The Haunted Fort by Franklin W. Dixon

According to a student, the conflagration had apparently broken out suddenly—on the lake itself.
    â€œWhich means somebody poured a kerosene slick on the water and ignited it,” Frank said.
    Chet nodded solemnly. “With the wind and floating pieces of burning wood, we’re lucky it didn’t spread along the whole shore front.”
    By now, most of the onlookers had dispersed and the fire trucks and police car were leaving.
    The Bayporters surveyed the grim, charred skeleton of the boathouse, wondering who the arsonist could have been, and what his motive was. Another attempt to discourage the Hardys from investigating Fort Senandaga?
    â€œIt wasn’t Ronnie Rush who set it, anyway,” Chet declared. “He was too busy making fun of my painting.”
    The three boys searched the burned wreckage for evidence. They found nothing but a fat, charred cork, smelling of kerosene, bobbing on the waterfront.
    â€œA pretty slim clue,” Joe muttered, stuffing the cork into his pocket. After supper they stopped in with Uncle Jim to see Mr. Davenport. He seemed inconsolable. The school’s exhibit was only two days away, and the blackened ruins would detract greatly from the estate’s appearance. Joe had an idea.
    â€œWe’ll begin clearing away the debris first thing tomorrow, and have the lake front in good shape by Senandaga Day.”
    Mr. Davenport brightened, and Uncle Jim said, “That would be a big help. At least the lake residents will be able to beach their boats.”
    â€œThere’s one person I suspect,” the art patron burst out angrily, “who would want to spoil our exhibit. A certain party down the lake.”
    The boys assumed he meant Chauncey Gilman, but somehow they could not picture the critic in the role of an arsonist.
    The brothers then told the others about the mysterious French flag they had seen at the fort. Mr. Davenport expressed complete bewilderment.
    â€œA flag over Senandaga!” he exclaimed incredulously. “It must be the work of some blamed touristl A trespasser!”
    Frank doubted this, saying that even a practical joker might not go to the trouble of climbing the fence.
    â€œDon’t tell me a ghost put up that flag,” Chet gulped.
    Mr. Davenport shook his head. “You can get to the fort by boat, too.”
    The Hardys left him, wondering if the strange incident was part of the puzzle they were trying to solve.
    Directly after breakfast the boys plunged into the task of cleaning up the dock site. With axes and wheelbarrows, charred wood was cut up and carted away, as well as burned shrubbery. Up to their waists in water, Frank and Joe hewed down the remaining boathouse supports and dock stakes.
    â€œWhew!” Chet exclaimed as noontime approached. “I feel as though I’d been building a fort.”
    Ronnie Rush came up just then and looked on smugly. “Want to help?” Joe asked him.
    â€œMy time is too valuable,” Ronnie said, and sauntered off.
    â€œHe may not have burned the docks, but he sure burns me up!” Chet muttered.
    At last the boys finished their project, having set up bright buoys offshore. After lunch they were summoned to Cedartown Police Headquarters, where the chief handed them a photograph. “Recognize him?”
    â€œThe picture and frame thief!” Joe exclaimed.
    â€œHis name’s Adrian Copier,” the chief informed them, adding that the man had a long criminal record as a thief, especially of art objects. There was no indication of his being an arsonist.
    â€œI wonder if he’s the brains behind the thefts at Millwood,” Frank said, “or if he’s working for a higher-up.”
    The chief shrugged. “Copler seems to be as elusive as he is clever. But I’ll keep men on the lookout.”
    Back at the school, the boys discussed their future trips to the fort. “The Queen’s still laid up and we can’t keep