I've never used a water glass before but I'm sure they're lots of fun. I didn't know that was what their name was either, it is obvious though, huh. The idea of the water clock really interests me (we're all budding horologists, huh?). I like the name clepsydra too (Gr: 'steal water'). I assume they were popular in Greece at some point, but clearly in China too. Apparently there are only very few working ones left. According to one link it was possibly the first means of time measurement to exist besides the very primitive and modestly accurate sun dial. I tried to find an illustration or photo, but found no pictures just this...

http://www.horology.com/htt-clep.html