THE AURORA BOREALIS, OR POLAR LIGHT: 



ITS PHENOMENA AND LAWS. 



BY ELIAS LOOMIS, 



PROFESSOR OF NATURAL PinLOSOPHY AND ASTRONOMY IN YALE COLLEGE. 



The Aurora Polaris is a Inminons appoarance frequently pcen near the horizon 

 as a diifuse light like the morning twilight, whence it has received the name of 

 aurora. . In the northern hemisphere it is usually termed aurora borealis, 

 because it is chiefly seen in the north. Similar phenomena are also seen in the 

 southern hemisphere, where it is called the aurora australis. Each of them 

 might with greater propriety be called aurora polaris or polar light. 



The aurora exhibits an infinite variety of appearances, but they may generally 

 be referred to one of the following classes : 



1. A horizontal light, like the morning aurora or break of day. The polar 

 light may generally be distinguished from the true dawn by its position in the 

 heavens, since in the United States it always appears in the northern quarter. 

 This is the most common form of aurora, but it is not an essentially distinct 

 variety, being due to a blending of the other varieties in the distance. 



2. An arch of light somewhat in the form of a rainbow. This arch frequently 

 extends entirely across the heavens from east to Avest, and cuts the magnetic 

 meridian nearly at right angles. This arch does not long remain stationary, but 

 frequently rises and tails ; and when the aurora exhibits great splendor, several 

 parallel arches are often seen at the same time, appearing as broad belts of light 

 stretching from the eastern to the western horizon. In the polar regions five 

 such arches have been seen at once,* and on two occasions have been seen nine 

 parallel arches separated by distinct intervals.! 



3. Slender luminous beams or columns, well defined, and often of a bright 

 light. These beams rise to various heights in the heavens, 30°, 50^, 70°, and 

 sometimes, though rarely, they pass the zenith. Frequently they last but a 

 few minutes ; sometimes they continue a quarter of an hour, a half hour, or even 

 a whole hour. Sometimes they remain at rest, and sometimes they have a quick 

 lateral motion. Their light is commonly of a pale yellow, sometimes reddish, 

 occasionally crimson, or even of blood color. Sometimes the tops of these beams 

 are pointed, and having a waving motion, they resemble the lambent flames of 

 half-extinguished alcohol, burning upon a broad, flat surface. Sometimes the 

 luminous beams are interspersed with dark rays, resembling dense smoke. 



4. The corona. Luminous beams sometimes shoot up simultaneously from 

 nearly every part of the horizon, and converge to a point a little south of the ze- 

 nith, forming a quivering canopy of flame, which is called the corona. The sky 

 now resembles a fiery dome, and the crown appears to rest on variegated fiery 

 pillars, which are frequently traversed by waves or flashes of light. This may 

 l3e called a complete aurora, and comprehends most of the peculiarities of tha 

 other varieties. 



*Frauklin's First Expedition, p. 588. 



t Voyages en Scandiuavio, 183d, pp. 170-171. 



