Creusot plates.

Certain general rules may be laid down for attacking armor. If the
armor is iron, it is useless to attack with projectiles having less
than 1,000 feet striking velocity for each caliber in thickness of
plate. It is unadvisable to fire steel or chilled iron filled shells
at thick armor, unless a normal hit can be made. When perforation is
to be attempted, steel-forged armor-piercing shells, unfilled, should
be used. They may be filled if the guns are of great power as compared
to the armor. Steel and compound armor are not likely to be pierced by
a single blow, but continued hammering may break up the plate, and
that with comparatively low-powered guns.

Wrought iron must be perforated, and hard armor, compound or steel,
must be broken up. Against wrought iron plates the projectile may be
made of chilled cast iron, but hard armor exacts for its penetration
or destruction the use of steel, forged and tempered. Against
unarmored ships, and against unarmored portions of ironclads, the
value of rapid-firing guns, especially those of large caliber, can
hardly be overestimated.

The relative value of steel and compound armor is much debated, and at
present the rivalry is great, but the weight of evidence and opinion
seems to favor the all-steel plate. The hard face of a compound plate
is supposed to break up the projectile, that is, make the projectile
expend its energy on itself rather than upon the plate, and the
backing of wrought iron is, by its greater ductility, to prevent the
destruction of the plate. It seems probable that these two systems
will approach each other as the development goes on. An alloy of
nickel and steel is now attracting attention and bids fair to give