On the front transom is fastened a wooden bolster, grooved to receive the elevating quoin. Notches, on the underside of the front and rear of the cheeks, give hold to the handspikes in throwing the carriage to the right or left.
40. Describe the coehorn mortar-bed.
It is made of a block of oak wood, in one piece, or two pieces joined together with bolts. A recess, for the trunnions and part of the breech is made in the top of the bed; and the trunnions are kept in their places by plates of iron bolted down over them. Two iron handles are bolted to the bed on each side, by which four men can carry the bed with the mortar in its place.
41. Describe the eprouvette mortar-bed.
It consists of a block of wood, on the top of which is countersunk and bolted the bed-plate, which is a heavy circular plate of cast-iron having a rectangular recess with sloping sides, so as to make it longest at the bottom. Into this recess the sole of the mortar slides. The wooden block is bolted to a stone block of the same size, which is firmly placed in the ground on a masonry foundation.
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42. Describe the heavy sea-coast mortar bed.
The bed for the heavy ten-inch mortar is the only one which has yet been adopted. The cheeks are of cast-iron, and somewhat similar in form to those in the beds of siege-mortars; but in the front, the cheeks turn up to receive between them the front transom, which has, countersunk in and bolted to it, an elevating screw-bed, through which works an inclined elevating screw, which rises or falls by turning the nut, fitted on it by means of a lever inserted into mortises cut in the direction of the radii of the circular nut.
Both the transoms are made of wood, connected with the cheeks by mortises and tenons, and secured by bolts running through, and nuts on the outside. One of these bolts at each end, is longer than the others, and the projecting ends are made use of as manœuvring bolts. Directly behind and underneath the position for the trunnions, a bronze bed-piece is placed to receive the shock of the piece. It consists of a large beam of bronze, with each end well let in to the face of the cheek. The use of the elevating screw instead of the quoin, is rendered necessary by the great mass of metal to be raised or lowered in sighting the piece.
43. What is a barbette carriage?