116 Prof. Thomson on the Dynamical Theory of Heat. 



39. Supplementary Table of the Motive Poicers of Heat. 



40. Taking the range 30^ to 140^ as an example suitable to 

 the circnnistanees of some of the best steam-engines that have 

 yet been made (see Appendix to Account of Carnot's Theory, 

 Sec. y.), we find in column III. of the supplementary table, 377 

 ft.-lbs. as the corresponding duty of a unit of heat instead of 

 440, shown in column III., which is Carnot^s theoretical duty. 

 We conclude that the recorded performance of the Fowey-Consols 

 engine in 1845, instead of being only 57^ per cent, amounted 

 really to 67 per cent., or f of the duty of a perfect engine with 

 the same range of temperature ; and this duty being -271 (rather 

 more than ^) of the whole equivalent of the heat used ; we con- 

 clude further, that ■^, or 18 per cent, of the whole heat sup- 

 plied, was actually converted into mechanical effect by that 

 steam-engine. 



41. The numbers in the lower part of the supplementary 

 table show the great advantage that may be anticipated from the 

 perfecting of the air-engine, or any other kind of thermo-dynamic 

 engine in which the range of the temperature can be increased 

 much beyond the limits actually attainable in steam-engines. 

 Thus an air-engine, with its hot part at 600°, and its cold part at 

 0° Cent., working with perfect oeconomy, would convert 76 per 

 cent, of the whole heat used into mechanical effect ; or working 

 with such oeconomy as has been estimated for the Fowey-Consols 

 engine, that is, producing 67 ])er cent, of the theoretical duty 

 corresponding to its range of temperature, would convert 51 per 

 cent, of all the heat used into mechanical effect. 



42. It was suggested to me by Mr. Joule, in a letter dated 

 December 9, 1848, that the true value of /x might be " inversely 



