[Illustration]

TOM SWIFT AND THE ELECTRONIC HYDROLUNG

[Illustration: _The grenade went streaking straight toward Tom!_]

THE NEW TOM SWIFT JR. ADVENTURES

TOM SWIFTAND THE ELECTRONICHYDROLUNG

BY VICTOR APPLETON II

ILLUSTRATED BY CHARLES BREY

NEW YORKGROSSET & DUNLAPPUBLISHERS

Copyright BY GROSSET & DUNLAP, INC. , 1961

[Transcriber's note: Extensive research did not uncover any evidence thatthe copyright on this publication was renewed. ]

ALL RIGHTS RESERVED

PRINTED IN THE UNITED STATES OF AMERICA

CONTENTS

CHAPTER PAGE

1 PIRATE MISSILE 112 UNDERSEA SURVEY 203 INVISIBLE SUB 314 AERIAL ATTACK 395 A HUNCH PAYS OFF 506 THE CAISSON CLUE 567 PORPOISE TAG 648 DATE TROUBLE 729 A MAGNETIC KIDNAPING 8110 TELEPHONE CODE 9011 SQUARE-DANCE HOAX 10012 DETECTION TEST 10913 ENEMY FROGMEN 11914 A PROPAGANDA BLITZ 13015 MOUNTAIN HIKE 14016 THE GUNMAN'S SURPRISE 14817 A MISSING AMULET 15618 SMILEY THE SEA COW 16619 FLASH FROM THE DEPTHS 17620 A LUCKY BLAST 183

CHAPTER I

PIRATE MISSILE

Tense, excited men gazed spaceward from the ships and planes of theSouth Atlantic task force. Other watchers waited breathlessly in thecontrol room of the ship _Recoverer_. Among these was Tom Swift Jr. 

"How close to earth is our Jupiter probe missile?" Bud Barclay asked Tomexcitedly. 

The lanky blond youth beside him, in T shirt and slacks, shot a glanceat the dials of the tracking equipment. "Eight thousand miles from thisspot, Bud. It should land here in fifteen minutes!"

Tom Jr. , his father, Bud, and a host of scientists, Navy officers, andnewsmen were crowded aboard a U. S. Navy missile launching ship. 

"Just think!" Bud exulted. "You'll have data from the planet Jupiterthat no one on earth has yet been able to get!"

"_If_ we recover the missile safely, " Mr. Swift spoke up hopefully. Theelder scientist's voice was quiet but taut with the strain of waiting. The two Swifts resembled each other closely--each had deep-set blue eyesand clean-cut features--although Tom was somewhat taller and rangier. 

"You're right, Dad, " Tom agreed. "If we don't snare the missile, ourwhole project will be a total loss to America's space program!"

At Tom's words, the watchers and crewmen who were crowded into the_Recoverer_'s control room stirred restlessly. Its bulkheads were bankedwith radar and telemetering devices. Tension had been mountingthroughout the morning aboard the ships and observation planes of thetask force as everyone awaited the return of the planet-circlingmissile--scientists' deepest penetration into space so far. 

"What do you mean, a total loss?" Bud argued. "Even if the recoveryoperation's a flop, the shot will still pay off in valuable information, won't it?"

Tom shook his head grimly. "The purpose of this unmanned, exploratoryflight around Jupiter was to take and record all kinds of data. But noneof the info is being radioed back to us. "

"How come?"

"If we had put in radio gear strong enough to relay signals back, itwould have cut down the amount of information-gathering equipmentaboard, " Tom explained. "We had to make every ounce count. "

Outwardly calm, Tom was seething with inner excitement. Although onlyeighteen--the same age as his husky, dark-haired pal and copilot, BudBarclay--Tom had been given the job of directing the recovery phase ofthe United States government's Project Jupiter survey. The Swifts andtheir rocket research staff had built the missile and engineered thespace probe for the government. 

"Whew!" Bud gave a nervous whistle. "I see what you mean, pal. With allour eggs in one basket, we sure can't afford to get butter-fingered withthe Jupiter prober. "

Admiral Walter, a tall, distinguished man, graying at the temples, smiled. "It's what we call in warfare a calculated risk, Bud, " he said. "But with Tom in charge, I believe we have nothing to worry about. "

Mr. Swift's eyes shone with fatherly pride at the admiral's remark. TomJr. 's pioneering rocket flights and inventions had won the youth a toprank in American space research. 

"Guess you're right, sir, " Bud agreed. "I'll back genius boy here anyday!"

Tom winced as Bud whacked him heartily on the shoulder. "Better saveyour orchids and keep your fingers crossed, fly boy, " the young inventoradvised. "That rocket's not home yet. "

Radio telescopes, both on land and aboard the ships of the task force, were following the missile's progress as it drew closer to earth. Allwere feeding a steady stream of information to the ships' computers. 

"How soon will you fire the retro-rockets, Tom?" Admiral Walter inquiredpresently. 

"In about ten seconds, sir, " Tom replied, eying the sweep second hand ofthe clock. 

Moments later, a red light flashed on the master control panel. Tom'sfinger stabbed a button. Far out in space, the retarding rockets in themissile's nose were triggered for a brief burst, slowing its high speed. Without this, the missile would hurtle to flaming destruction in theatmosphere. 

"We've picked it up!" shouted a radarman. 

Bud gave a whoop of excitement and everyone crowded around theradarscope. Tom's steel-blue eyes checked the blip. Then he threw aswitch which started an automatic plotting machine that had beenprepared with the landing plan, and noted that the missile was slightlyoff the correct path. A new flow of information now began pulsing in asother ships' tracking radars recorded its course. The data was being fedautomatically to the "capture" computer. This would analyze the correctflight path for the recovery missile, which would magnetically seize thereturning traveler from Jupiter and bring it safely home. 

Tom quickly read off the results from the computer's dials, then busiedhimself again with the retarding-rocket controls. 

"Everything going okay, skipper?" Bud asked. 

Tom nodded. "I've readjusted the retarding rockets. They'll fire at theproper intervals to slow down the missile still further and bring itback on beam. "

The excited buzz of voices in the compartment gradually quieted as theclock ticked steadily toward the next step in the recovery operation. 

"Stand by for missile firing!" Tom snapped. 

A seaman relayed the order over the ship's intercom. Tense silence fellas Tom's eyes followed the sweep of the second hand. 

"All clear for blast-off!" came the talker's report. 

Tom pressed the firing button. A split second later the listeners'eardrums throbbed to a muffled roar from topside as the slender recoverymissile shot skyward. The ship rocked convulsively from the shock ofblast-off. Then it steadied again as the gyros damped out thevibrations. 

"Wow!" Bud heaved a sigh of relieved tension. Then he dashed from thecompartment and up the nearest ladder for a quick look at the rocket asit disappeared into the blue. 

Tom watched the recovery missile intently on the radarscope. 

"Nice going, son, " said Mr. Swift quietly. 

In response to his father's reassuring grip on his arm, Tom flashed hima hasty smile. For the first time, the young inventor realized he wasbeaded with perspiration and that his pulse was hammering. 

"It's a case of wait and hope, " Tom murmured. 

[Illustration]

On every ship and plane in the task force, eyes were glued to the radarscreens. Two small blips were visible--one the Jupiter probe missile, the other the recovery missile--moving on courses that would soonintersect. 

Just as Bud returned to the compartment, several of the watchers gavestartled gasps. 

"Another blip--coming in from nine o'clock!" Admiral Walter exclaimed. "What's that?"

Tom stared at the new blip. It was moving steadily toward the meetingpoint of the first two missiles!

"It's a thief missile!" Tom cried out. "Some enemy's trying to steal ourprobe data!"

"Good night!" Bud gulped. "Who'd dare try that?"

"I don't know, " Tom muttered tensely. "But if those three missiles meet, our whole project will be wrecked!"

"Better tape all readings!" Mr. Swift advised. 

"Right, Dad!"

Admiral Walter had paled slightly under his deep tan. In stunnedsilence, the Navy officers and scientists watched as Tom's lean handsmanipulated two controls. 

"What are those for?" Bud asked. 

"One's to speed up our recovery missile, " Tom explained. "Looks like aslim hope, though, from the way that third blip is homing on target. This other control has just caused every instrument on this ship, andall the others in the task force, to make permanent records on magnetictape of all their readings. 

"If a collision occurs and the probe missile falls into the sea, " Tomwent on, "there's only one hope of recovery--to plot the exactgeographical position and then get to the spot before the enemy does!"

"Roger!" Bud agreed. 

It was obvious that Tom's fears about the missiles colliding were wellfounded. The mystery blip had veered as the recovery missile speeded up. Within seconds, the three blips met on the screen and fused into asingle spot of light. 

"The probe missile's no longer responding to control!" one of thetelemetering scientists called out. 

Admiral Walter, grim-faced, flashed a questioning look at Tom. "Thenrecovery has failed?"

"I'm afraid so, sir. "

The fused blip was still visible on screen as the radar dishes trackedit, moving in a way that indicated a steep downward plunge. 

For a moment Tom felt numb with despair. But he set his jaw firmly andturned to the admiral. 

"Sir, I'd like helicopters readied for take-off immediately, " Tom said. "As soon as the tracking instruments lose contact, have the recordingtapes picked up from every ship in the task force and brought here tothe _Recoverer_. "

Admiral Walter nodded tersely. "Very well. Then what?"

"I'll get to work right now, " Tom replied, "and lay out a computerprogram to process the readings. "

The data--consisting of millions of information "bits" from theshipboard instrument tapes--would be fed to an electronic brain. Thebrain would then calculate the probable location in latitude andlongitude of the sunken missile. 

As the admiral snapped out orders, Tom exchanged a brief worried glancewith his father. Each was pondering the same thought. 

_Could Tom find the lost Jupiter probe missile? Or would their enemylocate it first?_

CHAPTER II

UNDERSEA SURVEY

With an effort, Tom forced all thoughts of failure out of his mind andconcentrated on the job at hand. In an hour he had the computer programblocked out. 

Mr. Swift and several of the other scientists checked his work. Eachnodded approval. By this time, the fused blip had long since disappearedfrom the radarscopes, indicating that the Jupiter probe missile--or whatwas left of it--had plunged to the ocean bottom. 

"What's your next move, Tom?" Admiral Walter asked. 

"No point in wasting time waiting for the computer results, " Tomdecided. "Suppose Bud and I fly back to Swift Enterprises and organize asearch party. "

"Good idea. " As Admiral Walter extended a hand, his weather-beaten facesoftened. "And don't feel downhearted, son. You rate a Navy 'E' for theway you handled this operation. It would have succeeded if it hadn'tbeen for that confounded enemy missile!"

"Thank you, sir. " Tom managed a grateful grin, in spite of hisdiscouragement. 

Minutes later, the two boys embarked in a motor launch that took them toan aircraft carrier standing by in the vicinity. From the flattop theytook off in a Navy jet for Shopton. 

Meanwhile, Mr. Swift remained aboard the _Recoverer_ to supervise thedata processing. Tom, looking back from the soaring jet, could see oneof the helicopters on its way to the missile ship to deliver the firstbatch of tapes. 

It was late afternoon when the Navy jet touched down on the Enterprisesairfield. The Swifts' sprawling experimental station was a walled, four-mile-square enclosure with landing strips, work-shops, andlaboratories, near the town of Shopton. Here Tom Jr. And his fatherdeveloped their amazing inventions. 

Tom and Bud hopped into a jeep at the hangar and sped to theAdministration Building, where Tom shared a double office with hisfather. Bud sank down into one of the deep-cushioned leather chairs, while Tom adjusted the Venetian blinds to let in the afternoon sunshine. 

The spacious office was furnished with twin modern desks, conferencetable, and drawing boards which swung out from wall slots at the pressof a button. At one end of the room were the video screen and controlboard of the Swifts' private TV network. Here and there stood scalemodels of their inventions, a huge relief globe of the earth, and areplica of the planet Mars. 

"What are your plans for our search expedition, skipper?" Bud asked. 

Tom ran his fingers through his crew cut. "Let's see. We'd better takethe _Sky Queen_, I think, and also--"

Tom broke off as the desk intercom buzzed. Miss Trent, the Swifts'secretary, was on the wire. 

"Your father's calling over the radio, Tom. "

"Swell!" Tom flicked a switch to cut in the signal of his privatetelephone. "Hi, Dad! We just got back. Any news?"

"Yes, son. We have the computer results, " Mr. Swift replied. "Got apencil handy?"

Tom copied down the latitude and longitude figures as his fatherdictated. 

"According to the latest hydrographic maps, based on IGY findings, " Mr. Swift went on, "this area is a high plateau of the Atlantic Ridge--it'snear the St. Paul Rocks. "

"What about the depth?"

"It averages between a hundred and three hundred feet, " said the elderscientist. 

Tom gave a whistle. "Lucky break, eh?"

"Maybe and maybe not, " Mr. Swift said cautiously. "The bottom there isheavily silted. "

"Oh--oh. " Tom made a wry face. "In that case, we may have some diggingto do. "

"I'm afraid so. However, no use borrowing trouble. " After a shortdiscussion, the elder scientist added, "I'll probably fly home tomorrow, son. Give my love to Mother and Sandy. "

"Right, Dad. So long!" Tom hung up and reported the news to Bud. 

"What kind of underwater gear will we use?" Bud inquired. 

"I'm not sure myself, " Tom admitted. "Guess we'll have to take along avariety of equipment and play it by ear. "

Before proceeding with his search plans, Tom phoned home to inform hismother of his arrival. Mrs. Swift was sympathetic when she heard of thefailure to recover the probe missile. 

"I'm sure you'll locate it, " she said encouragingly. 

"Some of your cooking will sure help brighten the picture, " Tom repliedwith a grin. As he put down the receiver a moment later, he told Bud, "You're having dinner with us tonight, pal. Fried chicken and biscuits. "

Bud licked his lips. "Lead me to it!"

Chuckling, Tom began drawing up a list of supplies for the expedition. Bud helped with the details, after which Tom phoned the undergroundhangar and the Swifts' rocket base at Fearing Island to give the ordersfor the next day. Crewmen were also detailed for the trip. 

It was six o'clock when the two boys finally piled into Tom's low-slungsports car and drove to the Swifts' big, pleasant house on the outskirtsof Shopton. Sandra, Tom's blond, vivacious sister, greeted them at thedoor. 

"About time!" she teased. "We were beginning to think you two had takenoff somewhere. "

"Think I'd leave town while you and that fried chicken are in Shopton?"Bud grinned. 

"What a line!" Sandy's blue eyes twinkled. "I know it's the friedchicken you're really interested in. "

"Where's the rest of that 'we' you were referring to?" Tom inquired. 

"I'm sorry, Tom, " Sandy said in a mournful voice. "Phyl couldn't makeit. "

As Tom's face fell, she burst out giggling and a second later PhyllisNewton emerged from the kitchen. Brown-eyed, with long dark hair, Phylwas the daughter of Tom Sr. 's old comrade-in-arms and lifelong chum"Uncle Ned" Newton. Like Sandy, she was seventeen. 

"You didn't think I'd miss this rare evening, did you, Tom?" she said, laughing. "After all, it isn't often we see you two. "

Sandy and Phyl liked to needle the boys about their infrequent dates, due to Tom's and Bud's busy schedules. 

Mrs. Swift, slender and sweet-faced, gave Tom a hug and greeted Budwarmly. Over the delicious dinner, the conversation turned to themysterious thief missile. 

"Who on earth could have fired it?" Sandy asked. 

Tom shrugged. "No telling--yet. There's more than one unfriendly countrywhich would give a lot for the data picked up on our Jupiter shot. "

"You aren't expecting more trouble, are you?" Phyl put in uneasily. 

Tom passed the question off lightly in order not to alarm his mother andthe two girls. But inwardly he was none too sure of what his surveyexpedition might encounter in trying to locate the lost probe missile. 

Ever since his first adventure in his Flying Lab, the youthful inventorhad been involved in many daring exploits and thrilling situations. Timeand again, Tom had had to combat enemy spies and vicious plotters benton stealing the Swifts' scientific secrets. 

His research projects had taken him far into outer space and into thedepths of the ocean. With his atomic earth blaster, Tom had probed underthe earth's crust at the South Pole, and in other adventures he hadfaced danger in the jungles of Africa, New Guinea, and Yucatan. Hislatest achievement, receiving the visitor from Planet X, had been toconstruct a robot body for this mysterious brain energy from anotherworld. Now, Tom realized, he was on the brink of another adventure whichmight hold unexpected dangers. 

Early the next morning the majestic _Sky Queen_ was hoisted from itsunderground hangar berth and hauled by tractor to its special runway. This mammoth, atomic-powered airplane had been Tom's first majorinvention. A three-deck craft, it was equipped with complete laboratoryfacilities for research in any corner of the globe. Jet lifters in thebelly of the fuselage enabled the craft to take off vertically and alsoto hover. 

As Tom supervised the loading of the equipment, a foghorn voice boomed, "'Mornin', buckaroos!"

The chunky figure of Chow Winkler came into view. Formerly a chuck-wagoncook in Texas, Chow was now head chef on Tom's expeditions. As usual, aten-gallon hat was perched on his balding head and he was stomping alongin high-heeled boots. 

"Wow! A shirt to end all shirts!" Tom chuckled. 

"Real high style, eh?" Chow twirled about to display his latest Westerncreation. The shirt seemed to be made of silvery fishlike scales, whichglistened like a rainbow. 

"I figured as how this was just the thing fer an ocean jaunt, " Chowadded with a grin. "How soon do we take off, boss?"

"As soon as we get the rest of this gear stowed, " Tom replied. 

Twenty minutes later the _Sky Queen_ soared toward the ocean. Soon theycame in sight of Fearing Island rocket base, a few miles off the coast. Once a barren stretch of sand dunes and scrub-grass, the island was nowthe Swifts' top-secret rocket laboratory, guarded by drone planes andradar. It served as the supply base for Tom's space station and as thelaunching area for all space flights. Seacopters and jetmarines werealso berthed here. 

A radio call from Tom brought a sleek, strange-looking craft zooming upto join them. 

It was the _Sea Hound_, latest and largest model of Tom's amazing divingseacopter. It had an enclosed central rotor, powered by atomic turbines, with reversible-pitch blades for air lift or undersea diving. Superheated steam jets provided forward propulsion in either element. 

As the _Sea Hound_ streaked alongside the Flying Lab, two figures in theseacopter's flight compartment waved to Tom and Bud. One was HankSterling, the blond, square-jawed chief pattern-making engineer ofEnterprises. The other was husky Arv Hanson, a talented craftsman whotransformed the blueprints of Tom's inventions into working models. 

"All set, " Hank radioed. "Lead the way. "

"Roger!" Tom replied. 

Flying at supersonic speed, they reached the area of the lost missile inthe South Atlantic soon after lunch. Already on hand were ships of theNavy task force assigned by Admiral Walter to participate in the missilesearch. The _Sea Hound_ settled down on the surface of the water, whilethe _Sky Queen_ hovered at low altitude nearby. 

Tom contacted the government craft and learned that as yet no sign ofthe lost Jupiter prober had been detected. Then he made ready to beginhis own search. 

"Let's try the Fat Man suits first, " Tom told Bud. Turning to SlimDavis, a Swift test pilot who was in the crew, the young inventor added, "Take over, will you, Slim?"

"Righto. " Slim eased into the pilot's seat. 

"Got a job for me, skipper?" asked Doc Simpson, Swift Enterprises' youngmedic. 

"Yes. Help the boys, if you like, rig the undersea elevator, and thenassemble a tractorized air dome, " Tom suggested. 

"Will do, " Doc promised. 

A ladder was dropped. Tom and Bud excitedly descended to the _SeaHound_. The search for the lost missile was about to begin!

Once the boys were aboard, the seacopter submerged and dived quickly tothe ocean floor. Tom and Bud each climbed into a Fat Man suit and wentout through the air lock. The suits, shaped like huge steel eggs with aquartz-glass view plate for the operator seated within, had mechanicalarms and legs. 

The boys waddled about, the built-in searchlights of their suitspiercing the murky gloom. They saw nothing but the deep accumulation ofsilt on the ocean bottom, which made the going difficult. 

"This is too slow, " Tom called over his sonarphone. "Let's try the airdome. "

The dome was a huge underwater bubble of air, created by a repelatrondevice which actually pushed the ocean water away. The air supply insidewas kept pure by one of Tom's osmotic air conditioners which made use ofthe oxygen dissolved in the water. 

The air bubble, however, even with its jet-propelled platform, alsoproved inadequate for the research job. Its caterpillar treadsrepeatedly bogged down in the silt. 

"Maybe the seacopter itself is our best bet, " Bud suggested. 

"Worth a try, " Tom urged. 

