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XL VIII. Researches on Unipolar Induction, Atmospheric Elec- 

 t)'ieiti/, and the Aurora Borealis, By E. Edlund, Professor 

 of Physics at the Swedish Royal Academy of Sciences. 



[Continued from p. 306.] 



§ 2. Atmospheric Electricity and the Aurora Borealis. 



IT is known that the earth may be regarded as a relatively 

 good conductor of electricity ; while, on the other hand, 

 the atmospheric air, in the dry state and under the pressure 

 to which it is subject at the surface of the earth, is a very 

 bad conductor. Its conductivity, which depends almost ex- 

 clusively upon the relative quantity of humidity which it con- 

 tains, is consequently subject to incessant variations from the 

 double point of view of time and space. When the density 

 of the air diminishes, its conductivity increases ; consequently 

 there must exist at a considerable altitude above the terres- 

 trial surface a stratum of air the conducting-power of which 

 is better, yet without being particularly good. The terres- 

 trial surface, both solid and liquid, is therefore immediately 

 surrounded by a stratum of air endowed with feeble conducti- 

 vity and subject to incessant variations. To this stratum suc- 

 ceeds another, the conductivity of which is greater and, as 

 far as we know, sensibly invariable. The upper limit of 

 the atmosphere has been fixed by astronomic methods at an 

 altitude of between 70 and 80 kilometres. Truly speaking, 

 these determinations signify only that the atmosphere up to 

 that limit possesses sufficient density for its presence to be in- 

 dicated by the ordinary methods of determination. That the 

 atmosphere, though excessively rarefied, extends to a still 

 greater elevation is most evidently proved by the fact that 

 shooting stars have been observed at nearly 900 kilometres 

 above the surface of the earth. These small bodies evidently 

 can only become bright in consequence of a portion of their 

 vis viva, transformed into heat by the friction of the air, aug- 

 menting their temperature to such a degree that they begin 

 to shine. Now we can only perceive the falling body from 

 the moment when it becomes luminous ; and it is clear that 

 at that moment it will have already traversed a certain length 

 of path in the rarefied atmosphere before attaining so high a 

 temperature. Therefore the upper boundary of the atmosphere 

 must be situated at a much * greater distance from the earth 

 than has hitherto been admitted. 



The magnetic action of the earth cannot be explained en- 

 tirely and in detail by the assumption that its magnetic force 

 is due to a magnet of iron or steel situated in the earth, and 



