The cook flicked on the dial knob and the twangy strains of Hawaiian guitar music came throbbing out. A split second later the volume swelled as the same music echoed back to them from the two-room apartment adjoining the lab, where Tom ate and slept when engaged in some round-the-clock experiment. Chow was startled by the blare. "You got a stereo hookup here, boss?" he inquired. "Not exactly." Tom explained that the music had merely been picked up by the mike on his workbench, then fed into the adjoining apartment and amplified over a speaker there. Chow grinned, snapping his fingers to the catchy melody. "Comes out even louder'n it does from the radio!" "Yes, but the sound quality's not so good," Tom said. "You'd notice the difference with real stereo." Chow walked out with the portable, crooning contentedly to the music. Tom frowned, trying to get his train of thought to focus once more on the submarine problem. But for some reason the business with the microphone and the speaker in the next room kept lingering in his mind. Suddenly Tom exclaimed aloud, "Say! I wonder if that's how the enemy sub blinds our sonar?" The idea certainly seemed feasible. Suppose the submarine used a great many "microphones"--or receiving transducers--to pick up the sonar pulses beamed out by another craft trying to detect it? These impulses could then be passed on and sent out by speakers on the opposite side of the sub, and relayed along on their underwater path of travel. Thus the sonar waves would appear to be striking no obstacle--and no echo would return to the sonarscopes on the search craft! "Jumping jets!" Tom thumped his fist on the workbench in his excitement. "I'll bet that's the answer, all right!" He grinned. "Brand my boot heels, it's partly due to good old Chow!" He grabbed a pencil and began sketching his idea on paper. It would be necessary to spot the receivers and transmitters all over the hull of the submarine. Diagrams and pages of scribbled computations followed the rough sketches. An invisible sub--one that sonar pulses would seem to pass right through, as if nothing were there! "Seems so simple now that I have the key!" Tom said to himself elatedly. Hours ticked by while he analyzed the wave action mathematically, then worked out a typical hookup for one of his jetmarines in a set of precise schematic drawings. Finally the young inventor dropped his pencil, picked up the telephone, and dialed Bud Barclay. "Hop over here, fly boy," Tom told his chum. "Something hot on the griddle!" Bud arrived in a few moments. Tom showed him the drawings and explained his plan for dodging underwater detection. He also related how Chow's remarks about the radio music had sparked the idea. His chum slapped him on the back. "Good going, Tom!" "Let's fly right over to Fearing and see how it works on a jetmarine!" Tom proposed enthusiastically. Bud grinned but made no move. He stood looking at Tom, arms folded and feet wide apart.