The Sun.
London, England, 1867-12-26 00:00:00

PLYMOUTH BREAKWATER FORT.

A more serious subject is that of the Plymouth
Breakwater Fort, because contracts have been made
for its ironwork and the manufacture of its parts is
actually progressing. An experimental iron front
has been constructed to represent the Plymouth
fort as nearly as possible ; we say " as nearly as
possible," because, though the test of a simple shield
at Shoeburyness will give an accurate measure of
its strength wherever it may be placed, the condi-
tions are different when a whole fort has to be
tested. The strength of a house is not to be mea-
sured by that of one of its walls standing alone, and
some additions have been made to the target of
Shoeburyness to bring its stability more nearly on a
level with that of the fort itself.

Externally the front represents an iron plated
target, 21ft. 9in. long and about I?ift. high, but
the face is not vertical. It slopes back one foot in
the first 11 of its height, and, above that, the iron
plating, reduced to a thickness of 3ba., retires at an
angle of 45 deg.
The outer armour is composed of three 5-inch
plates, running horizontally, out of the lower two
of which the port, 2ift. wide and 3ft. high, is cut.
In rear of this armour is a layer of iron planks
about 16in. wide and Sin. thick, placed vertically.
Behind these is a layer of similar planks above
and plates below, placed horizontally, and so as
to break joint with the other planks and armour-
plates. The total thickness of iron is, therefore,
15in.

To support the armour, a number of iron girders,
five in the Shoeburyness front, run up the back of
the iron planks and armour, to which they are
bolted by through bolts. Each girder consists of
two iron planks, lit. wide and sin. thick, placed
6in. apart, and connected at the top and bottom by
cross bolts, while the bolts which pass through the
armour and between the girder planks are nutted
down to cross pieces of iron liin. thick, which
connect the girder planks and bind the whole iron-
work firmly together. But the fort is built of iron
on a granite foundation, and it .is necessary that
the tough plates be securely imbedded in the solid
stone. To this end an iron plate 2ft. 7in. wide and
24in. thick, is laid in a recess cut in the granite
floor, and extends along the whole front. It abuts
against the inside of the lowest armour-plate, and
carries the planks and back lower plates, their
edges being laid upon it. Holes cut through it
and about six inches deep into the granite receive
the feet of the girders, which are thus rooted in the
solid foundation, and serve to hold firm the foot of
the armour.

The plating and its upright girders abut at the
top against an iron plate connecting the ends of
longitudinal girders, which support the arches
of the casemate roof, and are in their turn held up
by a box girder, supported at its extremities upon
piers of brick in cement 4f t. thick, cased with !inc.
iron plates. Between these piers and the rear of
the armour plate girders a clear space is left, for
two reasons—first, to prevent any damage which
may have been done to the front from affecting in-
juriously the stability of the piers and roof ;
secondly, to allow the bolts passing through the
armour and its girders to be reached for removal or
alteration.

We have said that the experimental front differs
in some particulars from the arrangements in the
fort itself, and we now proceed to detail them.
The arches of the experimental front have tie rods
to compensate for the want of continuity of thrust
gained in the complete work.
The front of the true fort is oval ; that of the
experimental one is straight, to lessen the expense.
This involves a weakness, and obliges the shape
of the pier to be slightly. altered. An old piece of
masonry being utilised, there is not room for an
entire pier. Its dimensions are, therefore, reduced.
The spacing of the roof girders is not quite the
same, because the target, representing only a part
of the real fort, could not have its girders project-
ing in the air. The ends of the plates want the
continuity of support obtained in the fort. This
is partly met
~by increasing the size of the end
bolts, but in any case the ends must be weaker
than the same spots in the fort. Muzzle pivoting
carriages are to be used in the future, so the port is
left very small. This is an alteration from the origi-
nal design, but is ordered also for the fort. The
substitution of an iron plate in the hard thickness
instead of iron planks is also new, but ordered for
the fort. Spare test plates will supply the icon
wanted for the purpose. The experimental work has,
however, one advantage in this respect, that the
hind plate will be about 21ft. 9in., while in the real
fort the length is about 14ft.
It would be quite impossible to leave the piers
and iron roof supports as they are, and to substitute
other forms of armour for the one in favour at the
War-office, but it is estimated that any of the pro-
posed plans would involve an additional expense.
For instance, while the iron work for the experi-
mental front costs £2,500 per gun, plates and com-
pound backing such as those previously mentioned
in our description of the armoured casematea are
estimated to cost about £2,800 per gun ; and
Messrs. Brown and Co.'s proposal of solid plates
14in. or 15in. in thickness held between vertical
iron beams, with recesses to take the edges of the
plates, is calculated to involve an outlay of almost
£3,500 per gun for iron work alone. We give these
figures with some diffidence ; they are received as
fair estimates at the War-office, but are open to
correction from any firm which may offer to take
contracts at a lower price. Should cast-iron be
proved capable of making a good backing, an Bin.
plate could be backed by 18in. of cast-iron, with
an inner skin of tin. plates, for about £2,000 per
gun.