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able number of isolated and apparently instantaneous electrical discharges, the interval between the components being so small that, to the naked eye, they constituted a continuous act."

Several curious effects were observed in these experiments. Working with a disk having a single narrow opening, the multiple elements of the discharge were detected with great regularity, and Prof. Rood several times, instead of seeing the opening single, noticed that it had a form resembling the letter X or V, the lines in different positions of the disk having, as it were, got crossed in his eyes by their quick changes of position. On several occasions, when observing with the naked eye, the normal zigzag flashes lasted not less than a second, and the light seemed to pour steadily in a stream from the cloud to the earth. Observations made in the area occupied by a storm, out beyond its edge, and when it was quite distant, gave results that were identical, which the professor thinks furnishes an "argument in support of the hypothesis that zigzag lightning, heat and sheet lightning, etc., are really identical, being, in point of fact, due to the same cause but viewed under different conditions." As the result of these experiments, Prof. Rood concludes: "It is evident, from the foregoing, that the nature of the lightning-discharge is more complicated than has been generally supposed; it is usually, if not always, multiple in character, and the duration of the isolated constituents varies very much, ranging from intervals of time shorter than one one-thousandth of a second up to others at least as great as one-twentieth of a second; and, furthermore, what is singular, a variety of this kind may sometimes be found in the components of a single flash."

Such being the rough conclusions reached concerning the duration of the spark upon a grand scale, let us now consider the results of experiment upon it where all the conditions are in command. In 1835, Mr. Wheatstone attempted to measure the spark of a Leyden jar charged by a common frictional machine. The light from the spark was received upon a mirror mounted upon an axle capable of a high rate of revolution. The image of the spark, being thrown upon the mirror, was reflected to a distant point, and the time of the spark was inferred from the fixity or movement of the image. By using this arrangement, Mr. Wheatstone concluded that the discharge may take place within the millionth of a second; a result which was accepted by the scientific world for a quarter of a century. In 1858, a German named Feddersen, an accomplished physicist, dissatisfied with Wheatstone's results, entered upon a careful reexamination of the subject. He used the revolving-mirror arrangement with frictional electricity; but, as Wheatstone had driven his machinery by strings, Feddersen adopted a train of toothed wheels, and with this form of mechanism he found that the image of the spark was drawn out by the revolving mirror into a whitish streak which indicated that the time of the discharge was not less than the twenty-five-thousandth of a second, while