was a silvery, momentary sound that protested against the blasting thrum of power that shook the walls.

Manning fought along the floor to Russ's side. Russ roared in his ear: "Gravitational control! Concentration of gravitational lines!"

The papers on the desk started to slide, slithering onto the floor, danced a crazy dervish across the room. Liquids in the laboratory bottles were climbing the sides of glass, instead of lying at rest parallel with the floor. A chair skated, bucking and tipping crazily, toward the door.

Russ jerked the power lever back to zero. The power hum died. The liquids slid back to their natural level, the chair tipped over and lay still, papers fluttered gently downward.

The two men looked at one another across the few feet of floor space between them. Russ wiped beads of perspiration from his forehead with his shirt sleeve. He sucked on his pipe, but it was dead.

"Greg," Russ said jubilantly, "we have something better than anti-gravity! We have something you might call positive gravity ... gravity that we can control. Your grandfather nullified gravity. We've gone him one better."

Greg gestured toward the machine. "You created an attraction center. What else?"

"But the center itself is not actually an attracting force. The fourth dimension is mixed up in this. We have a sort of fourth-dimensional lens that concentrates the lines of any gravitational force. Concentration in the fourth dimension turns the force loose in three dimensions, but we can take care of that by using mirrors of our anti-entropy. We can arrange it so that it turns the force loose in only one dimension."

Greg was thoughtful for a moment. "We can guide a ship by a series of lenses," he declared at last. "But here's the really important thing. That field concentrates the forces of gravity already present. Those forces exist throughout all of space. There are gravitational lines everywhere. We can concentrate them in any direction we want to. In reality, we fall toward the body which originally caused the force of gravitation, not to the concentration."

Russ nodded. "That means we can create a field immediately ahead of the ship. The ship would fall into it constantly, with the concentration moving on ahead. The field would tend to break down in proportion to the strain imposed and a big ship, especially when you are building up speed, would tend to enlarge it, open it up. But the field could be kept tight by supplying energy and we have plenty of that ... far more than we'd ever need. We supply the energy, but that's only a small part of it. The body emitting the gravitational force supplies the fulcrum that moves us along."

"It would operate beyond the planets," said Greg. "It would operate equally well anywhere in space, for all of space is filled with gravitational stress. We could use gravitational bodies many light years away as the driver of our ships."

A half-wild light glowed momentarily in his eyes.

"Russ," he said, "we're going to put space fields to work at last."

He walked to the chair, picked it up and sat down in it.

"We'll start building a ship," he stated, "just as soon as we know the mechanics of this gravity concentration and control. Russ, we'll build the greatest ship, the fastest ship, the most powerful ship the Solar System has ever known!"

"Damn," said Russ, "that thing's slipped again."

He glared at the offending nut. "I'll put a lock washer on it this time."

Wilson stepped toward the control board. From his perch on the apparatus, Russ motioned him away.

"Never mind discharging the field," he said. "I can get around it somehow."

Wilson squinted at him. "This tooth is near killing me."

"Still got a toothache?" asked Russ.

"Never got a wink of sleep last night."

"You better run down to Frisco and have it yanked out," suggested the scientist. "Can't have you laid up."

"Yeah, that's right," agreed Wilson. "Maybe I will. We got a lot to do."

Russ reached out and clamped his wrench on the nut, quickly backed it off and slipped on the washer. Viciously he tightened it home. The wrench stuck.

Gritting his teeth on the bit of his pipe, Russ cursed soundlessly. He yanked savagely at the wrench. It slipped from his hand, hung for a minute on the nut and then plunged downward, falling straight into the heart of the new force field they had developed.

Russ froze and watched, his heart in his throat, mad thoughts in his brain. In a flash, as the wrench fell, he remembered that they knew nothing about this field. All they knew was that any matter introduced in it suddenly acquired an acceleration in the dimension known as time, with its normal constant of duration reduced to zero.

When that wrench struck the field, it would cease to exist! But something else might happen, too, something entirely unguessable.

The wrench fell only a few feet, but it seemed to take long seconds as Russ watched, frozen in fascination.

He saw it strike the hazy glow that defined the limits of the field, saw it floating down, as if its speed had been slowed by some dense medium.

In the instant that hazy glow intensified a thousand times—became a blinding sun-burst! Russ ducked his head, shielded his eyes from the terrible blast of light. A rending, shuddering thud seemed to echo ... in space rather than in air ... and both field and wrench were gone!

A moment passed, then another, and there was the heavy, solid clanging thud of something striking metal. This time the thud was not in space, but a commonplace noise, as if someone had dropped a tool on the floor above.

Russ turned around and stared at Wilson. Wilson stared back, his mouth hanging open, the smoldering, cigarette dangling from his mouth.

"Greg!" Russ shouted, his cry shattering the silence in the laboratory.

A door burst open and Manning stepped into the main laboratory room, a calculation pad in one hand, a pencil in the other.

"What's the matter?" he demanded.

"We have to find my wrench!"

"Your wrench?" Greg was puzzled. "Can't you get another?"

"I dropped it into the field. Its time-dimension was reduced to zero. It became an 'instantaneous wrench'."

"Nothing new in that," said Greg, unruffled.

"But there is," persisted Russ. "The field collapsed, you see. Maybe the wrench was too big for it to handle. And when the field collapsed the wrench gained a new time-dimension. I heard it. We have to find it."

The three of them pounded up the stairs to the room where Russ had heard the thump. There was nothing on the floor. They searched the room from end to end, then the other rooms. There was no wrench.

At the end of an hour Greg went back to the main laboratory, brought back a portable fluoroscope.

"Maybe this will do the trick," he announced bleakly.

It did. They found the wrench inside the space between the walls!

Russ stared at the shadow in the fluoroscope plate. Undeniably it was the shadow of the wrench.

"Fourth dimension," he said. "Transported in time."

The muscles in Greg's cheeks were tensed, that old flame of excitement burning in his eyes, but otherwise his face was the mask of old, the calm, almost terrible mask that had faced a thousand dangers.

"Power and time," he corrected.

"If we can control it," said Russ.

"Don't worry. We can control it. And when we can, it's the biggest thing we've got."

Wilson licked his lips, dredged a cigarette out of a pocket.

"If you don't mind," he said, "I'll hit for Frisco tonight. This tooth of mine is getting worse."