280 
POPULAR SCIENCE REVIEW. 
a down line,, with a signal station at each end, but no inter- 
mediate station. The fundamental law is, — ^^Two trains or 
engines are not to be allowed to run on the same line, between 
two signal stations at the same In order to carry out 
this important regulation, upon which the security of those 
who travel so largely depends, Every train or engine must 
be signalled out to the next station, before it leaves or ])asses a 
station.^^ So that when the business of the day, for instance, 
commences, station B knows that train No. 1 is asking per- 
mission to come to him from, station A ; and, to prevent all 
misunderstanding, ^‘’The train or engine must not be started, 
or allowed to pass, until the next station has talcen the out 
signal.^^ It is not enough for the first station to give the signal, 
the other station must talie it ; for no signal given by one 
station is complete until taken by the other station repeating 
it;^^ by which process a clear understanding is established 
between the two signallers, that the precise signal sent by one 
is received by the others. The next rule applicable is that 
Every train or engine that arrives at or passes a station is to 
be immediately signalled back in to the last station and it 
follows, from what has been already stated, that No second 
train or engine is to be allowed to follow until the in or arrival 
signal of the previous train has been taken, — that is to say, 
has been repeated back blow for blow. 
And in order that there may be no mistake as to whether or 
not the whole of the train is safely in, it is not to be signalled 
as until after it has been clearly ascertained by actually 
seeing the tail-lamps, or communication with the guards, that 
no portion of the train from any cause has been left behind.^^ 
These are the leading features of train- signalling, and they are 
quoted verbatim from well-digested regulations. It will be 
noticed that there is a double check against accident from 
collision, through a train following, — the in signal of the first 
train must be given and received ; the out signal of the second 
train must be answered, — must be repeated back. So that if 
from any oversight an attempt were made by station A to 
get permission to send a second train before the first was in, 
the attempt would be defeated by station B declining to 
answer. In addition to all this, and all of which is done in far 
less time than it takes to describe, the signals, and the time 
of making them, are entered in ruled columns by both stations. 
Signal-boxes are famished with good clocks, which are checked 
by a daily time-signal from the Royal Observatory, Green- 
wich. For the ordinary purposes of train- signalling, the 
greatest number of blows on the bell is three ; and this 
number may be made in about two seconds, or less. The- 
code, with its interpretation, is : — 
