[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.*