But the _Sea Hound_, too, had a serious drawback. Even with its powerfulsearch beam sweeping the ocean floor as it prowled along, the explorersfound their vision too limited. 

Finally Tom said, "Bud, we could skin-dive at this depth. "

"Let's give it a whirl, " Bud urged. 

The seacopter surfaced again, while the boys donned flippers, masks, and air lungs. Then they dropped over the side and made their way slowlydownward into the gray-green depths, accustoming themselves gradually tothe increased pressure. 

"A lot more freedom of action, " Tom thought. "If only we didn't have tocommunicate by signals!"

There was a sudden _swoosh_ somewhere on his right. A projectile, Tomrealized! Turning, his eyes widened in horror as he saw an uprush ofbubbles. 

Bud's air tank had been hit!

CHAPTER III

INVISIBLE SUB

Without wasting a moment, Tom lunged through the water toward hisstricken friend. Bud was floundering and thrashing about weakly. Heseemed dazed by the sudden shock of his plight. 

"Or maybe the impact of the projectile stunned him!" Tom surmised. 

Bud began groping his way upward just as Tom came alongside of him. Tomgrabbed him as best he could, hooking onto his belt. At the same time, the young inventor inhaled deeply, yanked out Bud's useless mouthpiece, and inserted his own in its place. 

Bud's eyes glowed with gratitude. 

"We'll have to get topside fast, " Tom thought, "even though it meansrisking the bends. "

He stroked upward and they shot toward the surface. Bud assisted to someextent, partly revived by the gulp of air. 

As they rose, fathom by fathom, their progress seemed to growmaddeningly slower. Tom had to let air bubbles escape constantly fromhis mouth. As the pressure decreased, due to the lessening depth of thewater, the air in his lungs expanded and he was forced to breathe out. 

Tom noticed with dismay that Bud was not responding very well, hisfeeble strokes were jerky and uncoordinated. "Must've lost pressure toofast when his tank was hit, " Tom realized. 

The water was growing greener and brighter now as they neared thesunshine. The _Sea Hound_'s shadowy outline loomed just above. With alast desperate burst of strength, Tom lunged upward and they brokewater. 

"H-h-help!" Tom gasped. 

There was no need for the cry. Hank and his crew, on the seacopter'sforward deck, had already grasped the situation. Strong arms reached outand hauled the two boys aboard. 

Both of them were shivering and writhing in pain, only half conscious. 

"They have the bends!" Arv Hanson cried in alarm. "Signal the _SkyQueen_ to drop a sling!"

The boys' masks were ripped off. Within moments, Bud had been tightlysecured to the sling, which was reeled back up into the plane. Tomfollowed in a few minutes. Doc Simpson took charge of the patientsimmediately. After a quick examination, he had the boys placed in asmall decompression chamber in the _Sky Queen_'s sick bay. 

"How are they?" Hank asked anxiously as he peered through the window ofthe chamber. The medic had given Bud a sedative and he was already fastasleep. Tom remained awake. 

"Aside from the pain, not in too bad shape, " Doc Simpson replied. 

It turned out that Tom's case was not so serious, but Bud had to stay inbed. With Tom, it was only a matter of decompression and he soon was upand about. 

Chow, in a chef's cap, with an apron around his paunchy stomach, hadcome stomping in hastily from the galley. "Pore lil ole boys, " hefussed. "Brand my snorkel, I never should've let you young'uns go pokin'around down below there without me around to keep an eye on things!"

Tom slapped the loyal old Texan on the back. "If you want a dive, comealong. "

"You're goin' back down?" Chow asked. 

"In the seacopter, " Tom replied. "To find out, if possible, who firedthat projectile at us. "

"Then count me in!" Chow declared, stripping off his apron. "I just hopeI get my hands on them sneakin' polecats!"

Slim Davis would pilot the _Sky Queen_ back to Shopton at once, becauseof Bud. Tom and Chow, meanwhile, would join Hank and his crew aboard the_Sea Hound_. 

Ten minutes later the sleek seacopter, its searchlight off to avoiddetection, was plummeting downward through water that changed beforetheir eyes from greenish blue to a deep-gray gloom. Iridescent fishdarted past the cabin window. 

"Think the enemy sub was searching for our Jupiter prober?" Hank asked. 

"It must have been, " Tom reasoned. 

Hank frowned. "Which means they must have figured out the missile'sposition as fast as our side did. "

"And they'll play rough to stop us from finding it, " Arv addedforebodingly. 

Within moments, the group clustered in the pilot's cabin felt a gentlebump as the _Sea Hound_ settled on the submerged plateau. Tom relaxed atthe controls but kept the rotors going so the craft would remainsubmerged. Meanwhile, the sonarman was probing the surrounding waters. 

"Any pings?" Tom asked. 

The man shook his head without taking his eyes from the sonarscope. "Nothing yet. "

Hank Sterling donned a hydrophone headset and listened intently. Thesilence deepened in the _Sea Hound_'s cabin. Suddenly Hank stiffened andthe sonarman cried out:

"A blip, skipper! At two o'clock!"

It was moving rapidly on the scope--something streaking toward theirstarboard beam!

"Good night! It's another missile!" Tom gasped. 

He darted back to the controls and gunned the reverse jets just in time!The missile flashed across their bow. 

"Great bellowin' longhorns!" Chow gasped weakly. His leathery face hadgone pale under its tan. "The yellow-livered drygulchers!"

"I don't get it, " Arv Hanson spoke up. "If they're in firing range, weshould have detected them, shouldn't we?"

Tom nodded grimly. "Whoever our enemies are, they must have perfected away to make themselves invisible to underwater detection. 

"_And we'll have to do the same!_" he vowed inwardly. Aloud, Tom said, "I hate to run from those sneaks, but if we stick around, we'll beasking for trouble. "

Slowing the rotors to permit the craft to rise, Tom guided the _SeaHound_ back to the surface. Then he reversed blade pitch for air flightand gunned the atomic turbines. The seacopter rose steeply above thebillowing South Atlantic. 

Tom radioed a terse report of their experience to the task-forcecommander and in turn was told that none of the naval craft had eithersighted or picked up any sign of a strange sub. 

As they streaked homeward, Chow was still fuming. "Why don't we post adummy sub there to scare off the varmints?"

"I'll pass the idea along to the Navy, " Tom said with a grin. 

Night had fallen when the searchers arrived back at Fearing Island. Tomcleared with the tower and landed, then went by jeep to baseheadquarters. He called Enterprises and learned that Bud's condition wasimproved, and that Mr. Swift had returned that afternoon. He spoke tohim about the mystery sub. 

"This is bad news indeed, son, " Mr. Swift said, after hearing how theattacker had defied detection. "You'd better inform Admiral Walter. Hehad to fly back to Washington. "

"I'll call him right away, " Tom promised. 

The admiral was equally disturbed when Tom succeeded in reaching him. "We must find that missile as soon as possible--at any cost, " he said. "Tom, you Swifts have had considerable experience in undersea dredging. Could you send a team of engineers to assist us in the work?"

"Yes, sir, " the young inventor replied. "I'll assign men to the jobfirst thing tomorrow. "

After hanging up, Tom hopped back to the mainland with Chow in a PigeonSpecial. This sleek little commercial plane was manufactured by theSwift Construction Company in charge of Ned Newton. 

Early the next morning Tom and his father drove to Enterprises, and theyoung inventor plunged into the job of organizing an engineering crewfor the missile hunt. Art Wiltessa, a crack underwater specialist aswell as engineer, was placed in charge. 

By noon the group had taken off for the South Atlantic in a Swift cargojet. A small portable model of Tom's atomic earth blaster was includedin their equipment. A jetmarine and a diving seacopter were alsodispatched from Fearing to assist in the operations. 

"It's apt to be a long-drawn-out job--and dangerous, " commented Mr. Swift as he lunched with Tom in their office. 

"Yes. Digging in that silt could be almost as bad as working inquicksand. "

Mr. Swift's deep-set blue eyes took on a thoughtful gleam. "Speaking ofsilt, son, I've found the ideal spot for my secret deep-sea farm. "

"You mean for growing those plants you use in making Tomasite?" Tomasked. 

The elder scientist nodded. Tomasite, a revolutionary plastic which Mr. Swift had developed, possessed amazing insulating properties againstboth heat and radiation. One of its secret ingredients came from certainplants found only in Far Eastern waters. Mr. Swift hoped to transplantthem locally. 

"The site is near Fearing Island--about fifty feet in depth, " he added. 

"You may have a tough time finding gardeners, Dad, " Tom pointed out. "Men can't work that far down for very long at one time. "

"It'll be a problem, " Mr. Swift conceded. He finished his coffee, thenlooked up with a twinkle in his eyes. "How about figuring out a solutionfor me, Tom?"

"A new kind of air lung?" Tom was intrigued!

CHAPTER IV

AERIAL ATTACK

"Yes, son, " Mr. Swift went on. "What's needed is a new type of breathingdevice--one that will eliminate bulky air tanks and permit a skin diverto stay down for long periods. "

"Quite an order, Dad. "

Grabbing a pencil, the young inventor began sketching. In both his FatMan suits and his osmotic air conditioner, Tom had already perfectedways of drawing oxygen from sea water. 

"But a small gadget for skin divers, " he said, "will take a fantasticjob of electronic miniaturization. " After a pause he added, "It couldreally speed up recovery of the Jupiter prober, though. "

Lunch over, Tom hopped a jet scooter and sped off to his privatelaboratory. The modernistic glass-walled structure--designed by Tomhimself--had every tool of modern scientific research, from electronicmicroscope to helium cryostat. 

As always, whenever he was absorbed in a new idea, Tom was eager to getto work. "Let's see what I'm shooting for. A small container, slungaround the diver's neck?. .. No, too dangerous. Better hook it to hisweight belt, with a tube to his face mask. "

Using a plastic foam "breadboard, " Tom began experimenting with variouscircuit designs. He worked through the afternoon and returned to theproblem early the next morning. 

He was interrupted by a message from Art Wiltessa, reporting no luck sofar in finding the missile. Later, shortly before lunch, Tom receivedanother call, this time from Admiral Walter. "Just wanted to keep youposted, Tom. Our task force reports no success on their part in findingthe buried missile. No sign of the enemy, either. "

"They'd probably hesitate to attack any official U. S. Navy units, " Tomsaid. "Or it might mean they've already found the missile themselves. "

"That's what I fear, " Admiral Walter confessed gloomily. "However, we'llcontinue searching. "

Tom promised to fly down to the site at the first opportunity, saying hewas developing a new device that might assist in the search. Aftersnatching a hasty lunch, Tom returned to work. 

Arv Hanson machined several parts and molded the plastic face mask toTom's specifications. By evening the new device was completed. 

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

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

When Sandy saw the power unit strapped to Tom's weight belt, sheexclaimed, "_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 dioxidethat I exhale with fresh oxygen drawn from the water. Otherwise, although the carbon dioxide I'd breathe out would be a very small amountat a time, it soon would make the air unfit. The nitrogen, which makesup much of the air we breathe, is chemically inert and can be used againand again. "

He pointed to a round screen on one side of the unit. "This is the waterintake, " Tom went on, "and this other screen is where the water comesout after we've removed its oxygen. "

Near the forward end of the unit, a semirigid plastic tube wasconnected, leading up to the face mask. At the rear was a power port forinserting a small solar battery. 

"What about this little tuning knob?" Sandy asked. 

"That's the rate control for adjusting the output frequency to thewearer's breathing rate. " Tom added, "I've decided to call the wholeapparatus 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 diveinto the tank. For the next ten minutes the girls and Chow watchedwide-eyed as he swam, walked around, and went through vigorous exercisesat the bottom of the tank without once coming up for air. 

"Whee!" Sandy exclaimed when Tom finally climbed out. "Make me one, so Ican 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, accompaniedby Doc Simpson, flew to Fearing Island so Tom could test his inventionin deep water. Boarding a small motor launch, with Doc at the helm, theycruised out to a suitable depth and dropped anchor. 

"Don't become too confident, Tom, " Doc warned. "I'll drop a signal lineover the side in case of emergency. "

Tom buckled on his equipment belt and adjusted the face mask. Then heheld up crossed fingers and back-flipped over the gunwale into thewater. Chow, Doc, and the girls watched his plummeting figure fade fromview. 

Tom, an expert skin diver, had never before felt such a sense of easeand freedom under water. He was moving, light and self-contained, in agreen, magical world. With no air tanks chafing his back, he felt akinto the fishes themselves. 

"Wish I'd brought a hook and line along. " He chuckled, as a school ofmackerel 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, thenstarted up again. Suddenly a stab of pain shot through his chest--awarning of nitrogen bubbles forming in his blood!

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

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

Doc and Chow hauled up frantically. Tom's face was contorted with painwhen 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 handsympathetically, while Doc Simpson injected a hypodermic to ease thepain. Chow steered the launch back to shore, and Tom was rushed to thebase infirmary in an ambulance. 

Here he was placed in a decompression chamber for several hours andlater 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 younginventor spent the evening sketching out an improved design to eliminatefuture accidents. 

"I'll install a special device to remove the nitrogen as the wearerexhales, " Tom explained. "Then a valve will feed in helium to replaceit. Since helium doesn't dissolve in the blood like nitrogen does, itwill not bubble out when the pressure is reduced. Should have thought ofthat 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 tosupply the wearer's needs. Remember, it can be used over and overagain. "

"Pretty neat, " Bud commented. 

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

In a few hours Tom had added a small fitting to his power unit toprovide for helium substitution. Then the two boys hopped back toFearing for a second deep-water test. This time, Tom was delighted tofind that he could operate comfortably at great depths, as well as riseor 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 andasked that a duplicate of the hydrolung be turned out in the shop assoon as possible. It was ready the following Monday morning, so Tomsuggested to his father that the two visit the proposed underwater siteand make some sample plantings. 

"Great idea, son, " Mr. Swift agreed. "I want to try out your new divingapparatus myself. If it's successful, we'll be able to tackle twoproblems 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 amile offshore. Each carried several of the valuable Far Eastern plants. 

[Illustration]

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

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

[Illustration]

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

Tom was thoughtful. "Dad, I wonder if the fish would eat those plantsfrom space which you've been growing under salt water?"

Tom was referring to certain strange plants rocketed to earth by unknownspace friends with whom the Swifts had been in communication. 

"I have a hunch, " Tom went on, "that the fish might be repelled by theunusual scent of those space plants. If so, we could scatter them amongthe earth plants to keep the fish away. "

Mr. Swift was impressed by Tom's idea. As soon as they had returned toEnterprises, he proposed that the experiment get under way. 

Tom volunteered to undertake the job at once with Bud. While the younginventor phoned his copilot, Mr. Swift went to his own laboratory toprepare the plants for shipment. 

Twenty minutes later the boys took off in a jet. The plants had beenparceled in transparent plastic film. Glistening with a red metallicsheen, they looked somewhat like tulips with honeycombed centers. 

"Scarecrow plants to drive off fishes, " Bud joked. "What will scientiststhink of next!"

Tom laughed, then abruptly frowned. "Hey! What's that character up to?"he said. "Trying to buzz us?"

A sleek gray jet without markings was arrowing in on them from threeo'clock. Bud flicked on the radio and barked a warning. The plane madeno response. As it kept coming, Tom increased speed--then rolled, dived, and changed course, but failed to shake off their pursuer. 

Bud, meanwhile, was frantically calling Enterprises and a nearbyairport, but getting no response. Yet their radio was working, for avoice suddenly crackled:

"_Follow the mystery plane for a landing and you won't be harmed!_"

CHAPTER V

A HUNCH PAYS OFF

Dismayed, Tom and Bud stared at each other. Apparently the enemy shiphad blanked out their radio communication to all points except themystery plane. 

"Who are you and what do you want?" Tom said into his microphone. 

The voice replied crisply, "_You'll find out when the time comes!_"

Tom flicked off his mike and exchanged another worried glance with Bud. "We seem to be in a spot, pal!"

"And how! Especially if that crate's armed!" Bud muttered. "But what arethey after?"

Tom shrugged. "The space plants maybe--or possibly our jet. "

"Might even be _us_ they want, " Bud said. "Got any tricks under yourmagician's hat?"

Tom's brain was already racing to figure a way out. Suddenly he snappedhis fingers. "Hey! I almost forgot!" he exclaimed. "Look in the locker, Bud, and see if we have the radio set that neutralizes allinterference!"

Bud's face brightened. "Now you're talking!"

The set had been perfected during Tom's _Cosmic Astronauts_ adventure, in defense against an Oriental enemy's jamming-wave generator. Bud foundit in the locker, dragged it out joyfully, and plugged it into the powersupply. 

Meanwhile, the mystery jet had banked in a wide circle and headed west. As Tom stalled for time, it swooped back again and the same voice camesnarling over the speaker. 

"_I warned you to follow us! Or would you prefer to be shot down?_"

As if to back up the threat, a burst of tracer fire grazed Tom's plane. 

He hastily switched on his mike. "Okay, hold your fire! I guess we haveno choice!"

The jet turned back on its westerly course, and Tom followed obediently. Meanwhile, Bud had warmed up the other radio and contacted Enterprises. Tom switched mikes long enough to report their position, course, andspeed, adding:

"Tell Security to alert Vignall Air Force Base pronto!"

"Roger Wilco!" the Enterprises operator responded. Even if the enemyship detected the call, Tom knew the automatic scrambling device wouldprevent the message from being understood. 

Minute after minute, the flight continued. "Where are they taking us?"Bud muttered. 

"Some out-of-the-way landing spot probably, " Tom conjectured. "I wonderhow soon those fighter boys will--"

Bud suddenly grabbed Tom's arm and pointed to starboard. "There theycome, skipper!"

Three gleaming specks had just burst through a cloud bank to the north. Closing in rapidly, they were soon visible as Air Force fighter jets, flying in V formation. 

"Fighter One to unmarked jet!" came the sharp command over the radio. "Can you read me?. .. You'd _better_ read me, pal! I order you to proceedto Vignall Air Base under our escort or take the consequences!"

The mystery pilot, evidently bewildered by the sudden onslaught, made afrantic effort to escape. But the fighters, with almost contemptuousease, quickly surrounded the plane and forced him to comply with orders. 

Bud whooped with laughter. "Just a sheep in wolf's clothing, eh, buster?"

Minutes later, all the planes, including Tom's, landed at the airfield. Four sullen-faced men, their hands up, emerged from the mystery jet. Military police with drawn automatics herded them to the commandant'soffice. Tom and Bud followed. 

"Attempted aerial piracy, eh?" the commandant said when he heard theboys' story. Turning to the prisoners, he snapped, "Who are you, andwhat's the meaning of all this?"

The crew captain, a hard-looking, stockily built man of aboutforty-five, rasped back, "We have nothing to say. "

The commandant wasted no words. "Search them, " he told the MP's. 

Their wallets and various other items revealed little. The crew captainwas carrying a private pilot's license on which he was identified as"Jack Smith. " The names of the others, as shown on identification papersof one kind or another, sounded equally false. 

"Probably all forged, " the commandant muttered, "but we'll check themout. "

He tried again to glean something from the prisoners, but they repliedwith sneering evasions. The commandant reddened with anger at theirstubbornness. "All right. Take them to the guardhouse, " he ordered. 

As the MP's marched the hijackers off, Tom asked how their case would behandled. 

"The crime is a federal offense, " the commandant explained. "Air ForceIntelligence will co-operate on the case, but the prisoners will beturned over to a federal marshal. "

Tom briefed him on the background of the situation, including theJupiter-probing missile mystery, then asked, "Could those men betransferred to the Shopton jail for the time being so our own securitysetup can take a hand in the investigation?"

The commandant nodded. "I'll arrange it. "

As the boys flew back to Enterprises, Bud threw Tom a quizzical glance. "How come you mentioned the Jupiter prober, skipper? Do you think thosehijackers were after information?"

Tom shrugged. "I'm wondering myself, Bud. If they were, it could meanour enemy hasn't found it yet!"

When they arrived at the experimental station, Tom made a full report toHarlan Ames, the slim, dark-haired security chief. Ames listenedthoughtfully but was as baffled as Tom. 

"Are the men Americans?" he asked. 

"I doubt it, " Tom said. "They speak English well enough, but with afaint accent. Somehow, I have a hunch they're Brungarians. "

Ames whistled. "That could spell trouble, skipper. " More than once, Brungarian rebel agents had engaged in brazen plots against America andthe Swifts. 

"Let's hope I'm wrong, " Tom said wryly. 

"Art Wiltessa--and the Navy--called again, " Ames added. "Still no luckon the missile search. "

The gloomy news did nothing to lift Tom's spirits. The next day, hopingto verify or disprove his suspicion, he drove to Shopton PoliceHeadquarters with Harlan Ames. The two talked briefly with Chief Slater, an old friend. Then a turnkey took them to the cell block. 

