342 STEAM ENGINE maining uncondensed vapor, and any air which may enter the condenser with the steam, are removed by the air pump p, and thrown into the hot well above the condenser whence they are taken by the hot water pump and dis- charged. Water is also taken from the hot well by the feed pump and fed to the steam boiler. Classification of Engines. Steam engines are designated as condensing or non-conden- sing, according as they are furnished with a condenser or as that detail is omitted. They are high pressure or low pressure, the former term being applied to engines supplied with steam of 50 Ibs. pressure to the square inch and upward, and the latter to engines working under 40 Ibs. pressure. The latter are almost invariably condensing engines, and high-pres- sure engines are very generally non-condensing. Reciprocating engines have pistons moving backward and forward in the steam cylinder, as in "Watt's engine. When they turn a shaft, they are sometimes called rotative. Rotary engines have a piston attached to a shaft and revolving with it within a cylinder of which the axis is parallel with the axis of rotation of the piston or vane. Engines are direct-act- ing where the piston rod acts directly upon the connecting rod, and through it upon the crank, without the intervention of a beam or lever. In back-acting or return connecting rod engines, the shaft lies between the cylin- der and the cross head, the connecting rod re- turning from the cross head to the crank. Beam engines have the working beam al- ready described. Side lever engines have two beams, one on each side of the steam cyl- inder, and below instead of above the cross head. Oscillating engines have their piston rods attached directly to the crank pin, and as the crank revolves the cylinder oscillates upon trunnions, one on each side of it, through which the teara enters and leaves the steam chest. The valves are within the steam chest, oscillating with the cylinder. In these engines the mechanism actuating the valves is seldom perfectly satisfactory in its operation. In com- pound or double-cylinder engines, the steam enters first a high-pressure cylinder, and there usually expands from its initial pressure of from 60 to 100 Ibs. down to a much lower den- sity ; it is then exhausted into a second steam cylinder, in which it expands still further while completing its work. Engines are also classi- fied, according to the use for which they are intended, as stationary, pumping, portable, lo- comotive, or marine engines. The locomotive engine is the simplest form. In it the con- denser and the governor are dispensed with, and the valve and its gearing are the simplest possible. The portable engine is usually very similar to the locomotive, and, like the latter, is attached to its steam boiler. It is sometimes provided with a heater to warm the feed water sent into the boiler, and is frequently provided with a governor. It is usually mounted on wheels. Both the locomotive and the portable engine employ high steam pressure without condensation. In both of these forms of en- gine are secured to the fullest extent lightness and simplicity, and, where properly construct- ed, cheapness, durability, compactness, and fair efficiency. Draft is usually secured in both by the blast of the exhaust steam. Engines of this class have attained the remarkable economical result of a horse power developed with the expenditure of less than three pounds of coal per hour. The oldest form of pumping engine still retained in use is the Cornish. In it the crank shaft and balance wheel are dis- pensed with, the end of the pump rod being attached directly to the end of the beam op- posite the steam cylinder. Steam is first ad- mitted above the piston, driving it rapidly downward and raising the pump rod. At an early point in the stroke the 'admission of steam is checked by the sudden closing of the induc- tion valve, and the stroke is completed under the action of expanding steam assisted by the inertia of the heavy parts already in motion. The necessary weight and inertia is afforded in many cases, where the engine is applied to the pumping of deep mines, by the immense- ly long and heavy pump rods. Where this weight is too great it is counterbalanced, and where too small, weights are added. When the stroke is completed, the "equilibrium valve" is opened, and the steam passes from above to the space below the piston, and an equilibrium of pressure being thus produced, the pump rods descend, forcing the water from the pumps and raising the steam piston. The absence of the crank or other device which might deter- mine absolutely the length of stroke compels a very careful adjustment of steam admission to the amount of load. Should the stroke be allowed to exceed the proper length, and should danger thus arise of the piston striking the cylinder heads, the movement is checked by buffer beams. The valve motion is actuated by a plug rod, as in Watt's engine. The regu- lation is effected by a "cataract," a kind of hydraulic governor, consisting of a plunger pump with a reservoir attached. The plunger is raised by the engine, and then automatically detached. It falls with greater or less rapid- ity, its velocity being determined by the size of the eduction orifice, which is adjustable by hand. When the plunger reaches the bottom of the pump barrel, it disengages a catch, a weight is allowed to act upon the steam valve, opening it, and the engine is caused to make a stroke. When the outlet of the cataract is nearly closed, the engine stands still a consid- erable time while the plunger is descending, and the strokes succeed each other at long intervals. When the opening is greater, the cataract acts more rapidly, and the engine works faster. This has been regarded until recently as the most economical of pumping engines, and it is still generally used in freeing mines of water, and in situations where exist- ing heavy pump rods may be utilized in con-
