NORTHERN LIGHTS. 803 



Many investigators consider the observations of Gassendi, 

 which relate chiefly to the phenomena of 1621, as the starting- 

 point toward a more correct conception of the nature of northern 

 lights. The first move toward a truly scientific investigation 

 into the matter was made by Halley, who in 1716 suggested that 

 auroras were a magnetic exudation from the northern pole of the 

 globe. His contemporaries did not share Halley's opinion. Wolf, 

 in Halle, maintained that the lights consisted of inflammable sul- 

 phurous fumes. Descartes and Triewald saw in them only a re- 

 flection of the snow and ice at the north pole. Mairan (1733) 

 considered them formed by a blending of zodiacal light with the 

 earth's atmosphere. The famous mathematician Euler adhered 

 to a sort of nebular hypothesis and declared the aurora to be a 

 phenomenon similar to that presented by the tails of comets. 

 Halley had arrived at his view through the observation that the 

 center line of the light-arc deviated to the west of the meridian 

 to about the same extent as the north pole of a magnetic needle. 

 This important discovery was followed by one made by Mairan, 

 that the crown of the northern lights lies in the (prolonged) direc- 

 tion of the dipping-needle ; and soon after this Hjoter, in Upsala, 

 demonstrated the influence of the aurora on a magnetic needle 

 placed horizontally (1741). 



It was by these discoveries that the relation between northern 

 lights and magnetism was established. Winkler (1746) and Van 

 Marum (1777) compared the former to the electric glow which can 

 be produced in rarefied air. The veil of the mystery had been 

 raised, but only to disclose a new query, for the demonstration of 

 the cause of these relations was a problem the solution of which 

 was reserved for modern science. How far the efforts in this 

 direction have been crowned by success we are now to consider. 



Auroras are most frequently seen in the cold and in the north- 

 ern temperate zone, rarely in the southern temperate zone, and 

 hardly ever in the tropics. The places where they most frequently 

 appear lie between the sixtieth and seventieth degrees of north 

 latitude. In the form of an oval, they include the geographical 

 as well as the magnetic north pole, which is to be found on the 

 peninsula Boothia Felix, Iceland, the Kara Sea, northern Siberia, 

 Bering Strait, Hudson Bay, Labrador, and Greenland. Northern 

 lights have been seen as far down as the twenty-fifth degree of 

 north latitude. In full splendor, however, they may be seen only 

 in the northern polar regions to the seventy-fifth degree of lati- 

 tude. Here Nature is displayed in all her grandeur. When the 

 sun has set, and the gray veil of twilight is cast over the earth, 

 the northern horizon grows darker and darker. Soon there may 

 be distinguished a segment of the sky more somber than its back- 

 ground ; this is hemmed in by white concentric arcs of light. 



