Rodman Gun

—The principal difficulty formerly experienced in manufacturing very large cast-iron cannon was the injurious the exterior. Gen. Rodman of the U. S. Ordnance Department developed a theory of 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 pa ing 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