152 COSMOS. 



Bossekop* as by Thienemann, Parry, Franklin, Richardson, 

 Wrangel, and Anjou. Bravais estimated the altitude of the 

 phenomenon to be fully 51,307 toises (or 52 geographical 

 miles), while an otherwise very careful observer, Farquhar- 

 son, considers that it scarcely amounts to 4000 feet. The 

 data on which all these determinations are based are very 

 uncertain, and are rendered less trustworthy by optical illu- 

 sions, as well as by erroneous conjectures regarding the posi- 

 tive identity of the luminous arch seen simultaneously at two 

 remote points. There is, however, no doubt whatever of the 

 inflaence of the northern light on declination, inclination, 

 horizontal and total intensity, and consequently on all the 

 elements of terrestrial magnetism, although this influence is 

 exerted very unequally in the different phases of this great 

 phenomenon, and on the different elements of the force. The 

 most complete investigations of the subject were those made 

 in Lapland by the able physicists Siljestrom and Bravaisf 

 (in 1^38-1839), and the Canadian observations at Toronto 

 (1840-184:1), which have been most ably discussed by Sa- 

 bine.f In the preconcerted simultaneous observations which 

 were made by us at Berlin (in the Mendel ssohn-Bartholdy 

 Garden), at Freiberg below the purface of the earth, at St 

 Petersburg, Kasan, and Nikolajew, we found that the mag- 

 netic variation was affected at all these places by the Aurora 

 Borealis, which was visible at Alford, in Aberdeenshire (57° 

 15' N. lat.), on the night of the 19-20th of December, 1829. 

 At some of these stations, at which the other elements of 

 terrestrial magnetism could be noted, the magnetic intensity 

 and inclination were affected no less than the variation.§ 

 During the beautiful aurora which Professor Forbes ob- 



* Voyages en Scandinavie, en Laponie, etc. (Aurores Bor^aks), p. 559; 

 and Martin's Tj-ad. de la Meteorologie de Kaemtz, p. 460. In refer- 

 ence to the conjectured elevation of the northern light, see Bravais, 

 0}^. clL, p. 549, 559. 



t Op, ciL, p. 462. 



X Sabine, Unusual Magnet. Disturbances, pt. i., p. xviii., xxii., 3, 

 54. 



§ 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 

 manifestation of the luminous magnetic storm does not by any means 

 always origiifcate in the magnetic pole itself; while, moreover, as Ar- 

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

 luminous arch is in some cases as much as 11° from the magnetic me- 

 ridian. 



