OF THE AURORA BOREALIS. 27 



the columns, and flow upwards in the direction of the columns towards the centre 

 of the corona. The velocity of their motions is amazing, the entire progress 

 being such as would carry them from the horizon to the zenith in a second of time, 

 passing over half that distance (their usual track) in half a second. The appearance 

 is that of undulatory rather than of progressive motion, resembling the waves 

 that sweep over tall, fine grass, when gently agitated by the wind. The finest 

 specimens of auroral waves were exhibited in the displays of November 17th, 1835, 

 April 22d, 1836, November 18th, 1841, and in all three of the great auroras of 

 September, 1851. l 



VII. Magnetic Phenomena. — The aurora borealis is accompanied by remarkable 

 magnt tic phenomena. 3 During the great exhibition of November 14th, 1837, accord- 

 ing to Mr. Herrick, the needle often moved thirty minutes in three seconds, and its 

 greatest deflection was nearly 0°. The disturbances of the dipping needle are also 

 frecpuently enormous during great auroras. The position of the streamers in direc- 

 tions parallel to the magnetic meridian, and the situation of the centre of the 

 corona at the pole of the dipping needle, are also facts constantly observed. There 

 does not, however, appear to be any established relation between the state of 

 intensity and the effect on the needle ; and there are singular local peculiarities in 

 respect to the accompanying magnetic phenomena. Professor Henry detected an 

 increase of horizontal intensity in the needle be/ore an aurora, and a diminution 

 of intensity at the maximum ; and the same fact is noticed by Hansteen (Kaimtz! 

 Meteorol., p. 461). , 



During the magnificent aurora of January 25th, 1837, while the maximum 

 declination at New Haven, according to Mr. Herrick, was one degree, at New 

 Britain, twenty-five miles north of New Haven, the change of declination was 

 nothing, according to Mr.. Burritt, who watched the needle attentively for two hours 

 during the most active part of the exhibition. Captain Parry also says that, in no 

 instance was the magnetic influence affected by the aurora borealis during the three 

 winters he passed within the polar circle. Captain Franklin states that, when the 

 aurora was streaming with prismatic colors, it had an obvious effect upon the 

 needle; but when it gave a steady, dense light, without motion, it produced no 

 effect on the needle. 3 



VIII. Geographical Relations. — The aurora borealis occurs most frequently in 

 the polar regions. It is only in the great periods that it descends much below the lati- 

 tude of 40°; but it descends lower on the western than on the eastern continent, and 

 prevails more in the northern than in (he southern hemispheres. Franklin observed, 

 in the polar seas, 142 auroras in six months ; and the French Commission, at 



1 Also in the recent exhibition of February 19, 1852. 



3 Dalton supposes that he was the first to discover the relation between the aurora borealis and the 

 earth's magnetism, in his observations on the exhibition of October 13, 1792 {Essays, second edition, 

 p. 147). But he seems to have been unacquainted with the fact that their connection had been detected 

 long before, being mentioned by Mairan, in the second edition of his TraiU de VAurore Boriale, 

 published in 1754, p. 450. 



3 Aorer. Jour. Sci., XVI, p. 148. 



