Clocks with no apparent mechanism are known as mystery clocks. They were invented by French clock-maker and magician Robert-Houdin [1]. He exhibited his first mystery clock at the Exhibition of the French Industry of 1839. Many other makers have created mystery clocks in popular decorative styles of their day [2].
[1] https://www.liveauctioneers.com/news/be-smart/robert-houdin-...
[2] http://www.roger-russell.com/mysteryclocks/mysteryclocks.htm