"After the invention of the steam-engine... by James Watt, the attention of engineers and of scientific men was directed to... its further improvement. ...Sadi Carnot, in 1824, published Réflexions sur la Puissance Motrice du Feu... [which] examined the relations between and the work done by heat used in an ideal engine, and by reducing the problem to its simplest form and avoiding...questions relating to details, he succeeded in establishing the conditions upon which the economical working of all heat-engines depends. ...Though the proof was invalid, the proposition remained true... Carnot's memoir remained for a long time unappreciated, and it was not until use was made of it by William Thomson... in 1848, to establish an absolute scale of temperature, that the merits of the method proposed in it were recognized. ...[H]e found that Carnot's proposition could no longer be proved by denying the possibility of "the ," and was led to lay down a second fundamental principle... now called the Second Law of Thermodynamics. ...It was published in March, 1851. In the previous year Clausias published a discussion of the same question... in which he lays down a principle for use in the demonstration of Carnot’s proposition, which, while not the same in form as Thomson’s, is the same in content, and ranks as another statement of the Second Law of Thermodynamics. Clausius followed up this paper by others, and subsequently published a book in which the subject of Thermodynamics was given a systematic treatment, and in which he introduced and developed the important function called by him the ."