724 THE AURORA BOREALIS. 



The most southerly arch approached within about 25° of the 

 zenith. It was abruptly terminated at its west extremity about 

 35° above the horizon ; as will be afterwards more particularly 

 described in discussing the question of the height above the earth. 

 This west abrupt extremity was a little to the north of the west. 

 Its east extremity was near the horizon in the north-east. The 

 streamers at the vertex of this arch were very short and compact, 

 and parallel to the magnetic meridian. From this point towards 

 both extremities the streamers gradually increased in length, and 

 being all directed to a point apparently 10° or 15° south of the 

 zenith, all formed angles with the general line of the arch, which 

 were more acute in proportion to the distance from the vertex. 



The arch might be about 10° broad, and speedily moved south- 

 ward, maintaining a parallelism with its first position. Its lateral 

 dimensions became gradually contracted. The streamers near 

 the zenith shortened into dense bundles, like sheaves of light, 

 parallel to the magnetic meridian, and consequently at right angles 

 to the general line of the arch ; and those towards the extremities 

 gradually diminished the angles which they made with that line 

 and approached to a parallelism with it. At length after reaching 

 the zenith the arch became diminished in breadth to about 3° or 4° 

 and coincided in its whole extent with the prime vertical to the 

 m.agnetic meridian, and the light at its vertex exhibited a nebulous 

 or mottled appearance, and that of the extremities of long streamers 

 or pencils of rays, now parallel to the arch itself. I had no oppor- 

 tunity, upon the present occasion, to witness the enlargement of 

 the breadth again, and the unfolding of the parallel streamers at 

 the vertex, which I had observed in former arches when they got 

 considerably beyond the zenith, for this arch gradually faded and 

 became extinct, about 10° or 12° southwards of the zenith. 



I now proceed to the question of the height of the Aurora 

 Borealis above the surface ctf the Earth. In the paper in the 

 Edinburgh Philosophical Journal, 1823, I had inferred, from the 

 bright phosphorescent light of a cloud apparently under an 

 Aurora, that they were in contact, or nearly so, with each other. 

 Another similar appearance, of a still more decided character, in 

 the autumn of 1825, but the precise date of which I have not 

 noted, confirmed in my mind the justness of the inference. In a 

 dark evening, without moon, an extended mass of clouds stretching 

 alono- the N. and N.E. quarter, not much raised above the visible 

 horizon, and having a clear sky above it, in which there was 

 playing a fine Aurora of vertical streamers with their lower ex- 

 tremities apparently touching it, was observed giving out at its 

 upper side a fitful but bright white light, more vivid and con- 

 spicuous amidst the darkness than if it had been illuminated by 

 the rising moon. Similar clouds in other parts of the horizon 

 exhibited no such light. It was impossible for a spectator to refer 

 the j^.urora to a distance more remote than that of the mass of 

 clouds, or to believe that the former and the light of the latter 

 were not part of the same phenomenon. Mr. Otley (Phil. 

 Trans. 1. c.) appears to have witnessed a similar phenomenon. 

 *' About 7 p.m. a dense cloud appeared in the horizon to the 



