Peocejsdixgs of the Polytechnic Association. 929 



the plienoiiienon is confined to tlie regions where haze and cirrus 

 clouds are wont to form; and from the abundant evidence we have 

 of their frequent presence in regions very low down, we are relieved 

 from the necessity of supposing that the rarity of the air in the high 

 regions is indispensable to the transmission of aiu'ora, although we 

 know that the more rare the air (within certain limits) the greater 

 is the fiicility oifered by it for the transmission. 



AuEORA NOT High Up. 



In proof that Aurora may not always be high up, I quote the fol- 

 lowing : " Captain Fraiiklin saw an aurora horealis, the light of 

 which appeared to him to illuminate the lower surface of a stratum 

 of clouds ; whilst twenty-five miles further on, Mr. Kendal, who had 

 watched the whole night without losing sight of the sky, did not 

 perceive any trace of light." 



" Captain Parry saw an aurora horealis display itself against the 

 side of a mountain." 



" Lieut. Hood and Dr. Eichardson, being placed at a distance of 

 about forty-three miles from each other in order to make simultaneous 

 observations, wlience they might deduce the parallax of the pheno- 

 menon, and, consequently, its height, were led to recognize that it 

 had not a greater elevation than fiive miles." 



Finally, " M. Liais, having had the opportunity of applying a 

 method of his to the measurement of an aurora seen at Cherbourg, 

 October 31, 1853, found that the arc of the aurora was about two 

 and one-half miles above the ground at its lower edge." 



The Koise of Aurora, 



The whizzing, or noise of crepitation, often heard by observers, in 

 high latitudes, although denied by some learned philosophers, present 

 to my mind ample evidence of the low position of many aurora. 



I quote further : 



"M. Yerder, on the night of October 13, 1819, being in the lati- 

 tude of Newfoundland, heard very distinctly a sort of crackling noise 

 or crepitation, when the building that he ascended was in the midst 

 of an auroraP 



" It is generally admitted by the inhabitants of the northern 

 regions, that when the aurora appears low, a crackling is Heard,, 

 similar to the electric spark." 



" M. Eamm, inspector of forests in iS"or\vay, wrote M. Hausteen- 



[Inst.] 59 



