370 
VELOCITY AND PEES SURE INSTRUMENTS. 
“The shape and size of the 'Rodman* coppers and knives render it 
imperative to place them at the upper extremity of the plug, and conse¬ 
quently on the exterior of the gun (if muzzle-loading); the gas has, 
therefore, a considerable space to travel through between the powder- 
chamber and the instrument. Thus, before reaching the latter, it has 
attained a high vis viva, especially in quick-burning powders; this is trans¬ 
ferred to the knife, and the recorded pressures are therefore always much 
higher than should be the case. This was clearly shown by placing a 
copper cylinder in one of the Rodman gauges, and comparing its com¬ 
pression with that of a similar cylinder taken from the crushing instrument. 
“ In the case of R.L.G. powder, the inner cylinder was compressed from 
0*500 in. to 0*285 in.; the area on which the gas acted being *167 of a 
square inch, and the corresponding pressure 22J tons per square inch. 
“On the other hand, the cylinder placed externally was reduced from 
0*500 to 0*251 in.; the gas acting on an area of 0*110 square inch, and 
the corresponding pressure being 40 tons per square inch. 
“The above obstacle to the attainment of correct pressure-indications 
does not exist in the crusher-gauge, as the small diameter of the working 
parts of this instrument permits of their being placed near the surface of 
the bore. 
“Another advantage due to its small dimensions consists in the readier 
attainment of uniformity in the quality of the metal upon which the pressures 
are recorded, as a very much larger number of ' crusher ’-cylinders than 
of ' Rodman’-plates can be produced from one and the same quality of 
metal.” 
The crusher-gauge has been in almost daily use since the date of this 
Report, and from its extreme simplicity, combined with very great accuracy, 
it has been found to be invaluable * 
When it is not considered desirable to bore holes for the gauges in service 
guns, the pressure has been obtained at three points—viz. (1) by a crusher 
in the base of the projectile, (2) by a copper cup, in which a gauge is 
fastened, at the end of the bore, (3) by inserting a gauge in the vent-bush 
hole, and firing the gun by electricity, wires being passed down the bore 
from the muzzle to the cartridge. By this means the pressure is approxi¬ 
mately arrived at; but since the copper cup is liable to a certain amount of 
motion, the record given by Ihe gauge inserted in it is not always reliable. 
The pressure recorded by the gauge in the base of the projectile, when 
charges of powder suitable to the calibre of the gun are employed, is always 
somewhat less than the maximum pressure in the powder-chamber as shown 
by the other gauges. This is probably due to the fact that the inertia of 
the shot is overcome, and it therefore begins to move, before the maximum 
pressure is reached, and, being in motion, the gauge is not influenced to the 
same extent as those that are immoveably fixed in the walls of the gun. 
To give some idea of the pressures recorded by the gauges in a large gun, 
* The following rifled guns have been fitted with these gauges in the manner shown at 
Plate V.:—9-pr. M.L., 16-pr. M.L., 25-pr. M.L., 40-pr. M.L., 80-pr. M.L., 7-in. B.L. (in vent- 
piece), 8-in. M.L., 10-in. M.L., 11-in. M.L., 12-in. M.L. (25 tons), 12-in. and 12'5-in. M.L. 
(38 tons), and 80-ton M.L. in the 14'5-in. calibre, about to be increased gradually to 15 and 16ins. 
