This device of the central battery was at first used solely for broadside guns. But the desire for ahead fire from behind armour soon caused the adaptation of the battery to allow it. Ports were cut in the two transverse bulkheads, the ship’s sides were indented, suitable gun-mountings were provided whereby some of the battery guns could be shifted from one porthole to another; and in this way it was secured that a fair proportion of the armament could be fired either on the beam or parallel with the keel-line of the ship. A power of offence was given in all directions, and no “point of impunity” existed. Ingenious were the arrangements resorted to, to obtain the maximum effect from the new medium-sized artillery which superseded the original truck-guns of the _Warrior_ and former warships. The armoured boxes, instead of being made with their sides respectively parallel, and at right angles, to the sides of the ship, were sometimes set diagonally, with their sides at forty-five degrees with fore-and-aft. Sometimes they were octagonal, sometimes with curved bulkheads, sometimes two batteries were superposed one on the other; but always the desire was to utilize each gun over as large as possible an arc of fire, and always the tendency was to augment the ahead fire. The central battery formed a powerful citadel covering the whole beam of the ship amidships. The guns of this citadel, by the power of manœuvring given by the adoption of twin-screw propelling machinery, could, it was argued, be brought to bear in any direction desired. Of all directions, “right ahead” was considered to be of the greatest importance. End-on fighting, it was assumed, would always be resorted to in future; and it was the power of keeping the ship end-on to the enemy which was the great military advantage conferred by twin screws. A further step in the direction of giving to each gun a large arc of fire was taken in the introduction of the sponson. By means of this circular platform, projecting from the vessel’s side, a gun could be carried so as to fire through an arc of 180 degrees. The same system obtained largely in the French ships of this period; by mounting guns in overhung circular turntables, one at each corner of the central battery _en caponière_, a large effective arc was obtained for them. Only one step more was necessary: that which would allow each gun to command the whole sweep of the horizon, and to be available for duty upon either beam and any bearing: the adoption of the _centre-line turret_. But before tracing the evolution of the turret, let us recapitulate the typical ships built between 1860 and 1873 which composed our central-battery fleet.