heel be held, the other
will revolve 100 times a minute; or, if one be slowed, the other will
increase its speed by a corresponding amount. The _average_ speed
remains 50. It should be mentioned that drum A has incorporated with it
on the outside a bevel-wheel (not shown) rotated by a smaller bevel on
the end of the propeller shaft.

THE SILENCER.

The petrol-engine, as now used, emits the products of combustion at a
high pressure. If unchecked, they expand violently, and cause a partial
vacuum in the exhaust pipe, into which the air rushes back with such
violence as to cause a loud noise. Devices called _silencers_ are
therefore fitted, to render the escape more gradual, and split it up
among a number of small apertures. The simplest form of silencer is a
cylindrical box, with a number of finely perforated tubes passing from
end to end of it. The exhaust gases pouring into the box maintain a
constant pressure somewhat higher than that of the atmosphere, but as
the gases are escaping from it in a fairly steady stream the noise
becomes a gentle hiss rather than a "pop." There are numerous types of
silencers, but all employ this principle in one form or another.

THE BRAKES.

Every car carries at least two brakes of band pattern--one, usually
worked by a side hand-lever, acting on the axle or hubs of the
driving-wheel; the other, operated by the foot, acting on the
transmission gear (see Fig. 48). The latter brake is generally arranged
to withdraw the clutch simultaneously. Tests have proved that even heavy
cars can be pulled up in astonishingly short distances, considering
their rate of travel. Trials made in the United States with a touring
car and a four-in-hand coach gave 25-1/3 and 70 feet respectively for
the distance in which the speed could be reduced from sixteen miles per
hour to zero.

SPEED OF CARS.

As regards speed, motor cars can rival the fastest express trains, even
on long journeys. In fact, feats performed during the Gordon-Bennett and
other races have equalled railway performances over equal distances.
When we come to record speeds, we find a car, specially built for the
purpose, covering a mile in less than half a minute. A speed of over 120
miles an hour has actually been reached. Engines of 150 h.p. can now be
packed into a vehicle scaling less than 1-1/2 tons. Even on touring cars
are often found engines developing 40 to 60 h.p., which force the car up
steep hills at a pace nothing le