282 
POPULAR SCIENCE REVIEW. 
knowing, on tlie instant, whetlier it is a train coming or one 
gone and safe in, and wkich train. It will be seen that the 
capabilities of the instruments used for signalling trains are 
very limited, and they can do only one particular kind of work, — 
the work for which they are constructed, and none other ; and 
this they must do not only with unerring accuracy, but very 
rapidly indeed : they have but little to say, few speeches to 
make; but what they say must be to the point, clear and 
explicit, and very quickly said. There is not time to spell out 
words and sentences, as with the ordinary speaking instru- 
ments, neither is there time for much manipulation to make 
signals, nor for much looking to read them ; the eye for the 
most part is otherwise occupied : a touch, and the signal 
must go ; a sound, and the ear must be arrested and the 
signal received. Let the reader cross the foot-bridge at 
Hungerford at any time, but especially at the busiest hours 
of days when traffic is heavy, and he will see how quickly 
the trains follow each other in safety, when protected as they 
are by a proper system of signals. As soon as one train has 
passed the signal-box on the Surrey side, a second may follow 
in safety from the Charing Cross Station. As many as four 
trains have here followed each other in five minutes, which would 
be impracticable without the aid of electricity. On the day 
of the Foresters^ fete (August 19, 1862) no less than 535 trains 
were signalled at one intermediate station in the London Bridge 
yard on one pair of bells from early morning till late at night, 
all the signals also being booked. 
There is a rule essential to intermediate stations, but of 
course not required at a terminus, that the ^^out signals of pass- 
ing trains are to bemade as they approach, in order to let them 
pass without check if the line is clear,’’ because unless the out 
signal, as we have already stated, be replied to, that is, unless 
the out signal, v/hich means May I send the train V’ be 
repeated back, which means — You may send the train,^^ — 
it would have to be checked ; for, under such circumstances, 
from whatever cause arising, ‘‘the train must be brought 
to a stand, and the driver cautioned to keep a good look-out, 
because he is running by sight and fixed signals, and not by 
telegraph signals."’^ 
I have heretofore confined myself to the code which is 
established for the simplest case of train- signalling, and which 
is applicable and is applied and in force on about five-sixths 
of the railway in question ; and, before passing on, to give one 
or two illustrations of special codes provided for mixed traffic, 
which requires distinctive signals, I may here give certain 
constant signals that are universal in their application, and 
may always have the same meaning. When a bell is struck 
