Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/302

Rh 288 G U N M A K I N G [ORDNANCE. but in tills case the advantage of being able to fire single shots would be lost, and the recoil would be increased. This gun would probably never be able to fire with such extreme rapidity as the experimental form of Gatling men tioned above, but it could be made to fire six or seven hundred rounds a minute. It is generally considered that a machine gun should be able to fire easily two hundred rounds in half a minute. ORDNANCE. ilnance. The manufacture of ordnance is a much more scientific and complicated study than that of small arms. As the forces increase in magnitude and intensity, while the ultimate strength of material remains constant, the nicety of adapt ation of means to ends grows pari passu with the guns. In producing a piece of ordnance, two distinct sets of conditions are involved those belonging to its actual construction, and those by which its proportions are regu lated. In constructing a gun, the material must be so selected and disposed as economically and safely to sustain the effect of the forces developed by the charge ; in design ing a gun, it is necessary to know the nature and direction of the forces which will combine to produce the desired ballistic results. The two sets of conditions are as distinct as those involved in the separate operations of writing and printing this article. nstruc- CONSTRUCTION. Nearlyall the accurate knowledge asyet a - obtained of the true action of gunpowder has been acquired within the last twenty years. The general idea previously held was that the explosion was instantaneous, and that the more violent the powder the greater would be the velocity of the projectile, The mode of proving the quality of the explosive was to place a small quantity in a short mortar, and to measure the distance to which it projected a light shell a test altogether wrong in principle, as will be shown later on. No accurate idea had been formed of the true pressure of the powder gas on the bore during discharge ; but it was understood that a gun was subjected to two principal strains orstresses a circumferential tension tending to split the gun open longitudinally, and a longi tudinal tension tending to pull the gun apart in the direc tion of its length. !omo- When guns are made of cast metal, and are, in fact, jneous homogeneous hollow cylinders, a limit is soon reached lus beyond which additional thickness is practically useless in giving strength to resist the circumferential tension. Supposing the metal employed to be incompressible, each concentric layer would take up a strain on discharge inversely proportionate to the square of its distance from the axis of the bore. Every metal, however, is compres sible as well as extensible, and hence the exterior always affords more support to the interior than the foregoing rule indicates. The great aim then of a gunmaker is so to select and arrange his material that the exterior shall take up as much as possible of the strain thrown upon the interior. In America, Captain Rodman introduced a method of casting guns hollow and cooling them down from the interior, so that the inner portions being first solidified were compressed and supported by the contraction of the outer parts when they subsequently cooled down. Thus, on discharge, the compressed inner portions expanded under the action of the powder gas to or beyond their natural diameter, throwing at once the strain on the outer portions which were already in a state of tension. But however well cast-iron may be disposed, it is naturally too weak and brittle for use with heavy guns, and those nations which employ it thus do so because it is comparatively cheap and easy to manufacture, and not because it is the best material. Austria and Russia have of late years made light guns of bronze cast in chill, and subjected to the wedging action of steel mandrils driven through the bore. The several layers of the metal are thus placed in a state of tension as regards the exterior, and of compression as regards the interior. At the present day systems of building up guns have been devised so that each portion of the metal is made to bear its fair share of the strain. The longitudinal tension is usually less important than the circumferential stress. It is considered to be provided against in homogeneous guns if the metal is as thick over the bottom of the bore as round the end. The strain is now measured by calculating the total pressure of the powder gas on the bottom of the bore, and comparing it with the area of the transverse section of the gun at the same place. This is not absolutely correct ; but, practically, the chief modes of gun construction now adopted leave a considerable margin of strength in this direction. Sir William Armstrong first successfully employed the Built- principle of initial tensions for all parts of a gun. In his guns, system, wrought-iron coils are shrunk over one another, so that the inner tube is placed in a state of compression and the outer portions in a state of tension, an endeavour being made to so regulate the amount of tension that each coil should perform its maximum duty in resisting the pressure from within. Further, he arranged the fibre of the several portions so as to be in the best positions for withstanding the pressures. It must be noted that wrought-iron bar is about twice as strong in the direction of the fibre as across it. He therefore constructed the exterior of the gun of coiled bars of wrought iron welded into hoops and shrunk one over the other, thus disposing the fibre to resist the circumferential strain. These outer coils were shrunk over a hollow cylinder of forged iron, having the fibre running lengthways so as to resist the longitudinal strain. Yithin this cylinder or forged breech-piece was placed a steel tube, gripped in like manner by shrinkage. This grand Slu-in principle of modern gun construction is carried out by turn- age. ing the inner coil in a lathe to an exterior diameter slightly greater than the interior diameter to which the outer coil is bored. The outer coil is expanded by the application of heat, and slipped over the inner one. It contracts on cooling, and if the strength of the two coils is properly adjusted, the outer will remain in a state of tension, and the inner in a state of compression. On this view, the ideal gun would be constructed of a vast number of exces sively thin rings so shrunk over each other that, on discharge, each should be equally strained. An attempt has been made by Mr Longridge, M.I.C.E., to approximate to this condition by winding steel wire under tension round a steel tube. This system, though possessing much ingenuity, has never made way, and might possibly be found wanting in longitudinal strength. Great success attended the early introduction of the coil principle. Guns of considerable size were made : the largest weighed as much as 23 tons, and projected a COO-lb shot with a fair velocity. It was found, however, that much difficulty attended the accurate shrinking of a number of thin rings, and that occasionally one or more of the outer ones would be strained to cracking, while the inner ones were intact. The original mode of construction was therefore modified, as experience was gained in the Govern ment factory at Woolwich. Acting underGeneral Campbell, R.A., Mr Fraser, M.I.C.E., thickened the coils, and extended their use to the breech-piece, it being found that the longi tudinal disposition of the fibre in that part was not required to sustain the longitudinal strain, and that the steel tube forming the bore was better supported by coils. The manufacture of ordnance at Woolwich and Elswick may be briefly described as follows. Steel cylinders, slightly larger than the exterior dimensions of the inner