166 0. N. Rood—Duration of Flashes of Lightning. 
durations of the total act were observed, even with the naked 
eye, which must have been as great as a second, and the light 
With a dise having only a single narrow, open sector, and 
making from 12 to 15 revolutions per second, the multiple 
character of the flashes was then studied. The flash consisted 
) 
taneous constituents was less than about ;,/55 of a secon 
On the other hand, sometimes the continuous duration of the 
constituents was such that the whole circumference of the dis¢ 
was surrounded by a bright or faint ring of light, according t 
the original luminosity of the flash, which gave a duration for 
the continuous act at least as great as ;'; of a second. Quite 
often I was able to notice that these continuous discharges did 
not make up all the elements of the flash, but that they wer 
terminated by an isolated and instantaneous discharge 
heard quite loudly and direct zig-zag flashes were occasionally 
seen ; rain also fell. The disc employed on this occasion ha 
only one square opening, the sides of which were seven m1 
meters (corresponding to about 12°), these dimensions having 
