The breech-loaders are slotted across the rear face in such a manner that the rear face of the block is entirely exposed, thus saving in length of breech of gun. The breech-block is cubical, and is traversed along the upper and lower surfaces by heavy threads set at an angle with the face of the block. These threads travel in heavy male threads in the slot, the system forming the support for the thrust on the block. Attached to the rear face of the block is a weighted crank which revolves a cogged wheel housed in the block and travelling in a rack in the rear of the lower side of the block-seat. In guns of heavy calibre the system is reversed, the crank and wheel housing in the rear of the gun and the rack in the block. By means of this gearing the block is moved transversely, masking and unmasking the bore, the left end of the block being cut for a loading-hole. A stop on the face of the breech locks the block when home and catches it at the proper point when open. [Illustration] The Whitworth groove is of a peculiar nature, being almost a perfect hexagon, and having an extremely sharp twist of from one turn in 2 feet in the 2-pdr. to one turn in 13 feet in the 9-inch. The projectiles are cut to fit the grooves, the armor-punching ones being of compressed steel. [Illustration: Whitworth Projectiles.] THE VAVASSEUR CONSTRUCTION. Vavasseur ordnance is used considerably in China, and has found some use in other quarters of the world. The gun belongs to the steel type. It consists of a mild-steel oil-tempered tube, reinforced by steel hoops, the hoops being narrow and numerous and shrunk on. The trunnion-hoop is of wrought iron. The gun has, properly speaking, ribs instead of grooves, the grooves being cut in the projectiles. The twist is a constant one. [Illustration}] [Illustration: Vavasseur Gun and Carriage.]