Aurora Polaris [pp. 531-534]

X URORA POLA RIS.
AURORA POLARIS.
HAT is the Aurora Polaris?
A luminous appearance in the
sky, exhibited as a horizontal band, an
arch, a streamer, or a corona, and limited chiefly to the colder latitudes: a
phenomenon which has attracted the attention of observers from a very early
date, but which has remained without
satisfactory explanation until our own
times. From the days of Aristotle until
the present, theories have been promulgated and books written to account for
the polar light; it has been referred to
the category of nebulous matter; to the
refraction of the sun's rays; to the admixture of the solar atmosphere and
that of the earth; to the formation of
nitrous vapors in the higher atmosphere; to the magnetic fluid, and to
that convenient agent for all meteorological phenomena-electricity. Pliny
alludes to it; Celsius discusses it;
Franklin conjectures; whilst all the accumulated knowledge of this century is
brought to bear upon it by the massive
intellects of Humboldt, De la Rive, Faraday and others. Since the laws of meteorology and electromagnetism have
become better known, and the practice
of recording meteorological observations
more widely extended, the appearance
of aurora has attracted more attention,
and its connection with disturbances of
the magnetic needle and the electrometer closely noticed. Such observations
have shown that whilst the auroral light
has been simultaneously perceived over
a very extended space, not only in the
northern hemisphere but also in the
southern, yet that the most frequent
and brilliant appearances present themselves between the parallels of fifty and
sixty-two degrees north latitude, and
fifty-five and sixty-seven degrees south
latitude; that in the tropics and extreme
polar regions it is almost unknown;
that its elevation varies from a few feet
to some sixty miles above the earth;
that its maximum brilliancy occurs about
midnight; that it exerts a considerable
influence over the magnetic needle and
upon the wire of the electric telegraph,
and that the auroral light is essentially
an electric light. Until lately the height
of the aurora has been supposed to be
very great-even beyond our atmosphere-and its existence in the lower
clouds and near the surface of the earth
denied; the first unexceptionable determination being that of Dalton, who
calculated the height of an aurora seen in
England, in I819, to be one hundred or
one hundred and two miles above the
earth. Wrangel, Struve, Parry, Fisher,
Farquharson, Hooker, Sabine and the
writer, have all noticed it at very inconsiderable heights, being but a few feet
removed from the surface of the earth.
With the exception of the observations
taken in Scotland, all the low exhibitions of aurora have been noticed in
the Arctic zone.
The experience of the writer during a
residence of two and a half years in the
polar regions, has led him to conclude
that the explanation which best coincided with the appearance of the aurora
in those latitudes will equally refer to
the same light wherever exhibited. In
the Arctic Sea there is always evaporation from the surface of the exposed
water, and according to the time of year,
the area of this exposed sea-surface
will be great or small. About the beginning of September, as the sun's altitude decreases and the nights become
colder, the surface of the sea is frozen
over, and the difference between the
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