Lou (water clock) is an idiom used by ancient Chinese equivalent to today’s “clock”. Zhonghu Dilou is also called clepsydra or water clock. The time-measuring device consists of several kettles filled with water, which are usually connected together. There are small holes on the bottom of upper kettles, so that once the kettles are filled up, the water will drop down stepwise and regularly to the lower kettles. The lowest kettle has an erect buoy in it carved with scale divisions. The clepsydra is a 24-hour calculagraph used to calculate time through observing the scale of buoy according to fluctuation of buoy enabled by changes in water level. It measures time day and night during both sunny and rainy days. However, there may be changes in the speed of dripping according to temperature condition, which lead to inevitable error or difference in telling time. Nevertheless, it can be corrected by using sundial during fine days.