vacuum in the cone E, causing air from the train pipe to rush into it
and be expelled by the steam blast.

Chapter XI.

RAILWAY SIGNALLING.

The block system--Position of signals--Interlocking the
signals--Locking gear--Points--Points and signals in
combination--Working the block system--Series of signalling
operations--Single line signals--The train staff--Train staff and
ticket--Electric train staff system--Interlocking--Signalling
operations--Power signalling--Pneumatic signalling--Automatic
signalling.

Under certain conditions--namely, at sharp curves or in darkness--the
most powerful brakes might not avail to prevent a train running into the
rear of another, if trains were allowed to follow each other closely
over the line. It is therefore necessary to introduce an effective
system of keeping trains running in the same direction a sufficient
distance apart, and this is done by giving visible and easily understood
orders to the driver while a train is in motion.

In the early days of the railway it was customary to allow a time
interval between the passings of trains, a train not being permitted to
leave a station until at least five minutes after the start of a
preceding train. This method did not, of course, prevent collisions, as
the first train sometimes broke down soon after leaving the station; and
in the absence of effective brakes, its successor ran into it. The
advent of the electric telegraph, which put stations in rapid
communication with one another, proved of the utmost value to the safe
working of railways.

THE BLOCK SYSTEM.

Time limits were abolished and distance limits substituted. A line was
divided into _blocks_, or lengths, and two trains going in the same
direction were never allowed on any one block at the same time.

The signal-posts carrying the movable arms, or semaphores, by means of
which the signalman communicates with the engine-driver, are well known
to us. They are usually placed on the left-hand side of the line of
rails to which they apply, with their arms pointing away from the rails.
The side of the arms which faces the direction from which a train
approaches has a white stripe painted on a red background, the other
side has a black stripe on a white background.

The distant and other signal arms vary slightly in shape (Fig. 91). A
distant signal has a forked end and a V-shaped stripe; the home and
starting signals are sq