Mortar-Beds.—Mortars are fired from a bed; in the U. S. service there are three kinds of mortar-beds in use in the siege service; the 8-inch, 10-inch, and the Coehorn; the first two differ only in dimensions. They are made of wrought iron and put together after the manner of the sea-coast gun-carriage. The different parts are the cheeks, which, like those of the gun-carriage, are triangular in shape, and two transoms connecting the cheeks together. At the end of each cheek are projections, called front and rear notches, underneath which the cannoneers embar with their handspikes to move the bed on the platform; there are also two front and two rear manœuvring-bolts for the same purpose. The elevation and depression are given as in the gun-carriage by embarring with the iron elevating bar through the fulcrum into the ratchets on the breech of the mortar. The Coehorn-bed is made of a block of oak wood, in one piece, or two pieces joined together with bolts. A recess for the trunnions and part of the breech is made in the top of the bed, and the trunnions are kept in their places by plates of iron bolted down over them. Two iron handles are bolted to the bed on each side, by which four men can carry the bed with the mortar in its place, the entire weight being only 296 pounds. Sea-coast mortar-beds are similar to those for siege purposes, but they have eccentric truck-wheels for manœuvring the mortar-bed on the platform and the manœuvring-bolts are omitted. The 13-inch sea-coast mortar is now mounted upon a centre pintle-carriage. The usual bed, now become the top carriage, is placed upon a chassis resting on a platform. The top carriage has a crane attached to the left cheek, and to the inside of the right cheek is attached a pawl worked from the front, for locking the eccentric axle in and out of gear, and the carriage is strengthened by an additional rear transom about 5 inches wide, the pipe being omitted. The chassis has the usual appliance for throwing this class of carriages into gear, and in addition an eccentric axle placed at right angles to and supported by a double front transom, and carrying a traverse wheel, by means of which motion is communicated to the chassis. The chassis is otherwise transomed and braced in accordance with the system. Heretofore nearly all sea-coast carriages were made of wood, but in consequence of the great difficulty of preserving this material from decay, especially when exposed to the dampness of casemates, they have nearly all been replaced by wrought iron. The carriages principally employed for the transportation of ammunition, implements, and materials for repairs, are caissons, mortar-wagons, forges, and battery-wagons.
The Caisson.—Caissons are used for conveying ammunition for a field-battery; all are similar in form. It is a four-wheeled carriage, consisting of two parts, one of which is a limber similar to that of the gun-carriage, and connected in a similar way by a wooden stock and lunette. On the axle-body of the rear part and parallel to the stock are placed three rails, upon which are fastened two ammunition-chests, one behind the other, and similar to the one on the limber; so that the caisson has three ammunition-chests, which will seat 9 cannoneers. The interior compartments of the ammunition-chests vary according to the nature of the ammunition with which they are loaded. In rear of the last chest is placed a spare-wheel axle of iron, with a chain and toggle at the end of it. On the rear end of the middle rail is placed a carriage hook similar to a pintle-hook, to which the lunette of a gun-carriage whose limber has become disabled may be attached, and the gun carried off the field. The caisson has the same turning capacity and mobility as the gun-carriage, so that it can follow the piece in all its manœuvres, if necessary. It also carries a spare-wheel, spare-pole, etc. The principal parts of the caisson are: stock, or middle-rail; it has an iron lunette on its front end; side-rails, front foot-board, rear foot-board, middle-chest, rear-chest, spare-wheel axle; it has a body, two ribs, and a chain and toggle to secure the wheel; there are also two stays for the axle; lock-chains, fastened to lock-chain bridles under the front ends of the side-rails, and held up by lock-chain hooks fastened to the outside of the side-rails; spare-pole, spare-pole key, key-plate, chain, and pin; the key-plate is fastened to the under side of the lunette; the key is attached to the left side of the stock by a chain and eye-pin; carriage-hook, for attaching a carriage that has lost its limber; wheel-guard plates, spare-pole ring, held by the axle-strap; ring-bolt for spare hand-spike, key-plate and key, on the right side of the middle-rail; key-plate, chain, and key for the shovel-handle, on the inside of the right side-rail; middle assembling-bar, of iron; it has two ears in the middle to serve as stay-plates for the middle-chests, and a slot for the axe on the right of the middle-rail; rear assembling-bar; it supports the spare-wheel axle, and has a slot for the pickaxe on the left of the middle-rail.Axle, the axle-body, being notched to receive the middle-rail and tenoned to fit into the notches in the side-rails; staples for tool-handles; they are driven into the top of the axle-body in front of the iron axle-tree, one for the shovel-handle near the right side-rail, the other for the handle of the pickaxe on the left of the middle-rail. Wheels of all artillery carriages are similarly constructed; they differ, however, in the size and strength of certain parts, depending on the size of the carriage to which they are attached. The principal parts are: the nave, the nave-bands, the nave-box, the spokes, the felloes, and the tire. The nave constitutes the central portion of the wheel, and distributes the pressure of the axle-arm to the spokes. It is generally made of a single piece of wood, and strengthened by four iron bands called the nave-bands. It is also pierced with a conical hole for the axle-arm; and to diminish wear and friction, it is lined with a box of brass or cast iron, called the nave-box. The spokes serve to transmit the pressure of the load to the rim of the wheel. In all artillery carriages there are seven felloes and fourteen spokes. The felloes are the wooden segments which form the rim, and are joined together at their ends by wooden pins, or dowels. The tire is a strong band of iron, shrunk tightly around the felloes, to hold them together, and protect the rim from wearing away by contact with the ground.
