Suddenly Astro jerked Tom by the sleeve and pulled him back into the restaurant to crouch behind the door.

"Hey, what's the matter with you?" growled Tom.

"Sh-h-h!" hissed Astro and pointed across the street. "Look!"

Tom poked his head around the corner of the doorway and quickly jerked it back again. Quent Miles was hurrying down the street.

"Wonder what he's doing around here?" whispered Astro, watching the black-clad s.p.a.ceman pa.s.s directly opposite them and continue down the street, seemingly unaware that he was being watched.

"He must be heading for the evacuated section," said Tom.

"How do you figure that?" asked Astro, as they peered cautiously around the edge of the doorway.

"He's wearing his oxygen mask."

"Come on!" said Astro. "Let's find out what that heel is up to."

Hugging the buildings, the two cadets walked down the street, following Miles. There was a puzzled frown on Astro's face as he stared at the s.p.a.ceman, a hundred feet away. "I swear, Tom," he complained, "I'm about to bust a rocket. Every time I see that guy, I think I know him, but when I try to pin it down, it slips away from me."

"Watch it!" cried Tom. "He's stopping."

The boys ducked behind a deserted jet car as Quent Miles suddenly spun around to stare suspiciously back down the street.

"I don't know if he saw us or not," whispered Tom.

"With that oxygen mask," replied the big cadet, "maybe he can't see very well."

"He's going on," replied Tom. "Come on. We've got to find out what he's up to. He wouldn't be concerned about someone following him if he weren't trying to hide something."

They slipped around the jet car and stepped back on the sidewalk. Ahead of them, Quent Miles was walking quickly, reading all the street signs.

Suddenly he turned down a side street, and the two cadets raced after him.

They were in the outskirts of the city now. Great areas were covered with rolling gra.s.s fields where the citizens of t.i.tan spent their leisure hours playing ball and picnicking, and it was easy for the cadets to follow the black-suited s.p.a.ceman. They had to put on their oxygen masks as the deadly fumes of the methane ammonia atmosphere began to swirl around them. They were near the outer limits of the atmosphere screen's effectiveness.

"I think he's going into that building up ahead, Astro," said Tom, his voice distorted to a low metallic hiss by the miniature amplifier in the face of the mask.

Astro nodded and they ducked into a gully as Quent Miles turned once again and glanced down the street.

"Wonder what's in that building?" mused Tom.

"One way to find out," said Astro. "Come on. He's moving again."

The gas began to thicken now, and the two cadets found it difficult to see more than a few feet ahead as they moved cautiously through the swirling death around them. After what seemed like an hour, but was actually hardly more than a few minutes, they found the building Miles had entered.

"I'd give two weeks' leave for a ray gun now," said Tom.

"Want me to try the door?" asked Astro.

"Go ahead. We can't learn anything standing out here."

Astro put his hand on the circular latch and twisted it slowly. The door slid back on rollers, exposing a dark interior. The two boys slipped inside.

"Better close the door, Astro," said Tom. "The ammonia doesn't seem to be so thick in here."

Astro twisted the latch on the inner side and the heavy door rolled back into place. They turned slowly and saw a room that was dark except for a single light gleaming weakly through the haze of the gas. When their eyes became adjusted to the semidarkness, they moved, searching for another door in the huge room.

"Are you sure this is the right place?" asked Astro.

"I can't be positive," said Tom. "The stuff outside was too thick--" He stopped, touched Astro on the arm, and pointed to his left. There was the sound of a door sliding back and light filtered into the murky room.

Quent Miles stood framed in the doorway, the unmistakable outline of a paralo-ray gun in his right hand.

"Drop to the floor," hissed Tom.

The two cadets dropped lightly to the floor and lay face down, while Quent Miles walked toward them fanning the gun around menacingly. Then, as he was about to step on 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 n.o.body 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 t.i.tan and lend his a.s.sistance 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 still have to wait for a report from the electronics sections, sir,"

said Howard, rubbing his eyes. He started to get up and then suddenly slumped to the floor.

"By the craters of Luna!" cried Walters, jumping to the young officer's side. Howard was picked up and placed on a nearby couch. While Strong and Kit loosened his clothing, Walters grabbed the nearest oxygen mask and slipped it over the s.p.a.ceman's face.