446
EXPOSITION AT VIENNA.
II. Signals giving knowledge of or control over the
POSITION OF SWITCHES OR SEMAPHORES NOT VISIBLE TO THE PERSON REQUIRING SUCH KNOWLEDGE OR CONTROL.
The apparatus classed under this head may he divided into :
1. Instruments simply giving information at a distance, by means of electricity, of the position of switches or semaphores, draw-bridges, etc. ; and
2. Instruments by which the position of semaphores may be changed or controlled at a distance through the medium of electricity.
In technical parlance, the former are designated, both by French and German engineers, by the name of " Control.” Since the introduction of the latter, however, a modification of terms would seem to be necessary.
While heavy switches are necessarily turned by hand, the lighter optical signals, consisting generally of a metal arm or disc placed at a convenient height at the side of the track, are frequently manipulated at a great distance from the stations by means of a wire running on pulleys at the top of posts some two feet above the ground. These semaphores are sometimes out of sight of the stations, and their position cannot always be known with certainty at the latter.
This arrangement, almost universal in Europe, has not been found of itself sufficient. When there are sharp curves in, the immediate vicinity of stations, neglect of the signal-men to perform their duty or failure of the signal to work properly has been a not infrequent cause of accident. Still, on busy lines, where the position of semaphores (which are necessarily placed some distance outside the switches), must be frequently changed in pursuance of advices received by speaking telegraph or by signals of the first class, they must be manipulated from the stations. The danger thus presented (in case of breakage of the wire or other failure of the distant semaphore), to a train lying quietly at a station of being run