THE 80-TON GUN. 
351 
results. The first five columns speak for themselves : the sixth gives 
the maximum pressure observed in the bore. It will be remarked that 
except in three rounds—all of whieh were fired with a very mild 
powder—the greatest strain occurs at A; that is, on the gauge placed 
in the axis, in the centre of the bottom of the bore. The gauges 
1, 2, 3, &c., are placed in the walls of the gun at 1, 2, 3, &c., feet from 
the bottom of the bore; and since the cartridge, when rammed home, 
takes up a little more or less than 3 ft.—according to the charge—the 
mean pressure in the powder-chamber will be gathered from the indica¬ 
tions given by the gauges A, 1, 2, and 3. Column 7 is thus formed. 
Before entering upon the information given by a comparison between 
columns 6 and 7, it will be convenient to describe the powders employed. 
The ingredients are in the same proportion and mixed in the same 
way as in the L.Gi., the R.L.G., and the Pebble powders used in the 
service. The differences consist in the density and size of grain. 
With regard to density, though it is found desirable in manufacture to 
vary it according to circumstances, yet in the large cubical powders 
now in question it never falls below 1'75, whereas the L.G. and R.L.G. 
powders are considerably lighter. Size of grain is the true controlling 
element, and the three sizes tried in the 80-ton gun rise from cubes 
of ljin. edge, weighing about six to the pound, to cubes whose edge 
is 2 ins., weighing about five to 2 lbs. 
Powder is not properly so much an explosive, as a substance burning 
with great rapidity. It offers in this respect a marked contrast to gun¬ 
cotton, dynamite, and other true explosives. If one of these agents be 
detonated, the detonation is immediately carried through the mass, what¬ 
ever its size, and the whole at once turns into gas. Gunpowder, on the 
other hand, as far as is known at present, cannot be detonated; but 
simply evolves its gas by burning in layers from the outside to the 
inside. It will be readily understood that a large grain will take longer 
to burn up and become entirely converted into gas than a small one 
will, and hence the new cubical grains of large size give off their gas 
with deliberation sufficient to permit the shot to move quietly away 
before excessive pressures can be set up. 
If a powder is of too violent or rapidly-burning a character, a 
vibratory or “ wave ” action will be induced in the chamber, and this it 
is particularly desired to avoid, as it is injurious to material and useless 
towards the production of velocity. The ideal powder would be one 
which should burn slowly and regularly, giving off its gas so as to yield 
gradually increasing pressures equal in all parts of the powder-chamber 
at the same time. In other words, the figures in column 6 and column 7 
of table 0 should be identical. The greater the difference the less 
suitable is the powder; though, since perfection can never be reached, 
a slight departure from uniformity must be tolerated if good velocity be 
obtained. 
It was decided by the Committee on Explosives that no charge should 
be considered admissible which gave a mean pressure in the powder- 
chamber exceeding 25 tons on the square inch; and the powder which 
would give the highest velocity, without exceeding this strain and 
without departing greatly from uniformity of action, would evidently be 
46 
