How did Count Rumford show that heat is a form of energy?

Count Rumford (Sir Benjamin Thompson) challenged the prevailing 18th-century belief that heat was a weightless fluid called caloric. Instead, he proposed that heat is a form of motion.1

His breakthrough came in 1798 while observing the boring of cannons in a Bavarian arsenal. He noticed that the friction generated during the process produced a continuous supply of heat as long as the work of boring continued. In his classic paper, "An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction," he argued that because the heat could be produced indefinitely through mechanical action, it could not be a material substance contained within the metal.1

By demonstrating that mechanical work could be converted into heat, Rumford provided early evidence for the mechanical equivalent of heat. This work laid the foundation for the modern understanding of thermodynamics, establishing that heat and mechanical energy are interconvertible forms of energy.12