1881.] 



011 ihe Phenomena of the Electric Discharrie. 



475 



Our experiments * enable us to throw sonic light on another 

 atmospheric electrical ])licnonienon — namely, the })robable height of 

 the aurora borealis, which the accompanying figure (Fig. 12) of a 

 discharge roughly resembles. I will now jiass the current of the 

 whole 14, -100 cells through tlie largo tube 

 199, containing a residual charge of atmo- 

 spheric air at a pressure of 1 millimetre, 

 and you will perceive a carmine luminosity 

 touching the positive pole and reaching half- 

 way down the tube. This reminds one of 

 those ruddy glows frequently seen in auroral 

 displays. Fig. 12 in the plate is copied 

 from a photograph since taken in my labo- 

 ratory of this appearance. Around the 

 bright luminosity is a dark band which 

 shuts oflf a portion of the fluorescence of the 

 glass tube, a blue fluorescence produced by 

 the ruddy light of the luminosity, showing 

 that around the luminosity there is an ab- 

 sorbent zone of less elevated temperature. 

 Many estimates have been made from time 

 to time of the height of aurorae, founded 

 upon observations made by persons at a 

 distance from each other, and supposed to be 

 observing the same feature in the display ; 

 but it must be remarked that there is always 

 much uncertainty in these estimates, from 

 the difl&culty of knowing whether the dif- 

 ferent observers have noticed the self-same 

 streamer. Frequently very considerable altitudes have been assigned 

 to these displays ; for example, as much as 281 miles. We shall 

 presently see that it is very improbable that any electrical discharge 

 could occur at such a height. We have calculated from experiment 

 that the pressure of least resistance for air is 0*397 millimetre, 

 498*6 millionths, and therefore in air it results that a maximum 

 electric discharge, and consequent brilliancy, of the aurora, would 

 occur at an elevation where the atmosphere has that pressure — 

 namely, 37 * 67 miles. The greatest exhaust we have produced — and 

 this has not been surpassed — is 0*000055 millimetre, 0*07 millionth, 

 which is the pressure the atmosphere would have at 81*47 miles; 

 and as 11,000 cells failed to produce a discharge even in hydrogen at 

 this low pressure, it may be assumed that at this height the discharge 

 would be considerably less brilliant than at 37*67 miles, should such 

 occur.f 



At a height of 281 miles the atmosphere would only 



♦ 'Roy. Soc. Proc' No. 203. 1880. 



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

 few thousand feet. 



