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 compre ion. As the cooling continues this operation is repeated, until the whole ma is brought to a uniform temperature, and the straining force is increased to an extent which depends on the size and form of the ma , 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 pa ing 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 ma 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 le 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: