realizing the Advantages of the Air -Engine. 11 



To illustrate the above law, the following table is added, 

 showing four examples of the efficiencies of theoretically perfect 

 engines working between limits of temperature to which there 

 will be occasion to refer in the sequel : the 7th column shows, 

 in each case, the maximum theoretical duty of a pound of coal, 

 supposing, as before, that the whole available heat of its 

 combustion is equivalent to 6,000,000 foot-pounds : the 8th 

 column shows, for each example, the corresponding minimum 

 theoretical consumption of coal per horse-power per hour : the 

 limits of temperature chosen in the five examples are respec- 

 tively as follows : — 



(1.) The limits of temperature for a condensing steam-engine, 

 with a pressure of 42 lb. per square inch in the boiler, and 

 2*9 lb. per square inch in the condenser : (in every instance in 

 this paper in which a pressure is mentioned, it is to be under- 

 stood to mean the total pressure and not the excess above the 

 pressure of the atmosphere). 



(2.) The limits of temperature for a non-condensing steam- 

 engine with a pressure of 153 lb. per square inch in the boiler. 



(3.) A probable estimate of the limits of temperature of 

 Ericsson's air-engine of 1852. 



(4.) The same for Stirling's air-engine, and also for that of 

 Napier and Rankine. 



The actual efficiency of these engines will be considered in 

 another part of this paper. 



Examples of Maximum Theoretical Efficiency. 



I 



M 



© 



Temperatures in degrees of 

 Fahrenheit. 



Maximum theo- 

 retical Efficiencies. 



Maximum theo- 

 retical Duties of 

 one pound of 

 Coal,ft.-lb. 



Minimum 



theoretical 



Consumption 



of Coal per 



horse-power 



per hour, lb. 



Ordinary. 



Absolute. 



Higher. 



Lower. 



Highei-. 



Lower. 



1 



2 

 3 



4 



270 

 360 

 480 

 650 



140 

 212 

 100 

 150 



731 



821 



941 



1111 



601 

 673 

 561 

 611 



iff — 0-178 



||f = 0-180 



f f-2 = 0-404 



yVVi = 0-450 



1,067,000 

 1,080,000 

 2,424,000 

 2,700,000 



1-86 

 1-83 

 0-82 

 0-73 



stated above are the most readily applicable to the performance of engines 

 worked by heat, and are therefore to be preferred in a paper such as the present. 

 See Trans. Royal Soc, Edin., vol. xx. (1850 to 1853), and Phil. Trans., 1854. 



Carnot was the first to perceive, that the maximum effect of the expendi- 

 ture of a given quantity of heat in a thermo-dynamic engine must be a func- 



