Chapter XXI. shows the usual form of construction. Three speeds and

reverse usually are provided, though some cars have four speeds. These gear sets are engaged successively until the desired speed is reached.

From the gear case the power is transmitted by the drive shaft, which has one or two universal joints to take up any misalignment and to enable installing the engine level and yet transmit the power to the driving axles, through the differential, at another level or angle, or at varying angles due to spring action.

Upon each of the wheels is a tire consisting of a rubber and fabric casing, enclosing a soft rubber tube with a valve by which it may be inflated. The purpose of the tires is to absorb road shocks and make riding easy, as well as to keep the car from jolting to pieces. The tires are of varying composition and form and are attached to the rims in differing ways. Many auto trucks use solid rubber tires.

Without going into detail, the foregoing gives the makeup of the car and the simple principles of its operation. To control the car requires various other parts. The first is the steering wheel and its mechanism. The wheel is mounted on a shaft running within a tube, and to the lower end of this shaft is a worm controlling a gear, and by levers and ball joints operating the steering knuckle on one of the front wheels, the other being operated simultaneously by the tie rod connection. On the steering wheel or steering column are mounted the spark-timing lever and the throttle lever. The spark lever regulates the time of the spark within the combustion chamber and the throttle the amount of mixture admitted to the combustion chamber.

At the feet of the driver are the pedal for engaging the clutch, that for applying the brake, and the accelerator pedal, which operates the throttle as well as the lever before mentioned. The throttle lever on the steering column is for the regular running adjustment, while the accelerator pedal is for temporary increase of the mixture in starting or in speeding up. There may also be a pedal for the self-starter, though the switch often is upon the dash. On the Ford and some other cars the gear shift is controlled by pedals, but usually the shift is made by a lever placed convenient to the right hand of the driver. With it is installed also the emergency brake lever, which is used for locking the car when standing and for alternating with the foot brake on long hills.

On the dashboard may be mounted the ignition and lighting switch, the speedometer, sight oil feed, and sundry other dials and switches, depending upon the fastidiousness of the owner. Where there is a self-starter there is a charge and discharge dial, the ammeter, which tells whether the generator is working, and how.