The rail-platform for siege-mortars consists of three sleepers and two rails for the cheeks of the mortar-bed to slide on, instead of the deck-plank, and is very strong, and easily constructed and laid. The pieces being notched to fit, are driven together at the battery, the distance between the centre lines of the rails being equal to that between the centre lines of the cheeks. The earth is excavated eight and a half inches, the depth of the sleepers, and the bottom made perfectly level. The directrix being exactly marked by stakes, the platform is placed in position, its centre line coinciding with a cord stretched between the stakes marking the line of fire. The earth is filled in as high as the upper surface of the sleepers, and firmly rammed; and stakes are driven in the rear angles formed by the sleepers and rails, and one at the rear end of each rail. [Sidenote: 122]] 13. Mention the parts of the _ricochet-platform_. 1 Hurter, 8 ft. long, 8 in. wide, and 8 in. thick. 3 Sleepers, 9 ft. “ 5½ “ “ “ 5½ “ 2 planks, 10 ft. 8 in. l’g, 13 “ “ “ 2¼ “ 1 plank, 7 ft. long, 13 “ “ “ 2¼ “ 1 piece plank, 2½ ft. long, 13 “ “ “ 2¼ “ And some stakes. 14. Describe the method of laying this platform. To lay this platform, place the hurter perpendicular to the line of fire, and secure it by four stakes, one at each end and two in front, 31½ inches from the middle towards each end: lay the three sleepers parallel to the hurter, the first 16 inches from the rear edge of the hurter, the second 43½ inches from the rear edge of the first, and the third 43½ inches from the rear edge of the second. Lay the plank 31½ inches from the directrix of the platform to the centre of the plank. Place the piece of plank 60 inches from the rear edge of the last sleeper, and bed it in the ground. Place on the last sleeper and this piece of plank, _the_ plank (7 feet long), its front edge 106 inches from the rear edge of the hurter. [Sidenote: 123]] PART XII. ARTILLERY CARRIAGES AND MACHINES. 1. What is meant by artillery carriages? Carriages of every description employed in the artillery service. 2. How are such carriages classified? Into two general divisions; first, those carriages on which artillery are mounted, either for firing or travelling; and secondly, such as are especially used for the transportation of artillery ammunition and stores. 3. What is a gun-carriage? It is the machine on which a piece is mounted for manœuvering and firing. 4. Into what classes may gun-carriages be divided? Into _movable_ and _stationary_ carriages. 5. What is the use of _movable_ carriages? They are used for the transportation of the pieces as well as for firing them, and are mounted on large wheels. They are furnished with limbers. 6. Describe the _movable_ carriage.