All muzzle-loading guns, except turret-guns and boat-guns, have six sights—three hind-sights (two tangent and one centre) and three trunnion-sights. Boat-guns have one centre hind and one fore sight, and turret-guns have turret-sights. All guns except boat-guns have wood scales, and all guns in covered batteries, except turret-guns, have graduated arcs on the pivot-circles. [Illustration: Barrel-Head Tangent-Sight—Breech-Loaders.] [Illustration: (Screw.)] [Illustration: (Drop.) Fore-Sight.] [Illustration: Sliding-Leaf Tangent-Sight—Breech-Loaders.] The tangent-sight is a rectangular steel bar rounded off on two sides, and having a gun-metal head in which slides a gun-metal leaf. The gun-metal leaf slides in a socket, and is provided with a simple clamp-screw. These sights are graduated on the four sides. In general, the forward side is graduated to 10´ from 0 up to 12°. The right side is marked from 100 up to 4000 yards, and for seconds of fuse from 1 to 30—both graduations for common shell with the full charge; the rear side from 100 to 4800 yards for Palliser shot or shell with the battering-charge; the left side from 100 to 4800 yards, and from 1 second to 34 seconds, for common shell with battering-charge. The centre hind-sight is a hexagonal gun-metal bar with the sliding leaf marked like the tangent-sights, but shorter. All hind-sights are set at a permanent angle of deflection. The fore or trunnion sights consist of a pillar and collar of gun-metal, a small steel leaf, and a screw for fixing the leaf. A gun-metal socket is fixed in the gun, and the sight secured in this socket by a double bayonet-joint, so that it may be readily removed or replaced without requiring a special adjustment. With turret-guns the sights are placed on top of the turret. Each turret is provided with a number of man-holes large enough to admit the head and shoulders. For each man-hole a pair of sights is adjusted, the line of sight being parallel to the axis of the gun. In some turrets, in order to prevent exposure, mirrors are arranged so that these sights may be used by persons in the turret. The wood scale is a wooden bar used in connection with marks on the rear face of the cascabel. This face is marked from 0 to 3½° for both elevation and depression. The wood scale is a square bar having its four sides marked to yards for different charges. When the gun is placed in position aboard ship, this scale is cut so that its zero and the cascabel zero shall coincide with the guns at level, on an even keel. The sights for breech-loaders are similar to those of muzzle-loaders, except that what is known as the barrel-headed sliding leaf is used, and there are no centre-sights. DISTINGUISHING MARKS OF PROJECTILES. The common shell is known from its carrying a fuse in the point which is cut off for that purpose. The double shell is known by its disproportion of length to diameter. The Palliser projectile is known from its sharp point.