In the first book of Hero's Pneumatics, chaps. 4–6, there is a description of a way in which it is possible to make water flow at a constant and adjustable rate. This is the old problem of the water-clock, in which the flow of water has to be constant during the day, but must be adjusted from day to day to suit the unequal length of the hours, which were reckoned from sunrise to sunset, and so were long in summer but short in winter. Hero solves the problem in the following way :—

In the container he places a siphon, through which the water flows ; this siphon he places on 2 float, which sinks down slowly as the water drains out. The rate of the flow depends upon the depth of the outer end of the siphon below the surface of the water of the container; as the float follows the surface, the depth is constant, and so is the rate of the flow.