AURORA BO REAMS. 185 



between the sun and a rainbow can see the latter. A rainbow 

 cannot be seen if it exists between the point of observation and 

 the sun. 



Why is it only on calm and clear evenings that auroras aw 

 seen 7 



Because like dew, they aro formed in consequence of a 

 reciprocal action between the lower and upper atmospheres ; a 

 process which can only exhibit its action as the aurora, when 

 the atmospheres are otherwise at rest, and when there are no 

 winds to dissipate the gases, or divert them from the natural 

 position necessary to form aurora ; otherwise they form clouds, 

 etc. 



The sensation of cold is caused by the quantity of mineral 

 gases mingling in the atmosphere. 



The colours of the aurora are produced by the variety, and 

 the different combinations, of the gases mingling ; the vegetable 

 gases causing the red, yellow and warm colours, and the mineral 

 gases showing the blue and cold colours. 



