"See that twist in the rope, just under the sandbags?" he said. "That much of the rope is in the influence of the gravitor's field, which is cancelling out the pull of the Earth's gravity. Now then, if it can influence that three-foot length of rope, what influence might it have on the air around it--and on the slipstream of air flowing over our ships, which is supposed to enter the air-vents and be blasted out the jets for propulsion of the ships?" "It could be scrambling our intake flow," Foster acknowledged pensively. "But would that condition alter in time?" Morrow shook his head. "I don't think it does--or that it would unless the gravitor's batteries were almost burned out. Then the field's influence might lessen a bit. Otherwise, no." "Then why is it that the jets' efficiency increases with time?" Foster asked. "How'd you get seventy miles an hour on the big ship, then ninety? And five hours' running built up the little ship's speed an additional four miles per hour, didn't it?" Smitty nodded. "It gets gradually better--but not much. If we knew how it happened and what it was doing to the air-flow, maybe we could design jet-pods with the right shape to use that air-flow and get good performance." Foster turned and peered sharply at Morrow. "Bill, doesn't that gravitor's field work by conductivity of some sort through the surrounding material?" "Uh?" Morrow started. "Yes, it--wait! You mean the ship's plastic hull?" "Right. And what about the polarization of that plastic?" Morrow pursed his lips, contemplatively. "Like all materials on Earth, it's polarized--if you want to use that word--to the gravitational and magnetic fields of Earth. I see what you're driving at, though--the gravitors establish a field in which the Earth's gravity and magnetism are cancelled out, or bent back upon themselves. The mechanism of the gravitors, the hull they support, everything within their field of influence is placed on a basis of its own gravity, mass-attraction, magnetism, what-have-you." "And that's gradually changing the polarization of those materials," Foster concluded. "And the gravitors' field, working through the material, is also affected. There's a gradual change in its influence on other surrounding matter--and on the slipstream flowing over the ship!" "We'd need a wind-tunnel to test that, wouldn't we?" Smitty asked dejectedly. "Yep," Foster agreed. "And wind-tunnels cost money. The only other way to test it would be to make a cross-country flight, and I wouldn't advise that." "What about a cross-country night flight?" Morrow wondered. Foster gave him a strange look. "You two haven't been reading the newspapers lately, have you?" * * * * * Morrow and Smitty exchanged glances of mingled surprise and guilt. "We've been rather busy out here," Morrow protested lamely.