PRESIDENTIAL ADDRESS. 



821 



Thomson, lie (loes not appear at tins date — JMay 17, 1853— to liavc realised 

 liimself the cfl'ect of the law of Carnot upon the theory of the heat engine, lie 

 clearly appreciated the first law, and gives the mechanical equivalent of heat as 

 determined by Joule at 770 foot-pounds, and by Thomson's formula as 772 foot- 

 pounds, but in his discussion of the principles of the heat engine he is of opinion 

 that a perfect engine is ideally possible giving 770 foot-pounds for each I'ahren- 

 heit heat-unit employed. This is clear from a table found on page 33 of the 

 paper, which I reproduce: — 



Siemens' Table of 1853. 



He apprehends the mechanical equivalent of heat, but he still appears under 

 the impression that if heat be added to a certain upper temperature and expansion 

 take place until the original temperature is reached, then he has a perfect engine 

 indicating the full result of .Toule's mechanical equivalent. He sees, however, 

 that the old theory of the regenerator is quite wrong. He states : — 



' The cause of the failure of Mr. Stirling's engine in practice may apparently 

 be traced chiefly to insufficiency of heating surface, occasioned apparently from 

 misapprehension of the principle involved, it having been thought that the same 

 heat would serve over and over again to produce power, and that the necessary 

 expenditure of heat consisted only in the mechanical loss by imperfect action of the 

 respirative plates, which were approached to each other to the utmost limits, 

 consistent with an unobstructed passage of the air. By the aid of the dynamical 

 theory of heat it has been shown that there is another and far more importact 

 expenditure of heat, which should have been provided for.' 



Siemens, in the discussion, rightly ujjheld the regenerator as useful, and 

 saw that there were limitations to its use. Mr. Hawksley contended that the 

 regenerator was useless. Mr. Pole considered that the regenerator was usefid, 

 but he did not definitely adopt the mechanical theory of heat. He stated :— 



' It must be allowed that the general action of caloric in producing power 

 was still involved in much obscurity. The heat was often considered in 

 reference to its quantity only, but it was certain also that its intensity performed 

 a very important part; and it had even been surmised that power might be 

 obtained by the reduction of intensity alone, without any change of quantity.' 



Armstrong concurred with Siemens and Pole. He believed in the utility of 

 the regenerator, limited as described by both. Mr. Edward Woods certainly 

 understood Siemens to have given 772 foot-pounds as the efficiency of an ideal 