The four prisoners had been confined in a single large cell. They seemedtense and angry--as if they had been quarreling among themselves. 

"Ready to talk yet?" Ames asked. Getting no reply, he repeated thequestion in Brungarian. 

Ames's ruse failed. "What language is that?" asked "Captain Smith"mockingly. "Pig Latin?"

As his cellmates grinned, Tom's eyes roved over their faces. Oneman--wavy-haired with penetrating dark eyes--seemed oddly familiar. Why?Suddenly the answer hit Tom like a flash. He resembled Streffan Mirov, the brilliant Brungarian rocket scientist who had tried to oust Tom'sexpedition from the phantom satellite Nestria. 

Playing a hunch, Tom said to him, "You know what your government does torebels and bunglers, Mirov. "

The man stiffened and paled. "We have not b-b-bungled!" he stutteredangrily. 

"Shut up, you fool!" their leader shouted. 

CHAPTER VI

THE CAISSON CLUE

"Captain Smith" had leaped to his feet, quivering with anger. But it wastoo late. His cellmate, by answering to the name of "Mirov, " had givenaway their nationality!

Tom and Ames exchanged grins of triumph. 

"No doubt you recall what happened to Streffan Mirov, " Tom went on, pressing his advantage. "Or should I say the _late_ Streffan Mirov? Ourlast report was that he had been tried and condemned by your owngovernment. Perhaps you can give us news of his fate?"

The wavy-haired prisoner's eyes blazed with hate. "Grin while you can, Tom Swift! Because of you, my brother Streffan is now serving a longprison sentence! But I, Dimitri Mirov, will get revenge!"

"You blame Tom Swift because your brother botched his job of claimingthe satellite Nestria by force and fraud?" Ames taunted. 

"Our space friends moved that asteroid into orbit around the earth, " Tomadded. "We claimed it by right of first landing. Even your own leaderscouldn't agree to Streffan's crazy scheme to destroy everything. "

Dimitri Mirov lost all control and burst into a volley of gutturalBrungarian abuse. 

"I warn you, Swift!" he choked. "Jailing us will not make you safe--oryour projects, either!"

A blow to the head from "Captain Smith" sent Mirov reeling back againstthe wall. "Fool! Maybe that will quiet you!" the pilot snappedviciously. "You have said too much already!"

"Let's go, Tom, " said Ames. "We've learned the information we came for. "

The prisoners could only glare in baffled rage through the cell bars asTom and the security chief turned their backs and walked away. 

"Nice going, Tom, " Ames murmured. "Your hunch certainly paid off. " ChiefSlater added his congratulations when he heard how Tom had trapped Mirovinto disclosing his identity. 

Both Tom and Ames were grave as they drove back to the plant. Neithertook Mirov's threats lightly. 

Tom pondered another angle. Were the Brungarian rebels perhapsresponsible for the attempted theft of the Jupiter-circling missile?

Ames was inclined to think so. "Moreover, " he forecast, "it's a cinchthey haven't thrown their last punch. I'll pass the word to the FBI andCentral Intelligence. "

After lunch Tom flew to Fearing Island with Bud, eager to tackle theirinterrupted job of rooting the space plants into the undersea silt beds. Zimby Cox, a sandy-haired, freckle-faced jetmariner, volunteered topilot a motor launch for them. 

They sped across the water, then dropped anchor at the farm site. Tomand Bud donned their hydrolung gear and went over the side, eachclutching containers of the space plants. 

Reaching bottom, they glided about in the shadowy green water, embeddingthe plants at far-spaced intervals. The Tomasite-producing plants hadbeen almost completely devoured. A few fish were darting about, but theyswam off quickly at the boys' approach. To Tom's delight, they showed nosign of returning. 

"Looks as if our keep-off signs are working, " Tom said with a pleasedchuckle when the boys finally surfaced and climbed back aboard the boat. 

Bud nodded. "Smart idea, all right. " Then he scowled thoughtfully. "Butif you ask me, skipper, fishes aren't the only thieves you'll have toguard against. "

"Meaning?"

"Mirov's pals, " Bud replied. "If it's the space plants they were afterwhen they pulled that aerial hijack attempt, they could take them easilyfrom these silt beds. "

Tom sobered. "You have a point there. I'd better have an audio screenset up around this whole area. That'll act as a burglar alarm--and helpdiscourage the fish, too. "

Twenty minutes later the boys were winging back to the mainland. WhenTom reached his office, he called in Gib Brownell, an Enterprisesengineer. 

"Got a job for me, skipper?"

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. Tomexplained his plan. "We'll use transmitter buoys, monitored by an alarmsystem at base headquarters on Fearing. "

Brownell studied the diagram and nodded. "Right. We can have it set upin 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. Oneof our subs has picked up a clue that someone has been operating in themissile search area. "

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

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

Tom thanked the admiral and hung up, feeling more uneasy than ever. Thereport 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 someflight-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 usedin making the caisson indicated that it was of Brungarian origin. Aspectrographic analysis of the steels confirmed the theory. Theirmetallurgical content agreed with known Brungarian steel formulas. 

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

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

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

"But why ditch it?"

Tom shrugged. "An optimistic guess is that they spotted our Navy searchforce 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 aroundif 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 getto work on a non-detectable sub, like the one that attacked us. Butmaybe 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 electronichydrolungs for scouting around!" he added eagerly. 

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

"Ion drive? For underwater?" Bud, who was familiar with ion propulsionfor spaceships, wrinkled his brow in a puzzled frown. 

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

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

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

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

Fascinated, the young inventor worked late into the evening, stoppingonly in response to a telephone plea from Mrs. Swift. By midmorning thenext day, Tom had assembled a pilot model of his ion-drive jet. Inappearance, it was a slender metal cylinder, two feet long, with aninner concentric tube projecting at each end. 

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

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

He climbed into the tank and slid the drive unit to one end of itstrack. Then Tom metered out power slowly. With a gentle _whoosh_, theion-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 upa little. "

He slid the drive unit back to starting position, then opened the switchwider. He had just started across the tank himself when suddenly hebecame powerless to move. 

Tom was pinned helplessly against the wall of the tank by the powerfulwater-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 tofree himself. 

The ion-drive unit had hurtled to the far end of the tank at the firstflick of power. But its exhaust tube was still jetting out a current ofwater with stunning force. Tom could feel the near-crushing pressureagainst 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 lastounce of breath were being squeezed out by the viselike pressure. 

Suddenly a gravelly Western voice reached him, singing "Home on theRange. " It drew closer, swelling into a foghorn drone as the lab doorswung 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 passingglance at the figure in the tank. 

[Illustration: _Tom squirmed desperately to free himself_]

"Soup's on, son!" Chow announced loudly. He began to ladle out a bowl ofoyster stew from a steaming pot. Evidently he had not realized the younginventor'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 awayfrom them chores an' dive in! Brand my cactus salad, if there's onething that riles a cook--"

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

At the strange sound of Tom's voice, Chow jerked around. His eyes buggedout at the look on the young inventor's face. Then he dashed to thepublic-address outlet on the wall and switched on the mike. 

"Help! Help!" Chow yelled. "Tom Jr. 's trapped in his lab!"

The roly-poly chef was quivering in panic. He dashed across the room andpaced helplessly about the tank. Within moments, excited men werecrowding into the laboratory. 

Mr. Swift, among the first to arrive, took in the situation at a glance. He dashed to the control board and slammed shut the main switch, thuscutting off power to the ion-drive jet. 

"Whew! Th-thanks, Dad!" Tom's chest was heaving as he gulped in air torelieve his tortured lungs. 

Tom Sr. Helped him climb out of the tank. 

"B-b-brand my rhubarb rockets, " Chow stuttered. "What in tarnationhappened?"

"Guess I gunned my new skin-diving jet a bit too hard, " Tom saidsheepishly. "It was almost a K. O. For me!"

Mr. Swift asked Tom about the invention. After explaining how it worked, Tom added with a grin, "Maybe you'd better hang around, Dad, until Iinstall some sort of density-control gadget for my hydrolung. Then I cango up or down, or stay at any level easily. "

Such a device, Tom felt, might prove to be a lifesaver if he should everbecome trapped under water--perhaps far from help. 

The elder scientist chuckled and threw an arm around Tom's shoulders. "I'd say you could design something like that with your eyes shut, son!"

Warmed by his father's appreciation, Tom set to work improving hisdiving apparatus. 

An hour later Bud came bursting into the laboratory. "Hey! What's this Ihear about your getting hammerlocked by a water jet?" the husky youngpilot asked. He had been on a test flight and just returned. 

Tom laughed good-naturedly. "Nothing serious. In fact, I felt prettysilly, " he told his chum. "I souped up our ion-drive gizmo a bit toomuch. "

Bud picked up the slender metal cylindrical assembly from the workbench. "This it?" he asked, his curiosity immediately aroused. 

Tom nodded and demonstrated the device in the test tank. 

Bud whistled with glee. "Boy! With this rig, we can scoot around like apair of barracudas!" he exclaimed. "What about that other thing you'reworking on?" Bud pointed to a small electronic chassis on the workbench, studded with a tangle of transistors, diodes, and condensers. 

"It's a density-control device, " Tom explained. "A substitute forballast tanks, you might say. It'll enable us to rise or sink to anydepth at will, simply by varying our underwater density. "

Tom said the device would be carried in a small case, hooked to thediver's belt, with a single tuning-knob control. The "throttle" or speedcontrol for the ion drive would be housed in the same unit. 

"I can't wait to try out the new diving gear, " Bud said excitedly. 

By four o'clock Tom had the apparatus perfected, and turned it over toArv Hanson for fast duplication. 

"We'll give it a shakedown tomorrow morning, " he told Bud. 

The duplicates of the ion drive and density control were ready andwaiting when the boys arrived at the plant next day. They immediatelyflew to Fearing Island and embarked in a motor launch, with Zimby Coxagain at the helm. 

This time they cruised out to deeper water. Tom and Bud donned flippersand belt, and helped each other strap on his ion-drive jet. 

"_Down_ we go, into the wilds of sharks!" Bud chortled lustily. "Watchyour step, Tom. "

"Just make sure you come up again in one piece, " Zimby said with a grin. "Also, don't get carried away with that ion squirt gun and take off on around-the-world underwater cruise. "

"Who knows?" Tom joked. Adjusting his face mask, he plunged over theside. Bud followed. 

Down they glided into the sea-green wilderness. Leveling off in sight ofthe ocean floor, they tried their drive jets. The effect was thrilling!_Zip . .. Whoosh!_ They darted to and fro like human torpedoes. 

Then Tom twirled the control knob of his density unit. Immediately hebobbed upward like a cork. A reverse twirl sent him plummeting towardthe bottom again. Bud, watching with wide-eyed excitement, beganexperimenting on his own. 

Soon the boys were engaging in all sorts of underwater acrobatics. Presently Bud felt a nudge in the back that sent him hurtling a dozenyards through the water. 

"Snuck up on me, eh, pal?" he thought with a chuckle. "Okay, Tom oldboy, here's where the undersea terror strikes back!"

Swooping around to return the compliment, Bud gulped in surprise. Instead of his chum, he found himself face to face with a bottle-noseddolphin!

"Good night!" Bud thought. "A porpoise! So you're the joker who nudgedme!"

With a playful toss of its comical-looking snout, the porpoise swam off, as if inviting Bud to join in the fun and games. A whole school of thecreatures cavorted into view. 

"Okay! If you want to play!" Chuckling, Bud darted in pursuit, whackedthe porpoise that had nudged him, and jetted off again. The porpoisegave chase, whistling and grunting audibly. 

Tom joined in the fun, and soon a rollicking game of underwater tag wasin full swing. The dolphins seemed as playful and mischievous as smallchildren. 

Twenty minutes later the boys surfaced and hauled themselves aboard. Both tore off their masks and flopped into the boat, shaking withlaughter, surfacing and diving. 

"What was so funny down there?" Zimby asked. 

When Tom told him about the dolphins, he too burst into laughter. Theporpoises rose into view and convoyed the launch all the way back to theisland. 

The boys were so jubilant over the performance of the new hydrolung gearthat Tom decided to press his search for the Brungarian sea-prowlersimmediately. Soon after lunch they took off in the _Sea Hound_ andheaded for the South Atlantic. Hank Sterling, Chow Winkler, and twocrewmen accompanied the boys. 

Dazzling afternoon sunshine sparkled over the sea when they reached themissile search area. Tom immediately contacted Art Wiltessa and thetask-force ships. They had no new developments to report. 

The young inventor gave orders to submerge. As soon as the seacoptertouched bottom, Tom and Bud swam out through the air lock with theirhydrolungs. 

They probed about for half an hour, ranging farther and farther from the_Sea Hound_. Then Tom felt a touch on his arm. He turned and saw Budpointing off excitedly to the right. 

A strange submarine was moving slowly toward them!

CHAPTER VIII

DATE TROUBLE

The boys exchanged looks of fear through their face masks as theknifelike hull and conning tower of the submarine loomed gray andghostly. 

Was the sub Brungarian? And what was it up to? Were the two young skindivers about to be run down or kidnaped?

_Or was its crew friendly?_

"Better not chance it, " Tom decided fast. He caught Bud's eye again andmotioned upward with a jerk of his thumb. "Topside, pal!"

"Roger!" Bud's lips shaped the word silently behind his face mask. 

In a twinkling both boys flicked their density controls and zoomedupward. The sub at once seemed to betray a hostile intent. It blew itstanks and planed upward in pursuit. But Tom and Bud easily pulled away. Their density units worked like magic, shooting them straight towardthe surface. 

"Wow!" Bud shoved back his face mask as they broke water. "That baby wasafter us and no mistake!"

Tom nodded, treading water. "Let's not stick around here, either! We'llsoon have company again if we do!"

Bud did not argue. "Where to, skipper?"

In the fresh salt air, with the sunshine sparkling on the waves, it washard to believe that an enemy submarine was hot on their trail. But bothyouths realized their peril was growing by the moment. 

"Back toward the _Sea Hound_, " Tom said, pointing north-northwest. "Submerge as we go!"

Bud circled his thumb and forefinger, then adjusted his mask, and thetwo boys plunged back in. On a sloping downward course, they sped alonglike undersea rockets, their ion jets functioning perfectly. Minuteslater, they sighted the seacopter. 

Hank waved to them through the cabin window as they glided past. The airlock opened speedily and the two boys entered. Both heaved sighs ofrelief when they were safely inside. 

"Somethin' wrong?" Chow asked, sniffing trouble. 

"A strange submarine, " Tom reported. "Brungarian more than likely. Itmay be heading this way if they've tracked us. "

"A sub?" Hank was startled. "We've picked up nothing on sonar!"

"Check again, " Tom ordered. 

The sonarman bent to his scope and Hank listened intently over thehydrophones. Neither could detect any sign of another craft. 

"Probably the same one that fired on us the last time, " Tom said grimly. "We'd better clear out before they take another pot shot at us. "

Hank sent the _Sea Hound_ zooming toward the surface while the boyschanged quickly into slacks and T shirts. Then Tom took over thecontrols for the flight home. 

"Brand my vitamin vittles! Are we just goin' to turn tail an' run everytime them varmints come skulkin' around?" Chow fumed as the seacopterarrowed northward. 

"Not if I can help it, " Tom vowed. "But first I must figure out a way tomake our own craft invisible, so to speak. It's the only way to protectour American crews, Chow, if we hope to do any secret digging for thatlost missile. "

"Want another suggestion, skipper?" Bud put in. "This one is about thehydrolung. "

"Sure. Speak up. "

"How about putting some sort of communications system into our hydrolunggear? If I hadn't been close enough to grab you when I spotted thatsub, it might have been curtains, pal!"

"You're right, " Tom agreed. "I'll get to work on it. "

It was sunset when Fearing Island came into sight. The boys flew aPigeon Special back to Enterprises, where Tom phoned a full report onthe mystery sub to the Navy Department. Then the two chums drove to theSwift home for a late supper. 

Phyl Newton was visiting Sandy that evening, but the girls displayed amarked coolness toward Tom and Bud. Instead of engaging in conversation, they retired to Sandy's room upstairs to play records, while Mrs. Swiftserved the boys a warmed-up but tasty meal of roast beef and mince pie. 

"What's wrong? Are we repulsive or something?" Bud asked as they ate. 

Tom shrugged, concentrating on a mouthful of roast beef. "Search me. Wesure don't seem very popular with the girls tonight. "

Mrs. Swift, overhearing their remarks in the kitchen, smiled butmaintained a diplomatic silence. 

Suddenly Bud slapped his forehead. "Good night! No wonder!"

Tom looked up with a grin of interest. "Well, what have we done?"

"It's what we _haven't_ done, pal!" Bud retorted. "We had a date thisafternoon, remember? That beach party and dance put on by Sandy andPhyl's school sorority!"

Tom gulped. "Oops! Boy, we really did pull a boner this time! Icompletely forgot!"

As they finished supper, the boys discussed various ways to make amends. Boxes of chocolates? Flowers? None of their ideas seemed to have theproper spark. 

"We'll have to come up with something super, " Bud said. 

"Right!" Tom agreed. "Let's sleep on it and see if we can't dream upsomething by tomorrow morning that'll really wow them. "

The next morning Tom had a flash of inspiration as he drove to the plantin his sports car. He hailed Bud at the first opportunity. 

"I have it, pal! What say we stage an old-fashioned square dance Tuesdaynight at the yacht club on Lake Carlopa?"

Bud's eyes lighted up. "Hey, that's a great idea! We'll invite a wholegang, get Chow to handle the refreshments, and make it a real shindig!"

The boys shook hands enthusiastically. Eager to patch matters up as soonas possible, they invited Sandy and Phyl out to lunch that day. Overdessert, the boys announced their plans for a square dance. 

"We--uh--realize we goofed yesterday on that beach party, " Tom saidsheepishly. "But we're hoping you'll give us another chance. "

The girls looked at each other, their eyes twinkling, then burst intogiggles. 

"You're forgiven completely!" Phyl declared. 

"Then it's a date?" Bud put in. 

"You bet it's a date, and don't you forget it!" Sandy warned. "Phyl andI are going right over to Dorman's Department Store and pick out somecute outfits for the dance!"

Tom and Bud chuckled over the success of their scheme as they drove backto Enterprises. Later that afternoon a telephone call interrupted Tom ashe worked in his lab on a sonic-communications system for the hydrolungapparatus. 

"This is Lester Morris, " said the voice at the other end of the line. The name did not register with Tom at first until his caller added, "Ihear you're planning a square dance Tuesday night at the yacht club. "

Suddenly Tom remembered. Lester Morris was a popular dance orchestraleader in and around Shopton. He was also much in demand as asquare-dance caller and fiddler. 

"That's right, " Tom said with a chuckle. "News must travel fast. We justphoned invitations to our friends. "

Morris asked if musicians had been hired for the evening. When Tom saidNo, his caller volunteered for the job, offering to provide a smallcombo of country-style players. His asking price sounded like a bargainrate, and Tom, knowing Morris's reputation, was only too glad to engagehim. 

"Lucky break, his calling, " the young inventor thought as he hung up. 

Bud was delighted to hear of the arrangement when he came into thelaboratory a while later. The boys talked over their dance plans for afew moments, then Bud asked:

"How's our underwater talkie system coming?"

Tom scratched his jaw thoughtfully. "A bit tricky but not toodifficult, " he replied. "It's mostly a job of adapting the sonarphonearrangement from our Fat Man suits--in miniature. "

A tiny mike, Tom explained, would be installed on the inside of eachface mask, with its output feeding to a sonar transducer on theexterior. The receiving transducers would feed from amplifiers toearphones. The hookup would be powered by the solar battery in thehydrolung power unit, by connecting wires through the breathing tube. 

"That's neat, Tom, " Bud said. "Need any help?"

"You can mold us a pair of new face masks--big enough to cover theearphones, " Tom suggested. He handed Bud a penciled sketch from theworkbench, adding, "Then drill the holes for the mikes andearphones--the dimensions are there on the drawing. But watch it soyou don't crack the plastic. "

[Illustration: _Chow was wild-eyed with panic!_]

While Bud complied, Tom began assembling the tiny electronic parts. Intwo hours the gear was ready for testing. 

Tom wiped his perspiring forehead and gave Bud a grin of satisfaction. "Go get your swim trunks, fly boy. Let's give it a tryout in the tank. "

"Swell idea! Be back in a jiff!"

After a quick change, the boys strapped on the new hydrolung equipment. Before adjusting his face mask, Tom mentioned that he had insertedscrambling circuits into the communicators to foil any enemyeavesdroppers. 

"If they do pick up anything, it'll sound like chop suey, " Tom endedwith a chuckle. 

