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.
In the construction 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 is termed 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. See [Ordnance, Construction of].
The best-known cannon of the built-up class 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.