Roumania is thinking of protecting a portion of the artillery of the
forts surrounding her capital by metallic cupolas. But, before deciding
upon the mode of constructing these formidable and costly affairs, and
before ordering them, she has desired to ascertain their efficacy and
the respective merits of the chilled iron armor which was recently in
fashion and of rolled iron, which looks as if it were to be the fashion
hereafter.
[Illustration: FIG. 1.--MOUGIN'S ROLLED IRON TURRET.]
The Krupp works have recommended and constructed a cupola of
casehardened iron, while the Saint Chamond works have offered a turret
of rolled iron. Both of these recommend themselves by various merits,
and by remarkably ingenious arrangements, and it only remains to be seen
how they will behave under the fire of the largest pieces of artillery.
[Illustration: FIG. 2.]
We are far in advance of the time when cannons with smooth bore were
obliged to approach to within a very short range of a scarp in order to
open a breach, and we are far beyond that first rifled artillery which
effected so great a revolution in tactics.
[Illustration: FIG. 3.]
To-day we station the batteries that are to tear open a rampart at
distances therefrom of from 1,000 to 2,000 yards, and the long, 6 inch
cannon that arms them has for probable deviations, under a charge of 20
pounds of powder, and at a distance of 1,000 yards, 28 feet in range, 16
inches in direct fire and 8 inches in curved.
The weight of the projectile is 88 pounds, and its remanent velocity at
the moment of impact is 1,295 feet. Under this enormous live force, the
masonry gradually crumbles, and carries along the earth of the parapet,
and opens a breach for the assaulting columns.
[Illustration: FIG. 4--STATE OF A CUPOLA AFTER THE ACTION OF
THIRTY-SEVEN 6 IN. PROJECTILES.]
In order to protect the masonry of the scarp, engineers first lowered
the cordon to the level of the covert-way. Under these circumstances,
the enemy, although he could no longer see it, reached it by a curved or
"plunging" shot. When, in fact, for a given distance we load a gun with
the heaviest charge that it will stand, the trajectory, AMB (Fig. 2), is
as depressed as possible, and the angles, a and a', at the start and
arrival are small, and we have a direct shot. If we raise the chase of
the piece, the projectile will describe a curve in space which would be
a perfect parabola were it not for the resistance of the air, and the
summit of such curve will rise in proportion as the angle so increases.
So long as the falling angle, a, remains less than 45°, we shall have a
curved shot. When the angle exceeds this, the shot is called "vertical."
If we preserve the same charge, the parabolic curve in rising will meet
the horizontal plane at a greater distance off. This is, as well known,
the process employed for reaching more and more distant objects.
[Illustration: Fig. 5.--STATE OF A CAST-IRON CUPOLA AFTER THE BREAKAGE
OF A VOUSSOIR.]
The length of a gun depends upon the maximum charge burned in it, since
the combustion must be complete when the projectile reaches the open
air. It results from this that although guns of great length are capable
of throwing projectiles with small charges, it is possible to use
shorter pieces for this purpose--such as howitzers for curved shots and
mortars for vertical ones. The curved shot finds one application in the
opening of breaches in scarp walls, despite the existence of a covering
of great thickness. If, from a point, a (Fig. 3), we wish to strike the
point, b, of a scarp, over the crest, c, of the covert-way, it will
suffice to pass a parabolic curve through these three points--the
unknown data of the problem, and the charge necessary, being
ascertained, for any given piece, from the artillery tables. In such
cases it is necessary to ascertain the velocity at the impact, since the
force of penetration depends upon the live force (mv²) of the
projectile, and the latter will not penetrate masonry unless it have
sufficient remanent velocity. Live force, however, is not the sole
factor that intervenes, for it is indispensable to consider the angle at
which the projectile strikes the wall. Modern guns, such as the Krupp 6
inch and De Bange 6 and 8 inch, make a breach, the two former at a
falling angle of 22°, and the latter at one of 30°. It is not easy to
lower the scarps enough to protect them from these blows, even by
narrowing the ditch in order to bring them near the covering mass of the
glacis.
The same guns are employed for dismounting the defender's pieces, which
he covers as much as possible behind the parapet. Heavy howitzers
destroy the _materiel_, while shrapnel, falling nearly vertically, and
bursting among the men, render all operations impossible upon an open
terre-plein.
[Illustration: FIG. 6.--STATE OF A CHILLED IRON CUPOLA BROKEN BY A 12
INCH BALL.]
