Turning from Beach to Cannon street, it is apparent that the train movement which has there to be accommodated would fall into inextricable confusion if it was attempted to manage it in the way which has been described. The number of trains is so great and the movement so rapid and intricate, that not even a regiment of employés stationed here and there at the signals and switches could keep things in motion. From time to time they would block, and then the whole vast machine would be brought to a standstill until order could be re-established. The difficulty is overcome in a very simple way, by means of an equally simple apparatus. The control over the numerous switches and corresponding signals, instead of being divided up among many men stationed at many points, is concentrated in the hands of two men occupying a single gallery, which is elevated across the tracks in front of the station and commanding the approaches to it, much as the pilot-house of an American steamer commands a view of the course before it. From this gallery, by means of what is known as the interlocking system, every switch and signal in the yard below is moved; and to such a point of perfection has the apparatus been carried, that any disaster from the misplacement of a switch or the display of a wrong signal is rendered impossible. Of this Cannon street apparatus Barry says, "there are here nearly seventy point and signal levers concentrated in one signal house; the number of combinations which would be possible if all the signal and point levers were not interlocked can be expressed only by millions. Of these only 808 combinations are safe, and by the interlocking apparatus these 808 combinations are rendered possible, and all the others impossible."[17] [17] _Railway Appliances_, p. 113.