Armor Plated Casemates.—­Fig. 7 shows the state of a chilled iron casemate after a vigorous firing.  The system that we are about to describe is much better, and is due to Commandant Mougin.

[Illustration:  Fig. 8.—­Mougin’s armor-plate casemate.]

The gun is placed under a vault whose generatrices are at right angles to the line of fire (Fig. 8), and which contains a niche that traverses the parapet.  This niche is of concrete, and its walls in the vicinity of the embrasure are protected by thick iron plate.  The rectangular armor plate of rolled iron rests against an elastic cushion of sand compactly rammed into an iron plate caisson.  The conical embrasure traverses this cushion by means of a cast-steel piece firmly bolted to the caisson, and applied to the armor through the intermedium of a leaden ring.  Externally, the cheeks of the embrasure and the merlons consist of blocks of concrete held in caissons of strong iron plate.  The surrounding earthwork is of sand.  For closing the embrasure, Commandant Mougin provides the armor with a disk, c, of heavy rolled iron, which contains two symmetrical apertures.  This disk is movable around a horizontal axis, and its lower part and its trunnions are protected by the sloping mass of concrete that covers the head of the casemate.  A windlass and chain give the disk the motion that brings one of its apertures opposite the embrasure or that closes the latter.  When this portion of the disk has suffered too much from the enemy’s fire, a simple maneuver gives it a half revolution, and the second aperture is then made use of.

The Schumann-Gruson Chilled Iron Cupola.—­This cupola (Fig. 9) is dome-shaped, and thus offers but little surface to direct fire; but it can be struck by a vertical shot, and it may be inquired whether its top can withstand the shock of projectiles from a 10 inch rifled mortar.  It is designed for two 6 inch guns placed parallel.  Its internal diameter is 191/2 feet, and the dome is 8 inches in thickness and has a radius of 161/2 feet.  It rests upon a pivot, p, around which it revolves through the intermedium of rollers placed in a circle, r.  The dome is of relatively small bulk—­a bad feature as regards resistance to shock.  To obviate this difficulty, the inventor partitions it internally in such a way as to leave only sufficient space to maneuver the guns.  The partitions consist of iron plate boxes filled with concrete.  The form of the dome has one inconvenience, viz., the embrasure in it is necessarily very oblique, and offers quite an elongated ellipse to blows, and the edges of the bevel upon a portion of the circumference are not strong enough.  In order to close the embrasure as tightly as possible, the gun is surrounded with a ring provided with trunnions that enter the sides of the embrasure.  The motion of the piece necessary to aim it vertically is effected around this axis of rotation.  The weight of the gun is balanced by a system of counterpoises and the chains, l, and the breech terminates in a hollow screw, f, and a nut, g, held between two directing sectors, h.  The cupola is revolved by simply acting upon the rollers.