The first half of the eighteenth century marks the real beginning of both the technology and the science of heat. In these fifty years it became clear that heat could be employed to do useful work, replacing that of men, horses, wind, or falling water. These had been the only available means to do significant work throughout human history. Theoretical ideas, which were clearly formulated by the end of the century, began to develop before 1750. The two most important of these were the suggestion that heat might be conserved and the distinction between amount of heat or quantity of heat and quality of heat. Quality of heat is what we now call temperature and the study of temperature is called thermometry. The study of amount of heat is called calorimetry.