[footnote] *Kamtz, 'Lehrbuch der Meteorologie', bd. iii., s. 498 and 501.
The process of the Aurora is, as has already been observed, the restoration
of a disturbed condition of equilibrium. The effect on the needle is
different according to the degree of intensity of the explosion. It was
only unappreciable at the gloomy winter station of Bosekop when the
phenomenon of light was very faint and aptly compared to the flame which
rises in the closed circuit of a voltaic pile between two points of carbon
at a considerable distance apart, or, according to Fizeau, to the flame
rising between a silver and a carbon point, and attracted or repelled by the
magnet. This analogy certainly sets aside the necessity of assuming the
existence of metallic vapors in the atmosphere, which some celebrated
physicists have regarded as the substratum of the northern light.
When we apply the indefinite term 'polar light' to the luminous phenomenon
which we ascribe to a galvanic current, that is to say, to the motion of
electricity in a closed circuit, we merely indicate the local direction in
which the evolution of light is most frequently, although by no means
invariably, seen. This phenomenon derives the greater part of its
importance from the fact that the Earth becomes 'self-luminous', and that as
a planet, besides the light which it receives from the central body, the
Sun, it shows itself capable in itself of developing light.
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