This need was a major motivation for the development of accurate mechanical clocks John Harrison created the first highly accurate marine chronometer in the mid-18th century The Noon gun in Cape Town still fires an accurate signal to allow ships to check their chronometers. Use of an atomic clock in radio signal producing satellites is fundamental to the operation of GPS (Global Positioning System) navigation devices. An alarm clock is a clock that is designed to make a loud sound at a specific time The primary use of these clocks is to awaken people from their sleep in order to start their days in the mornings, but can also be used for short naps; they are sometimes used for other reminders as well To stop the sound, a button or handle on the clock needs to be pressed, and some stop automatically after a few minutes if left unattended A classical analog alarm clock has an extra hand that is used to specify the time at which to activate the alarm. Traditional mechanical alarm clocks have one or two bells that ring, but digital alarm clocks can make other noises Simple battery-powered alarm clocks make a loud buzzing sound, or other similar noise to wake a sleeper, while novelty alarm clocks can speak, laugh, or sing Some alarm clocks have radios that can be set to start playing at specified times, and are known as clock radios A progressive alarm clock, still new in the market, can have different alarms for different times (see Next-Generation Alarms). In a mechanical bell-style alarm clock, a mainspring drives a gear that propels a clacker back and forth between two bells or between the sides inside a single bell In an electric bell-style alarm clock, the bell rings with an electromagnetic circuit and armature that turns the circuit on and off again repeatedly. The ancient Greek philosopher Plato (428–348 BC) was said to possess a large water clock with an unspecified alarm signal similar to the sound of a water organ; he used it at night, possibly for signalling the beginning of his lectures at dawn (Athenaeus 4.174c).[1] The Hellenistic engineer and inventor Ctesibius (fl 285–222 BC) fitted his clepsydras with dial and pointer for indicating the time, and added elaborate "alarm systems, which could be made to drop pebbles on a gong, or blow trumpets (by forcing bell-jars down into water and taking the compressed air through a beating reed) at pre-set times" (Vitruv 11.11).[2] The late Roman senator Cassiodorus (c 485–585) advocated in his rulebook for monastic life the water clock as a useful alarm for the 'soldiers of Christ' (Cassiod 30.4 f.).[3] The Christian rhetorician Procopius described in detail prior to 529 a complex public striking clock in his home town Gaza which featured an hourly gong and figures moving mechanically day and night.[3] In China, a striking clock was devised by the Buddhist monk and inventor Yi Xing (683–727). The Chinese engineers Zhang Sixun and Su Song integrated striking clock mechanisms in astronomical clocks in the 10th and 11th centuries, respectively.[5] A striking clock outside of China was the water-powered clock tower near the Umayyad Mosque in Damascus, Syria, which struck once every hour