Mortar-wagonsare designed for the transportation of siege-mortars and their beds, or of guns or large shot, and shells. A limber similar to the one for siege-gun carriages is used with it. The body consists of a platform of rails and transoms resting on an axle-tree. The stock is formed by prolonging the two middle-rails. The side-rails projecting to the rear form supports for the pivots of a windlass-roller. This roller is used to load guns and mortars on the wagon by drawing them up the stock. A muzzle-bolster on the stock near the limber, and a breech-hurter near the hind part of the wagon, are provided and used when long pieces are transported on it. Mortars are usually carried mounted on their beds.
Thetraveling-forgeis a complete blacksmith’s establishment, which accompanies a battery for the purpose of making repairs and shoeing horses. It consists of a body, upon which is constructed the bellows-house, etc., and the limber, which supports the stock in transportation. The body is composed of two rails, a stock, and an axle-tree. The bellows-house is divided into the bellows-room and iron-room. Attached to the back of the house is the coal-box, and in front of it is the fireplace. From the upper and front part of the bellows an air-pipe proceeds in a downward direction to the air-box, which is placed behind the fireplace. The vise is permanently attached to the stock, and the anvil, when in use, is supported on a stone or log of wood, and when transported is carried on the hearth of the fireplace. The remaining tools are carried in the limber-chest. When in working order the point of the stock is supported by a prop. Nomenclature of the traveling-forge body: Lunette, prop, vise, stock, wheel-guard plates, stock-stirrup, fireplace, back of fireplace, air-back, wind-pipe, bellows, ribs, hinges, hook, fulcrum, hook and staple, roof of bellows-house, bows, studs, girders, end-boards, bottom-boards, side-rail, lock-chain hook, coal-box, lid or roof, handles, hinges, turnbuckle, and hasp. A new pattern of field-forge has been proposed by Col. Laidley, U. S. Ordnance Corps.
Thebattery-wagonis employed to transport the tools and materials for repairs. Among the tools are those for carriage-makers, saddlers, armorers, and laboratorians’ use, scythes and sickles for cutting forage, and spare implements for the service of the piece. The body of the battery-wagon is a large, rectangular box, covered with a roof of painted canvas; and to the back part is attached a rack for carrying forage. The bottom of the body is formed of one middle- and two side-rails, resting on a stock and axle-tree, as in the traveling-forge. The tools and materials of the battery-wagon are carefully packed in the manner prescribed by the Ordnance Manual, in order that no difficulty may be experienced in finding a particular article when wanted. The smaller articles are carried in boxes properly lettered and numbered. The traveling-forge and battery-wagon are not confined to the service of field-batteries, but are used with siege and sea-coast carriages as occasion may require. Nomenclature of the battery-wagon body: Lunette, stock, wheel-guard plate, lock-chain, lock-chain bridle, lock-chain hook, studs, side-rails, upper rails, hinges, bows, cover-boards, cover-strap and turnbuckle, hasp, side-boards, stays, bottom-rails, bottom-boards, cross-bars, forage-rack, including chains, sides, and bars.
Ordnance, Construction of.The present condition of gun construction is mainly experimental. Iron in one form or another is the only material used for heavy artillery, but the particular form in which it is to be used, whether as cast, wrought, or steel, or whether in bars, coils, or ingots, or in combination,—as, for instance, steel or wrought iron interior and cast iron or wire-wrapped or hooped exterior,—is still undecided, and it is left for experiments which are still in progress, or to be made hereafter, to decide which is best. In the United States, cast iron is used for smooth-bore guns, and also for rifle guns, but as its use for the latter has not proved satisfactory, experiments are now being made with wrought iron lined and with wire-wrapped and other built-up guns, with fair prospect of success. In England, modern gun construction at one period inclined to the use of a steel or wrought iron interior tube, strengthened by an exteriorcasting of iron, which is the system of Palliser and Parsons. But the preference for the inventions of Sir William Armstrong, improved by those of Fraser, have resulted in the exclusive use, in that country at present, of the system of these two inventors. This method of gun construction is, in brief, a steel core (or body of the gun) strengthened by three or more exterior tubes of coiled wrought iron. This system is at present popularly known as the “Woolwich,” but sometimes called the “Elswick,” from the place where Sir William Armstrong’s works are now located. In Germany and Russia, and some other European nations, the Krupp system of heavy forgings of steel ingots is preferred. This last is by far the most expensive, and does not always produce the most durable guns. The question of breech- or muzzle-loading is still an undecided one. (SeeBreech-loadingandBreech-mechanism.) The Germans prefer the first named, as do the French, Austrians, and Russians, for large calibers and for most small guns, while the English, after several years’ trial of the first, have of late abandoned its use and returned to the muzzle-loader, though the question has again been recently agitated. In the United States, experiments still going on have not yet demonstrated which principle is the best suited to the gun construction used in America. The advantages of loading at the breech with heavy guns are numerous and great; but the serious mechanical difficulties (seeBreech-mechanism) of perfecting the movable breech attachment have militated against its adoption, especially in a country committed like the United States to the use of cast iron. During the half-decade (1855-60), and the succeeding decade (1860-70), enormous strides were made in gun construction and in that of carriages and projectiles, and the manufacture of gunpowder.
