“Sheet lightning” presents the appearance of a diffuse glow over the sky. When lightning of this character is seen playing about the horizon on summer evenings, in the absence of an audible thunderstorm, it is often called “heat lightning.” Most sheet lightning is probably a mere reflection of ordinary streak lightning below the horizon or hidden by clouds. Some authorities believe, however, that diffuse, silent discharges actually occur in the clouds. Balloonists claim to have encountered such discharges near at hand. An analogous phenomenon is the glowing of so-called “incandescent” or self-luminous clouds, to which several observers have called attention. A remarkable phenomenon of somewhat similar character has been reported by Dr. Knoche, late director of the Chilean meteorological service, who states that it occurs on a vast scale along the crest of the Andes during the warm season. The mountains seem to act as gigantic lightning rods, giving rise to more or less continuous diffuse discharges between themselves and the clouds, with occasional outbursts simulating the beams of a great searchlight. These displays are visible hundreds of miles out at sea. Something akin to this so-called “Andes lightning” has occasionally been reported from other mountainous regions, including the mountains of Virginia and North Carolina. “Beaded” lightning and “rocket” lightning are as rare as they are interesting. The former resembles a string of glowing beads, while the latter is a form of streak lightning that shoots up into the air at about the apparent speed of a skyrocket. “Ball lightning” takes the form of a fiery mass (not always globular), which generally moves quite deliberately through the air or along the ground, and in many cases disappears with a violent detonation. It occurs inside of buildings, as well as out of doors. In order that a discharge of electricity may break through the resistance of the air along paths as long as those commonly observed, enormous differences of potential must exist in the atmosphere during thunderstorms. How such conditions arise has been the subject of an immense amount of speculation. The explanation now generally accepted was proposed in the year 1909 by the English physicist and meteorologist, Dr. George Simpson. This hypothesis is based upon the fact, well attested by laboratory experiments, that the breaking up of drops of water involves a separation of positive from negative electricity; in other words, the production of both positive and negative ions. In this process the drops become positively charged; i. e., they retain a greater number of positive than of negative ions, the latter being set free in the air. About three times as many negative as positive ions are thus released.