“We might assume, then,” the FBI man said, “that on the past two Saturdays the men’s real purpose for going out in the boat was to drop other metal tubes overboard. Other tubes similar to this one.” “And remember,” Mr. Taylor said, “the first time Eddie and Teena saw them was the very Saturday after the isotope was stolen from the college.” “Right,” Mr. Jamison said. “So,” the FBI man picked up the line of thought, “the question is why the men dropped the metal tubes out on the sand bar. It’s a fairly safe bet that each tube contained a little of the radioactive material, plus other blueprints. Let’s assume that the reason behind the whole thing is to smuggle the blueprints out of the country.” “That would go for the isotope, too,” the man from Drake Ridge said. “It was a new secret isotope, you know. Various foreign governments would like to get their hands on it.” “But the men didn’t talk like foreigners,” Eddie said. “Of course, they wouldn’t,” Mr. Evans said. “Might not even be foreigners. Unfortunately, there are a few greedy people who will do almost anything for money.” “Even spy?” Teena said, aghast. “Even spy,” Mr. Evans said. “But what we need to find out is how anyone is managing to smuggle the stuff out of here.” “Probably by boat,” Mr. Jamison said. “The Coast Guard keeps close tabs on all boating,” Mr. Evans said. “And the bay’s too shallow to allow ocean-going ships inside.” A thought sprang into Eddie’s mind. “Mr. Evans,” he said, “I found a rubber strap on the beach last week. It looked like a strap broken off a swim fin or something like that. It—it has some foreign printing on it. I have it at home.” His announcement had an immediate effect. “That should give us a real clue,” the FBI man said quickly. “It makes sense, too, that the cylinders would be recovered by skin divers. Perhaps foreign divers similar to our own frogmen.” “It would have to be done after dark,” Mr. Ross said. “Otherwise they would be seen. And how could anyone locate a small cylinder like that under ten feet of water at night.” “I think I can answer that,” Eddie’s father said. “In fact, Eddie and Teena found that answer. It could be located with a Geiger counter.” “That’s it,” the FBI man agreed. “For instance, they could use a rubber boat to sneak in under cover of darkness. They would know the approximate location of the cylinder.” “How?” Teena’s father asked. “By some established plan. Probably by triangulation. They could use the lighthouse for one reference point. Perhaps some other signal light on shore would give them a second point.” Quickly, Eddie told him about seeing the heavy-duty battery lantern in the shack. “They might use it for a signal light,” he said.