"Now for a test," the young inventor said to himself.

Sandy Swift and Phyl Newton were eager to watch the test, so the next morning they drove to the plant in Phyl's white convertible. Tom, clad in swim trunks, was waiting for them with Chow near the edge of a mammoth concrete tank. Set in bedrock, at one end of the Enterprises grounds, the tank was used for submarine testing.

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 k.n.o.b?" 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.

Now came the real test. Deeper and deeper, Tom cleaved his way downward.

Reaching bottom, he prowled about the ocean bed for a while, then started up again. Suddenly a stab of pain shot through his chest--a warning of nitrogen bubbles forming in his blood!

Tom swam toward the signal cord, dangling dimly in the distance. By the time he reached it, his muscles were knotting with cramps.

"It's the bends again, all right!" Tom realized. Gritting his teeth, he yanked hard on the line, then summoned his strength to hang on.

Doc and Chow hauled up frantically. Tom's face was contorted with pain when they finally got him aboard and stripped off his mask.

"Oh! How awful!" Phyl gasped.

Sandy cradled Tom's head in her lap, and Phyl held his hand sympathetically, while Doc Simpson injected a hypodermic to ease the pain. Chow steered the launch back to sh.o.r.e, and Tom was rushed to the base infirmary in an ambulance.

Here he was placed in a decompression chamber for several hours and later transferred to a hospital bed. Bud Barclay came to visit him.

"We're a fine couple of fish," he said.

Tom chuckled wryly. "_Live_ fish, anyhow."

"In my case, thanks to you," Bud said.

"Forget it, pal. The score's about even, I should think," Tom said, recalling the many life-or-death adventures they had shared.

Bud was thrilled to hear of Tom's electronic hydrolung. The young inventor spent the evening sketching out an improved design to eliminate future accidents.

"I'll install a special device to remove the nitrogen as the wearer exhales," Tom explained. "Then a valve will feed in helium to replace it. Since helium doesn't dissolve in the blood like nitrogen does, it will not bubble out when the pressure is reduced. Should have thought of that before!"

"But you'll need a tank for the helium, won't you?" Bud objected.

Tom shook his head. "Enough can be compressed into a small capsule to supply the wearer's needs. Remember, it can be used over and over again."

"Pretty neat," Bud commented.

By morning Tom felt thoroughly recovered. He insisted upon flying back to Enterprises to make the necessary changes in his hydrolung. Bud accompanied him, eager to get back on the job.

In a few hours Tom had added a small fitting to his power unit to provide for helium subst.i.tution. Then the two boys hopped back to Fearing for a second deep-water test. This time, Tom was delighted to find that he could operate comfortably at great depths, as well as rise or descend suddenly without ill effect.

Bud was aglow with enthusiasm. "Boy, we can really explore now!"

After the boys had returned to Enterprises, Tom phoned Arv Hanson and asked that a duplicate of the hydrolung be turned out in the shop as soon as possible. It was ready the following Monday morning, so Tom suggested to his father that the two visit the proposed underwater site and make some sample plantings.

"Great idea, son," Mr. Swift agreed. "I want to try out your new diving apparatus myself. If it's successful, we'll be able to tackle two problems at once--recover the Jupiter prober and start the 'sea farm.'"

They flew to Fearing, then went by boat to the farm site, about half a mile offsh.o.r.e. Each carried several of the valuable Far Eastern plants.

[Ill.u.s.tration]

The silt beds which Mr. Swift had selected were just deep enough to keep the plants from being discovered, yet enable them to receive sufficient sunlight.

Tom and his father started their planting. But no sooner had the first plants been embedded than fish darted in to nibble them. Even the roots disappeared into their greedy maws.

[Ill.u.s.tration]

"Looks as though we'll have to build some sort of net enclosure around and over our farm," Mr. Swift said, after they had climbed back into the boat. "But at least your hydrolung device is a great success, son!"

Tom was thoughtful. "Dad, I wonder if the fish would eat those plants from s.p.a.ce which you've been growing under salt water?"

Tom was referring to certain strange plants rocketed to earth by unknown s.p.a.ce friends with whom the Swifts had been in communication.