awn
in; (2) prevents the opening of the inlet valve; or (3) throttles the
gas supply. The last is now most commonly used on motor cars, in
conjunction with some device for putting it out of action when the
driver wishes to exceed the highest speed that it normally permits.

[Illustration: FIG. 47.--One form of governor used on motor cars.]

A sketch of a neat governor, with regulating attachment, is given in
Fig. 47. The governor shaft is driven from the engine. As the balls, B
B, increase their velocity, they fly away from the shaft and move the
arms, A A, and a sliding tube, C, towards the right. This rocks the
lever R, and allows the valves in the inlet pipe to close and reduce the
supply of air and gas. A wedge, W, which can be raised or lowered by
lever L, intervenes between the end of R and the valve stem. If this
lever be lifted to its highest position, the governing commences at a
lower speed, as the valve then has but a short distance to travel before
closing completely. For high speeds the driver depresses L, forces the
wedge down, and so minimizes the effect of the governor.

THE CLUTCH.

The engine shaft has on its rear end the fly-wheel, which has a broad
and heavy rim, turned to a conical shape inside. Close to this,
revolving loosely on the shaft, is the clutch plate, a heavy disc with a
broad edge so shaped as to fit the inside of a fly-wheel. It is
generally faced with leather. A very strong spring presses the plate
into the fly-wheel, and the resulting friction is sufficient to prevent
any slip. Projections on the rear of the clutch engage with the gear-box
shaft. The driver throws out the clutch by depressing a lever with his
foot. Some clutches dispense with the leather lining. These are termed
_metal to metal_ clutches.

THE GEAR-BOX.

We now come to a very interesting detail of the motor car, the gear-box.
The steam-engine has its speed increased by admitting more steam to the
cylinders. But an explosion engine must be run at a high speed to
develop its full power, and when heavier work has to be done on a hill
it becomes necessary to alter the speed ratio of engine to
driving-wheels. Our illustration (Fig. 48) gives a section of a
gear-box, which will serve as a typical example. It provides three
forward speeds and one reverse. To understand how it works, we must
study the illustration carefully. Pinion 1 is mounted on a hollow shaft
turned by the clutch. Into the hollow shaft projects