"Don't be too sure it's simple," admonished Rob, as they silently rose from their crouching postures and took after the vague shadow; "this thing may turn out to be bigger than we thought." "Have you laid any plans as to what you will do if we do apprehend him in the act of transferring the plans to Mr. Barr's enemies?" "Not yet. There's no use crossing a bridge till you come to it." Through the night the boys pursued their quarry as silently as two snakes. At times they lost sight of him, but always his figure would loom up against the star-sprinkled sky as he topped a sand dune. At length they saw him pause and light the lantern, which he had used in the shed, and which he still carried. This done, he swung the light twice across his body, after the fashion of a brakeman signalling a train to come ahead. Instantly, out of the darkness, flashed an answering beacon--a red light. The boys clasped each other's arms. That they were on the brink of an exciting adventure they did not doubt. But in each lad's heart was a firm resolve that, come what might, they would do their duty by Uncle Sam. "Was that red light shown from the other island?" whispered Merritt. "No, I am inclined to think it came from that launch we saw sneaking in behind the island this afternoon just before the signalling commenced," was the response. "In that case, she must be still far out?" "Yes; but in any event they would have to send a boat ashore. That launch is too large to land on the beach directly." As if in answer to his opinion the watching boys presently saw a red light creeping over the water toward the island. Undoubtedly it came from a small boat, so low on the water was it. Before long they could detect the splash of oars, although whoever was rowing the boat was trying to make as little noise as possible. As the light drew close in shore, Merritt seized Rob's arm. "What's the next move?" he asked. "It looks as if it were ours," was the quiet, but determined, rejoinder. CHAPTER XVII. ROB'S BRAVE ACT. While the boys had been watching, Barton had lain down, as though tired, on the summit of a near-by dune. As the red light came close in shore, however, he arose, and once more waved his lantern. At the signal the course of the red light shifted and headed directly toward him. The boys' hearts beat thickly; the time for action was at hand. The bow of the boat they had seen approaching grated on the beach, and two figures sprang out while Barton advanced to meet them. "Get as close as you can," whispered Rob, as he wriggled forward; "we want to get every word." Merritt merely nodded; but his silent advance was as rapid as his leader's. Owing to the nature of the ground, they were able to run forward in an almost upright position when they reached the hollows of the dunes, being compelled to cast themselves down only when they topped a rise. Therefore, they were within ear shot when Barton greeted the two men who had disembarked from the boat.