334 



On the Height of the Aurora Borealis. [Apr. 22, 



positive source of the electric current, the positive luminosity fades 

 away gradually, and frequently becomes almost invisible at some 

 distance from its source ; as, for instance, in the hydrogen discharge 

 at a pressure of 2*3 millims., 3027 M, shown in the accompanying- 

 figure, H, which resembles in some respects the phenomena of the 

 Aurora. The discharge at the negative terminal in air is always of a 

 violet hue, and this tint in the Aurora indicates a proximity to the 

 negative source. 



The following table, with the exception of pressure O'OOOOOOOl 

 millim., exhibits deductions from actual observations. 



Pressure 

 mm. 



Pressure M- 



Height 

 miles. 



Visible 

 at miles. 



Remarks. 



o-oooooooi 



o -ooooi 



124 -15 



1061 



No discharge could occur. 



-000055 



0-066 



81 -47 



860 



Pale and faint. 



0-379 



499 -0 



37 '67 



585 



Maximum brilliancy. 



0-800 



1053 -0 



33-96 



555 



Pale salmon. 



1-000 



1316 -0 



32 -87 



546 



Salmon coloured. 



1-500 



1974 -0 



30-86 



529 



Carmine. 



3 -000 



3947 -0 



27-42 



499 



20-660 



27184 -0 



17-86 



403 





62-000 



81579 -0 



12 -42 



336 





118 -700 



156184 -0 



11 -58 



324 



Full red. 



It is conceivable that the Aurora may occur at times at an altitude 

 of a few thousand feet. 



