THE ROYAL ARTILLERY INSTITUTION. 
19 
the velocity of the shot per second. To use it, draw out the centre scale until 
the line representing the time occupied by the shot in passing from one 
screen to the other is opposite the line representing the distance between 
the screens; the velocity will be seen opposite the arrow on the lower 
scale. In the drawing, the distance between screens is 96 ft., the time 
80 thousandths of a second, the velocity, therefore, 1200 ft. per second. 
On the actual scale—which is four times the size of the drawing—the velo¬ 
cities can be read off to half a foot. 
For an Experiment, 
cover the cylinders with tissue paper (of the best quality, free from holes), and 
if the experiments are to be in the open air, with no cover for the instrument, 
it is advisable to choose a bright-colored paper, so that the spots formed by 
the spark burning away the carbon may be easily distinguished from the 
white specks of dust which settle on the cylinders. The paper should then 
be thinly, but evenly, covered with lamp-black, by holding the cylinders 
either over a smoky lamp or piece of burning camphor, and then carefully 
replacing them on the instrument. 
Place the induction coil close to the left side, and attach wires from the 
binding screws of its secondary wire to the binding screws (DD) on the 
Fig. 8. 
instrument, which are in connection with the cylinders; connect up the battery 
with the coil, and the screens with the primary wire of the same, in the 
manner shown in the sketch, so that the current shall take the following 
course—viz., from one pole of the battery (a) round the coil to binding 
screw ($),then through further screen ( cd ) to the first screen ( e ), back to the 
coil (,/), and the other pole of the battery (g). 
A second battery (h) must be placed in connection with the electro-magnet, 
