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Sadi Carnot investigated what limits the work obtainable by transferring heat between hot and cold bodies. His reversible-engine argument compares machines without tying the ideal performance to a particular working substance. A reversible reference cycle can run backward, so a supposedly better engine could be combined with it to produce an impossible net result. This reasoning underlies Carnot's theorem and the modern bound $\eta \le 1-T_c/T_h$ for reservoirs at absolute temperatures $T_h>T_c$. The temperature notation is modern, rather than Carnot's original formulation.^[ref]