36 W. H. WARREN. 
length, having a similar machine at each end, and the tablet or 
staff cannot be taken from the instrument at any station without 
the concurrence of the station-master at the station to which the 
train is going to proceed. For instance, assume a section between 
A and B; there is an instrument at each station containing a 
number of iron tablets or staffs. These two instruments are 
electrically interlocked, the one with the other, in such a manner 
that A cannot obtain a tablet from his instrument without per- 
mission from B; nor can B get one from his machine without the 
concurrence of A. When one tablet or staff has been withdrawn 
from either instrument, neither the station-master at A nor the 
station-master at B can obtain another until the one already re- 
moved has been restored, and until this has been done the section 
between A and B is completely locked up. Now, as no train is 
allowed to proceed from A to B (or vice versa) without carrying. 
the tablet, it is evident that only one train can be on that section 
of single line at one time. 
Under the old staff system, unless the trains ran with regularity 
there always existed a possibility that the train staff would not 
be in possession of the station which first required the use of the 
road, and as no means existed to restore the staff to the station 
requiring it, recourse had to be made to a system of procuring a 
“line clear” message to permit of the train passing over the 
section. 
By the electric tablet or staff any desired number of trains 
can be despatched consecutively in either direction, there being a 
number of tablets or staffs in the instruments at both ends of 
the section, but only one can be obtained at once, consequently 
only one train can be in that section of line at one time, and 
what is known as the “absolute block” is maintained. 
The progress made since 1888 may be better explained by the 
following table, which shows the advances made yearly :— 
Double Line. Single Line. 
October, 1888 ... 28 miles ... Nil. miles 
October, D389 623.47 Slyy,, eet 3) ae 
