ver 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 the
hatch 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 he
watched Tom installing the delicate transistors and other components
with an electric soldering gun.

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

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

After the meal, Hank Sterling asked, "How about a detection test to see
how 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." Tom
produced a hydrographic chart of the waters around Fearing and marked
out a test area. "Cruise around there for an hour and we'll try to spot
you in the _Sea Hound_."

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

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

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

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

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

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

Tom felt a glow of satisfaction. He explained, however, that the
jetmarine's transparent nose pane--which had to be left unprotected for
the 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. For
another ten or fifteen minutes