[Ill.u.s.tration: _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 they probed about with no sound trace of the "invisible" jetmarine.
But presently Arv snapped his fingers to catch Tom"s attention. "Got her, skipper!"
Tom took over the hydrophones. Sure enough, his ears could make out the faint hum of the jetmarine"s atomic turbines. Tom directed Hank toward the sound, then ordered him to switch on the _Sea Hound_"s powerful search 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 of view.
"Your system blinded our sonar okay, skipper," Hank commented, "but this proves she could still be spotted by enemy listening devices."
Tom refused to be discouraged. He ordered Hank to return to base and wait for Bud. Meanwhile, the young inventor applied himself to the problem of how to mask the sub"s noise.
"How about it, pal?" Bud asked, when he reported aboard the seacopter a while later.
Tom explained the results of the test and the need for an added safeguard 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 propulsion machinery," Tom began. "That goes without saying. So we"ll have to camouflage 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 the hydrophones, you know."
As Bud nodded, Tom went on, "So we simply step up the volume till the sub"s own noise gets drowned out or "wasted" in all the racket."
This could be done, he concluded, with fairly simple amplifying equipment. 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 c.o.x joined the crew. Bud suggested taking along hydrolungs in case of any need for tinkering with the transducers or amplifying equipment.
This time, the jetmarine scored perfectly on the test, successfully eluding all the _Sea Hound_"s efforts to detect it. Tom returned happily to base, feeling that the antidetection problem was now solved. The jetmarine, 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 would have switched off the antidetection gear by this time. But neither sonarscope nor listening devices revealed the slightest clue.
Tom, Hank, and Arv exchanged fearful glances. Had the jetmarine foundered on the ocean bottom--perhaps fouled somehow by Tom"s new invention? 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 jetmarine to the surface. But just as he was about to blow the ballast tanks, Mel Flagler 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 could be another sub."
Bud turned the controls over to Zimby c.o.x. Then he rushed to the scope and examined the blip. "Seems to be moving away from us on a westerly course. It"s about two miles from here."
He donned the hydrophone ea.r.s.et and listened. "It"s no seacopter, nor a jetmarine either," he announced presently.
"A Navy sub, maybe?" suggested Zimby.