TEX: P = \frac{w \ v}{d^2 \ 576}]

for plates less than two and one-half inches thick, and

w v
P = ------ - 1.5
d squared 400

[TEX: P = \frac{w \ v}{d^2 \ 400} - 1.5]

for plates more than two and one-half inches thick.

If [theta] be the angle between the path of the projectile and the
face of the plate, then v in the above formulae becomes v sin [theta].

When we come to back the plates, their power to resist penetration
becomes greater, and our formula changes. The Gavre formula, given
above, is used to determine the velocity necessary for a projectile to
pass entirely through an iron plate and its wood backing.

Compound and steel armor are said to give about 29 per cent. more
resisting power than wrought iron, but in one experiment at the
proving ground, at Annapolis, a compound plate gave over 50 per cent.
more resisting power than wrought iron.

The Italian government, after most expensive and elaborate comparative
tests, has decided in favor of the Creusot or Schneider all-steel
plates, and has established a plant for their manufacture at Terni,
near Rome.

The French use both steel and compound plates; the Russians, compound;
the Germans, compound; the Swedes and Danes use both. Spain has
adopted and accepted the Creusot plate for its new formidable armored
vessel, the Pelayo; and China too has recently become a purchaser of
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