Page:Popular Science Monthly Volume 80.djvu/80

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T 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 conditions within these two clouds, at the instant when the discharge