James Joule (1818–1889) was an English brewer's son who proved, with fanatically careful experiments, that heat and motion are just two faces of the same thing: energy. The unit we count energy in today is named after him.
Joule showed that when energy seems to vanish — say a churning paddle warming a tub of water — it
hasn't disappeared at all; it has simply changed form. This is the heart of
Joule was so obsessed with the tiny temperature rise a waterfall should cause that he reportedly lugged a thermometer along on his honeymoon in the Alps to test it. He ran his precise experiments partly in his family's brewery, using his brewer's instinct for exact measurement. For years the professional scientists ignored the amateur, until the young William Thomson (later Lord Kelvin) recognised the brilliance of his work — and the two became lifelong collaborators, cementing one of the great laws of physics.