THE AURORA BOREALIS
He must be a very careless observer who has not been struck with the
appearance of the streamers which occasionally light up the northern
heavens, and which farmers consider to be indicators of strong wind or
broken weather.
The time was when the phenomenon was considered to be supernatural and
portentous, as the chroniclers of spectral battles, when "fierce, fiery
warriors fought upon the clouds, in ranks and squadrons, and right form of
war." And even in the rural districts of Britain, the blood-coloured
aurora, of October 24th, 1870, was considered to be the reflection of an
enormous Prussian bonfire, fed by the beleaguered French capital.
In joyful spirit, the Shetlanders call the beautiful natural phenomenon,
"Merry Dancers." Burns associated their evanescence with the
transitoriness of sensuous gratification:--"they flit ere you can point
their place." And Tennyson spoke of his cousin's face lit up with the
colour and light of love, "as I have seen the rosy red flushing in the
northern night."
Yet this phenomenon is to a great extent under the control of cosmical
laws. One of the most difficult problems of our day has been to
disentangle the irregular webwork of aurorae, and bring them under a law of
periodicity, which depends upon the fluctuations of the sun's photosphere
and the variations on the earth's magnetism, and which have such an
important influence upon the fluctuations of the weather.
The name "Aurora Borealis" was given to it by Gassendi in 1621.
Afterwards, the old almanacs described it as the "Great Amazing Light in
the North." In the Lowlands of Scotland, the name it long went by, of
"Lord Derwentwater's Lights," was given because it suddenly appeared on
the night before the execution of the rebel lord. In Ceylon aurorae were
called "Buddha Lights."
The first symptom of an aurora borealis is commonly a low arch of pale,
greenish-yellow light, placed at right angles to the magnetic meridian.
Sometimes rays cover the whole sky, frequently showing tremulous motion
from end to end; and sometimes they appear to hang from the sky like the
fringes of a mantle. They are among the most capricious of natural
phenomena, so full of individualities and vagaries. To the glitter of
rapid movement they add the charm of vivid colouring. It is strongly
asserted that aurorae are preceded by the same general phenomena as
thunder-storms. This was borne out by Piazzi Smith (late Astronomer-Royal
for Scotland), who observed that their monthly frequency varies inversely
with that of thunder-storms--both being safety-valves for the discharge of
surplus electricity.
Careful observers have, moreover, noticed a remarkable coincidence
between the display of aurorae and the maxima of the sun's spots and of the
earth's magnetic disturbances. Some have supposed that the light of the
aurora is caused by clouds of meteoric dust, composed of iron, which is
ignited by friction with the atmosphere. But there is this difficulty in
the way, shooting stars are more frequent in the morning, while the
reverse is the case with the aurora. The highest authorities have
concluded, pretty uniformly, that aurorae are electric discharges through
highly rarefied air, taking place in a magnetic field, and under the sway
of the earth's magnetic induction. They are not inappropriately called
"Polar lightnings," for when electricity misses the one channel it must
traverse the other.
The natives of the Arctic regions of North America pretend to foretell
wind by the rapidity of the motions of the streamers. When they spread
over the whole sky, in a uniform sheet of light, fine weather ensues.
Fitzroy believed that aurorae in northern latitudes indicated and
accompanied stormy weather at a distance. The same idea is still current
among many farmers and fishermen in Scotland.
Is there any audible accompaniment to the brilliant spectacle? The natives
of some parts, with subtle hearing-power, speak of the "whizzing" sound
which is often heard during auroral displays. Burns tells of their
"hissing, eerie din," as echoes of the far-off songs of the Valkyries.
Perhaps the most striking incident which corroborates this opinion
occurred during the Franco-Prussian War. Rolier, a practised aeronaut,
left Paris in a balloon, on his mission of city defence, and fourteen
hours afterwards landed in Norway. He had reached the height of two and a
half miles. When descending, he passed through a peculiar cloud of
sulphurous odour, which emitted flashed light and a slight scratching or
rustling noise. On landing, he witnessed a splendid aurora borealis. He
must, therefore, have passed through a cloud in which an electrical
discharge of an auroral nature was proceeding, accompanied with an audible
sound. There is, moreover, no improbability of such sounds being
occasionally heard, since a somewhat similar phenomenon accompanies the
brush discharge of the electric machinery, to which the aurora bears
considerable resemblance.
Though no fixed conclusions are yet established about the causes of the
brilliant auroral display, yet, as the results of laborious observations,
we are assured that the stabler centre of our solar system holds in its
powerful sway the several planets at their respective distances, supplying
them all with their seasonable light and heat, vibrating sympathetic
chords in all, and even controlling under certain--though to us still
unknown--laws the electric streamers that flit, apparently lawlessly, in
the distant earth's atmosphere.