ing a little when the brakes are applied. Inside the
chamber is a cylinder, the piston of which is rendered air-tight by a
rubber ring rolling between it and the cylinder walls. The piston rod
works through an air-tight stuffing-box in the bottom of the casing, and
when it rises operates the brake rods. It is obvious that if air is
exhausted from both sides of the piston at once, the piston will sink by
reason of its own weight and that of its attachments. If air is now
admitted below the piston, the latter will be pushed upwards with a
maximum pressure of 15 lbs. to the square inch. The ball-valve ensures
that while air can be sucked from _both_ sides of the piston, it can be
admitted to the lower side only.

[Illustration: FIG. 85.--Vacuum brake "off."]

[Illustration: FIG. 86.--Vacuum brake "on."]

Let us imagine that a train has been standing in a siding, and that air
has gradually filled the vacuum chamber by leakage. The engine is
coupled on, and the driver at once turns on the steam ejector,[21]
which sucks all the air out of the pipes and chambers throughout the
train. The air is sucked directly from the under side of the piston
through pipe D; and from the space A A and the cylinder (open at the
top) through the channel C, lifting the ball, which, as soon as
exhaustion is complete, or when the pressure on both sides of the piston
is equal, falls back on its seat. On air being admitted to the train
pipe, it rushes through D and into the space B (Fig. 86) below the
piston, but is unable to pass the ball, so that a strong upward pressure
is exerted on the piston, and the brakes go on. To throw them off, the
space below the piston must be exhausted. This is to be noted: If there
is a leak, as in the case of the train parting, _the brakes go on at
once_, since the vacuum below the piston is automatically broken.

[Illustration: FIG. 87.--Guard's valve for applying the Vacuum brake.]

For ordinary stops the vacuum is only partially broken--that is, an
air-pressure of but from 5 to 10 lbs. per square inch is admitted. For
emergency stops full atmospheric pressure is used. In this case it is
advisable that air should enter at _both_ ends of the train; so in the
guard's van there is installed an ingenious automatic valve, which can
at any time be opened by the guard pressing down a lever, but which
opens of itself when the train-pipe vacuum is rapidly destroyed. Fig. 87
shows this device in section. Seated on the to