pump shall we use? It won't be
pushed by a piston, for it will leak through either the cylinder walls
or the piston. A centrifugal pump would be equally ineffective. A
mercury vapor pump will take it out, of course, and keep a high vacuum,
but we'd never make any progress.

"Our new machine gives us the answer. With it we can just have a number
of openings in the wall of the outer shell, and set in them one of these
molecular motion directors, and direct the molecules into the outside
air. They can't come in through it, and they will go out!"

"But," Morey objected, "the vacuum that keeps out the gas will also keep
out heat, as well! Since our generator is to run on heat energy, it will
be rather chilly inside if we don't remedy that. Of course, our power
units could be placed outside, where the blast of air will warm them,
but we really won't have a very good streamline effect if we hang a big
electric generator outside."

"I've thought of that too," Arcot answered. "The solution is obvious--if
we can't bring the generator to the air, we must bring the air to it."
He began sketching rapidly on the pad before him, "We'll have all the
power equipment in this room here in the back, and the control room up
in front, here. The relays for controlling will be back here, so we can
control electrically the operation of the power equipment from our warm,
gas-tight room. If it gets too warm in there, we can cool it by using a
little of the heat to help accelerate the ship. If it is too cold, we
can turn on an electric heater run by the generator. The air for the
generator can come in through a small sort of scoop on top, and leave
through a small opening in the rear. The vacuum at the tail will assure
us a very rapid circulation, even if the centrifugal pump action of the
enclosed generator isn't enough."

His thoughts began moving more rapidly than his words. "We'll want the
generator greatly over power to run tests over a greater range. Won't
need more than one hundred kilowatts altogether, but should install
about a thousand--A.C., of course. Batteries in the keel for starting
the generator.... Self-supporting when it's rolling....

"But let's set down some actual figures on this."

For the rest of the day the three men were working on the general plan
of the new ship, calculating the strengths needed, supplementing
mathematics with actual experiments with the machines on hand. The
calculating machines were busy continuo