The reason of this is clear: So brief is the moment for which the spark endures, that the disk, though in rapid motion, makes no sensible advance during that small fraction of time; therefore, in the image on the retina, the impression made by the white sectors remains distinct from the impression made by the black, and the eye sees the disk as it really is.
I may notice, in passing, a very interesting consideration, suggested by this experiment. A cannon ball is now commonly discharged with a velocity of about 1,600 feet a second. Moving with this velocity it is, as you know, under ordinary circumstances, altogether invisible to the eye. But suppose it were illuminated, in the darkness of night, by this electric spark, which lasts, we will say, for the millionth of a second. During the moment of illumination, the cannon ball moves through the millionth part of 1,600 feet, which is a little less than the fiftieth of an inch. Practically, we may say that the cannon ball does not sensibly change its place while the spark lasts. Further, the impression it makes on the eye, from the position it occupies at the moment of illumination, remains on the retina for at least the tenth of a second. Therefore, if we are looking toward that particular part of space where the cannon ball happens to be at the moment the spark passes, we must see the cannon ball hanging motionless in the air, though we know it is traveling at the rate of 1,600 feet a second, or about 1,000 miles an hour.
Brightness of a Flash of Lightning.—I should like to say one word about the brightness of a flash of lightning. Somewhat more than thirty years ago, Professor Swan, of Edinburgh, showed that the eye requires a sensible time—about the tenth of a second—to perceive the full brightness of a luminous object. Further, he proved, by a series of interesting experiments, that when a flash of light lasts for less than the tenth of a second, its apparent brilliancy to the eye is proportional to the time of its duration.[9] Now consider the consequence of these facts in reference to the brightness of our electric spark. If the spark lasted for the tenth of a second, we should perceive its full brightness; if it lasted for the tenth part of that time, we should see only the tenth part of its brightness; if it lasted for the hundredth part, we should see only the hundredth part of its brightness; and so on. But we know, in point of fact, that it lasts for less than the millionth of a second, that is, less than the hundred-thousandth part of the tenth of a second. Therefore we see only the hundred-thousandth part of its real brightness.
Here is a startling conclusion, and one, I may say, fully justified by scientific evidence. That electric spark, brilliant as it appears to us, is really a hundred thousand times as bright as it seems to be. We cannot speak with the same precision of a flash of lightning; because its duration has not yet been so exactly determined. But if we suppose that a flash of lightning, in a particular case, lasts for the thousandth of a second, it would follow, from the above experiments, that the flash is a hundred times as bright, in fact, as it appears to the eye.
Various Forms of Lightning.—The lightning of which I have spoken hitherto is commonly called forked lightning; a name which seems to have been derived from the zig-zag line of light it presents to the eye. But there are other forms under which the electricity of the clouds often makes itself manifest; and to these I would now invite your attention for a few moments. The most common of them all, at least in this country, is that which is familiarly known by the name of sheet lightning. This is, probably, nothing else than the lighting up of the atmosphere, or of the clouds, by forked lightning, which is not itself directly visible.
Generally speaking, after a flash of sheet lightning, we hear the rolling of distant thunder. But it sometimes happens, especially in summer time, that the atmosphere is again and again lit up by a sudden glow of light, and yet no thunder is heard. This phenomenon is commonly called summer lightning, or heat lightning. It is probably due, in many cases, to electrical discharges in the higher regions of the atmosphere, where the air is greatly rarified; and, in these cases, it would seem to resemble the discharges obtained by means of an induction coil in glass tubes containing rarified gases. But there is little doubt that in many cases, too, summer lightning, like ordinary sheet lightning, is due to forked lightning, which is so remote that we can neither see the flash itself directly, nor hear the rolling of the thunder.
Perhaps the most distinct and satisfactory evidence on this subject, derived from actual observation, is contained in the following letter of Professor Tyndall, written in May, 1883: “Looking to the south and south-east from the Bel Alp, the play of silent lightning among the clouds and mountains is sometimes very wonderful. It may be seen palpitating for hours, with a barely appreciable interval between the thrills. Most of those who see it regard it as lightning without thunder—Blitz ohne Donner, Wetterleuchten, I have heard it named by German visitors. The Monte Generoso, overlooking the Lake of Lugano, is about fifty miles from the Bel Alp, as the crow flies. The two points are connected by telegraph; and frequently when the Wetterleuchten, as seen from the Bel Alp, was in full play, I have telegraphed to the proprietor of the Monte Generoso Hotel and learned, in every instance, that our silent lightning co-existed in time with a thunderstorm more or less terrific in upper Italy.”[10]
Another form of lightning, described by many writers, is called globe lightning. It is said to appear as a ball of fire, about the size of a child’s head, or even larger, which moves for a time slowly about, and then, after the lapse of several seconds, explodes with a terrific noise, sending forth flashes of fire in all directions, which burn whatever they strike. Many accounts are on record of such phenomena; but they are derived, for the most part, from the evidence of persons who were not specially competent to observe, and to describe with precision, the facts that fell under their observation. Hence these accounts, while they are accepted by some, are rejected by others; and it seems to me, in the present state of the question, that the existence of globe lightning can hardly be regarded as a demonstrated fact. At all events, if phenomena of this kind have really occurred, I can only say that nothing we know about electricity, at present, will enable us to account for them.[11]
St. Elmo’s Fire.—A much more authentic and, at the same time, very interesting form, under which the electricity of the clouds sometimes manifests its presence, is known by the name of St. Elmo’s fire. This phenomenon at one time presents the appearance of a star, shining at the points of the lances or bayonets of a company of soldiers; at another, it takes the form of a tuft of bluish light, which seems to stream away from the masts and spars of a ship at sea, or from the pointed spire of a church. It was well known to the ancients. Cæsar, in his Commentaries, tells us that, after a stormy night, the iron points of the javelins of the fifth legion seemed to be on fire; and Pliny says that he saw lights, like stars, shining on the lances of the soldiers, keeping watch by night upon the ramparts. When two such lights appeared at once, on the masts of a ship, they were called Castor and Pollux, and were regarded by sailors as a sign of a prosperous voyage. When only one appeared, it was called Helen, and was taken as an unfavorable omen.
In modern times St. Elmo’s fire has been witnessed by a host of observers, and all its various phases have been repeatedly described. In the memoirs of Forbin we read that, when he was sailing once, in 1696, among the Balearic Islands, a sudden storm came on during the night, accompanied by lightning and thunder. “We saw on the vessel,” he says, “more than thirty St. Elmo’s fires. Among the rest there was one on the vane of the mainmast more than a foot and a half high. I sent a man up to fetch it down. When he was aloft he cried out that it made a noise like wetted gunpowder set on fire. I told him to take off the vane and come down; but, scarcely had he removed it from its place, when the fire left it and reappeared at the end of the mast, so that it was impossible to take it away. It remained for a long time, and gradually went out.”