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Meteorology; or Weather Explained

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<h2>CHAPTER XXI</h2> 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 auroræ, 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 auroræ 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 auroræ 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 auroræ 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 auroræ 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 auroræ 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 aëronaut, 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.     <hr style="width: 50%;" /> <div style="break-after:column;"></div>

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