196 COSMOS. 



attracted and repelled. The assertion made by Parry, on the 

 strength of the data yielded by his observations in the neigh- 

 bourhood of the magnetic pole at Melville Island, that the 

 aurora did not disturb, but rather exercised a calming influ- 

 ence 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 explosion. It was only un- 

 appreciable at the gloomy winter station of Bosekop when 

 the phenomenon of light 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 considerable 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 necessity of assuming the 

 existence of metallic vapours in the atmosphere, which some 

 celebrated physicists have regarded as the substratum of the 

 northern light. 



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

 nous 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 frequently, although by no means invariably, 

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

 ance 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 coloured radiation towards 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 dif- 

 ficulty. This almost uninterrupted development of light in the 

 Earth leads us by analogy to the remarkable process exhibited 



* KEraU, Lehrbuch, der Meteorologie, bd. iii. s. 498 und 501, 



