Part 88

The _vent_ (see VENT) is perpendicular to the axis of the piece, and the interior orifice is at a distance from the bottom of the chamber equal to a quarter of its diameter, or at the junction of the sides of the chamber with the curve of the bottom. Experiment has shown this position to be the most favorable to the full development of the force of the charge, and to be least injurious to the piece. The size of the vent should be as small as possible, in order to diminish the escape of the gas and the erosion of the metal which results from it. In the U. S. service all vents are 0.2 inch in diameter. Experiment has, however, shown that the actual loss of force by the escape of the gas through the vent, as compared to that of the entire charge, is inconsiderable, and in practice may be neglected. In the U. S. service some pieces are made with two unbushed vents which are situated in two vertical planes on opposite sides of and parallel to the axis of the bore, and at a distance from it of one-half the radius of the bore. The left vent is bored entirely through, the other stops one inch short of the surface of the bore. When the open vent is too much enlarged by wear for further use, it is closed with melted zinc, and the other is bored out. Each vent is calculated to endure at least five hundred service rounds. In English guns of old model, the vent is placed four-tenths of the length of the cartridge from the bottom of the bore. In most breech-loaders, as well as many large modern muzzle-loaders, the vent is in the axis of the piece through the breech.

_Seat of the Charge._--The form of the seat of the charge, or that part of the bore of a fire-arm which contains the powder, will have an effect on the force of the charge and the strength of the piece to resist it. The considerations most likely to affect the force of the powder are the form of the surface and its extent compared with the inclosed volume. To obtain the full force of the charge it is necessary that the inflammation be nearly completed before the gas begins to escape through the windage, and the projectile is sensibly moved from its place, and as the tension depends much upon the heat evolved by the combustion, the absorbing surface should be a minimum compared with the volume. In cannon where the charge of powder is large, the form of the seat of the charge is simply that of the bore prolonged; this arrangement, when compared with the chamber, makes the absorbing surface of the metal a minimum and reduces the length of the charge, so that its inflammation will be as complete as possible before the gas escapes and the projectile is moved. To give additional strength to the breech, and to prevent the angle formed by the plane of the bottom and sides of the bore from becoming a receptacle for dirt and burning fragments of the cartridge-bag, it is rounded with the arc of a circle, whose radius is one-fourth the diameter of the bore at this point. Instead of being a plane bottom it is sometimes made hemispherical, tangent to the surface of the bore. In all United States cannon of the most recent model, the bottom of the bore is a semi-ellipsoid; this is thought to fulfill the condition of strength more fully than the hemisphere. With light pieces, in which it is necessary to use small charges of powder, if the charge were made into a cartridge of a form to fit the bore its length would be less than its diameter, and being ignited at the top, a considerable portion of the gas generated in the first instance of inflammation would pass through the windage, and a part of the force of the charge would be lost. To obviate this defect, to give the cartridge a more manageable form in loading, and to make the surface a minimum as regards the volume, the diameter of this part of the bore is reduced so as to form a chamber. The shape of the chambers of fire-arms is either cylindrical, conical, or spherical; the effect of these different forms of chambers on the velocity of the projectile will be modified by the size of the charge and the length of the bore. Up to a charge of powder equal to one-seventh of the weight of the projectile, and a length of bore equal to 9 or 10 calibers, experience shows that the presence of a chamber is advantageous, but beyond these it possesses no advantages to compensate for its inconvenience. For very small charges of powder and short lengths of bore, the cylindrical chamber gives better results than the conical chamber. For the same capacity, the conical chamber gives a shorter cartridge, and is therefore better suited to the rapid inflammation of a large charge of powder than the cylindrical chamber.

The Gomer chamber belongs to this class. (See GOMER CHAMBER.) The spherical chamber was formerly used particularly in mortars, but owing to the inconveniences which attend its construction and use, and its liability to deterioration, it is now entirely abandoned. In all the regulation guns of the U. S. land service, the bottom of the bore is a semi-ellipsoid. The adoption of this form simplifies the whole subject of chambers, and it is found to give increased ranges for small charges. No very careful experiments have been made to determine in a general way the effect of chambers on the strength of cannon; but late experience indicates that cylindrical chambers in heavy iron guns have an injurious effect on their endurance, and they have consequently been abandoned in these pieces.

_The Bore_ (see BORE).--The length of the bore has an important effect on the velocity of the projectile, and it was formerly supposed that the longest pieces gave the greatest ranges; this belief was in a great measure due to the slow rate of burning of mealed powder, which was originally used in cannon, but was entertained even after gunpowder received its granular form. When a gun is discharged, the accelerating force is due to the expansive effort of the inflamed powder, which reaches its maximum when the grains of the charge are completely converted into vapor and gas. This event depends on the size of the charge, and the size and velocity of combustion of the grains. With the same accelerating force, the point at which a projectile reaches its maximum velocity depends on its density, or the time necessary to overcome its inertia. The retarding forces are:

(1) The friction of the projectile against the sides of the bore; this is the same for all velocities, but different for different metals.

