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



As compared, however, with a theoretically perfect engine, 

 working between the same temperatures, it appears that two- 

 thirds of its expenditure of heat was wasted. 



21. Causes of waste in Stirling 'a Air-Engine. — We shall 

 now investigate the causes of waste in Stirling's air-engine 

 according to the classification explained in article 12. 



(1.) Imperfect communication of heat from the burning 

 fuel to the working substance. — As the heating surface in 

 Stirling's air-engine consisted simply of the hemispherical 

 bottoms of the receivers, it was of the worst form possible for 

 exposing a large surface within a given space. A steam-boiler 

 of that form would occasion an enormous waste of fuel ; it is 

 probable, therefore, that this first cause of waste operated 

 powerfully in Stirling's engine. 



(2.) Imperfect abstraction of the lost heat. — It is probable 

 that Stirling's engine was comparatively free from this cause 

 of waste, for the cold-water tubes exposed a large surface, and 

 were abundantly supplied with water. 



(3.) The communication of heat to or from the working 

 substance at improper periods of the stroke. — This cause 

 must have operated powerfully to occasion waste of heat in 

 Stirling's engine, for the following reason : — It is obvious, 

 from the construction of the engine, that the air, whether 

 being expanded or compressed, must have been continually 

 circulating over the heated bottom of the receiver, and re- 

 ceiving heat through it from the furnace, at all periods of the 

 stroke. Now it is only while the air is being expanded that 

 the heat received by it is effective in producing power ; while 

 the air is being compressed, the heat received by it is de- 

 trimental. The heat received, therefore, by the air in Stir- 

 ling's engine during at least one-half of each stroke — that is 

 to say, probably one-half of the heat received — must have been 

 absolutely wasted : it would be simply carried to the cold 

 water tubes, and there abstracted, without producing any 

 work. It is probable that, in an air-engine free from such 

 cause of waste of heat, a much smaller. extent of cooling sur- 

 face would be found sufficient to abstract the lost heat. 



(4.) Expenditure of heat in elevating the temperature of the 

 working substance. — In the air-engine, the sensible heat of 



