FIG. 72.—_Section, Elevation, and Plan of Turret of H.M.S. Captain._ ] [Illustration: FIG. 73.—_H.M.S. Captain._ ] A new system of mounting very heavy naval guns was proposed by Captain Coles about 1861. This plan consists in carrying one or two very heavy guns in a low circular tower or turret, which can be made to revolve horizontally by proper machinery. The turret itself is heavily armoured, so as to be proof against all shot, and is carried on the deck of the ship, which is so arranged that the guns in the turret can be fired at small angles with the keel. The British Admiralty having approved of Captain Coles’ plans, two first-class vessels were ordered to be built on the turret system. These were the _Monarch_ and the _Captain_—the latter of which we select for description on account of the melancholy interest which attaches to her. On page 155 a diagram is given representing the profile of the _Captain_, in which some of the peculiarities of the ship are indicated—the turrets with the muzzles of two guns projecting from each being easily recognized. The _Captain_ was 320 ft. long and 53 ft. wide. She was covered with armour plates down to 5 ft. below the water-line, as represented by the dark shading in the diagram. The outer plating was 8 in. thick opposite the turrets, and 7 in. thick in other parts. It was backed up by 12 in. of teak; there were two inner skins of iron each ¾ in. thick, then a framework with longitudinal girders 10 in. deep. The deck was plated in the spaces opposite the turrets with iron 1½ in. thick. The _Captain_ was fitted with twin screws—that is, instead of having a single screw, one was placed on each side, their shafts being, of course, parallel with the vessel’s length. The object of having two screws was not greater power, for it is probable that a single screw would be more effectual in propelling the ship; but this arrangement was adopted because it was considered that, had only one screw been fixed, the ship might easily be disabled by the breaking of a blade or shaft; whereas in the case of such an accident to one of the twin screws, the other would still be available. The twin screws could also be used for steering, and the vessel could be controlled without the rudder, as the engines were quite independent of each other, each screw having a separate pair. The diameter of the screws was 17 ft. The erections which are shown on the deck between the turrets afforded spacious quarters for the officers and men. These structures were about half the width of the deck, and tapered off to a point towards the turrets, so as leave an unimpeded space for training the guns, which could be fired at so small an angle as 6° with the length of the vessel. Above these erections, and quite over the turrets, was another deck, 26 ft. wide, called the “hurricane deck.” The ship was fully rigged and carried a large spread of canvas. But the special features are the revolving turrets, and one of these is