Bron: Railway Appliances, John Wolfe Barry, London 1887
Special and audible signals are used in fogs to supplement the visible signals.
The ordinary fog signal (fig. 90) is made of detonating composition, enclosed in a flattish metal capsule. The capsule is furnished with two thin metal clips, which can be bent round the rail so as to prevent the capsule from slipping off accidentally, and the pressure of
a wheel causes the detonating composition to explode. In foggy weather a platelayer or some other person is stationed at the foot of the signal posts and at other convenient positions, and his duty is to place and keep one or two detonating signals on the rails whenever the semaphore arm is at ‘Danger!’, removing the detonators when the arm is at 'All right !' or ‘Caution !’
The detonating signals are used largely under other circumstances as (for example) in the case of a train breaking down, or during repairs to the permanent way. All guards and engine drivers are supplied with detonating signals for use in emergencies, and they are a most valuable addition to visible signals; for a man can shut his eyes, but he cannot shut his ears.
The only danger connected with detonating or other audible signals lies in the possible failure of the detonating powder or other means employed for making a noise. With proper care, however, the risk of failure is small, and in all cases detonating signals should be used in duplicate.
Means are sometimes adopted, although very rarely, of placing detonating signals on the rails by a lever connected to the semaphore arm, so that when the semaphore arm is put at ‘Danger’ the lever may place a detonator on the rails, and when the semaphore arm is put to ‘ Caution’ the detonator may be withdrawn.