Referring now to the construction of the engines of the earliest boats, Symington’s _Charlotte Dundas_ used a horizontal direct-acting engine, and the general arrangement of her machinery would be considered creditable even at the present day.[79] The engine of the _Savannah_ was of the inclined direct-acting type. The type of engine which Newcomen invented has been retained for many years, but the oscillating or walking beam which is such a conspicuous feature of nearly all the American river craft has been placed by engineers in this country below the crank axle instead of above. The type of engine with the beam below the crank axle is known as the side lever. It is a type peculiarly suitable to paddle-wheels, and this being the only method of propulsion adopted on this side of the Atlantic for many years, there was little change for a considerable period in the shape of the engines, which therefore attained to a high stage of perfection until the limit of their profitable employment was reached. When larger engines became necessary, in consequence of the rapidly increasing size of vessels, the great weight of the side-lever engines proved a serious drawback. [79] Sennet and Oram’s “The Marine Steam-Engine,” 1898. Engineers were not long in devising a more compact form of machinery, and direct-acting engines were introduced, these involving the abandonment of the use of the heavy side levers. As the side-lever engines were made larger it became customary to use two beams, one on each side, and a rod from one end of each of these connected with a cross-piece at the top of the piston-rod. The other ends of the double beam were united by a cross-piece which carried from its centre the rod or lever which worked the crank of the paddle-shaft. Where it became necessary to use two engines in one vessel, they were so arranged that while one rod and crank were at their period of least activity, the other pair were exerting their greatest effort. The system of condensation of steam, which it would take too much space to describe in detail, is also a matter of great importance in determining the power of the engine, but the principle upon which the condensation is effected is well known, and the various methods of condensation can easily be ascertained from the numerous handbooks on engineering. [Illustration: MAUDSLAY’S OSCILLATING ENGINE.]