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 eyes. He banged his hand on his faceplate in an unconscious attempt to wipe it off. He cursed silently, and wiped it off on the inside of his helmet again. This time, two drops ran down the inside of his faceplate.

"Still don't work," said Cade.

"Keep trying," Cowalczk ordered. "Lehman, get a Geiger counter and come with me, we've got to fix this thing."

Lehman and Cowalczk, who were already suited up started across to the reactor building. Cade, who was in the pressurized control room without a suit on, kept working the switch back and forth. There was light that indicated when the valve was open. It was on, and it stayed on, no matter what Cade did.

"The vat pressure's too high," Cade said.

"Let me know when it reaches six pounds," Cowalczk requested. "Because it'll probably blow at seven."

The vat was a light plastic container used only to decant sludge out of the water. It neither needed nor had much strength.

"Six now," said Cade.

Cowalczk and Lehman stopped halfway to the reactor. The vat bulged and ruptured. A stream of mud gushed out and boiled dry on the face of the Moon. Cowalczk and Lehman rushed forward again.

They could see the trickle of water from the discharge pipe. The motor turned the valve back and forth in response to Cade's signals.

"What's going on out there?" demanded McIlroy on the intercom.

"Scale stuck in the valve," Cowalczk answered.

"Are the reactors off?"

"Yes. Vat blew. Shut up! Let me work, Mac!"

"Sorry," McIlroy said, realizing that this was no time for officials. "Let me know when it's fixed."

"Geiger's off scale," Lehman said.

"We're probably O.K. in these suits for an hour," Cowalczk answered. "Is there a manual shut-off?"

"Not that I know of," Lehman answered. "What about it, Cade?"

"I don't think so," Cade said. "I'll get on the