This is clever — in 1893 Texas inventor Martin Everhart patented a clock-winding mechanism that’s driven by rainwater. The water fills a tank in the attic and then drops through a pipe into a pail in the clock. The pail is balanced with a counterweight, so it falls and rises continuously, accepting a new measure of water at the top and discharging it at the bottom. This motion winds the clock.

I guess the whole thing would stop eventually in a drought, but the clock can be wound by hand if necessary.