Fig. 158.—Cross-section view of cylinder and steam chest of a steam engine.
Fig. 159.—The steam drives the piston to the left.
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.
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.
194. The Mechanical Equivalent of Heat.—While watching workmen bore holes in cannon, Count Rumford, 1753-1814, noticed with much interest the large amount of heat produced in the process. He observed that the heat developed seemed to have some relation to the work done upon the drill in boring the holes. Later experiments performed by many men indicated that a definite relation exists between the heat produced by friction and the amount of work done in overcoming the friction. This discovery indicates that in some way heat is related to energy and that heat is probably a form of energy. Later experiments have confirmed this idea, and it is now considered well established that heat is a form of energy. Many attempts have been made to discover the relation between the units of heat energy and the units of mechanical energy. To illustrate one method employed, suppose one measures a given length in inches and in centimeters; on dividing one result by the other, it will be found that a certain relation exists between the two sets of measurements, and that in every case that 1 in. equals 2.54 cm. Similarly, when the same amount of energy is measured both in heat units and in work units a constant relation is always found between the units employed. One B.T.U. is found equivalent to 778 ft.-lbs. 1 calorie being equivalent to 42,700 g. cm. (427 g. m.). This relation is called the mechanical equivalent of heat, or in other words it represents the number of work units equivalent to one heat unit.
Fig. 162.—Apparatus for determining the mechanical equivalent of heat.