3 14 THE POPULAR SCIENCE MONTHLY. 



able number of isolated and apparently instantaneous electrical dis- 

 charges, 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. Work- 

 ing 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 sup- 

 port 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 experi- 

 ments, Pro£ 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 char- 

 acter, 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 some- 

 times 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 Wheat- 

 stone'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 dis- 

 charge was not less than the twenty-five-thousandth of a second, while 



