On the principle that water drips evenly, people can measure time by watching the number of grades on the arrow. As a time-measuring apparatus, the clepsydra is used more widely than the sundial.

• 箭，则是标有时间刻度的标尺。 漏刻是以壶盛水，利用水均衡滴漏原理，观测壶中刻箭上显示的数据来计算时间。