Page:Popular Science Monthly Volume 56.djvu/604

588 lie in a plane, but was concave toward a point at the observer's left. That the ribbon did not stand at right angles with the line of sight, but was nearer the observer at the right-hand edge, is also shown by the inequality of the lower termination of the six vertical lines referred to above. The ones at the left either rest upon or are hidden behind a rise of ground, whose crest can be traced for a little distance each side of the flash, while those at the right come lower, falling between the observer and the ground at that point. Probably, when measured upon this diagonal and curved line, the width of the flash was fifteen or twenty feet.

Mention has already been made of the fact that the accompanying thunder was comparatively light, and not at all like that ordinarily heard when lightning occurs within so short a distance. Possibly this, as well as the absence of marks at the point where it reached the earth, might have been because the discharge was of very low tension. [A very similar lightning flash was described and pictured in the issue of the Electrical World and Engineer for October 28, 1899, by A. E. Kennelly, who suggested the following explanation: A lightning flash passed through the air on the left-hand side of the ribbon of lightning (the wind was blowing from right to left) and broke a hole in the air along that line. This discharge may have been oscillatory, and may have lasted in. all any time up to about of a second. The discharge then ceased for lack of electricity, but a fresh charge from the cloud being gathered immediately afterward, or in about of a second from the first rupture, a new discharge passed through the same hole in the air, which had not had time to seal up. There might thus be fourteen successive flashes (this was the number of distinct flashes making up the ribbon in the photograph), each averaging about of a second apart, through the same hole, owing to the imperfect conducting qualities of the clouds overhead, meanwhile the hole having been carried from left to right in the picture, across the line of sight (by the wind), and thus producing the appearance of a broad ribbonlike flash. Professor Trowbridge, of Cambridge, has suggested the possibility that many of these apparently curious electrical phenomena may be of purely optical or physiological origin—that is, may arise through the abnormal behavior of the eye or the camera lens toward intense lines of light, such as lightning flashes.—]