POLAR-LIGHT APPARA TUS. 163 



attained, the reunion will take place as soon as this equilibrium 

 is disturbed, either by the insulating power being diminished or 

 the electricity of the conductor augmented. The first case, which 

 probably is the most ordinary, happens if a southerly wind carrying a 

 quantity of vapour attains the polar regions : for instance, the belt, 

 where the vapour, in consequence of the cold, is condensed into a fluid 

 form, reduces thereby considerably the insulating power of the air and 

 enables the electric current to flow through it. The same thing would 

 occur if a layer of clouds happened to enter into this belt ; the upper 

 end of the cloud would become negatively electric, the lower one 

 positive, and thus the distance between the two conductors would in 

 fact be diminished. The electric current would go from the conductor 

 of air to the cloud, and through this latter to the earth. Similar 

 phenomena are observed in the polar regions, or the upper edges of 

 the clouds are not unfrequently seen shining with a yellowish light 

 stretching considerably upwards, whilst no light is discernible under 

 the cloud because of the air there having attained a density sufficient 

 to prevent the current from producing light. 



For special knowledge of the polar light and its theory, we refer to 

 essays inserted in the Archives des Sciences Phys. et Natur. de 

 Geneve, 1875 (Sept. and Oct.), and in January, 1876, as well as to two 

 essays published in the years 1869 and 1873, in the same scientific 

 journal, all which articles are more or less the result of observations 

 made in the arctic regions. Besides these we may refer also to the 

 works upon polar light of the American natural philosopher Loomis, 

 Eep. of Smithsonian Inst., 1865, &c. 



The PRESIDENT : We must return our best tharks to Mr. Lem- 

 strom for his elaborate paper, and express a hope that he may some 

 day publish it, when we shall be better able to do justice to it than we 

 can to-day. 



In consequence of the lateness of the hour Baron Ferdinand de 

 Wrangell and II Commendatore Professore Blaserna will give their 

 communications in the afternoon. 



(The Conference then adjourned for luncheon.) 



The PRESIDENT : The clock having struck two, I will call on 

 Mr. De La Rue for his communication on astronomical photo- 

 graphy. 



