64 PRINCIPLES OP GUNNERY. 
f.s. 
(1) Increase in velocity due to lengthening the bore 3 ft.* ... = 54 
(2) Increase in velocity due to increase in calibre from 12 ins. 
to 125 ins. . = 30 
(3) Increase in velocity due to use of gas-check . = 38 
(4) Increase in velocity due to adoption of cubical pebble, and 
increasing charge from 110 lbs. P. to 130 lbs. P 2 . . = 35 
(5) Increase in velocity due to still greater increase of charge 
from 130 lbs. to 180 lbs. P. 3 , made practicable by 
allowing air-space and chambering . = 150 
position of The position of the vent, or the point of ignition of the cartridge, 
the vent. ma k es a difference (especially when R.L.G. powder is employed) in 
the rate of explosion of the charge, and influences the muzzle velocity 
of the projectile, and the pressure in the powder-chamber. 
All the heavy M.L. guns are vented on the same principle— i.e., the 
position of the vent (where the cartridge is ignited) is fixed at a 
distance of r V hs of the length of the battering charge from the 
bottom of the bore. This position of the vent was found to develop 
the greatest muzzle velocity, when firing large charges of R.L.G. 
powder. It was found with cubical pebble powder that there w~as no dif¬ 
ference in muzzle velocity, whether the cartridge was lighted at the rear 
or in the service position, but that there was a considerable increase in 
the chamber pressure near the base of the projectile when lighted at 
the rear. Arrangements have therefore been made in the 80-ton gun 
to light the cartridge at the service position (although using an axial 
vent), in order to keep the chamber pressure as low as possible. 
Field and siege guns are rear vented—(1) so as to ensure the ignition 
of the cartridge, when firing reduced charges, (2) so as to expel the debris 
of the cartridge of a preceding round, when firing service charges, in 
order to avoid an accident in re-loading. But if these guns were vented 
near the centre of the cartridge, as in the heavy guns, there would be 
a gain in muzzle velocity. Experiments have been carried on with 
rear and forward vents, with the 16-pr., 25-pr.,- and 40-pr. guns. 
Table showing the Effect of altering the Position of the Vent on Muzzle Velocity. 
Gun. 
Charge, 
R.L.G. 
Muzzle velocity 
with rear vent. 
Muzzle velocity 
with forward vent. 
Gain 
in M.V. 
lbs. 
f.s. 
f.s. 
f.s. 
16-pr. 
3 
1358 
1397 
39 
25-pr. 
5 
1332 
1430 
98 
40-pr. 
8 
1326 
1470 
144 
In the Armstrong B.L. guns the point of ignition is in the axis of 
the piece; so that the cartridge is ignited at one end, and probably 
takes a longer time to explode than if ignited near the centre. 
There are so many elements of variation with regard to the explosion 
of different charges of different powders fired from different guns, 
already pointed out, that sometimes apparently contradictory results 
are obtained; so that it may be concluded that probably the best 
position of vent in any gun is that one which offers the most convenience 
and safety in practice; and then, if necessary, the rate of explosion can 
That is, the bore was lengthened from 162 to 198 ins, 
