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 merely repeated in the interlocking of a large number of signals.
On entering a signal-box a visitor will notice that the levers have different