the eighty-fifth round, the hollow-cast at the two hundred and fifty-first round. Its endurance was thus three times that of the other.
Rodman's process was of fundamental importance, because it established experimentally the principle of initial exterior extension 
Rodman Fifteen-inch Gun. and interior compression. This principle is applied in all gun construction to-day, although the use of cast iron has been wholly discarded. Like many other ideas of great importance in the history of invention, it seems to have been evolved independently by several claimants. The names of Blakely, Whitworth, Armstrong, Longridge, Brooke, Treadwell, and Parrott are at once called to mind. To describe their inventions and discuss conflicting claims would require a volume. The discovery of such an important principle, followed by the outbreak of the American civil war, gave an impetus to the improvement of ordnance which was felt over the entire world.
Hitherto the materials used in gun construction were cast iron, wrought iron, and bronze, this last being an alloy of copper with ten per cent of tin. In tenacity bronze is superior to cast iron, but it is softer, more fusible, and more expensive. Cast iron is moderately fusible, but not fixed in composition, having a variable amount of carbon, silica, and other impurities diffused through its mass. Its properties are correspondingly variable, but it is in general hard, brittle, and more or less crystalline. Wrought iron is the result of oxidizing out all of the carbon by puddling, then squeezing out the silica, and rolling so as to develop a fibrous in place of crystalline structure. It is much more tenacious than cast iron, almost infusible, but capable of ready welding and forging. The admixture of carbon seems to confer the property of fusibility.
Steel is the product of the recombination of pure wrought iron with a very small percentage of carbon and sometimes of manganese or nickel. Like cast iron, it is fusible; like wrought iron, it can be readily forged; and it is superior to each in elasticity and tenacity. The idea long ago suggested itself that steel ought to be the best material for the construction of cannon. But the practical obstacle was the great difficulty of securing large enough forgings of steel, and this of sufficiently good quality. Only since 1860 have the methods of steel manufacture been so improved as to make this metal available on a large scale.
