James Prescott Joule (1818 - 1889) was a brewer who, due to his work studying heat, became one of the founders of the science of thermodynamics. He is best known for showing that mechanical work and heat are interchangeable, which he did by carefully measuring the tempearature increase in water stirred by paddle wheels.
Since he had no formal university training, "many British scientists were hesitant to accept his work, but Joule patiently persisted. New ideas often take time to gain acceptance, especially if they are put forward by an amateur in that field. Joule’s findings challenged the caloric theory of heat which most physicists believed in at that time. In the caloric theory, heat was believed to be a fluid substance."
The SI Unit of energy, the Joule, is named in his honor.