clocks and watches
Clocks and watches are devices to indicate or record the passage of time, which have become essential features of modern life. In prehistory, time could be gauged solely from the position of celestial bodies; a natural development was the sundial, initially no more than a vertical post whose shadow was cast by the Sun directly onto the ground. Other devices depended on the flow of water from a pierced container (see clepsydra); the rates at which marked candles, knotted ropes, and oil in calibrated vessels burned down; and the flow of sand through a constriction from one bulb of an hourglass to the other.
Mechanical clocks were probably known in ancient China, but first appeared in Europe in the thirteenth century AD. Power was supplied by a weight suspended from a rope; later from a coiled spring; in both cases an escapement mechanism being used to control the energy release. Around 1657–58, Christiaan Huygens applied the pendulum principle to clocks; later, around 1675, his hairspring and balance-wheel mechanism made possible the first portable clocks – resulting eventually in watches. Jeweled bearings, which reduced wear at critical points in the mechanism, were introduced during the eighteenth century, and the first chronometer was also devised in this century. Electric clocks with synchronous motors are now commonly found in the home and office, while the atomic clock, which can be accurate to within one second in 3 million years, is of great importance in science.