168 O. N. Rood—Duration of Flashes of Lightning. 
till the resistance became so great as to cut off the discharge. 
In every case the light appeared to be instantaneous, that is, its 
duration was so small as to preclude the idea of its affording 
an explanation of the long intervals occurring in the case of 
lightning flashes. - course, in all these experiments, the 
actual rate of the mirror was quite low. I then repeated the 
same experiment with the vapor of water at a tension of abou 
15 millimeters with the same result. | 
Afterward a jet of steam was directed across the path of the 
electric spark in the free air, with a similar negative result. 
inally, thinking that possibly the rain might in some way be 
concerned in the production of the prolonged durations, while 
sparks 10 to 15 millimeters in length, obtained from a smaller 
jar, were traversing the ordinary atmosphere, fine watery spray 
was directed across their path by the use of an “atomizer,” 
without sensibly increasing their duration. These experiments 
may be explained by the greater volume of the atmospheric 
electricity, but suggest on the other hand, the possibility of a 
real prolongation of some of the constituents of the flash. 
Recently the spectrum furnished by flashes of lightning has 
been examined by Dr. H. Vogel.* A number of lines were 
mates to one that is continuous. 
It will be noticed, that while making observations No. 5, I 
was in the area occupied by the storm, in No. 4 on its outer 
edge, in No, 1 out beyond the edge, and in No. 8 the storm 
was quite distant. Yet in all these cases the character of the 
* Pogg. Ann., 1871, No, 8, p. 653. 
