"Varrhus planned this," he said unsteadily. "He filled up his hollow cold bombs with solid iron. The heat that would come in would first melt and then vaporize the interior until the pressure inside was more than the still-solid crust could stand. And all that vaporized iron would burst out. What a fiend that man must be!" An hour later, baffled and discouraged, he was sitting in the laboratory with his head in his hands, trying desperately to grapple with this new problem. The new cold bombs apparently could not be assailed without destruction of those who attacked them. It was impossible to imagine that volunteers could be found to sacrifice their lives to destroy each new bomb as it was placed. The horror of being annihilated by a blast of metallic vapor would deter men who would not hesitate to face death in a less terrible form. And Varrhus was evidently able to place them again nearly as fast as they were blown up. Telegrams announcing the explosion of the Jacksonville and Charleston ice floes lay before Teddy, supplemented by a cablegram from Panama saying that the Miraflores Locks had been destroyed by the blast when the Panama cold bomb had burst. Teddy was nearly certain that the next morning would find the exploded bombs replaced. Varrhus' black flyer was evidently capable of carrying a great weight at an immense speed. It also seemed able to reach an almost incredible height, from the fact that the second cold bomb had been dropped in the Narrows in broad daylight without the flyer having been sighted. Evelyn turned from the instruments with which she had been working. She had scraped off a small bit of the lacquerlike surface of the silver bracelet, and had been analyzing it in the hope of finding what element or combination had been used to produce the mystifying heat-inductive effect. "Teddy," she said depressedly, "I can't find a thing. The lacquer effect seems to be simply the appearance of some way he has treated the metal. The surface gives just the same analysis as the filings from the inside of the metal. I took a spectro photo and it gives silver lines with a trace of lead. Analysis by arsenic reduction gives the same result." "Perhaps those detectives will be able to trace Varrhus by the mailing box they took," said Teddy, without much hope. "It's not very likely, though. We've _got_ to think of something!" Silence fell in the laboratory again, broken only by the faint whistling sound of the flame Evelyn had used in her analytical work. "The trouble is," said Teddy grimly, "that we've been _trailing_ Varrhus, instead of anticipating him. If we could know where he was going to be----" "He'll have to show up sooner or later," Evelyn commented. "We know, for instance, that he'll have to replace that bomb in the Narrows or let the harbor stay open. The use of these new explosive bombs means that he has to expose himself more than he'd have to with the old ones."