It is not improbable that, as
heavily-charged threatening clouds, owing to frequent transitions of the
atmospheric electricity to an opposite condition, are not always discharged,
accompanied by lightning, so likewise magnetic storms may occasion
far-extending disturbances in the horary course of the needle, without there
being any positive necessity that the equilibrium of the distribution should
be restored by explosion, or by the passage of luminous effusions from one
of the poles to the equator, or from pole to pole.
In collecting all the individual features of the phenomenon in one general
picture, we must not omit to describe the origin and course of a perfectly
developed Aurora Borealis. Low down in the distant horizon, about the part
of the heavens which is intersected by the magnetic meridian, the sky which
was previously clear is at once overcast. A dense wall of bank of cloud
seems to rise gradually higher and higher, until it attains an elevation of
8 or 10 degrees. The color of the dark segment passes into brown or
violet; and stars are visible through the cloudy stratum, as when a dense
smoke darkens the sky. A broad, brightly-luminous arch, first white, then
yellow, encircles the dark segment; but as the brilliant arch appears
subsequently to the smoky gray segment, we can not agree with Argelander in
ascribing the latter to the effect of mere contrast with the bright luminous
margin.
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