76 



THE POPULAR SCIENCE MONTHLY 



A FLASH OF LIGHTNING 



By Professor FRANCIS E. NIPHER 



WASHINGTON UNIVERSITY, ST. LOUIS 



IT is customary to classify lightning discharges into at least two 

 classes. This classification is based on the appearance of the 

 flash. One kind of lightning is called forked lightning and the other 

 sheet lightning. There has been some discussion concerning sheet 

 lightning, it being claimed by some that it is merely an illumination 

 due to a discharge which is hidden from view. 



The real fact appears to be that both ends of a lightning flash are 

 usually hidden from view within the two clouds. One of these clouds 

 contains falling drops of water which are overcharged with the negative 

 corpuscles which atoms of all kinds of matter contain when in normal 

 condition. The other cloud contains drops which have less than the 

 normal charge. This cloud has always been said to be positively 

 charged. 



The writer has sought to obtain photographic evidence of the con- 

 ditions within these two clouds, at the instant when the discharge 



Pig. 1. The Overcharged Cloud. An inflow of the negative fluid to the main 

 discharge channel, whose end is seen at the middle of the plate. 



