164 O. N. Rood—Duration of Flashes of Lnghtning. 
able to ascertain its duration.* Wheatstone, with a revolving 
dise of this kind, found the duration of the flash too short for 
measurement, and set it as less than ;;';; of a second. Cassel- 
character of the discharge, which indeed had previously been 
observed by Dove.t Faraday, in 1857, noticed that the 
duration of some flashes of lightning seemed to him fully 
as great as a second, if not more, and attempted to explain this 
by a phosphorescence of the portion of the cloud traversed by 
the flash.t CO. Decharme at Angers, in 1868, while a distant _ 
storm was raging, saw the heavens from time to time illumin- 
ated by a light which seemed to last from a half to an entire 
second.§ In these last two observations no apparatus was use 
for actual measurement. To the above, I may add a rough 
New Observations. 
(1.) Immediately after my own experiment, I arranged a small 
train of toothed wheels driven by a spring, so that it should be 
capable of rotating a circular paste-board disc which was pro 
vided with four open sectors of 8° each. This apparatus was 
constantly near me ready for use, but an entire year elapsed 
before an opportunity occurred. I noticed then upon one occa 
sion about midnight, that my room was from time to time 
illuminated by lightning-flashes, whose duration seemed as 
great as an entire second, and upon making an examination 
with the rotation apparatus, it was found that each flash con 
* Becquerel, Traité de l’Electricite et du Magnetisme, vol. vi, pp. 125; 129, 
Paris, 1840. 
der Physik, for 1847, p. 668. i Phil. Mag., IV, xiii, p. 5 
Fortschritte 06. 
Fortschritte der Physik, xxiv, p. 644 This Journ, IL, i, Jan., 1871. 
