222 Observations upon Auroral and Optical Phenomena. 



whereas the observed moiion was by estimation 51°, and possibly 

 rather more. Either therefore the arch was nearer the earth's sur- 

 face in its western parts, where it was also the brightest, or its visi- 

 ble extreme parts were in motion at unequal velocities. At this 

 time the great circle that would nearly contain the western limb of 

 the arch, would have its meridian about 12° east of north. In the 

 position last given the arch was stationary for ten minutes. It was 

 most brilliant at lOA. 15m. Two minutes afterwards it separated 

 into parallel /eeces, distinct from each other, and lying oblique to the 

 direction of the arch. These fleeces were in slow motion toward 

 the west, keeping the line of the arch. The motion was such as 

 made itself evident to the sight at the first instant of attentive obser- 

 vation. In other words it was sufficiently rapid to be just percepti- 

 ble directly to the sense. At \0h. 30m. the arch faded almost en- 

 tirely, so that I considered it as having vanished. Its position was 

 then 11|° south of Alpha Lyrae, — south also of Arcturus and the 

 Northern Crown, and south of its previous position in the east. On 

 looking afterwards. I found that the arch had completely formed 

 again in its last position, and was well defined, although faint. It 

 disappeared altogether about 10^. 40m. P. M. without having chan- 

 ged place. The steamboat at 9A. 55m., the time of my most defi- 

 nite observations, was nearly opposite Old Field Point, on Long Isl- 

 and. My position, as closely as I can estimate it, was twenty two 

 miles S. 8° W. from New Haven. 



Fortunately this phenomenon was viewed attentively at New Ha- 

 ven, by Prof. Olmsted, of Yale College, who obligingly put into my 

 hand the loose paper containing his minutes, made on the spot, from 

 which I extract so much as relates to the phenomenon during the 

 time of my own observations already detailed. 



" 3m. before 10 — arch — northern margin grazed Arcturus. Ver- 

 tex on the Arrow, Dec. 18°, — covered the Dolphin. Vertex ad- 

 vanced no farther south, but western limb moved to the head of the 

 Serpent, where it remained stationary till 25 minutes after 10, and 

 disappeared." 



" Western limb bent, being convex towards the south." 



" Northern margin well defined." 



" Parallel ridges N. E. and S. W. two to four degrees asunder, 

 like parallel drifts of snow." 



By attentively considering and comparing the two sets of obser- 

 vations given above, and made at points twenty two miles asunder, 

 we derive the following conclusions. 



