THE AURORA BOREALIS, 243 



byD. Kirkwood, Esq. The cases, in which one arch only is to be seen 

 by the same spectator are far more frequent, than those in which two or 

 more are visible at the same time. Captain Bonnycastle, who has given, 

 in the 30th volume of Silliman's Journal, an interesting description of a 

 very splendid aurora, as seen by him on the Northern shore of Lake 

 Ontario, and who asserts that the arch is a constant character of the au- 

 rora of the lakes, saw four at the same time. When several co-exist, 

 the more southern is mostly white ; the rest are sometimes party-col- 

 ored. The breadth of the arch, which is nearly uniform throughout its 

 whole extent, is from 3° to 12°, being different during different auroras, 

 and, we have every leason to believe, also for different observers or in 

 different localities, even during the same display. We are forced to this 

 last conclusion by the discordant estimates made by various careful ob- 

 servers of what was regarded the same arch. That local causes mate- 

 rially influence the appearance or non-appearance of the arch is more 

 than probable, from the fact stated above upon the authority of Captain 

 Bonnycastle, who saw three or four not noticed by others, and from the 

 fact that one observer may see two, whilst others, not more than a few 

 miles distant, will see only the brighter. 



Though sometimes irregularly bent in a portion of its course, the 

 arch is generally very regular and well defined, and lies nearly in a great 

 circle of the sphere. Its position is generally a little oblique to the nie- 

 redian. That of April 7th ult., for example, cut the eastern horizon at 

 about 15° south of east, and met the opposite horizon at about 15° north 

 of west. The position is perhaps at right angles to the magnetic not 

 the terrestrial region. 



As to its formation it is various. Sometimes it seems to be formed 

 by a portion of luminous cloud appearing in the eastern quarter of the 

 heavens, and gradually e.xtending itself westward by a kind of rolling 

 or wave-like motion. This motion, which by some has been compared 

 to that of forming snow-drifts is a very constant character of the arch. 

 Sometimes, however, it seems to start almost suddenly into existence, 

 and to disappear and reappear successively again at nearly the same 

 place in the heavens. And at other times it seems to arise from a por- 

 tion of the nebulous matter or vapor, described as lying in the northern 

 horizon, detached and impelled towards the equator by the same force 

 which impels the streamers. Indeed, becoming, after it has "moved some 

 distance from the north, the southern limit of the streamers, it appears 

 as if it weie thrust forward by them. This was beautifully exempli- 

 fied in the remarkable arch of the 7th of April last, which was so 

 extensively witnessed and admired in the northern United Slates. At a 



