Thunder-storms.—The phenomena of thunder-storms have been known ever since human beings peopled the earth. The cause or causes are still imperfectly known.
Thunder-storms derive their name from the reverberations and crashes of thunder following lightning discharges, which possess an intensity unknown except in nature. These discharges take place between cloud and earth, between earth and cloud, and between cloud and cloud. But the lightning discharges are not the cause of the storm; they are incidents merely in its progress; and except in intensity and volume the thunder does not differ from the snapping of an electric spark.
Several things take place in the formation of a thunder-
After Humphreys.
The movement of the wind in a thunder-storm; A, base of cumulo-nimbus cloud; B, ground level. A roll scud forms between the wind of updraught and that of a downdraught.
storm. A strong updraught of air and the shattering of rain- drops are among the features necessary to produce free electricity. The updraught of air is almost always a noticeable feature, and this takes place conspicuously in the cumulus thunder-head. Ordinarily the base of the cumulus cloud is less than I mile in height; but the updraught that precedes the thunder-storm, and is a potent cause of it, carries the cauliflower head of the cloud to a height of 4 or 5 miles. It is within this head that the potential electricity of the raindrops is changed to kinetic or free electricity.
Experiments have shown that a blast of air driven against drops of distilled water, with a force sufficient to blow them into spray, produces both positive and negative electricity-
