Until the early 1800's heat was believed to be a material substance called 'calorie'. It was assumed that heat 'flows' from hot bodies into cold bodies, the hot body transferring some of its calorie and warming to the cold body. It remained for the 18th century scientist Count Rumford ( Benjamin Thompson) to contribute significantly to what is currently accepted as the correct theory concerning the nature of heat. His experiments showed that motion-a form of energy could be transformed into is attached to one end of the string which passes over the pulley and the effort is applied at the other end.
The French scientist Jacques Charles discovered that heat could be transformed into motion. Sir James Prescott Joule, a 19th century physicist later proved that a given amount of mechanical energy always produced the same amount of heat.