364 Measurement of Powder Pressures in Cannon. [May, 
oil, and the breech-plug inserted afterward. A vent of the 
normal size was drilled just forward of the partition formed 
by the ring and disc just mentioned. 
It has already been intimated that the temperature at 
which the piezometer was set was that at which the gun was 
to be fired. In the earlier experiments the method by which 
it was intended to effect this was by inclosing the instru- 
ment, ready to be set, in the gun for a sufficient time to 
equalise their temperature — which was left to merely atmo- 
spheric influences. The instrument was then withdrawn, 
quickly set, and returned, and the gun fired without delay. 
It was afterwards found that the gun changed its tem- 
perature sensibly, from changes of wind and sky, in shorter 
intervals than was supposed. To avoid the errors liable to 
arise from this cause the practice was changed. The instru- 
ment was set at a determinate temperature, higher than 
that of the outside air, but lower than that of the arsenal 
workshops. The gun was run into the shops (near by) for 
a short time in the interval between the experiments, and 
made slightly warmer than the firing temperature. The 
piezometer was then properly inserted, the gun taken out- 
side, and when cooled to the proper point the thermometer 
was removed and the gun fired by a primer already inserted. 
The average pressure indicated at different distances from 
the bottom of the bore, the charge being in each case ij- lbs. 
of powder, in a cartridge-bag, and one 6-pounder shot, is 
given below — 
Distance from the Pressure per 
bottom of bore. sq. in. 
i-inch (bronze gun) 20,210 lbs. 
ij- „ (iron gun) 18,150 „ 
4 „ (bronze gun 16,510 „ 
The capability of the piezometer to measure pressures of 
much more brief duration than the time of the passage of 
the ball through the barrel will appear from a com- 
parison of the inertia to be overcome in each case. The 
weight of the piston and stem is 20 grs., and the resistance 
of the inertia of the oil is equivalent to that of about 36 
additional grs., while the weight of the pistol ball is 218 grs. 
After taking into account the differences of area and the 
mode of adbion in the two cases, I think it may be safely 
concluded that the piston will have reached its extreme 
position as soon at least as the ball has passed through the 
same distance. 
The superior practical importance of determining the 
greatest instead of the average pressure upon the sides of the 
