144 
POPULAR SCIENCE REVIEW. 
at regular and fixed speeds, if time-tables could be adhered to, 
if the line were not crowded with traffic, if the driver could 
always ensure a good view before him, if signals were near 
together and they were properly regarded, then a rigid interval 
of time might be maintained between following trains; but 
none of these elements of safety are constant. Fast expresses 
follow slow goods trains, now through a thick fog, now up a 
wet incline, at one moment in bright sunshine, at the next in 
a thick snowstorm ; creeping mineral trains break down in a 
long interval between two stations ; passengers rush in at the 
last minute, detain the train, and prevent the time-tables from 
being adhered to ; trains are so frequent at some places that 
the five minutes’ interval cannot be adhered to ; obstructions 
to view arise from curves or cuttings, or from atmospheric 
causes ; long lengths of line are unprotected by any signal at 
all, and signals themselves are too frequently neglected. Hence 
the system is brimfull of elements of danger, and the inexorable 
logic of facts has shown that the 44 time ” interval is illusory 
and the system unsafe. 
But when trains, however rapidly or slowly they may be 
running, however much punctuality has been infringed, how- 
ever crowded with traffic the line may be, are invariably kept 
apart by an interval of one or two miles, collision between them 
becomes impossible. This is the block system , which has, very 
improperly, been divided into two classes, the absolute and the 
permissive. The former is the block system proper, the latter 
is not a 44 block ” system at all, but a system introduced, not to 
secure the safety of trains, but to increase the capacity of the 
line for the transmission of increasing traffic. It is, doubtless, 
an improvement on the time system, but it bears little affinity 
to the block, and should certainly not be included in the same 
category. 
The block system is effectually carried out by means of 
electricity. 
The line is divided into sections, each having its own instru- 
ments for up and down trains respectively. Let a, b, c (fig. 1, 
PL CXXXIV.) represent three signal stations, and the space 
between them two sections of the line. Let the upper line be 
that for down trains, the lower that for up trains, and the centre 
the block signals. The connection between the latter exists 
only from one signal station to the other. Each line of rails 
has its own instrument, and each instrument its corresponding 
line, or out-door, signal. The block signals are for the guid- 
ance of the signalman ; the out-door signals for the guidance 
of the engine-driver. 
The electric signals consist of a miniature semaphore signal 
(fig. 2) capable of giving two signals — one, the arm raised to a 
