When steam had fairly established its place as the most effective power for ocean voyages of every duration, and through every zone and clime, improvements in the methods of harnessing it, and in the form and material of the ships that it was to drive, followed fast upon each other. As in the case of the invention of the steamboat, the public has commonly lightly awarded the credit for each invention to some belated experimenter who, walking more firmly along a road which an earlier pioneer had broken, attained the goal that his predecessor had sought in vain. So we find credit given almost universally to John Ericsson, the Swedish-born American, for the invention of the screw-propeller. But as early as 1770 it was suggested by John Watt, and Stevens, the American inventor, actually gave a practical demonstration of its efficiency in 1804. Ericsson perfected it in 1836, and soon thereafter the British began building steamships with screws instead of paddle-wheels. For some reason, however, not easy now to conjecture, shipbuilders clung to the paddle-wheels for vessels making the transatlantic voyage, long after they were discarded on the shorter runs along the coasts of the British isles. It so happened, too, that the first vessel to use the screw in transatlantic voyages, was also first iron ship built. She was the "Great Britain," a ship of 3,000 tons, built for the Great Western Company at Bristol, England, and intended to eclipse any ship afloat. Her hull was well on the way to completion when her designer chanced to see the "Archimedes," the first screw steamer built, and straightway changed his plans to admit the use of the new method of propulsion So from 1842 may be dated the use of both screw propellers and iron ships. We must pass hastily over the other inventions, rapidly following each other, and all designed to make ocean travel more swift, more safe, and more comfortable, and to increase the profit of the shipowner. The compound engine, which has been so developed that in place of Fulton's seven miles an hour, our ocean steamships are driven now at a speed sometimes closely approaching twenty-five miles an hour, seems already destined to give way to the turbine form of engine which, applied thus far to torpedo-boats only, has made a record of forty-four miles an hour. Iron, which stood for a revolution in 1842, has itself given way to steel. And a new force, subtile, swift, and powerful, has found endless application in the body of the great ships, so that from stem to stern-post they are a network of electric wires, bearing messages, controlling the independent engines that swing the rudder, closing water-tight compartments at the first hint of danger, and making the darkest places of the great hulls as light as day at the throwing of a switch. During the period of this wonderful advance in marine architecture ship-building in the United States languished to the point of extinction.