222 AURORA BOREALIS. 



ful drawings of numerous auroras were made at both stations whicB clearly 

 establis^icd this coiucidence. The parallaxes resulting from the observations 

 were !?ometimes positive and sometimes negative; the average parallax but 

 little exceeding one degree. These observations were i-egarded as demonstrating 

 that the average height of auroral arches is from 60 to 100 miles above the 

 earth's surfiice. 



Some persons maintain that the small height of auroras is proved by their 

 being sometimes seen between the observer and a cloud, or between the observer 

 and a hill of moderate elevation. Cases do unquestionably occur in which the 

 aurora appears to be situated between a cloud and the observer ; but this appear- 

 ance is believed to result from a cirrus or cirro stratus cloud of very small 

 density being strongly illumined by auroral light which shines through the cloud, 

 so as to produce the same appearance as if the aurora prevailed on the under 

 side of the cloud. 



Sometimes the lower extremity of an auroral streamer appears to be pro- 

 longed below the summit of a mountain or hill. Captain Parry states that on 

 one occasion he observed a bright ray of the aurora shoot suddenly downward 

 between him and the land, which was then distant only 3,000 yards * This ap- 

 pearance is believed to have been an illusion. A similar phenomenon was twice 

 noticed by the French observers at Bossekop, and was ascribed by them to the 

 reflection of the auroral light from the snow which covered the mountain. 



On the whole, then, we conclude that although it is possible the aurora may 

 sometimes descend nearly to the earth's surface, there is no sufficient evidence 

 to prove that the true polar light has ever descended so low as the region of 

 ordinary clouds. 



Nuise of the aurora. — There is no satisfactory evidence that the auroi'a ever 

 emits any audible sound. It is a common impression, at least in high -hititudes, 

 that the aurora sometimes emits sound. This sound has been caUed a rustling, 

 hissing, whizzing, crackling noise. But Scoi'esby, Ross, Back, and other dis- 

 tiuguisiied travellers who have spent several wiiaters in the Arctic regions, where 

 auroras are seen in their greatest brilliancy, have been convinced that this supposed 

 rustling is a mere illusion. The observers at Bossekop never heard any noise 

 which they could ascribe to the aurora, although their attention was specially 

 directed to this subject ; and they concluded that the sounds which have been 

 ascribed to the aurora mu!?t have been due to other causes, such as the whistling 

 of the wind, the whirling of the snow, the distant murmur of the sea, or the 

 cracking of the snow when it congeals after having been partially melted. 



When we see a brilliant light shooting like a rocket across the sky, it is na- 

 tural to expect an accompanying sound. People generally hear what they 

 expect to heir. Tacitus informs us that the ancient Germans heard S, noise 

 whenever the setting sun descended into the western ocean. t 



No observer has ever spoken of the interval that had elapsed between the 

 darting of the auroral rays and the alleged noise. But, on account of the 

 elevation of the aurora, this interval should be a long one. Sound requires four 

 minutes to travel a distance of fifty miles. It is probable, therefore, that the 

 sounds which have been heard during exhibitions of the aurora are to be as- 

 cribed to other causes than the aurora. 



Diurnal periodicity of auroras. — Auroras appear at all hours of the night, 

 but not with equal frequency. In Canada, the number of auroras increases 

 uninterruptedly from sunset till an hour before midnight, from which time the 

 number diminishes uninterruptedly till morning. At more northerly stations 

 in North America, auroras are most frequent at midnight; and at places still more 

 northerly, as far as the Arctic ocean, they appear to be most frequent an hour 



* Parry's Third Voytigo, p. GI. t Tacitus Germania, c. 45, 1. 4. 



