With regard to type, the adoption of armour inevitably gave a set-back to the value of the shell gun. Shells, which would rend and set on fire a wooden ship, would not pierce armour or inflame iron plates; of which facts Hampton Roads afforded a demonstration. It seemed clear also from that incident, to experts in this country and in France, that no extension of the Paixhans principle was likely to compete with armour in the future. The system of shell fire of General Paixhans, like the shot system of the inventor of the carronade, had relied on low muzzle velocities and curved trajectories, to effect its purpose. His shells were for lodgment rather than penetration, and did not gain their effect by their kinetic energy; and in view of this their inventor had himself conceived the use of iron armour as the very means whereby they might be countered. Nevertheless the Americans had been strongly attracted by the Paixhans principle, and with their Dahlgrens and Columbiads had extended it in practice to embrace the use of guns of the largest calibres. The action between the _Monitor_ and the _Merrimac_ did nothing to shake their faith in this class of ordnance. Subsequent experiments appeared to confirm the national predilection; and one of their writers, in giving credit to the navy chiefs for adhering to the principle of the large smooth-bore gun, recorded that the small-bore-and-high-velocity theory had received its quietus by the utter demolition of a 6-inch plate by a ball from a 15-inch gun at Washington in February, 1864.[165] In France and England it was held, and held rightly, that high velocities were necessary for the attack of armour.