TO MOUNT THE LOCK. 1. Put the upper piece and spring into their place in the face of the carrier and slide on the cover (6). 2. Place the carrier in its grooves, put in the piece which holds the carrier in its place, and drive in the pin (5) which secures the piece. 3. Put in the safety-sear and the firing-pin; drive in the tapered pin (4) that secures the safety-sear. 4. Put in the hammer, taking care that its point enters the slot in the firing-pin. Drive in the pin (3) that secures the hammer. When the pin (3) is in place, upset the point slightly by a few blows of a small hammer. 5. Put in the sear and the pin (2) which secures it. 6. Place the carrier-levers in their places; put in the mainspring, taking care its point enters the slot in the firing-pin; put on the clip and drive in the pin (1) which secures the mainspring and clip. NOTE.--There may be a little difficulty in entering the pin (1), but a special tool is supplied, which, being entered from left to right, will hold the clip and mainspring in their places, and then the pin (1) can be driven home from right to left, which, in its passage through the hole, will remove the tool. METALLIC CARRIAGE FOR MACHINE GUNS. This carriage, manufactured by the Ordnance Department, is designed for any machine-gun; each kind of gun to be provided with its own mount adapted to the carriage, which is made of steel. The design includes a spherical shield above and a plane apron below the axle for the protection of the cannoneers. The apron is hinged to the axle and is folded up and keyed to the under side of the trail when not in action. The spherical shield can be removed if desired. The body is the axle, and it and part of the trail make a chest for ammunition in which 1200 .45-calibre cartridges, packed in paper boxes, can be carried. Two doors, right and left of trail, on inner side, give access to the cartridge-space in the body, and a door on top of the upper end of trail to the cartridge-space in the trail, the two parts being continuous. Lower down in the trail is a tool-box to which the trail-seat, when raised, gives access. The top of the carriage makes a convenient table for tools, feed-guides, and ammunition when in action. The shield with aperture and disks are so combined that the gun can be readily pointed without exposure. CHAPTER X. Theoretical Gunnery. Definitions. General Principles. Probability of Fire. Burst of Shell. Burst of Shrapnel. DEFINITIONS. INTERIOR BALLISTICS. =Interior Ballistics.=--The effects produced on a projectile in the bore of a gun, and on the gun, when subjected to the action of the products of combustion of gunpowder or other explosives. =Gunpowder Explosion= is the rapid conversion of gunpowder into gases and solids with evolution of heat.