“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. 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.
“No, no, Mr. Cobb; do not be displeased. We really meant no discourtesy; but your remark is not what you would have made had you thought a moment, for we know you to be a man of education. We do not use wheels on the pneumatic roads. These trains run upon the many little runners which you saw under the cars. Were we to use wheels,” he continued, after a pause, “centrifugal force would tear them into pieces in no time. Take the case of a wheel four feet in diameter: the circumference of such a wheel is a little over twelve feet. At the rate of four miles per minute, it would have to revolve 1,760 times. No wheel that can be made would stand such a test. It would fly into fragments inside of the first mile. A wheel of the above dimensions and at that rate of revolution would have a centrifugal force equal to 1,000,000 pounds. Now, as the centripetal force is the tensile strength of the material only, and that of the best steel wire only 160,000 pounds, it will readily be seen that the centrifugal force would instantly cause the wheel to fly into fragments.”
“You are right,” Cobb answered, going over the figures in his mind. “Wheels would never do; I can see it plainly.”