The boys submerged in the test tank and proceeded to give the newunderwater communication system a thorough check-out. It workedperfectly. Ten minutes later Tom and Bud clambered out again, drippingwet but well satisfied. 

They had just peeled off their masks when Chow came charging into thelab, with a crowd of workmen and technicians at his heels. The cook waswild-eyed with panic. 

"What's wrong, Chow?" Tom asked in alarm. 

CHAPTER IX

A MAGNETIC KIDNAPING

"The space people or some enemy's invadin' us!" Chow shouted. "Take asquint through your telescope, boss! Brand my bazooka, they may belandin' any second!"

More people came streaming in, attracted by the chef's cries andgesticulations. Some were bewildered, a few frightened. Others werelaughing, thinking the whole thing a joke. The scene was rapidly takingon the proportions of a riot!

"Whoa! Slow down, Chow!" Tom ordered, trying to make himself heard abovethe din. 

"It--it's the truth, boss!" Chow stammered, mopping his brow with a hugered bandanna. "Why, sufferin' rattlesnakes, didn't I hear 'em spoutin'their space lingo with my own ears?"

"You heard _what_?" Bud said. 

"Spoutin' space talk!" the cook repeated. "It come right over theloud-speaker in the galley! They was chitter-chatterin' plottin' to blowus all to smithereens!"

"That's a fact! We heard it, too!" one of the workmen chimed in. 

Tom and Bud looked at each other blankly. Then suddenly Tom's eyeskindled with a dawning suspicion. Whirling around, he rushed over toinspect the public-address outlet on the wall. 

Meanwhile, Mr. Swift had just driven in through the main gate ofEnterprises. "What's going on?" he asked the guard at the gate, notingthe excited hubbub around Tom's laboratory. 

"Don't rightly know, sir, " the guard replied. "I was wondering myself. Iknow it sounds crazy, but I thought I heard someone yelling there wasgoing to be a space attack. "

Mr. Swift's eyebrows lifted in amazement. Without further discussion, hestepped on the accelerator and sped off along the paved drive. Secondslater, his car braked to a stop near Tom Jr. 's private laboratory. Thescientist jumped out and made his way through the milling crowd. 

"What's going on?" Mr. Swift stared in astonishment at Tom and Bud, whowere both doubled up with laughter. 

"A scrambled radio alert, Dad, " Tom gasped between chuckles. "Chowthought some Martian monsters were invading us, and sort of pushed thepanic button. "

The Texan blushed as Tom explained what had happened. Realizing Chow'sembarrassment, Tom tried to make his mistake sound understandable. 

Apparently the power line to the ion-drive control board had somehowpicked up the boys' scrambled conversation underwater. The signal hadbeen transferred by inductance in the wall wiring and amplified over thepublic-address system. 

"Our wall mike was on, " Tom added, "and it probably picked up some ofthe sound waves from the tank. Anyhow, " he concluded, slapping the cookaffectionately on the back, "I'm sure glad we have a wide-awake hombrelike Chow in the outfit. It wouldn't be the first time he's saved ournecks!"

Chow perked up, and the employees, reassured, returned to their jobs. 

"I have some news of my own, " Mr. Swift announced with a smile as theroom cleared. "But I'm afraid it'll sound pretty tame compared to aspace attack. "

"Let's hear it, Dad, " Tom said eagerly. 

"I've been conducting some experiments with those space plants, " theelder scientist said. "It looks as though they may prove to be avaluable nutritional source. "

The plants, Mr. Swift went on, showed promise of producing enormousamounts of protein quickly and cheaply--enough to increase the world'sfood supply by a sizable margin. Moreover, he had isolated a vitamin inthis protein not found in any of man's present foods. 

"Doc Simpson has been working with me, " Mr. Swift concluded. "He hasbeen doing some experiments of his own with a vitamin extract from thespace plants. He thinks it may prove highly beneficial to human beings. "

Tom was thrilled, and even Bud realized that Mr. Swift's cautious reportcould well turn out to be of history-making importance. 

"I'd say your news makes a phony space attack look pretty tame, Dad, "Tom said, his eyes flashing enthusiastically. "With the earth'spopulation increasing, this could be the answer to the food problem. "

"Don't tell Chow, " Bud added, "or we may find spaceburgers on the nextmenu!"

The Swifts chuckled. Chow's hobby of concocting weird dishes was astanding joke at Enterprises, and already had led to such dubioustriumphs as armadillo stew and rattlesnake soup. 

Monday morning Tom buckled down seriously to the job of designing anundetectable sub. His drawing board was littered with sketches anddiagrams when the phone rang, breaking in on his thoughts. Tom answeredit with a scowl of impatience. The caller was Lester Morris. 

"Could you meet me at the yacht club to talk over the dance program?"Morris asked. 

Tom hesitated. For Sandy's and Phyl's sakes he was eager to doeverything possible to make the square dance a success. But on the otherhand. .. . 

"I'm pretty busy today, " Tom said. "But my sister and my friend BudBarclay can tell you what we want--probably better than I can. Suppose Iask them to meet you there after lunch?"

There was a slight pause. "Very well, " Morris agreed, although hesounded a bit annoyed. 

After hanging up, Tom phoned Bud and asked him to keep the appointment. Bud was only too happy to oblige, jumping at the chance to take Sandyout to lunch beforehand. 

At one o'clock the husky young pilot and his date strolled into theyacht club lounge. Lester Morris was nowhere in sight, so they sat downto wait. Twenty minutes later the musician still had not appeared. 

"I hope he hasn't forgotten, " Sandy said, glancing at her wrist watch. 

"If he's a square-dance caller, his memory ought to be extra good, " Budjoked. "Fine thing if he can't even remember the time of day!"

After waiting a while longer, Bud decided to telephone Morris's home. But at that moment a thin, seedy-looking man came into the lounge. Hisclose-set eyes and loudly striped suit combined to give him a somewhatdisreputable appearance. 

"Good grief! Len Unger!" Sandy whispered. "What does he want with us?"

Unger was walking straight toward them. Both Bud and Sandy had met himoccasionally around town and found him obnoxious. 

"Sorry, but Morris got tied up, " Unger informed them. "He sent me totalk to you. "

Sandy's blue eyes met Bud's in a flicker of distaste, but she tried toconceal her feelings. "Please sit down, " she invited Unger politely. "What square-dance numbers does Mr. Morris do?"

Len Unger shrugged. "You name 'em. "

"But, my goodness, " Sandy said, puzzled, "how do we know he'll have thesquares I name?"

Unger stared at her as if he did not quite understand. "You mean, can hecall off the dances you want? If he can't, I'll let you know. "

"Does he do patter calls or singing calls?" Bud put in. 

Again Unger hesitated, then said, "Both. "

"Wonderful!" Sandy exclaimed gleefully. "I thought he only did singingcalls. " After a moment's thought, she went on, "Well, let's see. Whatabout 'Birdie in the Cage'?. .. And 'The Gal from Arkansas' . .. 'Uptownand Downtown'. .. . "

Unger jotted the names on the back of an envelope. Pausing a moment, heremarked, "Guess your brother was too busy to make it today, eh, MissSwift? What kind of ex-spearmints is he working on now?"

"I really couldn't say, " Sandy replied coldly. She always made it apoint not to discuss Tom Jr. 's or her father's research work withoutsiders. 

Unger persisted chattily, "I read where he handled that Jupiter probeshoot for the Navy. "

"Let's get back to square dancing, " snapped Bud. As he and Sandyfinished planning the program, Len Unger continued to drop remarks andquestions about "The Great Tom Swift" and his inventions. All pryingqueries were side-stepped. 

As soon as possible Sandy and Bud cut short the conversation and leftthe yacht club. Unger's face wore an angry sneer as they walked out. 

"What a creep!" Bud said, when he and Sandy were driving back in his redconvertible. 

Meanwhile, in his private laboratory at Enterprises, Tom was somewhatdiscouraged. He had tried several different experimental attacks on theproblem of an undetectable submarine. None had worked out successfully. 

"I thought that idea of a sonar-wave baffle might lead somewhere, " hemurmured, "but it looks as though I'm wrong. "

Flopping down on a stool at his workbench, Tom cupped his chin in hishands. He was frowning, deep in thought, as the pudgy figure of ChowWinkler came into the laboratory. 

"'Smatter, boss?" the cook inquired cheerfully. "Ain't your ole thinkbox workin' today?"

"Doesn't seem to be, " Tom confessed. 

"Give it time, son. Tomorrow's another day, " Chow said philosophically. "What you need is a haircut for the square dance. "

Tom laughed in spite of himself. "Maybe you're right, Chow. Might helpme think better. "

Tom got off the stool and stretched out the kinks in his legs. Hestrolled outside with Chow, then scootered to the parking lot and hoppedinto his sleek, silver sports car. 

A moment later he was whizzing off in the direction of Shopton. Nearingtown, Tom turned off on a side-road short cut. He noticed in his mirrorthat a truck behind him also turned off. 

"Really barreling along!" Tom thought. "If you're in such a hurry, theroad's yours, pal. "

He pulled over sharply, motioning the truck to pass. Instead, to Tom'ssurprise, it closed in straight behind him. The next moment, Tom saw aport open below the truck's hood and a strange-looking device pop out ona springlike steel cable. 

It clamped magnetically to Tom's rear bumper! His car was caught like afish on a line!

Tom stepped on the accelerator, trying to pull free. The truck at onceswerved off the road, steering around a utility pole. As the cabletautened, there was a sickening screech of metal and the sports car wasbrought to a crashing halt!

Tom's head slammed against the side window. With a groan, the younginventor blacked out. 

CHAPTER X

TELEPHONE CODE

As he regained consciousness, Tom's eyes fluttered open. Sparks of painshot through his head. A groan escaped his lips. 

"Oo-o! What hit me?" Tom wondered. 

He was lying on a sofa in a strange room. Someone was seated nearby, watching him. Tom tried to move his limbs and sit up. Then he discoveredthat his wrists and ankles were tied with sash cord. 

"Better lie still, sonny boy, " a gruff voice advised. "You ain't goin'nowhere. "

The man who had spoken got up from his chair and came over to the sofa. He was of medium height, very muscular looking, with cold, glitteringeyes. Rolled-up shirt sleeves revealed his powerful, hairy arms. 

"Where am I?" Tom asked, suddenly remembering the events on the roadbefore he blacked out. "And what's this all about?"

The man said with a mirthless grin, "You're a prisoner. And you're goin'to stay here until the cops let Dimitri Mirov go. It's up to you howfast they spring him. "

The huge man lifted a telephone from an end table adjoining the sofa andset it on the floor alongside Tom. 

"Here's a phone. Go ahead and use it, but don't try any funny stuff. "

In spite of his headache, Tom's brain was racing. What to do now? Heshut his eyes and screwed up his face in an expression of pain, pretending to be still groggy while he stalled for time to figure outhis next move. 

"How can _I_ get Mirov out of jail?" Tom faltered. 

"You figure it out!" the man snarled. "And you'd better get results ifyou want to stay healthy!"

Through half-slitted eyes, Tom noted the telephone number printed on thedial. Evidently his captor had not thought to remove it from theinstrument. A lucky break!

If only, Tom thought, he could devise some way to transmit the number toAmes without arousing his captor's suspicion--the phone's location couldthen be traced!

What about some sort of double-talk code? For instance, Tom toldhimself, keep slipping numbers into the conversation in order totransmit the digits of the telephone number. Would Ames catch on?

The number shown was BArwick 3-7156. BA on the dial would be the same as"2, 2. "

"Come on! Quit stalling!" the man said threateningly. 

"How can I dial with my hands tied?" Tom objected. 

"I'll do the dialing, wise guy!"

He lifted the phone from its cradle and extended it to his prisoner. Tomtold him the Enterprises number, then asked for Ames's extension as theswitchboard operator answered. A moment later the security chief's voicecame over the line. 

"Ames speaking. "

"This is Tom Jr. , Harlan. " His captor bent close to the receiver as Tomreplied, in order to overhear what was being said. "I've been thinking, "the young inventor went on, "that it might be smart to have Mirovreleased. "

"_Released!_" Ames gasped in surprise. "But why, skipper?"

"Well . .. Er . .. As a good-will gesture, " Tom said. "I think it mightprevent future trouble with the Brungarians, don't you?"

"I do not!" Ames exploded. "The idea sounds crazy!"

"I don't think it's _too_ crazy or _too_ risky, " Tom argued. Byemphasizing the words, he hoped to impress them on Ames's mind. 

[Illustration: _"Come on! Quit stalling, " the man threatened_]

Tom's tone of voice and the farfetched nature of what he was saying hadalready triggered the security chief's suspicions. "Where are youcalling from?" Ames asked after a tense pause. 

"Shopton, " Tom replied. "I just drove in for a haircut. " With a chuckle, he added, "Haven't had one in _three_ months. That's a whole _week_longer than I usually go!"

Would Ames understand that by "week" he meant _seven_ days?. .. "_It'sthe best I can do_, " Tom thought. 

"Look, skipper, are you sure you want Mirov let out?" Ames said slowly. "I still think it's unwise. "

"Consider it an order!" Tom snapped. "This is _one_ thing I insist upon, Harlan. Shouldn't take more than _five_ or _six_ hours, should it, evenif he has to wire the Brungarian Embassy to put up bail?"

"It can probably be handled faster than that--if he has any friendsaround town, " Ames said. 

Tom took the cue. "Could be, " he replied meaningfully. 

Tom's captor snatched the phone away and slammed it back on the hook. "All right, smart boy! That's enough!" he growled, glaring at Tom. 

Back at Enterprises, Ames hung up thoughtfully. Tom's reply to his lastquestion about Mirov having "friends around town" had convinced Amesthat the young inventor was a prisoner, speaking under duress. Moreover, it had seemed as if someone else's breathing was faintly audible in thebackground, close to the phone. 

_But what message had Tom tried to convey?_

As a routine security-department precaution, Ames's phone was connectedto a recorder which automatically taped all calls. Now, while hepondered the problem, Ames pressed a foot-treadle switch to play backthe conversation. 

Meanwhile, Tom and his captor waited tensely. From time to time thelatter glanced at his watch. "Better hope that call does the trick, Swift, " he muttered. "It's the only hope you got of leavin' here alive!"

"How will you know if they've turned Mirov loose?" Tom asked. He waswondering if he might persuade his captor to let him make a second call. 

"Don't worry. Mirov knows how to contact me. "

Half an hour dragged by--then forty minutes. Suddenly the door buzzerrang sharply. The man jerked to attention, obviously startled. Heglanced at Tom, then toward the direction of the sound, moistening hislips nervously. 

"He must have been expecting just a phone call, " Tom decided. 

The buzzer shrilled again. This time the man got up from his chair, gagged Tom hastily with a handkerchief, and went to the door. 

"Who's there?" he asked loudly. 

"Mirov! Let me in, Duffy!" replied an accented voice from outside. 

With a look of relief, Duffy started to open the door--then froze as hesaw not only Mirov, but two police officers and Ames accompanying him. 

"Are you the one who's going to put up bail?" one of the officersdemanded. 

Duffy floundered, scenting danger but unable to pick up any clue fromMirov's face. "Why--uh--yeah, maybe. How much is it?"

"Ten million! Can you raise it?" Ames snapped sarcastically. 

As Duffy gaped in confusion, the officers suddenly flung their weightforward. The door flew open and Duffy was thrown back, almost losing hisbalance. Beyond, through the small vestibule, Ames caught a glimpse ofTom on the sofa. 

"There he is!" Ames shouted. 

Moments later, Tom was untied. Mirov and Duffy were handcuffed together. 

The young inventor shook hands joyfully with his rescuers. "Nice going, Harlan! Boy, I was sweating icicles here, wondering if you'd be able todecipher all my double talk!"

"You made the numbers clear enough, " the security chief said with agrin, "but it took a while to guess what they stood for. And then, ofcourse, we had to trace the address through the telephone company. "

Eying the ugly bruise on Tom's forehead, Ames added, "Sure you're allright?"

"Right now I feel swell!" Tom declared, chuckling. He told of hiskidnaping, while one of the officers took down the details. 

The prisoners were taken off to jail in the police squad car. Tom andAmes, meanwhile, in the security chief's high-powered sedan, drove tothe scene of Tom's capture. 

They found his sports car badly damaged. The right side was wedgedagainst the utility pole, which was leaning at a crazy angle. 

Ames whistled and shook his head. "Boy! You're lucky you got off withjust a bruise, Tom!"

"You're telling me, " the young inventor agreed ruefully. 

After calling a repair garage to send out a wrecker, they drove to theSwifts' home. Mrs. Swift and Sandy, previously unaware of Tom's plight, were horrified to hear what had happened. The sight of Tom's bruise alsoupset them. 

Tom did his best to allay their concern, but finally allowed himself tobe hustled up to bed. Dr. Emerson, the Swifts' family physician, wasimmediately summoned to the house. He pronounced the bruise not serious, but advised that Tom remain quiet, at least for the rest of the day. 

Bud came to visit the young inventor that evening, just as Sandy wasbringing up a tray. On it was a sizzling T-bone steak. 

"Wow! Wish I could have that kind of service, " Bud said jokingly. Thenhe became serious. "I'd sure like to meet that creep who snagged you, Tom. What a fiendish trick! You realize you might have been killed?"

"I realize it, all right, " Tom said wryly. 

The next morning Tom felt no ill effects from his grim adventure andinsisted upon driving to Enterprises. He phoned Admiral Walter, whosereport was bleak--the searchers had still gleaned no trace of the buriedmissile. 

Refusing to be discouraged by the news, or lack of news, Tom went to hisprivate laboratory and applied himself once again to the problem ofbuilding an "invisible" submarine. But again success eluded him. 

At last Tom shook his head in disgust. "May as well get that haircut Istarted out for yesterday, " he decided. 

Before leaving, Tom phoned Phyl Newton to thank her for the gift offruit and nuts she had brought over the previous evening after learningof his dangerous experience. They chatted for a while and wound up bymaking a date for lunch. 

Tom drove back to town in the family car and got a haircut. Then hepicked up Phyl at her home and took her to the yacht club. Here theylunched on the terrace overlooking the sparkling blue waters of LakeCarlopa. 

The young inventor's spirits were high when he finally returned to hislaboratory and buckled down to work. 

"I'll lick this problem yet, " he muttered. "Those enemies of ours areclever, but if they can produce an undetectable sub, there's no reasonwhy I can't do the same. "

Deep in thought, Tom idly fingered a microphone on his workbench. 

"In fact, " the young inventor mused, "why not go them one better? I'llinvent a submarine that's not only invisible to sonar, but equipped to_see them_!"

CHAPTER XI

SQUARE-DANCE HOAX

Random hunches and circuit diagrams flashed through Tom's brain. "Thejob will boil down to blotting out sonar waves and piercing the enemy'sown 'wave-trap defense, '" the young scientist concluded. 

As Tom struggled with the problem, he lost all track of time. A doorswung open and high-heeled boots clumped on the floor tiles. Tom lookedup and saw the portly, aproned figure of Chow Winkler entering. 

"Hi, boss! Can I borrow a radio?" Chow asked. "Kinda like a lil musicwhile 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' themslick lil transistor doodads, eh?"

The cook flicked on the dial knob and the twangy strains of Hawaiianguitar music came throbbing out. A split second later the volume swelledas the same music echoed back to them from the two-room apartmentadjoining the lab, where Tom ate and slept when engaged in someround-the-clock experiment. 

Chow was startled by the blare. "You got a stereo hookup here, boss?" heinquired. 

"Not exactly. " Tom explained that the music had merely been picked up bythe mike on his workbench, then fed into the adjoining apartment andamplified over a speaker there. 

Chow grinned, snapping his fingers to the catchy melody. "Comes out evenlouder'n it does from the radio!"

"Yes, but the sound quality's not so good, " Tom said. "You'd notice thedifference 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 onthe submarine problem. But for some reason the business with themicrophone 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 subblinds our sonar?"

The idea certainly seemed feasible. Suppose the submarine used a greatmany "microphones"--or receiving transducers--to pick up the sonarpulses beamed out by another craft trying to detect it? These impulsescould then be passed on and sent out by speakers on the opposite side ofthe sub, and relayed along on their underwater path of travel. 

Thus the sonar waves would appear to be striking no obstacle--and noecho 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 bootheels, it's partly due to good old Chow!"

He grabbed a pencil and began sketching his idea on paper. It would benecessary to spot the receivers and transmitters all over the hull ofthe submarine. Diagrams and pages of scribbled computations followed therough sketches. 

An invisible sub--one that sonar pulses would seem to pass rightthrough, as if nothing were there! "Seems so simple now that I have thekey!" Tom said to himself elatedly. 

