One of my favorite clocks ever made is the Atmos clock by Jaeger-LeCoultre. The Atmos functions through daily temperature variations: a gas inside a flexible membrane expands as the temperature rises, compressing a spring that powers the pendulum. This is a true "perpetual" clock, designed to keep accurate time for years with no intervention—a true work of horological art!
This Atmos clock comes complete with the original instruction manual (see photos for details on how the mechanism works).
