n Astro's hand, he turned and walked quickly
back to the door. "You must be nuts, Charley," the two cadets heard him
say. "There's nobody here."

The door rolled closed and the light was cut off. Tom and Astro rose and
quietly made their way toward the door. They stopped, leaned against the
door, and tried to hear what was going on inside, but were unable to
distinguish more than a vague mumble of voices, because of their masks
and the thickness of the door. Suddenly, however, they were conscious of
footsteps approaching from the other side.

There was no time to hide. Each boy flattened himself against the wall
on opposite sides of the door and held his breath as the door opened
slowly.

* * * * *

"There can be no doubt about it, Steve," said Commander Walters to the
young captain. "What we need are more powerful pumping stations for
oxygen _and_ additional generators for supplying power to the force
field."

"How do you figure that, sir?" asked Strong.

"It's simply this," replied Walters. "The population here has nearly
tripled in the past two years. The force-field screens were set up
originally to accommodate only a minimum number of miners and their
families. With the heavy demand for crystal, and therefore, more
civilians to dig it out, the force field has been overloaded."

"But I still don't see how, sir," Strong protested.

"The more people, the more oxygen needed to keep them alive, right?"

Strong nodded.

"The force screens hold back the methane ammonia gas and create a vacuum
into which we pump oxygen, right?"

Again Strong nodded.

"Now we have a demand for more and more oxygen, and we pump it into the
vacuum, but eventually we arrive at the point where the pressure of the
oxygen inside is greater than the pressure outside. Therefore, the
screening force field is broken in its weaker points and the oxygen
escapes. When the balance is restored, the rupture isn't sealed and gas
seeps in."

Strong glanced questioningly at Captain Howard and at Kit Barnard, who
had been asked to remain on Titan and lend his assistance to the problem
of the screens.

"Well, gentlemen?" asked Walters, noticing Strong's glance. "That is my
theory. Do any of you have a better one? Or a more reasonable
explanation?"

Strong, Barnard, and Howard shook their heads. A complete check of every
possible source of trouble had been made by the four men and they had
found nothing.

"We