362 AURORA BOREALIS. 



and all gaseous bodies owe their volume and 

 elastic force to the agency of an igneous aether 

 which surrounds their atoms and that after 

 being greatly accumulated in aqueous vapour, 

 it is given out in the form of lightning on the 

 meeting of warm and cold currents of air. We 

 have also seen, that the great aerial ocean is in a 

 state of perpetual circulation, from the poles to 

 the equator by under currents ; and from the 

 equator to the poles by upper currents ; and that 

 the density of the atmosphere increases as we 

 advance from the tropics, to the points of lowest 

 mean temperature : from which it follows, that ca- 

 loric must be evolved by the tropical atmosphere 

 as it passes from a rarer to a denser state. It is 

 equally certain, that the tropical air must carry 

 with it a considerable proportion of aqueous va- 

 pour, that must be condensed into fogs or clouds, 

 rain or snow ; which accounts for a very common 

 attendant of the aurora, viz. a low black cloud, 

 apparently beyond it, and near to the horizon. 



M. Hansteen, who has travelled much in the 

 north of Europe and Asia, observes, " that while 

 the aurorae are streaming, the sky becomes 

 opaque or misty." He further states, that " it is 

 a matter of common observation in the arctic 

 regions, confirmed by long experience, that the 

 aurora is usually followed by intense cold, espe- 

 cially after a mild day ;" doubtless, for the same 

 reason, that warm and sultry weather in the 



