The necessity of providing a defence against shells was recognized both by England and France during the Crimean war, and a protection of armor was supplied to some floating batteries built at that time which were intended to operate before fortified positions; and at the conclusion of the war the English built the _Warrior_ and the French built _La Gloire_. These were the first specimens of iron-clad ships of war. They were capable of resisting successfully the entrance of shells from guns of the period. It is thus seen that almost coincident with the general adoption of horizontal shell-firing, naval construction entered a new phase, and a new problem was submitted to the naval artillerist. Against an iron-faced target the solid shot might be partially effective, but the impact of the spherical shell was harmless, and the explosive effect of the bursting charge enclosed in it would be superficial. This was amply demonstrated in actual practice during our war experience, notably at Mobile Bar, in the engagement with the Confederate iron-clad _Tennessee_, the roughly constructed armor of which vessel resisted a storm of our heaviest shells. [Illustration: HORIZONTAL SECTION OF MILLWALL SHIELD.] The impotency of the spherical shell against armor being recognized by foreign governments, they proceeded to develop the rifled cannon, which with its elongated projectile offered the means of effecting the object of the time—to perforate armor with an explosive projectile. Our authorities, however, persevered in their faith in the smooth-bore, and held that the _racking_ effect of a spherical projectile of sufficiently large calibre was superior to that produced by the perforation of a rifle projectile of inferior diameter. The 15-inch and 20-inch smooth-bore cannons were cast in accordance with this idea, and the racking side of the question was so obstinately held that the British government imported in 1867 from the United States a 15-inch gun for the purpose of determining by their own experiments what foundation there was for the advantages that were claimed for it. The gun was bought of Charles Alger & Co., of Boston; it weighed nineteen tons, and threw a cast-iron spherical solid shot of about four hundred and fifty pounds. It was mounted at Shoeburyness, and was fired in competition with English rifled cannons of 9-inch and 10-inch calibres. The result of the experiments went to show that against a target with a power of resistance inferior to the energy of the projectile the effect of the large sphere at short range is more disastrous than that of the elongated rifle projectile of the same weight; but that against a target able to resist the total energy of both the injury done by the rifle projectile is by far the greater. The comparative effect is well shown on a target called the “Millwall Shield,” consisting of a plate nine inches in thickness, backed by Hughes’s hollow stringers—an