742 THE AURORA BOREALIS. 



discharged in the form of Aurora, and not in thunder and light- 

 ning, is the permanent moisture of the air. Hygrometric 

 observations made during the Expedition of the " Sophie " show 

 that the air is constantly saturated with aqueous vapours, which 

 are condensed most frequently in clouds, more rarely in rain. It 

 is clear that this layer of moisture, a good conductor of electricity, 

 causes a slow discharge. 



M. de la Rive adds (p. 166) : — M. Lemstrom established by a 

 great number of facts that the Aurora is due to atmospheric 

 electricity, the presence of which in the Polar regions he has 

 proved, often in the region of the clouds, and sometimes nearer 

 the earth. He shows that this light is the consequence of electric 

 discharges, which in these regions, constantly charged with 

 moisture, operate slowly and continuously, instead of suddenly and 

 producing lightning as in the equatorial regions and mean latitudes. 

 He shows with reason that terrestrial magnetism, to which we had 

 attributed an exaggerated importance in the production of Aurora, 

 plays in this phenomena only a very secondary part. This consists 

 simply in giving to the luminous electric streamers a certain 

 direction which they are capable of taking as they are propagated 

 in a gaseous conductor. 



The electric discharges which take place in the Polar regions 

 between the positive electricity of the atmosphere and the 

 negative electricity of the earth, are the essential and unique cause 

 of the formation of the polar light — light the existence of which is 

 independent of terrestrial magnetism, which contributes only to 

 give to the polar light a certain direction, and in some cases to 

 give it motion. This is^what I have always maintained, contrary 

 to those who believe they see in terrestrial magnetism, or rather in 

 the induction currents which it is capable of developing, the origin 

 of the polar light. 



21. Observations on the Spectrum of the Aurora made by 



M. AuGUSTE WijKANDER and Lieutenant Parent, of the 



Swedish Arctic Expedition in 1872-73, under Professor 



Nordenskiold. — (Archives des Sciences Physiques et Natu- 



relles, vol. li., p. 25.) 



The Swedish Arctic Expedition under Professor Nordenskiold 



have made some valuable observations on the spectrum of the 



Aurora Borealis in Mussel Bay, on the north of Spitzbergen, 



during the winter of 1872-73. 



The instrument employed was a direct-vision spectroscope, con- 

 structed by Baron Wrede, and similar to that used by M. Lem- 

 strom in his observations on the spectrum of the Aurora. The 

 instrument consisted of two prisms, one of which sei'ves to refract 

 and disperse the luminous rays, whilst the other brings them back 

 to the first direction by total reflection. The prisms are so far 

 capable of motion, that the different parts of the spectrum can be 

 brought on the system of crossed spider lines placed at the focus 

 of the eye-piece. By a side opening and a mirror opposite to it 

 the spider lines could be uniformly illuminated when necessary ; 



