420 Mr. Dal ton on the Height of the Aurora 



illuminated as usual after an aurora borealis of the common 

 kind ; so that it seems impossible to doubt that the same arch 

 was seen at all the places of observation, and at the same time. 



A good description of the phenomenon was published by 

 Messrs. Coldstream and Foggo in the Edinburgh Journal of 

 Science for June 1826f it is as follows: 



" March 29th. Immediately after the fading of the evening 

 twilight, at 8 h 15 ra P.M., a bright luminous ray was seen to 

 rise from the eastern horizon, gradually to extend itself to- 

 wards the zenith, and thence towards the western horizon, 

 presenting, when completed, the appearance of an arch of sil- 

 very light, similar to that seen here on the 19th March, 1825. 



" When first formed it was a few degrees to the north of 

 the zenith of this place ; the light in the centre was rather dif- 

 fuse ; its edges were irregular ; and the western limb had, as 

 it were, a plumose appearance. It soon evinced a decided 

 motion towards the south, and in a few minutes reached our 

 zenith. Its edges were now sharply defined, and throughout 

 its whole course it was nearly uniform in appearance and 

 breadth ; the intensity of its light in the zenith had increased, 

 while in the same quarter the breadth had considerably di- 

 minished. 



" The direction it now had was very nearly at right angles 

 with the magnetic meridian. 



" At \ past 8, faint beams of the aurora began to rise from 

 the northern horizon, and at one time promised to form a 

 splendid display; but the corruscations never became very 

 vivid ; they were not rapid in their motions, and did not flit 

 along the horizon. 



"The arch still continued its motion towards the south, 

 and in 15 minutes passed through a space of about 20°. Its 

 southern edge reached a point about 24° or 25° south of the 

 zenith, beyond which it did not go. The light now became 

 gradually fainter, and at length disappeared. 



" Meanwhile the aurora in the north continued to play, but 

 with no increase of vividness. For some minutes, soon after 

 9 o'clock, we observed broad bands of light, having their 

 longer axes (which generally subtend angles of about 18° or 

 20°) parallel with the horizon, darting with great velocity 

 across the illuminated space from east to west and from west 

 to east. These formed, ran their course, and vanished in a 

 moment ; they had no vertical motion, but they appeared at 

 various degrees of elevation, never higher however than 30°. 

 Soon after this interesting (and perhaps unusual) display, the 

 beams disappeared, and nothing was left but a diffuse lumi- 

 nousness along the horizon." 



At 



