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 sulphurous fumes. Descartes and Triewald saw in them only a reflection 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) direction 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 northern 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 latitude. 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 background; this is hemmed in by white concentric arcs of light.
