The motive power is gunpowder, and the
article to be produced is perhaps a hole in an armor-plate, perhaps a
breach in a concealed escarp, or perhaps destructive effect on troops.
These articles are quite distinct, and though all guns are capable of
producing them all to some extent, no gun is capable of producing more
than one in the highest state of excellence.
Thus, for armor piercing, a long pointed bolt, nearly solid, is
required. It must strike with great velocity, and must therefore be
propelled by a very large charge of powder. Hence an armor-piercing gun
should have a large chamber and a comparatively small bore of great
length.
For breaching fortifications, on the other hand, curved fire is
necessary; the escarps of modern fortresses are usually covered from
view by screens of earth or masonry in front, so that the projectiles
must pass over the crest of the screen, and drop sufficiently to strike
the wall about half-way down, that is to say, at an angle of 15 deg. to 20 deg..
To destroy the wall, shell containing large bursting charges of powder
are found to be particularly well adapted. Now it is clear that, for a
shell to drop at an angle of 15 deg.
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