The changes which have now become completed in the marine steam-engine have been effected at a later date than those which produced the modern locomotive. On the American rivers the modification of the beam-engine since the time of Robert L. Stevens has been very slight. The same general arrangement is retained, and the details are little, if at all, altered. The pressure of steam is sometimes as high as 60 pounds per square inch. [Illustration: FIG. 130.--Beam-Engine.] The valves are of the disk or poppet variety, rising and falling vertically. They are four in number, two steam and two exhaust valves being placed at each end of the steam-cylinder. The beam-engine is a peculiarly American type, seldom if ever seen abroad. Fig. 130 is an outline sketch of this engine as built for a steamer plying on the Hudson River. This class of engine is usually adopted in vessels of great length, light draught, and high speed. But one steam-cylinder is commonly used. The cross-head is coupled to one end of the beam by means of a pair of links, and the motion of the opposite end of the beam is transmitted to the crank by a connecting-rod of moderate length. The beam has a cast-iron centre surrounded by a wrought-iron strap of lozenge shape, in which are forged the bosses for the end-centres, or for the pins to which the connecting-rod and the links are attached. The main centre of the beam is supported by a "gallows-frame" of timbers so arranged as to receive all stresses longitudinally. The crank and shaft are of wrought-iron. The valve-gear is usually of the form already mentioned as the Stevens valve-gear, the invention of Robert L. and Francis B. Stevens. The condenser is placed immediately beneath the steam-cylinder. The air-pump is placed close beside it, and worked by a rod attached to the beam. Steam-vessels on the Hudson River have been driven by such engines at the rate of 20 miles an hour. This form of engine is remarkable for its smoothness of operation, its economy and durability, its compactness, and the latitude which it permits in the change of shape of the long, flexible vessels in which it is generally used, without injury by "getting out of line." [Illustration: FIG. 131.--Oscillating Engine and Feathering Paddle-Wheel.]