It took 24 men working hand pumps for an hour and a half to expel the water from the caisson. The original wooden caisson lasted until 1901, when the steel caisson still in use today was completed. [Illustration: The Yard’s First Dry Dock] 1. After its 1858 launching, U.S.S. _Hartford_ is docked for installation of its steam engine. To empty the dock, workers opened the discharge gates (A), releasing water to flow (_red arrows_) down discharge culverts (B) (on both sides of dock) to fill the reservoir (C). 2. The pumphouse (D), its steam engine driving two pumps in underground wells (E), pumped the water from the dock via the reservoir and sent it through an underground culvert back to the harbor. 3. To fill the dock, the discharge gates were closed and the filling gates (F) were opened. Water flowed (_green arrows_) first to wells (G), then into the dock through the same culverts used to empty it. Steam windlass Timber dock Swing gates (backed up caisson) Caisson (“floating gate”) Ironclad Technology The clash of ships at sea embodies the ongoing technological battle between arms and armor: between deploying ever more destructive weapons and contriving ways to withstand them. As long as solid shot was the only way to attack a ship’s hull, heavy timbers were usually armor enough. Big wooden warships were rarely sunk by even the heaviest shot. (_Constitution_ is a particularly good example.) But the rules of the game changed with the coming of more powerful and more accurate guns, and especially with the development of the practical explosive shell in the 19th century. A shell could open a gaping hole in a heretofore impervious wooden hull. By 1860 France and then Britain had begun building ironclads. In Britain, especially, the rising cost of diminishing timber supplies was another incentive to experiment with iron, both as armor and for structural elements of the hull. But in the United States, wood was still cheaper than iron. Also, though the country had earlier experimented with ironcladding, the Navy resisted the new technology, putting emphasis on speed rather than armor. But it quickly made up for lost time after the beginning of the Civil War. The Confederacy took the lead, for the same reason that the United States had built “super-frigates” at the end of the 18th century. A country that could afford only a small navy had to build state-of-the-art warships. The blockade-breaker C.S.S. _Virginia_ showed the lethal effectiveness of its ironcladding on its first outing (_see page 28_). The next day U.S.S. _Monitor_ fought _Virginia_ to a draw in the first battle between ironclads (_right_). The encounter spurred European navies to accelerate their ironclad programs, but new breech-loading rifled guns were demonstrating greater armor-piercing ability. In response iron, and then steel, armor was made thicker and harder, leading to still more powerful guns. The gun designers generally stayed