The Sun.
London, England, 1868-06-23 00:00:00

PLYMOUTH BREAKWATER TARGET.

The result of the experiments last week against the
Plymouth Breakwater Target shew conclusively that the
normal resistance of its weakest part is just overcome by
the force exerted by a 600-pound shell fired from the
12-inch rifle gun of 25 tons, with a battering charge of 76
pounds of powder, from a range of 200 yards. At the
same time it was proved that this force is incapable of
penetrating or doing much injury to those portions
immediately in front of the strengthening uprights
in rear, or to that portion of the target over which the
addditional layer of 5-inch plate was fixed. So that,
either by the introduction of more uprights, or by an ad-
ditional course of plating over the whole surface of the
face of the fort, we may be assured that the work will be
impenetrable by the most powerful gun now in existence,
at any range whatever. But here another question arises.
Is it probable that any more powerful gun may, in the
future, be made ? The answer is in the affirmative. Four
main features have been revealed by the practice against the
target and the two solid 15-inch plates. In the first
Place, it has been shewn that 15 inches of iron armour,
whether in three layers of 5 inches each or in a solid plate,
are incapable of resisting the 12-inch rifle gun at 200 yards.
Secondly, it has been satisfactorily shewn that the
structure supporting the roof could not be damaged by
horizontal fire unless the armoured wall be first breached ;
and this system of roof construction is capable of general
application. Thirdly, the trials chewed that the
bolts were capable of holding together the armour
plates, and of connecting properly the armour with the
uprights in rear. Very few bolts were fractured during
the trials. Fourthly, the rope mantlet which, when in
position, would hang between the uprights and the box
girder which supports the roof girders, was shown to be
quite capable of intercepting any fragments of armour,
bolts, nuts, &c., that might be driven back. In one
instance it caught a fragment of armour weighing
upwards of 8001 b., and only had two of its strands
cut. It is, however, doubtful whether it could
stop the fragments of a shell after their passage
through the armour, provided they retain moderate
velocity. The cost of the Plymouth fort might be roughly
taken at £3,000 per gun, exclusive of foundations and
turrets ; and it would perhaps be best strengthened by
putting an additional upright between each pair of those
now provided for, except at the port, where the addition
should be another armour plate, five inches thick, inside
the port, and extending between the port uprights.
These additions could be made even were the work
partially built, and the extra cost would be about
100, making the total £3,400 per gun, to keep out 12-inch
shells at 200 yards. But, looking to the coming 11-inch
guns, and considering the very uncertain and occasionally
disastrous effects of shells bursting between the plates
of an iron target composed of plates crossing each
other, it is more than probable that a safer and
cheaper protection will be found by using iron plates
of moderate thickness backed by other materials ;
and as wood is undesirable from its perishable
nature, we have to look to masonry to act as backing
when combined with iron. No experiments have yet been
made in this country with such a combination, and many
will probably be necessary before the proper forms and
proportions of the different materials can be determined.
The experiments to be carried on this week include the trial
of a casemate in which the front piers and wall are built of
Portland cement concrete, contained within cells of thin
iron plate ; outside this is a skin of two-inch plating, on
which eight inches of iron are variously disposed; thus on one
Portion of the front there is a solid 8-inch armour plate ;
next, adjoining this, a 41-inch plate with bridge-rail
stringers equal in volume to a W.,,-inch plate ; then 6-inch
plates with other stringers equal to a 2-inch plate ; then a
4-inch plate with stringers equal to another 4 inches ; and,
lastly, a 4:1-inch plate partly supported on stringers equal
to 31 inches, and partly backed by concrete and attached
to the left pier.