ain quantity of
air rushes out of the train-pipe into the small chamber; a further
local reduction in the pressure of the train-pipe in that vehicle is
thereby effected, and this almost instantaneously actuates the
triple-valve of the succeeding vehicle, and so on throughout the
train. In this way, on a train 1800 ft. long, consisting of sixty
30-ft. vehicles, the brake-blocks may be applied, with equal force, on
the last vehicle about 2-1/2 seconds later than on the first.

High-speed air-brake.

Brake-blocks can be applied, without skidding the wheels, with greater
pressure at high speeds than at low. Advantage is taken of this fact
in the design of the Westinghouse "high-speed" brake, invented in
1894, which consists of attachments enabling the pressure in the
train-pipe and reservoirs to be increased at the will of the driver.
The increased pressure acting in the brake-cylinder increases in the
same proportion the pressure of the brake-shoes against the wheels.
Attached to the brake cylinder is a valve for automatically reducing
the pressure therein proportionately to the reduction in speed, until
the maximum pressure under which the brakes are operated in making
ordinary stops is reached, when this valve closes and the maximum safe
pressure for operating the brakes at ordinary speeds is retained until
a stop is made.

[Illustration: Fig. 2--Automatic Vacuum-Brake, showing its general
arrangement.]

Automatic Vacuum-Brake.

In the automatic vacuum-brake, the exhausting apparatus generally
consists of a combined large and small ejector (a form of jet-pump)
worked by steam and under the control of the driver, though sometimes
a mechanical air-pump, driven from the crosshead of the locomotive, is
substituted for the small ejector. These ejectors, of which the small
one is at work continuously while the large one is only employed when
it is necessary to create vacuum quickly, e.g. to take off the brakes
after a short stop, produce in the train-pipe a vacuum equal to about
20 in. of mercury, or in other words reduce the pressure within it to
about one-third of an atmosphere. The train-pipe extends the whole
length of the train and communicates under each vehicle with a
cylinder, to the piston of which, by suitable rods and levers, the
brake-shoes are connected. The communication between the train-pipe
and the cylinder is controlled