A mechanism for measuring and indicating the passage of time, and for recording the duration of intervals. Its essential elements are a source of energy to drive the mechanism, and a device to maintain a regular (usually stepwise) rate of motion. (In the earliest mechanical clocks, the striking of bells was a primary function.) The first clockwork escapement was Chinese, AD 724, and water-powered clocks are noted in China from the 760s. In mediaeval and many later clocks, the drive was a falling weight; from the 16th-c, it was the coiled mainspring; and from the late 19th-c, it was electric current from batteries or mains. Steady rate was provided first (very inadequately) by a verge escapement (projections on a freely oscillating arm engaging intermittently in the teeth of a crown wheel); then during the 17th-c by a pendulum, the accuracy of which was increased by successive refinements and corrections (eg for temperature change). From c.1670 a balance wheel was used for small clocks and watches. In modern times, clocks are powered by electric synchronous motors maintained by the mains frequency, by a maintained tuning fork, or by the oscillation of a quartz crystal. Observatory clocks use one of two methods: (i) the constancy of the difference in energy levels of different states of the cesium atom, a beam of the atoms divided by a magnetic field forming part of an electric circuit with self-correcting feedback; (ii) the natural frequency of inversion of the pyramidal ammonia molecule, the excitation radiation of the correct frequency being locked in by a feed-back circuit.