"summer lightning," takes place without any thunder; so that, in such cases, no actual thunder-storm is in progress.
3. "Globular Lightning."—This is a rare phenomenon, and one which no one has as yet been able to produce in the laboratory, whereas the phenomena of the two previous types are easily produced. The general description of the occurrence is that a luminous ball is seen, moving very slowly, not touching any object, and eventually breaking up with a violent explosion and the appearance of several flashes of ordinary lightning. It is reported that persons have gone out from a house into a street to follow such a ball and watch its movements, so that the occurrence must have lasted at least a number of seconds. Ordinary lightning, as is well known, is practically quite instantaneous. The size of the ball on different occasions has varied from that of an orange to that of a large glass lamp-globe, or even larger. Many physicists refuse to believe any accounts of this manifestation of the electrical discharge, but the reports of it are too numerous and circumstantial for us to consider them to be entirely baseless.
There is another way of classifying lightning flashes, and that is as to their color. The seven colors of the solar spectrum are well known, but the spectrum of the electric spark differs materially from the solar spectrum. It exhibits rays which extend far beyond the extreme violet of the solar spectrum. We see, therefore, that in the light of lightning a wide range of color is possible. If any of my readers have ever watched a storm carefully, they must have noted that some of the flashes were bluish, others reddish, etc. It is generally the blue tints which accompany the most destructive strokes.
Some attempts have been made to estimate the actual force exerted by a lightning flash. The late Mr. de la Rue constructed a magnificent electrical battery of many thousand cells. From experiments with this, the number of cells being raised to 15,000, and the "potential" of each being rather over one "volt," it was found that 9,700 "volts"—say 9,500 cells—were required to produce a discharge through one centimetre (·3937 inch). Starting from these data, the electro-motive force requisite to produce a flash of lightning one mile (63,360 inches) in length, at ordinary pressures, is 1,480,570,000 volts, practically given by a battery of fifteen hundred million cells.
A flash a mile in length is nothing very extraordinary, and it is therefore not to be wondered at that experiments to bring electricity down from the clouds are very dangerous, and have frequently had fatal results. Soon after Franklin, in the last century, had made his famous experiment with a kite, and proved that electricity existed in a thunder-cloud, natural philosophers generally began to imitate him. One of them in St. Petersburg, a