These simple, independent, hand-worked semaphore signals did good service for many years, but being independent and in no way physically connected with one another at junctions, or stations, or with the switches they were intended to control, it was quite possible for mistakes to arise where everything depended upon the accuracy and prompt decision of the signalman. The possibility that such mistakes could occur, and the certainty that they actually did occur, and too often with most disastrous results, led gradually to the grouping and interlocking of a large number of signal levers and switch levers together in one signal cabin. The advantages of the concentrating and interlocking of signals and switches are twofold. In the first place, one man in the signal cabin can work and control the levers for a large number of switches and signals, where formerly several men were required to be located at various places in the station-yard; but the second, and by far the most important advantage, is that with proper interlocking arrangements it is practically impossible to give conflicting signals. With a modern interlocking frame, and assuming the normal position of 315 all the signals to be at _danger_, then before a signal can be lowered for an approaching engine or train all the switches and corresponding signals, from any lines or sidings connecting with the line to be signalled _clear_ must first be set so as to prevent any engine or train coming out of such connecting lines or switches on to the line to be made clear. In a similar manner, before the points and signals can be set to permit an engine or train to pass from a siding on to the main line all the necessary signals must first be set to _danger_ to prevent the approach in either direction of any engine or train on the main line about to be occupied. The mechanical arrangements of the interlocking frame are so exact and complete as to effectually prevent any but the proper combination being made. An untrained or inexperienced signalman might inadvertently attempt to pull over a wrong lever, only to find it securely locked and immovable under the control of other levers. The proper sequence of levers must be made, and the accurately adjusted mechanism automatically prevents mistakes which formerly occurred with the old hand-worked signals from the oversight or confusion of the signalman. The interlocked switches or points are worked from the signal-cabin by light wrought-iron tubing (termed rodding) or channel-shaped iron bars supported on fixed iron rollers, and the signals by galvanized wires running over light pulleys. Modern signals are always weighted at the signal-post, so that in the event of the breaking of the pulling-wire they will fly back to their normal position of danger.