SIGNALS.
Signals are used to indicate to the station-officials and others on duty what is taking place on the road, that they may be able to communicate to the engine-drivers, by means of the code, such information and instruc tions as may be necessary to insure the safety of trains, etc.
Classes qf Signals.—The signals employed may be optical or acoustical, for it is only through the senses of sight and hearing- that rapid and sharply-defined impressions may be received. The signals, of whatever nature they may be, are transmitted by repetition. Inasmuch, however, as the acoustic signals are audible for only a limited distance, and as opti cal signals, under unfavorable atmospheric conditions, cannot be distin guished very far, electricity has come to be used exclusively as the means of transmitting signals over considerable distances.
Stephenson's Signal System.—The first railway opened for traffic, in 1829, between Manchester and Liverpool, had no other provision for sig nalling than that afforded by the steam-whistle of the locomotive. The first step toward the invention of a signal system was made somewhat later by Stephenson, who introduced, at the entrance and exit places of the sta tions and at curves, posts upon which were mounted disks with the oppo site faces of different colors. These disks could be shift,gd, and at night were replaced by lanterns of distinct colors.
The "Semaphore" Signal.—The Stephenson system reinained in use until the year 1841, when the English engineer Gregory introduced the " semaphore" signal, based upon the old system of telegraphing over short distances. For fixed signalling, this system, with numerous modifications in the mode of. operation, is in universal use on railways at the present day. The semaphore has two arms, for trains coining from either direc tion, and the position of the arms, whether vertical or inclined at an obtuse or acute angle, determines, according to a prearranged code, the character of the signal to be conveyed to the engine-driver of an approaching train. At night the semaphores or disks are illuminated by suitable lamps, by means of which lights of different colors (commonly red and green, signi fying respectively danger and caution) are exhibited.
A recent improvement of the semaphore, which is the best position signal yet devised, is a modification invented by Koyl, an American engi neer, which may be used by night, as well as by day, in the sameemanner (pi. 25, figs. 1, 2). A reflector of silvered glass is placed along the centre of the semaphore ann from end to end; but, as it would be impossible to illuminate a flat reflector with an ordinary lamp sufficiently for railway purposes, the flat semaphore arm is replaced by one cun-ed to a parabolic form (fis-. 2). This is mounted in the ordinary casting which supports the arm of the semaphore, and it rotates about the axis on the post, which is also the axis of the paraboloid, and in the focus of which the lamp is placed. The upper part of the lain]) being red and the lower part clear (or green; Jig. r), the arm, when horizontal, is illuminated with the reflected red light and appears as a red band, and, when the arm is dropped, white or g,reen as the case may be. The Koyl semaphore promises to be very useful, as it cannot be confounded with any of the numerous side-lights along the railway-tmck at night.
The "Block plan that has been found to afford the g,rreatest assurance of safety, and that is now very generally adopted, is the one in which the normal pbsition of the signal is at danger. When the telegraph informs the signal-man of the approach of a train, he must move his signal to the position indicating- " track clear " before the train is per mitted to pass the station. When the train has passed .the station, the signal returns automatically to its normal position indicating danger. The great advantage of this plan of operating the signal is that accidents from the neglect of the signal-man to attend to the setting of the signal are pre vented, since in case of such neglect the signal will remain at dang-er and bar the passage of the train.