Engineering Wonders of the World
Volume I

240 ENGINEERING WONDERS OF THE WORLD. Fig. 28.—GENERAL VIEW OF LOCOMOTIVE CAB', SHOWING AUDIBLE SIGNAL APPARATUS (A) IN- STALLED. efficient in fair and foggy weather ; (3) in the event of any part of the mechanism failing, the danger signal would be exhibited. The “ engine-driver’s cab signal ” or “ au- dible signalling system,” as it is variously called, provides in the cab of the engine an instrument on which are mounted an electric bell and a steam whistle (Fig. 27). In one side is a window at which a red or a white signal shows. Should the “ distant ” be at danger when the engine passes, the red signal, bearing the word “ Danger,” appears at the window, and the whistle blows, and continues to blow. until stopped by the driver ; if cleared, the safety indication appears, and the electric bell rings until switched off. So that in either case an audible as well as a visible signal is given. These operations are effected by the passage of a shoe on the engine over a curved ramp, about 60 feet long, placed on insulating supports between the two rails of the track near the distant signal. The ramp is con- nected by electric wires to the signal cabin, where is a battery of several Leclanché cells in circuit. The signalman closes the circuit when he moves his lever to “ clear,” and breaks it when he returns it to the “ danger ” position. On the engine is another battery, which, under normal conditions, energizes a magnet which keeps the steam whistle silent. If the shoe (referred to above) be lifted half an inch, this “ local ” circuit is broken, and the magnet releases its hold. The whistle sounds, and the visible danger signal is given in the cab. Also an electric relay is brought into action to keep the circuit permanently broken until closed by the driver. Should the signal be at “ safety,” the engine shoe, on passing over the ramp, will, besides breaking the local whistle circuit, complete an electric circuit, including the signal box battery and a second whistle magnet, so providing power to keep the whistle valve closed. At the same time a relay closes a separate bell circuit, and causes the appropriate sound signal to be given and the visible safety signal to appear at the indicator. Another relay keeps tho bell cir- cuit closed, so that the bell may continue to ring until it is stopped by the engine- driver. There can be little doubt that the audible signal apparatus is an efficient substitute for distant signals, or an admirable adjunct thereto—in each of which capacities it may- be used—and should prevent effectually the disregard or disobservance of signals by engine-drivers, and in foggy weather render unnecessary the employment of fogmen at distant signals.