Cast Metal Guns.—The principles which govern the construction of homogeneous cast metal guns as established by long practice will be considered under the following heads:
Exterior Form.—The exterior of cannon is generally divided into five principal parts, viz.: the breech, the first reinforce, the second reinforce, the chase, and the swell of the muzzle.
Thebreech(seeBreech) is the thickness of metal in the prolongation of the axis of the bore, and should be at least equal to one and a quarter times the diameter of the bore; a less thickness has been found insufficient for heavy iron guns.
Thefirst reinforce(seeReinforce) extends from the base-ring to the seat of the ball, and is the thickest part of the piece, for the reason that the pressure of the powder is found to be greatest before the projectile is moved far from its place. In shape this reinforce was formerly made slightly conical, under the impression that the pressure was greater at the vent than at the seat of the projectile; but it is now made cylindrical throughout. The thickness of bronze cannon at the seat of the charge is less than for iron guns.
Thesecond reinforce(seeReinforce) connects the first reinforce with the chase. It is made considerably thicker than is necessary to resist the action of the powder, in order to serve as a proper point of support for the trunnions, and to compensate for certain defects of metal liable to occur in the vicinity of the trunnions of all cast cannon, arising from the crystalline arrangement and unequal cooling of the different parts.
The Chase(seeChase).—From the extremity of the second reinforce cannon taper more or less rapidly to the vicinity of the muzzle; this part called the chase constitutes the largest portion of the piece in front of the trunnions. The thickness of metal in the chase should be sufficient to resist the striking of the ball against the side of the bore. This injury being greater in bronze and soft iron guns, their taper is less than in cast-iron cannon. In the construction of bronze guns, the thickness of metal at the neck or thinnest part is about five-elevenths of that at the first reinforce. All projections on the surface of cannon not absolutely necessary for the service of the piece are omitted in cannon oflate models. This omission simplifies their construction, renders them easier to clean, and obviates certain injurious strains that would otherwise arise from unequal cooling in fabrication.
Swell of the Muzzle.—The enlargement called swell of the muzzle was generally regarded as necessary, inasmuch as the metal situated immediately at the muzzle is supported only in rear, and it was thought necessary to increase its thickness in order to enable it to resist the action of the projectile at this point. At present, however, the tendency is to reduce the size of the swell of the muzzle and to omit it entirely on all sea-coast cannon.
Interior Form of Cannon.—The interior of a cannon may be divided into three distinct parts, viz.: the vent, or channel which communicates with the charge; the seat of the charge or chamber, if its diameter be different from the rest of the bore, and the cylinder, or that portion of the bore passed over by the projectile (see appropriate headings).
Thevent(seeVent) 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 themetal 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. (SeeGomer 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(seeBore).—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 thebody 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 ofhollow casting. The general form of the old casting is that of asolidfrustrum 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 theexterior. 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 inbronze, 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. (SeeOrdnance, 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.—SeeOrdnance, 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. SeeNapoleon 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 (seeOrdnance, 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.
In theconstruction of built-up guns, makers have aimed at the ideal gun which has its strength proportioned to the strain it is called upon to bear in all its parts. All parts of the sides of a cannon are not strained equally, and are therefore not brought to the breaking-point at the same time. Any arrangement of the parts by which the explosive strain is distributed equally over the entire thickness of the piece, necessarily brings a greater amount of resistance into play to prevent rupture. There are two general plans for accomplishing this, viz.: First, by producing a strain of compression on the metal nearest the surface of the bore. This istermed an “initial strain,” and is brought about by shrinking heated bands or tubes around the part to be compressed, or by slipping a tube into the bore, which has been slightly enlarged by heat. In either case it is apparent that the extent of the strain depends on the relative size of the fitting surfaces, and the amount of heat used to produce expansion. Sometimes the parts are forced together by hydraulic pressure after they have been carefully bored and turned to the proper size. The second plan is based on “varying elasticity,” and is accomplished by placing that metal which stretches most within its elastic limit around the surface of the bore, so that by its enlargement the explosive strain is transmitted to the outer parts. By the selection of suitable materials and their proper management, both of these plans may be combined in the same gun, and thereby give it increased strength. SeeOrdnance, Construction of.
The best-known cannon of thebuilt-upclass are:
Ames Gun.—The rifled guns made by Mr. Horatio Ames, of Falls Village, Conn., are made of wrought iron on the built-up principle. The wrought iron is in the form of rings, made by bending a bar around a mandrel and welding the ends. After turning them in a lathe, two or more of these rings are fitted one within another to form a disk. These disks are welded in succession to a concave breech-piece. Some of these guns have shown remarkable endurance. They are weakest against longitudinal strains.
