It is not proposed to enter at any length into the mechanical details of this appliance, which, however, must be considered as one of the three or four great inventions which have marked epochs in the history of railroad traffic.[18] As, however, it is but little known in America, and will inevitably within the next few years find here the widest field for its increased use, a slight sketch of its gradual development and of its leading mechanical features may not be out of place. Prior to the year 1846 the switches and signals on the English roads were worked in the same way that they are now commonly worked in this country. As a train drew near to a junction, for instance, the switchman stationed there made the proper track connection and then displayed the signal which indicated what tracks were opened and what closed, and which line had the right of way; and the engine-drivers acted accordingly. As the number of trains increased and the movement at the junctions became more complicated, the danger of the wrong switches being thrown or the wrong signals displayed, increased also. Mistakes from time to time would happen, even when only the most careful and experienced men were employed; and mistakes in these matters led to serious consequences. It, therefore, became the practice, instead of having the switch or signal lever at the point where the switch or signal itself was, as is still almost universally the case in this country, to connect them by rods or wires with their levers, which were concentrated at some convenient point for working, and placed under the control of one man instead of several. So far as it went this change was an improvement, but no provision yet existed against the danger of mistake in throwing switches and displaying signals. The blunder of first making one combination of tracks and then showing the signal for another was less liable to happen after the concentration of the levers under one hand than before, but it still might happen at any time, and certainly would happen at some time. If all danger of accident from human fallibility was ever to be eliminated a far more complicated mechanical apparatus must be devised. In response to this need the system of interlocking was gradually developed, though not until about the year 1856 was it brought to any considerable degree of perfection. The whole object of this system is to render it impossible for a switchman, whether because he is weary or agitated or actually malicious or only inexperienced, to give contrary signals, or to break his line in one way and to give the signal for its being broken in another way. To bring this about the levers are concentrated in a cabin or gallery, and placed side by side in a frame, their lower ends connecting with the switch-points and signals by means of rods and wires. Beneath this frame are one or more long bars, extending its entire length under it and parallel with it. These are called locking bars; for, being moved to the