realizing the Advantages of the Air-Engine. 21 



leads to an important practical advantage ; for it is this air 

 alone which comes into contact with the cylinder, the piston, 

 the packings of the piston and those of the plunger-rod, which 

 are consequently never exposed to a high temperature. 



It was, perhaps, mainly in consequence of this, that Stir- 

 ling's engine, with its final improvements, required less oil and 

 fewer repairs, worked with less friction, and was less liable to 

 get out of order, when properly managed, than any steam- 

 engine. 



Stirling's air-engine employed to drive the machinery of the 

 Dundee Foundry, was double-acting, having two receivers, one 

 connected with the top and the other with the bottom of the 

 cylinder. The plungers of those receivers were suspended by 

 their rods from the opposite ends of a small beam. A reci- 

 procating motion was given to that beam by means of a piece 

 of mechanism which possessed a power of regulating the 

 length of stroke of the plungers ; and in its effect, though not 

 in its construction, was analogous to the link motion. 



The testimony of Mr James Stirling to the advantages of 

 this engine was corroborated by that of the late Mr Smith of 

 Deanston and by that of Mr James Leslie. 



20. Efficiency of Stirling's Air-Engine. — According to Mr 

 Stirling, the air in his engine received heat at the temperature 

 of 650° Fahr., and discharged the lost heat at that of 150° 

 Fahr. The fourth example of the table in Article 10 shows 

 that the efficiency of a theoretically perfect engine, with those 

 limits of temperature, would be 0*45, and its consumption of 

 coal 0*73 of a lb. per horse-power per hour. 



It appears that the actual consumption of coal per horse- 

 power per hour was about 2-2 lb., being three times the con- 

 sumption of a theoretically perfect engine, and corresponding 

 to an actual efficiency of 0-15, or one-third of the maximum 

 theoretical efficiency. 



Stirling's air-engine, therefore, was more economical than 

 any existing double-acting steam-engine, — probably indeed 

 more economical than any possible double-acting steam-engine. 



tion, and the mean effective pressure diminished in the same proportion : so 

 that the mechanical effect remains unaltered. 



