_The block_ is rotated by means of a rack which engages teeth cut in the upper surface of the block at its rear end. The rack is actuated by a lug on the under side of the operating lever which engages a slot in the rack. The rack is located in the inside face of the block carrier. When the breech is tightly closed this lock bears against the breech face of the howitzer and is forced back against the rack lock spring leaving the rack free to move. As the breech starts to open the rack lock is forced up by its spring and locks the rack, preventing further rotary motion of the breechblock. _The operating lever_ is provided with an operating lever handle which is kept in its raised position by the operating lever handle spring. When the breech is closed and locked the lower portion of the operating lever handle engages the block carrier lever catch. When the breech is fully open the operating lever latch which extends through the operating lever, engages the operating lever catch and holds the breech in that position. [Illustration: _155-MM HOWITZER MODEL OF 1918_ (_SCHNEIDER_) _FIRING MECHANISM_] _The obturator spindle_ is of the _mushroom head type_. It passes through the center of the breechblock and is screwed into the front end of the firing mechanism housing, which fits into the hub of the block carrier. The obturator spindle is prevented from turning by the firing mechanism housing key spring. A vent for the passage of the primer flame is drilled through the center of the obturator spindle. The obturator spindle bushing is screwed into the front end of the obturator spindle and the obturator spindle plug into the rear end—the latter forming a chamber for the primer. The obturator spindle spring bears against the firing mechanism housing and the breechblock, keeping the head of the obturator spindle tightly against the _gas check pad_. The gas check pad or plastic obturator is composed of a mixture of one part asbestos and three parts nonfluid oil, contained in a canvas covering. The pad is protected by the front, rear and small split rings. A steel filling-in disk is placed between the gas check pad and the breechblock. =The firing mechanism= housing is provided with a firing mechanism safety plunger which is forced by the firing mechanism safety plunger spring against the inside circumference of a circular boss on the face of the breechblock. When the breechblock is rotated to its locked position, the plunger slips into a notch in the boss and permits the entrance of the firing mechanism block. When the breech is unlocked the lower end of the firing mechanism safety plunger extends into the firing mechanism housing and obstructs the entrance of the firing mechanism block. This safety device makes it impossible to unlock the breech while the firing mechanism block is in place or to insert the firing mechanism block while the breech is unlocked.