CHAPTER XXVIII. 



AUEOEA BOKEALIS. 



Visible at both poles. Mairan on the extent of the Sun's atmosphere. 

 Lardner on Auroras. M. Biot on Polar Volcanoes. Distance of 

 Auroras. Seen by Aeronauts below them. Facts. Caused by 

 mineral emanations from the Polar Latitudes. How they affect th 

 compasses. Why seen on Calm evenings. Dew. Cause of colour*. 

 Similarity between Auroras and Meteors. 



AURORAS are visible in the direction of both the Northern 

 and Southern poles various explanations have been given of 

 them, but they are all open to objection. 



A curious one is given by Mairan, who supposed the aurora 

 to proceed from the intermixture of the far extending atmos- 

 phere of the sun with that of the earth. While -we do not 

 altogether coincide with it, we are glad to get an incidental con- 

 firmation of our theory of tides and atmospheres from him. 



Lardner says : " Although the complete explanation of the 

 aurora has not been accomplished, the electricity and magnetism 

 of the earth and its atmosphere must now be regarded as its 

 source." This is partly correct, but as he does not apprehend 

 the properties or mode of action of either, of course he could 

 not understand the theory in its entirety. 



M. Biot supposed that metallic clouds thrown out of polar 

 volcanoes were the cause of auroras, and this accounted for 