Hours ticked by while he analyzed the wave action mathematically, thenworked out a typical hookup for one of his jetmarines in a set ofprecise 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 thegriddle!"

Bud arrived in a few moments. Tom showed him the drawings and explainedhis plan for dodging underwater detection. He also related how Chow'sremarks 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 andfeet wide apart. 

"Well, let's go, pal!" Tom urged impatiently, puzzled by Bud's lack ofresponse. 

"What about the square dance?"

Tom stopped short, feeling like a punctured balloon. He stared in dismayat 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 thegirls down twice. But at least let's get together all the gear we'llneed when we _do_ go to Fearing. "

"I guess we'll have time for _that_, " Bud conceded with a sympatheticgrin. 

Tom assembled a mass of electronic equipment and phoned variousEnterprises' departments for other items. Bud helped to collect them, and the boys trucked the paraphernalia out to a hangar to be loadedaboard a Whirling Duck. Then they scootered back to the lab for a quickshower and change. 

Twenty minutes later, in sport jackets, checked shirts, and slacks, thetwo chums hopped into Bud's red convertible. They picked up Sandy andPhyl and drove a little way into the country for dinner at a huge oldfarmhouse restaurant. 

"Well, the evening's off to a good start, " Sandy said with a happy laughas 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 thesquare dance, " Bud remarked. 

Tom chuckled. "Don't worry, pal. You always untangle those feet of yourswhen the fiddle strikes up!"

The blazing lights of the yacht club were reflected in the blue-blackmirror of the boat basin. Bud parked and they went inside. 

"Welcome, buckaroos!" Chow Winkler greeted them with an enthusiasticbellow as they entered the dance room. 

The old cowpoke was splendidly dressed in a maroon satin shirt and whitewhipcord breeches tucked into shiny new boots. But instead of his usualsombrero, a chef's cap was perched on his head. 

"Chow! You look marvelous!" Sandy said. 

The cook blushed with pleasure. "You gals look purty enough to charm ahoot owl right off'n his perch!" he shot back. Both Phyl and Sandy werewearing gay calico dresses that had full swirling skirts. 

The room was decked out with colored bunting and twisted crepe-paperstreamers. And at one end of the dance room, Chow had rigged up a modelof a Western chuck wagon. 

"Real atmosphere!" Tom said admiringly. "Chow, you've done us proud!"

"Thanks, boss. " The cook, who had asked especially to take charge of thedecorations, glowed at the praise. Then he became serious. "But what'skeepin' that dad-blamed fiddler?"

The guests soon began to stream in, but half an hour went by, and LesterMorris and his fellow musicians had not arrived. 

"I'd better phone his house, " Tom decided worriedly. 

Mrs. Morris answered. She seemed surprised at Tom's call. "Why, myhusband's playing at a party over in Carterton this evening, " she said. "Are you sure you engaged him for tonight?"

"I'm positive, " Tom replied. 

"Just a moment, please. I'll look in his date book to see if there'sbeen a mistake. "

A minute later her voice came over the line again. "I'm terribly sorry, Mr. Swift, but your name isn't listed anywhere on Lester's schedule. "

The others saw from Tom's face as he hung up that something was wrong. 

"What gives?" Bud asked anxiously. 

"No music for one thing. " Tom reported what Mrs. Morris had told him. 

"But you hired the guy!" Bud protested. "And Sandy and I talked to hisagent!"

Tom was already piecing together the mystery. He shook his headthoughtfully. "I'm sure now the whole deal was a hoax, Bud, " hedeclared. "Both the first call that supposedly came from Lester Morris, and the second one asking me to come here and talk things over. "

By not responding to the second call in person, Tom went on, he hadprobably saved himself from being waylaid or kidnaped by his enemies. 

"Thank goodness!" Sandy exclaimed. "Still, that creepy Len Unger wastrying to get information from us. "

"But how did your enemy know about the dance, Tom?" Phyl Newton put in. 

Sandy snapped her fingers. "I know! I'll bet it was when we wentshopping for our dresses, Phyl, right after the boys invited us! Thedepartment store was full of people--almost anyone might have heard usdiscussing the dance!"

"Especially if he was already trailing you to pick up bits of usefulinformation, " Tom agreed. 

Bud whipped out a handkerchief and mopped his face nervously. "Thequestion is what do we do now, chums? A roomful of guests and no music!"

"Relax, pardners!" Chow broke in cheerfully. "Just keep things goin' fora spell, an' I'll fix things up pronto!"

Doffing his chef's cap, Chow hustled out to his parked jeep and took offwith a roar. Mystified but hopeful, Tom, Bud, and Phyl did their best toentertain the guests. Sandy had rushed to the telephone. In twentyminutes Chow came rushing back. 

"Hey! He has a fiddle!" Bud exclaimed. 

Mounting the platform, the stout cowpoke raised his hands and shoutedfor attention. 

"Ladies an' gents, we'll start off with that good old dance known as theTexas Star!"

As everyone took his place, Chow tuned up hastily. Then he tucked thefiddle under his chin, stomped out the rhythm, and launched into alively rendition of "Turkey in the Straw" while he called out theaccompaniment:

 "_Gals to the middle, then back so far! Gents step up for a clockwise star! Now shift hands and twirl t'other way, We'll keep on dancin' till the break o' day. .. . _"

The dance number finally ended to thunderous applause. Chow, puffing andred-faced but wreathed in smiles, was soon ready for another. Half anhour later, a dance band of high school boys, hastily summoned by Sandy, arrived to spell the Texan. 

The irrepressible chef, however, continued to call out most of thenumbers and proved to be the hero of the evening. He gained even moreacclaim for his delicious French fried potatoes and "steerburgers"served during the pause for refreshments. 

"Oh, Chow! What would we ever do without you?" Sandy said, and the cookbeamed. 

Suddenly, in the midst of the lively chatter and laughter, the dancefloor was plunged into total darkness!

Phyl clung fearfully to her escort. "Tom!" she gasped. "This is anothertrick of your enemy's to harm you!"

CHAPTER XII

DETECTION TEST

"Don't worry, Phyl. It may be only a blown fuse, " Tom tried to assurethe fearful girl. 

But Tom was worried himself. Not only might he be in danger, but itcould involve his friends!

Nevertheless, he raised his voice above the excited babble. "Please becalm, everyone! We'll have the lights on again in a jiffy!"

Taking Phyl by the hand, Tom groped his way toward the main door. 

"Let's check the switch, " he murmured, and ran his hand over the wallnear the door. He located the metal plate and flipped the switch. 

The lights went on! Good-natured cheers arose. Bud, grinning butpuzzled, left Sandy's side long enough to come over and speak to Tom. 

"What happened?"

"I guess some practical joker clicked off the switch. "

Bud suddenly caught sight of a stout youth in a plaid shirt and bluejeans, who was standing in a nearby corner. He was shaking all over withhalf-stifled merriment. 

"There's the wise guy! Rock Harriman!"

Rock, an all-star tackle on the Shopton High football team, was wellknown for his pranks and practical jokes. Bud rushed over. 

"Okay! Confess!" the husky young flier roared in a jokingly ferocioustone. 

"Don't get sore!" Rock gasped between chuckles. "I couldn't resist. Boy, did you hear everyone squeal when the lights went out?"

Tom grinned in relief. "How about another dance, Phyl?"

As the music struck up again, he squeezed Phyl's hand. "I sureappreciate your concern, even if I didn't rate it. "

Phyl blushed as she returned the squeeze. "You rate with me, " sheconfided shyly. 

The festivities finally ended after a thoroughly enjoyable evening. BothSandy and Phyl declared to their dates that it more than made up for theforgotten beach party. 

"But let's not wait too long for the next date, " Sandy warned playfully. 

"Okay, that's a deal, " Bud promised. 

The next morning at the plant Tom called on Harlan Ames. He told of thesinister hoax by the caller who had passed himself off as Lester Morris. The security chief promised to investigate. 

"I'll tip off the police about Len Unger, " Ames added. "If they can findhim, we may be able to crack this case wide open. "

Tom telephoned Bud, Hank Sterling, and Arv Hanson to meet him at thehelijet hangar. The four took off in one of the Swifts' Whirling Ducks, which was standing by loaded and ready. Soon they landed on FearingIsland, where Tom would try out his antidetection invention. 

"What'll we use for a test sub, skipper?" Hank asked as they drovetoward the docks. 

"A jetmarine, " Tom replied. 

A truck with engineers and technicians was following the jeep. Itcarried the equipment which Tom and Bud had assembled the previous day. 

When they arrived at the docks, Tom gathered the men in a loading shed. He showed them his drawings and explained how his "sonar-blinding" setupwould operate. 

"Don't let the diagrams fool you. The basic idea is very simple. Weabsorb all sonar impulses that hit the ship and transmit them out theopposite side of the hull, instead of letting a ping bounce back andshow up on the sonarscope of any hostile sub on the lookout for us. "

Most of the job, he went on, would be tedious detail work. It wouldconsist of attaching hundreds of mikes and speakers all over the hull topick up and transmit the sonar pulses. The mikes would be receivingtransducers and the speakers would be transmitting transducers. 

"The leads from them, " Tom ended, "will be centralized in a singleelectronic control unit inside the ship. I'll handle that part of it. "

"Great idea, Tom!" Arv Hanson said admiringly. 

"But what a job it'll be rigging those transducers, " put in one of thetechnicians. 

Tom nodded wryly. "You're right, Danny. If this experiment works out, though, I think I can lick that problem on future installations. "

The young inventor explained that he hoped to find a way to mold thetransducers into a continuous plastic sheet. This could be applied tothe hull of a submarine in a single operation. 

"But this time we'll have to do it the hard way, " Tom added with anapologetic grin. 

A jetmarine was hoisted into drydock and the work crew swarmed over it, rigging the transducers. Would his experiment succeed? Tom wondered. Hopefully, he set to work assembling the electronic control unit. 

Bud helped the men on the hull for a while, then descended through thehatch to see how Tom was progressing. 

[Illustration: _Would his experiment succeed? Tom wondered_]

"I'd go gaga trying to keep track of those circuits, " Bud said, as hewatched Tom installing the delicate transistors and other componentswith an electric soldering gun. 

The young inventor grinned. "It'll be simple enough when the controlunit's all put together, " he replied. "Just a single on-off switch andone test circuit. "

By noon, after working at a frenzied pace, the job was done. Tom thankedeach one of the men personally. Then everyone went to eat lunch. 

After the meal, Hank Sterling asked, "How about a detection test to seehow she works?"

"Coming right up, " Tom said. "Want to skipper the jetmarine, Bud?"

"Sure do!"

"Okay. Pick out a couple of men for a crew and take her down. " Tomproduced a hydrographic chart of the waters around Fearing and markedout a test area. "Cruise around there for an hour and we'll try to spotyou in the _Sea Hound_. "

"Hide and seek, eh?" Bud grinned and snapped a salute, then left tosupervise the relaunching of the jetmarine. 

For his crew, Bud chose Mel Flagler and another man. Mel was anexperienced jetmariner who had gone on the Swift expedition to AurumCity, the underwater ruins of a lost civilization. Here Tom had used hisspectromarine selector to restore the ancient buildings. 

Tom, Hank, and Arv went back to the airfield and soon took off in thediving seacopter. Landing on the water, they submerged and began theundersea detection test. 

Tom manned the sonarscope personally, eager to conduct as careful asearch as possible. 

"Getting any blips, skipper?" Hank called out from his post at the _SeaHound_'s controls. 

"Not a ping, Hank. The system seems to be working out even better thanI'd hoped. "

Tom felt a glow of satisfaction. He explained, however, that thejetmarine's transparent nose pane--which had to be left unprotected forthe pilot's visibility--offered one vulnerable spot to sonar detection. 

"But a little smart maneuvering can cover up that angle, " Tom added. "Try the hydrophones, Arv, and see if you can hear 'em. "

The chief modelmaker slipped on the earphones and listened intently. Foranother ten or fifteen minutes they probed about with no sound trace ofthe "invisible" jetmarine. 

But presently Arv snapped his fingers to catch Tom's attention. "Gother, skipper!"

Tom took over the hydrophones. Sure enough, his ears could make out thefaint hum of the jetmarine's atomic turbines. Tom directed Hank towardthe sound, then ordered him to switch on the _Sea Hound_'s powerfulsearch beam. 

The light cut a path of radiance through the murky dark-green waters. Dead ahead, the jetmarine could be seen gliding across their field ofview. 

"Your system blinded our sonar okay, skipper, " Hank commented, "but thisproves she could still be spotted by enemy listening devices. "

Tom refused to be discouraged. He ordered Hank to return to base andwait for Bud. Meanwhile, the young inventor applied himself to theproblem of how to mask the sub's noise. 

"How about it, pal?" Bud asked, when he reported aboard the seacopter awhile later. 

Tom explained the results of the test and the need for an addedsafeguard against hydrophone detection. "Think I see a simple way out, though, " he added with a pleased chuckle. 

"Natch! With a brain like yours, it's a cinch, " Bud quipped. "Explain, professor. "

"Well, we can never do away with the noise of a sub's propulsionmachinery, " Tom began. "That goes without saying. So we'll have tocamouflage it--lose it in the underwater jungle noises, so to speak. "

Bud scratched his head. "How do we do that?"

"By amplifying the natural undersea sounds all about it, " Tom explained. "Fish and all forms of underwater life make a background noise over thehydrophones, you know. "

As Bud nodded, Tom went on, "So we simply step up the volume till thesub's own noise gets drowned out or 'wasted' in all the racket. "

This could be done, he concluded, with fairly simple amplifyingequipment. Bud, Hank, and Arv were jubilant at the idea. 

"Nice going, " Bud said. "How soon can we give it a try?"

"Soon as I can rig up the amplifier, " Tom promised. 

In less than two hours they were ready to submerge again. Zimby Coxjoined the crew. Bud suggested taking along hydrolungs in case of anyneed for tinkering with the transducers or amplifying equipment. 

This time, the jetmarine scored perfectly on the test, successfullyeluding all the _Sea Hound_'s efforts to detect it. Tom returned happilyto base, feeling that the antidetection problem was now solved. Thejetmarine, however, failed to appear. 

"That's funny. The test was over at four-fifteen, " Tom murmured. 

"Maybe Bud surfaced out at sea somewhere, " Arv Hanson suggested. 

Repeated radio calls brought no response. Tom, now seriously worried, took the seacopter down again for another search, hoping that Bud wouldhave switched off the antidetection gear by this time. But neithersonarscope nor listening devices revealed the slightest clue. 

Tom, Hank, and Arv exchanged fearful glances. Had the jetmarinefoundered on the ocean bottom--perhaps fouled somehow by Tom's newinvention? Or had Bud and his crew fallen victim to the enemy?

CHAPTER XIII

ENEMY FROGMEN

At the end of the test period, Bud had prepared to bring the jetmarineto the surface. But just as he was about to blow the ballast tanks, MelFlagler sang out a warning from the sonarscope. 

"Whoa! Hold it, skipper! I think we have company on the starboard beam!"

Bud jerked his head around in surprise. "You mean the _Sea Hound_?"

"No, she surfaced, " Mel reported. "Can't make this out yet, but it couldbe another sub. "

Bud turned the controls over to Zimby Cox. Then he rushed to the scopeand examined the blip. "Seems to be moving away from us on a westerlycourse. It's about two miles from here. "

He donned the hydrophone earset and listened. "It's no seacopter, nor ajetmarine either, " he announced presently. 

"A Navy sub, maybe?" suggested Zimby. 

Bud shrugged. "Let's find out. " He ordered a change of course, hard tothe right, and gunned the jets to bring the jetmarine directly on themystery object's trail. 

"It's a sub, all right, " he said a short time later, listening againover the hydrophones. 

"Pretty close to Fearing Island, isn't it?" put in Mel Flagler. "That'sa government-restricted area. "

Bud nodded grimly. "But staying just out of sonar range from the base. "

The jetmarine closed steadily on its quarry. In a few minutes they wereable to make it out dimly through the cabin window, dead ahead. 

"That's sure no U. S. Navy sub that I know of, " Bud said. "Probably anenemy snooper. "

"What if they spot us?" Zimby asked. 

Bud chuckled. "That's the beauty of it, pal! Don't forget. With this newantidetection gear we're invisible to them. At least as long as theydon't run into us or we into them, " he added. 

"Or unless they have superdetection equipment we don't know about, "cautioned Mel Flagler. 

As they talked, the unidentified submarine was bearing steadily towardthe mainland. Fathometer soundings showed it was on a steep upward slopeof the continental shelf. 

Presently a foaming gush of bubbles showed that the sub ahead wasblowing its tanks. The jetmarine followed as it surfaced and Bud hastilymanned the periscope. 

"What're they up to?" Mel asked tensely. 

"Don't know yet, but the hatch is opening, " Bud reported. Suddenly hegave an excited gasp. "Jumpin' jets! They're sending out a couple offrogmen!"

Bud's companions were electrified by the news. 

"Spies!" Zimby exclaimed. 

"What do we do now?" piped up Mack Avery, the third man in Bud's crew. "Hadn't we better radio the Coast Guard and the FBI?"

Bud wrenched away from the eyepiece. "I have another idea! Any of youfellows game to go with me and capture those spies?"

All three of his companions volunteered eagerly. Bud chose Mel Flagler, then took another sight through the periscope. 

"The sub's submerging again, " he reported. "That'll give us a clearfield. Zimby, you and Mack keep an eye on that baby while we're gone, and be plenty careful she doesn't spot you!"

"Roger! And take this roll of wire to tie up your prisoners. "

Hastily Bud and Mel changed into swimming trunks and donned hydrolungs. They went out through the air lock, plunged into the bracing salt water, and switched on their ion-drive units. 

"Can you see 'em?" Mel asked over his mike. 

"Not yet. Let's speed up before we lose 'em completely!"

Both pushed their ion drives to capacity, scanning the water ahead inall directions. 

"There they are!" Bud exclaimed presently. He pointed to two tinyfigures, barely visible in the distance. 

"Wow! They're sure not wasting any time!" Mel muttered. "Let's step onit, Bud! They'll be ashore in a minute!"

A darting school of sea bass screened the figures briefly from view. Asthe fish flickered past, Mel and Bud saw the frogmen breast-stroke uptoward the surface and break water. 

Bud and Mel followed. Ahead lay a barren stretch of beach, humped withsand dunes. It was skirted beyond by a thick fringe of trees. 

"They certainly picked a perfect spot for a sneak landing!" Bud thought. The beach seemed totally deserted, with no sign of human habitation. 

By this time, the frogmen were scrambling ashore. Within moments, Budand Mel were on their heels. The raiders whirled in dismay as theycaught the sound of footsteps rushing up behind them through the sand. 

Bud and Mel hurled themselves forward, each dropping a man with a flyingtackle. All four went down in a struggling, kicking tangle of arms andlegs. 

The battle was rough but short. Bud and Mel had the advantage ofsurprise, and soon pommeled and grappled their foes into submission. 

Bud, astride his opponent's chest with knees pinning the man's arms, unlooped from his belt the wire he had brought. 

"Here! Take some of this and wire your man's wrists together!" Bud toldMel. 

When the frogmen were safely bound, Bud and Mel allowed them to standup. Neither captive tried to escape. 

"Now, my sneaky friends, talk!" Bud snapped. "What kind of a sightseeingtrip did you plan?"

The frogmen's jaws remained tightly clamped. Both looked flushed andsullen as they faced their captors. 

"Got their lips zipped, I guess, " Mel said disgustedly. 

Bud decided to try another tack. "Doesn't matter, " he said carelessly. "We know they're pals of the Mirovs. "

Both men started as if they had been stung. Bud followed up quickly, hoping to prod them into some unguarded remark. 

"Just as we thought!" he snarled. "A couple of low-down Brungarianrebels! And up to their usual amateurish spy stunts!"

The raiders' eyes blazed, but they maintained silence. Both, however, kept darting looks of keen interest at the Americans' hydrolung gear. 

Just as Bud was wondering how he could get the prisoners to the nearestpolice headquarters, a jeep came bouncing into view across the sand. 

"Hey! Police!" Mel exclaimed with a happy grin. 

"We're in luck, " Bud said. "They can take these creeps off our hands. "

The jeep braked to a halt a few yards away, and two uniformed officershopped out. 

"What's going on here?" said one, who was wearing a sergeant's stripes. The jeep had the words BEACH PATROL stenciled on it in white paint. 

"We just nailed these two Brungarian frogmen, " Bud explained. "A sub putthem ashore--probably as spies or saboteurs. They won't talk to us, butmaybe you can pump them at headquarters. "

The startled sergeant turned a cold eye on the two prisoners. "Gotanything to say for yourselves?" When neither answered, he unholsteredhis revolver and covered them. "Better take off those wires and putbracelets on them, Mike, " he told his fellow officer. 

