As the trains when going at a considerable speed cannot be
suddenly stopped, it is necessary to have signals placed in certain
conspicuous positions, that the engine driver may begin to stop the
train (when necessary) in time; this he effects by what is called
a “break,” a contrivance by which two pieces of wood are made (by
turning a screw) to grasp firmly each of a pair of wheels, and so
prevent them turning round, this produces so much friction against
the trams that (after the steam is turned off) the onward motion of
the train is soon stopped. The signals are of three kinds generally,
a red flag to indicate danger, a green one to caution, and a white
one to show that the way is clear; these are (on most occasions)
held by a man and waved to and fro to attract attention, but there
are however a great many occasions for fixed signals, as at stations
and bends in the line where the engine driver can only see a short
distance ahead; these fixed signals consist of tall posts placed
where they can be seen at a considerable distance. These posts have
an arrangement at the top consisting of a lamp with a “bull’s-eye” or
lens at each side pointing up and down the line, and a pair of arms
capable of being let down into the post, raised at right angles with
it, or into a position midway between the post and a right angle (as
shown in figs. 11 and 12). One side of each arm is painted red, the
other white, one arm serving for a signal up the line and the other
down; attached to the joint of each arm, close to the post, are two
iron frames each holding a piece of colored glass, one red the other
green, and so arranged that when the arm is at right angles to the
post, the red glass is before the lamp and when the arm is let half
way down the green glass comes in front of the lamp (fig. 13), thus
the same action serves both for day and night signals. When the arm
showing the red side projects in a horizontal direction, it indicates
(in the day) “danger,” and so does the red light at night; when the
arm is let down half way, it shows that caution is required, and the
green glass then before the lamp shows the same signal at night; when
the arm is let quite down out of sight, it shows safety, and so does
the white light of the lamp thus freed from both screens of colored
glass.