Ctesibius invented the Improved Clepsydra (Water Clock): A …

Ctesibius invented the Improved Clepsydra (Water Clock): A more accurate water clock for measuring time, which became widely used in antiquity.
Basically, that apparatus had 2 sections. One water buffer and a collector. The buffer was dropping, water drops, that mathematically was used to count the time in the collector or reservoir of the drops.
Altough he didn't know Calculus, it was an empirical way to make linear the rate of filling of the reservoir used to collect the drops.
The pace of that filling was proportional to the time, and thus it was a "water clock" Here an example .