These projectiles are differently mounted for the different models. For the model 1870 the mounting consists of a forward ring of zinc or cast-iron just back of the shoulder of the shell, slightly smaller than the diameter of the bore across the lands. Its object is simply to keep the forward end of the projectile centred. A rear ring of copper, of a diameter slightly greater than that of the bore across the grooves. This ring brings up in its seat in the shot-chamber, and on firing the bands are scored and rotate the projectile. Around the surface of this ring are two or three grooves, cut to receive the metal displaced by the lands on firing. These projectiles carry at their base a light wire grommet to facilitate handling them. For the model 1864-67 the projectiles are provided with studs, there being two rows. The forward studs take in the grooves and give the twist to the projectile; the rear ones hold the base of the projectile centred, and according to the calibre of the piece they travel in the grooves or on the lands. For the heavy projectiles, where the rear studs travel on the lands, there are provided three small intermediate studs which bring up in false grooves provided for the purpose, keeping the projectile from going too far forward in the bore when loading. These studs are sheared off on firing. The studs are made either of zinc or bronze. Projectiles for the bronze muzzle-loaders are similar to the model 1864. Case-shot are of the ordinary type, and have neither studs nor belts. Sea-weed wads are used with all projectiles except those of the model 1870. These are used in order that the powder-charge may act with a more gradual effect on the projectile at the start. Placed between the charge and the projectile, their rapid compression gives room for the powder to expand and burn more slowly at the first instant. FUSES. Percussion fuses are used in all shell except the 25-pdr. and 8-pdr. boat-guns, which use time-fuses. The percussion fuse consists of a hollow cylindrical bronze body having a solid conical head. At the bottom of the cylinder is screwed a copper firing-pin. The centre of the cylinder is occupied by a brass plunger carrying a charge of hunting powder. The forward end of this plunger carries a wooden plug which is hollow and filled with detonating composition. The plunger is held in position by two iron pins upon which the base rests, and two leaden stops through the wall of the fuse entering the shell of the plunger. These stops are not broken by the start of the projectile, but by its shock on striking an object. The bottom of the fuse, being covered by shellac-paper, is blown away by the charge of powder in the plunger, and the flame is communicated to the charge. [Illustration: Percussion Fuse.] [Illustration: Time-Fuse.]