188 



that the aurora borealis was a magnetic phenomenon, has 

 acquired empirical certainty from Faraday's brilliant discovery 

 of the evolution of light by magnetic forces. The northern 

 light is preceded by premonitory signs. Thus in the morning 

 before the occurrence of the phenomenon, the irregular horary 

 course of the magnetic needle generally indicates a disturbance 

 of the equilibrium in the distribution of terrestrial magnetism.* 

 When this disturbance attains a great degree of intensity 

 the equilibrium of the distribution is restored by a dis- 

 charge attended by a development of light. " The auroraf 

 itself is therefore not to be regarded as an externally mani- 



* [The aurora borealis of October 24, 1847, which was one of the moai 

 brilliant ever known in this country, was preceded by great magnetic 

 disturbance. On the 22nd of October the maximum of the west decli- 

 nation was 23 10'; on the 23rd the position of the magnet was con- 

 tinually changing, and the extreme west declinations were between 

 22 44' and 23 37' ; on the night between the 23rd and 24th October, 

 the changes of position were very large and very frequent, the magnet 

 at times moving across the field so rapidly that a difficulty was expe- 

 rienced in following it. During the day of the 24th of October, there 

 was a constant change of position, but after midnight, when the aurora 

 began perceptibly to decline in brightness, the disturbance entirely 

 ceased. The changes of position of the horizontal-force magnet were aa 

 large and as frequent as those of the declination magnet ; but the ver- 

 tical-force magnet was at no time so much affected as the other two 

 instruments. See On the Aurora Borealis, as it was seen on Sunday 

 evening, October 2Uh, 1847, at Blackheath, by James Glaisher, Esq., of 

 the Koyal Observatory, Greenwich, in the London, Edinburgh, and 

 Dublin Philos. Mag. and Journal of Science, for Nov. 1847. See 

 further, An Account of the Aurora, Borealis of October the 24th, 1847, 

 by John H. Morgan, Esq. We must not omit to mention, that magnetic 

 disturbance is now registered by a photographic process : the self- 

 registering photographic apparatus used for this purpose in the obser- 

 vatory at Greenwich, was designed by Mr. Brooke, and another ingenious 

 instrument of this kind has been invented by Mr. F. Ronalds of the 

 Richmond Observatory.] Tr. 



t Dove, in Poggend. Annalen, bd. xx. s. 341, bd. xix. s. 388. 

 "The declination needle acts in very nearly the same way as an 

 atmospheric electrometer, whose divergence in like manner shews the 

 increased tension of the electricity, before this has become so great as to 

 yield a spark." See also the excellent observations of Professor Kamtz 

 in his Lehrbuch der Meteorologie, bd. iii. s. 511-519, and Sir David 

 Brewster, in his Treatise on Magnetism, p. 280. Regarding the mag- 

 netic properties of the galvanic flame, or luminous arch from a Bunsen'g 

 carbon and zinc battery, see Cassclmann's Beobachtungen ^Marburg, 

 1844), &. 56-62. 



