Hall of Union and Peace was built in the Ming Dynasty. In this building the Qing emperors conferred honorable titles on their empresses. The empresses also held their birthday celebrations here. Since the reign of the Qing Qianlong emperor, 25 jade seals representing imperial authority have been kept in this hall. Legend has it that twenty-five indicated that the imperial power could last 25 generations. After the Oing emperor Qianlong, only six emperors were on the throne. Legend continues that the heavenly number was 25, because it is the sum of the odd numbers: one, three, five, seven and nine. The 25 imperial seals were the certificates for the emperor to exercise his power.
On the east is a water clock made in 1745. Water clocks had been used for quite a long time before the mechanical clock was introduced into China. Chinese people invented the method of keeping time by the method of dripping water more than 2, 500 years ago. The timepiece consists of five bronze vessels. Each vessel has a small hole at the bottom. When the uppermost vessel is filled with water, then water begins to drip evenly through the holes. There is a figurine in the lowest container. The calibrated scale in his hands floats up with the rising water, and time is indicated on the markings.
To the west is a chiming clock made in 1789 by the Workshop of the Board of Imperial Household.