Having treated at some length the apparatus used for converting water into high-pressure steam, we may pass at once to a consideration of the mechanisms which convert the energy of steam into mechanical motion, or _work_. Steam-engines are of two kinds:--(1) _reciprocating_, employing cylinders and cranks; (2) _rotary_, called turbines. RECIPROCATING ENGINES. [Illustration: FIG. 17.--Sketch showing parts of a horizontal steam-engine.] Fig. 17 is a skeleton diagram of the simplest form of reciprocating engine. C is a _cylinder_ to which steam is admitted through the _steam-ways_[3] W W, first on one side of the piston P, then on the other. The pressure on the piston pushes it along the cylinder, and the force is transmitted through the piston rod P R to the _connecting rod_ C R, which causes the _crank_ K to revolve. At the point where the two rods meet there is a "crosshead," H, running to and fro in a guide to prevent the piston rod being broken or bent by the oblique thrusts and pulls which it imparts through C R to the crank K. The latter is keyed to a _shaft_ S carrying the fly-wheel, or, in the case of a locomotive, the driving-wheels. The crank shaft revolves in bearings. The internal diameter of a cylinder is called its _bore_. The travel of the piston is called its _stroke_. The distance from the centre of the shaft to the centre of the crank pin is called the crank's _throw_, which is half of the piston's _stroke_. An engine of this type is called double-acting, as the piston is pushed alternately backwards and forwards by the steam. When piston rod, connecting rod, and crank lie in a straight line--that is, when the piston is fully out, or fully in--the crank is said to be at a "dead point;" for, were the crank turned to such a position, the admission of steam would not produce motion, since the thrust or pull would be entirely absorbed by the bearings. [Illustration: FIG. 18.--Sectional plan of a horizontal engine.] DOUBLE-CYLINDER ENGINES. [Illustration: FIG. 19.] [Illustration: FIG. 20.] Locomotive, marine, and all other engines which must be started in any position have at least _two_ cylinders, and as many cranks set at an angle to one another. Fig. 19 demonstrates that when one crank, C_1, of a double-cylinder engine is at a "dead point," the other, C_2, has reached a position at which the piston exerts the maximum of turning power. In Fig. 20 each crank is at 45 deg. with the horizontal, and both pistons are able to do work. The power of one piston is constantly increasing while that of the other is decreasing. If _single_-action cylinders are used, at least _three_ of these are needed to produce a perpetual turning movement, independently of a fly-wheel. THE FUNCTION OF THE FLY-WHEEL.