ively small degree by the screw-down brakes on the
engine and guard's van. The goods train of to-day in many cases still
observes this practice, long obsolete in passenger traffic.

An advance was made when a chain, running along the entire length of the
train, was arranged so as to pull on subsidiary chains branching off
under each carriage and operating levers connected with brake blocks
pressing on every pair of wheels. The guard strained the main chain by
means of a wheel gear in his van. This system was, however, radically
defective, since, if any one branch chain was shorter than the rest, it
alone would get the strain. Furthermore, it is obvious that the snapping
of the main chain would render the whole arrangement powerless.
Accordingly, brakes operated by steam were tried. Under every carriage
was placed a cylinder, in connection with a main steam-pipe running
under the train. When the engineer wished to apply the brakes, he turned
high-pressure steam into the train pipe, and the steam, passing into the
brake cylinders, drove out in each a piston operating the brake gear.
Unfortunately, the steam, during its passage along the pipe, was
condensed, and in cold weather failed to reach the rear carriages. Water
formed in the pipes, and this was liable to freeze. If the train parted
accidentally, the apparatus of course broke down.

Hydraulic brakes have been tried; but these are open to several
objections; and railway engineers now make use of air-pressure as the
most suitable form of power. Whatever air system be adopted, experience
has shown that three features are essential:--(1.) The brakes must be
kept "off" artificially. (2.) In case of the train parting accidentally,
the brakes must be applied automatically, and quickly bring all the
vehicles of the train to a standstill. (3.) It must be possible to apply
the brakes with greater or less force, according to the needs of the
case.

At the present day one or other of two systems is used on practically
all automatically-braked cars and coaches. These are known as--(1) The
_vacuum automatic_, using the pressure of the atmosphere on a piston
from the other side of which air has been mechanically exhausted; and
(2) the _Westinghouse automatic_, using compressed air. The action of
these brakes will now be explained as simply as possible.

THE VACUUM AUTOMATIC BRAKE.

Under each carriage is a vacuum chamber (Fig. 85) riding on trunnions, E
E, so that it may sw