128 COSMOS. 



traordinary disturbance in its direction, which was frequent- 

 ly connected with a vibratory, trembling, and fluctuating 

 motion. It became customary to ascribe this phenomenon 

 to some special condition of the needle itself, and this was 

 characteristically designated by French sailors Vaffolement de 

 raiguille, and it was recommended that une aiguille affolee 

 should be again more strongly magnetized. Halley was cer- 

 tainly the first who inferred that polar light was a magnetic 

 phenomenon — a statement* which he made on the occasion 

 of his being invited by the Royal Society of London to ex- 

 plain the great meteor of the 6th of March, 1716, which was 

 seen in every part of England. He says " that the^ meteor 

 is analogous with the phenomenon which Gassendi first des- 

 ignated in 1621 by the name o^ Aurora Borealis'^ Although, 

 in his voyages for the determination of the line of variation, 

 he advanced as far south as 52°, yet we learn, from his own 

 confession, that he had never seen a northern or southern 

 polar light before the year 1716, although the latter, as I 

 can testify, is visible in the middle of the tropical zone of 

 Peru. Halley, therefore, does not appear, fromihis own ob- 

 servation, to have been aware of the restlessness of the nee- 

 dle, or of the extraordinary distiirbimces and fluctuations 

 which it exhibits at tiie periods of visible or invisible north- 



* Halley, Accozmt of the late stirprising A'ppearance of-Lightsin the 

 Air, in the Phil. Transact., vol. xxix., 1714-1716, No. 347, p. 422- 

 428. Halley's explanation of the Aurora Borealis is unfortunately 

 connected with the fantastic hypothesis which had been enounced by 

 him twenty-five years earlier, in the Phil. Transact, for 1693, vol. 

 xvii., No. 195, p. 563, according to which there was a luminous fluid 

 in the hollow terrestrial sphere lying between the outer shell which we 

 inhabit and the inner denser nucleus, Avhich is also inhabited by hu- 

 man beings. These are his words: "In order fo make that inner 

 globe capable of being inhabited, there might not improbably be con- 

 tained some luminous medium between the balls, so as to make a per- 

 petual day below." Since the outer shell of the earth's crust is far 

 less thick in the region of the poles of rotation (owing to the compres- 

 sion produced at those parts) than at the equator, the inner luminous 

 fluid (that is, the magnetic fluid), seeks at certain periods, more espe- 

 cially at the times of the equinoxes, to find itself a passage in the less 

 thick polar regions through the fissures of rocks. The emanation of 

 this fluid is, according to Falley, the phenomenon of the northern 

 light. When iron filings are strewn over a spheroidal magnet (a te- 

 rella), they serve to show the direction of the luminous colored rays of 

 the Aurora. "As each one sees his own rainbow, so also the corona 

 appears to every observer to be at a different point" (p. 424). Regard- 

 ing the geognostic dreams of an intellectual investigator, who display- 

 ed |uch profound knowledge in all his magnetic and astronomical la- 

 bors, see Cosmos, vol. i., p. 171. 



