334 RAILWAY CONSTRUCTION. 



Whichever of the points be used, they must be set, and bolt 

 locked for the main line before the train staff and its key can be 

 withdrawn from the ground frame and restored to the engine- 

 driver. As the siding is trapped at F and G, it is impossible 

 for any vehicles to be moved out on to the main line except 

 through the medium of the train staff and key. The same 

 arrangement of detached lock is equally available for a single 

 siding with only one set of points. 



Electric Repeater. It will sometimes occur that on account 

 of a curve or other obstacle, the arms and back lights of a 

 distant or other signal cannot be seen from the signal cabin, and 

 it is necessary to introduce an electric repeater. This little 

 instrument consists of a miniature semaphore signal fixed in a 

 metallic box with a glass front, and placed on a stand about a 

 foot above the floor level immediately in front of the signal lever 

 for which it is intended to serve as an indicator. Like the 

 signal proper, the normal position of the miniature semaphore is 

 at danger, but when the signal lever is pulled over in the cabin, 

 the rod that pulls down the arm on the signal post effects a 

 contact with an electric circuit which lowers the arm of the 

 miniature semaphore at the same moment that the signal arm 

 proper is lowered, and gives visible indication in the cabin that 

 the signal is working. Fig. 508 is a sketch of one form of 

 electric repeater. 



Detonators or fog signals are largely used in foggy weather 

 and snowstorms, when the out-door signals cannot be seen from 

 an approaching train. At such times the atmosphere is so dense, 

 and the surrounding objects so obscured, that an engine-driver 

 is totally unable to distinguish the usual landmarks which guide 

 him on the approach to a station or semaphore, and he might 

 easily pass by a signal unless he received an audible signal to 

 indicate the position of the one that is invisible. Detonators 

 are usually made in the form of a circular tin or metallic case 

 about two inches in diameter, and three eighths of an inch thick, 

 with soft metal clips on opposite sides for bending over and 

 securing to the rails. The case is filled with detonating powder, 

 which is crushed by the first wheel passing over it, and explodes 

 with a loud report. It is customary to use these detonators in 

 pairs placed a short distance apart in case one of them should 

 fail to explode. 



Fog-signalling regulations vary on different railways, but 