(2) The shocks of the projectile striking against the sides of the bore; these will vary with the angle of incidence, which depends on the windage and the extent of the injury due to the lodgment and balloting of the projectile.

(3) The resistance offered by the column of air in front of the projectile; this force will increase in a certain ratio to the velocity of the projectile and length of the bore. As the accelerating force of the charge increases up to a certain point, after which it rapidly diminishes as the space in rear of the projectile increases; and as the retarding forces are constantly opposed to its motion, it follows that there is a point where these forces are equal, and the projectile moves with its greatest velocity; it also follows that after the projectile passes this point its velocity decreases, until it is finally brought to a state of rest, which would be the case in a gun of great length. Elaborate experiments have been made in this country and abroad to determine accurately the influence which the length of the piece exercises on the velocity of its projectile. The experiments made by Maj. Mordecai of the U. S. Ordnance Department with a 12-pounder gun, show that the velocity increases with the length of the bore up to 25 calibers; but that the entire gain beyond 16 calibers, or an addition of more than one-half to the length of the gun, gives an increase of only one-eighteenth to the effect of a charge of four pounds. It follows from the foregoing that the length of bore which corresponds to a maximum velocity depends upon the projectile, charge of powder, and material of which the piece is made, and taking the caliber as a unit of measure, it is found that this length is greater for small-arms which fire leaden projectiles than for guns which fire solid iron shot, and greater for guns than for howitzers and mortars, which fire hollow projectiles. For the same charge of powder it may be said that the initial velocity of a projectile varies nearly with the fourth root of the length of the bore, provided the variation in length be small.

_Manufacture of Cannon._--Cannon for the U. S. service are made by private founders. The material and product of the casting are under the supervision of an ordnance officer, who receives the pieces only after they have satisfied all the conditions imposed by the regulations of the service. There are several foundries for making cast-iron cannon. Wrought-iron field cannon are principally made at the Phœnixville Iron-Works, Pa. There are also several private establishments where special cannon are made. The several operations of manufacturing cannon are, molding, casting, cooling, and finishing.

_Molding_, in general terms, is the process by which the cavity of the form of the gun is obtained by imbedding a wooden model in sand, and then withdrawing it. The wooden model is technically called the pattern, and the sand is confined in a box, which is divided into two or more parts for convenience in withdrawing the pattern. The pattern of the piece to be cast, somewhat enlarged in its different dimensions, is composed of several pieces of hard wood, well seasoned, or, for greater durability, of cast iron. The first piece of the model comprises the body of the piece from the base-ring to the chase-ring; the swell of the muzzle, and the sprue, or dead-head, are formed of the second piece; the breech, of the third; and the trunnions, of the fourth and fifth pieces. The sprue, usually called the “head,” is an additional length given to the piece, for the purpose of receiving the scoria of the melted metal as it rises to the surface, and furnishing the extra metal needed to feed the shrinkage. Its weight also increases the density of the lower portion of the piece. The breech is slightly lengthened in the direction of the knob of the cascabel, to form a square projection by which the piece can be held when being turned and bored. The best material for the mold is dry, hard, angular, and refractory sand, which must be moistened with water in which strong clay has been stirred, to make it sufficiently adhesive; when not sufficiently refractory, the sand is vitrified by the high temperature of the melted metal, and protuberances--not easily removed--are formed on the casting. When not sufficiently coarse and angular, the materials cannot be so united as to preserve the form of the molds. The mold is formed in a case of cast iron, and termed the “box,” or the “flask,” consisting of several pieces, each of which has flanges perforated with holes for screw-bolts and nuts, to unite the parts firmly. To form the mold, the pattern for the sprue and muzzle, previously coated with pulverized charcoal or coke, moistened with clay-water to prevent adhesion, is placed vertically on the ground, muzzle part up, and carefully surrounded by the corresponding parts of the jacket. When properly adjusted, the sand, prepared as above, is rammed around it. The model for the body of the piece is then placed on the top of this, and the corresponding parts of the jacket correctly secured, and filled in succession with the molding composition. The patterns for the trunnions and rimbases are bolted to the model of the piece, and when the sand is rammed firmly around these, the bolts are withdrawn, this part of the mold completed, and the end-plates screwed on. After completing the mold for the body of the piece, the model for the cascabel is properly adjusted and the mold completed. Care is taken to cover each portion of the model with the coke-wash mentioned above, and to sprinkle dry sand upon the top of the mold in each piece of the jacket, to prevent adhesion, so that the portions of the mold may be separated. In the body of the sand, a channel for the introduction of the metal is formed in the same manner as the mold cavity. It enters at the bottom of the mold, to prevent the bottom from being injured by the falling metal, and in an oblique direction, to give a circular motion to the metal as it rises in the mold, and thereby prevent the scoria from adhering to the sides. When the mold is completed, the parts of the flask are carefully taken apart, and the pieces of the model withdrawn from the mold contained in them. If any portions of the mold be injured in withdrawing the model, they are repaired, and the interior of the mold is covered with coke-wash; after which the several parts are placed in an oven to be gradually and perfectly dried. When this is accomplished, the parts are carried to a pit, where they are united and secured in a vertical position, with the breech below. Any portion of the sand broken off during the movements and adjustments should be replaced, and the whole of the interior covered with coke-wash. The object of coke-wash is to prevent the sand from adhering to the melted metal, which, when prepared, is made to flow in at the entrance of the side-channel. As the metal rises in the mold, a workman agitates it with a long pine stick, to cause the scoria and other impurities to rise to the surface, and brings them toward the centre of the mold, to prevent their entering the cavities for the trunnions.

