(5) Driving gear, differential, clutch, etc. As in the case of the gas tractor, the machine consists simply of a steam engine and its boiler that drive the road wheels of the tractor through a gear train. With the steam tractor the gearing is simplified as the reverse is performed by the engine’s valve motion, and not through gearing. There is no need of speed changing transmission gears in the steam tractor as the engine is sufficiently flexible to provide an innumerable number of speeds by simple throttle control. While the fuel most commonly used is coal, straw and wood, crude oil is often used, the fuel being determined principally by the location of the engine, and by its cost on the job. The matter of fuel should be taken into consideration when the engine is purchased as the different grades demand different fire box and boiler construction. When it is possible to obtain crude oil at a reasonable figure, it certainly should be used in preference to all others as liquid fuel is the most compact, most easily controlled, and efficient of any. The subject of oil burners is taken up later in this chapter, a number of types of which are clearly illustrated. (148) The Cylinder and Slide Valve. The steam engine cylinder consists essentially of a smoothly bored iron casting in which a plunger called the “piston” slides to and fro, the cylinder acting not only as a container for the steam acting on the piston but as a guide and support as well. Needless to say, the contact or fit between the piston and cylinder walls must be as perfect as possible, tight enough to prevent steam passing the piston, and free enough to allow the piston to slide without unnecessary friction. The reciprocating piston is connected to the crank through a connecting rod by which the pressure on the piston is communicated to the crank arm. The pressure exerted on the crank pin by the piston depends on the area of the piston (in square inches) and the pressure of the steam on each square inch of the area. With a given steam pressure, the greater the area, the greater the force tending to turn the crank. As power is the rate or distance through which the force acts in a unit of time it is obvious that the power developed by the engine is equal (in foot pounds) to the force in pounds multiplied by the velocity of the piston in feet per minute. Since there are 33,000 foot pound minutes in a horse-power, the power developed by such a cylinder is equal to the force multiplied by the piston velocity, divided by 33,000. [Illustration]