The frogmen were handcuffed with cool efficiency and bundled into thejeep. Meanwhile, the sergeant turned back to Bud and Mel. 

"You fellows come along too, " he ordered. 

"But we haven't got time, " Bud protested. "Our own sub's waiting rightoffshore and we want to tail the sub that brought those guys here!We're from the Swift rocket base. "

"Any identification?" the sergeant asked. 

"How _could_ we have in this getup?" Mel retorted. 

"That's what I thought. So get moving, " the sergeant barked. 

Reluctantly, Bud and Mel hopped onto the running board and clung to thebouncing jeep as it sped to the nearby town of Sandbank. At headquartersthey were questioned by the local police chief. 

"If you'll call Swift Enterprises at Shopton, sir, Mr. Swift--or HarlanAmes of the plant security department--will vouch for us, " Bud said. 

The chief picked up the telephone and soon had Mr. Swift on the line. After speaking to him briefly, he passed the phone to Bud so thescientist could identify his voice. 

"That's Bud Barclay, all right. He's one of our most trusted employees, "Mr. Swift told the chief after hearing Bud's story. 

The officer promised to release Mel and Bud at once. Before doing so, however, he took them into the adjoining office where the two frogmenwere being questioned. 

"Any luck?" the chief asked the sergeant. 

Sergeant Gryce shook his head in disgust. "Not much. They did admit theycame in a sub, but they claim it didn't wait to pick them up. "

The police chief shot a few questions of his own at the men, but theyanswered either in curt monosyllables or not at all. 

"Look, sir, " Bud put in, "if they're telling the truth about their subnot waiting, our jetmarine may have chased it. That means Mel and I arestranded here. Could you have your men wait for us on the beach till wefind out?"

"Gladly, " the chief replied. "You two have done a fine day's work. "

After the prisoners had been locked up to be handed over to the FBI, thetwo Beach Patrol officers drove Bud and Mel back to the area where theyhad landed. Just as the jeep turned down the dirt road leading to theshore, Bud's keen eyes spotted a lurking figure in the distance. 

"Stop, please!" Bud said, tapping the driver on the shoulder. 

As the jeep halted, Bud pointed toward the beach. A man was crouchingbehind a sand dune, with a large fish basket beside him. The sergeant, puzzled, took out a pair of binoculars to study the situation. Fortunately, the jeep was still screened by trees, and the crouching manevidently did not realize he had been seen. 

"What's in the basket?" Bud asked. "Could it be clothes?"

"Sure looks like it, " the sergeant said, passing over the binoculars. 

After a brief look, Bud explained the hunch that had occurred to him. "I'll bet that guy's waiting with clothes for the frogmen. He probablygot here late and doesn't realize they've been nabbed!"

"Well, he'll soon find out, " the police driver said grimly. He was aboutto start up the jeep when Bud stopped him again. 

"Wait! You have no proof that's what he's here for, " Bud pointed out. 

The pilot suggested that the police keep out of sight while he and Melapproached the man in their swimming gear. "If that stranger takes thebait, we'll really have the goods on him!" Bud concluded. 

"Smart idea, son, " the sergeant said with a dry chuckle. "Go to it!"

Bud and Mel circled widely through the trees, took a quick dip in thewater, then approached along the beach as if they had just landed andwere searching for someone. 

To their delight, the man rose from behind the sand dune and hailedthem. Bud and Mel hurried over to him. 

"You have clothes for us?" Bud asked. "We just came ashore from thesub!"

"Yeah, right here, " the man said in English with no trace of an accent. "Thought I'd missed you. "

"Thanks, pal--that's all we want to know!"

The man gaped in comic dismay as Bud pounced on him and pinned him tothe ground. Moments later, the two police officers rushed up andhandcuffed him. 

[Illustration]

"Hey! What's the big idea?" the man stammered. "I ain't done nothing. Just got a phone call this morning, offering me fifty bucks to bring twosets of clothes down to the beach at five o'clock for a couple ofdivers. "

"Tell that to the FBI!" snapped the sergeant. 

When the officers had departed with their new prisoner, Bud and Mel, both grinning, dived into the surf and headed out to sea. 

In a few minutes they were sure they were at the right spot to meet thejetmarine. But it was gone!

CHAPTER XIV

A PROPAGANDA BLITZ

As the _Sea Hound_ returned to Fearing Island from its search for Bud'sjetmarine, Tom was beside himself with worry. Had his experiment costthe lives of his best friend and the other crewmen aboard?

"I'll never forgive myself if anything's happened to them!" Tom mutteredbleakly. 

Hank Sterling squeezed the young inventor's arm. "You know Bud's highspirits, skipper, " he said. "He may have taken off on some crazy lark. "

"Sure! A whale hunt, maybe!" Arv Hanson wisecracked, trying to lightenthe gloom. 

Tom forced a grin, but he remained heavy-hearted as they neared thebase. His only hope now was that a radio message from the jetmarinemight have been picked up while they were gone. 

As soon as the seacopter was moored, Tom leaped ashore. The crewmen onthe docks had no news to report, so Tom piled into a jeep with Arv andsped off to the Fearing communications center. Hank remained aboard the_Sea Hound_ to secure all gear. 

Churning along the graveled road, Tom and Arv passed the launching area. Huge, needle-nosed cargo rockets and the mighty spaceship _Titan_ loomedagainst the sky. Tom's moon-voyaging _Challenger_ and his more recentspace craft the _Cosmic Sailer_ were also based there. 

"Going to alert the Navy for a search?" Arv inquired as they reached thecommunications building. 

Tom nodded and braked the jeep to a screeching halt. "Coast Guard too. They can pass the word to commercial shipping to be on the lookout. "

A telephone rang as he hurried into the office. 

"For you, " the clerk said, looking up at Tom. "Nice timing!"

Tom grabbed the phone. His face widened into a grin. "Bud! You seagoingjet stream! What happened?"

Arv grinned, too, in relief. 

"Your antidetection gear worked so well we vanished right out of theocean!" Bud replied with a chuckle. Turning serious, he reported how hisjetmarine had trailed the mysterious intruder and how he and Mel hadcaptured the two Brungarian frogmen and their shore contact. 

"Nice going, pal!" Tom exclaimed. 

"But here's the catch, " Bud went on. "When we took off again in ourhydrolungs to go back aboard ship, the jetmarine was gone!"

"Maybe she's trailing the enemy sub, " Tom conjectured. 

"That's what I'm hoping, " Bud said uneasily. "Trouble is, our subsaren't armed, and who knows about that Brungarian job? The way theysling missiles around, anything could happen if she spots thejetmarine. "

Tom frowned. "I'll organize a search right away. Where are you callingfrom?"

"Police headquarters at Sandbank. "

"Okay. Take it easy, and I'll send a whirlybird to pick you up, " Tompromised. 

"And don't forget some clothes, " Bud added with a chuckle. "Mel and Iare getting chilly. "

"Right!" Tom hung up and gave Arv Hanson a quick briefing. 

Then he phoned the base airfield to dispatch a helicopter. He alsocontacted the nearest Coast Guard station and put through along-distance call to Navy Headquarters in Washington to request help insearching for the jetmarine. Finally he and Arv headed back to thesubmarine docks in the jeep. 

A flurry of activity followed as Tom detailed ships for the search androunded up crews. He was interrupted by a phone call in the loadingshed. It was the control-tower operator. 

"One of our drone planes has spotted a sub approaching, skipper, " theoperator reported. 

"What bearing?" Tom demanded excitedly. 

"One-seven-six. " Tom was about to hang up and grab a pair of binocularswhen the operator added hastily, "Wait! It's responding to our radiochallenge!. .. That's ours, all right!"

Tom dashed out of the shed and scanned the sea to the southward. Sureenough, a jetmarine had surfaced and was speeding toward the sub docks. Minutes later, Tom was shaking hands warmly with Zimby Cox and MackAvery. 

"Is Bud okay?" was Zimby's first question. 

"Right! I just heard from him, " Tom replied. "He and Mel captured thoseenemy frogmen and a copter's on the way to pick them up. What happenedto you fellows?"

Zimby confirmed Bud's guess that they had taken off in pursuit of theenemy craft. 

"We figured Bud and Mel could make out on their own, " Zimby explained. "And we thought the sub's course or actions might tip us off to itsnationality. Also, if it tried any sabotage or mine-planting, we couldradio the Navy. "

Instead, Cox went on, the mysterious craft had proceeded to a pointabout ten miles offshore where it rendezvoused with another submarine. 

"And get this, skipper!" Mack Avery put in. "The other sub wasundetectable! We were close enough to get a peek at it, but we couldn'tping it on the sonarscope. "

"That figures, " Tom said grimly. "Those frogmen were apparentlyBrungarians. "

Zimby Cox related that a man had transferred from the undetectablesubmarine to the one they had been following. The first sub had thenheaded out to sea, as if to cross the ocean back to its home base. Theother had departed on a course toward the South Atlantic. 

"Probably back to the lost missile area. At least that's the way wefigured it, " Zimby added. 

"And neither sub spotted you?" Tom questioned. 

Zimby grinned wryly. "We might not be here if they _had_ detected us. But I'm pretty sure they didn't. Anyhow, they gave no sign. "

Tom was doubly elated at the news. His antidetection gear had evidentlyworked perfectly in a showdown test with the enemy, even at close range. Moreover, if the second sub was returning to the South Atlantic, itseemed likely that the enemy, too, had not yet located the preciousmissile with its data from Jupiter. 

"You guys rate Navy medals, " Tom told Zimby and Mack jubilantly. "Comeon back to Shopton with me and I'll buy you the juiciest steaks intown!"

Before leaving the base, Tom called the Coast Guard and the Navy tocancel his search request. He also telephoned a full report on the enemysubmarines to Admiral Walter. 

After hanging up, Tom decided on another move. "Our antidetection gearseems to have panned out pretty well, " he told Hank. "I think we shouldmake use of it right away. By sending that jetmarine to the SouthAtlantic, we might get a line on enemy activities down there. "

Hank was in favor of the idea. He volunteered to prepare the jetmarinefor a cruise and take off from Fearing that very night. 

"Thanks, " Tom said with a parting handshake. "Keep us posted if youlearn anything. "

Meanwhile, Bud and Mel Flagler had arrived at the base by helicopter. They and their two shipmates flew back to the mainland with Tom and Arvfor a celebration dinner in town. 

The next morning found the young inventor hard at work in his privatelaboratory. He was tapping his head with his slide rule and frowning ata blackboard scrawled with equations when Bud dropped in for a visit. 

"What now, inventor boy?" his copilot asked. "Don't you ever give thatbrain of yours a rest?"

"Oh, hi, Bud!" Tom looked around absent-mindedly. "I'm just trying tofigure out a way to crack the Brungarians' antisonar system. "

"Good night!" Bud sank down on a lab stool. "You've come up with a wayto make our own subs undetectable. Isn't that enough?"

Tom shook his head. "Not if we want to keep track of those sneaks. And Ithink I see a way to do it. "

"How?"

"So far, I have been thinking about refining our own search sonar. " Tomexplained that the new system he had in mind would send out a _complex_pulse--that is, an underwater sound wave with many harmonics instead ofa single tone, sharp-peaked sound impulse. 

"This will make it less likely that their antidetection gear will absorball of it, " Tom went on. "What's not absorbed will return as an echo. I'm also going to modify our receivers. But I've still not worked thatout. "

Bud nodded, his forehead puckered in a look of concentration. "So--?"

"So our sonar picks up all that hash, and by means of a computer setupfilters out the sub's real echo from the shadow reflections. "

"Hey! Sounds pretty cute, " Bud said. 

Tom broke into a dry chuckle. "Right--_if_ I can do it. " After that job, Tom added, he hoped to adapt his own antidetection methods to makehydrolung wearers safe from underwater detection. "And if the Jupiterprober hasn't been found by that time, Bud, I'm going to request theNavy to let us take over the search alone. "

Bud gave a whistle of excitement at the possibility of new underseaadventures ahead. "Count me in, pal!"

The two boys broke off their conversation a short time later and wentback to the Administration Building for lunch with Tom's father. 

Mr. Swift greeted them with a smile as they entered the big doubleoffice. "Glad you could join me, boys! Chow's laid out quite a feast forus today. "

Three places had been set at the conference table, and an appetizingrepast of sizzling ham and sweet potatoes waited in covered dishes on alunch cart nearby. 

"Mmm!" Bud inhaled the aroma. "Good chow from good old Chow!"

Tom switched on the videophone screen to a private channel to catch thenoon news while they ate. The newscaster wore a look of excitement as hespoke without pausing for the usual commercial. 

"The Brungarian government has just scored a propaganda bombshell!" hereported. "In a news announcement released less than half an hour ago, they stated that their Navy has perfected an _undetectable submarine_!"

The Swifts and Bud froze, openmouthed, at the newscaster's words. 

"No need to tell you what this could mean to American security, " he wenton. "If enemy subs slipped through our continental defenses, theirmissiles could devastate the United States with scarcely an instant'swarning! The whole country's been rocked by the announcement. Anofficial comment by our Defense Department is expected at any moment. "

"Sufferin' satellites!" Bud gulped. 

Mr. Swift nodded. "It's a great propaganda stroke. But I wonder whythey've chosen to reveal their secret at this time. "

Tom said thoughtfully, "Dad, do you suppose they've realized the factthat we _know_ about their antisonar gear?"

"Could be, son. They may figure that since the secret is out already, they may as well play it up for all it's worth. " The elder scientistpaused and frowned. "Or it might be intended to force our hand. "

"You mean in hopes of getting us to reveal whether or not we have anantidetection system ourselves?" As his father nodded, Tom scowled. "Ifso, that sub yesterday may have been observing our tests. "

The telephone rang and Tom leaped to answer it. The caller was DanPerkins of the _Shopton Evening Bulletin_. 

"You can guess why I'm calling, Tom, " the editor said. "How about astatement from you Swifts on this Brungarian sub story?"

"We found it very interesting, " Tom said politely but noncommittally. Parrying further questions, he hung up as soon as possible. 

Mr. Swift approved Tom's policy of silence. Almost immediately the phonebegan ringing again with a succession of calls from other newspapers andwire services. Tom dashed off a brief, general statement and instructedMiss Trent to give it to all further callers. 

"Maybe this is a good time to make a private announcement to youfellows, " Mr. Swift said to the two boys, his eyes twinkling. "Do yourecall my telling you that Doc Simpson had isolated an unknown vitaminfrom the space plants? Well, we've now discovered that this vitamin cancondition the human body to stay under water indefinitely. Doc isputting some up in capsule form. "

Both Tom and Bud gave whoops of glee at this news. 

"Dad, you've helped overcome one of the big problems in our search forthe lost missile!" Tom exclaimed. 

CHAPTER XV

MOUNTAIN HIKE

"After adequate doses of your space vitamin, Dad, a skin diver couldtackle almost any undersea job in my hydrolung!" Tom exclaimed. "Hewouldn't be subjected to any antiosmosis troubles with his bodytissues. "

His father nodded. "For the first time, man might become a _truly marinecreature_!"

"Wow! Think of it!" Bud gasped excitedly. "With Tom's hydrolung and aknife to hunt his own food, he could practically live in the sea!"

"That's no farfetched dream, Bud. " Tom's steel-blue eyes flashed at thethought of new fields of scientific conquest. "This discovery of Dad'sand Doc Simpson's opens up some really amazing possibilities. "

Most important at the moment, the vitamin would be a great boon incarrying out search and digging operations for the Jupiter prober. Withfresh enthusiasm, Tom returned to his laboratory to work on the newsonar gear. In his own mind, he had already named it a "quality analyzersonar, " since that exactly described the way it would function. 

"Hmm, let's see, " Tom mused as he settled down at his workbench, pencilin hand. "Besides a regular sonarscope, I'll need at least three unitsfor the gear. "

First, he would need an oscillator to produce the complex pulse. Next, of course, an oscilloscope to check the pulse as it was beamed out. Last--but highly important--a correlation calculator. 

This latter unit would compare the original pulse with the returningechoes. If an echo had a high enough "standard of acceptance"--that is, if its quality was very near the original pulse, it would show up on thescreen in the normal way. If the echo came back blurred, or if "shadowechoes" showed up, these would be separated and appear on the screencolored red. 

"Whew!" Tom sighed as he realized the complicated job of circuit designthat lay ahead. "This sure is going to burn some midnight oil!"

The young inventor worked all afternoon at a furious pace, breaking offtoward dinnertime to telephone his mother that he would be stayingovernight at the lab. After a hasty meal, he resumed his layout job atthe drawing board and by midnight had finished designing his qualityanalyzer sonar. 

Whipping off his eyeshade, Tom went into the apartment next door andstretched out to snatch a few hours' sleep. But as usual when in themidst of an exciting new project, he was too keyed up to rest for long. 

Before daylight, Tom was back at his workbench ready to begin assemblingthe units of his new sonar gear. Later he phoned Chow but scarcelypaused to eat when the cook arrived with his order. 

"Brand my solar stovepipe!" Chow scolded. "Take time to eat your vittlesproperly, boss!"

"Hmm?. .. Oh, sure. " Tom looked up and grinned. 

The stout old Texan stomped out, shaking his head. 

As the morning wore on, the pace at which Tom had been working began totell on the young inventor. His head nodded again and again. Graduallyhe fell forward into an exhausted doze. 

The next thing Tom knew, he was sailing through the air, high aboveSwift Enterprises. Lake Carlopa was a tiny blue puddle below, and thetown of Shopton a mere cluster of toy buildings in the distance. 

"Good grief!" Tom exclaimed with a gulp. "What's keeping me up?"

He was floating freely, without the support of any aircraft--or even oneof his amazing force-ray repelatrons!

The discovery triggered off disaster. Like a character in a moviecartoon, now that he knew he had nothing to support him, Tom instantlywent plunging downward--down, down, straight into the lake!

_Splash!_

Tom gasped and shuddered and shook his head like a drenched terrier. 

_Another splash!_ As Tom brought his eyes into focus, he realized he wasback at his workbench in the laboratory. Chow was standing in front ofhim, holding a half-empty pail of water, ready to splash him again!

"Hey! Cut it out!" Tom cried out, jerking bolt upright. Then, as he sawthe disturbed look on Chow's face, Tom burst out laughing. "Okay. Relax, old-timer! Guess I was dreaming. "

"Brand my snake oil!" Chow said. "You looked so pale an' pasty, you hadme plumb scared, Tom! I couldn't wake you nohow!" Worriedly the cookadded, "What you need is a good beefsteak and some sunshine. You beenunder water too long. "

"In more ways than one!" Tom chuckled as he grabbed a towel and driedhimself off. 

The beefsteak, with crisp golden-brown French fried potatoes, wasalready at hand on Chow's lunch cart. Tom ate with a hearty appetiteand the stout chef went off, secretly plotting to arrange the secondhalf of his prescription. 

When he reached the galley, Chow plucked the wall phone off its hook andcalled Bud at an airfield hangar. After a brisk conversation, he hungup, grinning contentedly. 

At one o'clock Bud came bursting into Tom's laboratory. "Snap to, skipper!" he announced. "You have company!"

Tom looked up from his work in surprise. 

"_Ta-daaa!_" Bud sang out, imitating a trumpet flourish. 

Sandy and Phyl Newton marched in, smiling. 

"Boy, this _is_ a surprise!" Tom got up to greet them. "A mightypleasant one. But what's the occasion?"

"The occasion is that you're coming on a mountain hike with us, out inthe nice fresh air and sunshine!" Sandy informed him. 

"And please don't argue, " Phyl said with a giggle. "It's for your owngood--not to mention ours. "

"I suppose Chow Winkler put you up to this. " Tom grinned. 

"Never mind that, " Sandy said sternly. "Just come along quietly. It's abeautiful day. "

Tom glanced at his workbench cluttered with drawings and electronicgear. "Well, okay, since you're twisting my arm, " he agreed. "I guess itmight clear my brain at that. "

"Now you're talking. " Bud clapped Tom on the back and propelled himtoward the two girls, who promptly seized his arms before he mightchange his mind. 

On their way to the door, however, the telephone rang. Tom insisted uponanswering it, in spite of the girls' scolding. 

"Tom Swift Jr. Talking. "

"This is Chief Slater, Tom, " said the voice at the other end of theline. "Dimitri Mirov wants to see you. I don't know what's up, but hemight be ready to tell something worth while. Could you drop by?"

"Sure thing, Chief. Right away!" Tom hung up, excited by the thoughtthat the Brungarian might be about to reveal an important secret. "Mindstopping by police headquarters first?" he asked his friends. 

Minutes later, Bud's red convertible pulled up in front of the graystone building. Tom jumped out and dashed up the granite steps. 

