The 19th century mystery watch was a genuine engineering puzzle: a pocket watch whose face was entirely transparent.
Several European exhibitions of the late 19th century CE displayed a curious and confusing feat of horology: a pocket watch with a completely transparent face. The watch’s hands appeared as if suspended in the middle of the watch. The timepiece functioned completely normally, and the hands turned like any other watch or clock, but they did so unattached to any visible mechanism.
The mystery watch was one of many wonders on display at the famous 1889 Exposition Universelle in Paris (the same world’s fair that gave us the Eiffel Tower). It so impressed the visiting Shah of Persia – Mozaffar ad-Din Shah Qajar – that he knighted the watch’s designer, Hugues Rime.
So how does it work? The actual technique is elegant and simple. The actual mechanism is hidden behind the solid metal rim around the face of the watch. The hands of the watch are mounted on a small circular pane of glass. The edges of the glass disk, which you cannot see because they’re hidden under that rim, have metal gear teeth.
The watch mechanism in the metal crescent turns the transparent glass disk via those teeth. The disk turns the minute hand. Tiny gears underneath the minute hand turn the hour hand. And so you have hands that appear to float and turn without any impetus.
French jeweller Cartier used this trick in the early 20th century to make mystery clocks for the Rockefellers and still make mystery watches today using disks of sapphire: