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



The Absolute Zero of Heat on a perfect-gas thermometer, 

 is that point on its scale which corresponds to total absence of 

 heat ; and from this point absolute temperatures are under- 

 stood to be reckoned, in the law stated in the next article. 



According to the latest determination, from the experiments 

 of Messrs Joule and Thomson, the Absolute Zero of Heat does 

 not differ by any amount appreciable in practice from the A b- 

 solute Zero of Pressure, being the temperature at which (if it 

 were possible for a perfect gas to preserve its properties at so 

 intense a degree of cold) the product of its pressure and volume 

 would be reduced to nothing ; and this point is 493° of Fahren- 

 heit below the temperature of melting ice ; that is to say, 

 493°-32° = 461 below Fahrenheit's ordinary zero.* 



10. The Law of the Maximum Efficiency of Thermo- 

 dynamic Engines is expressed by the following proportion : — 

 As the absolute temperature of receiving heat 

 is to the difference between the absolute temperatures 



of receiving and discharging heat, 

 so is the whole heat received 



to the portion of heat permanently converted into me- 

 chanical power ; 

 that is to say, 



so is unity to the efficiency of the engine. 

 This proportion may be otherwise expressed, as follows : — 

 As the absolute temperature of receiving heat 

 is to the absolute temperature of discharging heat, 

 so is the whole heat received 

 to the necessary loss ofheat.\ 



* The product of the volume in cubic feet of one pound of atmospheric 

 air, by its pressure in pounds on the square foot, at the temperature of melting 

 ice, is 26,214 foot-pounds. The corresponding product, at any other tempe- 

 rature, is found with a degree of accuracy sufficient for practical purposes, by 



26,214 

 multiplying the absolute temperature by =: 53*172 foot-pounds per 



degree of Fahrenheit. In the detailed investigations already referred to, the 

 absolute zeros of heat and of pressure have had various positions assigned to 

 them as the most probable, within a range of about 4° Fahrenheit, according 

 to the degree of precision of the experimental data, existing at the periods 

 when the several papers were written. This range of variation, however, is 

 not sufficient to cause any error of practical importance in calculations re- 

 specting engines. 



t There are other forms in which .this law might be expressed ; but those 