"I've had Mirov transferred to a cell by himself, " Chief Slater said ashe took Tom back to see him. "Figured he might talk more freely awayfrom his pals. "

The prisoner, however, showed no eagerness to do so at Tom's arrival. Heremained slouched on his bunk as the young inventor pulled a chair up tothe cell bars. His only response was a slight curl of the lips. 

"Have you heard about my country's new submarine?" Mirov inquired afterChief Slater left. 

Tom nodded curtly. 

"When are _you_ going to build one?" Mirov prodded slyly. 

"Look!" Tom snapped. "You asked to see me. Here I am. What is it youwant?"

Mirov shrugged with a look of amusement. "To make a bargain with you, "he replied casually. "I know the secret of that sub. Get me and myfriends released and I'll give it to you. "

Tom had no intention of doing so, but he parried the offer, hoping todraw Mirov out further. The prisoner, however, would say nothing more. 

At last Tom gave up and rose to leave. "I'll think over yourproposition, " he said. 

He heard Mirov chuckle as he walked away. Somewhat puzzled, Tom reportedthe conversation to Chief Slater and also telephoned the plant to informAmes. 

Then he hurried back to the car. Bud frowned upon hearing Tom's story. 

"Do you think he's on the level?"

Tom shrugged as they headed out into the countryside. "I may be wrong, but the whole thing sounded fishy. "

"Now look!" Sandy said severely. "If we're going to enjoy this hike, we're _not_ going to talk about Brungarians or inventions or that lostmissile. From now on, it will cost anyone five cents every time hebreaks the rule!"

The boys chuckled and agreed. But agreeing proved easier than keepingthe rule. Again and again, either Tom or Bud would inadvertently drop aremark about their submarine experiments or the search in the SouthAtlantic. By the time they had parked in the hills and started climbing, Sandy's and Phyl's pockets were jingling with coins. 

"What are you going to do with it all?" Bud asked jokingly. 

"_Give it to us!_" snapped a strange voice. 

As the four young people turned with a start, they saw two men burstfrom the shrubbery just behind them. 

Both were holding guns!

CHAPTER XVI

THE GUNMAN'S SURPRISE

Sandy and Phyl were terrified by the sudden appearance of therough-looking pair with their drawn revolvers. Tom and Bud remainedcool, eying the men warily. 

"What's the big idea?" Tom asked. 

"Shut up and hoist your mitts!" the bigger of the men snarled. As theboys obeyed, he muttered to his partner, "Keep these two punks covered, Mugs, while I take their cash!"

"Right, Packy! I'll watch 'em!"

Sandy and Phyl emptied their pockets. Then Packy took the boys' walletsand change. 

"Now turn around and march!" Packy snapped. 

Bud took the lead, followed by the two girls, with Tom bringing up therear. They plodded up the brushy slope in silence for several minutes. Presently a weather-beaten cabin in a grove of trees came into view. 

"You intend to hold us there?" Tom asked. 

"You'll find out soon enough!" Packy answered. "We'll teach you tointerfere with the Mirovs!"

_The Mirovs!_ Like pieces of a jigsaw puzzle, the whole picture suddenlyfell into place. It was clear to Tom now how the capture had beenarranged. 

The call to the jail from Dimitri Mirov had been a hoax. Its purpose hadreally been to get Tom away from Enterprises--thus giving the two thugsa starting point from which to follow him. The mountain hike, organizedby Bud and the girls, had played right into their hands! As Tom sized upthe situation, seeking a way out, the group reached the cabin. 

"What are your terms for letting us go?" Tom asked their captors, stalling for time. 

The man named Packy gave an ugly chuckle. "None yet, " he said. "We mayjust decide to set the cabin on fire. "

Sandy uttered a gasp as his words sank home. Phyl Newton had turneddeathly pale. Packy now told his partner to unlock the cabin. Mugsstepped to the door. 

At that moment Tom caught Bud's eye. _It was now or never!_

Tom whirled and smashed a stiff handblow to Packy's wrist, knocking thegun from his hand. Bud hurled himself on Mugs. 

Taken off guard, the shorter thug staggered and went down under a hailof punches. Bud grabbed his wrist and twisted it mercilessly while hepinned him to the ground. 

Mugs screeched with pain. "C-c-cut it out!"

"Then drop your gun!" Bud snapped. 

Tom, meanwhile, had followed up his first advantage with a stunning blowto the solar plexus. Packy grunted for breath, then came back viciouslywith several well-aimed punches that staggered Tom. 

As the young inventor stumbled backward, Packy dived for his gun. Thoughstill groggy, Tom managed to kick the weapon out of reach. Before Packycould straighten up, Tom followed with a sweeping uppercut that caughthim squarely on the chin. 

Packy went down like a felled tree!

Tom picked up the gun before his groaning victim could recover. By thistime, Bud had pounded his own opponent into submission. Within a fewmoments, both thugs were lined up against the wall of the cabin. Theirwrists were tightly strapped behind them with their own belts. 

"Oh . .. Thank goodness!" Sandy gasped. 

Tom gave the girls a reassuring grin. "Are you two all right?"

"I g-guess so. " Phyl gave a nervous smile. 

Now that the tables were turned, it was the thugs' turn to "march. "The boys herded them warily back down the hillside toward the road, where Bud had parked his red convertible. Sandy and Phyl followed closebehind. 

[Illustration: _Tom and Bud hurled themselves at the thugs_]

Like all cars belonging to the Swifts' key personnel, Bud's was equippedwith a two-way shortwave radio. Tom switched it on and radioed ShoptonPolice Headquarters. Chief Slater promised to send a squad car at once. 

Minutes later, they heard it approaching. Two husky police officersleaped out as the car braked to a halt, and took charge of theprisoners. Scowling and sullen, they were driven off to jail. 

"Well, " said Bud jokingly, "what about that relaxing hike we werestarting?"

Phyl sighed. "I'm afraid you two boys just can't get away from_in_ventions and _ad_ventures. "

Sandy added, "I suggest we go home for a nice safe dinner. "

Later, at the Swifts' house, Tom received a telephone call from ChiefSlater. He reported that the two prisoners were known hoodlums from anearby city. 

"They claim they were hired for this job last night by a stranger whospoke with an accent, " Slater went on. "According to their story, theynever even got a look at his face, and they had no idea he was an enemyagent. "

"Sounds reasonable, " Tom agreed. "It's not likely Mirov's Brungarianhenchmen would endanger their whole setup by taking any cheap gunmeninto their confidence. "

Chief Slater also reported that Len Unger was still at large. "But theFBI will probably pick him up soon, " he added. 

"I sure hope so, " Tom said. 

A ten-hour sleep that night proved a fine tonic. Tom awoke the nextmorning feeling entirely refreshed, and after a hearty breakfast, hurried off to the plant. Here he plunged into work on his qualityanalyzer sonar. 

Much of the circuitry was assigned to the electronics department. Thefinished boards and sub-assemblies were fed back to Tom in his privatelaboratory. He himself assembled the major units. 

At lunchtime, over a bowl of chili and crackers, Tom recalled anotherproblem. "We'll need an undetectable sub to test my analyzer, " he mused. "That means a repeat job of rigging all those transducers. Whew! I'dbetter get busy on that plastic sheathing. "

As soon as he had eaten, Tom phoned Arv Hanson, who arrived at the labin a few moments. 

"You remember that idea I mentioned to Danny about molding all thetransducers into a single continuous plastic sheet?" As Arv nodded, Tomwent 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 asclosely together as possible, with minimum clearance, the plasticcoating could do an even better job of absorbing sonar pings than thehand-rigged model. 

"And the leads from all the transducers can be combined into a singleflat tape, " Tom ended. "That'll make it simple to hook up with theelectronic control unit inside. "

"Got it, skipper, " Arv said tersely. "It'll take overtime to set up thejob in the plastics department. But we ought to be rolling out thesheeting Tuesday. "

"That's swell, Arv! Thanks!"

By midmorning Tuesday, Tom had his quality analyzer sonar completed andwas showing Bud how the units worked. 

"Boy, it looks simple enough the way you explain it, prof!" Bud saidadmiringly. "How soon can we try it?"

"Depends on Arv, " Tom replied. He picked up the phone and called theplastics department. To his delight, the sheathing was already beingrolled out in quantity. Arv promised that by noon he would have enoughof it available to coat a jetmarine. 

"Nice going!" Tom said. "Shoot it out to the cargo-jet hangar as soon asit's ready!"

Soon after lunch, Tom, Bud, and Arv took off for Fearing Island. Whenthey arrived at the base, the plastic coating with its myriad tiny"mikes" and "speakers" was speedily applied to a jetmarine under Arv'ssupervision. Tom, meanwhile, wired the control unit and also installedthe analyzer sonar in the _Sea Hound_. 

"Want to be 'It' for another underwater game of hide-and-seek?" Tomasked Bud with a grin. 

"Sure, but don't tag me with a torpedo!"

Minutes later, the jetmarine slipped off into the depths with Bud andtwo other crewmen aboard. Tom and Arv followed in the seacopter. Thequality analyzer sonar worked even better than Tom had hoped. He notonly tracked the jetmarine on its outward course, but located it threedifferent times after shutting off the analyzer long enough for Bud toseek a new location. 

"How'd you like to relieve Hank in the South Atlantic?" Tom asked Budupon their return. 

Bud gave a whoop of excitement. "Roger!"

Tom slapped him on the back. "You can take off as soon as your ship'sprovisioned. I'll join you later--but first, " Tom added mysteriously, "Ihave another job to attend to. "

CHAPTER XVII

A MISSING AMULET

Bud's curiosity was instantly aroused. "Don't tell me you have a newtrick up your nautical sleeve to fox the Brungarians?"

Tom grinned. "That's the general idea. I hope to give hydrolung diversthe same protection that your jetmarine has. "

"You mean make them invisible to sonar?"

"Yes, " Tom replied, "and also give them personal spy gear to probe thewaters around them and spot an 'undetectable' enemy. "

Bud whistled. "Do that, and I'll say you're _really_ a magician, skipper!"

Tom himself transferred the analyzer from the _Sea Hound_ to Bud'sjetmarine. On a chance that it might become necessary to operate atgreater depths--either in searching for the lost missile or in shadowingthe enemy--Tom also assigned Arv Hanson the job of rigging the _SeaHound_ and another seacopter with his new inventions. 

Four crewmen volunteered for the cruise. When the jetmarine was ready, Tom and Bud exchanged tight handshakes. 

"Good luck!"

"Thanks, Tom. "

The young inventor waved as Bud disappeared down the hatch. As soon asthe craft had submerged, Tom went back to Shopton. That evening theSwifts were enjoying a quiet dinner at home when a loud, growling buzzshattered their mealtime conversation. 

"Oh!" Sandy gasped. "The burglar alarm!"

The Swifts' house and grounds were protected by a secret magnetic field. Any intruder breaking the barrier touched off the automatic alarmsystem. To avoid the buzzing, the family and their close friends worewrist watches containing tiny neutralizer coils. 

"I'll see who it is, " Tom said, and hurried to the door, feeling atwinge of apprehension. 

_Was this a new attempt by Brungarian agents?_

He switched on the porch light and peered out cautiously through theone-way glass pane in the door. A slim, hatless figure in a dark suitwas just coming up the steps. Tom gave a smile of relief. 

It was Harlan Ames!

"Hi, Harlan!" Tom opened the door before Ames had a chance to ring thedoorbell. "We heard you coming!"

The security chief was startled when he realized he had activated thealarm system. 

"That's strange, " he said uneasily. "Tom, I wonder if--"

He broke off to dart a quick glance at his wrist. Then his face relaxedinto a look of chagrin. 

"Great! I forgot my wrist watch!" he murmured. "Haven't visited yourhouse in so long I neglected to wear it. "

The other Swifts smiled in amused relief, and Mrs. Swift invited him tojoin them for dessert. Ames, however, declined politely. 

"Thank you, but I just finished dinner myself, " he explained. "I droppedby to--"

Once again Ames's voice trailed off in midsentence, as he reached intothe side pocket of his coat. 

"My amulet!" he gasped. "It's gone!"

"Are you sure?" Tom said with quick concern. 

Ames nodded as he frantically tried all his other pockets. Theelectronic amulet to which he referred had been issued to allEnterprises personnel and family visitors who used the private gate. Theamulets were contained in slender bracelets and were designed to trapradar impulses. This prevented them from showing up as blips on thegiant detector radarscope mounted on the main building. The purpose ofthe scope was to reveal unauthorized visitors or spies. 

"My bracelet broke this afternoon, " Ames said. "I slipped it into mypocket to have it repaired. But it's not there now!"

Tom grabbed a flashlight and dashed outside for a hasty check of thewalk. Ames followed, to look inside his black sedan. But the amulet didnot come to light. 

"Did you go home after you left the plant today?" Tom asked. 

Ames shook his head worriedly. "No, I stopped at a restaurant. Mind if Iuse your phone?"

"Go ahead. "

The security chief called Enterprises and asked his assistant, PhilRadnor, who was on night duty, to make a thorough search. While awaitingthe results, Ames also called the restaurant, but learned that no suchitem had been turned in. 

Half an hour later Radnor called back to report no luck. "The amulet mayshow up yet, Harlan, " he said. "But I'll alert the guards at the plantto be on the lookout for an unauthorized visitor. "

"Thanks, Phil. " Ames hung up and turned away from the telephone with anembarrassed look. "Fine example I'm setting as head of plant security, "he murmured. "Let's hope the amulet wasn't stolen. "

Excusing themselves from Mrs. Swift and Sandy, Tom Sr. And Jr. Retiredwith Ames to Mr. Swift's study to discuss the news he had brought. 

"I had a late call from Admiral Walter this evening, " Ames explained. "The Navy's getting pretty desperate over that lost missile. They'reready to co-operate with any moves you care to make. I take it you'reprepared to carry out a search on your own, Tom?"

The young inventor nodded. "Yes, as soon as I've perfected all the gearI'll need--which won't be long, I hope. "

Ames added, unhappily, that certain papers and news commentators hadbeen making snide remarks about the Swifts' failure to match theBrungarians' submarine achievement. 

"I think Tom has that situation pretty well in hand, " Mr. Swift remarkedwith a smile. 

Tom gave Ames a full report on his own apparatus for rendering asubmarine invisible to underwater detection. Ames grinned at the news. The grin grew even wider as he heard of the successful test of thequality analyzer sonar. 

"Bud Barclay's on his way to the South Atlantic right now with a fullyequipped jetmarine, " Tom ended. 

The next morning he eagerly tackled the job of adding sonar protectionand sonar detection features to his electronic hydrolung. What anamazing fish man the wearer would be, Tom thought, if his projectsucceeded!

It would enable a skin diver to operate indefinitely under water atjet-propelled speed--invisible to enemy "eyes, " yet be able to spy outany hostile undersea prowlers, including supposedly "undetectable"submarines!

Tom chuckled wryly as he mulled over the difficulties ahead. "Bud wasn'tkidding when he said it would take a magician!"

Besides his mask, electronic breathing device, density-control unit, andion drive, the wearer would now need at least three majoradditions--first, sonar-blinding equipment with electronic control;second, amplifying equipment to camouflage the wearer's noise underwater; and, third, a portable quality analyzer sonar. 

"Whew! The miniaturizing job alone will be a king-sized headache!" Tomsaid to himself. "I'd better start with a skin-diving suit made of thatmolded plastic Arv is turning out. "

After having some of the Tomasite sheathing, with its embeddingtransducers, sent over from the plastics department, Tom cut out a suitfrom a pattern and welded the seams electronically. He had just finishedwiring the control unit when Chow wheeled in a lunch cart. 

"Got some _dee_licious steak-and-kidney pie today, " the cook announced, setting it out. 

"Swell, " Tom said absent-mindedly. 

Chow frowned but left without interrupting the young inventor. Twentyminutes later the cook poked his head into the laboratory again. Tom hadnot yet touched his lunch. 

"Brand my vitaminnies, start eatin', boss!"

"Sure, Chow. "

By this time, however, Tom had become so absorbed in the task ofassembling some tiny monolithic blocks for the computer circuits of hisanalyzer, that the lunch remained untasted. When Chow returned a thirdtime, Tom was startled by his bellow:

"Get your nose out o' that work, buckaroo, and _eat_!"

Realizing Tom's pie had cooled off, Chow had brought another serving, hot from the oven. Seeing the stern look on the Texan's face, Tom burstout laughing and obeyed meekly. 

"I declare!" Chow chuckled. "One o' these days I'll have to force-feedyou if you won't pay no mind to your own nourishment!"

"Sorry, old-timer. " Tom smiled. "Sometimes I do get a bit wrapped up, Iguess. "

Hour after hour, Tom stayed glued to his workbench, sometimes busy withdelicate electronic gear, sometimes lost in thought as he pondered atricky problem in circuit design. It was long after dark when he drovehome from the experimental station, yet he was back on the job in hislaboratory early the next morning. 

By lunchtime Tom had all the apparatus assembled. He was just trying onthe plastic suit, with all its accompanying paraphernalia, when Chowmade his usual appearance. 

"Great sufferin' snakes!" the cook gasped. "You ain't goin' divin' in_that_ getup, I hope! You look like a Christmas tree, boss!"

Tom nodded glumly. "Know something, Chow? That's just what I wasthinking myself. "

The young inventor's suit was loaded down with the various electronicunits and festooned with wires. Even taking a few steps around the labconvinced Tom that the design was too unwieldy. 

"I'd probably either get tangled in seaweed or sink from sheer weight, "he muttered. 

Changing back to his slacks and T shirt, Tom began eating abstractedlyas Chow hovered around. 

"If fishes could talk, I reckon you'd scare 'em half to death in thatrig!" Chow said, trying to cheer Tom. 

"Fish do talk, " the young inventor said. "At least they make noises. Don't you remember that emergency fish-talk code we used when we were--"

Suddenly Tom paused, his mouth dropping open. "_Chow!_ You've justsolved my problem!" he exclaimed. 

"I have?" Chow goggled at the young inventor. 

"You sure have!" Tom bounced off his stool and began pacing about. "Now, take porpoises. They utter all sorts of sounds--grunts, squeals, jawclaps--and one particularly characteristic sound, like the grating ofa rusty hinge. "

Chow scratched his chin uncertainly. "Wal, what about it?"

"Suppose I used that rusty-hinge noise to mask the diver's noise. " Tomturned and stabbed the air with his finger. "I could _also_ use thatsame sound output as the search pulse for my quality analyzer sonar!"

In this way, Tom explained, he could eliminate part of his bulkyequipment and do an even better job of making the diver "invisible. "

Bubbling with enthusiasm, Tom decided to buy a live porpoise at once andmake an exact recording of its sounds. As soon as he had finished lunch, he put in a number of calls to suppliers of marine specimens. But nonecould provide a porpoise on short notice. 

"Guess I'll have to catch one myself!" Tom told Chow. 

He drove out to the airfield and took off in a Whirling Duck for FearingIsland. At the base, both Mel Flagler and Zimby Cox were eager toaccompany the young inventor when he told them about the trip he had inmind. 

Tom chose the _Sea Hound_ as the fastest and best suited craft for hispurpose. With Mel's and Zimby's help, he quickly rigged a plastic "tank"in the stern cabin. Minutes later, the seacopter zoomed skyward, headingfor the Florida Keys. 

The flight was a short one at transonic speed. Tom chose a sparklingstretch of open water, a mile or so offshore from a palm-green islet. Zimby agreed to stay aboard and tend ship while Tom and Mel went overthe side in hydrolungs. 

The two glided about in the translucent blue depths, keeping in closerange of each other. The sea was alive with shimmering fish of everyhue, darting among the coral. Suddenly, as Tom veered around to rejoinMel, his eyes widened in horror. 

A vicious-looking hammerhead shark was zeroing in, directly behind hisfriend!

"Look out!" Tom yelled over his microphone. 

CHAPTER XVIII

SMILEY THE SEA COW

Mel turned in the nick of time. The monster shark was bearing down onhim like an undersea express train. Overcoming a moment of panic, Melgunned his ion drive to dodge the attack. 

As Tom watched in agonized suspense, he saw the shark's jaws open andshut in a lightning snap at Mel's outstretched arm. Its razor-sharpteeth missed their target by inches!

Mel's gasp of relief was audible over Tom's earphones. "Let's get out ofhere!" he cried, arrowing away from the man-killer. 

Suddenly Tom realized the full extent of their peril. A long, sweepingcoral reef, which extended above water, lay between them and the _SeaHound_. Unless they could round the reef in time, the shark had themtrapped!

"Quick! This way!" Tom exclaimed. 

The shark was moving at blinding speed. As if sensing the boys' plan ofescape, it launched itself in a wide curving sweep to cut them off. 

"We can't make it!" Tom gasped. "We'll have to fight!"

