James Prescott Joule, an English scientist, was the first to empirically prove the interchangeability of heat and work, laying the groundwork for the First Law of Thermodynamics. His most important experiment investigated the relationship between mechanical work and heat using a setup that incorporated falling weights to turn paddles in water. Joule discovered that the mechanical energy lost by the weights was proportional to the rise in temperature in the water. This led to the development of the idea that energy, whether in the form of heat or mechanical work, is conserved and interchangeable.