<P> Water clocks, or Fenjaan, in Persia reached a level of accuracy comparable to today's standards of timekeeping . The fenjaan was the most accurate and commonly used timekeeping device for calculating the amount or the time that a farmer must take water from a qanat or well for irrigation of the farms, until it was replaced by more accurate current clock . Persian water clocks were a practical and useful tool for the qanat's shareholders to calculate the length of time they could divert water to their farm . The qanat was the only water source for agriculture and irrigation so a just and fair water distribution was very important . Therefore, a very fair and clever old person was elected to be the manager of the water clock, and at least two full - time managers were needed to control and observe the number of fenjaans and announce the exact time during the days and nights . </P> <P> The fenjaan was a big pot full of water and a bowl with small hole in the center . When the bowl become full of water, it would sink into the pot, and the manager would empty the bowl and again put it on the top of the water in the pot . He would record the number of times the bowl sank by putting small stones into a jar . </P> <P> The place where the clock was situated, and its managers, were collectively known as khaneh fenjaan . Usually this would be the top floor of a public - house, with west - and east - facing windows to show the time of sunset and sunrise . There was also another time - keeping tool named a staryab or astrolabe, but it was mostly used for superstitious beliefs and was not practical for use as a farmers' calendar . The Zeebad Gonabad water clock was in use until 1965 when it was substituted by modern clocks . </P> <P> Joseph Needham speculated that the introduction of the outflow clepsydra to China, perhaps from Mesopotamia, occurred as far back as the 2nd millennium BC, during the Shang Dynasty, and at the latest by the 1st millennium BC . By the beginning of the Han Dynasty, in 202 BC, the outflow clepsydra was gradually replaced by the inflow clepsydra, which featured an indicator rod on a float . To compensate for the falling pressure head in the reservoir, which slowed timekeeping as the vessel filled, Zhang Heng added an extra tank between the reservoir and the inflow vessel . Around 550 AD, Yin Gui was the first in China to write of the overflow or constant - level tank added to the series, which was later described in detail by the inventor Shen Kuo . Around 610, this design was trumped by two Sui Dynasty inventors, Geng Xun and Yuwen Kai, who were the first to create the balance clepsydra, with standard positions for the steelyard balance . Joseph Needham states that: </P>

Things used to measure time in modern days