_Condition of Greatest Effect_. It is a well-settled rule, that the
penetration projectiles is proportionate directly to their weight
and diameter, and to the square of their velocity. For example, the
10-1/2-inch Armstrong 150-pound shot, thrown by 50 pounds of powder at
1,770 feet per second, has nearly twice the destructive effect upon
striking, and four times as much upon passing its whole diameter through
armor, as the 15-inch 425-pound shot driven by the same powder at 800
feet. The American theory is, that very heavy shot, at necessarily low
velocities, with a given strain on the gun, will do more damage, by
racking and straining the whole structure than lighter and faster shot
which merely penetrate. This is not yet sufficiently tested. The late
remarkable experiments in England--firing 130-and 150-pound Whitworth
steel shells, holding 3 to 5 pounds of powder, from a 7-inch Armstrong
gun, with 23 to 27 pounds of powder, through the Warrior target, and
bursting them in and beyond the backing--certainly show that large
calibres are not indispensable in fighting iron-clads. A destructive
blow requires a _heavy charge of powder_; which brings us to
_The Strain and Structure of Guns, and Cartridges_. The problem is, 1st,
to construct a gun which will stand the heaviest charge; 2d, to reduce
the strain on the gun without reducing the velocity of the shot. It
is probable that powder-gas, from the excessive suddenness of its
generation, exerts a percussive as well as a statical pressure, thus
requiring great elasticity and a certain degree of hardness in the
gun-metal, as well as high tensile strength. Cast-iron and bronze are
obviously inadequate. Solid wrought-iron forgings are not all that could
be desired in respect of elasticity and hardness, but their chief defect
is want of homogeneity, due to the crude process of puddling, and to
their numerous and indispensable welds. Low cast-steel, besides being
elastic, hard, tenacious, and homogeneous, has the crowning advantage of
being produced in large masses without flaw or weld. Krupp, of Prussia,
casts ingots of above 20 tons' weight, and has forged a cast-steel
cannon of 9 inches bore. One of these ingots, in the Great Exhibition,
measured 44 inches in diameter, and was uniform and fine-grained
throughout. His great success is chiefly due to the use of manganesian
iron, (which, however, is inferior to the Franklinite of New Jersey,
because it contains no zinc,) and to skill in heating the metal, and to
the use of heavy hammers. His heaviest hammer weighs 40 tons, falls 12
feet, and strikes a blow which does not draw the surface like a light
hammer, but compresses the whole mass to the core. Krupp is now
introducing the Bessemer process for producing ingots of any size at
about the cost of wrought-iron. These and other makes of low-steel
have endured extraordinary tests in the form of small guns and other
structures subject to concussion and strain; and both the theory and all
the evidence that we have promise its superiority for gun-metal. But
another element of resistance is required in guns with thick walls. The
explosion of the powder is so instantaneous that the exterior parts of
the metal do 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 without greatly reducing
the velocity of the shot has become imperative. It would be impossible
even to recapitulate the conflicting arguments of the experts on this
subject, within the limits of this paper. It does appear from recent
experiments, however, that this result can be accomplished by
compressing the powder, so that, we will suppose, it burns slowly and
overcomes the inertia of the shot before the whole mass is ignited; and
also by leaving an airspace around the cartridge, into which the gases
probably expand while the inertia of the shot is being overcome, thus
avoiding the excessive blow upon the walls of the gun during the first
instant of the explosion. Whatever the cause may be, the result is of
the highest importance, not only as to cast-iron guns, but as to all
ordnance, and warrants the most earnest and thorough investigation. The
principles of the Armstrong gun differ in some degree from all those
mentioned, and will be better referred to under the head of _Heavy
Ordnance Described_. The Armstrong gun is thus fabricated. A long bar of
iron, say 3 by 4 inches in section, is wound into a close coil about 2
feet long and of the required diameter,--say 18 inches. This is set upon
end at a welding heat under a steam-hammer and "upset" into a tube which
is then recessed in a lathe on the ends so as to fit into other tubes.
