12 W. J. M. Rankine on the Means of 



In order to show the manner in which the pressure and 

 volume of elastic substances vary, in producing the maximum 

 theoretical efficiency of a thermo-dynamic-engine, so as to 

 verify in every case the general law, a supplement is added 

 to this section, containing detailed computations for three ex- 

 amples of theoretically perfect engines : viz., a steam-engine 

 working between 270° and 140°, an air-engine working be- 

 tween the same temperatures, and an air-engine working 

 between 650° and 150°. 



11. As the law above stated is true for all substances what- 

 soever in all conditions, it is obvious that, in a purely theo- 

 retical point of view, the only reason for preferring any one 

 substance to another, as the agent in converting heat into 

 mechanical power, is the greater ease and safety of causing it 

 to expand by heat at a high temperature. In this point of 

 view, permanent gases are preferable to vapours rising from 

 liquids ; for the density of a permanent gas can be regulated at 

 pleasure so as to limit its pressure at any temperature, how 

 elevated soever, to a safe and manageable amount ; whereas a 

 given vapour, while in contact with its liquid, has but one pos- 

 sible density for each given temperature, and consequently 

 but one possible pressure ; and as the pressures of the vapours 

 of all easily obtainable fluids increase very rapidly with the 

 temperature, it would be unsafe to use vapours at temperatures 

 at which it is safe and easy to use permanent gases. For ex- 

 ample, at the temperature of 650° Fahr. (measured from the 

 ordinary zero), a temperature up to which air-engines have 

 actually been worked with ease and safety, the pressure of 

 steam is 2100 pounds upon the square inch ; a pressure which 

 plainly renders it impracticable to work steam-engines with 

 safety at that temperature. 



Supplement to Section I. — A. Example of the Computation of 

 the power produced by the combustion of one pound of Coal in a 

 theoretically perfect Steam-Engine, working between the tempera- 

 tures of 270° and 140° of Fahrenheit. 



Data. 

 Mechanical equivalent of the whole available heat obtained 



by the combustion of 1 lb. of coal, 6,000,000 ft.-lb. 



tion of the temperatures of receiving and discharging heat; but the hypothesis 

 of the substantiality of heat misled him as to the nature of the function. 



