The instant this is done, the air is permitted to enter _in front_ of the train, and then gradually shut off until, the train having acquired its normal speed, the valves are closed altogether, and the air permitted to enter the tunnel _behind_ the train only. It is very simple, and works to perfection. There are inlets through the rear shield of the train, to which are connected tubes running to each car. These are the air-tubes of the train. As the pressure of the air against the rear shield is one pound per square inch, a like pressure is exerted at the orifice of each tube; but, as there is no resistance to its ingress, it passes through into the cars, causing an internal pressure of the atmosphere of nearly one pound per square inch. Valves opening in _front_ of the rear shield, and at a pressure of a little less than one pound per square inch, permit of the escape of the vitiated air into the tunnel _ahead_ of the rear shield. Thus a steady stream of pure air is maintained throughout the whole train. The trains are received at their destination upon compressed-air receivers, and gradually come to a stand-still. At Salt Lake, forty five minutes are allowed for this train to transfer passengers and for supper, and then the train starts onward for Omaha. At that city the train is again made up and starts upon its new course for Chicago, New York, New Orleans, Minneapolis, or other point, as the case may be. Now, our train was placed, as I said, in an auxiliary tunnel, which was, by simple mechanical means, brought into position as the segment of the main tunnel. You, of course, noticed that each car was fitted at its end with a circular disc, covering the whole end excepting the door which leads into the next car. Well, this circular disc covers the end car completely. When our train was brought into the main tunnel, the pressure upon its end-section would have been, if suddenly exerted, so great that we would have started off with a great shock, but the air is allowed to enter behind the car gradually, as I have explained. When the full momentum is reached the full pressure of the external air is allowed to exert itself against the end of the train." "And how long does it take to gain this full momentum?" Cobb asked. "But a few moments. Are you aware that you are now traveling at the rate of two hundred and forty miles per hour, or four miles per minute?" He smiled at the look of incredulity which his words evoked. Cobb was loath to believe he was in earnest, for he felt no shock of starting, nor did he experience any motion such as he would naturally associate with such a terrific speed. "Such a rate must make the wheels spin," from Cobb. Lyman looked at him, while Rawolle burst into a laugh. "I do not see anything to laugh at," the other retorted, a little nettled.