STEAM ENGINE 345 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. 7 is an outline sketch of this engine as built for a steamer plying on the Hudson river. This class of engine is usu- ally adopted in vessels of great length, light draught, and high speed. But one steam cyl- inder 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 oppo- site 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 independent eccentrics, the latter being set in the usual manner, opening and closing the ex- haust passages just before the crank passes its centre. The steam eccentric is so placed that the steam valve is opened as usual, but closed when but about one half the stroke has been made. This result is accomplished by giv- ing the eccentric a greater throw than is required by the mo- tion of the valve, and FIG. 7. Beam Engine. the form known as the Stevens valve gear, an " ivention of Kobert L. and Francis B. Stevens, steam and exhaust valves are worked by permitting it to move through a portion of its path without mov- ing the valve. Thus in fig. 8, if A B be the FIG. 8. direction of motion of the eccentric rod, the valve would ordinarily open the steam port when the eccentric as- sumes the position O C, closing when the ec- centric has passed around to O D. "With the Stevens valve gear, the valve is opened when the eccentric reaches E, and closes when it arrives at O F. The steam valve of the opposite end of the cylinder is open while the ec- centric is moving from O M to O K. Between K and E, and between F and M, both valves are seated. H B is proportion- al to the lift of the valve, and O H to the motion of the valve gear when out of contact with the valve lifters. While the crank is moving through an arc E F, steam is entering the cyl- inder ; from F to M the steam is expanding. At M the stroke is completed and the other steam valve opens. The ratio is the ratio of expansion. EM The condenser is placed imme- diately beneath the steam cyl- inder. 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 23 m. 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." For p a( jclie engines of large vessels, the favorite type has been the side lever engine, which is now rarely built. For smaller vessels, the oscilla- ting engine with feathering paddle wheels is still largely employed in Europe. This style
