"But the other setting doesn't turn the device off, as I thought at first. Instead it -- neutralizes... the..."

He tailed off, staring at the wreck. Only Krupp was still out there; as a nominal penalty for his prank the Captain had set him the chore of dumping the instruments' data into the desk.

Krupp moved behind a blackened rib. Ballantine glanced at me, his face empty, then ran jerkily down the ramp towards the wreck.

Intrigued, I stayed to watch. Ballantine walked to the center of the nullifier disc and turned the two-way switch. Then he hoisted up the data desk's one percent weight and set it on his shoulder. He posed like a parody of Krupp, grinning coldly--

-- until Krupp himself came back into view. The big man stared, amazed. Then he strode up behind Ballantine and gave him a shove that sent him sprawling. The desk tumbled in the air; Krupp caught it neatly.

Ballantine hauled himself stiffly to his feet and brushed purple dust from his suit.

Krupp laughed at him. "Leave men's work to the men," he said harshly. "Turn that gravity thing off, Ballantine, and I'll carry the desk back to the ship."

Ballantine knelt and deftly turned the switch to its second setting.

Krupp gasped; his knees buckled. With a grunting effort he straightened up. I watched, bewildered. Ballantine approached Krupp and stared up into his face. "What's the matter, big man? Can't hold a little weight?"

Krupp looked as if he might drop the desk -- but while Ballantine taunted he had to stand there, legs shaking.

Something was wrong, I realized. Shouting for help I ran to the wreck; I brushed Ballantine aside and turned the switch. As the weight lifted from him, Krupp sighed. His blood-swollen face smoothed over and he fell back into the dust.

It took three of us to carry him back to the ship.

The Captain spent a long time grilling Ballantine, but she came away frustrated. What was there to find out? Krupp had hoisted one load too many, crushed a few vertebrae--

The Captain filed a report, and Krupp started to learn to use crutches.

I spent a long time thinking it all over.

We lifted off, and I found myself standing once more with Ballantine, this time at a port. We watched the planet recede. I began: "You were saying?"

His bony head swiveled towards me.

"On the ramp," I prompted. "Remember? You said that switch wasn't on-off..."

He turned away, but I grabbed one sharp-boned shoulder. "You see, I've worked it out. You said there were three gravity forces, two positive and one negative. One setting of the switch canceled out the positive Yukawa, leaving zero overall.

"But the other setting didn't switch the device off. It canceled out the other Yukawa. The negative one. And that left two positives..."

Ballantine grinned abruptly, showing crooked teeth.