AURORA BOREALIS. 363 



middle latitudes is rendered cool by a thunder 

 storm ; viz. by the mixture of cold with warm 

 air. 



M. Hansteen has given a singular explanation 

 of the aurora, which shews how vague and inde- 

 finite the views of philosophers still are in regard 

 to the origin and nature of this beautiful meteor. 

 He observes, that " it is probably the result of a 

 struggle of powers put in activity by the vari- 

 ously constituted substances composing the mass 

 of the earth, which we may one day, perhaps, 

 learn to know." He thinks, that " it produces 

 the arctic fogs, by decomposing the aqueous 

 vapour of the polar atmosphere." (Edin. Phil. 

 Journal, vol. 12.) 



It has been asked, why the aurora are not 

 continually formed, as the atmosphere is con- 

 stantly flowing from the tropical to the higher 

 latitudes? We might as well be asked, why 

 there is not perpetual thunder and lightning 

 while the atmosphere is moving from the sea 

 over the land, charged with aqueous vapour. 

 The condensations which occur in both cases are 

 local and temporary, and depend on the relative 

 temperatures of different aerial masses. When 

 the upper current of rarefied air from the tropics, 

 charged with vapour, comes in contact with the 

 cold air of the polar regions, it gives out a large 

 portion of caloric or electric matter, where, not 

 being compressed as in the lower atmosphere, it 



