It must be convincing. It should appear to radiate impossibly, or to destroy energy without radiation. But it must actually produce a broadcast signal of this exotic type--here the sergeant described with shaky precision the exact constants of the wave to be generated--and the broadcaster from nowhere must not be able to deduce those constants or that wave-type from the diagram until he had built the transmitter and tried it. "I know it can't be done!" said the sergeant desperately. "I know it can't! But it's gotta be! Or they'll parachute a transmitter down on us sure." Graves smiled a quick and nervous smile. He began to sketch a circuit. It was a wonderful thing. It was the product of much ingenuity and meditation. It had been devised--by himself--as a brain-teaser for the amusement of other high-level scientific brains. Mathematicians zestfully contrive problems to stump each other. Specialists in the higher branches of electronics sometimes present each other with diagrammed circuits which pretend to achieve the impossible. The problem is to find the hidden flaw. Graves deftly outlined his circuit and began to fill in the details. Ostensibly, it was a circuit which consumed energy and produced nothing--not even heat. In a sense it was the exact opposite of a perpetual-motion scheme, which pretends to get energy from nowhere. This circuit pretended to radiate energy to nowhere, and yet to get rid of it. * * * * * Presently Lecky could be heard expostulating gently: "But of course we are willing to give you the circuit by which we communicate with the year 3020! Naturally! But it seems strange that you suspect us! After all, if you do not tell us how to meet the danger your broadcasts have told of, you will never be born!" Sergeant Bellews mopped his face and moved into the screen's field of vision. "Doc," he said, laying a hand on Lecky's arm. "Doc Graves is sketchin' what they want right now. You want to come show it, Doc?" Graves took Lecky's place. He spread out the diagram, finishing it as he talked. His nervous, faint smile appeared as the mannerism of embarrassment it was. "There can be no radiation from a coil shaped like this," he said embarrassedly, "because of the Werner Principle.... Yet on examination ... input to the transistor series involves ... energy must flow ... and when this coil...." His voice flowed on. He explained a puzzle, presenting it diffidently as he had presented it to other men in his own field. Then he had been playing--for fun. Now he played for perhaps the highest stakes that could be imagined. He completed his diagram and, smiling nervously, held it up to the communicator-screen. It was instantly transmitted, of course. To nowhere. Which was most appropriate, because it pretended to be the diagram of a circuit sending radiation to the same place. * * * * *