1880.] On the Height of the Aurora Borealis. 



333 



have this pressure (neglecting change of temperature), and therefore 

 the display at this elevation would be of maximum brilliancy and 

 would be visible at a distance of 585 miles. 



• The greatest exhaust that we have produced, 0*000055 millim., 

 0*066 M, corresponds to a height of 81 "47 miles, and as 11,000 cells 

 failed to produce a discharge in hydrogen at this low pressure, it may 

 be assumed that at this height the discharge would be considerably less 

 brilliant, especially in air, than that at 37*67 miles, the height of maxi- 

 mum brilliancy. 



At a height of 124*15 miles the pressure would be only 0*00000001 

 millim., 0*00001 M, and it is scarcely probable that an electric dis- 

 charge would occur with any potential conceivable at such a height. 



The colour of the discharge varies greatly with the tenuity of air or 

 other gas with the same potential. Thus in air at a pressure of 62 

 millim., 81579 M, the discharge has the carmine tint which is so fre- 

 quently observed in the display of the Aurora; this corresponds to an 

 altitude 12*4 miles, and would be visible at a distance 336 miles. At 

 a pressure of 1*5 millims., 1974 M, corresponding to a height of 30*86 

 miles, the discharge becomes salmon coloured, having completely lost 

 the carmine tint. At a pressure of 0*8 millim., corresponding to 

 33*96 miles, the tint of the discharge is of a paler salmon colour, and 

 as the exhaust is carried further it becomes a pale milky white. The 

 roseate and salmon coloured tints are always in the vicinity of the 



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