68 Modern Cannon Powder. [January, 
the sparks on the discs is read off, each spot being brought 
in succession exactly opposite the discharger belonging to 
the disc it is on: the speed at which the discs are moving 
being known, the time occupied by the shot in passing from 
one point to another is readily ascertained, and its velocity 
of translation calculated. 
In order to test the accuracy of the instrument, it is only 
necessary to cut the whole of the primary wires simulta- 
neously, when the whole of the sparks should be in one 
straight line, and the deviations from a straight line, that is, 
from an absolutely simultaneous record, give the instru- 
mental errors. 
Great difficulties were experienced in securing a simul- 
taneous rupture of the primary wires, and only two methods 
were found at all satisfactory. One arrangement was to cut 
the wires by a flat-headed bullet fired from a rifle, across 
the muzzle of which they were all tightly stretched ; in the 
other they were all wound round a detonating fuze, the ex- 
plosion of which severed them almost instantaneously. A 
number of the observations thus obtained are given in the 
Report, and the errors—including those due to the impossi- 
bility of obtaining an absolutely instantaneous rupture of all 
the wires—seldom exceed 0°000003 second, while the maxi- 
mum error is only 000002 second! 
In addition to the holes tapped to receive the cutting- 
plugs already described, the gun is also bored to take a 
number of Rodman or ‘“‘crusher” gauges. When any of 
these holes, which are twenty-one in number, were not re- 
quired in the experiments, they were filled with solid steel 
plugs. The Rodman pressure gauge is shown in position in 
the gun (Plate II.): it consists of a piston, working in a 
hollow screw plug open to the bore, the outer end of which 
carries a pointed knife, against which a piece of copper is 
placed. When the gun is fired, the gaseous pressure on the 
base of the piston forces the knife into the copper, and the 
indent is a measure of the pressure which has acted on the 
base of the plug. In this instrument the gas has a consi- 
derable space to travel between the powder chamber and 
the piston ; thus, before reaching the latter it attains a high 
vis viva, especially in quick burning powders, and a¢ts upon 
the piston more like a blow than a pressure, and the records 
are therefore much higher than should be the case. 
To remedy this defe@t the “‘ crusher” gauge was devised 
by the Committee (see Fig. 3, Plate I.) : the reduced dimen- 
sions of this instrument allow it to be placed so close to the 
bore of the gun that the gas has no space to travel before 
