Engineering Wonders of the World
Volume I

226 ENGINEERING WONDERS OF THE WORLD. Fig. 2.—OLD DISC SIGNAL AT SPETISBURY, SOMER- SET AND DORSET RAILWAY. of indications of the arm of a semaphore such as had been first used for naval and military signalling by a Frenchman named Claude Chappe in 1796. Such a signal was erected at New Cross late in 1841 or early in 1842. Only three intimations were called for—- “ danger,” “ caution,” and “ clear.” To give the first, the arm was set horizontally ; lowered to an angle of 45° for “ caution ; ” and lowered vertically for “ clear.” For night work a Fig. 3.—“distant” (fork-ended) and “stop” (square-ended) semaphore arms. lamp was provided with red, green, and white lenses. This was on a separate shaft, and originally was turned independently of the operation of the arm. These devices were all for stop signals, as distant signals had not been introduced. The latter came subsequently, and were first known as “ auxiliary ” signals. Semaphore signals were adopted very slowly for stop signals, and discs were long retained for distant signals. The Great Northern line, when opened in the early ’fifties, was the first to have Semaphore Signals. semaphore distant signals throughout. No distinction was made in the shape of the two types of signals. Later, by cutting a notch and giving a fish-tail to those distant signal arms that were placed near stop signals, the one class of signal could be distinguished from the other. This led to the adoption of the fish-tail arm now universal. See Fig. 3. Fig. 4.—“ STEPPED ” SIGNALS AT A JUNCTION. The greater the elevation of an arm the more important the track to which it refers. An improvement in the design of signals is the “ stepping,” whereby they are made more readable. Stepping J . Signals. At the approach to a junction, for instance, the arm for the most impor- tant line is the highest, and that for the next