Armstrong Gun.—Is so much like theWoolwich, which it preceded, that a separate description is unnecessary. SeeWoolwich Gun.
Blakely Gun.—The most approved pattern of the gun invented by Capt. Blakely combines in its construction the principles of “initial tension” and “varying elasticity,” the object of which is to bring the strength of all the metal of the piece into simultaneous play to resist explosion. It is made of several tubes or barrels, the inner one of which is of low steel, having considerable but not quite enough elasticity. The next tube is made of high steel with less elasticity, and is shrunk on the barrel with just sufficient tension to compensate for the insufficient difference of elasticity between the two tubes. The outer cast jacket, to which the trunnions are attached, is the least elastic of all, and is put on with only the shrinkage by warming it over a fire. The steel tubes are cast hollow and hammered over steel mandrels under steam-hammers; by this process they are elongated, and at the same time the tenacity of the metal is increased, all the steel parts are annealed. Other combinations of iron and steel are used, except wrought iron, which is regarded as objectionable on account of its tendency to stretch permanently. Blakely guns were rifled with one-sided grooves, and are fired with expanding projectiles. This gun is no longer made under that name. As now made it is called the
Vavasseur Gun, and is manufactured by Messrs. J. Vavasseur & Co. of the London Ordnance-Works. It is made entirely of the best Sheffield cast steel, except the trunnions, which are wrought iron, and consists of an interior tube and outer tube and a number of hoops. The inner tube is forged from a solid ingot. It is rough bored and turned and then oil tempered. The outer tube and rings are cast hollow and hammered over steel mandrels. They are heated and shrunk on. Theoretically, it is difficult to pick a flaw in the construction of this gun. The rifling used is anomalous. It consists of threeribsinstead of grooves projecting into the bore. The projectile has corresponding grooves. These guns have found quite a market in the South American republics.
Brooke Gun.—This gun was made after the plan of Capt. Brooke for the Confederate service; it resembles Parrott’s in shape and construction, except that the reinforcing band is made up of iron rings not welded together. The rifling is similar to that used in the Blakely guns.
Fraser Gun.—SeeWoolwich Gun.
Gatling Gun.—SeeGatling Gun.
Krupp Gun.—SeeKrupp Gun.
Lancaster Gun.—This gun is now little used; it was made of wrought iron. The bore was cut in a spiral form with an elliptical cross-section, and the projectile shaped to fit it, by which means a rotary motion was imparted.
Palliser Gun.—Maj. Palliser of the British service is the inventor of a system which has been successfully applied in England to utilize smooth-bore cast-iron guns by converting them into rifles. By his plan the gun is first bored to a cylinder or finely tapering cone, then lined with a tube of coiled wrought iron, the breech end of which is shrunk on; the exterior of the barrel has a uniform diameter throughout. The tube is double at this part to obtain the benefit of the tension and to enable any fracture of the inner layer to be made known without bursting the gun. The bottom of the barrel is closed by a wrought-iron cup screwed in. The tube is inserted into the gun from the muzzle without the application of heat. A small amount of play is allowed between the barrel and the cast-iron body; this disappears, or is much reduced by a “setting up charge,” which expands the barrel against the cast iron. The end of the barrel is made to bear accurately against the cast-iron breech. A collar screwed into the muzzle secures the tube in position, and prevents it from being thrust forward by the compression of the metal by repeated firing. In front of the trunnions a pin is screwed in through the cast iron, to resist the tendency of the tube to be turned by the bearing of the projectile in the grooves. On the exterior of that portion of the inner tube that is covered by the second tube is cut a spiral gaschannel; this communicates with a tell-tale hole drilled through the cast-iron breech, by which gas can escape and announce the fracture of the inner tube. The venting and rifling are similar to those employed in the Woolwich guns. In the larger guns Maj. Palliser proposes to use two or more concentric tubes, in some the exterior one to be of steel. This system is being applied in the United States with the most promising results in the conversion of 10-inch Rodman guns into 8-inch rifles. The rifles thus obtained, though giving to a projectile a less muzzle velocity than does the 10-inch smooth-bore, has, on account of the increased weight of shot, greater penetrating power at all ranges, being doubled at some and trebled at others. Its accuracy is three times greater, and the capacity of its shell twice that of the original gun.
Parsons Gun.—The system upon which Mr. Parsons makes his guns is similar to that of Maj. Palliser. (SeePalliser Gun.) It depends upon the principle of varying elasticities, and is based upon the fact that wrought iron may be stretched three times as much as cast iron, and will offer three and a half to six times the resistance within the limit of its elasticity. These well-known gun constructions, known asconverting systems, both consist in lining a cast-iron case with a wrought-iron or steel tube. In the Palliser or English method the tube is inserted from the muzzle. In the Parsons or American method, through the breech. In both nearly the whole of the longitudinal strain is transferred to the cast-iron case. Both systems were first perfected in England. Col. Crispin (U. S. Ordnance Corps) deserves the credit of introducing them into the U. S. service in constructing the newexperimental rifles. The Parsons system is better adapted to constructing breech-loaders.
