not have time to act before the inner parts are strained
beyond endurance. In order to bring all parts of a great mass of metal
into simultaneous tension, Blakely and others have hooped an inner tube
with rings having a successively higher initial tension. The inner tube
is therefore under compression, and the outer ring under a considerable
tension, when the gun is at rest, but all parts are strained
simultaneously and alike when the gun is under pressure. The Parrott and
Whitworth cannon are constructed on this principle, and there has been
some practice in winding tubes with square steel wire to secure the most
uniform gradation of tension at the least cost. There is some difficulty
as yet in fastening the wire and giving the gun proper longitudinal
strength. Mr. Wiard, of New York, makes an ingenious argument to show
that large cannon burst from the expansion of the inner part of the gun
by the heat of frequent successive explosions. In this he is sustained
to some extent by Mr. Mallet, of Dublin. The greater the enlargement of
the inner layer of metal, the less valuable is the above principle of
initial tension. In fact, placing the inner part of the gun in initial
tension and the outer part in compression would better resist the
effect of internal heat. But Mr. Wiard believes that the _longitudinal_
expansion of the inner stratum of the gun is the principal source of
strain. A gun made of annular tubes meets this part of the difficulty;
for, if the inner tube is excessively heated, it can elongate and slip
a little within those surrounding it, without disturbing them. In fact,
the inner tube of the Armstrong gun is sometimes turned within the
others by the inertia of the rifled projectile. On the whole, then,
hooping an inner steel tube with successively tighter steel rings, or,
what is better, tubes, is the probable direction of improvement in heavy
ordnance. An inner tube of iron, cast hollow on Rodman's plan, so as to
avoid an inherent rupturing strain, and hooped with low-steel without
welds, would be cheaper and very strong. An obvious conclusion is,
that perfect elasticity in the metal would successfully meet all the
foregoing causes of rupture.

In America, where guns made entirely of cast-iron, and undoubtedly the
best in the world for horizontal shell-firing, are persisted in, though
hardly adequate to the heavy charges demanded by iron-clad warfare, the
necessity of decreasing the strain on the gun withou