"Is there any difference between walls pulling and singularity pushing?"

Farley didn't answer for a moment. "Damned if I know. The singularity stretches down the middle of the corridor within the tube. There's speculation it has something to do with maintaining this plasma but. . . honest, we're all ignorant here. You have a wide-open field to explore."

Patricia reached out with her hand. The twisted-mirror surface reached back to her with an out-of-focus something, not a hand. The hand and its opposite met. She felt a tingling resistance and pressed harder.

Her hand was gently pushed down the length until she lifted it away. Patricia—somewhat to her surprise—understood the principle immediately.

"Of course," she said. "It's like touching the square root of space-time. Try to enter the singularity, and you translate yourself through a distance along some spatial coordinate."

"You slide along," Farley said.

"Right."

Patricia maneuvered herself to the rounded beginning—or was it the end?—of the singularity, then reached around the zone with both arms as if to hug it. Her fingers squeezed the twisted surface and she was pulled against the base, then bounced back.

"Touch it," Patricia said, "and it repels the pressure with a force parallel to the axis." She touched it twice in succession. The ring and cable stopped her reacting twist. "I pinch it at this angle, I'm propelled by the singularity, going north. The opposite angle, south. No torque—unidirectional. Either I'm pushed straight outward, or I'm shunted along the line."

Farley smiled enviously through her faceplate. "You catch on fast."

"Glad you think so," Patricia said. She sighed and backed away. "Okay. Let's go back. I'm going to have to think this over."

Farley took hold of her shoulder and directed her back along the cage, down the scaffold and into the airlock. Patricia was already glassy-eyed, musing.

She hardly noticed the elevator ride. At the tent camp, she sat down with the slate and Carrolson's processor. Farley wandered off for a few minutes to eat. When she returned, the linked processor and slate were flashing requests for the next sequence of instructions. Vasquez appeared to be napping. Farley glanced at the slate display.

From the—a future (?) Singularity. Longer—passing through the asteroid wall. Inverse-square repulsion increasing. Where did the Stoners go? Why, down the corridor, of course.

No set curvature near the twisted mirror. Must have the multi-meter to check that out—certainly seems likely, however. If I regard the setup as predesigned technology, technology manipulating geometry, use of spaces and altered geodesics as a tool. A singularity, perhaps infinitely long, beginning here, just before the boundary where chamber and corridor meet.

Energy to maintain plasma tube in the corridor. Could that be made a function of the separate universe the corridor obviously is? Where did the matter come from—all that dirt, and the atmosphere? Not from the Stone, not all of it; that's obvious.

The warm air coming down the corridor lapped at the tent, brushed the grass near the camp and mingled with the cold air pouring down from the cap, forming dust-devils.