Ocean steamships have gradually evolved into two types. The freighter, broad in beam and capacious, is built to carry an enormous amount of freight at a moderate speed. The White Star liner Celtic is a vessel of this class; her schedule time between New York and Liverpool is about nine days. The Philadelphia of the American line, though not the fastest steamship, makes the same trip in an average time of five and one-half days.[7] Twin-screws, instead of a single propeller, are employed on nearly all the large liners. The gain in speed is not greatly increased, but the vessel is far more manageable with two screws than with one; moreover, if one engine breaks down, the vessel can make excellent time with the other. Triple-expansion engines are almost universally used on modern steamships, and a pound of coal now makes about three times as much steam available as in the engines formerly used. As a result a bushel of wheat is now carried from Fargo, N. Dak., to Liverpool for about twenty-one cents--less than one-half the freight tariff of 1876. [Illustration: THE SCHOONER THOMAS A. LAWSON. THE FIRST SEVEN-MASTED SAILING-VESSEL] The fastest liners consume from three hundred and fifty to more than four hundred tons of coal a day, and for each additional knot of speed the amount of coal burned must be greatly increased. Freighters like the Celtic consume scarcely more than half as much as those of the Kaiser Wilhelm II. type. =Sailing-Craft.=--In spite of the growth and development of steam-navigation, a large amount of freight is still carried by sailing-craft; moreover, it is not unlikely that the relative proportion of ocean freight carried by sailing-vessels will increase rather than decrease, especially in the case of imperishable freight. The square-rigged ship, or bark, has been very largely replaced by the fore-and-aft, or schooner-rigged vessel. A large full-rigged ship requires a crew of thirty to thirty-six men; a schooner-rigged vessel needs from sixteen to twenty. These vessels are commonly built with three and four masts; some of the largest have six or seven. They carry as many as five thousand tons of freight at a speed of about ten knots--only a trifle less than that of an ordinary tramp freighter. Some of the larger vessels are provided with auxiliary engines and propelling apparatus, which enables them to enter or to leave port without the assistance of a tug. Donkey-engines hoist and lower the sails, and perform the work of loading and unloading. They are admirable colliers and grain-carriers. At the beginning of the twentieth century, about ninety thousand sailing-craft and thirty-five thousand steam-vessels were required to carry the world's commerce. Of this number, Great Britain and her colonies register nearly thirty-five thousand, and the United States over twenty thousand.