QUICKER THAN LIGHTNING. 313 



tering upon the inquiry, he soon found himself involved in preliminary- 

 difficulties with the spark itself. His phosphorescent investigations 

 remain yet to be carried out, but the results obtained relative to the 

 electric flash are of extreme interest. The full account of the research 

 is given in a series of papers published in Sillimari's Journal, and, if 

 the reader finds the following statement insufficient in its details, he 

 will know where to go for further explanations. 



Since the time of Franklin, the lightning-flash has been regarded as 

 a gigantic electric spark produced in the atmosphere ; the inquiry, 

 therefore, involved the nature of the meteorological discharge, as well 

 as of the spark artificially produced. Various attempts to determine 

 the duration of lightning have been made, with varying results. Fara- 

 day observed it, without any instruments for measuring the time, which 

 seemed to last for a second, but he was doubtful if part of the effect 

 was not due to the lingering phosphorescence of the cloud. Decharme 

 observed the lightning-flashes from a distant storm, which also ap- 

 peared to last for from a half to an entire second. Prof. Dove employed 

 a revolving disk with colored sectors, and satisfied himself that single 

 flashes of lightning often consisted of a number of instantaneous dis- 

 charges. It is well known that, when a rapidly-moving train of cars 

 is illuminated at night by lightning, it seems to stand still, that is, the 

 duration of the flash is so brief that no motion of the train is percep- 

 tible while it lasts. The wheels are sharply defined as if perfectly 

 motionless, but if they had a blurred aspect we should know that the 

 illumination lasted sufficiently long to render the motion perceptible. 

 Prof. Rood extemporized a simple contrivance for observing lightning, 

 which acted upon this principle. It consisted of a white card-board 

 disk, five inches in diameter, with a steel shawl-pin for an axis, on which 

 it was made to revolve by striking the edge. He traced black figures 

 near the circumference of the disk, and when it was in rapid motion 

 these figures were sometimes seen as sharply as though they had been 

 stationary, although they were often blurred as though the disk had 

 moved through a few degrees during the act of discharge. He then 

 cut narrow, radial apertures into the circumference of the disk, and 

 observed the lightning through these openings. Here, again, the 

 apertures were sometimes seen quite unchanged, but they were more 

 frequently elongated into well-defined streaks some degrees in length. 

 He afterward measured the average rate of rotation imparted to the 

 disk in this way, and arrived at the conclusion that the lightning-flashes 

 on the occasion referred to had a duration of about one five-hundredth 

 cf a second. Dissatisfied with the roughness of these observations, 

 Prof. Rood arranged a small train of toothed wheels driven by a spring, 

 which rotated a circular pasteboard disk with four open sectors. This 

 instrument gave more regular and precise results; and, while it was 

 shown that the flash sometimes lasts for a whole second, the suggestion 

 of Dove was clearly verified that each flash "consisted of a consider- 



