Page:The New International Encyclopædia 1st ed. v. 02.djvu/301

AURORA. of Omaha; on the Burlington and Missouri River Railroad (Map: Nebraska, G 3). It is in a farming and dairying region, and has foundries and machine shops, flouring mills, and extensive creamery interests. Population, in 1890, 1862; in 1900, 1921.

AURORA. A town in Cayuga County, N. Y., settled about 1775, 16 miles southwest of Au- burn; on Cayuga Lake, and the Lehigh Valley Railroad. It is a beautiful residential town, in the centre of an agricultural region which produces grain, hay, live-stock, grapes, and apples. Aurora has a public library, and is the seat of Wells College for women, founded 1868. Town-meetings are held every other year to elect officers. Population, about 1000.

AURORA (Lat. from the root urcre, to burn, Skt. iixus, dawn, Gk. 'Hcis, or "Eois, Eos, or HcuH). The goddess of the dawn, or 'morning redness.' According to Hesiod, she was the daughter of Hyperion and Thia, sister of Helios and Selene, and wife of the Titan Astræus. Zephyrus, Boreas, Notus, Hesperus, and the other stars were her children. She rises from Oceanus, and her rays are the first indication of the coming of Helios (the sun). In legend Aurora appears chiefly as the abductor of beautiful youths, Tithonus, Orion (q.v.), and Cephalus, the hunter. By Tithonus she became the mother of Memnon (q.v.), and by Cephalus of Phaëton. In the earlier poets Aurora is never more than the early morning, and Euripides first identified her with 'll/iifm, UCmera (the day). In art she appears as j)resent at the death and burial of Memnon, as carrying away her favorites, and as a goddess of light, either as a winged maiden, or as in a chariot with winged horses preceding Helios.

AURORA. A renowned ceiling painting by Guido Reni in the Casino Rospigliosi at Rome. Aurora precedes the chariot of the sun god, scattering flowers in his path. The fresco is notable for its coloring, and is the greatest work of the painter. For illustration see.

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 Hemisphere 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 an 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 100 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, several 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- tudes 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 auroræ generally present a variety of interesting optical phenomena that may generally be analyzed into component parts; viz., long, slender beams of light, usually yellow, but sometimes 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 appear to form cylindrical bars, waved surfaces, rich drapery, and small clouds of light. Inasmuch 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. Observation 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 common to say that auroral beams are parallel to the magnetic needle: and this fact suffices to assure us that the aurora is composed of luminous magnetic matter acted upon by the so-called earth's magnetism, but, more properly, by the magnetic field surrounding the earth, just as is the compass needle.

. The literature relative to the aurora borealis is very extensive and widely diffused. Excellent general summaries of our knowledge are: Fritz, Das Polarlicht (Leipzig, 1881); Angot, Aurora Borealis, International Scientific Series, Vol. 81 (London, 1896); Lemstrom, L'aurore boréale (Paris, 1886). 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 Bo-