Observations upon Auroral and Optical Phenomena. 221 



exactly similar and similarly inclined. At those portions of the 

 streamers which were 20° above the horizon the obliquity to the verti- 

 cal was about 17°. These streamers increased steadily in brightness, 

 and their remarkable similarity and correspondence of position led 

 me to look above, in expectation of finding them to be part of a con- 

 nected line. Immediately over head I at once perceived an arch 

 forming, — in appearance like a thin whitish vapor — well defined but 

 narrow, and so faint that it would not have attracted notice, had not 

 its existence been suspected and sought for. This circumstance 

 seemed at first to suggest a probability that the bow might be arran- 

 ged in a horizontal stratum of vapor, too faintly luminous, and too 

 thin to make a strong impression on the eye looking directly through 

 its substance, and requiring to be viewed through a section oblique 

 to its length, and thus to have its successive parts accumulated upon 

 each other, in order to give the impression of brightness. Soon, 

 however, the parts over head increased in brightness, as did also 

 those towards the horizon, — the part in the west being the brightest, 

 and all having the apparent density of a common cirro-cumulous 

 cloud. At 9h. 55m. I find by my minutes, made at the time, that 

 the arch was centered over head in the star Alpha Lyrae, which ap- 

 peared undiminished in brightness. At the same time its centre 

 passed, in the east, through a star which I take to have been Mar- 

 Jcab. and in the west about 5^° to the north of Arcturus. It was 

 therefore at this time disposed nearly in a great circle, whose merid- 

 ian passed 15° east of north, instead of about 6° west of north, as 

 the magnetic direction of the region would have led us to expect.* 

 At lOA. 5m. the arch was centered in Arcturus, — then about 20° 

 . high in the west. Higher up it cut the southern edge of Corona. 

 Over head it had moved 5|° southward in Lyra. In the east it 

 had also advanced southward, but not more than 2° opposite the 

 star before mentioned, then estimated to have 25° of altitude. It 

 thus appeared evident that the western parts of the arch were in a 

 state of more rapid apparent motion than the eastern, or even than 

 the vertex, — since a motion of 5|° over head would correspond 

 (supposing the arch horizontal) to about 2° at an altitude of 20°, 



* This refers to the western limb of the arch. The eastern limb would have its 

 proximate great circle a few degrees nearer the east and west points. The arch 

 was certainly not disposed in a great circle, as is evident from the fact that the ex- 

 tremities were much inclined to the vertical, while the arch passed through my 

 zenith. It was much more probably disposed in a small circle. 



