LIGHTNING AND THUNDER. 209 



and, when brought within proper distance, a discharge 

 takes place from the cloud of higher to that of lower 

 potential : just as a similar discharge takes place be- 

 tween the sliding electrodes of the machine : and the 

 result is chain lightning, of which the spark of the ma- 

 chine is an exact type. 



The distance, through which this discharge takes 

 place, depends on the quantity and intensity of the 

 charge, and the difference of potential between the 

 clouds. It may be any distance, from a few yards to 

 several miles. Observation on discharges between 

 clouds overhanging fixed localities, as two mountain 

 peaks, shows that they are sometimes from three to five 

 miles or more in length. 



We have seen how sparks, eight to ten inches in 

 length, are produced by the machine ; and have tested 

 their energy. If we compare such a discharge to that 

 produced between two clouds, whose magnitude and 

 potential, as compared with those of the machine, are 

 almost infinite, we can form some adequate conception 

 of the enormous energy of the lightning. 



When the line of discharge is concealed by inter- 

 vening clouds, and we see only the illumination result- 

 ing from it, the phenomenon is known as sheet light- 

 ning. We have the same result, when the spark from 

 the machine, occurring in a dark room, is concealed. 

 Hence, we may reasonably infer, that the discharge be- 

 tween the clouds, like that between the electrodes of the 

 machine, would always present the appearance of chain 

 lightning, if the line of discharge were always visible. 



The contorted and bifurcated discharges, known as 

 zigzag lightning, and forked lightning, like similar dis- 

 charges in the machine, are doubtless due to differences 



