Potential energy transferred to internal energy in a waterfall
From the results of his experiments, Joule came to the conclusion that the water at the bottom of a waterfall ought to be slightly warmer than that at the top. At the top of the fall the water possesses potential energy, which becomes transferred to kinetic energy as it descends. Part of this kinetic energy becomes transferred to internal molecular energy when the motion of the water is arrested at the bottom. Joule decided to carry out a test on a particular waterfall in Switzerland. Unfortunately, even with a very sensitive thermometer he failed to detect the small temperature rise expected, since the water was too broken by spray.