484
EXPOSITION AT VIENNA.
But half of the spindle, where touched by T, is cut away; hence, only one current is transmitted, and,'as the armature of E 2 requires alternate positive and negative currents to attract and repel it, rocking the escapement, the index is not moved either at the transmitting or the receiving station. The only effect, therefore, of turning the crank when the upper plungers are depressed is to ring the bell at the station above or below, as the case may be, thus advising the signalman of the approach of a train.
As far as safety is concerned, the apparatus of Messrs. Siemens seems to fulfil the requirements of a block-system more completely than any other yet introduced in Europe. No accidental signals can be given, and neglect of duty on the part of the signal-man causes no danger, but only delay. Danger may arise, as in any of the systems yet described, from the train breaking in two without the knowledge of the engineer or of the signal-man, Avho, on its passage, would unblock the signal of the preceding station while cars might be standing on the track between the two. There is also the chance, always remote, that the signal-man will wilfully do wrong, and, his own block being on, signal to the preceding station that the train has passed, when, in fact, it has not. The adoption of induction instead of galvanic currents, the control by one signal-man over the semaphore of another, and the use of but one wire, give this system an advantage over the preceding block instruments. It has, however, in common with them, the disadvantage of being quite expensive, and requiring the constant attention of a signal-man at each station.
These last two considerations would probably* alone be sufficient to prevent the adoption of this or indeed any of the block systems yet described, on American railways.
Within the past five or six years, the automatic signal of Mr. Hall, before described, has been, by a slight change in the relative position of the semaphores and circuit-closers, made to serve as a block system. The semaphores, enclosed in suitable cases, are placed at intervals of about a mile along the track. Thus, when a train passes one of the signal- stations, the wheels of the locomotive will depress a lever, close an electric circuit, and display a danger-signal, which is