Two tubes set end to end are heated to welding, squeezed together by
a heavy screw passing through them, and then hammered lightly on the
outside without a mandrel. Other short tubes are similarly added. Five
tubes of different lengths and diameters are turned and bored and shrunk
over one another, without successively increasing tension, however, to
form a gun. The breech-end of the second tube from the bore is forged
solid so that its grain will run parallel with the bore and give the
gun longitudinal strength. Both the wedge and the screw breech-loading
apparatus are employed on guns of 7 inches bore (110-pounders) and
under. It will thus be seen that the defects of large solid forgings are
avoided; that the iron may be well worked before it is formed into a
gun; and that its greatest strength is in the direction of the greatest
strain; and on the other hand, that the gun is weak longitudinally and
excessively costly, (the 7-inch gun costs $4,000, and tin 10-1/2-inch,
$9,000,) and that the material, although strong and pretty trustworthy
in the shape of bars, has insufficient elasticity and hardness. Still,
it is a formidable gun, especially when relieved of the weak and complex
breech-loading apparatus, and used with a better system of rifling and
projectiles than Armstrong's. The 110-pounder Armstrong rifle has 99-1/2
inches length and 7 inches diameter of bore, 27 inches maximum diameter,
and weighs 4-1/3 tons. The "300-pounder" smooth-bore has 11 feet length
and 10-1/2 inches diameter of bore, 38 inches maximum diameter, and
weighs 10-1/2 tons. The Mersey Iron-Works guns are of wrought-iron, and
are forged solid like steamboat-shafts, or hollow by laying up staves
into the form of a barrel and welding layers of curved plates upon
them until the whole mass is united. But few of these guns have
been fabricated. The most remarkable of them are, 1st, the Horsfall
smooth-bore, of 13 inches bore, 44 inches maximum diameter, and 24
tons weight,--price, $12,500; 2d, the "Alfred" rifle, in the recent
Exhibition, of 10 inches bore,--price, $5,000; 3d, the 12-inch
smooth-bore in the Brooklyn Navy-Yard, which, though very light, has
fired a double 224-pound shot with 45 pounds of powder: if properly
hooped, it would make the most formidable gun in America. Blakely has
constructed for Russia two 13-inch smooth-bore guns, 15 feet long and 47
inches maximum diameter, of cast-iron hooped with steel: price, $10,000
each. He has also fabricated many others of large calibre, on the
principles before mentioned. The 15-inch Rodman smooth-bore cast-iron
gun is of 48 inches maximum diameter, 15 feet 10 inches long, and weighs
25 tons. The cost of such guns is about $6,000. The Dahlgren 15-inch
guns on the Monitors are about four feet shorter.
_Results of Heavy Ordnance_. The 10-1/2-inch Armstrong gun sent a round
150-pound shot, with 50 pounds of powder, through a 5-1/2-inch solid
plate and its 9-inch teak backing and 5/8-inch iron lining, at 200
yards, and one out of four shots with the same charge through the
Warrior target, namely, a 4-1/2-inch solid plate, 18-inch backing, and
5/8-inch lining. The Horsfall 13-inch gun sent a round 270-pound shot,
with 74 pounds of powder, entirely through the Warrior target at 200
yards, making an irregular hole about 2 feet in diameter. The same
charge at 800 yards did not make a clean breach. The Whitworth
shell burst in the backing of the same target has been referred to.
Experiments on the effect of the 15-inch gun are now in progress. Its
hollow 375-pound shot (3-inch walls) was broken without doing serious
damage to 10-1/2-inch laminated armor backed with 18 inches of oak.
The comparative test of solid and laminated armor has already been
mentioned. The best 4-1/2-inch solid plates, well backed, are
practically proof against the guns of English iron-clads, namely,
68-pounder smooth-bores and Armstrong 110-pounder rifles, the service
charge of each being 16 pounds.
_Rifling and Projectiles_. The spherical shot, presenting a larger area
to the action of the powder, for a given weight, than the elongated
rifle-shot, has a higher initial velocity with a given charge; and all
the power applied to it is converted into velocity, while a part of the
power applied to the rifle-shot is employed in spinning it on its axis.
But, as compared with the rifle-shot, at long ranges, it quickly loses,
1st, velocity, because it presents a larger area to the resisting air;
2d, penetration, because it has to force a larger hole through the
armor; and 3d, accuracy, because the spinning of the rifle-shot
constantly shifts from side to side any inaccuracy of weight it may
have on either side of its centre, so that it has no time to deviate in
either direction. Practically, however, iron-clad warfare must be
at close quarters, because it is almost impossible to _aim_ any gun
situated on a movable ship's deck so that it will hit a rapidly moving
object at a distance. It is believed by some authorities that elongated
shot can be sufficiently well balanced to be projected accurately
from smooth-bores; still, it is stated by Whitworth and others that a
spinning motion is necessary to keep an elongated shot on end while
passing through armor. On the whole, so far as penetrating armor is
concerned, the theory and practice favor the spherical shot. But a more
destructive effect than mere penetration has been alluded to,--the
bursting of a shell within the backing of an iron-clad vessel. This
can be accomplished only by an elongated missile with a solid head for
making the hole and a hollow rear for holding the bursting charge. The
rifle-shot used in America, and the Armstrong and some other European
shot, are covered with soft metal, which in muzzle-loaders is expanded
by the explosion so as to fill the grooves of the gun, and in
breech-loaders is planed by the lands of the gun to fit the
rifling,--all of which is wasteful of power. Whitworth employs a solid
iron or steel projectile dressed by machinery beforehand to fit the
rifling. But as the bore of his gun is hexagonal, the greater part of
the power employed to spin the shot tends directly to burst the gun.
Captain Scott, R.N., employs a solid projectile dressed to fit by
machinery; but the surfaces of the lands upon which the shot presses are
radial to the bore, so that the rotation of the shot tends, not to split
the gun, but simply to rotate it in the opposite direction.
_Mounting Heavy Ordnance_, so that it may be rapidly manoeuvred on
shipboard and protected from the enemy's shot, has been the subject of
so much ingenious experiment and invention, that in a brief paper it can
only be alluded to in connection with the following subject:--