The tunnel in which they then were was, like the great lower chambers, well lighted up. At one side, and opposite to where they stood, was a recessed chamber containing what appeared to be very powerful machinery. Cobb saw the motor disconnect from the train at this point, but he was not permitted to notice further the working of this most remarkable invention, for the guards ordered them into the car, and the door was closed and bolted. Going back to the smoker, they lighted their cigars and settled themselves comfortably among the cushions. "Now," exclaimed Rawolle, sending up a cloud of smoke, "now I am at your service." "Then, tell me all about that which I have seen," Cobb impatiently asked. "Don't you see how anxious I am?" "Very well. Let us commence at the beginning: In the first place, this that you have seen is the pneumatic railway. Its official designation is 'The Central Pneumatic.' There are, in the United States, quite a number of these roads. From San Francisco run three, as follows: one to the north, one to the south, and this one to the east. Here is a map showing all these roads in the country;" and he took from his pocket an official railway guide, and handed it to his listener. "As the word implies, air is the motive power--not compressed, but atmospheric pressure against a surface, on the other side of which a partial vacuum has been created by exhaustion. This is the method in the tunnels only. After the trains leave the great tunnels, they are moved about the yards, which you saw were all underground, by electric motors. Hydraulic lifts take them up to the station and lower them again. Everything is underground until the train rises through its opening in the floor of the depot. When the guard ordered us into the car, and bolted the door, we had been pushed into the receiving section of the main tunnel. The main tunnel is a complete iron and stone structure, extending between San Francisco and Salt Lake without break. At Salt Lake are the engines which exhaust the air from this tunnel, the pressure of the external air being the propelling power to move the train forward to its destination. The tunnels are twelve feet in diameter, and the rear car of the train carries a shield, or end-piece, which almost fills the cross-section of the tunnel; in fact, there is but the hundredth part of an inch between the edge of the shield and the interior side of the tunnel. The engines, as I said, are constantly pumping out the air, but this is carried to such a degree that the external pressure on the tubing of the tunnel is always under one pound per square inch. A series of valves at the end of the tunnel farthest away from the engines, permits ingress to the air which acts against the rear end of the train to move it forward. The train is first placed in a movable section of this tunnel, and, everything being ready, this section is moved upon rollers into connection with the main tunnel--a sort of valve action.