210 ELEMENTS OF STATIC ELECTRICITY. 



of resistance in the air, to the induction of surrounding 

 clouds, and to the mutual repulsion of the molecules of 

 air and vapor within the line of discharge ; which, be- 

 iug electrified to the same potential, tend to separate and 

 form resultant lines, under the influence of forces act- 

 ing at right angles to each other. 



Observation shows, that there is usually a succession 

 of discharges between the two clouds, similar to the 

 repeated discharges from a Holtz machine : in which, 

 after the initial charge, electricity is generated by in- 

 duction alone. This action begins when the edges 

 of the two clouds, at different altitudes, approach 

 within discharging distance, and come into vertical 

 line ; and the effect of induction is to accumulate 

 the electricity of the cloud of higher potential at 

 the end nearest to the other cloud, while the elec- 

 tricity of the latter is repelled to the remote end ; 

 just as a similar effect is produced by the mutual 

 approach of two differently charged conducting plates 

 or cylinders ; the difference of potential between the 

 adjacent parts being thus greatly increased. 



The discharge produces a momentary equilibrium, 

 which is again disturbed by induction, as larger areas 

 of the two clouds approach more closely: the residual 

 becoming the initial for a new charge, further conden- 

 sation taking place, and fresh supplies of electricity flow- 

 ing in from the surrounding atmosphere. In this way 

 the series of discharges continues, till the clouds unite, 

 and complete equilibrium takes place. 



When several such clouds, at different potentials and 

 different altitudes, collect in each other's vicinity ; as 

 is usually the case in a thunder storm of much magni- 

 tude ; the mutual inductive effect is greatly intensified. 



