by a ball-valve, one form of which is
shown in fig. 2. The release-valve is for the purpose of withdrawing
the ball from its seat when it is necessary to take off the brakes by
hand; it is made air-tight by a small diaphragm, the pressure of
which, when there is vacuum in the pipe, pulls in the spindle and
allows the ball to fall freely into its seat. When air is exhausted
through the train-pipe it travels out from below the piston direct,
and from above it past the ball, which is thus forced off its seat, to
roll back again when the exhaustion is complete. In this state of
affairs the piston is held in equilibrium and the brake-blocks are
free of the wheels. To apply them, air is admitted to the train-pipe,
either purposely by the guard or driver, or accidentally by the
rupture of the train-pipe or coupling-hose between the vehicles. The
air passes to the lower side of the piston, but is prevented from
gaining access to the upper side by the ball-valve which blocks the
passage; hence the pressure becomes different on the two sides of the
piston, which in consequence is forced upwards and thus applies the
brakes. They are released by the re-establishment of equilibrium (by
the use of the large ejector if necessary); when this is done the
piston falls and the brakes drop off. The general arrangement of the
apparatus is shown in fig. 2. To render the application of the brakes
nearly simultaneous throughout a long train, the valve in the guard's
van is arranged to open automatically when the driver suddenly lets in
air to the train-pipe. This valve has a small hole through its stem,
and is secured at the top by a diaphragm to a small dome-like chamber,
which is exhausted when a vacuum is created in the train-pipe. A
gradual application destroys the vacuum in the chamber as quickly as
in the pipe and the diaphragm remains unmoved; but with a sudden one
the vacuum below the valve is destroyed more quickly, and with the
difference of pressure the diaphragm lifts the valve and admits air. A
rapid-acting valve (fig. 3) is sometimes interposed between the
train-pipe and the cylinder on each vehicle. In the normal or running
position, a vacuum is maintained below the valve A and above the
diaphragm B, while the chamber below B and above A is at atmospheric
pressure. For an emergency application of the brake, air is suddenly
admitted to the train-pipe and thus to