cheaper of the two, for the reason that, although the wires cost more, the instruments cost less. In addition to these, there is a single-line block telegraph, which is worked by two wires, one for the block instrument and one for the bell.
All the different kinds of block telegraph apparatus may be worked either on the "absolute" or "permissive" system, according to the requirements of the case
Fig. 12.
Three-wire "Tell-tale" Block Telegraph Instrument.
and the regulations in force; the meaning of the "absolute block " being that there can only be one train in a given section at the same time, while, under the "permissive" system, there may be more than one train in the section. Prior to the year 1879 it had not been found practicable to work with the block telegraph through busy station yards, and the passage of the train was, in
