and the Aurora borealis. 429 



atmosphere have been charged, in the way indicated, with po- 

 sitive electricity, other clouds may in turn be negatively elec- 

 trified by influence. 



As was said above (p. 364), the electropositive fluid (the 

 aether) flows from the upper strata of the atmosphere to the 

 earth in the direction of the dipping-needle. The vertical 

 component of the induction-force in general diminishes as we 

 remove from the equator towards the pole, while the density 

 of the electric fluid present in the atmosphere increases with 

 the latitude. On arriving near enough to the pole for that 

 component and the electric resistance of the air to be no longer 

 capable of opposing a sufficient obstacle, the positive fluid 

 flows down into the electronegative earth. The localities where 

 this takes place form a continuous zone surrounding, in the 

 northern hemisphere, both the magnetic and the astronomic 

 pole, and descending in America to lower latitudes than in the 

 Old World. In my opinion, to the passage of these currents 

 through the rarefied air we must ascribe the production of the 

 aurora borealis*. 



greater in the former season than in the latter. Now, as every one knows, 

 what takes place is precisely the contrary. The positive electricity pro- 

 duced by evaporation, chiefly in the torrid zone, would rise into the at- 

 mosphere with the ascending aqueous vapour, would then be conducted 

 by the upper currents of air (the counter-trades) towards higher latitudes, 

 where it would form the aurora borealis by its descent to the earth. But 

 these currents are at an insignificant height from the ground in compa- 

 rison with that of the aurorse boreales ; and, besides, they descend to the 

 surface of the earth long before arriving at the regions marked by the 

 principal frequency of these phenomena. Although complete confidence 

 cannot be accorded to the measurements of the altitude of the aurora 

 borealis, we are certain that it is sometimes very considerable. Some de- 

 terminations made during the above-mentioned French expedition indi- 

 cate a height of 150 kilometres ; and the height has been found still 

 greater on other occasions. (At the highest latitudes, however, the 

 aurora borealis may from time to time appear at a short distance from the 

 terrestrial surface, as is proved by the observations of Farquharson, 

 Wrangel, Parry, Lemstrom, and several other arctic voyagers.) At so 

 great an elevation the extremely rarefied air is assuredly not troubled by 

 winds ; and it is difficult to conceive how that rarefied air could become 

 electric through the evaporation produced in the equatorial regions. If 

 there existed no special force to raise the electropositive fluid into the at- 

 mosphere, this fluid must immediately descend to the earth, and the atmo- 

 sphere would certainly exhibit no traces of electricity. 



* On the passage of electric currents through the air M. Lemstrom 

 has made some experiments which throw much light upon the pheno- 

 menon in question (Archives cles Sciences Phys. et Nat. t. liv. pp. 72, 

 162). With the aid of a Holtz machine he kept at a determined electric 

 charge a metal knob furnished with some metallic points. With this 

 view the knob was connected by a conducting wire to one of the poles of 

 the machine, while the other pole was in communication with the earth. 



