262 On the Aurora Borealis. 



M. Becquerel objects to this theory that the existence of 

 metal, in that uncombined form, in which alone it has the con- 

 ducting power — in volcanic eruptions — is not yet proved. In 

 explanation of which objection, it should be added that M. 

 Biot's theory supposed the electricity to proceed from polar 

 volcanoes. 



Professor Faraday, in vol i. of his Researches, remarks : — 

 " I hardly dare venture, even in the most hypothetical form, to 

 ask whether the Aurora Borealis and Australis may not be the 

 discharge of electricity thus urged towards the poles of the earth, 

 from whence it is endeavouring to return by natural and ap- 

 pointed means above the earth to equatorial regions." 



Humboldt says : — " The Aurora Borealis has not been de- 

 scribed merely as an external cause of a disturbance in the 

 equilibrium of the distribution of terrestrial magnetism, but 

 rather as an increased manifestation of telluric activity, amount- 

 ing even to a luminous phenomenon, exhibited on the one 

 hand by the restless oscillation of the needle, and on the other, 

 by the polar luminosity of the heavens. The polar light appears, 

 in accordance with this view, to be a kind of silent discharge 

 or shock, at the termination of a magnetic storm, the disturbed 

 equilibrium of the electricity is renewed by a development of 

 light by lightning, accompanied by pealing thunder." 



M. De La Bive, after speaking of the two electricities of the 

 earth and atmosphere, and the recomposition going on between 

 them, and stating that the great electrical discharge takes place 

 at the poles, proceeds : — " This discharge, when it has a cer- 

 tain degree of intensity, will be luminous, especially if, as is 

 nearly always the case near the poles, and in the higher regions 

 of the atmosphere, it meet on its way those extremely attenu- 

 ated frozen particles out of which the loftier clouds and mists 

 are formed." More lately still he expresses similar and more 

 elaborate views. (See abstract in the Intellectual Observer 

 for August.) 



In the Arctic seas there is always more or less evaporation 

 from the surface of the exposed water, and according to the 

 time of year the area of exposed sea surface will be great or 

 small. Towards the end of August and beginning of Septem- 

 ber, as the sun's altitude decreases, the nights become gradu- 

 ally colder, the surface of the sea is frozen over, and the differ- 

 ence between the temperature of the air and water increases. 

 [For my purpose I will speak of the sea of Baffin's Bay and 

 Davis's Strait.] With the advance of the season, the evapora- 

 tion, which in summer appears as fog, in winter takes a different 

 form ; for wherever a space of water appears, and the tempera- 

 ture of the air is colder than that of the water, the vapour of 

 the water, in rising from its surface, becomes visible as a dense 



