METEOROLOGY. 



433 



take place in air. We may therefore assume that the pressure of least 

 resistance for air is (O.C42xl3)-^22 = 0.379 millim., 498.C M. 



"At a height of 37.67 miles above the sea-level, the atmosphere would 

 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 millimeters; 

 0.066 M, corresponds to a height of 81.47 miles, and as 11,000 cells failed to 

 l)roduce 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 maximum bril- 

 liancy. 



"At a height of 124.15 miles the pressure would be only 0.00000001 

 millimeter ; and it is scarcely probable that an electric discharge would 

 occur with any potential conceivable at such a height. 



"The color 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 

 millimeters, 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 of 12.4 miles, and would be visible at a distance of 330 miles. 

 At a pressure of 1.5 millimeters, 1974 M, corresponding to a height of 

 30.86 miles, the discharge becomes salmon-colored, having completely 

 lost the carmine tint. At a pressure of 0.8 millimeter, corresponding 

 to 33.96 miles, the tint of the discharge is of a paler salmon-color, and 

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

 roseate and salmon-colored tints are always in the vicinity of the positive 

 source of the electric current; the positive luminosity fades away grad- 

 ually, and frequently becomes almost invisible at some distance from 

 its source. 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 neg- 

 ative source. 



"The following table, with the exception of pressure 0.00000001 milli- 

 meter, exhibits deductions from actual observations: 



" It is conceivable that the aurora may occur at times at an altitude of 

 a few thousand feet." {Nature, 1880, xxii, pp. 33, 34.) 

 H. Mis. 26 28 



