=192. Heat Engines.=--At the present time there is a great variety of _heat engines_ in use such as _steam_, _hot air_, _gas_, and _gasoline_ engines, all using _heat energy_ to produce motion. The expansive power of steam when confined has been observed for hundreds of years and many different machines have been invented to use it in doing work. [Illustration: FIG. 158.--Cross-section view of cylinder and steam chest of a steam engine.] [Illustration: FIG. 159.--The steam drives the piston to the left.] [Illustration: FIG. 160.--External view of steam engine.] =193. The Steam-engine.=--The man who perfected the steam-engine, and devised its modern form was _James Watt_ (1736-1819). The essential parts and the action of the steam engine may be readily understood by studying a diagram. In Fig. 158, _S_ stands for _steam chest_, _C_ for _cylinder_, _P_ for _piston_ and _v_ for _slide valve_. The first two are hollow iron boxes, the latter are parts that slide back and forth within them. The action of the steam engine is as follows: Steam under pressure enters the steam chest, passes into the cylinder and pushes the piston to the other end. The slide valve is moved to its position in Fig. 159. Steam now enters the right end of the cylinder, driving the piston to the left, the "dead" steam in the left end of the cylinder escaping at _E_ to the air. The slide valve is now shifted to its first position and the process is repeated. It will assist the student to understand this action if he makes a cardboard model of these parts, the piston and slide valve being movable. In practical steam-engines, the piston rod is attached to a _crank rod_ fastened to a crank which turns a wheel. (See Fig. 160.) The back and forth, or _reciprocating_ motion of the piston is by this means transformed into _rotary_ motion, just as in the sewing-machine the back-and-forth motion of the treadle produces rotary motion of the large wheel. Upon the shaft of the steam engine is fastened an _eccentric_ (see Fig. 163) which moves the slide valve. The steam engine acts continuously as long as steam is supplied to it. Since it shifts the position of the slide valve automatically, it is called an automatic steam engine. And because the team drives the piston both ways, it is called a _double-acting_ steam engine. See Fig. 161 for a length-section of a modern locomotive. [Illustration: FIG. 161.--Length-section of modern, fast-passenger locomotive. _A_, cylinder valve--piston type valve; _B_, cylinder--piston at out end of stroke; _C_, boiler tubes--flues from fire-box; _D_, fire-tube type superheater; _E_, draught screen; _F-A_, fire-brick arch to protect tubes from direct heat; _F-B_, firebox; _G_, grate; _H_, exhaust nozzle; _I_, safety valve nest; _T_, throttle lever; _R_, throttle rod; _Y_, throttle valve.]