Random hunches and circuit diagrams flashed through Tom's brain. "The job will boil down to blotting out sonar waves and piercing the enemy's own 'wave-trap defense,'" the young scientist concluded.  
As Tom struggled with the problem, he lost all track of time. A door swung open and high-heeled boots clumped on the floor tiles. Tom looked up and saw the portly, aproned figure of Chow Winkler entering.
 
"Hi, boss! Can I borrow a radio?" Chow asked. "Kinda like a lil music while I wrassle them pots an' pans in the galley."
 
"Sure, pardner." Tom pointed toward a portable radio on a shelf nearby.
 
Chow's leathery face broke into a grin as he picked it up. "One o' them slick lil transistor doodads, eh?"
 
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.
 
"Well, let's go, pal!" Tom urged impatiently, puzzled by Bud's lack of response.
 
"What about the square dance?"
 
Tom stopped short, feeling like a punctured balloon. He stared in dismay at his smiling, dark-haired copilot. "Good night! I forgot again!"
 
With a sigh, Tom added, "You're right, of course. We sure can't let the girls down twice. But at least let's get together all the gear we'll need when we do go to Fearing."
 
"I guess we'll have time for that," Bud conceded with a sympathetic grin.
 
Tom assembled a mass of electronic equipment and phoned various Enterprises' departments for other items. Bud helped to collect them, and the boys trucked the paraphernalia out to a hangar to be loaded aboard a Whirling Duck. Then they scootered back to the lab for a quick shower and change.
 
Twenty minutes later, in sport jackets, checked shirts, and slacks, the two chums hopped into Bud's red convertible. They picked up Sandy and Phyl and drove a little way into the country for dinner at a huge old farmhouse restaurant.
 
"Well, the evening's off to a good start," Sandy said with a happy laugh as they headed back along the lakeshore road to the yacht club.
 
"Hope I didn't put away too much fried chicken to sashay properly at the square dance," Bud remarked.
 
Tom chuckled. "Don't............