The effect of 6 and 8 inch rifled mortars is remarkable. The Germans
have a 9 inch one that weighs 3,850 pounds, and the projectile of which
weighs 300. But French mortars in nowise cede to those of their
neighbors; Col. De Bange, for example, has constructed a 10½ inch one of
wonderful power and accuracy.
Seeing the destructive power of these modern engines of war, it may well
be asked how many pieces the defense will be able to preserve intact for
the last period of a siege--for the very moment at which it has most
need of a few guns to hold the assailants in check and destroy the
assaulting columns. Engineers have proposed two methods of protecting
these few indispensable pieces. The first of these consists in placing
each gun under a masonry vault, which is covered with earth on all sides
except the one that contains the embrasure, this side being covered with
armor plate.
The second consists in placing one or two guns under a metallic cupola,
the embrasures in which are as small as possible. The cannon, in a
vertical aim, revolves around the center of an aperture which may be of
very small dimensions. As regards direct aim, the carriages are
absolutely fixed to the cupola, which itself revolves around a vertical
axis. These cupolas may be struck in three different ways: (1) at right
angles, by a direct shot, and consequently with a full charge--very
dangerous blows, that necessitate a great thickness of the armor plate;
(2) obliquely, when the projectile, if the normal component of its real
velocity is not sufficient to make it penetrate, will be deflected
without doing the plate much harm; and (3) by a vertical shot that may
strike the armor plate with great accuracy.
General Brialmont says that the metal of the cupola should be able to
withstand both penetration and breakage; but these two conditions
unfortunately require opposite qualities. A metal of sufficient
ductility to withstand breakage is easily penetrated, and, conversely,
one that is hard and does not permit of penetration does not resist
shocks well. Up to the present, casehardened iron (Gruson) has appeared
to best satisfy the contradictory conditions of the problem. Upon the
tempered exterior of this, projectiles of chilled iron and cast steel
break upon striking, absorbing a part of their live force for their own
breakage.
In 1875 Commandant Mougin performed some experiments with a chilled iron
turret established after these plans. The thickness of the metal
normally to the blows was 23½ inches, and the projectiles were of cast
steel. The trial consisted in firing two solid 12 in. navy projectiles,
46 cylindrical 6 in. ones, weighing 100 lb., and 129 solid, pointed
ones, 12 in. in diameter. The 6 inch projectiles were fired from a
distance of 3,280 feet, with a remanent velocity of 1,300 feet. The
different phases of the experiment are shown in Figs. 4, 5, and 6. The
cupola was broken; but it is to be remarked that a movable and
well-covered one would not have been placed under so disadvantageous
circumstances as the one under consideration, upon which it was easy to
superpose the blows. An endeavor was next made to substitute a tougher
metal for casehardened iron, and steel was naturally thought of. But
hammered steel broke likewise, and a mixed or compound metal was still
less successful. It became necessary, therefore, to reject hard metals,
and to have recourse to malleable ones; and the one selected was rolled
iron. Armor plate composed of this latter has been submitted to several
tests, which appear to show that a thickness of 18 inches will serve as
a sufficient barrier to the shots of any gun that an enemy can
conveniently bring into the field.
[Illustration: FIG. 7.--CASEMATE OF CHILLED IRON AFTER RECEIVING
NINETY-SIX SHOTS.]
_Armor Plated Casemates_.--Fig. 7 shows the state of a chilled iron
casemate after a vigorous firing. The system that we are about to
describe is much better, and is due to Commandant Mougin.
[Illustration: FIG. 8.--MOUGIN'S ARMOR-PLATE CASEMATE.]
The gun is placed under a vault whose generatrices are at right angles
to the line of fire (Fig. 8), and which contains a niche that traverses
the parapet. This niche is of concrete, and its walls in the vicinity of
the embrasure are protected by thick iron plate. The rectangular armor
plate of rolled iron rests against an elastic cushion of sand compactly
rammed into an iron plate caisson. The conical embrasure traverses this
cushion by means of a cast-steel piece firmly bolted to the caisson, and
applied to the armor through the intermedium of a leaden ring.
Externally, the cheeks of the embrasure and the merlons consist of
blocks of concrete held in caissons of strong iron plate. The
surrounding earthwork is of sand. For closing the embrasure, Commandant
Mougin provides the armor with a disk, c, of heavy rolled iron, which
contains two symmetrical apertures. This disk is movable around a
horizontal axis, and its lower part and its trunnions are protected by
the sloping mass of concrete that covers the head of the casemate. A
windlass and chain give the disk the motion that brings one of its
apertures opposite the embrasure or that closes the latter. When this
portion of the disk has suffered too much from the enemy's fire, a
simple maneuver gives it a half revolution, and the second aperture is
then made use of.
