ly as he felt safe in doing. Running would use up
oxygen too fast. He was halfway there when the pressure warning light
went on, and the signal sounded inside his helmet. He turned on his
ten-minute reserve supply, and made it to the tractor with about five
minutes left. The air purifying apparatus in the suit was not as
efficient as the one in the tractor; it wasted oxygen. By using the suit
so much, Evans had already shortened his life by several days. He
resolved not to leave the tractor again, and reluctantly abandoned his
plan to search for a large bubble.

* * * * *

The sun stood at half its diameter above the horizon. The shadows of the
mountains stretched out to touch the shadows of the other mountains. The
dawning line of light covered half of Earth, and Earth turned beneath
it.

Cowalczk itched under his suit, and the sweat on his face prickled
maddeningly because he couldn't reach it through his helmet. He pushed
his forehead against the faceplate of his helmet and rubbed off some of
the sweat. It didn't help much, and it left a blurred spot in his
vision. That annoyed him.

"Is everyone clear of the outlet?" he asked.

"All clear," he heard Cade report through the intercom.

"How come we have to blow the boilers now?" asked Lehman.

"Because I say so," Cowalczk shouted, surprised at his outburst and
ashamed of it. "Boiler scale," he continued, much calmer. "We've got to
clean out the boilers once a year to make sure the tubes in the reactor
don't clog up." He squinted through his dark visor at the reactor
building, a gray concrete structure a quarter of a mile distant. "It
would be pretty bad if they clogged up some night."

"Pressure's ten and a half pounds," said Cade.

"Right, let her go," said Cowalczk.

Cade threw a switch. In the reactor building, a relay closed. A motor
started turning, and the worm gear on the motor opened a valve on the
boiler. A stream of muddy water gushed into a closed vat. When the vat
was about half full, the water began to run nearly clear. An electric
eye noted that fact and a light in front of Cade turned on. Cade threw
the switch back the other way, and the relay in the reactor building
opened. The motor turned and the gears started to close the valve. But a
fragment of boiler scale held the valve open.

"Valve's stuck," said Cade.

"Open it and close it again," said Cowalczk. The sweat on his forehead
started to run into his e