The machinery of these ships was, for that time, remarkably powerful and efficient. The engines were of the side-lever type, as illustrated in Fig. 92, which represents the engine of the Pacific, designed by Mr. Charles W. Copeland, and built by the Allaire Works. [Illustration: FIG. 92.--The Side-Lever Engine, 1849.] In this type of engine, as is seen, the piston-rod was attached to a cross-head working vertically, from which, at each side, links, _B C_, connected with the "side-lever," _D E F_. The latter vibrated about a "main centre" at _E_, like the overhead beam of the more common form of engine; from its other end, a "connecting-rod," _H_, led to the "cross-tail," _W_, which was, in turn, connected to the crank-pin, _I_. The condenser, _M_, and air-pump, _Q_, were constructed in the same manner as those of other engines, their only peculiarities being such as were incident to their location between the cylinder, _A_, and the crank, _I J_. The paddle-wheels were of the common "radial" form, covered in by paddle-boxes so strongly built that they were rarely injured by the heaviest seas. These vessels surpassed, for a time, all other sea-going steamers in speed and comfort, and made their passages with great regularity. The minimum length of voyage of the Baltic and Pacific, of this line, was 9 days 19 hours. During the latter part of the period the history of which has been here given, the marine steam-engine became subject to very marked changes in type and in details, and a complete revolution was effected in the method of propulsion. This change has finally resulted in the universal adoption of a new propelling instrument, and in driving the whole fleet of paddle-steamers from the ocean. The Great Britain was a screw-steamer. The screw-propeller, which, as has been stated, was probably first proposed by Dr. Hooke in 1681, and by Dr. Bernouilli, of Groningen, at about the middle of the eighteenth century, and by Watt in 1784, was, at the end of the century, tried experimentally in the United States by David Bushnell, an ingenious American, who was then conducting the experiments with torpedoes which were the cause of the incident which originated that celebrated song by Francis Hopkinson, the "Battle of the Kegs," using the screw to propel one of his submarine boats, and by John Fitch, and by Dallery in France. Joseph Bramah, of Great Britain, May 9, 1785, patented a screw-propeller identical in general arrangement with those used to-day. His sketch exhibits a screw, apparently of very fair shape, carried on an horizontal shaft, which passes out of the vessel through a stuffing-box, the screw being wholly submerged. Bramah does not seem to have put his plan in practice. It was patented again in England, also, by Littleton in 1794, and by Shorter in 1800.