1879J Measurement of Powder Pressures in Cannon. 361 
so as to stand upright within it : and a pair of wooden 
pinchers for handling the piezometer, which instrument 
could be inserted in them in such a way as to be nearly 
enveloped and yet to leave the adjusting-screw and piston 
readily accessible. The piezometer, seized in the pinchers, 
is placed at the bottom of the cup in company with the tools 
to be used in setting it, and is covered with a loose wad of 
cotton. The cup is set in the middle of the bath and sur- 
rounded with water, kept as nearly as possible at the desired 
temperature, for a sufficient time to impart, as nearly as 
appreciable, the same to the instrument. It is then with- 
drawn, the screw loosened, the piston depressed a little to a 
regulated depth with a special tool, the screw tightened, 
and the piston rotated a few degrees, which completes the 
setting. The objedt of this last movement is to inscribe a 
transverse line on the stem, affording a starting point in 
measuring the length of the stroke. 
Small changes of temperature after the instrument is set 
are of no consequence, as the oil will of course return to the 
same volume, and the piston stand at the same place, on 
returning to the same temperature. 
Before placing the piezometer in the hollow plug, a thin 
leather envelope, kept saturated with oil, is drawn upon it 
(with the intention of affording protection against the shock 
of firing), and when inserted the remaining space within the 
plug is filled with oil, which is retained by stopping the 
opening through the retaining-ring (which forms the com- 
munication with the bore of the gun), with a loosely-fitted 
disc of cork or leather. 
One particular to be noted is the position of the eye of 
the piston with reference to the line in which the gun will 
recoil on firing. The metal surrounding the eye occupies a 
position at one side of the piston’s axis of rotation in the 
barrel, thereby throwing the centre of gyration out of that 
line ; and if that centre be so situated as to fall outside of a 
plane coincident with the line of recoil, it is evident that 
the piston will have a tendency to rotation when the gun is 
fired. It was apprehended that this rotation might interfere 
with the proper marking of the stem, or accurate measure- 
ment of the mark, and for that reason care was usually taken 
to place the piston in such position that its centre of gyra- 
tion should be nearly in the plane just mentioned and for- 
ward of its axis of rotation. 
After firing, the length of the stroke was measured under 
a compound microscope by the application of a scale divided 
into thousandths of an inch and capable of being read to ten- 
thour.andths. 
