AURORA BOREALIS.. 201 



is not found to be changed during the most intense Aurora ; 

 but, on the other hand, the three expressions of the poAver of 

 terrestrial magnetism, decHnation, inchnation, and intensity, 

 are all affected by polar light, so that in the same night, and 

 at different periods of the magnetic development, the same 

 end of the needle is both attracted and repelled. The asser 

 tion made by Parry, on the strength of the data yielded by 

 his observations in the neighborhood of the magnetic pole at 

 Melville Island, that the Aurora did not disturb, but rathei 

 exercised a calming influence on the magnetic needle, has been 

 satisfactorily refuted by Parry's own more exact researches,* 

 detailed in his journal, and by the admirable observations of 

 Richardson, Hood, and Franklin in Northern Canada, and 

 lastly by Bravais and Lottin in Lapland. The process of the 

 Aurora is, as has already been observed, the restoration of a 

 disturbed condition of equilibrium. The effect on the needle 

 is different according to the degree of intensity of the explo- 

 sion. It was only unappreciable at the gloomy winter station 

 of Bosekop when the phenomenon of Hght was very faint and 

 low in the horizon. The shooting cylinders of rays have been 

 aptly compared to the flame which rises in the closed circuit 

 of a voltaic pile between two points of carbon at a considera- 

 ble distance apart, or, according to Fizeau, to the flame rising 

 between a silver and a carbon point, and attracted or repelled 

 by the magnet. This analogy certainly sets aside the neces- 

 sity of assuming the existence of metallic vapors in the atmos- 

 phere, which some celebrated physicists have regarded as the 

 substratum of the northern light. 



When we apply the indefinite term polar light to the lumin- 

 ous phenomenon which we ascribe to a galvanic current, that 

 is to say, to the motion of electricity in a closed circuit, we 

 merely indicate the local direction in which the evolution of 

 light is most freqilently, although by no means invariably, 

 seen. This phenomenon derives the greater part of its im- 

 portance from the fact that the Earth becomes self-luminous, 

 and that as a planet, besides the light which it receives from 

 the central body, the Sun, it shows itself capable in itself of 

 developing light. The intensity of the terrestrial light, or, 

 rather, the luminosity which is diffused, exceeds, in cases of 

 the brightest colored radiation toward the zenith, the light 

 of the Moon in its first quarter. Occasionally, as on the 7th 

 of January, 1831, printed characters could be read without 

 difficulty. This almost uninterrupted development of light 



* Kamtz, Lehrbuch der Metecrologie, bd. iii., s. 498 und 501. 



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