ATMOSPHERIC ELECTRICITY. 105 



influence the phenomena we have discussed, and it seems probable that 

 we should have some evidence derived from observation if the aurora 

 was always accompanied by discharges through the earth's surface. 

 Except in the polar regions, these aurorse do not seem to affect the 

 normal fall of potential. There is a third view we may take as to the 

 circulation of electric currents indicated by the aurora — the return cur- 

 rent may take place in space outside the earth's atmosphere. A good 

 deal might be said in favor of this view, and the rotation of the earth's 

 magnetic field in space might be a sufficient cause for the production 

 of these currents; but this is not the place to enter further into this 

 question. 



Calculations made from observation on the height of the aurora have 

 generally resulted in an altitude of from 100 to 200 miles, except in the 

 polar regions, where the aurora seems occasionally to descend to a 

 much lower level. It has also been noticed that aurorae are associated 

 with certain bands of cirrus clouds, and this seems to indicate that 

 although the luminous phenomena is sufficiently intense to be observed 

 at only great heights, yet the electric phenomena may descend to the 

 level of the cirrus. 



As regards the connection between the aurora and the sun-spot 

 period, further observations in the polar regions are needed. On the 

 one hand, we have Paulsen's 1 statement, derived from observations in 

 Greenland, to the effect that the greatest number of auroroe are seen 

 when sun spots are at their minimum — that is, at a time when in our 

 own latitudes the number is smallest; and, on the other hand, we have 

 Nordenskiold's observations, which seem to point in the opposite direc- 

 tion. In a publication which contains much important matter on the 

 geographical distribution and form of the aurora borealis, Nordenskiiild 

 contrasts the appearances he has observed in the Vega during the 

 winter of 1878-79, passed in the Bering Straits, with that previously 

 observed in 1872-73 to the north of Spitzbergen. According to this 

 author, the aurorre, during the minimum sun-spot period in 1878-79, 

 were "hardly worthy of his notice by the side of those observed in 

 1872-73." But although only faintly luminous, the aurora? of 1879 were 

 persistent and regular in shape. They did not affect the magnetic field, 

 and seemed to show a regular and continuous, though weak, electric 

 discharge. The arc and streamers in 1872 were much more brilliant 

 and much more irregular. Some objection may be raised against these 

 observations, in so far as they refer to different places, and local cir- 

 cumstances may have affected the phenomenon; but, in the face of the 

 very careful description he gives us, we can not as yet accept Paulsen's 

 results without further confirmation. 



The problem of atmospheric electricity, like that of terrestrial mag- 

 netism, presents special features in the arctic regions, and until we 



Paulsen, "Danske Videnskab. Selskabs Forhand./' 1889. (I have not seen the 

 original memoir, but only an abstract in the Jabrbiich, der Astronomie und 

 Geophysik, 1890.) 



