18 A GENERAL VIEW OF THE SUBJECT, [CHAP. 1.
end. It is to be noted that the diagram in Fig. 10 does not corre- spond, in the timing of its events, with the valve-action shown in Fig. 9. (g) MEASUREMENT OF POWER OF ENGINE.-The steam diagram, besides showing how the valve is working and furnishing part of the data for a discussion of the thermodynamic behavior of the steam, also gives a measurement of the power developed by the engine. Since the abscissa represents distance moved by the piston, and the ordinate pressure acting, the area of the figure represents work. (h) DIVISIONS OF THE SUBJECT.-The preceding general descrip- tion of the steam-plant and of the engine forms a sufficient introduc- tion to the detailed study of the several parts of the subject. This will follow the lines already laid out, taking up first the relations between work and heat, then the mechanics of the engine the latter including the action of the forces in the machine and the working of the auxiliary mechanisms, the valve-gear and the governor. With this will be given a systematic description of the several parts of the apparatus, with representative examples of different types of design in each department. § 4. Classification of Steam-engines. (a) SERVICE. Steam-engines may be classified according to service, general form, mechanical features, and the manner of using the steam. As to service, we have first the great variety of stationary engines, used for driving machinery by means of the different kinds of power-transmission apparatus, or more directly applied to such work as turning electric generators, or hoisting- drums at mines, or metal-rolls in mills: an even more direct and intimate combination of prime-mover and load is found in steam- pumps, blowing-engines, and air-compressors. The locomotive type including the traction-engine and the steam automobile- forms another class; and the marine engine a third. (b) OTHER BASES OF CLASSIFICATION.-As to general form and mechanical features, the engine may be horizontal or vertical or of special shape; and may have one of several different styles of
