The evolution from sail to steam involved changes in the design of the hull of a ship. At first it was considered that to turn a sailing-ship into a steamer it was simply necessary to fit a hull designed for a sailing-vessel with a steam engine. It was soon, however, discovered that the fine lines and deep keel required to carry sail were not required in a steamship, and in course of time full-bodied hulls with square bilges without keels were adopted. An iron steamer is but a rectangular girder or tank with the ends sharpened, the co-efficient of fineness varying from 62 to 78 degrees, according to the speed or deadweight capacity required. In 1860 Sir Edward Harland, with a view to easy propulsion, introduced steamers into the Mediterranean trade with a length of ten times their beam. These were so successful that when he built the fleet for the White Star Line he carried out the same principle, thereby also securing steady sea boats. He also introduced central passenger saloons and cabins, which speedily made the White Star ships very popular. Cabin accommodation placed in the centre of the ship has now become general. Some further modifications in design have taken place; ships have, relatively, now less length and more beam, and the cabin accommodation is built up citadel fashion in the middle of the ship. The most notable evolution has, however, been in size and speed. The "Britannia," built in 1840, was 1,200 tons, with 8½ knots speed. She was followed by the "Great Britain," in 1843, 3,270 tons; she was, however, too large for the times, and did no good. The "Great Eastern," built in 1855, was of 18,915 tons, and 12 knots speed, and was also a failure, although if she had been given sufficient power she would probably have hastened the era of large and fast vessels. The demand for speed was for some years the governing feature in the design of steamers in the Atlantic trade, and to a smaller extent in the Eastern trades, in which the carrying of coal for long voyages has also to be considered. The increase in power required to obtain high speeds necessitated the adoption of twin screws, and with the still higher powers required by the "Mauretania," "Olympic," etc. (60,000 h.p.), four propellers are found necessary. In the Atlantic trade, the "Arizona," built by John Elder & Co. for the Guion Line, was the first of the "Atlantic greyhounds." She was quickly followed by the "Alaska" and the "Oregon," the latter being built in 1882, with a speed of 19 knots. She was the fastest ship of her time, and became the property of the Cunard Company. She was again eclipsed by the Cunard ships "Umbria" and "Etruria," in 1885, with a speed of 19½ knots. In 1888 the "City of Paris" and "City of New York" had attained a speed exceeding 20 knots. For some years no improvement in speed was obtained until the advent of the "Campania" and "Lucania," in 1893, with a tonnage of 12,900 and a speed of 22 knots. [Illustration: SS. "Britannia," 1840]