Quantitative studies by Joule from 1843 onwards provided soundly reproducible phenomena, and helped to place the subject of thermodynamics on a solid footing. In 1843, Joule experimentally found the mechanical equivalent of heat. In 1845, Joule reported his best-known experiment, involving the use of a falling weight to spin a paddle-wheel in a barrel of water, which allowed him to estimate a mechanical equivalent of heat of 819 ft·lbf/Btu (4.41 J/cal). This led to the theory of conservation of energy and explained why heat can do work.