The "Tell-tale" instruments are somewhat similar in construction to the three-wire, the difference being that the commutator is provided with a revolving disc, lettered "line closed" (indicator vertical), "line clear," and "train on line," and Nos. 2 to 6 on the remaining five sections of the disc. Up to the point of getting one train in the section, the operation is exactly the same as with the ordinary three-wire instruments, but as each additional train is admitted into the section a corresponding disc is brought forward; and also as the trains pass out of the section the same disc is moved back, section by section, until the line is again clear, and the indicator is left at the normal position of "line closed."
It only remains to say that the single-line block telegraph is worked by a similar kind of instrument to the three-wire (absolute biock), only there is a commutator at each end, which is blocked over by the station towards which the train is running, keeping the indicators at both signal boxes at "train on line" until the arrival of the train, when they are again released.
Important as the block telegraph is, it is, of course, only an auxiliary to the working of the outdoor semaphore signals, without which it would be impossible to conduct the traffic; and the maintenance of these in a state of perfect efficiency has much to do with the punctual running of the trains. It not infrequently happens that a train will run from London, say, to Liverpool or Manchester without a single "danger" signal being exhibited, or, in other words, that there is a clear road throughout. This efficiency is to a great extent secured by the application of electricity. As is well known, expansion and contraction of the signal wires take place owing to change of temperature, the
