340 SEC. 10. - ELECTRICITY. 



The manner in which the electricity divides itself on the two conductors 

 depends on their reciprocal position to each other as well as on their form. 

 The earth might, without a remarkable difference, be considered as a sphere, 

 and likewise the conductor of air, but in their reciprocal position to each 

 other it appears that the space of rarefied air of 5 mra approaches much nearer 

 to the earth at the poles than at the equator, principally in consequence of the 

 inequality of the temperature of the air in the two places. If we assume the 

 mean temperature of the air round the equator to be 25, at the poles 12, 

 and everywhere on the conductor of air 60, and we suppose at the same 

 time the air everywhere half saturated with moisture, and that the temperature 

 is reduced in proportion to the elevation, we find, if the above-mentioned 

 formula of Laplace (1*) is applied, that the conductor of air, at the equator, 

 must be at an elevation of 37 '47 kilometers, and at the poles but 34 '25. 



In consequence of this relative position, and if the two conductors are 

 regarded as conducting surfaces, the electric density on them both becomes 

 about 9 per cent, greater at the poles than at the equator, and the power, 

 by which the two electricities endeavour to join again, at least 20 per 

 cent., but probably 30 or 40 per cent, greater, if all the circumstances are 

 considered, at the poles than at the equator. It is in these facts we have 

 to seek the principal cause of the accumulation of electricity at the poles 

 of the earth and of the phenomenon that occurs there, and is called polar- 

 light or aurora borealis. 



It is a remarkable fact that the thunderstorms diminish as well in number 

 as in intensity in proportion as we remove from the equator, and that at the 

 70th degree of latitude they cease completely, after having shown once 

 more in the highest north vestiges of their primitive intensity. In Fin- 

 nish Lapland, for instance, thunderstorms are very uncommon, but when 

 they occur they are extremely intense, and are almost always accompanied by 

 thunderbolts. This peculiarity has probably its cause in the fact that the 

 region of thunderclouds lowers towards the earth in accordance with the 

 .same rule as the before-mentioned conductor of air. The reduced number of 

 thunderstorms is caused by the fact that the very source of electricity in 

 the atmosphere, that is to say, the evaporation, is very much reduced; 

 however, another important cause is here active, namely, the heightened 

 conductive power that the air possesses in consequence of its greater quantity 

 of moisture, whereby the electricity becomes unable to keep itself, beyond a 

 certain latitude, upon the clouds, until it has attained a greater tension, 

 but is conducted down to the earth in form of a slow current, visible in the 

 polar-light. 



It results from experience, with a high degree of probability, that the 

 polar-light is an electric phenomenon, for its effects are of the same nature 

 as those of the electric currents. Thus the polar-light causes disturbances 

 in terrestrial magnetism, induces currents in the telegraphic wires, and 

 furnishes a spectrum of nine bands, which coincide, except one, with the 

 spectral lines produced if an electric current goes through a rarefied space of 

 air. Thus there is no doubt that the polar light is caused by an electric 

 current going down from the upper rarefied layers of air to the earth ; this 

 current, during its passage through the rarefied air, produces light phenomena 

 that cannot arise in denser layers of air. 



The polar-light apparatus now exhibited shows that an electric current 

 flowing out from an isolated body does not produce any light phenomena in 



(1*) X= 18-393 metres (1 + 0'002837 Cos. 2 <) where X signifies the 



elevation, < the latitude, T the temperature at the surface of the earth, t at the upper 

 point H, and Ji the stand of the barometer for the same points, but duly reduced. 



