B, if the previous train has passed his cabin and he knows the section between A and B is clear, repeats this signal. The train is then despatched from A, the signalman at A gives the signal "Train on line," the signalman at B acknowledges this by moving his own indicator and the one at A to "Train on line," and at once gives the "Be ready" signal to C, and so on throughout. As soon as the train has passed B the man at B moves his own indicator and the one at A over to "Line clear," and
Fig. 13.
Set of Three-wire Block Telegraph Instruments with Bell.
upon this being acknowledged by A the indicator is left vertical, signifying "Line closed," and thus the operation has been completed so far as that particular train and that particular section are concerned. Of course it will be apparent that if this system is properly carried out, it is an absolute impossibility for two trains to be between A and B at the same time.
A new form of instrument devised by Mr. G. E.
