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 Cox 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 Cox. 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 earset and listened. "It's no seacopter, nor a jetmarine either," he announced presently. "A Navy sub, maybe?" suggested Zimby. Bud shrugged. "Let's find out." He ordered a change of course, hard to the right, and gunned the jets to bring the jetmarine directly on the mystery object's trail. "It's a sub, all right," he said a short time later, listening again over the hydrophones. "Pretty close to Fearing Island, isn't it?" put in Mel Flagler. "That's a government-restricted area." Bud nodded grimly. "But staying just out of sonar range from the base." The jetmarine closed steadily on its quarry. In a few minutes they were able to make it out dimly through the cabin window, dead ahead. "That's sure no U.S. Navy sub that I know of," Bud said. "Probably an enemy snooper." "What if they spot us?" Zimby asked. Bud chuckled. "That's the beauty of it, pal! Don't forget. With this new antidetection gear we're invisible to them. At least as long as they don't run into us or we into them," he added.