$4 (b)] CLASSIFICATION OF STEAM-ENGINES. 19
framework. Besides the usual form of the main mechanism, there are several other types of construction, of which the oscillating engine is, perhaps, the most distinctive. The engine may be single or complex-in the latter case, having two or more complete engines combined in one, these elements being either simple or compound; thus in pumps and in locomotives duplex compound engines are quite common. The type of valve-gear and the kind of controlling apparatus likewise form grounds of distinction, as also the question whether the engine runs in only one direction, or reverses. (c) DIFFERENT WAYS OF USING THE STEAM.-In the matter of steam-working, engines may be either simple or multiple expansion; that is, the steam may pass through only one cylinder, or through several cylinders, of increasing size, in succession. There is great variety in the arrangement of the cylinders of compound engines. The distinction between condensing and non-condensing engines has been already pointed out. (d) ROTARY ENGINES AND STEAM TURBINES.-It is to the engine with the usual device of cylinder and piston for utilizing steam- force that the name steam-engine is commonly applied. Other forms of the steam machine, the rotary and turbine classes, are known by their distinctive names. Thermodynamically, they perform the same function, though with some modifications in the operation; mechanically, they very properly fall into entirely separate classes.
