506 Magnitude of the Unipolar Induction of the Earth. 



water which issues mixed with a certain quantity of lique- 

 fied water becomes electro-positive under ordinary conditions. 

 Armstrong mentions, in the account which he gives of his 

 experiments, that if we place the metallic points intended to 

 collect the electricity near the orifice of issue, we obtain a 

 large quantity of electricity, but of a very small tension. If, 

 on the other hand, we put the points three or four feet off 

 from the orifice, the quantity of electricity is smaller, but the 

 tension, by way of compensation, is now much greater than 

 at the shorter distance. Armstrong does not point out the 

 cause of this phenomenon, but it is easy to discover it, by 

 applying to the case the principles which have been explained. 

 In the immediate neighbourhood of the orifice of issue, the 

 vapour contains an insignificant quantity of liquefied water ; 

 at a greater distance, on the other hand, a considerable pro- 

 portion of the aqueous vapour has become liquefied in conse- 

 quence of the cooling which occurs during the passage ; it 

 follows hence that the electricity conveyed by the vapour has 

 attained a higher tension, although a portion has been lost 

 during transport. 



Another well-known circumstance favours intimate con- 

 nexion between the liquefaction of aqueous vapour and a great 

 increase in the electric tension of the air, viz. that we never have 

 thunder in those regions of the earth's surface where no fall 

 of atmospheric water occurs, nor formation of clouds. Such 

 is, for example, the case in Peru, in the Sahara, and in the 

 deserts of Asia. In general, thunderstorms are more frequent 

 after midday and in the evening. 



The ascending currents, which occur especially during the 

 morning, carry a considerable quantity of aqueous vapour into 

 the higher regions of the atmosphere ; there this vapour 

 quickly becomes cooled in consequence of the low temperature 

 which exists there, and forms cumuli, which resolve themselves 

 into rain or hail if the liquefaction continues, and are distin- 

 guished by their high electric tension. If the terrestrial sur- 

 face is strongly heated by the solar rays, the ascending currents 

 of air become greater, and carry with them a larger quantity 

 of moisture. It will be understood without trouble that these 

 currents facilitate the transport of electricity into the higher 

 regions. A thunderstorm which occurs during the evening 

 is therefore often preceded by a hot morning, and the air is 

 saturated with moisture which renders it oppressive and suf- 

 focating. The rapid liquefaction of the vapour, attended by 

 a heavy fall of atmospheric water in the form of rain or hail, is 

 produced also in cyclones, whence it follows that these pheno- 

 mena are also accompanied by thunder. 



