Below the C class in England appeared the Opal class of 1900 tons, a modification of the earlier Blanche type corresponding and following close after the American Plymouth type. At the same time a corresponding type appeared in France and the United States. In the former the Rigault de Genouilly has a displacement of 1640 tons, an increase of 400 tons over her immediate predecessors, a battery of eight 5½-inch guns, and a speed of 15 knots, with a steaming capacity of 4000 miles at 10 knots. In America the Marion has a displacement of 1900 tons, a battery of one 8-inch and seven 5½-inch guns, and a speed of 12 knots, with a steaming capacity of 2500 miles at 10 knots. The French in this class show the same difference in qualities from the English and American types that appeared twenty years before. The English development closes with the gun-boats whose types come more directly from those commenced during the Crimean war. The displacement of these vessels ranges from 400 to 900 tons, with a diversity that scarcely admits of a distinct classification. The American gun-boats are reductions of the Iroquois class of corvettes, exceeding the English in average displacement, or rather showing no types below 500 tons. The French gun-boats are more closely allied to English than to American types, although there is but little difference between the three, the English showing greater diversity, the Americans greater measurements, and the French greater precision of rating. [Illustration: _Chart of Architectural Development.—Unarmored Vessels._] CONSTRUCTIONAL DEVELOPMENT. Previous to 1857 wood construction was universal in the building of war-vessels. About this date England introduced iron frames in her first armored vessels, passing rapidly to the full development of iron construction in armored ships, composite construction in the medium and light classes of unarmored vessels, and iron or steel sheathed with wood in the first and second unarmored rates. By 1867 the old wood construction had been entirely discarded. [Illustration: French Corvette La Clocheterie. (Wooden System.)] [Illustration: United States Corvette Plymouth. (Wooden System.)] In France the wooden construction was almost exclusively used until the development of the programme of 1873. Iron was then introduced in the armored hulls, iron sheathed with wood in the first and second rate unarmored ships, and composite construction in the gun-boats of less than 700 tons, leaving the light second-rates and the third-rates to the old wood construction.