STEAM ENGINE tact, as the arm swings around with the valve stem, with a cam adjusted by the governor. Tliis adjustment permits the steam to follow the piston further when the engine is caused to "slow down," and thus tends to restore the proper speed. It disengages the steam valve earlier, and expands the steam to a greater ex- tent, when the engine tends to run above the proper speed. "When the catch is thrown out, the valve is closed by a weight or a strong spring. To prevent jar when the motion of the valve is checked, a "dash pot" is used, in- vented by F. E. Sickels. It is a vessel having a nicely fitted piston, which is received by a "cushion" of water or air when the piston suddenly enters the cylinder at the end of the valve movement. In the original water dash pot of Sickels, the cylinder is vertical, and the plunger or piston descends upon a small body of water confined in the base of the dash pot. In the Greene steam engine, fig. 6, the valves are four in number, as in the Corliss. The cut-off gear consists of a bar, A, moved by the steam eccentric in a direction parallel with the centre line of the cylinder and nearly coinci- dent as to time with the piston. On this bar are tappets, C 0, supported by springs and ad- justable in height by the governor, G. These FIG. 6. Greene Engine. tappets engage the arms B B, on the ends of rock shafts E-E, which move the steam valves and remain in contact with them a longer or shorter time, and opening the valve during a greater or less part of the piston stroke, as the governor permits the tappets to rise with di- minishing engine speed, or forces them down as speed increases. The exhaust valves are moved by an independent eccentric rod, moved by an eccentric set, as is usual with the Corliss and with other engines generally, at right angles with the crank. This engine, in con- sequence of the independence of the steam eccentric, and of the contemporary movement of steam valve motion and steam piston, is capable of cutting off at any point from begin- ning to nearly the end of the stroke. The usual arrangement, by which steam and exhaust valves are moved by the same eccentric, only permits expansion with the range from the beginning to half stroke. The Wright engine has an adjustable expansion valve gear also, and the point of cut-off is determined by the governor. In this machine the steam valves are opened by a cam of such form that when the cam shaft is moved longitudinally, the valve is held open a longer or a shorter time. The position of the cam shaft is adjusted by the governor. Its motion is obtained by gear- ing it to the main shaft. The Babcock and Wilcox engine has a cut-off valve on the back of the main valve, which is moved by a small steam cylinder. The point of cut-off is deter- mined by the governor also, by varying the time of admission of steam into the auxiliary cylinder. This engine has the same latitude of expansion as the Greene engine. The char- acteristics of fhe American stationary engine are high steam pressure without condensation, an expansion valve gear with drop cut-off ad- justable by the governor, high piston speed, and lightness combined with strength of con- struction. In other countries this engine is now rapidly coming into general use, but abroad the valve most generally adopted is the form usual in other styles of engine, expansion being obtained by a cut-off valve on the back of the main valve, and regulation secured by attach- ing the governor to a throttle valve. The Marine Steam Engine. Marine engines have a great variety of forms, but general practice has now indicated a few which are preferred. They are almost invariably fitted with con- densers. Until recently they were usually driven by steam of moderate pressure, but within a few years the pressure of steam, which in the time of Watt was usually from 5 to 10 Ibs. above the atmosphere, has risen to 60 Ibs. In the earlier days of steam navigation, the paddle wheel was exclusively used. Ke- cently the screw has become the sole instru- ment of propulsion, where deep water permits its use. In shallow water the paddle wheel is still employed. . Marine engines are therefore divided into paddle engines and screw engines. The most common forms of paddle engines in the United States are the overhead beam engine, driven by steam of from 20 to 50 Ibs. pressure, and fitted with a jet condenser, and the high-pressure and non-condensing direct- acting engine, used principally on the western rivers. The latter is driven by steam of from 100 to 150 Ibs. pressure, and exhausts its steam into the atmosphere. It is the simplest pos- sible form of direct-acting engine. The valves
