LIGHTNING AND THUNDER. 211 



Suppose three clouds, arranged in a series, end to 

 end, and so graduated as to potential, that the central 

 cloud is at a mean between the other two. Let a dis- 

 charge take place from the cloud of highest potential 

 to the central one ; a second discharge must quickly 

 follow, from the central cloud to the one of lowest po- 

 tential : since the first discharge has greatly increased 

 their difference of potential. This second discharge 

 would renew the difference of potential between the 

 first and central clouds, and prepare the way for another 

 series of similar discharges. 



The most careless observer cannot fail to have noticed 

 such series of discharges, following each other in 

 rapid succession, in different parts of the sky, during a 

 violent thunder storm. 



Observation also shows, that during a thunder show- 

 er, there is always an increase of rain-fall, and an en- 

 largement of the drops, within a few seconds after each 

 electric discharge ; the time being just sufficient for 

 the rain to descend, if it left the cloud at the moment 

 of the discharge. From which we may infer, that con- 

 densation is a result of the discharge ; that, in the mo- 

 mentary equilibrium which follows it, the small drops, 

 which were before kept apart by mutual repulsion, 

 from being highly charged and at the same potential, 

 now coalesce, and form the large drops ; which, being 

 too heavy to be sustained in the atmosphere, fall. 



THUNDER. As the spark from the machine is the 

 type of lightning, so the snap represents tlmnder ; 

 which is undoubtedly due to the same cause the sud- 

 den and intense vibratory motion of the air, in the line 

 of discharge, producing violent undulations in the sur- 

 rounding air. A cause which will appear sufficiently 



