214 REPORT—1874. 
of telegraphic signals and interlocking mechanisms for the working of the points 
and signals. 
In former times it was customary to allow a certain number of minutes to elapse 
after a train passed any station, or junction, or level crossing, or other point where 
a servant of the company was stationed, before the succeeding train was allowed to 
pass the same place. Thus at numerous points along the line a time interval was 
preserved between successive trains. It was quite possible, however, that the fore- 
most of the two trains, after passing any of these places where signals were given, 
might become disabled, or might otherwise be made to go slowly, and that the fol- 
lowing train might overtake it, and come into violent collision with it from behind. 
In order to provide against the occurrence of such accidents, a system was intro- 
duced called the Block System; and its main principle consists in dividing the line 
into suitable lengths, each of which is called a block section, and allowing no engine 
or train to enter a block section until the previous engine or train has quitted that 
portion of the line. In this way a space interval of at least the length of a block 
section is preserved between the two trains at the moment of the later train’s passing 
each place for signalling; and the risk of this space interval becoming dangerously 
small by negligence or other accidental circumstances, as the later train approaches 
the next place for signalling, is almost entirely avoided. 
Further, at each signalling-station, the various levers or handles for working the 
points, and those for working the semaphore signals for guiding the engine-drivers, 
instead of being, as was formerly the case, scattered about in various situations 
adjacent to the signalling-station, and worked often some by one man and some by 
another, without sufficient mutual understanding and without due harmony of action, 
are now usually all brought together into one apartment called the signal-cabin. This 
cabin, like a watch-tower, is usually elevated considerably above the ground, and is 
formed with ample windows or glass sides, so as to afford good views of the railway 
to the man who works the levers for the semaphores and points, and who transmits 
by electricity signals to the next cabins on both sides of his own, and, when neces- 
sary, to other stations along the line of railway. 
The interlocking of the mechanisms for working the points and for working the 
semaphores, which, by the signals they show, control the engine-drivers, consists in 
having the levers by which the pointsman works these points and signals so con- 
nected that the man in charge cannot, or scarcely can, put one into a position which 
would endanger a train without his having previously the necessary danger-signal 
or signals standing so as to warn the engine-driver against approaching too near to 
the place of danger. 
The latest important step in the development and application of the block system 
is one which has just now been made in Scotland, on the Caledonian Railway. 
Before explaining its principle, I have first to mention that a semaphore arm raised 
to the horizontal position is the established danger-signal, or signal for debarring an 
engine-driver from going past the place where the signal is given. Now the ordi- 
nary practice has been, and still is, to keep the semaphore arm down from that level 
position, and so to leave the line open for trains to pass, except when the line is blocked 
by a train or other source of danger on the block section in front of that semaphore, 
and only to raise the semaphore arm exceptionally as a signal of danger in front. 
The new change, or improvement, now made on the Caledonian Railway consists 
mainly in arranging that along a line of railway the semaphore arms are to be 
regularly and ordinarily kept up in the horizontal position for prohibiting the 
passage of any train, and that each is only to be put down when an approaching 
train is, by an electric signal from the cabin behind, announced to the man in charge 
of that semaphore as having entered on the block section behind, and when, further, 
that man has, by an electric signal sent forward to the next cabin in advance, 
inquired whether the section in advance of his own cabin is clear, and has received 
in return an electrical signal meaning “ The line is clear; you may put down your 
debarring signal, and let the train pass your cabin.” The main ettect of this is 
that along a line of railway the signals are to be regularly and ordinarily stand- 
ing up in the debarring position against allowing any train to pass; but that just 
as each train approaches, and usually before it has come in sight, they go down 
almost as if by magic, and so open the way in front of the train, if the line is ascer- 
