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."
It was not long before nearly $1600 worth of lead pipes were recovered in Key West, where they had been taken; and when Frank went home late in the fall, as strong and brown as any mother could ask to see her son, he found a big letter waiting for him, without any stamps on the envelope, but printed in the corner, "War Department, Adjutant-General's Office. Official business."
"DEAR SIR,--The Secretary of War has learned from Dr. R. D. Murray, and from other official sources, of the recovery, through your efforts, of a large amount of government property stolen from Fort Jefferson.