16'S COSMOS. 



the other elements of terrestrial magnetism could be noted, 

 the magnetic intensity and inclination were affected no less 

 than the variation. 19 



D-uring the beautiful Aurora, which Professor Forbes ob- 

 served at Edinburgh on the 21st of March, 1833, the inclina- 

 tion was strikingly small in the mines at Freiberg, while the 

 variation was so much disturbed that the angles could scarcely 

 be read off. The decrease in the total intensity of the mag- 

 netic force which has been observed to coincide with the in- 

 creasing energy of the luminosity of the northern light is a 

 phenomenon which is worthy of special attention. The mea- 

 surements which I made in conjunction with Oltmanns at 

 Berlin during a brilliant Aurora on the 20th of December, 

 1806, M and which are printed in Hansteen's " Unter- 

 suchungen iiber den Magnetismus der Erde," were con- 



19 Dove, in Poggend. Ann. Bd. xx, s. 333 341. The unequal influ- 

 ence which an Aurora exerts on the dipping needle at points of the 

 earth's surface, which lie in very different meridians, may in many 

 cases lead to the local determination of the active cause, since the mani- 

 festation of the luminous magnetic storm does not by any means always 

 originate in the magnetic pole itself ; while, moreover, as A rgelander 

 maintained and as Bravais has confirmed, the summit of the luminous 

 arch is in some cases as much as 11 from the magnetic meridian. 



20 "On the 20th of December, 1806, the heavens were of an azure 

 blue, with not a trace of clouds. Towards 10 P.M. a reddish-yellow 

 luminous arch appeared in the N.N.W., through which I could distin- 

 guish stars of the 7th magnitude in the night telescope. I found the 

 azimuth of this point by means of a Lyrse, which was almost directly 

 under the highest point of the arch. It was somewhat further west 

 than the vertical plane of the magnetic variation. The Aurora, which 

 was directed N.N.W., caused the north pole of the needle to be deflected, 

 for, instead of progressing westward like the azimuth of the arch, the 

 needle moved back towards the east. The changes in the magnetic 

 declination, which generally amount to from 2' 27" to 3' in the 

 nights of this month, increased progressively and without any great 

 oscillation to 26' 28" during the northern light. The variation was the 

 smallest about 9h. 12m. when the Aurora was the most intense. We 

 found that the horizontal force amounted to 1' 37".73 for 21 vibrations 

 during the continuance of the Aurora, while at 9h. 50m: A.M., and con- 

 sequently long after the disappearance of the Aurora, which had en- 

 tirely vanished by 2h. 10m. A.M. it was 1' 37".17 for the same number 

 of vibrations. The temperature of the room, in which the vibrations of 

 the small needle were measured, was in the first case 37.76 F. and 

 in the second 37.04 F. The intensity was therefore slightly diminished 

 during the continuance of the northern light. The moon presented no 

 coloured rings." From my magnetic journal, see Hansteen, s. 4d&. 