Both swimmers were armed with skin diver's knives as a precaution. Thetwo maneuvered to meet the killer's onslaught. 

This time its broad nightmarish head was aiming straight at Tom. Hejetted off to the right, but the monster veered instantly. Its lashingtail gave Mel a stunning blow. 

As the shark's jaws gaped for a bite, Tom zoomed underneath theman-eater and slashed its belly with his knife. 

The shark, maddened, thrashed the water in a frenzy. Tom moved likelightning to dodge a deadly blow from its bony tail. Again and againthey felt the horrifying brush of the killer's fins or armor-tough hide. By this time, Mel had revived. Repeatedly the two boys dived to jab andslash at the shark's soft underbelly. 

Both were nearly exhausted when the monster at last went limp andfloated slowly up toward the surface. Pale with shock and fright, Tomand Mel jetted back to the _Sea Hound_. 

Zimby Cox was startled by their faces when they clambered aboard andripped off their masks. "What happened to you two?"

Tom told him. "Good night!" Zimby cried out. 

After resting, Tom and Mel dived in again. This time luck was withthem. In less than twenty minutes they sighted a small porpoise. 

"Think we can lure it back toward the _Sea Hound_?" Mel queried. 

"We'll try, " Tom replied. 

The creature with the bottle-shaped snout was as friendly and playful asmost of its fellow dolphins. Too playful, Tom concluded, after vainlytrying to tease it into chasing them. Instead of following, it would"tag" Tom or Mel quickly, then swim away, evidently expecting to bechased in turn!

"I give up!" Mel snorted in disgust. 

Tom grinned and bobbed to the surface. He waved his hand several timesin a prearranged signal. Zimby at last spotted him and brought the _SeaHound_ to the scene. 

Raising his mask, Tom called, "Let's have the net!"

Zimby lowered a nylon net and some pieces of fish to the two swimmers asthey came alongside. With the food as bait they tried to lure theporpoise to the seacopter. But just as they thought they had it, themonster would scoot off. 

"It's just laughing at us!" Mel fumed. 

At last, after winning its confidence with several bits of fish, theboys succeeded in snaring the porpoise. Tom clambered onto the _SeaHound_'s deck and helped Zimby haul their catch aboard. "Quacking"reproachfully, it was lowered through the hatch and placed in the tank, which was then pumped full of salt water. 

As soon as the _Sea Hound_ arrived at Fearing, Tom phoned Chow Winklerat Enterprises and asked him to fly out to the base. 

"Pardner, how'd you like to ride herd on this critter and gentle it downfor me?" Tom asked, when he showed Chow the porpoise. 

It had been transferred to a huge, glass-paneled tank which had been setup just outside Tom's Fearing Island laboratory during his flight to theFlorida Keys. 

"Reckon I kin try makin' friends with it, " Chow declared. 

The porpoise stared morosely at Chow. The kindly old Texan's heart wastouched by the odd creature. To his delight, it soon responded to hisfriendly overtures and quickly recovered its good nature. By the nextmorning the porpoise was playing catch with Chow, or else swimming overto have its back scratched. The cook named it Smiley. 

"She's kind of a sea cow, " he told Tom, "and you got to talk to mySmiley like any cow!" Tom grinned and refrained from explaining to Chowthat a real "sea cow" was a walrus. 

Meanwhile, the young inventor was busy with his own experiments. Bymeans of a microphone placed in the tank, he made exact recordings ofSmiley's "talk. " Using Mel Flagler as a subject, Tom also tape-recordedthe sound of a skin diver propelled through the water by ion drive. 

The next step was to compare the sound pattern of the tapes. Tomfiltered out the difference in the two sounds with the correlationcalculator unit of one of his quality analyzer sonars. 

"Uh-huh. So you got the difference betwixt Smiley's talk an' the noiseMel made, " muttered Chow as he watched the jagged lines of lightflashing on the pulse-check oscilloscope. "Now what're you fixin' to dowith it?"

"This will be fed into the diver's sonar along with his own noiseoutput, " Tom said, "to make him sound like a porpoise. "

Chow howled. "That I've got to hear!"

The young inventor worked feverishly throughout the day and into thenext, perfecting his new "porpoise sonar. " Using microelectroniccomponents, he was able to reduce all the units to amazingly small size. 

Next, Tom began tailoring himself a completely new skin-diving suit. Mask, ion-drive jet, and the various hydrolung units were molded intothe plastic, with no loose wires or tubes showing. 

Monday morning he was ready to try the outfit. The sonarscope with itstiny viewing screen was strapped to his left forearm. Another small unitwas fastened to the inside of his wrist, with four plungers infinger-tip reach. 

"What in tarnation's that?" Chow asked. 

"Simplified controls, " Tom explained. "One's for breathing adjustment, one's for the density unit, one is my ion-drive 'throttle, ' and thislast is for the sonar pulse--which will duplicate the porpoise sound. "

The suit worked perfectly in a tank test. Chow was amazed as he listenedto Tom gliding about, via an underwater microphone. 

"If that don't beat all!" he declared. "Can't tell the difference 'twixtyou an' Smiley!"

As Tom emerged from the tank, the portly cook rolled up his own pantlegsand waddled up the metal ladder to the tank brim. He summoned theporpoise with a whistle and straddled its back. 

"What in the name of aquanautics do you think _you're_ doing?" Tomgasped. 

"I'll show you a real broncobustin' act in the water, " Chow bragged. 

Smiley glided off gently at first, Chow fanning the air with his hat andyipping like a rodeo star. He did, in fact, cling to his slippery perchwith considerable skill. 

But suddenly Smiley began bobbing and humping like an eel. Chow's facefroze in alarm. A moment later the porpoise dived and the cook let out ayell of terror, "He-e-elp!"

Roaring with laughter, Tom dived in and rescued him. "Guess he ain'tquite broke yet, pardner!"

"Reckon not. "

Now that Tom had all his technical problems solved, he plunged eagerlyinto the job of fitting out his expedition to the South Atlantic tosearch for the lost Jupiter missile. 

[Illustration]

Besides the _Sea Hound_ and the other diving seacopter which had alreadybeen rigged with antisonar and antidetection equipment, Tom ordered alarge cargo jetmarine to be similarly equipped. 

[Illustration]

Then he drew up a list of supplies and underwater search gear neededfor the missile hunt. Tom phoned orders to a dozen differentdepartments. Food, space-plant pills, extra clothing, tools, including amidget atomic earth blaster, grappling hooks--nothing was overlooked. 

"I'd better take along a Damonscope too, " Tom reflected. "Judging bythose Navy reports, ordinary Geiger counters haven't revealed anything. "

Tom's Damonscope, one of his early inventions, was a photographic devicewhich worked on fluorescent principles. It was amazingly sensitive toany form of radioactivity--and the missile, of course, would be "hot"from exposure to cosmic rays. 

Meanwhile, Tom had ordered his new hydrolung suit, with its four-plungercontrol unit and porpoise sonar, to be flown back to Enterprises. ArvHanson had promised to make up several duplicates with a team oftechnicians working on all-night shifts. 

Late the next afternoon Tom returned to the mainland to confer with hisfather. Mr. Swift reviewed the expedition plans with approval. 

"Suppose we call Admiral Walter now and set a time for the Navy to moveout of the missile area, so you can take over, " his father said. 

Tom agreed, and his father placed the long-distance call to Washington. Moments later, Admiral Walter came on the line. Mr. Swift talked to himbriefly, then turned the phone over to Tom, who described hispreparations for the missile hunt. A time schedule of operations andcommunications was quickly laid out. 

The admiral was amazed to learn that Bud Barclay was already patrollingthe area. "Our ships haven't seen or heard him!" the officer exclaimed. Suddenly Admiral Walter broke off. "Hold it, please, Tom! A code call isjust coming in!"

His voice was grave as he returned to the Swifts' line. "That messagewas from your friend, Bud Barclay, " Admiral Walter reported. "It looksas if our enemy has found the missile!"

"Oh, no!" Tom groaned. 

CHAPTER XIX

FLASH FROM THE DEPTHS

Tom was stunned by the news. "There's no chance of a mistake?"

"Judge for yourself, " Admiral Walter replied. He read the message:

 HAVE JUST SIGHTED ENEMY CRAFT DREDGING OUT METAL OBJECT

Tom repeated the information to his father. Both Swifts were silent fora moment, exchanging dejected looks. Then Mr. Swift remarked evenly:

"The game's never lost till it's over, son. "

"You're right, Dad!" Tom exclaimed. Turning back to the telephone, hesaid, "Admiral, I'm not quitting. We'll take off as soon as I can getback to the base!"

With a hasty good-by to his father, and farewells to his mother, Sandy, and Phyl by phone, Tom dashed out of the building. He sped to ArvHanson's workshop, and the new hydrolung suits were loaded onto a smallpickup truck and taken to the airfield. While flying back to FearingIsland in a helijet, Tom received a radio flash from his father. 

"Another message from Bud. He says the object dug up by the Brungarianswas _not_ the missile. It appeared to be the metal section of a ship'sprow, from some hulk buried in the silt!"

Tom was jubilant. "Terrific news, Dad! Our luck may be turning!"

At the rocket base Tom detailed crews for the three undersea craft whichwere to take off on the expedition. Arv Hanson would captain oneseacopter, Mel Flagler the jetmarine, while Zimby Cox, Chow, and fourcrewmen would accompany Tom in the _Sea Hound_. 

Because of their sonar-blinding systems, Tom realized there was a chanceof the ships losing contact with one another--especially if theiranalyzer sonars developed trouble. He therefore plotted their course tothe South Atlantic carefully, and issued orders for the antidetectioncircuits to be switched off every half-hour for a position check. 

"Report to your ships, " he now ordered. 

As Tom was about to leave base headquarters, Harlan Ames telephoned fromShopton. "Bad news, Tom. Dimitri Mirov has broken jail!"

"Good night!" Tom stifled a groan of dismay. "How did it happen?"

Ames said the Brungarian had somehow fashioned a crude weapon andoverpowered the turnkey. Disguising himself in the guard's uniform, hehad slipped out before his victim was discovered. 

"He must have had outside help within close call, " Ames ended, "becausehe seems to have made a clean getaway. The State Police have spread adragnet, but it doesn't look hopeful. "

"He'll probably duck out of the country pronto, " Tom surmised. "Anyhow, this won't stop us, Harlan. "

By nightfall the little fleet of three undersea craft was speedingsouthward at periscope depth. Tom alternated at the controls with Zimby, two hours on and two hours off. Sleep came in snatches, the crewmenflopping on their bunks as the chance offered. Chow's tasty meals helpedbreak the monotony. 

It was the following day when they reached the missile search area. Tomsurfaced the _Sea Hound_ and reversed blade pitch, then gunned the rotorturbines for an aerial reconnaissance flight, while the jetmarine and theother seacopter stood by in the water. 

"Brand my guppies, it's some ocean, eh, boss?" Chow remarked in an awedvoice. 

"Big enough, all right, " Tom agreed with a grin. "And plenty of water tosearch in. "

"No sign of the Navy, " Zimby said. 

Tom nodded. "They pulled out on schedule. "

"What about them Brungarian sidewinders?" put in Chow. 

"That's the question!" Tom swooped down to rejoin the other two craft. "We'll keep an eye out for enemy blips while we do our prospecting. "

Rather than lose time trying to contact Bud, Tom decided to let him findthe _Sea Hound_. Accordingly, he switched off the antidetection systemand ordered all ships to submerge. Arv's seacopter and Mel's jetmarinewere to maintain close formation and stand guard while Tom's craft didthe actual searching. 

Now the missile hunt began. Tom had plotted a concentric search pattern, focused on the probable position worked out by the task-force computers. After checking his fix on the automatic navigator, Tom switched on theDamonscope and steered the _Sea Hound_ on a gradually circling course. 

The Damonscope was mounted in a blister on the hull, its camera lenspointing toward the ocean floor. The automatic developing film wouldrecord any trace of fluorescence, and a red light would signal thisresult to the pilot's cabin. 

Minutes went by as the _Sea Hound_ nosed slowly along through thegray-green gloom, its sister craft flanking it a hundred yards on eitherside. They were moving only a fathom or so above the bottom. 

"A blip at eleven o'clock!" the sonarman called out suddenly. Tom'spulse quickened. "Moving straight toward us, " the sonarman added. 

Tom surrendered the controls to Zimby long enough to dart over and studythe sonarscope. "I've a hunch it's Bud, " he told the others. 

His guess proved correct when the unmistakable outline of a jetmarineloomed into view. Tom flicked on the search beam for a moment, and Budcould be seen waving through the cabin window. Then the yellow glarewent off, and Bud's jetmarine glided away to take up a scouting positionahead of the _Sea Hound_. 

An hour went by, then another. Suddenly a flash of light stabbed throughthe murk from dead ahead. 

"It's a signal from Bud!" Zimby exclaimed. 

Tom nodded grimly. "He's spotted trouble--probably an enemy sub. "Silence settled over the cabin as Tom reached out to switch on theantisonar circuits. 

At that same instant a red light flashed on the control panel. "TheDamonscope!" Tom cried out. "We may be over the Jupiter prober!"

Cutting off the steering jets, Tom gave a brief flick on the reversejets to halt the craft. Then he turned over the controls to Zimby andbegan stripping down to don a hydrolung suit. 

"Gallopin' guppies! What're you aimin' to do?" Chow exploded. 

[Illustration]

"Go out and look for that missile, " Tom said calmly. "It's what we camefor. "

"Are you loco, boss? What about that sub Bud just spotted? Mebbe it'sMirov's bunch!"

Tom refused to be dissuaded. After swallowing a space-plant pill, hearmed himself with an underwater flashlight. 

"Think it's safe to show that light, skipper?" a crewman asked uneasily. 

"If the enemy spots it, I'm hoping they'll think it's coming from aschool of lantern fish or sea anglers, " Tom explained. He picked up athree-pronged digging fork with his other hand and went out through theair lock. 

Tom glided back to the spot which the _Sea Hound_ had just passed overand began digging into the silt. Presently he felt the fork strikesomething hard. 

"An obstruction!" Tom thought excitedly. 

He probed deeper. Bit by bit, a smoothly contoured and still-shiny metalsurface became visible. "I've found it!" Tom's eyes flashed in triumph, his heart pounding. 

There was no doubt he had uncovered the nose cone of the missile whichhad re-entered the earth's atmosphere tailfirst!

Meanwhile, Bud, keeping watch on the enemy submarine, had seen a shadowyfigure glide from its air lock and head in Tom's direction. Bud donned ahydrolung and followed. 

"What's that he's carrying?" Bud wondered. 

Suddenly the answer came to him--a self-propelled underwater grenade!Horrified, Bud jetted forward, tackling the diver at full speed. 

A split second too late! The grenade went streaking straight toward TomSwift!

CHAPTER XX

A LUCKY BLAST

Tom's earphones caught the hiss of the approaching grenade. Instantlyhis eyes darted to the sonarscope on his wrist. 

A tiny blip of light was moving on the screen!

Tom whirled about, then gunned his ion drive. He pushed out of the pathof the grenade, which nevertheless grazed him as it streaked past. 

Seconds later, the grenade struck bottom. A shattering _bo-o-oom_reverberated through the depths, and clouds of silt darkened the waterinto Stygian gloom. 

Tom, knocked off balance, was tumbled about helplessly by the train ofshock waves. As they died away, he gradually recovered his bearings andpressed the throttle control of his ion drive. It coughed and stuttered!For a moment Tom felt a surge of panic, but the jet motor smoothed intoa steady purr of power. 

"Whew!" he thought in relief. "At least I can still get around at fullspeed if anything else comes at me!"

He had clung to the flashlight and fork despite the explosion. The blasthad hurled him away from the spot where the missile was buried, so Tombegan trying to locate it again. 

But he soon realized that his efforts were hopeless. He must wait untilthe silt which clouded the water cleared. Now Tom feared that theexplosion might have reburied the nose cone. 

Suddenly a new worry gripped him. _Had the missile's precious contentsbeen destroyed by the blast?!_ Slowly he began making his way back tothe _Sea Hound_. 

Unknown to Tom, Bud was fighting a desperate battle with his adversarybarely fifty yards away. The divers grappled each other in anoctopuslike duel. At such depths, their movements were impeded, as if byoil. 

The Brungarian pulled out the knife at his belt. Bud, a skilled wrestlerfrom high-school days, managed to twist his foe's knife arm behind hisback--then applied a punishing judo hold! The Brungarian gave an audiblescreech of pain and dropped the knife. 

"Now you're coming along with me!" Bud muttered. He gunned his jet, forcing himself and his adversary toward the _Sea Hound_. 

Moments later, they passed the seacopter's cabin window. Reaching theair lock, Bud hammered for admission. The hatch opened quickly and hisprisoner was hauled inside. Bud followed. 

Tom greeted him with a bear hug. "Hi, Bud, you old devilfish!" Turningto the prisoner, Tom added "Who's this?"

"The rat who fired that grenade at you!"

The prisoner was wearing a frogman costume and a mask which hid thelower part of his face. The man's dark eyes glittered in hate, as Tomordered him to remove his mask. Sullenly the prisoner obeyed. 

Tom gasped. "_Dimitri Mirov!_" The name sent a shock through theAmericans aboard. 

"Wal, I'll be jing-whistled!" Chow declared, then broke into a gleefulcackle. 

Under their scornful gaze, the Brungarian's own eyes wavered and hisshoulders slumped in an attitude of defeat. "What is the use?" hemuttered. "Again I have failed. My career is over now, just like mybrother's. "

Tom seized the opening. "In that case, maybe you're ready to do sometalking now. "

Mirov shrugged. "What do you wish to know?"

In answer to Tom's questions, Mirov admitted that his group, composed ofBrungarian rebel Navy men and rocket engineers, had sabotaged thereturning Jupiter probe missile, hoping to obtain its data for their ownuse. 

Their key agent in America was the man who had posed over the phone asLester Morris and masterminded the other attempts to kidnap Tom. He hadalso taken the amulet bracelet from Ames's jacket in a restaurant. 

Mirov himself had been given the bracelet after his jail break. Pullingback the sleeve of his frogman suit, he displayed it with a momentarysmirk of pride. 

"I even got inside the grounds of Swift Enterprises and stole a planethat same night, " Mirov boasted. 

Tom was startled. "How did you manage that?"

"Very simple. I thumbed a ride with one of your trusted workers on thelate shift and showed him the amulet to identify myself as a Swiftemployee. The guard at the gate was fooled the same way. "

Tom nodded thoughtfully. "They were instructed to look for a man tryingto sneak past alone. Seeing you in the same car with a known employee, he probably assumed you were all right. "

Mirov was allowed to change into dry clothes, then his hands were boundbehind his back. When the water cleared, Tom and Bud ventured outsideagain. First they headed for Bud's jetmarine to reassure his crew. Herethey learned that the mystery submarine had vanished. 

"Good riddance!" Bud exclaimed jubilantly. "They probably didn't evenrealize you had found the missile!"

"_Had found_ is right--past tense, " Tom said wryly. "It's no doubtburied again. But at least we have the right spot. "

They emerged from the jetmarine and headed back toward the site. As theyglided astern of the _Sea Hound_, Tom uttered a cry over his suit mike. 

"_Bud! There it is!_"

Both boys darted ahead at increased speed, and Tom played his flashlightbeam over the precious treasure. Instead of burying the missile deeper, the grenade explosion had uncovered the entire nose cone and part of thesection behind it!

"Sizzlin' squids! What a break!" Bud whooped. 

The boys jetted back to the _Sea Hound_ to announce the good news. Zimbyand two other crewmen were dispatched in hydrolungs to inform the otherships. Tom requested them to remain submerged and guard the site. 

Twenty minutes later the _Sea Hound_ was zooming up to the surface. Tomhoisted the craft's aerial and radioed word to his father, who wasoverjoyed. Mr. Swift, in turn, had news--that the rebels' key man andLen Unger had been seized by the FBI. Tom's next call was to AdmiralWalter. 

"Tom, this is wonderful news!" the admiral exclaimed. "I'll have ourNavy ships routed back there immediately--and I intend to fly outmyself as soon as I can board a plane!"

As Tom waited for the task force to arrive, his thoughts turned to newinventions to tackle. But he could not anticipate what would happen tohim in his _Triphibian Atomicar_. 

Within hours, the task force arrived at the site and recovery operationsgot under way. The missile was hoisted to the surface by cables attachedto submarines, then hauled aboard the tender. Tom himself supervised thejob of extracting the sealed data section. 

"You've done a tremendous job, Tom Swift, and our whole country will beproud of you!" Admiral Walter declared before sailing home. 

Tom grinned as he prepared to descend the ladder over the side. "Let meknow about life on Jupiter, sir. I may go there myself one of thesedays!"