Parrott Gun.—The Parrott rifled gun is a cast-iron piece of about the usual dimensions, strengthened by shrinking a coiled band or barrel of wrought iron over that portion of the reinforce which surrounds the charge. The body of the larger Parrott guns are cast hollow, and cooled from the interior on the Rodman plan. The barrel is formed by bending a rectangular bar of wrought iron spirally around a mandrel, and then welding the mass together by hammering it in a strong cast-iron cylinder, or tube. In bending the bar, the outer side being more elongated than the inner one, is diminished in thickness, giving the cross-section of the bar a wedge shape, which possesses the advantage of allowing the cinders to escape through the opening, thereby securing a more perfect weld. The barrel is shrunk on by the aid of heat, and for this purpose the reinforce of the gun is carefully turned to a cylindrical shape, and about one-sixteenth of an inch to the foot larger than the interior diameter of the barrel in a cold state. To prevent the cast iron from expanding when the barrel is slipped on to its place, a stream of cold water is allowed to run through the bore. At the same time, and while the band hangs loosely upon it, the body of the gun is rotated around its axis to render the cooling uniform over the whole surface of the barrel. The proof of the Parrott guns consists in firing each piece 10 rounds with service charges.
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 (seeOrdnance, Strains upon), and as a remedy for them proposed that cannon should be cast on 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 many of the guns employed in the field service.
Whitworth Gun.—These guns are made of a species of low steel; the smaller are forged solid, the larger are built up with coils or hoops; the hoops are forced on by hydraulic pressure, and for this purpose are made with a slight taper and with the design to secure initial tension. The ends of the hoops are joined by screw-threads. The hoops are first cast hollow, and then hammered out over a steel mandrel. Before receiving their final finish they are subject to an annealing for some three or four weeks, which makes the metal very ductile, but at the same time slightly impairs its tenacity. The system differs from Krupp’s in the smaller masses used and the greater number of hoops. The process for making the hoops is better calculated to develop their tensile strength. The breech-pin is made with offsets in such a way as to screw into the end of the barrel and the next two surrounding hoops. The cross-section of the bore of the Whitworth gun is a hexagon with rounded corners. The twist is very rapid and the projectiles are made very long.
Woodbridge Gun(invented by Dr. Woodbridge, of Little Falls, New York).—The system of construction consists essentially of a thin steel barrel over which wire is wound, barrel and wire being subsequently consolidated into one mass by a brazing solder melted and poured into the interstices. The following brief description is extracted from one of the inventor’s letters to the chief of ordnance: “Square wire is wound upon a steel core somewhat longer than the intended bore of the gun, a sufficient number of wires being wound at once side by side to produce the required obliquity of the turns. The successive layers have opposite twists. When the mass has reached the required dimensions, it is inclosed in an air-tight caseto protect it from oxidation, and is heated therein to a temperature somewhat above that required for the fusion of the soldering metal. The soldering metal having been melted is run in, filling all the interstices of the mass. When cooled the gun is bored and finished as usual.” The invention dates back to about 1850. A small gun made in this way was tested by Maj. Laidley (U. S. Ordnance Corps) in 1865. It endured 1327 rounds with excessive charges, when the attempt to burst it was abandoned on account of the breaking off of the trunnions. The only large gun ever made—a 10-inch gun—was fabricated at Frankford Arsenal. It was not entirely finished till April, 1876, soon after which it was displayed at the Centennial Exhibition in Philadelphia. Certain defects in its manufacture prevent it from fairly representing the Woodbridge system.
Woolwich Gun.—The Woolwich or Fraser gun is in its construction a modification of the Armstrong plan, which latter had been previously used in Great Britain; the principal difference is in substituting for a number of single coils and a forged breech-piece a few long double and triple coils, and in using a cheaper quality of wrought iron. The number of pieces employed in the construction depends upon the size of the gun; an 8-inch rifled gun is composed of the inner tube (barrel) of steel, the muzzle-coil (trousers), the breech-coil (jacket), and the cascabel-screw. The barrel is made from a solid forged cylinder of cast steel, drawn by heating and hammering; it is turned, bored, and chambered; then heated to a uniform temperature in a vertical furnace and plunged into a covered tank of rape-oil, where it cools and soaks. The muzzle-coil is constructed of two single coils welded together endways. Each coil is formed by heating a long bar and wrapping it about a mandrel; this is next heated in a reverberatory furnace and welded under a steam-hammer. Before being united the two cylinders are turned and bored. The breech-coil is composed of a triple coil, a trunnion-ring, and a double coil welded together. The double coil is formed by placing a single coil, when cold, on a mandrel and winding over it, but in the reverse directions to break joints, a second bar; if over this a third bar is immediately wound in the same direction as the first, a triple coil will result. These coils are welded by being heated and hammered on the end and on the sides. The trunnion-ring is made by welding slabs of iron together on the flat end of a bar, and gradually forming a ring by driving through the centre wedges and mandrels increasing in size; the trunnions, one of which comes from the bar, are at the same time hammered into shape. The coils and the ring having been turned and bored, the latter is placed on a shoulder of the triple coil, the double coil is dropped through the trunnion-ring on the triple coil, and the joints welded in this position. The cascabel is forged of good scrap-iron; the different parts having been formed are accurately turned and bored with a slight taper. The muzzle-coil tube being heated is dropped over the barrel, which is stood in a pit, a stream of cold water circulating through the bore. The half-formed gun is then placed on its muzzle, water forced through the bore, and the breech-coil heated and slipped into position. The cascabel is screwed into the breech-coil abutting against the barrel, great care being taken that the contact is perfect. A tell-tale hole is cut along the thread on the cascabel to give warning by the escape of gas should the barrel break in firing. The vent is bored through hardened copper; it enters near the centre of the service cartridge. This gives greater velocity, but also greater pressure. The large guns have from seven to ten grooves. The twist is uniformly increasing; the shape of the grooves is circular, with curved edges.
