The disappearing floor / Franklin W. Dixon.

Once again Frank and Joe Hardy accept the challenge of a puzzling case when their famous detective father asks the boys to assist him in tracking down a notorious jewel thief and his accomplices. The trail leads to the outskirts of the Hardys' home town and to a weirdly guarded mansion on the old Perth estate. With their chubby, ever-hungry friend Chet Morton, Frank and Joe tackle another mystery -- one which has baffled the town of Bayport for many years: What caused the sudden death of old man Perth's nephew who inherited the mansion when his uncle died? A disappearing floor, a huge, savage-looking hound, a galloping ghost, a college professor's startling invention are just a few of the strange elements that complicate the boys' efforts to solve both mysteries. Before Frank and Joe finally discover the mysterious circumstances under which Perth's nephew died and also bring the jewel thieves to justice, the young detectives need all their sleuthing instincts to extricate themselves from one of the most harrowing situations they have ever faced. - Flyleaf