Cartier’s clock whose hands appear to float without visible mechanisms is called a mystery clock.
The name describes the visual illusion: the clock’s movement is hidden, so the hands seem disconnected from any mechanism. In Cartier examples, the hands can be mounted on transparent crystal discs or wheels with concealed teeth. Gearing hidden in the case or base turns those components while preserving the appearance of free-floating hands.
Cartier became especially associated with mystery clocks during the early twentieth century. Louis Cartier collaborated with clockmaker Maurice Couët, whose designs helped turn the illusion into one of the maison’s celebrated objets d’art. Cartier’s clocks often combined the mechanism with rock crystal, hardstone, gold, enamel and diamonds.
The term is broader than Cartier alone. Mystery watches and clocks had earlier precedents, and the English Wikipedia article uses “mystery watch” as the main title while treating “mystery clock” as an alternative name. “Pendule” is a French word for a clock, not the canonical name of this floating-hands design.