If you’ve been following this blog, you’ll remember that a couple of posts ago I ranted about how Egyptian water-clocks seemed impractical and I didn’t see how they could even function as clocks at all.
Well, apparently back around 270 bc, an inventor named Ctesibius (Teh-SEE-bee-us) of Alexandria thought the same thing. He identified two problems:
One) The water-clock wasn’t a clock, but rather a timer. It only worked while there was water in it.
Two) The water came out of the bung-hole at the bottom at different rates of speed: quickly when the jar was full, slowly as it grew empty. That’s because the weight of the water on top pushed down on the water that was escaping—less water, less pressure, slower dripping of water. That made it unreliable for keeping time.
So how did Ctesibius fix these problems? Well, he figured in order to keep constant pressure on the hole at the bottom, the water clock should always be full. So he set up a second jar of water to keep the first one filled. The second jar had a hole at the bottom that leaked water into the first jar.
THEN, a third, empty jar was placed under the first jar. Instead of telling the time by how much water had leaked out, this empty jar told time by how much water had leaked into it.
Ctesibius even made a float to put into the empty jar. As the water level rose, an arrow—attached to the float—pointed to the hour.
A tip of the hat to Heidi K. for sending me a link to this video!