Sutcliffe Gun.—This invention, by E. A. Sutcliffe of New York City, relates to a breech-mechanism for cannon. SeeBreech Mechanism.
Griffin Gun.—Name sometimes given to the 3-inch rifled field-piece in the U. S. service. It is made of wrought iron. The method of fabrication is to wrap boiler-plate around a mandrel and to weld it.
Ordnance, Metals for.The only metals ordinarily used for cannon are cast and wrought iron, steel, and an alloy of copper and tin, or a combination of these metals. Cannon metals should be able to resist the corroding action of the atmosphere, the heat and the products of combustion of the powder; should be susceptible of being easily bored and turned, and should not be too costly. The qualities necessary in cannon metals are strength to resist the explosion of the charge, weight to overcome severe recoil, and hardness to endure the bounding of the projectile along the bore. The shape of the bore would otherwise be rapidly altered by the action of the projectile. This quality is particularly necessary in rifled cannon. The term strength as applied to cannon metal is not confined to tensile strength alone, but embraces also elasticity, ductility, and crystalline structure, which affect its power to resist the enormous and oft-repeated force of gunpowder. (SeeOrdnance, Strains upon.) Each discharge of a cannon, however small, impairs its strength, and repeated a sufficient number of times, will burst it; this arises from the fact that the feeblest strains produce a permanent elongation or compression of iron; this is technically known as the permanent set, and the same is probably true of all other metals. The property of ductility is of importance in enabling a metal to resist rupture after it has passed its elastic limit. The size and arrangement of the crystals of a metal have an important influence in its strength to resist a particular force. A metal will be strongest when its crystals are small, andthe principal faces parallel to the straining force, if it be one of extension, and perpendicular to it, if it be one of compression. The size of the crystals of a particular metal depends on the rate of cooling; the most rapid cooling giving the smallest crystals.
Cast ironis very generally employed, notably in the United States, in the fabrication of heavy cannon for siege and sea-coast purposes. It possesses the very important qualities of tenacity, hardness, and cheapness, and with proper care is not seriously affected by rust. Its principal defect is an almost entire want of elasticity, in consequence of which its tenacity is destroyed after a certain number of applications of the straining force. But little is known of the causes which affect the quality of the cast iron used for cannon metal. The amount of carbon, the state of its combination, together with the ore, fuel, and fluxes, and the process of manufacture, all materially affect the quality of the iron. All that is known is, that certain ores treated in a certain way make cast iron suitable for cannon, and the fitness of a particular kind of cast iron for artillery purposes can only be determined by submitting it to the tests of the service. After this is known, a knowledge of certain physical properties, such as tenacity, hardness, density, and color, form and size of crystals presented in a freshly fractured surface, will be useful in keeping the metal up to the required standard. The pig-iron from which cannon are made should be soft, yielding easily to the file and chisel; the appearance of the fracture should be uniform, with a brilliant aspect, dark gray color, and medium-sized crystals. When remelted and cast into cannon, it should have about sufficient hardness to resist the file and chisel, but not to be so hard as to be bored and turned with much difficulty; its color should be a bright gray, crystals small, structure uniform, close, and compact. The density of gun metal should be about 7.25, and its tenacity about 30,000. There are several varieties of cast iron differing from each other by almost insensible shades. The principal divisions are, however, gray and white. Gray iron is softer and less brittle than the white, is slightly malleable and flexible, and does not resist the file. It has a brilliant fracture of a gray or bluish-gray color. This iron melts at a lower temperature than white iron and becomes more fluid, contracts less and contains fewer cavities; it fills the mold well, the edges of a casting are short, and the surface smooth, convex, and covered with carburet of iron. Gray iron is the only kind suitable for making castings which require great strength, such as cannon. White iron is very brittle, resists the file and chisel, and is susceptible of high polish, the surface of a casting is concave, the fracture presents a silvery appearance. Its qualities are the reverse of those of gray iron; it is therefore unsuitable for ordnance purposes. Mottled iron is a mixture of white and gray; it has a spotted appearance, and flows well. The casting has a plane surface with edges slightly rounded. It is suitable for making shot and shells. Besides these general divisions, there are several other varieties of iron whose qualities depend upon the proportion of carbon, and the state in which it is found in the metal. The color and texture of cast iron depend greatly on the size of the casting and the rapidity of cooling. SeeOrdnance, Strains upon.
