Part 8 (1/2)
The heaviest of our up-to-date ordnance is of moderate calibre, the largest breech-loaders being 12-inch, 10-inch, and 9.2-inch guns. But the elaborateness of its manufacture is such that one big gun takes nearly as long to ”build up” as the s.h.i.+p for which it is destined.
Each weapon has to pa.s.s through about sixteen different processes:--
(1) The solid (or hollow) ingot is _forged_.
(2) _Annealed_, to get rid of strains.
(3) It is placed horizontally on a lathe and _rough-turned_.
(4) _Rough-bored_ in a lathe.
(5) _Hardened._ Heated to a high temperature and plunged, while hot, into a bath of rape oil kept cold by a water-bath. It cools slowly for seven to eight hours, being moved about at intervals by a crane. This makes the steel more elastic and tenacious.
(6) _Annealed_, _i.e._ reheated to 900 Fahr. and slowly cooled. Siemens' pyrometer is used in these operations.
(7) _Tested_ by pieces cut off.
(8) _Turned_ and _bored_ for the second time.
(9) Carefully turned again for _shrinkage_. Outer coil expanded till large enough to fit easily over inner. Inside, set up vertically in a pit, has outside lowered on to it, water and gas being applied to make all shrink evenly. Other projections, hoops, rings, &c., also shrunk on.
(10) Finish--_bored_ and _chambered_.
(11) _Broached_, or very fine bored, perhaps _lapped_ with lead and emery.
(12) _Rifled_ horizontally in a machine.
(13) Prepared for breech fittings.
(14) Taken to the Proof b.u.t.ts for trial.
(15) Drilled for sockets, sights, &c. Lined and engraved.
Breech fittings, locks, electric firing gear, &c., added. Small adjustments made by filing.
(16) _Browned_ or _painted_.
When worn the bore can be lined with a new steel tube.
These lengthy operations completed, our gun has still to be _mounted_ upon its field-carriage, naval cone, or disappearing mounting, any of which are complicated and delicately-adjusted pieces of mechanism, the product of much time and labour, which we have no s.p.a.ce here to describe.
Some account of the princ.i.p.al parts of these guns has already been given, but the method by which the breech is closed remains to be dealt with.
It will be noticed that though guns now barely reach half the weight of the monster muzzle-loaders, they are even more effective. Thus the 46-ton (12-inch) gun hurls an 850-lb. projectile with a velocity of 2750 foot-seconds, and uses a comparatively small charge. The famous ”81-ton” needed a very big charge for its 1700-lb. sh.e.l.l, and had little more than half the velocity and no such power of penetration.
This change has been brought about by using a slower-burning explosive very powerful in its effects; enlarging the chamber to give it sufficient air s.p.a.ce, and lengthening the chase of the gun so that every particle of the powder-gas may be brought into action before the shot leaves the muzzle. This system and the subst.i.tution of steel for the many layers of welded iron, makes our modern guns long and slim in comparison with the older ones.
To resist the pressure of the explosion against the breech end, a tightly-fitting breech-plug must be employed. The most modern and ingenious is the Welin plug, invented by a Swedish engineer. The ordinary interrupted screw breech-plug has three parts of its circ.u.mference plane and the other three parts ”threaded,” or grooved, to screw into corresponding grooves in the breech; thus only half of the circ.u.mference is engaged by the screw. Mr. Welin has cut steps on the plug, three of which would be threaded to one plane segment, each locking with its counterpart in the breech. In this case there are three segments engaged to each one left plane, and the strength of the screw is almost irresistible. The plug, which is hinged at the side, has therefore been shortened by one-third, and is light enough to swing clear with one touch of the handwheel that first rotates and unlocks it.
The method of firing is this: The projectile lifted (by hydraulic power on a s.h.i.+p) into the loading tray is swung to the mouth of the breech and pushed into the bore. A driving-band attached near its base is so notched at the edges that it jams the sh.e.l.l closely and prevents it slipping back if loaded at a high angle of elevation. The powder charge being placed in the chamber the breech-plug is now swung-to and turned till it locks close. The vent-axial or inner part of this breech-plug (next to the charge), which is called from its shape the ”mushroom-head,” encloses between its head and the screw-plug the de Bange obturator, a flat canvas pad of many layers soaked with mutton fat tightly packed between discs of tin. When the charge explodes, the mushroom-head--forced back upon the pad--compresses it till its edges bulge against the tube and prevent any escape of gas breechwards.
The electric spark which fires the charge is pa.s.sed in from outside by means of a minute and ingenious apparatus fitted into a little vent or tube in the mushroom-head. As the electric circuit cannot be completed till the breech-plug is screwed quite home there is now no more fear of a premature explosion than of double loading. If the electric gear is disordered the gun can be fired equally well and safely by a percussion tube.