ELECTRICITY OF THE ATMOSPHERE AND AURORA BOREALIS. 235 



needle of inclination, and oujjTit consequently to diverge from all 

 sides ; a circumstance unfavorable to their union in perspective at the 

 magnetic zenith. But it is not proved that the radiation has always for 

 center the magnetic pole ; it may very well have another central jwint, 

 as was the case during the observation I made on the 18th of October. 

 On that occasion, in fact, the ring could not have been completely visible 

 if it had had for center the magnetic pole. Besides, this variation of posi- 

 tion is more in conformity with the manner in which the electric charges 

 operate, and with the peculiarities observed in the local appearances of 

 polar light.* 



Eeturuiug to the lights observed around the elevated peaks of the 

 Spitzbergen, I ought to say that this phenomenon has been noticed be- 

 fore. The learned philologist of Finland, Cashen, witnessed it in his 

 journeys to Siberia, and his description of it exactly accords with what 

 I have myself observed. Similar light has been seen in South America 

 above the peaks of the Cordilleras, and in several other localities men- 

 tioned by M. Delleman. The Archives des Sciences pliysiqiies et natu- 

 relles (tome xxxi, p. 15) contain an article by M. H. de Saussure, in 

 which are described a great number of phenomena, belonging, without 

 doubt, to the same category. But it is in the arctic regions, above all 

 others, that we find circumstances most favorable to observations of 

 this kind, and it is much to be desired that future expeditions will un- 

 dertake them. 



If we seek for the reason why the clouds of the upper latitudes dis- 

 charge themselves under the form of polar light, and not that of thun- 

 der and lightning, we find it in the permanent humidity of the air. The 

 hygrometric observations, made during the expedition of the Sophie, 

 show that the air is constantly saturated with aqueous vapors, which 

 condeuse frequently into clouds, more rarely into rain. It is clear that 

 this stratum of humidity, a good conductor of electricity, determines 

 a slow discharge. If between the poles of an electrical machine, not 

 sufficiently near together to produce a discharge, we project, by means 

 of an atomizer, some water in spray, we see the discharge under the 

 forai of brilliant rays. It is the same in a glass cylinder in which the 

 air has been rarified by several strokes of a pistouj a rarefaction suf- 

 ficient to produce a mist. The discharge, which at first appears as a 



*The author remarks here that the appearance of polar li^ht is always accompanied by a 

 dark segment, througrh which the stars are visible. He mentions in this connection the ex- 

 periment of M. de la Rive, who, in transmitting the discharge of an induction coil through 

 very rarefied air, proved the existence of a very remarkable dark band near the negative 

 electrode. The author cites an experiment made by himself in the presence of M. Edlund, 

 dixring which he obtained by means of an electrophorus, in a tube of very rarefied air, a 

 bluish light, followed by a dark band strongly marked around the negative electrode, and a 

 kind of crown of rays around the positive electrode* In the phenomenon of polar light, the 

 earth constitutes the negative electrode, the rarefied air of the higher regions of the atmo- 

 sphere the positive electrode, and the dark segment bears a strong resemblance to the dark 

 band of the preceding experiment. 



