imple
valves, we let out a little of whichever we wish--release it a grain
at a time, if that is best. We let out a few grains of Grah, and Oro,
being stronger, draws us upward; or we let a little of the Oro escape,
and we fall downward swiftly. You see it is simple, as I said."

Rawson's reply was not an answer to Gor so much as it was an argument
with himself. "Heavy," he said. "Specific gravity beyond anything
we've ever known. Osmium, the heaviest substance we have, would be
light as a feather compared to this. But wait. This Grah, as you call
it, falls downward, but that means it falls toward the outside of the
earth. With us it would be light--light! And Oro would be heavy. New
substance--new matter! One feels only the attraction of our normal
gravitation; the other doesn't react to that at all, but is driven
outward with tremendous force by counter-gravitation, the repulsion of
this Central Sun. You've used it cleverly, but we'd have done more
with it up on top."

* * * * *

He was lost in thought for some minutes, muttering figures and
calculations half aloud. "Two thousand miles from the Central Sun to
us; two thousand more through the solid earth. And if that repelling
force follows Newtonian laws it will decrease as the square.... But,
coming down from up on top, normal gravity would decrease directly as
the distance!" He made scratches with one small stone upon a larger
one in lieu of paper and pencil, but, to his listeners, his muttered
words could have meant nothing.

"Around six seventy-six hundred and seventy miles to the neutral zone,
the Zone of Fire. And a column of water--it would carry on by, plug
the shaft, check the back-pressure, and then...." For the first time
since that night when the mole-men had poured out into the crater, his
eyes were alight with hope, though his face seemed tense and grim.
Then the lines about his lips relaxed; he smiled at Loah.

"I would like to investigate this under-world," he said, "--not very
far down. Will you take me?"

The girl's adventurous spirit had led her on many exploring trips in
that subterranean world. She laughed happily when Rawson told her what
he wanted. "But, yes," she said; "of course I know such a place." And
from some two or three miles below, after anchoring the _jana_
securely, she led him through a winding tunnel where he knew he was
steadily climbing.

* * * * *

It was