At lunchtime, over a bowl of chili and crackers, Tom recalled another problem. "We'll need an undetectable sub to test my analyzer," he mused. "That means a repeat job of rigging all those transducers. Whew! I'd better get busy on that plastic sheathing." As soon as he had eaten, Tom phoned Arv Hanson, who arrived at the lab in a few moments. "You remember that idea I mentioned to Danny about molding all the transducers into a single continuous plastic sheet?" As Arv nodded, Tom went on, "Let's try it, using Tomasite as the plastic." Tom picked up a pencil and quickly sketched out the production steps. By machine-spacing the transmitting and the receiving transducers as closely together as possible, with minimum clearance, the plastic coating could do an even better job of absorbing sonar pings than the hand-rigged model. "And the leads from all the transducers can be combined into a single flat tape," Tom ended. "That'll make it simple to hook up with the electronic control unit inside." "Got it, skipper," Arv said tersely. "It'll take overtime to set up the job in the plastics department. But we ought to be rolling out the sheeting Tuesday." "That's swell, Arv! Thanks!" By midmorning Tuesday, Tom had his quality analyzer sonar completed and was showing Bud how the units worked. "Boy, it looks simple enough the way you explain it, prof!" Bud said admiringly. "How soon can we try it?" "Depends on Arv," Tom replied. He picked up the phone and called the plastics department. To his delight, the sheathing was already being rolled out in quantity. Arv promised that by noon he would have enough of it available to coat a jetmarine. "Nice going!" Tom said. "Shoot it out to the cargo-jet hangar as soon as it's ready!" Soon after lunch, Tom, Bud, and Arv took off for Fearing Island. When they arrived at the base, the plastic coating with its myriad tiny "mikes" and "speakers" was speedily applied to a jetmarine under Arv's supervision. Tom, meanwhile, wired the control unit and also installed the analyzer sonar in the _Sea Hound_. "Want to be 'It' for another underwater game of hide-and-seek?" Tom asked Bud with a grin. "Sure, but don't tag me with a torpedo!" Minutes later, the jetmarine slipped off into the depths with Bud and two other crewmen aboard. Tom and Arv followed in the seacopter. The quality analyzer sonar worked even better than Tom had hoped. He not only tracked the jetmarine on its outward course, but located it three different times after shutting off the analyzer long enough for Bud to seek a new location. "How'd you like to relieve Hank in the South Atlantic?" Tom asked Bud upon their return. Bud gave a whoop of excitement. "Roger!" Tom slapped him on the back. "You can take off as soon as your ship's provisioned. I'll join you later--but first," Tom added mysteriously, "I have another job to attend to." CHAPTER XVII A MISSING AMULET