ng 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 plen