O. N. Rood on the discharge of a Leyden jar. 155 
found by him, that an increase in the size of the coating of the 
jar or of the “striking distance,” prolonged the duration of the 
discharge ; so, for example with a spark 3-75 millimeters long, 
the duration of the whitish portion was 00007 of a second. 
A series of observations detailed at the end of this article, 
will point out a probable explanation of the great discrepancy 
existing between the results of Feddersen, which are undoubt- 
edly correct, and those of Wheatstone. Finally, Feddeisen 
in the course of his investigation found abundant proof of 
the oscillatory nature of the electric discharge, which had 
been predicted from the results of a set of experiments by 
Prof. Joseph Henry as far back as 1842. 
Apparatus.—I arranged in the first place an apparatus like 
that of Feddersen, but it was soon found that to attain a rota~ 
tion of one hundred per second, it was necessary in my case to 
employ a weight of 200 pounds, and in addition when using 
this form, I labored under the disadvantage of not being able 
to vaty the size or curvature of the mirror readily ; in short it 
was found to lack flexibility, a quality more necessary in the 
present case than in the investigation which engaged the atten- 
tion of the German physicist. On this account I decided to 
employ a plane revolving mirror with a stationary achromatic 
a lens. The addition of an extra wheel, with a few other me- 
2 
chanical changes, converted the train of wheels furnished by 
Duboscq for Becquerel’s phosphoroscope, into an admirable 
means tor ‘driving the revolving mirror, and with a weight of 
30 pounds, 300 rotations per second were readily obtained, the 
_ motion being smooth and sufficiently uniform. In order to 
measure the rate of rotation, the cylinder on the lowest wheel 
_ Was made to wind up a fillet of paper, upon which dots were 
- made by an electro-magnetic apparatus regulated by a seconds 
pendulum, when a simple calculation furnished the rate of the 
wheel to which the mirror was attached. By this method it 
became possible to put to a sharp test the regularity of the rate 
of the train, which was found in all cases to be considerably 
greater than was at all necessary. These determinations were 
afterwards made with a watch having large second spaces, the 
dots being made by hand, as the refinement of a seconds pend- 
ulum and battery was found for my purpose to be superfluous. 
rs employed were always plane, being sometimes — a a 
1 
_ The mirro rs 
silvered at the back, while for the most delicate observations 
the rear surface was painted with black varnish, so as to de-— - 
stroy the second reflection. They varied insize from 1‘5inches 
. 3 e on x 
square, down to 2 by ‘5 in., and were used sometimes double | 
the axis back to back, and at other times four mirrors were em- 
