Tom handed him a hastily scribbled diagram of the audio-screen setup.

"One of those hurry-up deals, Gib," he said with an apologetic grin. Tom explained his plan. "We'll use transmitter buoys, monitored by an alarm system at base headquarters on Fearing."

Brownell studied the diagram and nodded. "Right. We can have it set up in twenty-four hours."

As Brownell left the office, the telephone jangled. Tom reached for it.

"Admiral Walter calling." His voice was tense. "Important news, Tom. One of our subs has picked up a clue that someone has been operating in the missile search area."

"What sort of clue, sir?" Tom asked.

"A compressed-air caisson for underwater work. It had been driven into the silt and then abandoned." Admiral Walter added that photographs and a section of the caisson were being flown to the Naval Research Laboratory for careful study. "I'll have a full report transmitted to you by video as soon as it reaches my desk."

Tom thanked the admiral and hung up, feeling more uneasy than ever. The report came through the following morning. Tom absorbed the contents, then gave a low whistle.

"Trouble?" asked Bud, who had just dropped into the office with some flight-test data on a new Swift superjet.

"Our old enemies again." Tom shoved the papers across his desk.

The report stated that both the design and manufacturing techniques used in making the caisson indicated that it was of Brungarian origin. A spectrographic a.n.a.lysis of the steels confirmed the theory. Their metallurgical content agreed with known Brungarian steel formulas.

"The sneaky rats!" Bud cried out. "Well, at least we know now who sabotaged our missile recovery."

As Tom paced about the office, Bud added, "What do you suppose they were using the caisson for?"

"Probably as a base for some heavy, rotating search equipment," the young inventor surmised.

"But why ditch it?"

Tom shrugged. "An optimistic guess is that they spotted our Navy search force and pulled out quickly, fearing a surprise attack."

"What's a pessimistic explanation?" Bud asked.

"Mission completed," Tom said grimly. "No need for them to stick around if they'd already snagged the missile."

Bud scowled at the thought. "Oh, no! That mustn't be true!"

Tom plopped down at his desk, frowning. "Bud, I've been itching to get to work on a non-detectable sub, like the one that attacked us. But maybe it would be smarter to get a line on Mirov's pals first."

"You mean down in the South Atlantic?"

Tom nodded. "I'd sure like to know if they found that missile."

"You and I both, pal!" Bud agreed. "Hey! We could use the electronic hydrolungs for scouting around!" he added eagerly.

"I intend to," Tom said. "But we'll need speed to cover the area. So first I want to add an ion drive to our equipment."

"Ion drive? For underwater?" Bud, who was familiar with ion propulsion for s.p.a.ceships, wrinkled his brow in a puzzled frown.

"A goofy idea just occurred to me, but I think it may work out," Tom replied. He seized a pencil and began explaining what he had in mind.

The drive unit would take water into itself, separate the ionized molecules, and expose them to an electric field. Thus a stream of water would be forced out. This procedure, in turn, would set up a siphoning action through a central tube--in effect, creating a small but powerful water-jet motor.

"We'll be human submarines!" Bud exclaimed.

By the time Bud left the laboratory half an hour later, Tom had already plunged into work on his newest invention. The idea was simple enough in itself, Tom felt. The main problem would be the design job--laying out a compact, lightweight unit which a swimmer could easily carry on his back.

Fascinated, the young inventor worked late into the evening, stopping only in response to a telephone plea from Mrs. Swift. By midmorning the next day, Tom had a.s.sembled a pilot model of his ion-drive jet. In appearance, it was a slender metal cylinder, two feet long, with an inner concentric tube projecting at each end.

Tom had ordered a tank set up in his laboratory to test the unit. The tank was filled chest-deep with water, and the ion drive was mounted on a unitrack running the length of it. Tom set up his control board alongside, with the main power switch within easy reach. The drive unit was connected to the board by a suspended cable.

"Boy, this'll be like playing with a speedboat in a bathtub!" Tom thought with a chuckle as he changed into swim trunks.

He climbed into the tank and slid the drive unit to one end of its track. Then Tom metered out power slowly. With a gentle _whoosh_, the ion-drive unit whizzed along the unitrack to the other end of the tank.

"Not bad," Tom muttered, a pleased grin on his face. "Now I'll rev it up a little."

He slid the drive unit back to starting position, then opened the switch wider. He had just started across the tank himself when suddenly he became powerless to move.

Tom was pinned helplessly against the wall of the tank by the powerful water-jet exhaust! And the control switch was beyond his reach!

CHAPTER VII

PORPOISE TAG

"Good grief! I'm trapped!" Tom squirmed desperately in a vain attempt to free himself.

The ion-drive unit had hurtled to the far end of the tank at the first flick of power. But its exhaust tube was still jetting out a current of water with stunning force. Tom could feel the near-crushing pressure against his chest, even the full length of the tank away!

"H-h-help!" Tom gasped.

Moments dragged by with agonizing slowness. Tom felt as if his last ounce of breath were being squeezed out by the viselike pressure.

Suddenly a gravelly Western voice reached him, singing "Home on the Range." It drew closer, swelling into a foghorn drone as the lab door swung open.

"Good old Chow!" Tom thought. "Thank heavens!"

The grizzled, bowlegged cook ambled cheerfully into the laboratory, pushing a lunch cart. But, to Tom's dismay, he cast only a pa.s.sing glance at the figure in the tank.

[Ill.u.s.tration: _Tom squirmed desperately to free himself_]

"Soup's on, son!" Chow announced loudly. He began to ladle out a bowl of oyster stew from a steaming pot. Evidently he had not realized the young inventor's dilemma!

"Extra good today too, if I do say so myself!" the old Texan went on, setting out the rest of the lunch. "Well, come on, buckaroo! Break away from them ch.o.r.es an' dive in! Brand my cactus salad, if there's one thing that riles a cook--"

Summoning all his strength, Tom croaked out weakly, "Chow!... Get help!"