LIGHTNING (ME. lightnyng, illumination. from lighten. light). A brilliant flash of light between clouds, or between a cloud and the earth. These flashes were shown by Benjamin Franklin to be simply enormous electrical discharges. The length of such a flash may he several miles. When the flashes are between the lower clouds and the earth they are comparatively narrow and brilliant and are accompanied by thunder. but when they take place in the upper cloud region they become more diffuse and thunder is rarely heard. Three general classes of lightning are recognized. The first of these is the forked or zigzag lightning. which is a line of light of in tense brilliancy, appearing to the eye as a single blinding streak of light, which sometimes breaks into one or two branches and is often of a rosy or violet tint. The second class of lightning is called sheet lightning: this has no definite form. It is generally of a rosy or red tint and appears in the distant horizon lighting up the clouds. dust. or haze in the atmosphere. A true sheet-lightning discharge is rare, but the ordinary appearance of sheet lightning is very common, being due to the illumination of the clouds and haze by flashes of lightning which may be be yond the horizon, and thus become manifest al though distant one or two hundred miles. The third kind of lightning is the so-called ball lightning, which is said to appear like a small globe of brilliant light moving slowly through the air at a short distance above the ground or even rolling along the ground itself. Reliable observers have stated that on some occasions ball lightning has been seen to roll slowly into a house through an open door or window; it gen erally breaks up with an explosion, which, how ever, is not very destructive or dangerous. The descriptions of this phenomenon present many points difficult to understand, and it is only of late years that electricians have been willing to admit that we have here a true but peculiar form of electric discharge that demands further ex perimental investigation. Very minute discharges analogous to ball lightning have heen produced in the physical laboratory. The silent electric discharges known as Saint Elmo's fire (q.v.) are not usually included under lightning.
The thunder that accompanies lightning seems to be satisfactorily explained by the fact that the electric discharge in forcing its way through the atmosphere heats the air and the vapor lying in its path to a very high temperature. causing a very violent expansion along the whole length of the flash, simultaneously followed by an rapid contraction and the production of a wave of expansion and compression. or what is the same thing, a noise. Owing to the refraction of sound as it travels through the atmosphere, and especially the irregular refraction due to the temperature and wind, thunder does not travel very far before it begins to rise above the ground. so that observers frequently see flashes
of lightning without hearing the thunder which has passed over their heads. Thunder is there fore rarely heard at a distance of 15 or whereas discharges of artillery may be beard 30, 50. or 100 miles.
Beginning with the researches of the late Prof. Ogden Y. Rood. of Columbia College. and with the results obtained from the repent applications of photography. a few definite facts have been added to our knowledge of the nature of light ning. Professor Rood was able to show that a so-called single flash which may last several tenths of a second is simply an irregular suc cession of elementary flashes each of which lasts but a few thousandths of a second. or even less. Prof. .Tohn Trowbridge. of Harvard 1 Di versity, has constructed a storage battery of minute cells, by means of which he has been able to imitate true lightning flashes of sev eral yards in length. Prof. .1 OCe ph Henry was able to show that all electric discharges are of an oseillating or alternating nature. The indi vidnal oscillations take place in a few millionths of a second and rapidly die away in intensity so that the whole discharge is accomplished in a very short time, depending on the size and dis tance of the electrified bodies. Now as lightning is evidently a simultanemis discharge from myriads of electrified drops toward numerous, spots on the earth's surface. or toward a similar electrified cloud at a distance. it is therefore a rational hypothesis to assume that the numerous discharges observed by Rood as constituting one flash were simply a grand example of Henry's oscillatory di•harge. The truth of this hypoth esis is confirmed by the numerous photographs of lightning that have been taken the past (VW ua•, Une of the most remarkable of these was taken by W. H. Jennings, of Philadel phia, August 12. 1892, from the rear platform of a railroad train in full motion crossing the prairie of North Dakota; this was simply an example of what is now called 'ribbon lightning.' A line illustration of a ribbon flash is shown in the accompanying figure. This photograph was taken by J. .11. Justice, of Franktord. Pa., between 9:00 and 9:30 P.m., August 10. 1899. The plate shows a small single flash to the left of the centre and a vivid broad multiple flash a lit tle to the right of the centre. The latter flash was between 2000 and 3000 feet distant from the camera ; it descended from the cloud to the earth at an inclination of from 20 to 40 degrees, ap parently striking the ground within a mile to the north. The angular breadth of the flash, as photographed, is about 32 minutes of are near its summit and about 18 minutes near the horizon, representing a linear breadth of between 14 and 24 feet.