Motion
renders the phenomenon more visible. Round the point in the vault of heaven
which corresponds to the direction of the inclination of the needle, the
beams unite together to form the so-called corona, the crown of the northern
light, which encircles the summit of the heavenly canopy with a milder
radiance and unflickering emanations of light. It is only in rare instances
that a perfect crown or circle is formed, but on its completion the
phenomenon has invariably reached its maximum, and the radiations become
less frequent, shorter, and more colorless. The crown and the luminous
arches break up, and the whole vault of heaven becomes covered with
irregularly-scattered, broad, faint, almost ashy-gray luminous immovable
patches, which in their turn disappear, leaving nothing but a trace of the
dark, smoke-like segment on the horizon. There often remains nothing of the
whole spectacle but a white, delicate cloud with feathery edges, or divided
at equal distances into small roundish groups like cirio-cumuli.
This connection of the polar light with the most delicate cirrous clouds
deserves special attention, because it shows that the electro-magnetic
evolution of light is a part of a meteorological process. Terrestrial
magnetism here manifests its influence
p 197
on the atmosphere and on the condensation of aqueous vapor.
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