What is this and what purpose does it serve?

This is the replica of the very first of a very important modern device. It was made by a Chinese Chan Deng(78-139 AD). It is a bronze vessel about 2m in diameter. At 8 points around the top are dragons heads holding bronze balls. The mechanism was that the ball would fall from the dragon's mouth into the toad's open mouth. What was the use of this device??

Try to think of a good answer if you do not know this. Googling will give you the answer but not the satisfaction.