The superior fitness for cannons of steel over cast-iron was recognized many years ago, but the difficulty of casting steel in large masses prevented the introduction of steel guns, and the generally acceptable treatment of cast-iron made it answer satisfactorily the demands for gun-metal not subjected to unusual strains. Mr. Frederick Krupp, of Essen, in Germany, was the first steel manufacturer who succeeded in casting steel in large masses, and he produced a number of steel guns cast from crucibles in solid ingots, which were bored, turned, and fashioned as in the case of cast-iron smooth-bore guns. These guns held a position in advance of other manufactures on the score of strength of material. But the introduction of the rifle system, the call for higher velocities, the increased charges of powder, with the consequent increase of strain, enhanced by the friction attending the passage of the projectile forced along the bore, had the effect of calling attention to the weakness that was inherent in the method of construction of cannons. It is well known that an explosive force operating in the interior of a hollow cylinder of any thickness is not felt equally throughout the wall of metal; the parts near the seat of explosion are called upon to do much more work in restraining the force generated than are the parts more remote. It has been determined that the strain brought upon the portions of the wall is in inverse proportion to the squares of their distances from the seat of effort. Thus, in a gun cast solid, if we take a point two inches from the bore, and another four inches from the bore, the strain felt at those points respectively will be inversely in the proportion of four to sixteen, or, in other words, the metal at two inches from the bore will be strained four times as much as that at the distance of four inches. From this it can be seen that the metal near the seat of effort may be strained beyond its tensile strength, while that more distant is only in partial sympathy with it. Rupture thus originates at the interior portion, and the rest of the wall yields in detail. No additional strength of material can change this relationship between the parts; they result from a law, and show that this method of construction for a cannon is untrustworthy where the strains approach the tensile strength of the material.
The means of providing against this successive rupture of over-strained parts is found in the “built-up gun,” in which an interior tube is surrounded by encircling hoops of metal, which are shrunk on at sufficient tension to compress the portions which they enclose. This is the principle of “initial tension,” which is the basis of the modern construction of cannons. By adopting this method, an ingot to form a tube to burn the required amount of powder can be cast of a light weight in comparison with what would be needed for a complete gun, and the strength and number of reinforcing rings to be shrunk around it can be readily determined, proportioned to the known strain that will be brought upon the bore of the piece. The late developments in the manufacture of steel by the open-hearth process remove all difficulty to procuring the necessary metal in masses suitable for all parts of the heaviest guns.
BREECH-LOADING RIFLE-TUBE READY FOR RECEIVING JACKET.
The built-up steel gun is the one now adopted in Europe by the leading powers, and it is the gun with which the United States navy will be armed; but, before its final adoption, efforts were made to convert old smooth-bore cast-iron guns into rifles, and to construct new guns partially of steel and partly of wrought-iron. As some of these methods of conversion offered an economical means of acquiring rifled cannons, our naval authorities were led into the error of countenancing the effort to a moderate degree.
BREECH-LOADING RIFLE-JACKET, ROUGH-BORED AND TURNED.
The system that was adopted was that originally suggested by Mr. P. M. Parsons in England, which was afterwards patented by Major Palliser, R.A., and bears his name. It consisted in enlarging the bore of a cast-iron gun, and inserting a tube of wrought-iron formed of a bar arranged in the form of a coil when heated. The tube was expanded by firing charges of powder, and afterwards rifled. The guns are muzzle-loaders, and are not increased in length beyond that of the cast-iron gun which forms the casing for the tube. The length is thus limited in order to preserve the preponderance of the piece, and because of the want of longitudinal strength in the coil, which cannot be depended on beyond a few tons’ strain; the arrangement of metal in a coil provides very well for circumferential or tangential strains, but in the Palliser conversion the longitudinal strength depends on the cast-iron casing. The idea of the coiled wrought-iron tube originated with Professor Treadwell, of Harvard University, in 1841. He utilized it by enclosing a tube of cast-iron or steel in the same manner as it is applied in the wrought-iron Armstrong and Woolwich guns.
