O. N. Rood—Duration of Flashes of Lightning. 169 
lightning, as far as could be observed, was quite identical, 
furnishing, as it seems to me, argument in support of the 
pothesis that zig-zag lightning, heat and sheet lightning, etc., 
are really identical, being in point of fact due to the same cause, 
but viewed under different conditions. 
Best form of apparatus for the study of Lightning Flashes.— 
From the contents of this article, it will be seen that additional 
observations are still desirable, and hence I wish to describe 
more definitely the apparatus which after many trials answered 
best, as well as to suggest one of a more perfect form. At the 
practicable with storms in our latitudes, though the frequency 
of the flashes in the Tropics might render possible its employ- 
ment. A spring rotation apparatus, on account of portability, 
vided with some contrivance by which the observer might 
always be able to ascertain to what extent the clock-work at 
