A detachable wrench _e_ may be applied either to the screw _d b_ to turn it to lock or unlock, or to the traversing screw _c_, which, by engaging with a nut (not shown), runs the breech block in and out. [Illustration: FIG. 268.--INTERRUPTED THREAD BREECH MECHANISM.] By far the most popular principle of the breech block, however, is that of the interrupted thread, shown in Fig. 268, in which the plug, when closed, has its axis in alignment with the axial bore of the gun. Its threads are interrupted by longitudinally arranged channels, and the breech of the gun has corresponding threads and channels. When the plug is pushed into the gun, the screw threads of the plug enter the channels of the breech, and a rotary turn of the screw plug then locks its threads into those of the breech. The screw plug is supported by a carrier hinged at one side to the gun, and arranged to swing the plug into axial alignment with the bore, or be thrown to one side to admit the charge. The patents to Chambers, No. 6,612, July 31, 1849 (re-issue No. 237, April 19, 1853), and to Cochran, No. 26,256, November 29, 1859, are the earliest American examples of this principle of action, and are believed to be the original inventions of the same. In one form or another this construction enters into most all modern breech mechanisms. Among the forms used by the United States are the Driggs-Seabury, the Dashiell, and the Vickers-Maxim. To prevent the expanding gases from driving through the crevices of the breech block, expanding or swelling rings, known as gas checks, are arranged on the front of the breech block. De Bange’s patent, No. 301,220, July 1, 1884, covers the most popular form. [Illustration: FIG. 269.--SIGHTING A SIX-INCH RAPID FIRE GUN.]