m as
follows:--Starting and home; _then_ distant. And restore them--distant;
_then_ starting and home. If a signalman were quite independent, he
might, after the passage of a train, restore the home or starting, but
forget all about the distant, so that the next train, which he wants to
stop, would dash past the distant without warning and have to pull up
suddenly when the home came in sight. But by a mechanical arrangement he
is prevented from restoring the home or starting until the distant is
at danger; and, _vice versa_, he cannot lower the last until the other
two are off. This mechanism is called _locking gear_.

LOOKING GEAR.

There are many different types of locking gear in use. It is impossible
to describe them all, or even to give particulars of an elaborate
locking-frame of any one type. But if we confine ourselves to the
simplest combination of a stud-locking apparatus, such as is used in
small boxes on the Great Western Railway, the reader will get an insight
into the general principles of these safety devices, as the same
principles underlie them all.

[Illustration: FIG. 93.--A signal lever and its connections. To move the
lever, C is pressed towards B raising the catch-rod from its nick in the
rack, G G G, guides; R R, anti-friction rollers; S, sockets for
catch-rod to work in.]

The levers in the particular type of locking gear which we are
considering have each a tailpiece or "tappet arm" attached to it, which
moves backwards and forwards with the lever (Fig. 93). Running at right
angles to this tappet, and close to it, either under or above, are the
lock bars, or stud bars. Refer now to Fig. 94, which shows the ends of
the three tappet arms, D, H, and S, crossed by a bar, B, from which
project these studs. The levers are all forward and the signals all
"on." If the signalman tried to pull the lever attached to D down the
page, as it were, he would fail to move it on account of the stud _a_,
which engages with a notch in D. Before this stud can be got free of the
notch the tappets H and S must be pulled over, so as to bring their
notches in line with studs _b_ and _c_ (Fig. 95). The signalman can now
move D, since the notch easily pushes the stud _a_ to the left (Fig.
96). The signals must be restored to danger. As H and S are back-locked
by D--that is, prevented by D from being put back into their normal
positions--D must be moved first. The interlocking of the three signals
described is merel