, for the reason given
above.

[Illustration: FIG. 89.--Brake "on."]

The latest form of triple-valve includes a device which, when air is
rapidly discharged from the train pipe, as in an emergency application
of the brake, opens a port through which compressed air is also admitted
from the train pipe _directly_ into D. It will easily be understood that
a double advantage is hereby gained--first, in utilizing a considerable
portion of the air in the train pipe to increase the available brake
force in cases of emergency; and, secondly, in producing a quick
reduction of pressure in the whole length of the pipe, which accelerates
the action of the brakes with extraordinary rapidity.

It may be added that this secondary communication is kept open only
until the pressure in D is equal to that in the train pipe. Then it is
cut off, to prevent a return of air from B to the pipe.

An interesting detail of the system is the automatic regulation of
air-pressure in the main reservoir by the air-pump governor (Fig. 90).
The governor is attached to the steam-pipe leading from the locomotive
boiler to the air-pump. Steam from the boiler, entering at F, flows
through valve 14 and passes by D into the pump, which is thus brought
into operation, and continues to work until the pressure in the main
reservoir, acting on the under side of the diaphragm 9, exceeds the
tension to which the regulating spring 7 is set. Any excess of pressure
forces the diaphragm upwards, lifting valve 11, and allowing compressed
air from the main reservoir to flow into the chamber C. The air-pressure
forces piston 12 downwards and closes steam-valve 14, thus cutting off
the supply of steam to the pump. As soon as the pressure in the
reservoir is reduced (by leakage or use) below the normal, spring 7
returns diaphragm 9 to the position shown in Fig. 90, and pin-valve 11
closes. The compressed air previously admitted to the chamber C escapes
through the small port _a_ to the atmosphere. The steam, acting on the
lower surface of valve 14, lifts it and its piston to the position
shown, and again flows to the pump, which works until the required
air-pressure is again obtained in the reservoir.

[Illustration: FIG. 90.--Air-pump of Westinghouse brake.]

[21] This resembles the upper part of the rudimentary water injector
shown in Fig. 15. The reader need only imagine pipe B to be connected
with the train pipe. A rush of steam through pipe A creates a partial