"All right, old timer. You remember that the Tube was widened at the sides in order that we could make two circular tubes side by side—one going each way."
"I had forgotten that they were circular."
"That is because of the pressure. A circle presents the best resistance," and picking an odd envelope from his pocket, he made the following sketch and passed it to me.
I nodded as I recognized the cross-section.
"Now the plan of the thing is like this," he added, putting aside his pipe and pulling a sheet of paper from the corner of his desk.
Rapidly, with all his old accuracy, he sketched the main plan and leaned over as he handed it to me.
"You see," he explained, picking up his pipe again, "both pumps work at one time—in fact, I should say all four, because this plan is duplicated on the English side. On both ends then, a train is gently pushed in by an electric locomotive. A car at a time goes through the gate so that there is a cushion of air between each car. The same thing happens at Liverpool. Now, when the due train comes out of the suction tube, it goes on out the gate, but the air behind it travels right on around and comes in behind the train that is leaving."
"But how are you assured that it will not stall somewhere?"