288 
POPULAR SCIENCE REVIEW. 
meclianical arrangement of springs and studs^ so connected 
^vitli the battery, the semaphores, and wires, as to distribute 
the required currents in the required directions at the will of 
the operator. It is placed within easy reach of the signal-man. 
In this instance there are two keys, that on the right hand 
for Blackfriars signals, that on the left for Belvedere signals. 
Each key, as here arranged, has an upper and a lower knob, 
the respective duties of which are simple enough ; the current 
sent by pressing the upper knob puts up the semaphore arm, 
that sent by the lower knob lowers it. By this arrangement 
the man has perfect control over the bell and red arm of the 
semaphore at the next station, and its companion white arm 
at his own station, and can send, on the instant, signals to put 
an arm up or down, as he requires it; or he can make any 
amount of bell- signals, in reply or otherwise, without moving, 
if he does not wish to move the arms above mentioned. Over 
the red arm at his own station, and its companion white arm 
at the other station, as we have said, he has no power. 
I have endeavoured thus far to explain the science of train- 
signalling, in so far as the signal-men themselves are con- 
cerned. I have described the apparatus with which they 
interchange ideas, the manner in which they use it, and the 
rules by which they are guided. But they are only middle- 
men, as it were; the knowledge they thus acqume is not for 
themselves, but for the engine-drivers, to whom they have to 
communicate it, as they drive up one after the other with the 
successive trains. This is done by the aid of the long levers 
that are shown in the centre of Plate XI. 
When a train is approaching, its name or destination 
having preceded it by bell-signals, the first step is to prepare 
the road for it, if not already prepared ; and this is done by 
pulling forward a lever, attached to the proper points or 
switches, which is known from the labels that are seen in front 
of them. When this lever is out of the way, in other words, 
when the proper road is prepared, and not till then, two other 
levers are unlocked, and can be used; one of them puts the 
arm down of a distant signal, in the direction of the coming 
train ; and the other puts the arm down of the post over the 
signal-box itself. If, from oversight, the signal-man were to 
have attempted to put down the arm belonging to a road, 
which was not prepared for a train, he would have failed ; for 
he would have found it locked. When the train has passed, the 
levers, and with them the visual- signal, are restored to their 
original position, and locked there. When Saxby^s system 
is in use, as it is on the Charing Cross extension, the security 
obtained by this arrangement is obvious. The signal-man 
is unable to set the points for one road, and signal the engine- 
