manageable substance, in the hope of improving its uniformity, and
rendering it thoroughly trustworthy. The difference in strength, when
both are sound, is great. Roughly, gun steel is about twice as strong as
wrought iron.

I must now say a few words on the nature of the strains to which a piece
of ordnance is subjected when fired. Gunpowder is commonly termed an
explosive, but this hardly represents its qualities accurately. With a
true explosive, such as gun-cotton, nitro glycerine and its compounds,
detonation and conversion of the whole into gas are practically
instantaneous, whatever the size of the mass; while, with gunpowder,
only the exterior of the grain or lump burns and gives off gas, so that
the larger the grain the slower the combustion. The products consist of
liquids and gases. The gas, when cooled down to ordinary temperature,
occupies about 280 times the volume of the powder. At the moment of
combustion, it is enormously expanded by heat, and its volume is
probably somewhat about 6,000 times that of the powder. I have here a
few specimens of the powders used for different sizes of guns, rising
from the fine grain of the mountain gun to the large prisms and
cylinders fired in our heavy ordnance. You will readily perceive that,
with the fine-grained powders, the rapid combustion turned the whole
charge into gas before the projectile could move far away from its seat,
setting up a high pressure which acted violently on both gun and shot,
so that a short, sharp strain, approximating to a blow, had to be
guarded against.

With the large slow-bursting powders now used, long heavy shells move
quietly off under the impulse of a gradual evolution of gas, the
presence of which continues to increase till the projectile has moved a
foot or more; then ensues a contest between the increasing volume of the