The elevating arrangement consist of an elevating screw, working into a screw-bed, which slides in a vertical box, and carries on the top of it a movable pawl to fit into the notches cut in the breech of the gun, in order to give considerable elevations. For the purpose of transferring the pawl from one notch to the next, it has a slit in it, through which the elevating bar is passed, and the gun supported by making use of the edge of the elevating-box as a fulcrum. This arrangement is over the rear transom.
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59. Describe the chassis of the columbiad-carriage.
It consists, like those used with other barbette-carriages, of two rails connected by three transoms; but the tops of the rails are shod with iron plates, and the rear hurters are the large heads of heavy bolts which pass entirely through the rails. The front hurters are fixed to the front transom by a heavy plate and bolt.
Traverse-wheels are placed under both front and rear transoms, and the chassis moves on a pintle passing through the middle transom. Two of these wheels are placed under each end of the chassis, their axes being kept in place by straps bolted to the transoms. Recesses are cut in the underside of the transom, for the wheels to turn in. This chassis has no tongue.
60. Of how many parts are casemate carriages composed?
Like barbette-carriages, of a gun-carriage and chassis.
61. Describe the gun-carriage.
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It consists of two cheeks, joined together by as many transoms, and supported in front by an axle-tree on truck wheels, and in rear on the rear transom, which is notched to fit the tongue of the chassis. Each cheek is formed of two pieces, one on top of the other, and connected by dowels and bolts. On the underside, near the front, a notch is cut for the reception of the axle-tree, which is of oak; and nearly over the axle, on the upper side of the cheek, the trunnion-bed is placed. The rear of the upper piece of the cheek is cut into steps, which give a better hold for the assembling-bolts, than a uniform slope, and give purchases for the handspikes, in elevating the piece. On the inside of each cheek, just in rear of the axle, a vertical guide is fixed to keep the carriage on the chassis. It is of wood, and bolted to the front transom and axle-tree. The top of the front transom is hollowed out, to admit the depression of the piece. Behind the rear transom and at the notch cut in it, there is an eccentric-roller, so arranged as to bear the weight of the rear part of the carriage, or not, according as it is thrown in or out of gear.