I reckon you'd scare 'em half to death in that
rig!" 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 just
solved 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 of
a rusty hinge."

Chow scratched his chin uncertainly. "Wal, what about it?"

"Suppose I used that rusty-hinge noise to mask the diver's noise." Tom
turned and stabbed the air with his finger. "I could _also_ use that
same sound output as the search pulse for my quality analyzer sonar!"

In this way, Tom explained, he could eliminate part of his bulky
equipment and do an even better job of making the diver "invisible."

Bubbling with enthusiasm, Tom decided to buy a live porpoise at once and
make 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 none
could 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 Fearing
Island. At the base, both Mel Flagler and Zimby Cox were eager to
accompany the young inventor when he told them about the trip he had in
mind.

Tom chose the _Sea Hound_ as the fastest and best suited craft for his
purpose. With Mel's and Zimby's help, he quickly rigged a plastic "tank"
in the stern cabin. Minutes later, the seacopter zoomed skyward, heading
for the Florida Keys.

The flight was a short one at transonic speed. Tom chose a sparkling
stretch 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 over
the side in hydrolungs.

The two glided about in the translucent blue depths, keeping in close
range of each other. The sea was alive with shimmering fish of every
hue, darting among the coral. Suddenly, as Tom veered around to rejoin
Mel, his eyes widened in horror.

A vicious-looking hammerhead shark was zeroing in, directly behind his
friend!

"Look out!" Tom yelled over his microphone.

CHAPTER XVIII

SMILEY THE SEA COW

Mel turn