J4 Dugald Clerk, The Work and Discoveries of Joule 



sion which followed them. Many distinguished men joined 

 in the discussions, including James Stirling, Robert Stephen- 

 son, Sir George Cayley, C. W. Siemens, Professor Faraday, 

 and Isambard K. Brunei, the designer of the " Great 

 Eastern." 



The 1845 paper was a '* Description of Stirling's Improved 

 Air Engine " by James Stirling, M.Inst.C.E., a brother of the 

 inventor, the Rev. Dr. Stirling, a Scottish clergyman, who 

 produced his first regenerative hot air engine in the year 18 15. 

 The paper described James Stirling's later development, which 

 consisted in the use of air at a greater density than the atmo- 

 sphere within the engine which was in operation for some 

 years at the Dundee Foundry Company's Works. Two 

 engines were built of 21 and 45 H.P. respectively, the smaller 

 engine consumed, it was stated, only 2 \ lbs. of coal per H.P. 

 hour. This was an extraordinarily good result — about one- 

 half of the most economical steam engine mentioned by 

 Joule. The 1853 meetings were occupied with four papers : 

 (1) " On the use of Heated Air as a Motive Power," by 

 Benjamin Cheverton ; (2) "On the Caloric Engine," by 

 Charles Manby; (3) "On the Principle of the Caloric Air 

 Heated Engine," by James Leslie; and (4) " On the Conver- 

 sion of Heat into Mechanical Effect," by Charles William 

 Siemens, A.M.I.C.E. 



The readers of the papers, and those who joined in the 

 discussion, except Siemens, clearly had no information as to 

 Joule's determinations, and they all misunderstood the action 

 of the heat regenerator. Stirling said : — 



"And it thus appears that by applying air successively 

 to a series of bodies regularly increasing in temperature, 

 and moving it alternately from one end of the series to the 

 other, it may be 'heated and cooled ten times, with an 

 expenditure of caloric which would barely have heated it 

 once, if it had been applied at once, to the hottest body. 



Nay it is evident that by multiplying the members 



of the series indefinitely air could be heated and expanded 

 and made to do work at no appreciable expense." 



Nearly all those present had no idea that heat disappeared 

 in performing work, and they agreed with Sir William 

 Thomson's 1848 paper, that " the conversion of heat (or 

 caloric) into' mechanical effect is practically impossible, cer- 

 tainly undiscovered." That is, that the source of power in 

 heat engines is not in conversion or absorption but merely in 



