Although Wilkinson's iron vessels continued to ply upon the Severn, more than twenty years elapsed before another shipbuilder ventured to follow his example. But in 1810, Onions and Son, of Brosely, built several iron vessels, also for use upon the Severn. Then, in 1815, Mr. Jervons, of Liverpool, built a small iron boat for use on the Mersey. Six years later, in 1821, Mr. Aaron Manby designed an iron steam vessel, which was built at the Horsley Company's Works, in Staffordshire. She sailed from London to Havre a few years later, under the command of Captain (afterwards Sir Charles) Napier, RN. She was freighted with a cargo of linseed and iron castings, and went up the Seine to Paris. It was some time, however, before iron came into general use. Ten years later, in 1832, Maudslay and Field built four iron vessels for the East India Company. In the course of about twenty years, the use of iron became general, not only for ships of war, but for merchant ships plying to all parts of the world. When ships began to be built of iron, it was found that they could be increased without limit, so long as coal, iron, machinery, and strong men full of skill and industry, were procurable. The trade in shipbuilding returned to Britain, where iron ships are now made and exported in large numbers; the mercantile marine of this country exceeding in amount and tonnage that of all the other countries of the world put together. The "wooden walls"[3] of England exist no more, for iron has superseded wood. Instead of constructing vessels from the forest, we are now digging new navies out of the bowels of the earth, and our "walls," instead of wood, are now of iron and steel. The attempt to propel ships by other means than sails and oars went on from century to century, and did not succeed until almost within our own time. It is said that the Roman army under Claudius Codex was transported into Sicily in boats propelled by wheels moved by oxen. Galleys, propelled by wheels in paddles, were afterwards attempted. The Harleian MS. contains an Italian book of sketches, attributed to the 15th century, in which there appears a drawing of a paddle-boat, evidently intended to be worked by men. Paddle-boats, worked by horse-power, were also tried. Blasco Garay made a supreme effort at Barcelona in 1543. His vessel was propelled by a paddle-wheel on each side, worked by forty men. But nothing came of the experiment. Many other efforts of a similar kind were made,--by Savery among others,[4]--until we come down to Patrick Miller, of Dalswinton, who, in 1787, invented a double-hulled boat, which he caused to be propelled on the Firth of Forth by men working a capstan which drove the paddles on each side. The men soon became exhausted, and on Miller mentioning the subject to William Symington, who was then exhibiting his road locomotive in Edinburgh, Symington at once said, "Why don't you employ steam-power?"