1872.] Modern Cannon Powder. 67 
a thousand parts, a linear representation is thus obtained at 
the circumference of the discs of intervals of time as minute 
as the one-muillionth part ef a second. Asa small variation in 
speed would affect the relations between the several records 
obtained, the uniformity of rotation is ascertained, on each 
occasion of experiment, by three observations,—one imme- 
diately before, one during, and one immediately after, the 
experiment, the mean of the three observations being taken 
as the average speed. The accuracy of the workmanship 
in the instrument is shown by the great degree of uniformity 
at which the speed is maintained. The Report gives the 
observations in six consecutive rounds ; in two of these the 
speed was absolutely uniform, while the greatest variation 
in any round is as follows :— 
Ist observation, 625 revolutions made in 21°2 seconds. 
and % r» »» 209,» 
3rd 2? 29 a) 20°7 39 
The arrangement for obtaining the electrical records is as 
follows :—The edges of the discs are covered with a strip of 
white paper, and each is connected with one of the secondary 
wires, G, of an induction coil. The other secondary wire, 
H, carefully insulated, is brought to a discharger, I, opposite 
the edge of its corresponding disc, and is fixed so as to be 
just clear of the latter. The surface of the paper on the 
discs is coated with lamp-black, so that the passage of a 
spark from the discharger to the disc burns away the black, 
and marks the spot perforated by exposure of the white 
paper beneath. 
In order to connect the primary wires of the induction 
coils with the bore of the gun, so that they may be cut by 
the shot in its passage, the gun has been tapped ina number 
of places (see Plate II.) for the reception of hollow steel 
plugs, carrying at the end next the bore a cutter which pro- 
jects slightly into the bore. This cutter is held in position 
by the primary wire, which is carefully insulated and passed 
down the plug, through the cutter, and back out of the 
plug, the ends being conne¢ted to the main wires leading to 
the induction coils. When the shot reaches the point where 
a plug is screwed in, it presses the cutter in flush with the 
bore, and, by so doing, cuts the primary circuit, thereby 
causing an induced spark to pass from one of the dischargers 
to the corresponding disc. As each plug is reached, a spark 
is delivered on the disc in conne¢tion with it, and thus the 
passage of the shot up the bore is recorded at regular inter- 
vals. By means of the micrometer, v, the distance between 
