TRAIN- SIGNALLING IN THEORY AND PRACTICE. 
287 
ing as the roads are multiplied. The greatest number of Bell- 
Semaphores in any box in the London yard is six ; and they 
are worked there in a modified form^ in order to meet the 
speciahty of the place. In one of these boxes^ 650 trains or 
engines were signalled^ and mostly by bell alone_, on Whit- 
Monday^ 1864^ May 16_, between 5.80 a.m. and midnight_, 
all accurately entered in the books^ twelve figures being 
required for the entry of each train or engine^, — amounting in 
all to 7_,800 figures^ — so much care is taken for the protection 
of the public^ and so much serious work is quietly and success- 
fully carried on for their advantage and security. 
In stating the mutual relations between voltaic currents 
and electro -magnets_, in order to explain the action of the 
signal-bell; it was enough to say that an electro-magnet has 
the power of attracting the iron keeper; whenever a voltaic 
current traverses the wire that surrounds the iron core. This 
single property will not suffice to the twofold action of the 
Electro-Magnetic Telegraph Semaphore; — the putting a signal- 
arm up; and then again putting it down. When a current 
circulates round a bar of iron in one known direction; 
the near end of the iron becomes a north pole; when the 
direction is reversed; it becomes a south pole. The north pole 
lias the property of attracting the south end of a permanent 
stoel magnet; and of repelling the north end ; — the south pole 
attracts the north and repels the south. Advantage is taken 
of this property in order to produce the two required motions. 
A couple of bar-magnets are so mounted in a movable system 
in relation to the two poles of an electro-magnet; that the 
action of the latter on the four poles of the magnets is com- 
bined to produce motion in one direction with one current 
of electricity; and in the reverse direction with the reverse 
current ; and the motion is transferred in a very simple 
manner to the arms of the semaphores. 
And the arrangements are such that the arms are firmly 
loclced in the position into which they have been placed by 
tiie electric current; so that no amount of vibration from 
passing trainS; and no external violence; accidental or other- 
wise; can displace them ; nothing; in fact; can alter their posi- 
tion; save an electric current sent in the reverse direction. 
The locking-plate is an armature; acting independently of the 
system of magnetS; which releases them by the same current 
that is used for shifting them. 
It remains to mention that for the simple key or contact- 
maker; previously referred tO; and used for sending only one 
kind; — either kind of current for mere bell-signals; is substi- 
tuted the Semaphore key;^"’ shown on the post in the centre of 
the Waterloo boX; Plate XI.; which is nothing more than a 
