p of an upright pipe is a
valve, _A_, connected by a bolt, B, to an elastic diaphragm, C, sealing
the bottom of the chamber D. The bolt B has a very small hole bored
through it from end to end. When the vacuum is broken slowly, the
pressure falls in D as fast as in the pipe; but a sudden inrush of air
causes the valve A to be pulled off its seat by the diaphragm C, as the
vacuum in D has not been broken to any appreciable extent. Air then
rushes into the train pipe through the valve. It is thus evident that
the driver controls this valve as effectively as if it were on the
engine. These "emergency" valves are sometimes fitted to every vehicle
of a train.

When a carriage is slipped, taps on each side of the coupling joint of
the train pipe are turned off by the guard in the "slip;" and when he
wishes to stop he merely depresses the lever E, gradually opening the
valve. Under the van is an auxiliary vacuum chamber, from which the air
is exhausted by the train pipe. If the guard, after the slip has parted
from the train, finds that he has applied his brakes too hard, he can
put this chamber into communication with the brake cylinder, and restore
the vacuum sufficiently to pull the brakes off again.

When a train has come to rest, the brakes must be sucked off by the
ejector. Until this has been done the train cannot be moved, so that it
is impossible for it to leave the station unprepared to make a sudden
stop if necessary.

THE WESTINGHOUSE AIR-BRAKE.

This system is somewhat more complicated than the vacuum, though equally
reliable and powerful. Owing to the complexity of certain parts, such as
the steam air-pump and the triple-valve, it is impossible to explain the
system in detail; we therefore have recourse to simple diagrammatic
sketches, which will help to make clear the general principles employed.

The air-brake, as first evolved by Mr. George Westinghouse, was a very
simple affair--an air-pump and reservoir on the engine; a long pipe
running along the train; and a cylinder under every vehicle to work the
brakes. To stop the train, the high-pressure air collected in the
reservoir was turned into the train pipe to force out the pistons in the
coach cylinders, connected to it by short branch pipes. One defect of
this "straight" system was that the brakes at the rear of a long train
did not come into action until a considerable time after the driver
turned on the air; and since, when danger is imminent, a ver