and the Aurora borealis. 425 



in consequence perpetually goes on increasing. The forces 

 tending to conduct the electric molecules in the vertical and 

 horizontal directions are very feeble above and around the 

 magnetic poles ; and consequently the electric density of the 

 atmosphere attains a maximum at a certain distance from 

 those poles. As we have seen, this maximum of electric den- 

 sity forms a zone enclosing, in the northern hemisphere, both 

 the magnetic and the astronomic pole ; and doubtless it is the 

 same in the southern hemisphere. The magnetic forces act 

 always with equal intensity, as the relatively slight variations 

 undergone by the terrestrial magnet from one period to another 

 can be neglected. If, then, the conductivity of the air were 

 equally invariable, the electrical tension of the lower strata of 

 the atmosphere would remain always the same ; but as this 

 condition is by no means fulfilled, that tension must necessa- 

 rily vary. It is evident that the electric tension at a given 

 point near the surface of the earth does not depend solely on 

 the conductivity of the air around that point, but also on that 

 of the higher strata, up to the height where the conductivity 

 becomes sufficiently good in consequence of the rarefaction of 

 the air. Suppose, for example, that a fog envelops the terres- 

 trial surface, and that above the fog is a stratum of dry air 

 impenetrable to electricity ; the force of magnetic induction, 

 still active, will then direct the electric molecules from the 

 earth into the fog, which will soon show itself electropositive, 

 as observations have proved. If, on the contrary, the fog ex- 

 tended up to the higher, conductive strata of the air, doubtless 

 the electric charge of the lower strata would disappear sud- 

 denly. From this we see how hazardous it is to attribute the 

 greater or less electric charge of the air at a given place to the 

 greater or less humidity of the air observed at the same place. 

 In my opinion it is highly probable that the periodic varia- 

 tions, both the diurnal and the annual, in the electric state of 

 the lower strata of the air have their cause in the variations of 

 the conductivity of the atmosphere ; but certainly it is not 

 enough to take into consideration only those variations which 

 happen upon the spot where the observations of atmospheric 

 electricity are made. 



The electric condition of the air in the polar regions is espe- 

 cially interesting. Scoresby, in spite of reiterated trials, 

 found it impossible to discover in those regions the slightest 

 trace of electricity in the air*. The French expedition of the 

 corvet ' La Recherche,' which passed the winter of 1838-39 

 at Bossekop in the Altenfjord (about 70° N. lat.), applied 

 themselves repeatedly to the examination of the electricity of 

 * An Account of the Arctic Regions, vol. i. p. 382 : Edinburgh, 1820. 



