were approaching from the right, he would want to
be certain that no train on the other line was converging on his. Danger
is avoided and assurance given by interlocking the points and signals.
To the left of the junction the home and distant signals are doubled,
there being two semaphore arms on each post. These are interlocked with
the points in such a manner that the signals referring to either line
can be pulled off only when the points are set to open the way to that
line. Moreover, before any shifting of points can be made, the signals
behind must be put to danger. The convergence of trains is prevented by
interlocking, which renders it impossible to have both sets of distant
and home signals at "All right" simultaneously.

WORKING OF BLOCK SYSTEM.

We may now pass to the working of the block system of signalling trains
from station to station on one line of a double track. Each signal-box
(except, of course, those at termini) has electric communication with
the next box in both directions. The instruments used vary on different
systems, but the principle is the same; so we will concentrate our
attention on those most commonly employed on the Great Western Railway.
They are:--(1.) Two tapper-bell instruments, connected with similar
instruments in the adjacent boxes on both sides. Each of these rings one
beat in the corresponding box every time its key is depressed. (2.) Two
Spagnoletti disc instruments--one, having two keys, communicating with
the box in the rear; and the other, in connection with the forward box,
having no keys. Their respective functions are to give signals and
receive them. In the centre of the face of each is a square opening,
behind which moves a disc carrying two "flags"--"Train on line" in white
letters on red ground, and "Line clear" in black letters on a white
ground. The keyed instrument has a red and a white key. When the red key
is depressed, "Train on line" appears at the opening; also in that of a
keyless disc at the adjacent signal-box. A depression of the white key
similarly gives "Line clear." A piece of wire with the ends turned over
and passed through two eyes slides over the keys, and can be made to
hold either down. In addition to these, telephonic and telegraphic
instruments are provided to enable the signalmen to converse.

SERIES OF SIGNALLING OPERATIONS.

[Illustration: FIG. 100.--The signaling instruments in three adjacent
cabins. The featherless arrows show the co