Page:Handbook of Meteorology.djvu/128

 oscillatory discharge which does not differ, except in intensity, from the undamped spark of a wireless transmitter, the several oscillations producing separate but interfering sets of sound waves. A more satisfactory theory makes the extreme and sudden heating of the air, with its moisture content practically an explosion with compression waves identical with those caused by instantaneous explosions. The reflection of sound also may be a factor in reverberation.

Forms of Lightning.—The most common form of discharge is shown in the accompanying illustration. The discharge merely follows the line of least resistance. The zig-zag discharge, with sharp angles and saw-teeth points, once patronized by artists in order to give effect to their illustrative work, has never been discovered in photographs of lightning discharges. The most extraordinary effects of lightning are the dark flashes occasionally caught in photographs of lightning.

Sheet lightning is generally regarded as the reflection of distant flashes from the surface of clouds. On various occasions the exchange of electricity takes the form of a bluish glow between the earth and a low cloud. This form of discharge is rare; probably it does not differ from the brush-shaped discharge visible when a static generator is operated in the dark. The St. Elmo fire is a discharge of this sort. During its occurrence, the peaks of roofs, the limbs of trees, flag-poles, church spires, and weather vanes are tipped with coronal circles of electricity. The St. Elmo fire is of rare occurrence. It sometimes follows thunder-storms.

Ball lightning has been observed so many times that its existence seems to be established beyond doubt. It has been