What scientist first showed a clear connection between mechanical energy and heat?

The scientist who first demonstrated a clear connection between mechanical energy and heat was James Prescott Joule. In the mid-19th century, Joule conducted experiments that quantified the relationship between mechanical work and heat, leading to the formulation of the first law of thermodynamics. His work established that energy is conserved and can be transformed from one form to another, specifically showing how mechanical energy could be converted into thermal energy.