After a brilliant aurora borealis, we have been able to recognize, on the following morning, trains of clouds, which, during the night, had appeared as so many luminous rays.
The absolute height of aurorae boreales has been very variously estimated by different observers. It has long been thought that we might determine it by regarding, from two places widely distant from each other, the same part of the aurora,--the corona, for example. But we have started from a very inaccurate assumption, namely, that the two observers had their eyes directed to the same point at the same time,--whilst it is now well proved that the corona is an effect of perspective, due to the apparent convergence of the parallel rays situated in the magnetic meridian; so that each observer sees his own aurora borealis, as each sees his own rainbow. The aspect of the phenomenon depends also upon the positions of the observers. The seat of the aurora borealis is in the upper regions of the atmosphere; though sometimes it appears to be produced in the less elevated regions where the clouds are formed. This, at least, is what follows from some observations, especially from those of Captain Franklin, who saw an aurora borealis the light of which appeared to him to illuminate the lower surface of a stratum of clouds; whilst some twenty-five miles farther on, Mr. Kendal, who had watched the whole of the night without losing sight of the sky for a single moment, did not perceive any trace of light. Captain Parry saw an aurora borealis display itself against the side of a mountain; and we are assured that a luminous ring has sometimes been perceived upon the very surface of the sea, around the magnetic pole. Lieutenant Hood and Dr. Richardson, being placed at the distance of about forty-five miles from each other, in order to make simultaneous observations, whence they might deduce the parallax of the phenomenon, and consequently its height, were led to the conclusion that the aurora borealis had not a greater elevation than five miles. M. Liais, having had the opportunity of applying a method, which he had devised for measuring the height of aurorae boreales, to an aurora seen at Cherbourg Oct. 31, 1853, found that the arc of the aurora was about two and a half miles above the ground, at its lower edge.