This
ring possesses a movement of rotation around the magnetized cylinder,
sometimes in one direction, sometimes in another, according to the
direction of the discharge and of the magnetization. Finally, some more
brilliant jets seem to come out from this luminous circumference
without being confounded with the rest of the group. Now the magnetic
pole exercises over the luminous haze which we have mentioned as always
present during an aurora precisely the same action which the pole of
the electro-magnet exercises in the experiment just described; and what
takes place on the small scale of the experiment is precisely what
takes place on the large scale of the phenomenon of the aurora
borealis.
The arc of the aurora borealis is a portion of a luminous ring, the
different points of which are sensibly at equal distances from the
earth, and which centres upon the boreal magnetic pole, so as to cut at
right angles all the magnetic meridians that converge towards this
pole. Such a ring, seen by an observer placed at the surface of the
earth, evidently presents to him the known arc of the aurora; and its
_apparent_ summit is always necessarily situated in the magnetic
meridian of the place.
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