23G ELECTRICITY OF THE ATMOSPHERE AND AURORA BOREALIS.^ 



spark, is gradually transformed into a luminous current, exhibiting the 

 ordinary- colors observed in polar light. 



The more the relative degree of the humidity of the air increases an 

 augmentation, which at the surface of the earth proceeds from the 

 equator to the poles, the more easily the electric discharge acts under 

 the form of polar light. But there may be a limit beyond whicLi the 

 humidity is so great that the discharge takes place without the accom- 

 paniment of light. Such would seem to be indicated hy the table prepared 

 by M. Loomis, of the geographical extension of polar light ; according to 

 which there must be a zone, comprised between the 08th and 7Gth de- 

 gree of latitude in Europe, and between the 50th and 64th in America, 

 in which the greatest number of phenomena of polar light are produced. 

 This very interesting peculiarity has been proved by observations made 

 during our expedition ; at Spitzbergen the polar light always appeared 

 to the south, while at a lower latitude, the G9th degree, it appeared at 

 the zenith or to the north. 



I now return to the question of spectral analysis ; and for the moment 

 notice only the mobility, so marked, of the light of the yellow ray, 

 which seems to indicate a discontinuous luminous source which is evi- 

 dently formed by an infinite number of sparks, succeeding each other 

 in rapid succession. 



As to the crackling or rattling noise which accompanies the appear- 

 ance of polar light, I cannot say anything positively, since, on the oc- 

 casions when I made my observations, the combined noise of the sea 

 and the wind was of such a nature as to drown the faint crepitation of 

 an electric discharge. It is very jirobable that such a noise can be heard 

 under certain circumstances, for instance when the discharge takes place 

 at a minimum height, and also when it is made between small particles 

 of ice, which produce longer, and consequently stronger, sparks than 

 those formed between i^articles of water. As these circumstances which 

 arfe necessary for the production of this sound rarely occur, we can un- 

 derstand why observers do not agree in regard to the fact of its exist- 

 ence. 



Some remarks upon the memoir of M. Lemstrom, hy Professor De la Rive. 



I find in the observations made by M. Lemstrom, in the polar regions, 

 such a complete confirmation of the views I have expressed on several 

 occasions in regard to the cause and explanation of polar auroras, thai 

 I cannot refrain from noticing very briefly some of the points upon which 

 observation and theory completely agree. I have generally found myself 

 in accord with observers, whether such as Parry, Franklin and Ross, or 

 Bravais and Martins ; it is rather between the theorists and myself that 

 there has been occasionally some difference. I ought to thank M. Lem- 

 strom for the pains he has taken, on every occasion, to mention my ex- 

 periments, and the consequences I have deduced from them, the accu- 

 racy of which he has confirmed. 



