In the adjacent diagram, from Carnot's 1824 work, Reflections on the Motive Power of Fire, there are "two bodies A and B, kept each at a constant temperature, that of A being higher than that of B. These two bodies to which we can give, or from which we can remove the heat without causing their temperatures to vary, exercise the functions of two unlimited reservoirs of caloric. We will call the first the furnace and the second the refrigerator.”English translation by Thurston (Carnot, 1890, p. 51-52). Carnot then explains how we can obtain motive power, i.e., “work”, by carrying a certain quantity of heat from body A to body B.
