The steam tube consists of a fixed tube and an outer tube, termed the slide valve, so arranged as to slide freely along the fixed tube. In the fixed tube there is a hole near each end, and a third hole in the threaded portion in front, to connect with the steam escape hole which is bored in the solid part of the front end of the barrel casing. This tube is fixed into the solid end of the barrel casing, and is retained in position by a screw which, being kept in adjustment by a keeper screw, ensures that the third hole coincides with the steam escape hole. At the breech end it fits into a recess. If the gun is fired with elevation, the valve slides backwards, and, closing up the hole at the rear end of the tube, prevents the water entering; at the same time it leaves the front hole uncovered, which, being above the water level, allows the steam to enter the tube and escape through the steam escape hole in the barrel casing. Similarly, if the gun is fired with depression, the valve slides forward and allows the steam, but not the water, to escape through the rear hole. In the lower part of the barrel casing is the ejector tube through which the empty cartridge cases are ejected from the gun. The tube is fitted with a spring, which prevents the cases falling backwards into the gun. The breech casing consists of two outside plates, a bottom plate which is riveted to them, and the rear cross piece, the whole being closed by a cover. The outside plates are dovetailed into the barrel casing, and, together with the cover, are secured by means of a cover joint pin. On the outside of the right hand plate there are the following fittings:—(1) a socket and stud for securing and supporting the buffer spring; (2) the resistance piece, and (3) the stud for the check lever. On the outside of the left hand plate are three studs for holding the fusee spring box, the rear one being on the slide mentioned below; there are also two other studs on this plate for fixing a shoulder piece to the gun if required. In both plates are slots partly closed by slides, in which the crank bearings move, and on the inside of both plates are solid cams which control the path of the extractor. Below these, and supporting the side plates, are rests, along which the recoiling portion travels. Along the bottom plate lies the trigger bar, and underneath is a bracket to which the elevating gear is attached by means of the elevating joint pin.