338 RAILWAY CONSTRUCTION. 



they can be equally well arranged for an oblique crossing, and of 

 widths to suit the locality. 



Block-Telegraph Signalling. However complete the outdoor 

 signals and interlocking at any station, they can only control 

 the movement of trains within their range, and something more 

 is requisite to ensure the safe working of the traffic over the 

 long lengths of line between stations. For some years a time- 

 interval was allowed for the working of trains following one 

 another on the UP and DOWN lines of a double line railway, no 

 train being allowed to leave a station sooner than a fixed 

 number of minutes after a previous train had started in the 

 same direction. With this system there was always the risk 

 that the first train might be overtaken and ran into by the 

 second, and especially in the night time, or when the atmo- 

 sphere was at all foggy. The electric telegraph was then called 

 in to assist in the train-working, and brief telegrams were passed 

 between the stations announcing the departure and arrival of 

 trains. The increased security and convenience thus obtained 

 led to the introduction of special electric telegraph instruments, 

 devoted to the exclusive duty of train-working. These instru- 

 ments, termed block telegraph instruments, are now almost 

 universally used on all double lines of railway, and have largely 

 contributed to the safe and efficient working of an ever in- 

 creasing traffic. They are made in various forms, but the 

 object of each is to ensure that before any train is allowed to 

 start from, or pass any station, the signalman at that station 

 shall receive from the signalman in the cabin in advance a dis- 

 tinct visible signal that the line is clear, and free of any train 

 up to the cabin in advance ; and also that after the train has 

 been despatched, the signalman in the rear shall be at once 

 advised when the train has arrived at the signal-cabin in 

 advance. Fig. 511 is a sketch of one type of block-telegraph 

 instrument, in which the leading feature is the miniature 

 signal-post with its two arms, an arrangement which readily 

 appeals to the eye of the signalman as being so similar in form 

 and action to the fixed signals in the station. Each instrument 

 is supplied with a bell or gong, by which the adjacent signalmen 

 can communicate with each other, in accordance with a fixed 

 code of signals which defines the relative numbers of strokes of 

 the bell or gong, to represent certain regulation calls and 

 answers. In the signal-cabins of the intermediate stations, two 