Wrought ironwas among the earliest metals employed in the construction of cannon, but in consequence of the defects which almost invariably accompany the forging of large masses, it was superseded by bronze and cast iron to a great extent. Wrought iron is softer than cast iron, and, being pure iron, is more liable to be corroded by the action of the atmosphere and products of combustion of the powder; it possesses also considerable ductility. The tensile strength of wrought iron, which under the most favorable circumstances is double that of the best cast iron, depends on the character of the crystalline structure, and the manner of applying the tensile force, or in other words, wrought iron offers the greatest resistance to a force of extension when the structure is fibrous, and the force acts in the direction of the fibres. The practical difficulties of rapidly cooling large masses so as to form small crystals, and compressing them by hammering, rolling, or otherwise to develop and give a particular direction to the fibre, have not thus far been wholly surmounted. On the contrary, large masses are generally found to contain such internal defects as false welds, cracks, and a spongy and irregularly crystalline structure, arising from the more rapid cooling of the exterior surface.
Steelis a compound of iron and carbon, in which the proportion of the latter seldom exceeds 1.7 per cent. It may be distinguished from iron by its fine grain, its susceptibility of hardening by immersing it when hot in cold water, and with certainty by the action of diluted nitric acid, which leaves a black spot on steel, and on iron a spot which is lighter colored in proportion as the iron contains less carbon. For the construction of cannon, steel may be divided into high and low steel, the difference being that the former contains more carbon than the latter. High steel is very hard and has great ultimate tenacity. It has but little extensibility within or without the elastic limit, and is therefore too brittle for use in cannon, unless used in such large masses that the elastic limit will not be exceeded by the explosive force of the powder. It melts at a lower temperature than wrought iron and is difficult to weld, as its welding temperature is but little less than that at which it melts. Low steel is often known as “mild steel,” “soft steel,” “homogeneous metal,” and “homogeneous iron,” and is made by fusing wrought iron with carbonin a crucible; after which it is cast into an ingot and worked under a hammer. As it contains less carbon than high steel, it has greater specific gravity. It can be welded without difficulty, although overheating injures it. It more nearly resembles wrought iron in all its properties, although it has much greater hardness and ultimate tenacity, and a lower range of ductility depending on its proportion of carbon. It has less extensibility within the elastic limit than high steel, but greater beyond it, or in other words, greater ductility. Its great advantage over wrought iron for general purposes is that it can be melted at a practicable heat, and run into large masses possessing soundness and tenacity. Its advantages for cannon are greater elasticity, tenacity, and hardness. Its tenacity when suitable for cannon is three times as much as cast gun iron, and one-half more than the best wrought iron. The principal varieties of steel are:
Natural Steel.—This is made principally in Germany, and is used for making files and other tools. It is obtained by reducing the rich and pure kinds of iron ore with charcoal, and re-fusing the cast iron so as to bring it to a malleable state. The India steel, or Wootz, is a natural steel containing a small proportion of other metals.
Blistered Steel.—This is prepared by exposing alternate layers of bar-iron and charcoal in a close furnace for several days. When taken out the bars are brittle in quality and crystalline in appearance. The purpose for which the steel is to be used determines the degree of carbonization. The best qualities of iron (Russian and Swedish) are used for the finest kind of steel.
Tilted Steel.—This is blistered steel moderately heated and subjected to the action of a tilt-hammer, by which means its density and tenacity are increased.
Shear Steel.—A blistered or natural steel refined by piling thin bars into fagots, and then rolling or hammering them into bars, after they have been brought to a welding heat in a reverberatory furnace. The quality is improved by a repetition of this process, and the steel is known accordingly by the names, half shear, single shear, double shear, etc.
Cast Steel.—This is made by breaking blistered steel into small pieces, and melting it in close crucibles from which it is poured into iron molds. The ingot is then reduced to a bar by hammering or rolling with great care. Cast steel is the finest kind of steel, and is best adapted for most purposes; it is known by a very fine, even, and close grain, and a silvery homogeneous fracture. The most remarkable specimen of cast steel for tenacity which is on record was manufactured at Pittsburgh, Pa. It was tested at the Washington Navy-Yard, and found to sustain 242,000 pounds to the square inch. The strength of cast steel usually runs from 70 to 140,000 pounds.
Bessemer Steel.—This steel is produced by forcing air into melted iron, by means of which the carbon and silicon of the crude cast iron is oxidized. The essential difference between this process and the ordinary puddling is mechanical, and consists in the intense and violent stirring of the Bessemerized iron, to which alone is due the production and maintenance of a temperature, without any other fuel than the carbon and silicon contained, that keeps the metal fluid so that it can be cast into homogeneous malleable ingots. When decarburation has been carried far enough, the current of air is stopped, and a small quantity of white pig-iron containing a large amount of manganese is dropped into the liquid metal. No very large cannon have yet been made wholly of Bessemer steel, but several small ones have, which have shown great endurance. Experiments at the Woolwich Arsenal have shown that the tenacity of this steel is more than doubled by hammering.
