"Good night!" Bud sank down on a lab stool. "You've come up with a way to make our own subs undetectable. Isn't that enough?"

Tom shook his head. "Not if we want to keep track of those sneaks. And I think I see a way to do it."

"How?"

"So far, I have been thinking about refining our own search sonar." Tom explained that the new system he had in mind would send out a _complex_ pulse--that is, an underwater sound wave with many harmonics instead of a single tone, sharp-peaked sound impulse.

"This will make it less likely that their antidetection gear will absorb all 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 that out."

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 setup filters 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 make hydrolung wearers safe from underwater detection. "And if the Jupiter prober hasn't been found by that time, Bud, I'm going to request the Navy to let us take over the search alone."

Bud gave a whistle of excitement at the possibility of new undersea adventures ahead. "Count me in, pal!"

The two boys broke off their conversation a short time later and went back to the Administration Building for lunch with Tom's father.

Mr. Swift greeted them with a smile as they entered the big double office. "Glad you could join me, boys! Chow's laid out quite a feast for us today."

Three places had been set at the conference table, and an appetizing repast of sizzling ham and sweet potatoes waited in covered dishes on a lunch 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 the noon news while they ate. The newscaster wore a look of excitement as he spoke without pausing for the usual commercial.

"The Brungarian government has just scored a propaganda bombsh.e.l.l!" he reported. "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 went on. "If enemy subs slipped through our continental defenses, their missiles could devastate the United States with scarcely an instant's warning! The whole country's been rocked by the announcement. An official 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 why they've chosen to reveal their secret at this time."

Tom said thoughtfully, "Dad, do you suppose they've realized the fact that 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 scientist paused 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 an antidetection system ourselves?" As his father nodded, Tom scowled. "If so, that sub yesterday may have been observing our tests."

The telephone rang and Tom leaped to answer it. The caller was Dan Perkins of the _Shopton Evening Bulletin_.

"You can guess why I'm calling, Tom," the editor said. "How about a statement 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 phone began ringing again with a succession of calls from other newspapers and wire services. Tom dashed off a brief, general statement and instructed Miss Trent to give it to all further callers.

"Maybe this is a good time to make a private announcement to you fellows," Mr. Swift said to the two boys, his eyes twinkling. "Do you recall my telling you that Doc Simpson had isolated an unknown vitamin from the s.p.a.ce plants? Well, we've now discovered that this vitamin can condition the human body to stay under water indefinitely. Doc is putting 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 for the lost missile!" Tom exclaimed.

CHAPTER XV

MOUNTAIN HIKE

"After adequate doses of your s.p.a.ce vitamin, Dad, a skin diver could tackle almost any undersea job in my hydrolung!" Tom exclaimed. "He wouldn't be subjected to any antiosmosis troubles with his body tissues."

His father nodded. "For the first time, man might become a _truly marine creature_!"

"Wow! Think of it!" Bud gasped excitedly. "With Tom's hydrolung and a knife 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 the thought of new fields of scientific conquest. "This discovery of Dad's and Doc Simpson's opens up some really amazing possibilities."

Most important at the moment, the vitamin would be a great boon in carrying out search and digging operations for the Jupiter prober. With fresh enthusiasm, Tom returned to his laboratory to work on the new sonar gear. In his own mind, he had already named it a "quality a.n.a.lyzer sonar," since that exactly described the way it would function.

"Hmm, let's see," Tom mused as he settled down at his workbench, pencil in hand. "Besides a regular sonarscope, I'll need at least three units for 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 returning echoes. 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 the screen in the normal way. If the echo came back blurred, or if "shadow echoes" showed up, these would be separated and appear on the screen colored red.

"Whew!" Tom sighed as he realized the complicated job of circuit design that lay ahead. "This sure is going to burn some midnight oil!"

The young inventor worked all afternoon at a furious pace, breaking off toward dinnertime to telephone his mother that he would be staying overnight at the lab. After a hasty meal, he resumed his layout job at the drawing board and by midnight had finished designing his quality a.n.a.lyzer sonar.

Whipping off his eyeshade, Tom went into the apartment next door and stretched out to s.n.a.t.c.h a few hours' sleep. But as usual when in the midst 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 a.s.sembling the units of his new sonar gear. Later he phoned Chow but scarcely paused to eat when the cook arrived with his order.

"Brand my solar stovepipe!" Chow scolded. "Take time to eat your vittles properly, boss!"

"Hmm?... Oh, sure." Tom looked up and grinned.