208 ELEMENTS OF STATIC ELECTRICITY. 



currents, and brought into contact with elevated parts of 

 the surface, and with trees, buildings, and other elevated 

 objects, must generate electricity by friction, much in the 

 same way as the carriers on the revolving plate of the 

 machine. And, as the vapor forms into clouds, they be- 

 come the accumulators of this electricity, in the same way 

 that it is accumulated by the plates and Leyden jars of 

 the machine. And this concentration of electricity in 

 the clouds raises their electric potential; and makes 

 them the nuclei to which the rising vapor is attracted in 

 consequence of its lower potential. 



Each infinitesimal drop of vapor is a sphere with its 

 electric charge on the surface ; and as these drops 

 coalesce, and form larger ones in the cloud, the charge 

 on each new drop accumulates on the surface ; and as the 

 increase of volume is greatly in excess of the increase 

 of surface, the electric surface density must increase 

 in nearly the same ratio ; the volume representing 

 electric quantity, which is thus condensed on a reduced 

 surface, producing a corresponding increase of intensity. 



Thus as a large body of invisible vapor forms first 

 into light fleecy clouds; and these collect into denser 

 masses; there is a constant reduction of volume, and 

 increase of electric intensity; till the fully formed 

 thunder cloud is the result. 



DISCHARGE BETWEEN CLOUDS. Two or more such 

 clouds, formed in different localities, often many miles 

 apart, and electrified in this manner, must, almost inevi- 

 tably, be at different electric potentials. And when car- 

 ried to\vards each other by opposite atmospheric currents, 

 at different altitudes, and brought within the sphere of 

 mutual electric influence, strong inductive effects are 

 produced ; their approach is accelerated by attraction, 



