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 times as many negative as positive electrons. It has been found also that a velocity of 25 feet (8 meters) per second, or more, will cause the larger drops to be shattered and beaten into spray. That is, if the drops falling in still air reach a velocity of 25 feet per second, they will be broken into smaller drops; or if the updraught exceeds 25 feet per second, the drops cannot fall against it; they will be shattered and carried upward until the velocity of the updraught is much reduced.

“Clearly,” Dr. Humphreys states, “the updraughts within a cumulus cloud frequently must break up, at about the same level, innumerable drops, which, through coalescence, have grown beyond the critical size and thereby according to Simpson’s experiments, produce electrical separation within the cloud itself. Under the choppy surges of a thunder-storm, the drops may undergo disruption and coalescence many times, and with each disruption a correspondingly increased electrical charge. Hence, once started, the electricity of a thunder-storm rapidly grows to a considerable maximum. After a time, the larger drops here and there reach places below which the updraught is slight; then they fall as positively charged rain. The negative electrons in the meantime are carried up into the higher part of the cumulus where they unite with the particles of cloud matter and thereby facilitate their coalescence into negatively charged drops. Hence the heavy rain of a thunder-storm should be positively charged—as almost always it is—and the gentler portions negatively charged—which frequently is the case.”

The falling rain—and also the hail which occasionally attends a thunder-storm—cools the air through which it passes and the cold air sinks to the earth with a considerable velocity. As it reaches the earth the down-rush plows underneath the warm, moist air in front of the storm, lifting it and thereby aiding the updraught. As the cold air spreads over the ground its velocity is great enough to raise clouds of loose dust that almost always precede the fall of rain.

As in the case of other storms, the latent heat set free by the condensation of moisture is the fuel of the thunder-storm,