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 by water-tight bulkheads, and, having a reserve of flotation, the stability of the ship is not lost, even though the parts above the protective deck, forward and aft, be destroyed or filled with water.  The guns are protected by turrets or barbettes.  The deflective system consists in inclining the armor, or in so placing it that it will be difficult or impossible to make a projectile strike normal to the face of the plate.  A plate that is inclined to the path of a projectile will, of course, offer greater resistance to penetration than one which is perpendicular; hence, when there is no other condition to outweigh this one, the armor is placed in such a manner as to be at the smallest possible angle with the probable path of the projectile.  This system is designed to cause the projectile to glance or deflect on impact.  Deflective armor should be at such an angle that the projectiles fired at it cannot bite, and hence the angle will vary according to the projectile most likely to be used.  In the usual form of deflective deck the armor is at such a small inclination with the horizon that it becomes very effective.  Turret and barbette armor may be considered as deflective armor.  The term inclined armor denotes deflective armor that is inclined to the vertical.  The kinds of armor that are in use may be designated as rolled iron, chilled cast iron, compound, forged and tempered steel, and nickel steel.  Iron armor consists of wrought iron plates, rolled or forged, and of cast iron or chilled cast iron, as in the Gruson armor.  Compound armor consists of a forged combination of a steel plate and an iron plate.  Steel armor consists of wrought steel plates.  Nickel-steel armor consists of plates made from an alloy of nickel and steel.