VELOCITY AND PRESSURE INSTRUMENTS. 369 
This exceedingly simple machine is shown in Plate IY. The idea of ascer¬ 
taining the pressure by the amount of “ crushing ” or compression of small 
cylinders of copper, originated, I believe, in the Royal Gun Factories—in 
which department these gauges were first manufactured. The instrument 
consists of a steel bush (Fig. 1), of the same dimensions as the vent-bush 
used for all our service guns, provided with a moveable base or nozzle screwed 
on to the bush. The interior of the nozzle is hollowed out so as to form a 
chamber, in which is placed a steel “ anvil/'’ with grooves along its surface com¬ 
municating with a channel corresponding with the vent-channel in a vent-bush. 
Thus any gas that may, by accident, find its way into the crusher-chamber has 
a free escape into the air. This channel has been omitted in the crushers 
for the 80-ton gun. In this chamber is placed a small cylinder of copper, 
A (Figs. 1 and 2), half-an-inch in length and inch in diameter. This 
cylinder is held lightly (but not prevented from expanding laterally) in the 
centre of the chamber, by a small watch-spring. One end rests against 
the anvil, while the other end is acted on by a piston, C (Fig. 1), which is 
moveable in the nozzle, and is pressed against the copper cylinder by a small 
spring. The lower end of this piston is -J- th inch in area (under ordinary 
circumstances) and is open to the pressure of the powder gas; a small brass 
cup, or “ gas-check,” D , being inserted to prevent the gas from penetrating 
past the piston. 
The apparatus is used as follows:—A hole is bored in the gun in the 
required position (see Plate V.), into which the gauge is screwed until the 
end of the plug is flush with the surface of the bore. On the explosion of 
the charge, the copper cylinder is crushed between the piston and the anvil, 
and the amount of compression is an indication of the pressure exerted by 
the gas at that point. The actual pressure corresponding to any given 
compression of the crusher-cylinder is arrived at by compressing a series of 
similar copper cylinders in the statical testing machine in the Royal Gun 
Factories, and tabulating the results. It is found that, with proper care in 
the selection and testing of the copper, the results are very reliable. Where 
extreme accuracy is required—as in the late experiments with the 80-ton 
gun—each copper cylinder is tested previous to being inserted in the 
gauge. This is done by crushing them all up to a given pressure (say 
18 tons on the square inch)—‘rather less than the pressure which the 
crusher-gauge is expected to record—which method of procedure has the 
additional advantage of eliminating some irregularities to which the crusher- 
gauge has been found, under certain circumstances, to be liable. The 
compression of the cylinder in the crusher-gauge is ascertained by removing 
the plug from the gun, unscrewing the nozzle, taking out the cylinder, and 
measuring its length by means of a micrometer reading to the one-thousandth 
of an inch. A reference to the tabulated results of the previous compression 
of similar coppers in the testing machine at once gives the pressure in tons 
on the square inch without any calculation. The gauge is cleaned, a fresh 
copper cylinder is inserted in the chamber, and the nozzle is screwed home, 
when the apparatus is ready for immediate use. 
In their first Progress Report, dated 5th February, 1870, the Committee 
on Explosives state “ that much greater dependence could be placed upon 
the results furnished by this apparatus than upon those obtained with the 
‘ Rodman 9 gauge, and for the following reasons :— 
