It has seen a subject of some dispute whether any sound accom panies the development of the terrestrial light. The Greenland sledgers and Siberian fox-hunters are positive in their assertions that there la ; and intelligent observers engaged in the Arctic magnetical and meteorological expeditions have recorded their evidence In the affirmative. On the other hand, Parry, Franklin, Richardson, and others state to having witnessed thousand. of aurora without perceiving any attendant lesenda.
According to Dalton, the southern lights have been frequently seen In England ; and the northern lights were seen in 45 S. lat. on January 14, 1831. I lumboldt remarks, however, that "It is necessary to distinguish between the sphere of a simultaneous apparition of the phenomenon and the zone of the earth in which the phenomenon is I displayed almost every night of the year. As each observer sees his own rainbow, so also, doubtless, does he see his own polar light. A great portion of the earth engenders the radiating light-phenomenon at the same time." The intensity of the terrestrial light diminishes with the decrease of latitude, or, more strictly, with the decrease of magnetic latitude. In Iceland, Greenland, and on the banks of the Slave Lake, it appears at certain seasons every night. In Italy it is rare. On the shores of Siberia there appear to be "special regions of the northern lights." There is great difficulty in determining the altitude of the terrestrial light, in consequence of the incessant oscillations of the luminous rays; so that the results of different observations "vary between several miles and three or four thousand feet." Moreover, it is probable that its altitude differs at different times.
Our observations and knowledge of the terrestrial light leads us to the important and interesting conclusion that our earth itself is luminous ; and It is supposed that the degree of luminosity is is little greater than that of the moon in her first quarter. These considerations, compared with the observed fact that the planet Venus " glows occasionally with a proper phosphorescent gleans in those parts which are not illumi nated by the sun, lead us to inquire whether the moon and planets may not likewise be magnetic, thus keeping up a mutual influence between themselves and our earth.
Why the phenomenon of the terrestrial light should confine itself especially to the polar regions is readily explained on the supposi tion that it is a luminous discharge of superabundant magnetism ; and since heat is destructive of magnetism, the magnetic intensity will be greater in the colder regions of the earth. [TERRESTRIAL Illeoxrrtsist.] An illustration of this may be given by taking a bar magnet and dipping it in iron filings, when the filings will be found to arrange themselves near the poles of the magnet, presenting the appear ance as shown in the following figures.
Whether the magnetic condition of our earth has undergone any recent change we cannot say ; but it is remarkable that we can find no very early accounts of the phenomenon we have been describing. It certainly appears to have been less frequent before the 18th century than it has been since. Torfceus, who wrote in 1706, relates that even in his own time the aurora was an object of terror in Iceland. For some time it was thought that there was no aurora australis, or terrestrial lights in the south ; and though the fact is now well established, yet the earliest account Is e have of such an occurrence is given by Don Antonio Ulloa, who saw it at Cape Horn in 1745.
From the writings of Aristotle, Cicero, Pliny, and others, we read of appearances in the heavens which we conclude to have been aurorae, and which were viewed with the same superstition as comets.
For further information on this subject, the reader is referred to the writings of M. de Mairau (1754) ; DaltonHumboldt's Cosmos ; ' Halley; Forbes ; Expeditions of Parry, Franklin, Richardson, and Henderson ; Kaemtz's Complete Course of Meteorology' (Loud., 1845); Reports of the British Association for the Advancement of Science ; and the Philosophical Transactions of the Royal Society.