Flags of different colours, held by flagmen stationed at suitable places, answered the purpose for a while, or so long as the authorized running speed did not prevent the train being brought to a stand after sighting a flag warning the engine-driver to stop. As speeds were increased, a longer or more distant view of signals became imperative, and tall posts, or semaphore signals, were introduced. Well-defined blades or discs placed on high posts were easily worked from the ground-level, and could be seen for long distances, thus enabling the trains to be controlled or brought to a stand before reaching the signal. The efficiency of the principle once recognized, improvements and additions were made from time to time, until we have the simple acting tall semaphore signal so universally in use at the present time. The position of the signal arms or blades in the daylight, and the colours shown by the lamps at night, form the code of signals for the proper working of the train service; and as the signal arms and lamps are both worked simultaneously by the same gearing, it is only 314 necessary to light the lamps to put the signals in complete condition for night-working. For some years, when the traffic was small, with trains at low speeds and at considerable intervals, one double-arm semaphore signal-post at a station was made to serve for all purposes; but as the train service became more frequent and more rapid, it was found that another semaphore or tall post signal, was necessary to give warning to the engine-driver some distance back before reaching the station or _home signal_. More particularly was this necessary at those stations where it was not intended that every train should stop. This new signal, called the _distant signal_, very soon came into general use. It was placed at distances varying from 400 to 800 yards away back from the station or home signal, and was worked by a long strained wire extending from the distant signal to a ground-lever placed near the home signal, the levers for these distant signals and home signals being thus near together and under the control of one man. More recently it was found necessary to introduce another important wire-worked signal called a _starting signal_, which is placed at the outgoing or departure end of the passenger platforms, lines, or station sidings, to prevent any train or engine starting or proceeding on its journey until such starting signal is lowered to indicate that the line is clear.