This figure shows the railroad track divided into sections, a mile more or less in length, the section rails being insulated from adjacent sections. A light battery has its terminals connected to the opposite rails at one end of the section and at the other end a relay magnet has its terminals connected to the opposite rails. Thus the current passes through the whole length of the section, keeping the relay on continuously closed circuit and magnetized as its normal condition. The relay thus keeps the secondary circuit, which directly controls the signal, normally closed, whereby the signal is normally held in a position indicating safety. When a train enters upon the section, the wheels and axles, connecting the opposite rails thereof, short circuit the current from the relay, which instantly releases its armature, thus opening the signal circuit. The signal is then instantly thrown to the danger position by means of a counterbalance. The signal may be of the enclosed disk type, electro-mechanical, electro-pneumatic, electrically controlled gas, or of any other kind. The Robinson patented system is broad, basic and a generic creation; it is not limited to any specific construction or arrangement of signal but covers all kinds. In expounding the early history of the art of automatic signaling, the following photographic reproductions from some of Robinson's early circulars and patents will be of interest. The following sections on CURVE, TUNNEL, STATION, SWITCH and DRAW-BRIDGE SIGNALS are a photographic reproduction from a circular issued by Mr. Robinson in 1870: CURVE AND TUNNEL SIGNALS. A train approaching a curve will throw up a red signal around the curve as a warning to trains from the opposite direction, and will also exhibit a signal in its rear. Thus, collisions from front or rear are guarded against. These signals may be used throughout the whole extent of a road. In entering a tunnel a train will exhibit a signal at the other end to indicate its entry, and when it gets through it will lower the signal and ring a bell at the opposite end to indicate its exit. STATION SIGNALS. A train when it leaves a station, and at various points as it passes, will indicate to the stations along the line, its Location, Direction, Rapidity and Length. Thus all necessary information regarding moving trains will be automatically announced every few minutes at the stations. SWITCH AND DRAW-BRIDGE SIGNALS. If a switch or draw-bridge is misplaced an approaching train will set an alarm ringing at the station and will also exhibit a red signal ahead of the train as a warning to the engineer that the switch is misplaced. The following heading and sections are photographic reproductions of parts of a circular issued by Mr. Robinson at the time of its date, "September, 1872."