194 COSMOS. 



terrestrial inaffiietism.* When this disturbance attains a g^reat 

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

 stored by a discharge attended by a development of light 

 ' Tiie Auroraf itself is, therefore, not to be regarded as an ex 

 ternaily manifested cause of this disturbance, but rather as v 

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

 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 ^ 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 thc- 

 limits of which the condition of -the atmospheric electricity 

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



* [The Aurora Borealis of October 24tii, 1847, Vvbich was one of the 

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

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

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

 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 largo and very frequent, the magnet 

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

 enced in following it. During the day of the 24th of October there wa6 

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

 i:au perceptibly to decline in brightness, the disturbance entirely ceased. 

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

 ;i8 frequent as those of the declination magnet, but the vertical-force 

 m:^iriiet was at no time so much atfected as the other two instruineuto. 

 See On the Aurora Borealis, as it was seen on Sutidai/ evening, October 

 'lUk, 1847, at Blackheath, by James Glaisher, Esq.. of the Royal Observa- 

 tory, Greenwich, iu the London, Edinhirgh, and Dublin Philos. Mag. 

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

 Aurora Borealis of October the 'Mth, 1847, by John H. Morgan, Esq. 

 We must not omit lo mention that magnetic disturbance is now regis- 

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

 ])aratus used for this purpose in the Observatoiy at Greenwich ^yas de- 

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

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



Tr. 



t Dove, iu Poggend., Annalert,, bd. xx., s. 341 ; bd. xix., s. ;388. 

 " The declination needle acts in very nearly the same \A'ay as an atmos- 

 pheric electrometer, whose divergence in like manner shows the m- 

 ci-eased 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 Lehrbvch der Meieorologie, bd. iii., s. ^l\-:Ad, and Sir Dim i! 

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

 iietic properties of the galvanic flame, or luminous arch from a Buii- 

 Hen'h carbon and zinc battery, see Casaelmann's Bcobachiungen (Mar- 

 bur-. 1844). s. .56-(i2. 



