194 COSMOS. 



terrestrial magnetism * When this disturbance attains a great 

 degree of intensity, the equilibrium of the distribution is re- 

 stored by a discharge attended by a development of light 

 ' The Aurorat itself is, therefore, not to be regarded as an ex 

 ternally manifested cause of this disturbance, but rather as a 

 result of telluric activity, manifested on the one side by the 

 appearance of the light, and on the other by the vibrations of 

 the magnetic needle." The splendid appearance of colored 

 polar light is the act of discharge, the termination of a mag 

 netic storm, as in an electrical storm a development of light — 

 the flash of lightning — indicates the restoration of the disturb- 

 ed equilibrium in the distribution of the electricity. An elec- 

 tric storm is generally confined to a small space, beyond the 

 limits of w^hich the condition of the atmospheric electricity 

 remains unchanged. A magnetic storm, on the other hand, 



* [The Aurora Borealis of October 24th, 1847, which was one of the 

 most brilliant ever known in this country, was preceded by gi-eat mag- 

 netic disturbance. On the 22d of October the maximum of the west 

 declination was 23° 10' ; on the 23d the position of the magnet waa 

 continually changing, and the extreme west declinations were between 

 22° 44' and 23° 37' ; on the night between the 23d and 24th of October, 

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

 at times moving across the field so rapidly that a difiSculty was experi- 

 enced in following it. During the day of the 24th of October there was 

 a constant change of position, but after midnight, when the Aurora be- 

 gan perceptibly to decline in brightness, the disturbance entirely ceased. 

 The changes of position of the horizontal-force magnet were as large and 

 as frequent as those of the declination magnet, but the vertical-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, Octohei 

 24iA, 1847, at Blackheath, by James Glaisher, Esq., of the Royal Observa 

 tory, Greenwich, in the London, Edinburgh, and Dublin Philos. Mag. 

 and Journal of Science for Nov., 1847. See further, An Account of iht 

 Aurora Borealis of October the 2Ath, 1847, by John H. Morgan, Esq. 

 We must not omit to mention that magnetic disturbance is now regis 

 tered by a photographic process : the self-registering photographic ap- 

 paratus used for this purpose in the Observatory at Greenwich was de- 

 signed by Mr. Brooke, and another ingenious instrument of this kind 

 has been invented by Mr. F. Ronalds, of the Richmond Observatory.]— 



#I)ove, in Poggend., Annalen, bd. xx., s. 341 ; bd. xix., s. 388. 

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

 pheric electrometer, whose divergence in like manner shows the in* 

 creased 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 Ban- 

 sen's carboi\ and zinc battery, see Casselmann's Beobachtungen (Mar- 

 burg, 1844), s. 56-62. 



