[footnote] **The remarkable connection between the curvature of the magnetic lines and that of my isothermal lines was first detected by Sir David Brewster. See the 'Transactions of the Royal Society of Edinburgh', vol. ix., 1821, p. 318, and 'Treatise on Magnetism', 1837, p. 42, 44, 47, and 268. This distinguished physicist admist two cold poles (poles of maximum cold) in the northern hemisphere, an American one near Cape Walker (73 degrees lat., 100 degrees W. long.), and an Asiatic one (73 degrees lat., 80 degrees E. long.); whence arise, according to him, two hot and two cold meridians, i.e., meridians of greatest heat and cold. Even in the sixteenth century, Acosts ('Historia Natural de las Indias', 1589, lib. i., cap. 17), grounding his opinion on the observations of a very experienced Portuguese pilot, taught that there were four lines without declination. It would seem from the controversy of Henry Bond (the author of 'The Longitude Found', 1676) with Beckborrow, that this view in some measure influenced Halley in his theory of four magnetic poles. See my 'Examen Critique de l'Hist. de la Geographie', t. iii., p. 60.
The bold conjecture hazarded one hundred and twenty-eight years since by Halley,* that the Aurora Borealis was a magnetic phenomenon, has acquired empirical certainty from Faraday's brilliant discovery of the evolution of light by magnetic forces.
[footnote] *Halley, in the 'Philosophical Transactions', vol. xxix. (for 1714-1716), No. 341.
The northern light is preceded by premonitory signs. Thus, in the morning before the occurrence of the phenomenon, the irregular horary course of the magnetic needle generally indicates a disturbance of the equilibrium in the distribution of p 194 terrestrial magnetism.*
[footnote] *[The Aurora Borealis of October 24th, 1847, which was one of the most brilliant ever known in this country, was preceded by great magnetic disturbance. On the 22d of October the maximum of the west declination was 23 degrees 10'; on the 23d the position of the magnet was continually changing, and the extreme west declinations were between 22 degrees 44' and 23 degrees 37';on the night between the 23d and 24th of October, the changes of position were very large and very frequent, the magnet at times moving across the field so rapidly that a difficulty was experienced in following it. During the day of the 24th of October there was a constant change of position, but after midnight, when the Aurora began perceptibly to decline in brightness, the disturbance entirely ceased. The changes of position of the horizontal-force magnet were as large and as frequent as those of the declination magnet, but the vertical-force magnet was at no time so much affected as the other two instruments. See 'On the Aurora Borealis, as it was seen on Sunday evening, October 24th, 1847, at Blackheath,' by James Glaisher, Esq., of the Royal Observatory, Greenwich, in the 'London, Edinburgh, and Dublin Philos. Mag and Journal of Science for Nov.', 1847, by John H. Morgan, Esq. We must not omit to mention that magnetic disturbance is now registered by a 'photographic' process: the self-registering photographic apparatus used for this purpose in the Observatory at Greenwich was designed by Mr. Brooke, and another ingenious instrument of this kind has been invented by Mr. F. Ronalds, of the Richmond Observatory.] — Tr.
When this disturbance attains a great degree of intensity, the equilibrium of the distribution is restored by a discharge attended by a development of light "The Aurora* itself is, therefore, not to be regarded as an externally manifested cause of this disturbance, but rather as a result of telluric activity, manifested on the one side by the appearance of the light, and on the other by the vibrations of the magnetic needle."
[footnote] *Dove, in Poggend., 'Annalen', bd. xx., s. 341; bd. xix., s. 388. "The declination needle acts in very nearly the same way as an atmospheric electrometer, whose divergence in like manner shows the increased tension of the electricity before this has become so great as to yield a spark." See also, the excellent observations of Professor Käwmtz, in his 'Lehrbuch der Meteorologie', bd. iii., s. 511-519, and Sir David Brewster, in his 'Treatise on Magnetism', p. 280. Regarding the magnetic properties of the galvanic flame, or luminous arch from a Bunsen's carbon and zinc battery, see Casselmann's 'Beobachtungen' (Marburg, 1844), s. 56-62.
The splendid appearance of colored polar light is the act of discharge, the termination of a magnetic storm, as in an electrical storm a development of light — the flash of lightning — indicates the restoration of the disturbed equilibrium in the distribution of the electricity. An electric storm is generally confined to a small space beyond the limits of which the condition of the atmospheric electricity remains unchanged. A magnetic storm, on the other hand, p 193 shows its influence on the course of the needle over large portions of continents, and, as Arago first discovered far from the spot where the evolution of light was visible. It is not improbable that, as heavily-charged threatening clouds, owing to frequent transitions of the atmospheric electricity to an opposite condition, are not always discharged, accompanied by lightning, so likewise magnetic storms may occasion far-extending disturbances in the horary course of the needle, without there being any positive necessity that the equilibrium of the distribution should be restored by explosion, or by the passage of luminous effusions from one of the poles to the equator, or from pole to pole.
In collecting all the individual features of the phenomenon in one general picture, we must not omit to describe the origin and course of a perfectly developed Aurora Borealis. Low down in the distant horizon, about the part of the heavens which is intersected by the magnetic meridian, the sky which was previously clear is at once overcast. A dense wall of bank of cloud seems to rise gradually higher and higher, until it attains an elevation of 8 or 10 degrees. The color of the dark segment passes into brown or violet; and stars are visible through the cloudy stratum, as when a dense smoke darkens the sky. A broad, brightly-luminous arch, first white, then yellow, encircles the dark segment; but as the brilliant arch appears subsequently to the smoky gray segment, we can not agree with Argelander in ascribing the latter to the effect of mere contrast with the bright luminous margin.*
[footnote] *Argelander, in the important observations on the northern light embodied in the 'Vorträgen gehalten in der physikalish-okonomischen Gessellschaft zu Konigsberg', bd. i., 1834, s. 257-264.