420 
MINUTES OF PROCEEDINGS OF 
Charge in 
grains. 
Ratio of 
charge to 
space. 
■j 
Pressure in tons per 
square inch. 
Measured. 
Calculated. 
1 
•039 
0-5 
0-5 
2 
•078 
1-2 
1-1 
3 
•117 
1-5 
1-8 
4 
•156 
2-5 • 
2-6 
5 
•195 
3-7 
3-6 
6 
•234 
4-6 
4-8 
7 
•273 
5-4 
6-2 
8 
•312 
7-8 
7*8 
9 
•351 
10-3 
9-8 
10 
•390 
12-6 
12-1 
11 
•429 
14-8 
14-9 
12 
•468 
17-1 
18-1 
13 
•507 
21-9 
22-0 
14 
•546 
26-7 
26-5 
15 
•585 
31-5 
31-9 
16 
•624 
47*3 
38-2 
17 
•663 
45-6 
18 
•702 
73*2 
54-3 
19 
•741 
64-5 
20 
•780 
76-5 
26* 
1* 
194-5 
The curve in Tig. 2 shows the calculated pressures. They agree very 
well with the measured pressures up to 15 grains. The pressures afterwards 
are calculated without taking into account the higher pressures, which 
appeared to manifest themselves as the cannon began to fail. If these 
higher pressures were taken into account, Bumford considered that* 100,000 
atmospheres, or nearly 700 tons, would not be too great an estimate of the 
force of powder exploded in its own space.'* Piobert, however, considers 
that the higher pressures were probably due to the yielding of the metal at 
the top as it began to fail. 
Observing that when the space is three times that of the charge the 
pressure is, in round numbers, 10 tons; when it is twice, 20 tons; once 
and a half, 40 tons ; and once and a quarter, 80 tons; a very simple empirical 
formula, applicable to guns, may be given, founded on the supposition that 
* Some experiments have been made on this point by Captain Noble, of Elswick, who has fired 
by electricity as much as fib. of powder confined in cylinders of steel tempered in oil. The cylinders, 
which were 2 ins. internal diameter and If calibres thick, usually expanded •002 // or •003", 
and one as much as , 02 // external diameter. To produce this effect would, I consider, require an 
internal pressure of at least 100 tons per square inch; for the outside, where the tension would 
necessarily be least, would not yield permanently with a less strain than 30 tons, while the internal 
portions, being more expanded, would be under a greater strain. Some “ crusher gauges” were placed 
inside, and from them, I believe. Captain Noble deduced a maximum pressure of 40 tons. I do not 
know, however, what reliance is to be placed on their indications under such extreme pressures as 
they must have been subjected to; nor can I reconcile their indications with the expanding of the 
cylinders, which is more in harmony with Romford's results. 
