It's an Illusion
Mystery clocks are tricks. They're illusions and spectacles of magic, so it's no wonder that their history is entwined with that of famous magicians.
The mystery lies in the movement of the clock's hands. The creator of a mystery clock fools the observer by hiding the workings so that the hands appear to move on their own. Over the years, clever clockmakers have devised many ways to achieve this illusion. Mechanisms were closely guarded secrets -- in fact, they were often patented.
Fittingly, Jean Eugene Robert-Houdin, a 19th-century French magician and clockmaker, is widely hailed as the inventor of the mystery clock, but even his name is something of an enigma. (Houdin's last name at his birth in 1806 was Robert. Young Jean Eugene added "Houdin" to his name when he married the daughter of a prominent Paris maker of clocks and watches with that name.) Coincidentally, he had also become interested in conjuring and magic. He combined his two passions in the making of mystery clocks. His first mystery clock won a bronze medal at the Exhibition of the French Industry of 1839. Houdin used various optical tricks in his mystery clocks, including a rod that ran up through the ornate clock base and along the right of the top of the case, attaching to a screw that was connected to a second, invisible glass dial that turned behind the visible dial [source: Horologist].
In many of his mystery clocks, Houdin used some variation of secreting the mechanism the clock base and attaching the hands to a second, transparent, serrated-edge dial that was turned by pinions inside the frame of the case [source: Kolesnikov-Jessop]. Many other clock makers have used similar tricks involving transparent dials that look round but really have cogs hidden inside the clock's frame.
Keep reading to learn more about the tricks and treasures of mystery clocks.