"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.