Enough has been written to indicate in some detail the progress made in steam-ship construction. Wood was the material chiefly used until near the middle of the nineteenth century. Iron then began to take its place and the screw-propeller to supersede the paddle-wheel. Some iron screw steamers have already been mentioned, but this was inevitable, as no hard and fast line can be drawn across the history of invention and commercial enterprise, to separate iron from wood and screw from paddle. The screw propeller had actually been tried by Stevens in 1802, and iron boats for inland waters were built as early as 1787. But the general adoption of iron for building steam-ships and of the screw for the propulsion of ocean-going ships marks a new era in the history of steam-ship building. CHAPTER VIII EXPERIMENTAL IRON SHIPBUILDING The suitability of iron for shipbuilding purposes had been admitted long before the construction of wooden vessels reached its limit as a profitable undertaking. The first experiments with iron were on a small scale, but they demonstrated the theory of displacement, so that observant marine builders had it borne in upon them that flotation depended rather upon the displacement of the floating body than upon the specific gravity of the material for which the floating body was constructed. But the general public was unconvinced, and making deductions from a limited knowledge of the subject, cried: “Put a piece of iron on the water and see if it will float.” With the increase in the size of wooden steamers and sailing vessels there came the demand for stronger, heavier, and thicker timbers for all parts. This meant so much more unremunerative weight of hull to be carried and so much less space available in proportion to the size of the vessel; so that in time the limit of carrying cargo at a profit and of staunchness of construction was bound to be reached. In wooden steam-ships the limit of length was about 275 feet over all; the _Great Eastern_, built in 1858, proved that there was apparently no limit to the length of the iron ship.[78] [78] Mr. John Ward’s Presidential Address to the Institution of Engineers and Shipbuilders in Scotland, 1907. This length has been exceeded by a few American wooden sailing vessels. The largest square-rigged vessel ever built in America, the shipentine _Shenandoah_, was of wood; her dimensions being 299·7 feet, beam 49·1 feet, and depth 19·9 feet; 3407 tons gross and 3154 net. She was built at Bath (Maine) in 1890 for Messrs. A. Sewall and Co., and was acquired a couple of years ago by the United States Government for a hulk at San Francisco, but has since been recommissioned. Though not a clipper in the strict sense of the word, she was a fast sailer and is sometimes called the last of the Yankee wooden clippers.