motionless, but move with such rapidity that in some parts of the world they are known as the "merry dancers." The following description of an aurora seen by Captain Peary, during one of his voyages for the discovery of a Northwest Passage, will give an excellent idea of some of the phenomena. "The aurora began to show itself as soon as it was dark. Innumerable streams of white and yellowish light occupied the heavens to the southward of the zenith, being much brighter in the southeast, from which it often seemed to emanate. Some of these streams were in right lines, others crooked, and waving in all sorts of irregular figures, moving with inconceivable rapidity in various directions. Among them might frequently be observed shorter bundles of rays, CHAPTER XXX THE AURORA BOREALIS The free electricity of the atmosphere besides manifesting itself as a bright lightning flash, or as the faintly luminous tongues of flame known as St. Elmo's fire, also produces what is known as the aurora borealis, when it occurs in the Northern Hemisphere, and the aurora australis, when it occurs in the Southern Hemisphere. Unlike the lightning, or the St. Elmo's fires, this phenomenon is not limited to small portions of the heavens, but often covers a large part of the sky. The aurora borealis is one of the most impressive of the luminous phenomena of the atmosphere. It takes its name from the Latin word aurora, the morning hour; for its light, first seen in the sky near the horizon, is not unlike the light that occurs at daybreak. This phenomenon assumes a variety of forms. Sometimes huge pillars of fire move rapidly across the sky. At other times, the entire northern heavens are lighted up, as if by a huge storm of luminous snow. Generally, however, a corona, or auroral arch, is seen near the northern horizon, with its highest part immediately under the north magnetic pole of the earth. The height of the arch varies with the latitude, being greater in regions near to the poles of the earth. As the aurora progresses, the arch rises in the heavens and streams of different colored lights, white, red, purple, and sometimes yellow and green, dart up suddenly through the arch, as though coming from the centre of the sphere of which the arch is a segment. These streamers are not motionless, but move with such rapidity that in some parts of the world they are known as the "merry dancers." The following description of an aurora seen by Captain Peary, during one of his voyages for the discovery of a Northwest Passage, will give an excellent idea of some of the phenomena. "The aurora began to show itself as soon as it was dark. Innumerable streams of white and yellowish light occupied the heavens to the southward of the zenith, being much brighter in the southeast, from which it often seemed to emanate. Some of these streams were in right lines, others crooked, and waving in all sorts of irregular figures, moving with inconceivable rapidity in various directions. Among them might frequently be observed shorter bundles of rays, which, moving even with greater velocity than the rest, have acquired the name of 'merry dancers.' In a short time the aurora extended itself over the zenith, about half way down to the northern horizon, but no further, as if there were something in that quarter of the heavens that it did not dare to approach. About this time, however, some long streamers shot up from the horizon in the northwest, but soon disappeared. While the light extended over part of the northern heavens, there were a number of rays assuming a circular or radiated form, near the zenith, and appearing to have a common centre near that point, from which they all diverged. The light of which these were composed appeared to have inconceivably rapid motion in itself, though the form it assumed, and the station it occupied in the heavens, underwent little or no change for perhaps a minute or more. This effect is a common one with the aurora, and puts one in mind, as far as its motion alone is concerned, of a person holding a long ribbon by one end, and giving it an undulatory motion through its whole length, though its general position remains the same. When the streams or bands were crooked, the convolutions took place indifferently, in all directions. The aurora did not continue long to the north of the zenith, but remained as high as that point for more than an hour; after which, on the moon rising, it became more and more faint, and at half-past eleven was no longer visible. FIG. 61. ELECTRIC "The color of the light was most frequently yellowish white, sometimes greenish, and once or twice a lilac tinge was remarked, when several strata appeared, as it were, to overlay each other by very rapidly meeting, in which case the light was always increased in intensity." The aurora presents many different appearances and changes rapidly from time to time. As the under part of the corona, or arch, is dark by contrast with the lighter margin, it looks as though a huge dark orb, covered by a bright outer layer of light, was slowly rising in the northern sky, especially when the streamers dart up through the arch, as though they all came from the centre of a huge globe, of which the corona appears to be a part. At other times, a series of parallel rays appears as though hanging down from the clouds in the shape of a luminous curtain. This so-called auroral curtain really consists of several "curtains," one back of the other, extending across the sky from east to west. This curtain is often colored by horizontal bands bright red on the lower margin, yellow and green in other parts. The parallel streamers of which the curtain is composed do not remain in FIG. 62. DISCHARGE IN VACUUM a fixed position, but move in a hori zontal direction from the right or the left, thus giving the curtain the appearance of a ribbon or flag shaken by the wind. Sometimes, however, an auroral band, consisting of a single streamer, extends across the heavens from east to west. It is a popular error that auroras are seen most frequently in the immediate neighborhood of the poles. It is true that auroras are very seldom seen in the equatorial regions; indeed, they seldom occur nearer the equator than |