uare-ended, with straight stripes. When the arm
stands horizontally, the signal is "on," or at "danger"; when dropped,
it is "off," and indicates "All right; proceed." At the end nearest the
post it carries a spectacle frame glazed with panes of red and green
glass. When the arm is at danger, the red pane is opposite a lamp
attached to the signal post; when the arm drops, the green pane rises to
that position--so that a driver is kept as fully informed at night as
during the day, provided the lamp remains alight.

[Illustration: FIG. 91.--Distant and home signals.]

POSITION OF SIGNALS.

On double lines each set of rails has its own separate signals, and
drivers travelling on the "up" line take no notice of signals meant for
the "down" line. Each signal-box usually controls three signals on each
set of rails--the distant, the home, and the starting. Their respective
positions will be gathered from Fig. 92, which shows a station on a
double line. Between the distant and the home an interval is allowed of
800 yards on the level, 1,000 yards on a falling gradient, and 600 yards
on a rising gradient. The home stands near the approach end of the
station, and the starting at the departure end of the platform. The last
is sometimes reinforced by an "advance starting" signal some distance
farther on.

It should be noted that the distant is only a _caution_ signal, whereas
both home and starting are _stop_ signals. This means that when the
driver sees the distant "on," he does not stop his train, but slackens
speed, and prepares to stop at the home signal. He must, however, on no
account pass either home or starting if they are at danger. In short,
the distant merely warns the driver of what he may expect at the home.
To prevent damage if a driver should overrun the home, it has been laid
down that no train shall be allowed to pass the starting signal of one
box unless the line is clear to a point at least a quarter of a mile
beyond the home of the next box. That point is called the _standard
clearing point_.

Technically described, a _block_ is a length of line between the last
stop signal worked from one signal-box and the first stop signal worked
from the next signal-box in advance.

[Illustration: FIG. 92.--Showing position of signals. Those at the top
are "off."]

INTERLOCKING SIGNALS.

A signalman cannot lower or restore his signals to their normal
positions in any order he likes. He is compelled to lower the