Page:The American Cyclopædia (1879) Volume II.djvu/133

 AURORA AURORA BOREALIS 121 ACRORA, a city of Kane county, 111., on Fox river and the Chicago, Burlington, andyuincy railroad, 40 m. W. by S. of Chicago; pop. in 1860, 6,011 ; in 1870, 11,162. It contains 14 churches, a handsome city hall, a college, and many important manufactories, the power for which is furnished by the Fox river. The construction and repair shops of the railway situated here employ about 700 men. A semi- weekly newspaper, and 3 weeklies, one of which is German, are published here. AURORA BOREALIS (more correctly Aurora Polaris, since the phenomenon is not confined to northern latitudes), called also NOETHERN STREAMERS and NORTHERN LIGHTS, a luminous appearance, associated with energetic disturb- ances of the earth's magnetism and electrical condition. It is seldom seen save in high lati- tudes, though occasionally the tropics are visit- ed by auroral displays. . In polar regions au- roras are very common, and usually far more brilliant than in the temperate zones. Hum- boldt gives the following description of the appearances presented when the auroral phe- nomena are fully developed, although it must be understood that there is considerable variety in these displays : " An aurora borealis is always preceded by the formation of a sort of nebular veil which sl'owly ascends to a height of four, six, eight, or even to ten degrees. It is toward the magnetic meridian of the place that the sky, at first pure, commences to become brownish. Through this obscure segment, the color of which passes from brown to violet, the stars are seen as through a thick fog. A wider arc, but one of brilliant light, at first white, then yel- low, bounds the dark segment. Sometimes the luminous arc appears agitated for entire hours by a sort of effervescence and by a continual change of form, before the rising of the rays and columns of light, which ascend as far as the zenith. The more intense is the emission of the polar light, the more vivid are its colors, which from violet and bluish white pass through all the intermediate shades to green and purple red. Sometimes the columns of light appear to come out of the brilliant arc mingled with blackish rays similar to a thick smoke. Some- times they rise simultaneously in different parts of the horizon; they unite themselves into a sea of flames, the magnificence of which no painting could express, and at each instant rapid undulations cause their form and bril- liancy to vary. Motion appears to increase the visibility of the phenomenon. Around the point in the heavens which corresponds to the direction of the dipping needle produced, the rays appear to assemble together and form a boreal corona. It is rare that the appearance is so complete and is prolonged to the forma- tion of the corona; but when the latter ap- pears, it always announces the end of the phe- nomenon. The rays then become more rare, shorter, and less vividly colored. Shortly nothing more is seen on the celestial vault than wide, motionless nebulous spots, pale or of an ashen color ; these disappear while the traces of the dark segment whence the phenomenon originated remain still on the horizon." Al- though auroras are more commonly seen in high latitudes than near the tropics, it is not toward the true poles of the earth that the increase takes place, nor does the increase continue after certain high latitudes have been reached. Thus the frequency of auroras is different at different stations in the same latitude ; and in passing poleward from places in a given lati- tude, the region of maximum frequency is reached more quickly in some longitudes than in others. Thus an inhabitant of St. Peters- burg would have to travel to lat. 71 N. before reaching the place of greatest auroral activity ; while an inhabitant of Washington need travel northward only to lat. 56 to reach the region where auroral displays are most frequent. The zone on the earth's northern hemisphere where auroras occur most commonly and attain their greatest splendor, may be represented by con- structing a ring of card or paper, of such di- mensions as to agree with the 60th parallel of north latitude, and then pushing the ring south- ward on the side of America and northward on the side of Asia, until it passes through the most southerly part of Hudson bay and the most northerly part of Siberia. The position of the corresponding zone in the southern hemisphere has not yet been determined ; but it is believed that the southern zone of maxi- mum auroral frequency is nearly antipodal to the northern zone. From what we kno.w of the connection between the occurrence of au- roras and disturbances of the earth's magnet- ism, we have every reason to believe that as the magnetic poles of the earth are slowly shifting, so the zone of maximum auroral fre- quency must also change in position. It can- not be doubted, for example, that in the 17th century, when the northern magnetic pole lay between England and the north pole, terrestrial conditions were more favorable for the occur- rence of auroras in England than they now are, or than they then were in corresponding latitudes in North America. At present, on the contrary, the northern magnetic pole lies between the north pole and the northwestern extremity of the American continent; hence auroras are more frequent and more brilliant in North America than in corresponding lati- tudes iA Europe. To the description given by Humboldt we should add that sometimes in high latitudes, instead of extending from the horizon, the auroral arch appears in the form of a complete oval. Hansteen relates that at Christiania he twice saw the auroral arch in this form. Sometimes more than one arch has been seen. Thus the observers who were sent by the French government to winter at Bos- sekop in Finland, saw on one occasion no fewer than nine arches, separated by dark spaces, " and resembling in their arrangement magnifi- cent curtains of light, hung behind and below each other, their brilliant folds stretching com-