RAILWAY TRAVELLING AND ELECTRICITY. 
147 
signal box (fig. 7), one pole of which is to earth, the 
other is connected to the instrument, from which a wire 
is carried to the signal post, where it is joined to a 
spring, a, in close proximity to the back of the arm to be 
repeated. To the back part of the arm itself is fixed a piece 
of metal, B, and in contact with it a spring, c. When the 
arm is raised fully to the danger position, the spring a is 
free from the plate B, but the slightest depression of the arm 
forms a connection between them, and so completes the electric 
circuit. The arm of the repeater is maintained at danger by 
gravity, and is lowered by the electric current. This arrange- 
ment can of course be reversed, i.e. the repeater arm may be 
raised by the current and lowered by gravity if required, or the 
movement of the arm in its several stages of danger, caution, 
and clear, can be recorded with the same wire. 
The record of the light requires a separate and independent 
wire and battery. One pole of a battery is carried to the in- 
strument, the other being put to earth, and a wire is then 
carried from the ^instrument to the insulated stud d (fig. 8) 
of an iron frame, I, fixed within the lamp at the signal post, in 
such a manner that the hollow tube, n, shall stand immediately 
over the flame. This tube, n, is firmly fixed to the iron frame 
at K, but is free to move at, and in the direction of, G. G F is 
a small metal lever centred at F, kept by means of a spring in 
contact with the adjusting screw, e. To the frame I is attached 
a wire connected with the earth. If, now, a flame be brought 
beneath the tube H, the heat from it will cause the metal 
to expand. In expanding, it will move in the direction 
of G, press against that end of the lever g f, and so carry 
its opposite extremity free of the adjusting and contact 
screw, E. In other words, it will break the. electric circuit 
at E. The recording instrument is provided with an indi- 
cator having on it the words out and in, which is so weighted, 
or adjusted, that when not under the influence of an elec- 
tric current, the word in shall be brought up to the aperture 
in front of the instrument, making it then read, as in the 
engraving, light in. This, then, is what happens. The 
battery is always in circuit ready for action. So long as 
the light is burning, the tube h is expanded and breaks down 
the circuit. The moment the light goes out or gets dim, the 
tube again contracts, and the electric circuit being restored, the 
, current flows through the instrument, carrying over the indica- 
1 tor so as to exhibit the word out instead of in, and at the same 
time sets in motion a bell which continues to ring until the 
light has attention, or until the bell is turned out of circuit by 
means of the switch (on) in front of the instrument. 
The operation of scientific thought has introduced many 
