[Illustration: Fig. 1.]

European nations took up the subject of armor and energetically
conducted experiments which have cost large sums of money, but have
given much valuable data. For a long time wrought iron was the only
material used for armor, and the resisting power depending on the
thickness; and the caliber and penetration of guns rapidly increasing,
it was not long before a point was reached where the requisite
thickness made the load of armor so great that it was impracticable
for a ship to carry it. The question then arose as to what were the
most important parts of a ship to protect. The attempted solutions of
this question brought out various systems of distributions.

Armored ships were formerly of two classes; in one the guns were
mounted in broadside, in the other in turrets. Every part of the ship
was protected with iron to a greater or less thickness. In more modern
ships the guns are mounted in an armored citadel, in armored barbettes
or turrets, the engines, boilers and waterline being the only other
parts protected. There may be said to be three systems of armor
distribution. The belt system consists in protecting the whole
waterline by an armored belt, the armor being thickest abreast of the
engines and boilers. The guns are protected by breastworks, turrets or
barbettes, the other parts of the ship being unprotected. The French
use the belt system, and our own monitors may be classed under it. The
central citadel system consists in armoring that part of the waterline
which is abreast of the engines and boilers. Forward and aft the
waterline is unprotected, but a protective deck extends from the
citadel in each direction, preventing the projectiles from entering
the compartments below. The hull is divided into numerous compartments