"We don't know," answered Tom. "We are afraid they are up to some trick." "A trick?" repeated Sam. Then he gave a gasp. "The room--isn't there a back door, leading out to the shed?" "I don't know," answered Dick. "I'll run and see." Sam was gone less than two minutes when they heard a cry, and then he pounded on the door they had so carefully guarded. "There is a back door and it is wide open. The men have gone!" was his dismaying announcement. CHAPTER VI THE STORY OF A TREASURE It was a disheartening discovery, but the three Rover boys did not stop to think it over. Throwing open the bolted door, Tom and Dick joined Sam, and in the darkness made their way to the rear of the room in which they had held Cuffer and Shelley prisoners. In a minute more they were outside, under the trees at the rear of the old mill. "Which way did they go?" Tom asked the question, but nobody could answer it. The moon had now gone under some clouds and it was so dark they could scarcely see ten feet in any direction. "Perhaps they took to the river again," suggested Sam. "It is not likely," answered his big brother. "But we can take a look." They ran around to where the men had landed. Their boat was still in its place, tied to a tree. "Listen!" cried Sam. "Somebody is shouting, and there is a light." "It is Jack Ness," said Tom. The boys set up an answering shout, and soon a boat came up to the shore. It contained the hired man and the two Ditwolds. They had a lantern with them and also an old-fashioned single-barrel shotgun. The situation was quickly explained, and then the party of six began a systematic search of the woods and the various roads in the vicinity of Henderson's mill. This search lasted until morning, but nothing came of it. "We may as well give up," said Dick, at last. "They have gotten away and that is all there is to it." The boys were completely tired out when they got home. Their uncle and aunt were much worried over their prolonged absence and overjoyed to see them return unharmed. "I was so afraid one of you might get shot or something," said Mrs. Rover. "Some of those chickens thieves are desperate characters." "Those men were more than chicken thieves," answered Dick. And he told his uncle and aunt of the conversation overheard at the old mill. "It is a great pity that they got away," said Randolph Rover. "What do you imagine they are after. Uncle Randolph?" questioned Sam. "I do not know, excepting it may be some mining stocks or a deed to some property. Perhaps your father will be able to explain it when he gets back."