The sundial was one of the earliest devices for measuring time. But it can work only on a day with
plenty of sunlight. Early peoples also used ropes with knots tied at regular intervals. In the ninth century
candles were marked with regularly spaced lines, but this was not very accurate as a draught could
cause the candle to burn more quickly. When burned, such devices measured time. An hourglass or
sandglass tells time by means of sand trickling through a narrow opening. A water clock, clepsydra,
measures time by allowing water to drip slowly from one marked container into another. By the 1700s,
people had developed clocks and watches that told time to the minute. Modern electronic and atomic
clocks can measure time with far greater accuracy.