2S6 
POPULAR SCIENCS REVIEW. 
train is in. The semaphore arm has two motions, and com- 
mnnicates alternately and puts on record two facts only, and 
most important facts they are, — either that a train is on 
the line, or that a train is not on the line ; and as either one 
or other of these things must always be the case, it is a 
standing record, at any and at all times, of the actual state of 
the line, and is of very great service to the signal-man, in 
regulating the enormous traffic with which he has to deal. The 
combined office of the bell and semaphore is now evident. It 
is not enough, as we have seen, that the arm of the semaphore 
indicates the line clear : before a train is allowed to go, ideas 
must be interchanged by bell-signal ; and if the reply is 
combined with stiU displaying the all-clear signal, that is the 
red arm down, the train may proceed; nor is this enough, 
for it would be a contradiction after the train had passed, 
and is therefore on a certain length of rail, for the all-clear 
signal to remain displaj^ed ; and hence it is that a second 
interchange of out bell-signals is made, this time indi- 
cating that the train had come on as allowed; and the 
reply in recognition of this is combined with raising the 
red arm behind the train, and retaining it there until the 
train is safely in. It is then lowered without any further 
interchange of signals; and so on. It takes far longer to 
describe these operations than to perform them. The bell- 
semaphore signal is the result of a single act, — one and the 
same pressure on the key sends a bell signal and raises or 
depresses the semaphore arm as the case may require, — a 
single telegraph wire only being required for the combined 
system, as for the more simple bell system. 
This arrangement carried on thus far is all very well for the 
Charing Cross line proper, which at present consists of only an 
up and a down line ; but at the London Bridge Station, the 
several kinds of down trains have to be sorted out, and switched 
to the lines leading to their various destinations ; and on the 
other hand, up trains from different lines have to be collected 
and switched one after the other, onward on the one line to 
Charing Cross. The first distinctive change is that the two lines 
of rails as they enter the London Bridge Station become four, 
and are protected by two pairs of electro-magnetic semaphores 
instead of one pair; and have of course bell-codes modified 
to suit the necessities of the case. And then, when well in 
the yard of this station, the roads increase in number and in 
intricacy. Any attempt to describe them and to enter into the 
details of the telegraph arrangements provided for their pro- 
tection would be more tedious to the general reader than 
profitable. The whole is conducted on the principles so fully 
explained, — the apparatus beiug increased in number accord- 