_The Schumann-Gruson Chilled Iron Cupola_.--This cupola (Fig. 9) is
dome-shaped, and thus offers but little surface to direct fire; but it
can be struck by a vertical shot, and it may be inquired whether its top
can withstand the shock of projectiles from a 10 inch rifled mortar. It
is designed for two 6 inch guns placed parallel. Its internal diameter
is 19½ feet, and the dome is 8 inches in thickness and has a radius of
16½ feet. It rests upon a pivot, p, around which it revolves through the
intermedium of rollers placed in a circle, r. The dome is of relatively
small bulk--a bad feature as regards resistance to shock. To obviate
this difficulty, the inventor partitions it internally in such a way as
to leave only sufficient space to maneuver the guns. The partitions
consist of iron plate boxes filled with concrete. The form of the dome
has one inconvenience, viz., the embrasure in it is necessarily very
oblique, and offers quite an elongated ellipse to blows, and the edges
of the bevel upon a portion of the circumference are not strong enough.
In order to close the embrasure as tightly as possible, the gun is
surrounded with a ring provided with trunnions that enter the sides of
the embrasure. The motion of the piece necessary to aim it vertically is
effected around this axis of rotation. The weight of the gun is balanced
by a system of counterpoises and the chains, l, and the breech
terminates in a hollow screw, f, and a nut, g, held between two
directing sectors, h. The cupola is revolved by simply acting upon the
rollers.
[Illustration: FIG. 9.--THE SCHUMANN-GRUSON CUPOLA.]
_Mougin's Rolled Iron Cupola_.--The general form of this cupola (Fig. 1)
is that of a cylindrical turret. It is 12¾ feet in diameter, and rises
3¼ feet above the top of the glacis. It has an advantage over the one
just described in possessing more internal space, without having so
large a diameter; and, as the embrasures are at right angles with the
sides, the plates are less weakened. The turret consists of three plates
assembled by slit and tongue joints, and rests upon a ring of strong
iron plate strengthened by angle irons. Vertical partitions under the
cheeks of the gun carriages serve as cross braces, and are connected
with each other upon the table of the hydraulic pivot around which the
entire affair revolves. This pivot terminates in a plunger that enters a
strong steel press-cylinder embedded in the masonry of the lower
concrete vault.
The iron plate ring carries wheels and rollers, through the intermedium
of which the turret is revolved. The circular iron track over which
these move is independent of the outer armor.
The whole is maneuvered through the action of one man upon the piston of
a very small hydraulic press. The guns are mounted upon hydraulic
carriages. The brake that limits the recoil consists of two bronze pump
chambers, a and b (Fig. 10). The former of these is 4 inches in
diameter, and its piston is connected with the gun, while the other is 8
inches in diameter, and its piston is connected with two rows of 26
couples of Belleville springs, d. The two cylinders communicate through
a check valve.
When the gun is in battery, the liquid fills the chamber of the 4 inch
pump, while the piston of the 8 inch one is at the end of its stroke. A
recoil has the effect of driving in the 4 inch piston and forcing the
liquid into the other chamber, whose piston compresses the springs. At
the end of the recoil, the gunner has only to act upon the valve by
means of a hand-wheel in order to bring the gun into battery as slowly
as he desires, through the action of the springs.
[Illustration: FIG. 10.--MOUGIN'S HYDRAULIC GUN CARRIAGE.]
For high aiming, the gun and the movable part of its carriage are
capable of revolving around a strong pin, c, so placed that the axis of
the piece always passes very near the center of the embrasure, thus
permitting of giving the latter minimum dimensions. The chamber of the 8
inch pump is provided with projections that slide between circular
guides, and carries the strap of a small hydraulic piston, p, that
suffices to move the entire affair in a vertical plane, the gun and
movable carriage being balanced by a counterpoise, q.
The projectiles are hoisted to the breech of the gun by a crane.
Between the outer armor and turret sufficient space is left for a man to
enter, in order to make repairs when necessary.
Each of the rolled iron plates of which the turret consists weighs 19
tons. The cupolas that we have examined in this article have been
constructed on the hypothesis than an enemy will not be able to bring
into the field guns of much greater caliber than 6 inches.--_Le Genie
Civil_.
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