THE ROYAL ARTILLERY INSTITUTION. 
425 
The former, Bodman accounts for by the greater heat developed in a large 
than in a small charge. The small bore, too, abstracts a greater proportion 
of heat by means of the walls of the gun, and also allows a greater propor¬ 
tion of gas to escape by means of the vent. The increase of pressure towards 
the muzzle he believes “ to be due to the more violent and sudden contrac¬ 
tion in the thin than in the thick part of the gun. . . . Eor in the 
thick part of the gun the pressure is much less rapidly developed, and 
subsides much more gradually, the contained gas forming an elastic cushion, 
which would, if the bore were long enough, allow this part of the gun to 
return from its strained to its free condition, without any vibration at all: 
while in the model used in these experiments the pressure is almost instan¬ 
taneously developed and as suddenly subsides in the chase of the gun, so 
that while the indenting piston is on its way outward, it is suddenly met by 
the returning specimen, which is drawn in along with the housing by the 
contraction of the gun, with such violence as to amount in effect to a smart 
blow of the indenting tool against the specimen. Close examination shows 
a number of marks or cuts of the indenting tool on the specimen in this 
part of the gun, caused by the tool not striking in the same place at each 
vibration of the gun." 
If this explanation be the true one, it may account for the unsatisfactory 
results given by his gauge as to absolute pressures. That it does not give 
the absolute pressures is manifest, for in the 11-inch gun we find the pres¬ 
sure per square inch throughout the bore more than double that of the 
7-inch, and yet the resulting velocities remain the same, or nearly so. It 
is possible that it may represent the destructive action on the gun, and may 
be accounted for by a sudden pressure which sets up vibration in the 
powder-chamber to be continued throughout the bore, the greater intensity 
towards the muzzle being due to the same cause which makes the waves of 
the sea more violent as they come into shallow water.* That, however, the 
destructive action on the walls is the same in amount as that indicated by 
the gauges cannot be relied on, because the action shown by the gauges is 
the result of the compound vibration of both walls and indenting tool. 
He also tried the effects of varying the size of the grain of powder with 
the following results :—■ 
“ Table showing the velocities of shot and the pressure of gas, due to equal 
charges of powder, of the same composition, and differing only in size of grain— 
each result being the mean of five fires—with the 11-inch gun, the same shot being 
used in all the fires. 
1 
1 
i Diameter 
Weight of 
Weight of 
Initial 
Pressure of gas in tons. 
j of grain. 
charge. 
shot. 
velocity. 
At bottom. 
At 14". 
I 
At 28". 
! ins. 
•6 
lbs. 
| 12-67 
lbs. 
186-3 
f.s. 
933 
9-5 
4-6 
3-6 
; -5 
// 
n 
932 
9*5 
5-0 
3-3 
j *4 
n 
II 
881 
11-4 
4-8 
3-2 
1 '3 
u 
II 
890 
15-8 
4-8 
3-0 
j -3f 
a 
II 
912 
29-4 
6-6 
3-7 
* I have heard Professor Rankine give the explanation in a lecture on waves, at Newcastle- 
on-Tjne. 
f Powder of 1859, hut not so hard pressed as that of 1860. 
