(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: