Beneath the carrier-block everything is open, to allow the cartridges or shells which are withdrawn by the extractors from the barrels to fall to the ground. Within the cylindrical breech-case attached to the frame a heavy ring, not quite the length of the lock-cylinder, is fastened to the case and diaphragm, which nearly fills the space between the inside of the case and the cylinder. Portions of the inside of this ring are so cut away as to leave a cam projecting from the inner surface of the ring, having two helicoidal edges inclined to each other, and united by a short, flat plane. Against these edges the rear ends of the locks continually bear, there being room enough for the locks to lie loosely within the parts of the ring which are cut away. Each lock is held back against the cam by a lug projecting laterally from the end of the lock, and entering a groove formed at the base of the cam, in the thin part of the ring. A device for throwing the cocking-piece in and out of gear, worked by a knob on the right of the breech-casing, permits the gun being used for drill purposes without snapping the locks. When the arrow-head points to the front, it is in position for firing. With the head pointing to the rear it is out of action. On the left of the breech-casing, just under the hopper, is a movable section with three wedge-shaped prongs, which keep the cartridge-cases in the grooves of the carrier until they are ejected. It is called the shell-guide. The gun can be unloaded of any cartridges not fired by removing the feed-case, opening the hopper, and reversing the motion of the crank. The locks can be removed and inserted without taking off the cascable-plate; and the absence of one or more plugs does not affect the working of the gun, except to diminish the intensity of the fire. For each lock removed one unexploded cartridge falls to the ground at each revolution of the gun. The gun is encased in a frame which has trunnions, and is mounted in the ordinary way, like a field-piece. The screw for elevating and depressing the breech works in a nut attached to the trail of the carriage in the usual way. In the model of 1883 the trunnions of the gun are placed two inches below its centre, and the elevation and depression are given by means of an arc connected at both extremities with the gun, which may be elevated 74° or depressed 78°. A lateral movement is given the gun by means of a hand-lever which fits into a square mortise cast on the under side of the breech-casing. On top of the breech-casing is a spirit-level placed parallel to the axis of the piece, and on the right side of the cascable-plate is another at right angles to the first. [Illustration: FIG. 89.] ACTION OF THE MECHANISM. In Fig. 89 _cd_, _c′d′_ are the developments of the right-and left-hand sides of the elliptical groove; _cc′_ and _dd′_ are developments of the circular arcs.