/ELECTRICITY OF THE ATMOSPHERE AND AURORA BOREALIS. 237 



M. Lemstrom proves by a great number of facts, supported by incon- 

 testable reasoning, that polar light is due to atmospheric electricity, of 

 which he has proved the presence in the polar regions, often in the 

 region of clouds, and sometimes even nearer the earth. He shows, as 

 I have done, that this light is the consequence of electric discharges, 

 which in these regions, constantly pervaded with humidity, operate ia a 

 slow, continuous manner, instead of instantaneously by shocks producing 

 lightning, as takes place in equatorial regions and middle latitudes. 



He shows, with truth, that terrestrial magnetism, to which an exag- 

 gerated importance has been attributed, in the production of xjolar light, 

 has only a very secondary part in this phenomenon. This part consists 

 simply in giving to the luminous electric jets a certain direction they can 

 follow because of their flexibility, which depends upon whether the 

 medium through which they pass is gaseous. In sup[)ort of his views in 

 this respect, he refers to my experiments, by which I have demonstrated 

 this influence, and the law by which, according to Plucker, it is governed. 



One very essential point upon which M. Lemstrom insists, and which 

 has been noticed by several observers, particularly by Bravais, is that 

 the crown formed in some cases by the rays of polar light is very far 

 from having always for center the magnetic zenith; that is to say, the 

 vertical line passing through the magnetic pole of the earth. In fact, 

 although the formation of this crown depends upon the directing influ- 

 ence of the magnetism upon the electric currents which form the lumin- 

 ous jets, and is not, as M. Lemstrom very well proves, a simple effect 

 of perspective, it must also depend upon the direction of the passage 

 of the electric discharges through the atmosphere, a direction which 

 itself changes with the conductibility more or less variable of the differ- 

 ent atmospheric strata, so that the united effect of these two influences 

 ought to give to the rays a curvature and a position which cannot always 

 be the same. 



In short, the electric discharges which take place in the polar regions 

 between the positive electricity of the atmosphere and the negative 

 electricity of the terrestrial globe are the essential and only causes of 

 the formation of polar light — light, whose existence is independent of 

 that of terrestrial magnetism, which only imparts to it a certain direc- 

 tion, and in some cases a movement. These views I have always main- 

 tained in opposition to those who think they find in terrestrial magnet- 

 ism, or rather, in the currents of induction which it can develop, the 

 origin of polar light. 



I will not dwell upon various interesting circumstances, such as the 

 presence of a dark segment at the base of the luminous arcs of the 

 aurora borealis, in which M. Lemstrom sees, as I do, an analogous effect 

 to the dark band produced at the negative-electrode, in electric dis- 

 charges through rarifled air ; or such as the iufluence of particles of ice 

 suspended in the atmosphere, which I have also noticed. I will confine 

 myself to one point which, I confess, had completely escaped me, and 



