Once this fact was understood, control was easy. A single variable inductance in series with the windings handled everything. In a certain sense, the gadget was a magnet with negative--minus--self-inductance. When a plus inductance in series made the self-inductance zero, neither plus nor minus, the immensely powerful device became docile. A small current produced a mild thrust, affecting only part of the random heat-motion of atoms and molecules. A stronger current produced a greater one. The resemblance to an electromagnet remained. But the total inductance must stay close to zero or utterly violent and explosive forward thrust would develop, and it was calculable only in thousands of gravities. Burke had worked for three weeks to make the thing, but he developed a control system for it in something under four hours. That same night they got the bronze shaft into the ship. It fitted perfectly into the place left for it. Burke knew now exactly what he was doing. He set up his controls. He was able to produce so minute a thrust that the lath-and-plaster mould merely creaked and swayed. But he knew that he could make the whole mass surge unstoppably from its place. Holmes sent his workmen home. Sandy and Pam went to the movies with two very nice men who pumped them deftly of all sorts of erroneous information about Burke and Holmes and Keller and what they were about. The nice men did not believe that information, but they did believe that Sandy and Pam believed it. For themselves, the combination of an object made by Burke which flew thirty miles plus the presence of Holmes, who built plastic yachts, and the arrival of Keller to adjust instruments of which they had a complete list--these things could not be overlooked. But they did feel sorry for two nice and not over-bright girls who might be involved in very serious trouble. Holmes and Burke installed directional controls, wiring, recording instruments, etc. Stores and water and oxygen, for emergency use only, went into the lath-and-plaster construction. Holmes took a hammer and chisel and painstakingly cracked the mould so that the top half could be lifted off, leaving the bottom half exposed to the open air and sky. Then the broadcast from space cut off. It had been coming continuously for something like five weeks; one sharp, monotonous note every two seconds, with a longer, fluting broadcast every seventy-nine minutes. Now a third, new message began. It was yet another grouping of the musical tones, with a much longer interval of specific crackling sounds. Keller had adjusted every instrument and zestfully retested them over and over. Burke asked him to see if the third space message compared in any way with the second. Keller put them through a hook-up of instruments, beaming to himself, and the answer began to appear.