to the South Atlantic, it
seemed likely that the enemy, too, had not yet located the precious
missile with its data from Jupiter.

"You guys rate Navy medals," Tom told Zimby and Mack jubilantly. "Come
on back to Shopton with me and I'll buy you the juiciest steaks in
town!"

Before leaving the base, Tom called the Coast Guard and the Navy to
cancel his search request. He also telephoned a full report on the enemy
submarines to Admiral Walter.

After hanging up, Tom decided on another move. "Our antidetection gear
seems to have panned out pretty well," he told Hank. "I think we should
make use of it right away. By sending that jetmarine to the South
Atlantic, we might get a line on enemy activities down there."

Hank was in favor of the idea. He volunteered to prepare the jetmarine
for a cruise and take off from Fearing that very night.

"Thanks," Tom said with a parting handshake. "Keep us posted if you
learn 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 Arv
for a celebration dinner in town.

The next morning found the young inventor hard at work in his private
laboratory. He was tapping his head with his slide rule and frowning at
a blackboard scrawled with equations when Bud dropped in for a visit.

"What now, inventor boy?" his copilot asked. "Don't you ever give that
brain of yours a rest?"

"Oh, hi, Bud!" Tom looked around absent-mindedly. "I'm just trying to
figure out a way to crack the Brungarians' antisonar system."

"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 rea