When Sandy saw the power unit strapped to Tom's weight belt, she exclaimed, "_That_ little gadget will supply all the air you need? Why, it's no bigger than a pocket transistor radio!" Tom grinned. "I hope it will. That's what I intend to find out." "How does it work?" Phyl asked, fascinated. Tom explained, "Actually its function is to replace the carbon dioxide that I exhale with fresh oxygen drawn from the water. Otherwise, although the carbon dioxide I'd breathe out would be a very small amount at a time, it soon would make the air unfit. The nitrogen, which makes up much of the air we breathe, is chemically inert and can be used again and again." He pointed to a round screen on one side of the unit. "This is the water intake," Tom went on, "and this other screen is where the water comes out after we've removed its oxygen." Near the forward end of the unit, a semirigid plastic tube was connected, leading up to the face mask. At the rear was a power port for inserting a small solar battery. "What about this little tuning knob?" Sandy asked. "That's the rate control for adjusting the output frequency to the wearer's breathing rate." Tom added, "I've decided to call the whole apparatus an 'electronic hydrolung.'" Chow pushed back his ten-gallon hat and scratched his head dubiously. "Wal, I'm keepin' a net handy to drag you out, boss, just in case." Tom chuckled and fitted the mask over his face, then made a clean dive into the tank. For the next ten minutes the girls and Chow watched wide-eyed as he swam, walked around, and went through vigorous exercises at the bottom of the tank without once coming up for air. "Whee!" Sandy exclaimed when Tom finally climbed out. "Make me one, so I can take up skin diving!" "It's wonderful!" Phyl added admiringly. Tom took off his mask. "I'm pretty pleased with it myself," he admitted, grinning. The girls stayed at Enterprises for lunch. Then the group, accompanied by Doc Simpson, flew to Fearing Island so Tom could test his invention in deep water. Boarding a small motor launch, with Doc at the helm, they cruised out to a suitable depth and dropped anchor. "Don't become too confident, Tom," Doc warned. "I'll drop a signal line over the side in case of emergency." Tom buckled on his equipment belt and adjusted the face mask. Then he held up crossed fingers and back-flipped over the gunwale into the water. Chow, Doc, and the girls watched his plummeting figure fade from view. Tom, an expert skin diver, had never before felt such a sense of ease and freedom under water. He was moving, light and self-contained, in a green, magical world. With no air tanks chafing his back, he felt akin to the fishes themselves. "Wish I'd brought a hook and line along." He chuckled, as a school of mackerel darted past.