Carnot asks his reader to picture an ideal steam engine—one that, for a given flow of heat from a hot place to a cold one, can produce the maximum amount of motive power possible, i.e., it can lift a given weight to the greatest possible height. (For simplicity, I’ll refer to the source of the heat as the furnace and the cooler place where the heat ends up as the sink.) Next, Carnot proposes a hypothetical machine that does the same process in reverse—i.e., it uses up motive power to move heat from a cool place to a warmer one. In the modern world, we call such devices heat pumps or
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