Even if the new Whitworth system were adopted, the utilization of the old smooth-bore cannon which formed the existing national armament of ships and fortresses was not secured. Neither the Armstrong nor the Whitworth system provided an expedient for converting to rifled ordnance the thousands of cast-iron guns in which the defence of the country was invested. Efforts were therefore made to reinforce the old pieces so that, when rifled, they would be sufficiently strong to withstand the greater stresses entailed. Greater stresses in the metal, due to higher chamber pressures of the powder gases, were almost a necessary concomitant of rifling. For, apart from the increase in the size and mass of the projectile and its greater initial resistance to motion, pressures tended to increase in a greater ratio than the size of the pieces themselves; the mass of the projectile increased as the cube, the propulsive force of the gases as the square, of the diameter of the bore; hence to attain a given velocity, the larger the bore the higher the pressure required to propel it with a given type of powder,--other things being equal. No limit, therefore, could be assigned to the strength and power required of heavy ordnance. Moreover a struggle had begun in ’59, with the building of the _Gloire_ and _Warrior_, which was already foreshadowing tremendous developments both of guns and of armour. The experiences of America in this connection were not encouraging. The civil war served as an incentive to the Americans to rifle all their large calibre guns as quickly as possible. In ’62 large numbers of cast-iron cannon were rifled and reinforced by external hoops of iron. The results were deplorable. A great number of pieces burst; and experience made it clear that “a gun made up of a single homogeneous casting soon reaches a limit of resistance to internal pressure beyond which the addition of extra metal has little or no effect.” Two improvements must be mentioned as having more than a passing effect on the progress of ordnance in America: first, the adoption of compressed and perforated powder which, by prolonging the combustion period, caused a more even distribution of stresses over all sections of the barrel; second, the casting of guns hollow and the chilling of their interiors, so as to form on the inside of the piece a hardened stratum on which the outer parts of the casting contracted as they slowly cooled, thus giving it support. But in spite of these inventions it became apparent that cast iron was in its nature unsuited as a material for rifled ordnance.