the north, 

 rising ordinarily to the height of from five to ten degrees. This dark segment is 

 not a cloud, for the stars are seen through it as through a dense smoke. 



M. Struve says, " the stratus that rests on the northern horizon, and appears 

 to be the base of all the aurora? boreales that I have seen for a long time at 

 Dorpat, (latitude 58° 21' N, ) is not a cloud, but merely the sky somewhat darkened.' 

 Very freq[uently, when it was cj^uite black, and very high above the horizon, we 



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