Siemens-Martin.—In this process the ingredients of cast steel are melted together on the open hearth of a reverberatory furnace of special construction, and a certain proportion of manganese necessary to make a sound and practically malleable steel added. This steel is, however, little used in gun construction.
Semi-Steel.—If in the process of puddling or decarbonizing cast iron the process be stopped at a particular time, determined by indications given by the metal to an experienced eye, an iron is obtained of greater hardness and strength than ordinary iron, to which the name of semi-steel, or puddled steel, has been given. The principal difficulty in its manufacture is that of obtaining uniformity in the product, homogeneity and solidity throughout the entire mass. It is much improved by reheating and hammering under a heavy hammer; but it has not been found a reliable material for even cannon of small caliber. The celebrated guns made by Mr. Krupp of Germany are of cast steel, made from puddled steel, and of peculiar character, combining great tensile strength with the property of stretching to a great extent without breaking. Sir Joseph Whitworth improves the qualities of steel for his more recent guns by casting it under hydraulic pressure.
Chrome Steel.—An alloy of iron and chromium, which is not steel in the ordinary sense, but which possesses many of its characteristics. The tensile strength and resistance to crushing is much higher than ordinary cast steel. This material has been largely used in bridge-building, but has not yet been applied to cannon-making.
Bronzefor cannon (commonly called brass) consists of 90 parts of copper and 10 of tin, allowing a variation of one part of tin more or less; by increasing the proportion of tin, bronze becomes harder, but more brittle and fusible; by diminishing it it becomes too soft for cannon, and at the same time loses a partof its elasticity. Bronze is more fusible than copper, much less so than tin. It is harder, less susceptible of oxidation, and much less ductile than either of its constituents. Its fracture is of a yellowish color, with little lustre, a coarse grain, irregular, and often exhibiting spots of tin which are of a whitish color. The density and tenacity of bronze when cast into the form of cannon, are found to depend upon the pressure and mode of cooling. In consequence of the difference of fusibility of tin and copper, the perfection of the alloy depends much on the nature of the furnace and the treatment of the melted metal. By these means alone the tenacity of bronze has been carried up to 60,000 pounds. Bronze is but slightly corroded by the action of the gases evolved from gunpowder, or by atmospheric causes; but its tin is liable to be melted away at the sharp corners by the great heat generated in rapid firing. It is soft, and therefore liable to serious injury by the bounding of the projectile in the bore. This injury is augmented as the force of the rebound is increased by the elasticity of the metal. It was established by experiments of Maj. Wade of the U. S. Ordnance Corps more than twenty years ago that the tensile strength of bronze is related to its density. It has been discovered since that this density can be produced by artificial compression. Two men claim the honors of the invention—Gen. Uchatius of the Austrian army, and S. B. Dean, an American inventor. The methods are essentially the same. After the gun is cast, steel mandrels slightly conical in shape are driven through the bore by hydraulic pressure,—each being succeeded by one slightly larger,—thus enlarging the bore and compressing the metal surrounding it. It is claimed that the bronze is thus rendered harder and stronger, and the defects above cited in a large measure obviated. The term “steel bronze” or “bronze steel” has been applied to the metal so treated. Many guns have been made of it for the Austrian service,—the largest of which is a 6-inch breech-loader throwing a projectile of 85 pounds. This gun has proved itself slightly superior in power to the same sized Krupp gun of steel.
Aluminium Bronze.—An alloy of 90 parts of copper and 10 of aluminium. It is harder than ordinary bronze; much stronger, being 100,000 pounds to the square inch; it does not tarnish readily. Its properties would seem to especially fit it for a gun metal.Phosphor bronzeis an alloy with very similar properties.
Combined Metals.—Numerous trials have been made to improve the strength of cannon by combining two or more metals in such a way that the good qualities of one will counteract the defects of the others. But the only metals used to any extent are those described above. Steel is constantly gaining in favor as a cannon metal. It is now almost exclusively employed throughout Europe, and wherever the Krupp gun is used. The great perfection arrived at by Krupp and others in the manufacture of steel seems to place that metal above all others for gun construction, whilst the difficulty of handling large masses has been overcome by the enormous power of the machinery used. Steel is also sparingly employed both in the United States and England for converting smooth-bore guns into rifles according to the Palliser method, but experiments in the United States have shown that it is inferior to wrought iron for this purpose. SeeOrdnance, Construction of.
Wrought and cast iron are much used in this way for cannon in both the United States and in England. In the former, all the larger cannon belonging to the official system (both siege and sea-coast) are made of the cast metal, whereas the Parrott gun and the new rifled pieces are a combination of both. (SeeOrdnance, Construction of.) The metal chiefly employed in England is wrought iron, in combination with steel; the largest guns made at the Woolwich Arsenal are of this nature. Bronze, except as modified by the Austrians, has now nearly entirely gone out of use as a cannon metal. In France and the United States, field-pieces, mortars, and howitzers are still made of this material.