AURORA B0REALI3. 195 



shows its influence on the course of the needle over large por- 

 tions of continents, and, as Arago first discovered, far from 

 the spot where the evolution of light was visible. It is not 

 improbable that, as heavily-charged threatening clouds, owing 

 to frequent transitions of the atmospheric electricity to an op 

 posite condition, are not always discharged, accompanied b) 

 lightning, so likewise magnetic storms may occasion far-ex 

 tending disturbances in the horary course of the needle, with 

 out there being any positive necessity that the equilibrium of 

 the distribution should be restored by explosion, or by the 

 passage of luminous efTusions from one of the poles to the 

 equator, or from pole to pole. 



In collecting all the individual features of the phenomenon 

 in one general picture, we must not omit to describe the origin 

 and course of a perfectly developed Aurora Borealis. Low 

 down in the distant horizon, about the part of the heavens 

 which is intersected by the magnetic meridian, the sky which 

 was previously clear is at once overcast. A dense wall or 

 bank of cloud seems to rise gradually higher and higher, until 

 it attains an elevation of 8 or 10 degrees. The color of the 

 dark segment passes into brown or violet ; and stars are visi- 

 ble through the cloudy stratum, as when a dense smoke dark- 

 ens the sky. A broad, brightly-luminous arch, first white, 

 then yellow, encircles the dark segment ; but as the brilUant 

 arch appears subsequently to the smoky gray segment, we can 

 not agree with Argelander in ascribing the latter to the effect 

 of mere contrast with the bright luminous margin.* The 

 highest point of the arch of light is, according to accurate ob- 

 servations made on this subject,! not generally in the magnet- 

 ic meridian itself, but from 5° to 18*^ toward the direction of 

 the magnetic declination of the place.J In northern latitudes, 



* Argelander, in the important observations on the northern light 

 embodied in the Vortrdgen gehalten in der physikalisch-okonomischen 

 Gessellschaft zu Konigsberg, bd. i., 1834, s. 257-264. 



t For an account of the results of the observations of Lottin, Bravais, 

 and Siljerstrom, who spent a winter at Bosekop, on the coast of Lap 

 land (,70° N. lat.), and in 210 nights saw the northern lights 160 times, 

 see the Comftes Rendus de V Acad, des Sciences, t. x., p. 289, and Mar- 

 lins's Met^orologie, 1843, p. 453. See, also, Argelander, in the Vortra- 

 gen geh. in der Konigsberg Gessellschaft, bd. i., s. 259. 



X [Professor Challis, of Cambridge, states that in the Aurora of Oc- 

 tober 24th, 1847, the streamers all converged toward a single point of 

 the heavens, situated in or very near a vertical circle passing through 

 the magnetic pole. Around this point a corona was formed, the rays 

 of which diverged in all directions from the center, leaving a space free 

 from light: its azimuth was 18° 41' from south to east, and its altitude 

 69° 54'. See Professor Challis, in the Aihenteum, Oct. 31, 1847.'}— Tr 



