Thenceforward the story of the steam-ship progressed decade by decade. Between 1855 and 1865 paddle-wheels gave place to screw propellers, and the need for engines of a higher speed, which the adoption of the screw brought about, distinguished the following decade as that in which the “compound engine” was evolved. Put shortly, “compounding” means the using of the waste steam from one cylinder to do further work in a second cylinder. The extension of this system to “triple expansion,” whereby the exhaust steam is utilised in a third cylinder, the introduction of twin screws, and the substitution of steel for iron in hull-construction, were the chief innovations between 1875 and 1885. The last fifteen years of the century saw the tonnage of the world’s shipping doubled, and the main features of mechanical progress during that period were another step to “quadruple expansion” and the application of “forced draught,” which gives a greater steam-pressure without a corresponding increase in the size of the boilers. The first decade of the present century has been already devoted to the development of the “turbine” engine. I have to thank the Institute of Marine Engineers at Stratford, E., for much valuable assistance and for placing its _Transactions_ at my disposal; if I have not acknowledged every item derived therefrom I trust that this general acknowledgment will suffice. To Mr. J. Kennedy, author of “The History of Steam Navigation”; Mr. A. J. Maginnis, author of “The Atlantic Ferry”; and Captain James Williamson, author of “The Clyde Passenger Steamer,” I am greatly indebted for their kind permission to draw freely upon their books: and to the publishers of the two latter, Messrs. Whittaker and Co. and Messrs. MacLehose and Sons respectively, for the loan of illustrations. Special thanks are also due to Mr. E. A. Stevens of Hoboken, New Jersey, U.S.A., not only for information concerning the experiments made by Colonel Stevens with the screw propeller, but also for the loan of some unique photographs of early American boats. Mr. A. J. Dudgeon, M.I.N.A., M.I.C.E., son of the well-known Thames shipbuilder, has revised a large amount of my work, and was good enough to place at my disposal his valuable scrap-books, from the pictures in which my friend Mr. Ernest Coffin has drawn several charming line-illustrations and the initial letters to the chapters. For various assistance I have to thank other friends and correspondents: Mr. James A. Smith, M.I.N.A; Mr. Harry J. Palmer, formerly of _Shipping Illustrated_, New York, and now assistant to Captain Clark, Lloyd’s agent at New York; Mr. J. W. Little, of Messrs. Little and Johnson; and Mr. James Gallagher of Paris for his researches at the Academy of Sciences and elsewhere.