LIGHTNING AND Franklin in 1751 proved that lightning was simply a visible display of electricity— an ex tended spark; and until a generation ago little was added to his exposition to define the differ entia of this phenomena. The origin of atmos pheric electricity is believed to be simple fric tion. Faraday showed that a powerful current could be excited by steam-driven spray against a water surface, and the friction of wind-driven mist on the earth's surface may produce a great difference of potential between the latter and the upper air, possibly though not probably as sisted by the friction on dust particles in the air. In any event, rain conducts a portion of it to earth; so that a period of dry weather causes a great accumulation of electricity, the particles of air distributing their charges through each other. This would make the earth and sky, in Lodge's comparison, the two coat ings of a Leyden jar, but ordinarily the dis tance is too great for a spark to pass. The effect of electrical discharges on vapor, how ever, is to condense it into larger globules; this causes it to sink toward the earth as cloud, and the enormous tension relieves itself by passing from one cloud to another or to the earth or objects upon it, preferably projections of some height. If conduction through the cloud were instantaneous, it would be drained of its'eharge in one immense flash; but it being a poor con ductor, several flashes at different points are common.
The discharge is determined by the tension of the air, the maximum of which without rupture is one-half gramme per square centi meter. If the rupture is local, there is no flash, but only a brush; but it is often the case that when the weakest spot has given way, a gen eral breach follows for a long distance, some times miles, creating the flashes which pass either from cloud to earth or from cloud to cloud; and as the discharge of this portion draws the remaining current toward it, a second flash or set of flashes is made probable. But this analysis shows what experiment proves, that this flash is not a single discharge, but the successive discharges of a countless number of vapor particles or raindrops toward the earth or other electrified particles in the air, with such rapidity of progress that they seem simultane ous; since it is most improbable - that if vast numbers of points gave way at once, they should all give way in the same line. It has been further proved by Prof. Ogden N. Rood that the flash is not a single sequence merely too swift for the eye to individualize; but although it lasts only a fraction of a second, it is itself composed of primary flashes in irregular se quence, each lasting but from a thousandth to a few thousandths of a second. This result was
obtained by photography, which has invaluably supplemented laboratory experiment. By the latter, Prof. John Trowbridge has formed arti ficial flashes of lightning many feet long, made up of a combination of small discharges from a great number of petty cells. It had been long before proved by Joseph Henry that every elec tric discharge is an alternating or oscillating current, the periods of oscillation being only a few millionths of a second each and diminish ing very rapidly in intensity; the entire dura tion depending on the magnitude and distance of the bodies. Hence it has been inferred that the small primary flashes are instances of these alternating discharges. Photography steadily tends to confirm these views. • Lightning according to its manifestations Is divided into three classes. The oflash* or lightning is the one had in mind when the name is used without qualification; it ap pears either as a sharp zigzag line of extreme brilliancy, or the same forked, but as more clearly revealed by photography shows a wavy line oscillating with enormous rapidity, or in tree shapes with endless branches, or ribbon shape, or in a mass of strands of close but dis tinct parallel flashes like unraveled rope, or still other shapes. Dark flashes in photographs are only photo-chemical decompositions on the plate. The second sort is sheet lightning, a sudden glow of a golden or reddish tinge on the horizon, with no definite shape or bounds. It is not usually an actual discharge in that form, the very genesis of lightning making it rare; but is the reflection of lightning flashes out of sight beyond the horizon, cast on the clouds or atmospheric haze, and visible some times for many scores of miles beyond the place of the actual storm which causes the flashes. A third is ball lightning, which for a long time was not admitted as a genuine form of electric discharge, and is still a very difficult and in some points unexplainable phenomenon. It has • not been photographed, though some thing like it has been produced on a small scale in the laboi-atory. It is described as a ball or globe of brilliant light moving slowly a short distance above the surface of the earth, as if rolling on an invisible support a few feet high, and it has been said on occasion to float through an open door or window into a house, as though drawn in by a draft of air; it usually explodes, but without doing much damage. Un der the head of lightning is sometimes included Saint Elmo's fire or corposant —jets and brushes of light seen not only at the tips of masts and yards of a ship in a thunderstorm, but on mountain tops, in hissing tongues of brilliant white and blue light several inches long.