A device designed to indicate the time. The earliest forms of clock were clepsydras, gnomons, sundials and hourglasses. The mechanical clock appeared in the early 14th century. Balance-spring clock. The regulating organ in watches is the balance and spring assembly which is also used in medium-sized clocks known as pendulum clocks or carriage clocks. The atomic clock or, more correctly, the molecular clock is a quartz clock in which the oscillation of the quartz crystal is stabilised by that of the nitrogen atom of an ammonia molecule; this latter oscillation is independent of temperature, pressure, shocks and ageing. Clocks are also named according to their style of construction, their regulating organ or their intended purpose.