388 SEC. 10. - ELECTJKlCiTiT. 



submits almost to the same laws as if it were in a real conductor, and must 

 thus be distributed in a manner depending on the influence of the electro- 

 negative earth. Part of the electricity, conducted by the vapours, remains on 

 the clouds in the atmosphere and discharges in form of lightning ; another 

 part attains the region of rarefied air, owing to the fact that the vapour itself, 

 submitting to well-known physical laws, rises to this elevation, and also 

 because electricity tends to distribute itself on a conductor. 



The manner in which the electricity distributes itself between the two 

 conductors depends on their relative position as well as on their form. The 

 earth may, with sufficient approximation, be considered as spherical, as well 

 as the air conductor, but in their position relatively to each other it appears 

 that the region of rarefied air at 5 limi approaches much nearer to the earth at 

 the poles than at the equator, principally in consequence of the difference of 

 the temperature of the air in the two places. If we assume the mean tempe- 

 rature of the air round the equator to be 25, at the poles 12, and every- 

 where on the air conductor 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 air conductor must at the equator be at an 

 elevation of 37 "47 kilometers, and at the poles 34 '25 kilometers. 



In consequence of this relative position, 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 unite, at least 20 per cent., but probably, if 

 all the circumstances are considered, 30 or 40 per cent, greater at the poles 

 than at the equator. In these facts then must be sought the principal cause 

 -of the accumulation of electricity at the poles of the earth and of the phe- 

 nomenon occurring there, called polar-light or aurora boreaiis. 



It i* a remarkable fact that thunderstorms diminish in number as well as in 

 intensity in moving from the equator towards the poles, and that at the 70th 

 degree of latitude they cease completely, exhibiting once more in the highest 

 north vestiges of their primitive intensity. In Finnish 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 descends 

 towards the earth in accordance with the same rule as the before-mentioned 

 air conductor. 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 con- 

 sequence of its greater saturation with moisture, Avhereby the electricity 

 becomes unable beyond a certain latitude to remain upon the clouds, until it 

 has attained a greater tension, but is conducted down to the earth in form of 

 n 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 telegraphic wires, and furnishes 

 a spectrum of nine bands, which, with one exception, coincide with those 

 produced by an electric current passing through rarefied air. Thus there is 

 no doubt that the polar-light is caused by an electric current passing from 



(1*) X= IS'393 metres (1+0'002837 Cos. 2 $) where X signifies the 



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

 v point H, and h the reduced height of the barometer for the same points. 



