One other feature was put forward to complete this scheme of re-armament, the importance of which it is unnecessary to emphasize. M. Paixhans explored the possibility, by the sacrifice of a tier or more of guns, of rendering all classes of ships invulnerable by casing their sides with iron plates. Although rejected at the time, and as the result of trials which he himself carried out, this suggestion was destined to be carried into effect in startling fashion some thirty years later: with what consequences to naval architecture we shall presently see. In connection with the scheme of re-armament outlined by M. Paixhans in 1822 the suggestion was important in that there was implied in it an admission of one of the two weak features of the inventor’s system. The shell gun would lose its superiority over the shot gun, and might indeed be reduced to absolute impotence if, in imitation of France, the enemy also cased his ships of war with iron. The solid shot gun would once again have the advantage; in fact, that very equilibrium of relative values which M. Paixhans was endeavouring to destroy would once more obtain between the navies of the two rival powers. For this reason, presumably, and because the shell gun system contained, though in a less degree, the disability inherent in the carronade system--inferior ranging power, enabling a clever opponent armed with long solid shot guns to fight at a range which was too great for shell--the Paixhans scheme was not adopted in its entirety by the French government of the time. But the principle of unity of calibre was acclaimed and approved almost immediately, applied to solid shot guns. The French 30-pounder was chosen as the unit. In 1829 guns of this calibre, made on several different models to suit the various decks and classes of ship, were mounted in their fleets.[107]