From these lists the aims of the constructors in France and the United States may be seen. In the former, displacement was kept at a medium whilst speed was developed to the extreme, the balance in battery-power being sought in the introduction of rifles. With the latter, displacement and battery-power were carried to the extreme, speed being sacrificed, although in this respect great attention was paid to retaining fine under-water lines and a maximum of sail-power. From 1860 to 1873 an interregnum in the development of French wooden types occurs corresponding to the length of time intervening between the fleet programmes. At the outbreak of the civil war, the Hartford, Shenandoah, and Iroquois types were being built upon slowly, with every prospect of completing a small but compact and efficient cruising fleet. Whilst, however, this fleet had been designed especially for ocean cruising, the unforeseen exigencies of this war demanded the immediate introduction of a type of light-draught gun-boats for river service, as well as an immediate increase in the numbers of vessels for blockade duty. During the first two years blockading and river vessels were extemporized from whatever material could be found in the merchant service. It was this war, however, which gave birth to the Saco type of gun-boats, these vessels being of a greater tonnage and better fitted for blockade duty on the open coast than the gun-boats of foreign types. Although the vessels of this type, hastily constructed and of poor material, were completely worn out in five or six years’ service, the type was renewed and has remained in the service. Two types of river gun-boats, both of which passed out of existence with the war, demand attention from their great usefulness. The first of these was the ordinary river ferry-boat. These vessels, having a displacement of less than 300 tons and a draught of water of seven feet, possessed two valuable qualifications for river fighting. They were built to run either end foremost with equal facility, their speed being moderate and manœuvring qualities excellent. Their decks, intended to carry heavy moving weights, needed no especial bracing to prepare them for heavy batteries. These ferry-boats, without undergoing any transformations except those made necessary for the proper accommodation of the crew and the manœuvring of the guns, carried successfully throughout the war heavier proportional batteries than any vessels afloat before or since. The type itself, enlarged and modified so as to permit the vessel to do cruising duty as well as river service, appeared in 1863 in what was known as the double-ender, a vessel standing between the gun-boat and the second rate, but not to be classed with the regular third-rate cruiser.