AURORA BO'REA'LIS (Lat., 'northern dawn or light'). A luminous phenomenon seen during the night time in the heavens, most fre quently in Europe, to the north of the observer's zenith, but sometimes covering the whole sky. By connecting together the statistics from all possible observers, Fritz was able to show that there is in the Northern a belt of greatest frequency. The centre of this belt passes through the southern part of Hudson Bay. a little south of Cape Farewell and Ice land. through North Cape, Norway. over the mouth of the Lena River, over Point Barrow, Alaska, thence to the centre of Hudson Bay. Observers who are north of this belt see the aurora more frequently to the south than to the north, and also see it less and less frequently the farther north they go; observers who are south of the belt see the aurora to the north of them, but see it less frequently as they are nearer to the equator. A similar rule obtains in the Southern 'Hemisphere. There can be no doubt that the aurora is the result of a discharge of electricity through the atmosphere. It has been supposed that since a very thin atmosphere, such as that remaining in a glass jar or receiver of all air-pump, converts, the short bright elec tric spark into a lengthened delicate discharge, analogous to the aurora, the existence of the au rora itself necessarily implies a very thin atmos phere, and that therefore its light must emanate from a region 50 or 1(10 miles above the earth's surface. But if we consider the aurora as a dis charge through aqueous vapor or other rare gas, then we have nothing to do with the gaseous atmosphere as such, but with the aqueous com ponent only, and at moderate altitudes the density of the aqueous vapor is so slight that it must act as a very light gas, similar to that present in the vacuum chamber. As regards the height of the aurora above the earth's surface, scveral methods have been devised for calculat ing it; but all the trigonometrical calculations, based upon most careful observations, seem to show that the definite features that we see in the aurora light are perspective phenomena, and that the calculation of their height cannot be safely made by the method of simultaneous ob servations at two stations and measured alti tude,: and azimuth. In fact, the argument for the existence of the auroral light quite close to the earth's surface is too strong to be ignored. The character of this light is partially deter mined by means of the spectroscope, which es tablishes the fact that it comes from luminous, particles of vapors or gases. The most promi nent line of its spectrum is a yellow line, nearly identical with the yellow line known as the air line of the atmosphere. The special lines of the auroral spectrum apparently belong to the nitrogen.
The aurorae generally present a variety of inter esting optical phenomena that may generally be analyzed into component parts; viz., loug, slen der beams of light, usually yellow, but some times green, purple, violet, or rose. These beams are, by observation, ascertained to be approximately parallel to the free magnetic needle; by their association together they ap pear to form cylindrical bars, waved surfaces, rich drapery, and small clouds of light. lnas mwch as the slender beams are parallel to each other, it can easily happen that an observer may be so located as to see some of them end on; these will be scarcely visible. Other beams near by will appear greatly foreshortened, and pointing directly to the vacant centre. Those at a distance will appear less foreshortened, but also pointing toward the centre. There is thus formed a so-called corona, or circlet, or rays surrounding the central region. I tbservation has shown that its position corresponds very closely to that point in the sky toward which the free magnetic needle would point. It is therefore com mon to say that auroral beams are parallel to the magnetic needle; and this fact suffices to assure its that the aurora is composed of luminous Inag netic matter acted upon by the so-called earth's magnetism, but, more properly, by the magnetic field surrounding the earth, just as is the com pass needle.
BIBLIOGRAPHY. The literature relative to the Bibliography. The literature relative to the aurora borealis is very extensive and widely diffused. Excellent general summaries of our knowledge are: Fritz, Dos Polorliehi ( Leipzig, 1SS ) Argot, Borealis, Internal hum! Scientific Series, Vol. SI (London. Lem strum, I:11 urnre bOr (Paris, 1!=IS6) The largest collections of observations arranged in the order of date will be found in the works of: Lovering, On the Periodicity of the Aurora Be. i•ccuis (Boston, 1873) ; Fritz, Vcrzeichniss von Deobachtungen des Polarlichtes (Vienna, 1873) ; ltubenson, Catalogue des aurores boreales oh serr('cs en Suede (Stockholm, 1879-82). The reports of the various international expeditions for the observation of meteorology and magnet ism in the polar regions during 1881-83 fill about 30 volumes, of which one-fourth treat of the aurora. The Belgian and other expeditions to the Antarctic contain a small hut relatively im portant contribution from that region. The pioneer work in this line was that done by the French Exp&lition (lc :Vord (1S3S-42), whose volumes on auroras were prepared by Bravais and Martins. The question of the altitude of the aurora has been reviewed by C. Abbe in an article under that title, published in Terrestrial Magnetism (Baltimore, 1900 ) .