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THE POPULAR SCIENCE MONTHLY. 



spark was absolutely instantaneous, its image thrown upon the ground 

 glass would be exactly the same, whether the mirror was motionless or 

 was revolving at the highest speed. But, if the spark had an appre- 

 ciable duration, its image would be prolonged or drawn out into a 

 streak, the length of which must depend upon the time of discharge. 

 The rate of the mirror's rotation being known, also the distance, m i 9 

 and the length of the streak, it was easy to calculate the total duration 

 of the spark. 



Fig. 4. 



Revolving-Mirror Arrangement. 



Prof. Rood now had the subtile agent he was pursuing pretty effect- 

 ually in his grasp, and the results that came out were very striking. 

 The ordinary spark was found to be a highly-complex effect ; to con- 

 sist of diverse and successive elements, and, in fact, to have its periods 

 and orderly history just like the geology of the globe. But, while the 

 '* vast durations " of Lyell and Dana are vague and inferential, these 

 infinitesimal periods could be demonstrated with the greatest exact- 

 ness. The previous discordant results were reconciled, Feddersen be- 

 ing justified in assigning a longer period for the total duration of 

 the spark, and Wheatstone's time holding true of its elements. 



With a Ley den jar of about a quart capacity (114.4 square inches 

 of coating), and all the connections as short as possible, so as to offer 

 the least amount of resistance to the electric flow, with brass balls as 

 electrode?, with a striking distance of about the twenty-fifth of an inch, 

 and the velocity of the mirror up to 223 per second, the image of the 

 spark thrown upon the ground glass and viewed by the naked eye was 

 drawn out into a streak one and a half or two inches long, the length, 

 however, varying with the speed of the mirror. The aspects of the 

 image are represented in Fig. 5. The first part was pure white, which 

 shaded into a brownish-yellow tint, passing on into a pretty distinct 

 green. When a polished plate of glass was substituted for the ground 

 glass, and a small magnifier was used to observe the image, a series 

 of bright points, on each side of the streak, became visible, in the po- 

 sitions indicated by the dots in Fig. 5. With high velocities, this suc- 

 cession of points was beautifully developed, and it consists of a series 

 of separate discharges following the first. It was thus found that the 



