means for providing for the safety of the line was by erecting fixed signals at certain points, and keeping them at "danger" for a specified time after the passage of one train, before another was allowed to follow. Obviously, under a system of this kind, only a very limited number of trains could be dealt with within a given period of time, and as the traffic increased and trains were multiplied it was found quite inadequate, and some better plan had to be devised. The electric telegraph had already been utilised for transmitting from station to station the times of departure of the trains, and about the year 1853 Mr. Edwin Clark introduced the absolute block telegraph system, by means of which the number of trains which may be passed over a given section of line, with perfect safety, may, by the multiplication and shortening of the sections, be said to be almost unlimited.
Mr. Clark's apparatus took the form of what is known as the "three-wire" block telegraph for double lines, and this form has held its own up to the present time—that is, there is one wire with an instrument at each end of the section for the up line, the same for the down line, and one wire for the bell circuit, the latter being common to both lines. Besides this, there is in use a "one-wire" system, also for double lines, but which shows only two positions of the indicator instead of three. The advantages claimed for the "one-wire" system are (1) that on railways where there are four lines of rails only two wires are required, instead of the six that would be needed under the "three-wire" system; and (2) that where the sections are long, the "one-wire" system is less expensive, although in sections half a mile or less in length the "three-wire" proves to be the
