rect them. He's got a vibration plant that's
the handiest little war machine that ever existed."

Greg suddenly wheeled and walked to a wall cabinet. From it he took a
box and, opening it, lifted out a tiny mechanism.

He chuckled deep in his throat. "The mechanical shadow. The little
machine that always tells us where Craven is--as long as he's wearing
his glasses."

"He always wears them," said Russ crisply. "He's blind as a bat without
them."

Greg set the machine down on the table. "When we find Craven, we'll find
the contraption that's blanketing Jupiter and its moons."

Dials spun and needles quivered. Rapidly Russ jotted down the readings
on a sheet of paper. At the calculator, he tapped keys, depressed the
activator. The machine hummed and snarled and chuckled.

Russ glanced at the result imprinted on the paper roll.

"Craven is out near Jupiter," he announced. "About 75,000 miles distant
from its surface, in a plane normal to the Sun's rays."

"A spaceship," suggested Greg.

Russ nodded. "That's the only answer."

The two men looked at one another.

"That's something we can get hold of," said Greg.

He walked to the ship controls and lowered himself into the pilot's
chair. A hand came out and hauled back a lever.

The _Invincible_ moved.

From the engine rooms came the whine of the gigantic power plant as it
built up and maintained the gravity concentration center suddenly
created in front of the ship.

Russ, standing beside Greg at the control panel, looked out into space
and marveled. They were flashing through space, their speed building up
at a breath-taking rate, yet they had no real propulsion power. The
discovery of the gravity concentrator had outdated such a method of
driving a spaceship. Instead, they were falling, hurtling downward into
the yawning maw of an artificial gravity field. And such a method made
for speed, terrible speed.

Jupiter seemed to leap at them. It became a great crimson and yellow
ball that filled almost half the vision plate.

* * * * *

The _Invincible's_ speed was slacking off, slower and slower, until it
barely crawled in comparison to its former speed.

Slowly they circled Jupiter's great girth, staring out of the vision
port for a sight of Craven's ship. They were nearing the position the
little mechanical shadow had indicated.

"There it is," said Russ suddenly, almost breathlessly.

Far out in space, tiny, almost l