_Cooling._--After the mold is placed properly in the pit, it is usual to surround the box with sand, at least as high as the trunnions of the gun. This is done to prevent rapid cooling. With guns as heavy as 24-pounders, this sand is not removed for three days, and as the gun is heavier the time is prolonged, and is from seven to eight days for the 10-inch columbiad. At the proper time the sand is removed, and the gun, still imbedded in the box and sand of the mold proper, is hoisted out, the box taken off, and when nearly cold, the gun cleaned of the sand.

_Boring and Turning._--A cannon is bored by giving it a rotary motion around its axis, and causing a rod armed with a cutter to press against the metal in the proper direction. The piece, supported in a rack, is carefully adjusted, with its axis horizontal, and made to revolve on this axis by machinery attached to the square knob on the cascabel. After adjustment, the sprue-head is first to be cut off. This is effected by placing a cutter opposite the point at which the section is to be made, and pressing it against the metal whilst the piece is turning. The head being cut off, and the cutter removed, the boring is commenced by placing the boring-rod, armed with the first cutter, called the piercer, in the prolongation of the axis of the piece, and pressing it against the metal. The piercer is used till it penetrates to the bottom of the chamber, after which a second cutter, or reamer, is attached to the boring-rod, and with this the boring is made complete to the round part of the chamber. The reamer is then removed and its place supplied by the chamber-cutter, which gives the necessary form and finish to that part of the bore. In hollow-cast cannon the piercer is dispensed with. Whilst the boring is taking place the workman contrives to finish the turning of all the exterior of the piece except the portion between the trunnions, which is afterwards planed off in another machine. These operations having been completed, the piece is placed in the trunnion-machine, and the trunnions are turned down to the proper size. Care is taken to make the trunnions of the same diameter, and perfectly cylindrical. Their axes should be in the same right line, perpendicular to the axis of the piece and intersecting it.

_Boring the Vent._--Whilst in the trunnion-lathe, the axis of the piece is inclined to the horizon at the angle the vent is to make with it. A drill is placed vertically over the point where the vent is to be bored, and pressed against the metal whilst a rotary motion is given to it by hand or machinery. The time required to finish a cannon, ready for inspection, depends upon its size, or from three to four weeks for a 24-pounder gun, and six weeks for an 11-inch gun.

_Cast Metal Guns, Modern Improvements in._--The first great step in this direction was taken by Gen. Rodman of the U. S. Ordnance Corps. It was his investigation into the crystallization of cast iron which led to the abolition of sharp angles or projections in the form of cannon. His reputation, however, rests mainly upon the principle of _hollow casting_. The general form of the old casting is that of a _solid_ frustrum of a cone; it is therefore cooled from the exterior, which causes the thin outer layer to contract first, and forces the hotter and more yielding metal within towards the opening of the mold. Following this the adjacent layer cools and tends to contract, but the exterior layer to which it coheres has become partially rigid and does not fully yield to the contraction of the inner layer. The result is, the cohesion of the particles of the inner layer is diminished by a force of extension, and that of the outer layer increased by a force of compression. As the cooling continues this operation is repeated, until the whole mass is brought to a uniform temperature, and the straining force is increased to an extent which depends on the size and form of the mass, the rapidity with which it is cooled, and the contractibility of the particular metal used. The foregoing considerations led Rodman to cast the gun hollow and to cool it from the interior, to reverse the strains by external cooling, and make them contribute to the endurance rather than to the injury of the piece. The method employed is to carry off the internal heat by passing a stream of water through a hollow core, inserted in the centre of the mold cavity before casting, and to surround the flask with a mass of burning coals, to prevent too rapid radiation from the exterior. Results show that cast-iron cannon made by this plan are not only stronger, but are less liable to enlargement of the bore from continued firing. All large American guns of cast iron, including the cases for the experimental rifles, are now cast on the Rodman plan. The plan has also been adopted by most of the nations of Europe that use cast-iron guns,--France, Sweden, Italy, etc.

