Page:Popular Science Monthly Volume 80.djvu/81

This page has been proofread, but needs to be validated.
77
A FLASH OF LIGHTNING
PSM V80 D081 Outflow into the cloud which has less than its normal charge.png
Fig. 2. The Outflow into the Cloud which has less than its Normal Charge.

occurs. This evidence is presented in the figures, which are reproduced from photographic plates.

Fig. 1 represents in cross-section the cloud which is overcharged with the negative fluid. The cross-section is at right angles to the end of the long flash which connects the two clouds. The discharge lines on this plate resemble a system of rivers and tributaries, which penetrate the cloud. These drainage lines elongate up stream. Some of them are sharply defined. Others, for reasons which will be explained, are seen only in shadowy outline.

Before the flash occurred, the falling drops, which were all highly charged, repelled each other. After the discharge those drops which happened to lie in the path of some one of these tributary discharge lines, have lost their overcharge. There is then an attraction between such drops and those which were slightly outside of these drainage lines, and which are therefore still overcharged. These two groups of drops are intimately commingled, as is shown by the intricate nature of the system of drainage channels. As they continue their fall to earth, they coalesce, and a brief dash of unusually large drops of rain is observed.

The discharge pours through the long hole in the air, in which the conditions are like those which exist in a vacuum tube. The conditions which exist in the cloud at the other end of the flash are shown in Fig. 2.