Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/305

CONSTRUCTION.] GUN MAKING 291 venting escape of gas at the breech, and a special arrangement is therefore provided for this purpose (fig. 45). The end of the bore is enlarged, and into the recess thus formed fits a Broadwell ring, against which the face of the wedge abuts when forced home. The ring is so formed that it must always fit against the wedge, and be pressed firmly to it by FIG. 44. Krupp s breech action, cylindro-prismatic wedge, the action of the gas on firing. This is accomplished by making the recessed surface of the gun and the exterior surface of the ring portions of a sphere. In spite of the theoretical perfection of the system, and the excellence of manufacture attained, it has always been found that after a time after firing a very variable number of rounds gas would begin to escape, and then speedily cut a channel between the ring and the wedge. Krupp has fully recog nized this, and has been succcessful in minimizing and localizing the injury thus caused. The gas has two ways of escape : it can pass either between the ring C and the body of the gun D, and so out to Y ; or from X to Y. Several forms of Broadwell ring have been tried by Krupp ; the form shown in fig. 45 is now in use. When the gun is fired the gas acts on the rounded surface of the ring C, press- ing it down hard against the Fl0 " 45. -A steel facing-plate ;BB . . , , copper discs ; C, steel Broadwell lacing -plate A, and also ex- ring . D; body of gun; E, wedge; panding it against the body X, interior of gun ; Y, exterior of the gun D. The tendency of uu - of the gas is also to expand the body of the gun away from the ring, and many bronze guns have been destroyed from this cause, having expanded more than the ring was able to follow. A recess is cut in the exterior of the ring, partly to give it greater spring and partly to afford a relief channel for any gas that may have forced its way in. The form of ring here shown appears to have overcome the efforts of the gas to escape in this direction. Having stopped the gas from getting round the outside of the ring and injuring the gun, we have now to consider the results of escape from X to Y. A channel cut through here would disable ring and wedge, which, though not so bad as disabling ring and gun, would yet be productive of much inconvenience. The insertion of the steel facing-plate A at once saves the wedge and localizes the injury. It allows, moreover, of the introduction of copper discs behind the plate, for the purpose of making up for wear, compres sion of metal of wedge, &c. Two relief channels are cut round that surface of the ring which abuts on the facing- plate. Spare Broadwell rings and facing-plates are supplied with the guns ; should wear set in and escape of gas ensue, the defective fittings can be removed from the gun and new ones inserted in a couple of minutes. Each set should last on the average for several hundred rounds. In the French system no slot is cut in the gun (fig. 46). Frein The breech is closed by a screw plug from the rear, which breec swings on a hinge to the side when withdrawn, The male ^ screw on the plug and the female screw in the gun are "" divided circumferentially into six parts, of which three have the threads cut away, so that the surface is alternately screw and plain cylinder. When the threads of the plug coincide with the plain parts in the gun, the plug can be moved straight in or out. When pushed in, one- sixth of a turn en gages the threads, which thus give half the bearing of an ordinary screw. This is called the interrupted screw system. The plug FIG. 46. French breech action, is larger than the interrupted screw. bore, so as to afford room for the shell and cartridge to be thrust in. The end of the bore is recessed to take a ring against which the end of the plug abuts, somewhat as in Krupp s guns ; or a cup of steel is fixed on the end of the plug, which is slightly convex ; the pressure of the gas drives the flat back of the cup firmly against the con vex surface, bending out the lips, which are at the same time pressed against the sides of the gun by the gas. This method has been lately introduced by the Elswick Company. It is difficult to award superiority to the Krupp or the French system. Each appears to possess some little advan tage over the other, and both have attained great success. The Krupp system has undergone the more thorough testing in the field, and has the merit of allowing the parts which become damaged by the escape of gas to be replaced with greater ease and quickness. The French system, on the other hand, takes up less of the gun, so that in a piece of given length the bore may be upwards of one calibre longer than with Krupp s. In the French system, also, the breech- fittings are less exposed to damage by the enemy s fire, being behind the gun instead of at the side. DESIGNING. A gun, like all other machines, must be Gun designed to fulfil certain definite conditions. Its projectile s g may be required to pierce a given thickness of armour at a given distance ; or weight of piece may be the limit, and it may be wished to throw the most powerful shell or shrapnel to a given distance with a given elevation, con sistent with that limit. To work out problems of this nature, it is all-important to possess an accurate knowledge of the action of the charge inside the bore. By means of the Noble chronoscope and the crusher gauge (sec GUNNERY), this knowledge is obtained, and we shall now explain how the indications of these instruments are employed to assist in determining the proportions of ordnance. Gunpowder is not properly so much an explosive as a substance burning and giving off gas with great rapidity. It offers in this respect a marked contrast to gun-cotton, dynamite, and other true explosives. If one of these agents be detonated, the detonation is immediately carried through the mass, whatever its size, and the whole at once turns into gas. Gunpowder, on the other hand, as far as is known at present, cannot be detonated, but simply evolves its gas by burning in layers from the outside to the inside.

Thus a large grain will take longer to burn up and become