AURORA BOREALTS. 2."^ 9 



occasional deviations of two or throe degrees to the cliangos in the position of 

 the magnetic needle which are observed daring great auroral exhibitions ; but 

 permanent deviations, indicate the operation of some constant cause. The fol- 

 lowing is substantially the explanation suggested by M. Bravais.* The direction 

 of the magnetic needle at any place is determined mainly by its position with 

 respect to the magnetic poles of tlie earth, but partly by local causes, such as 

 the conformation of the land and sea, the structure of the earth in that vicin- 

 ity, &:c. In consequence of these local causes, the direction of the magnetic 

 needle at some places differs several degrees from what it would be if it were 

 controlled entirely by the magnetic poles. Now this local and disturbing influ- 

 ence probably diminishes as we rise above the earth's surface, so that at the 

 height of one or two hundred miles the direction of the magnetic needle may 

 differ several degrees from that at the surface of the earth. In northern Europe 

 the north end of the magnetic needle points several degrees more easterly than 

 it should if the magnetic meridians were entirely symmetrical. Hence it seems 

 not improbable that in this regio^i the declination of the magnetic needle 

 increa&es as we rise above the earth's surface at the rate of one degree to about 

 ten miles' deviation, and this supposition will reconcile our theory with the 

 observations. 



13. The flashes of light so frequently observed in great auroral displays are 

 due to inequalities in the motion of the electric currents. In consequence of the 

 imperfect conducting power of the medium through which it passes, the flow of 

 electricity through the upper regions of the air is not perfectly uniform. It 

 experiences more or less resistance to its motion, and hence escapes through the 

 air by paroxysms. The flashes of the aurora are therefore feeble flashes of 

 lightning. 



. 14. Cause of the magnetic disturhances. — The disturbance of the magnetic 

 needle during an aurora is due to the cm-rents of electricity flowing through 

 the atmosphere or through the earth. A magnetic needle is deflected from its 

 mean position by an electric current flowing near it through a good conductor, 

 like a copper wire. A stream of electricity flowing through the earth or the 

 atmosphere must produce a similar effect. The direction in which the magnet 

 is deflected may always be known from the rule given by Arapere : " If you 

 conceive yourself lying in the direction of the current, the stream of positive 

 electricity flowing through your head towards your feet, with the north pole of 

 the magnet before you, the north pole will always be deviated toward the right." 

 It is probable that the directive power of the magnetic needle is due to elec- 

 tric currents circulating round the globe from east to west. If there were such 

 electric currents circulating round the globe in planes parallel to the magnetic 

 equator, the effect of such currents would be everywhere to cause the magnetic 

 needle to assume a position corresponding very nearly with what is actually 

 observed. M. Lament, of Mrmich, thinks he has proved, by direct observation, 

 the existence of such currents constantly circulating from east to west, over the 

 surface of the earth.f 



According to the theory of Professor De la Ilive, already explained, there 

 is a general system of circulation of positive electricity from the equator towards 

 either pole, through the upper regions of the atmosphere ; in the higher lati- 

 tudes this positive electricity makes its way to the earth, and it travels 

 thence towards the equator to restore the equilibrium which is continually dis- 

 turbed by evaporation from the waters of the equatorial seas. This current 

 through the earth from the north polar regions southward, must modify the reg- 

 ular current of electricity which we suppose is constantly circulating from east 

 to west. Hence during the period of great auroraUdisplays we should expect a 



* Voyages en Scandinavie, p. 4.58. 



tBib. Uuiv. de Geneve, 1861, v. 12, p. 357. 



