For the simplicity's sake, I've used the familiar word
catapult. Historians prefer to use the French word
trebuchet. The correct term for the counterbalanced
lifting device is swape. The swape lets you divide
the force of lifting in two. Typically, to lift 80
pounds of water, you would push the bucket down
into the water with a 40 pound force. Then it would
take a 40 pound force in the other direction to lift
the filled bucket.

I remember seeing "well sweep" for long lever for raising water from well. There is
mention of it, and a picture (not very good) at
http://www.dabar.org/Breasted/SECTION-5/P36-BREASTED.html
The first quote came from http://www.uh.edu/engines/epi458.htm