This grouping of the levers in the tower simplified the control of the yard and placed the responsibility upon a more intelligent and more highly paid man than the average switchman, and consequently broadened the margin of safety. But terminals grew and yards grew and switches increased in number, until even this system was unable to meet the demands made upon it. It was at a time when this state of affairs seemed seriously to threaten the safety of operation of great terminals that some genius invented the pneumatic control. Instead of a row of great levers requiring the strongest muscles, the towerman found himself confronting a battery of tiny ones, operated by the touch of a finger. And that finger-touch against the slender lever is instantly magnified to the pull of a giant arm against a switch half a mile or more away. How? By a bewildering intricacy of cogs and valves, by the aid of the electric current and of compressed air, for, in order to perfect this mechanism, man has harnessed the whirlwind and the lightning. That finger-touch brings instantly an electric touch; the electric touch raises a valve which releases the compressed air from a cylinder into which it has been pumped; and the air thus withdrawn from the cylinder in the tower basement is also in the same instant withdrawn from a cylinder opposite the switch-point, by means of a slender pipe which connects the two; and a plunger in the cylinder at the switch moves the switch-point and the signals which protect it. That seems enough for any mere machine to do—but it does much more. For, by a series of interlocking devices, the switches are so controlled by each other that no signal for a train to proceed can be given until all the other switches over which the train will pass have been properly set and locked, nor can any switch be moved as long as any signal is displayed which gives right of way over it. Thus was the margin of safety further broadened, and the control of a great terminal brought down to three men on each eight hour trick, representing the very cream of their profession. Approaching trains announce their coming by ringing an electric gong, the chief towerman, of course, knowing just which train is due at that particular instant—knowing, too, if any train is late, and how late and with what other trains it conflicts. He must know the precise second of departure of each train from the shed, and every train must glide smoothly in and out without let or hindrance. He must _know_; he mustn’t merely think he knows, for this is one of the positions in which a man never has a chance to make two mistakes. For, while the tower machinery is wonderfully adapted to its purpose, it is, after all, the mind of the chief towerman which controls and directs it. * * * * *