For improvements in _bronze_, see the methods of Dean and Uchatius, ORDNANCE, METALS FOR.

The following are among the best known of cast metal homogeneous guns:

_Columbiad._--The columbiads are a species of sea-coast cannon containing certain qualities of the gun, howitzer, and mortar; they are long, chambered pieces capable of projecting solid shot and shells with heavy charges of powder, at high angles of elevation. The columbiad was invented by Col. Bomford, late of the U. S. service; the model was afterwards changed by lengthening the bore and increasing the weight of metal. (See ORDNANCE, HISTORY OF.) It was afterwards discovered that these pieces did not possess the requisite strength, and they were degraded to the rank of shell guns, and their places supplied by pieces of improved model. The change consisted in giving greater thickness of metal in the prolongation of the axis of the bore, which was done by diminishing the length of the bore itself; in substituting a hemispherical bottom to the bore, and removing the cylindrical chamber; in removing the swell of the muzzle and base-ring, and in rounding off the corner of the breech. In 1860 the model prepared by Capt. Rodman was adopted for all sea-coast cannon, and is essentially the same as the one described below.

_Paixhan Gun._--See ORDNANCE, HISTORY OF.

_Dahlgren Gun._--The guns constructed after the plan of Admiral Dahlgren of the U. S. navy, are used principally in the U. S. sea service. Those of large caliber are made of cast iron, solid, and cooled from the exterior. To produce uniformity in the cooling, the piece is cast nearly cylindrical, and then turned down to the required shape. The thickness of the metal around the seat of the charge is a little more than the diameter of the bore, as is true of nearly all the cast-iron guns. The chase, however, tapers more readily than in other cast-iron guns; they are smooth-bored, and the chamber is of the Gomer form. The principal guns of this system are of 9- and 11-inch caliber. A piece of 10-inch caliber has, however, been introduced into the navy for firing solid shot. The 15- and 20-inch naval guns are shaped exteriorly after the Dahlgren pattern, but are cast hollow, and have the elliptical chamber of the Rodman system.

_Napoleon Gun._--A bronze field-piece in the U. S. service. See NAPOLEON GUN.

_Rodman Gun._--The principal difficulty formerly experienced in manufacturing very large cast-iron cannon was the injurious strain produced by cooling the casting from the exterior. Gen. Rodman of the U. S. Ordnance Department developed a theory of the strains produced by cooling a casting like that of a cannon (see ORDNANCE, STRAINS UPON), and as a remedy for them proposed that cannon should be cast with a hollow core and cooled by a stream of water or air passing through it. This new mode of casting was afterwards adopted by the War Department. By this system of casting, guns of greatly-increased size and endurance are fabricated. The largest guns employed in the U. S. service (20-inch) are made on the Rodman plan, as well as the 15-inch, 13-, 10-, 8-inch, etc. The external form of Rodman guns is striking, as they are much larger at the seat of the charge than elsewhere. Their outline is made up of curved lines. This form has been almost universally adopted for U. S. guns. The Dahlgren, which preceded it, has nearly the same shape.

The great power demanded at the present day in heavy ordnance, however, cannot be attained by the use of cast iron alone. The difficulties of constructing homogeneous guns of the stronger metals--wrought iron and steel--have given birth in modern times to

_Built-up Guns._--The term “built-up” is applied to those cannon in which the principal parts are formed separately, and then united together in a peculiar manner. One object of this mode of manufacture is to correct the defects of one material by introducing another of opposite qualities, as for instance, trials have been made to increase the hardness, and therefore endurance, of bronze cannon by casting them around a core of steel which formed the surface of the bore. Built-up cannon are not necessarily composed of more than one kind of metal. Some of the most noted are made of steel or wrought iron alone. In this case the defects which we have seen accompany the working of large masses of wrought iron (crystalline structure, cracks, false welds) are obviated by first forming them in small masses, as rings, tubes, etc., of good quality, and then uniting them separately. The mode of uniting a built gun may be by welding the parts, by shrinking, or forcing one over the other, or by screwing them together.