t is Grah," he said, when he showed Rawson a little scattering of black dust within the box. "Always it tries to fall back under the ground. Both Oro and Grah grow deep down near the Zone of the Fires; we find them in the caves, Oro on one side and Grah on the other. Oro is as heavy in its upward falling as Grah is in its downward.
"Then"—he pointed to the central vertical tube in the shell—"we put both of them in here, bringing it a few grains at a time. One falls to one end and the other to the other. And then, with these simple 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."
e 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?"