The Caisson

—Cai ons are used for conveying ammunition for a field-battery; all are similar in form. It is a four-wheeled carriage, consisting of two parts, one of which is a limber similar to that of the gun-carriage, and connected in a similar way by a wooden stock and lunette. On the axle-body of the rear part and parallel to the stock are placed three rails, upon which are fastened two ammunition-chests, one behind the other, and similar to the one on the limber; so that the cai on has three ammunition-chests, which will seat 9 cannoneers. The interior compartments of the ammunition-chests vary according to the nature of the ammunition with which they are loaded. In rear of the last chest is placed a spare-wheel axle of iron, with a chain and toggle at the end of it. On the rear end of the middle rail is placed a carriage hook similar to a pintle-hook, to which the lunette of a gun-carriage whose limber has become disabled may be attached, and the gun carried off the field. The cai on has the same turning capacity and mobility as the gun-carriage, so that it can follow the piece in all its manœuvres, if nece ary. It also carries a spare-wheel, spare-pole, etc. The principal parts of the cai on are: stock, or middle-rail; it has an iron lunette on its front end; side-rails, front foot-board, rear foot-board, middle-chest, rear-chest, spare-wheel axle; it has a body, two ribs, and a chain and toggle to secure the wheel; there are also two stays for the axle; lock-chains, fastened to lock-chain bridles under the front ends of the side-rails, and held up by lock-chain hooks fastened to the outside of the side-rails; spare-pole, spare-pole key, key-plate, chain, and pin; the key-plate is fastened to the under side of the lunette; the key is attached to the left side of the stock by a chain and eye-pin; carriage-hook, for attaching a carriage that has lost its limber; wheel-guard plates, spare-pole ring, held by the axle-strap; ring-bolt for spare hand-spike, key-plate and key, on the right side of the middle-rail; key-plate, chain, and key for the shovel-handle, on the inside of the right side-rail; middle a embling-bar, of iron; it has two ears in the middle to serve as stay-plates for the middle-chests, and a slot for the axe on the right of the middle-rail; rear a embling-bar; it supports the spare-wheel axle, and has a slot for the pickaxe on the left of the middle-rail. Axle, the axle-body, being notched to receive the middle-rail and tenoned to fit into the notches in the side-rails; staples for tool-handles; they are driven into the top of the axle-body in front of the iron axle-tree, one for the shovel-handle near the right side-rail, the other for the handle of the pickaxe on the left of the middle-rail. Wheels of all artillery carriages are similarly constructed; they differ, however, in the size and strength of certain parts, depending on the size of the carriage to which they are attached. The principal parts are: the nave, the nave-bands, the nave-box, the spokes, the felloes, and the tire. The nave constitutes the central portion of the wheel, and distributes the pre ure of the axle-arm to the spokes. It is generally made of a single piece of wood, and strengthened by four iron bands called the nave-bands. It is also pierced with a conical hole for the axle-arm; and to diminish wear and friction, it is lined with a box of bra or cast iron, called the nave-box. The spokes serve to transmit the pre ure of the load to the rim of the wheel. In all artillery carriages there are seven felloes and fourteen spokes. The felloes are the wooden segments which form the rim, and are joined together at their ends by wooden pins, or dowels. The tire is a strong band of iron, shrunk tightly around the felloes, to hold them together, and protect the rim from wearing away by contact with the ground.