ELECTEICITY OF THE ATMOSPHERE AND AURORA BOREALIS. 231 



ring- gradually elougated, and suddenly meeting around the zenith, 

 formed for a few moments a boreal crown of perfect regularity and the 

 most brilliant colors. 



On our arrival at Tromso, I examined with the spectroscope a beauti- 

 ful polar light, which appeared on the 21st of October, commencing- at 

 the north. The first rays clearly displayed the yellow ray in question. 

 The phenomenon becoming more brilliant, a variegated band was formed 

 toward the south, of yellow, red, and green, which gave, 1st, the yellow ray; 

 2d, in the blue part, a very distinct and very clear ray ; 3d, two lines of 

 the breadth of a hair, showing very decided horizontal stripes by the 

 side of the yellow. I ought to say that the yellow ray in every case 

 seemed to me peculiar in being variable in intensity ; sometimes more, 

 sometimes less vivid. On the 27th of October, we were enabled to de- 

 termine more accurately the position of the rays, and we found that the 

 yellow line furnished by the yellow light of chloride of sodium was 

 Cl.O ; in the auroral light, the yellow line was at 79.9 ; the blue at 65.90. 

 The first of the shaded lines at 125.0 ; and the second shaded line at 

 about 105.0. 



II. Admitting, in general, the mostof theopinions advanced in the work 

 of M. Loomis, in regard to the aurora borealis,* which, in many respects, 

 accord with the theory given by M. de la Eive, and supported by experi- 

 ment, I have still, after the observations described, some doubts in re- 

 gard to certain parts of this work, especially those which relate to the 

 particular nature of the phenomenon. 



The fact that polar light is an electric phenomenon, taking place in 

 the atmosphere, is well established by the analogy which exists between 

 its effects and those of electric currents. It produces, as they do, per- 

 turbations of the magnetic needle and currents in a good conductor. 

 The luminous phenomenon itself exactly resembles the light produced 

 by electric discharges in a damp' atmosphere, or between two electrodes 

 placed in rarified air. The results of spectral analysis, as well as those 

 discovered by M. Angstrom, and the new rays I have described, give 

 further proof of the electric origin of polar light ; for, in order to obtain 

 a spectrum with a gas, the latter must be incandescent, and electricity 

 is the only source of heat which can produce this incandescence in the 

 aerial molecules and other particles which constitute the atmosphere. 



The question as to the height at which polar light is manifested has 

 been the subject of much controversy. From observations made in 

 America with great care and in several separate localities, M. Loomis 

 calculated the height of the beautilnl aurora borealis of the 2d of Sep- 

 tember to be at its lower limit 45 to 50 English miles, and at its upper 

 limit from 400 to 500. Mr. Potter, in England, in 1833, gave, as height 

 of the aurora, 03 miles, and Dalton, in 1820, estimated it at 100. Dur- 

 ing the French expedition, from 1838, 1839, observations made simulta- 

 neously at two stations — Bossekop and Jupwig — gave 00 to 100 miles 



* Smithsonian Report for 18(35. 



