The Breguet Metronome Clock (1798)
"The Metronome Clock is the final Breguet clock in the Royal Collection. The chronometer clock’s simple case consists of an ormolu frame housing five glass panels, the top secured by four small acorn finials. Two dials are positioned one above the other. The lower is white porcelain with Arabic numerals and 7 inches in diameter and a subsidiary dial at 12 o’clock. Above is a silvered brass dial that is 4-3/4 inches in diameter, with an outer zone divided 0-60, an inner zone divided 0-4, with two concentrically-pivoted register hands.
"At its heart beats an eight-day going barrel movement with a pinwheel escapement. The primary pendulum is 11-inches in length and suspended on a knife’s edge. A second pendulum consisting of a 1-1/4 inch brass ball suspended by a chain. Pulling a small knob between the dials disengages the primary pendulum, then engages the small brass ball that beats to the cadence determined by the register hands. This incredibly complex clock not only functions as a metronome to measure musical beats, but also contains an elaborate compensation mechanism so that precision time-measuring resumes even though the primary pendulum is stationary."