An old wooden boat is suspended from a metal frame like a swing, moving forward and backward in an unnaturally slow and almost imperceptible motion. The boat neither assumes the bouncing rhythm of a wave, nor the smooth alter- nation of a pendulum, but instead seems to hover. This condition is generated by two water tanks at either end of the boat, into which water is slowly pumped back and forth. The shifting weight of the water lends the boat its gradual, creaking movement.
Source:http://juliusvonbismarck.com/bank/index.php?/projects/hoch-und-runter/