TERRESTRIAL MAGNETISM. 153 



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

 ation 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 magnetic force, which has been observed to coincide with 

 the increasing energy of the luminosity of the northern light, 

 is a phenomenon which is worthy of special attention. The 

 measurements which I made in conjunction with Oltmanns 

 at Berlin during a brilliant aurora on the 20th of Decem- 

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

 suchungen tiber den Magnetismus der Erde," were confirmed 

 by Sabine and the French physicists in Lapland in 1838.f 



While in this careful development of the present condition 

 of our positive knowledge of the phenomena of terrestrial 

 magnetism, I have necessarily limited myself to a mere ob- 

 jective representation of that which did not even admit of 

 being elucidated by merely theoretical views, based only 

 upon induction and analogy ; I have likewise purposely ab- 

 stained in the present work from entering into any of those 

 geognostic hypotheses in which the direction of extensive 



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

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

 luminous arch appeared in the NN.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 Lyrre, which was almost directly 

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

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

 was directed NN.W., caused the north pole of the needle to be de- 

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

 arch, the needle moved back toward the east. The changes in the 

 magnetic declination, which generally amount to from 2' 27" toj^' 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 vi- 

 brations during the continuance of the aurora, while at 9h. 50m. A.M., 

 and consequently long after the disappearance of the aurora, which 

 had entirely 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 o- 04 F. The intensity was, therefore, 

 slightly diminished during the continuance of the northern light. 

 The moon presented no colored rings." From my magnetic journal, 

 see Hansteen, s. 459. 



f Sabine, On Days of Unusual Magn. Disturbances, pt. i., p. xviii. 

 "M. Bravais concludes from the observations made in Lapland that 

 the horizontal intensity diminishes when the phenomenon of the Au- 

 rora Borealis is at its maximum" (Martins, p. 461). 



G2 



