Frank was familiar with all the voices in the fort, but this was a strange voice. Evidently strangers had landed; but what could they mean by going to work at night? He still waited to listen, but instead of more words he heard the tread of feet overhead. Could they be going away? Whoever the men were, he must have help, and he would have called out in a second more if-- Hark! There was a grating on the stones above, then a glimmer of light, as if from a lantern. Then another strange thing happened. By the dim light he saw the end of a ladder come down through the trap--not the trap over his head, but two casemates further down, nearer the sally-port. By the same dim light Frank saw that the tanks were connected by broad brick arches, through which the water flowed from one to another. The ladder was let all the way down, and down it came four men, one after another, one carrying the lantern, all carrying hammers and saws, and all strangers. Frank was so surprised that he could do nothing but stand still and watch. There was no danger of his being seen, for he was in the deep gloom; but he could see every move the men made, as they carried the light. The men seemed to know the ground thoroughly, for they waded off through the water without hesitation, going in the opposite direction from Frank. Through two of the brick arches they went, then up to the front wall of the tank, and began to use their hammers and saws briskly. Frank saw that the wall was covered with a perfect maze of pipes, both iron and lead, and that made it all plain to him. These men were thieves, and they were cutting away the lead pipes to steal them. Perhaps the strange situation sharpened his wits. At any rate, Frank saw that the men were all absorbed in their work two casemates beyond the ladder, and without waiting a moment longer he waded silently but swiftly down to the foot of the ladder, flew up its rounds like an athlete, and drew the ladder up after him. That left the thieves securely trapped in the tanks. The stars were shining brightly now, and half wild with joy at his release Frank rushed across the enclosure. "Help! help! help!" he shouted again. "Thieves! thieves!" His friends could hear him plainly enough now that he was outside; and when Frank told his story they made short work of capturing the burglars and taking possession of their sloop that waited by the wharf. "Those fellows have been here before," the men reported who were sent down into the tanks: "they have cut away miles of lead pipe." The doctor saw that Frank was nearly used up with the excitement, and insisted upon his having a cup of hot coffee and going to bed. "You have made an important discovery," he said, "and the War Department ought to have something to say to you for it. Those fellows must have taken hundreds of dollars' worth of pipes, and I think we can recover them. I shall make a report to the Department, of course."