GUNS. 382 GUNS. of the disks are hollowed out so that a section of one side is somewhat like luiK of the letter 1. Wlien the powder pressure acts on the mushroom head the pad is squeezed and forced out against (he surface of the bore, and at the .same time the outer lips of the disks are also forced outward against it; this combination of metallic and pad contacts cuts ofl' all escape of gas to the rear. In the latest types of gas-checks used in United States naval guns the shape of mushroom head, gas-cheek, and rings has been modified, but the principle remains the same. The front disk (and in some cases the other as well) is now usually split on one side by a jilane passing through the centre of the disk and making an angle of about 75 degrees with the axis of the bore; this per- mits of a close fit without danger of jamming or sticking. In guns using fixed ammunition the cartridge-case acts as a gas-check, its thin walls ex]janding under the action of the powder gas until they press against the walls of the chamber so tightly that no gas escapes. Naval guns are fired by electric, percussion, , combination (electric and percussion), or friction primers. In the United States service only the first three varieties are used, and all guns are being fitted to use the combination primers a3 rapidly as opportunity offers. The electric and percussion primers are small tubes of copper an inch and a half long with enlarged heads not imlike in shape the bottle-neck small-arm car- tridges if we imagine the part of the cartridge which is of large diameter to be very short : the electric primer has a small wire projecting from it which connects to a firing circuit : the percus- sion primer has only a cap in the head like a centre-fire cartridge. The combination primer i.^ similar to a 32-calibro cartridge shell, but much longer; the cap in the head, if struck, will cause the primer to explode : and it is insulated from the shell of the primer so that if touched by the point of an electric contact piece the current will iow thrcug'i the cap, then through a bridge of fine platinum wire, thence to the shell of the primer, to the gun, and to earth. As the current passes through the platinum bridge it heats it white hot, igniting the priming composition, and that in turn ignites the charge, as with other primers. Primers are fired in locks, of which there are three styles in use. The oldest, called Mark VI., is fitted for electric or percussion firing, but to change from one to the other it is necessary to remove the wedge or breech-block of the lock. Mark VIII. is fitted for use with combination primers; to change from one method of firing to the other, it is only necessary to connect or dis- connect the electric firing wire. The only other lock in service uses the combination primer, and differs from Mark VIII. in automatically ejecting the primer after firing. It is nsed on quick-fire guns. In all cases, the lock screws on the rear end of the mushroom stem. Somewhat similar locks are used in foreign navies. To prevent premature firing of guns^ — for it is convenient to insert the primer while loading is going on — the electric circuit is not complete through the primer until the breech-block is closed and fully turned into place, so that the contacts placed in the proper position are pressed together; the pressure of the firing key will then complete the circuit. In quick-fire and rapid-fire gims (see Rapid-Fiee Gun.s), a detent holds the percus- sion firing pin back so that it cannot strike the primer until the breech is closed and locked. The electric current for firing is obtained from elec- tric batteries at the guns or a current of reduced voltage derived from the djnamo. The rifiing of United States naval guns is poly- groove with a twist which increases from zero to one turn in twenty-five calibres; that is to say, the projectile starts along grooves which at first are parallel to the axis of the bore, but which almost immediately begin to incline to form a spiral on its inner surface, the inclination increasing to a point near the muzzle from which the spiral is uniform ; the projectile is thu^ caused to rotate faster and faster, until, when it leaves the muzzle, it is spinning at a rate that will make it revolve once while moving a dis- tance equal to twenty-five "times its diameter. SECTION OF UNITED ST.TES NAVY 6-INCH GFX SHOWING RIFLING. ALSO ENLAESED PLAN OF RIFLING (a. a). The grooves are .05-incli in depth and several times as wide, but decrease in width as the muz- zle is approached, and their number is usually four times the calibre expressed in inches (for a 0-inch gun, 4 X 6 = 24) ; but there are some exceptions. In the design of naval guns the weights are kept down as much as possible, as every addi- tional ton of weight carried involves three or four tons extra displacement in the ship. In guns for forts and fixed defenses on shore, small sav- ings in weight are unimportant. The largest guns now being made for ships of any navy are of 12-inch calibre, though many very new 13-inch are carried on recently built ships of the United States Navy and many 13.5-inch guns are in service in tJie British and French navies. The heaviest guns ever put on board ship were the 16.25-ineh, 110-ton guns of the British battle- ships Mctoria, Sanspnreil. and Benhoic. The first-named was lost in collision, so that the Hanspareil and Benboir are the only vessels now caiTying them. Aside from their unwieldy size, which made them undesirable, these guns were faulty in design and were poorly constructed, so that they never gave satisfaction. Guns of 100 tons weight were mounted in Italian battleships many years ago, but some of these have been removed and replaced by others of less weight and smaller calibre, and the others are likely to be if the vessels are not condemned as not worth
