ssed it to me.

[Illustration:--CROSS-SECTION OF TUBE--]

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.

[Illustration:--PLAN OF UNDER-SEA TUBE--]

"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?"

"It won't be likely to with pressure pumps going behind it and suction
pumps pulling from in front. We can always put extra power on if
necessary. Thus far the road has worked perfectly."

"How much power do you need to send it through, under normal
conditions?"

"Our trains have been averaging about fifty tons, and for that weight we
have found that a pound pressure is quite sufficient. Now, taking the
tunnel's length as four thousand miles (of course it is not that long,
but round figures are most convenient) and the tube width eleven and one
quarter feet each and working this out we have 3,020,000 cubic feet of
free air per minute or 2,904,000 cubic feet of compressed air, which
would use about 70,000 horse power on the air compressor."

"But isn't the speed rather dizzy?"

"Not any more dizzy, Bob, than those old fashioned money-carrying
machines that the department stores used to use--that is in comparison
to size. The average speed is about 360 feet a second. Of course, the
train is allowed to slow down toward the end of its run, even before it
hits the braking machinery beyond the gate."

"But how much pressure did you say would be put on the back of the
diaphragm--I remember that each car has a flat disc on the back that
fits fairly tightly to the tube ..."

"The pressure on the back is less than seven tons. However, the disc
does not fit tight. There are several leaks. For instance, the cars are
as you know, run on the principl