524 



NATURE 



[September 24, 1908 



their reach, in order to the calm and deliberate discussion 

 of the question." This resulted in the further Meeting of 

 May 17, 1S53. when Papers were read by Manby, Leslie, 

 and Siemens. The Paper by Manby consists of the 

 summarv of a discussion by M. Galy-Cazalet, which tool-: 

 place in Paris in 1852. M. Galy-Cazalet comes to the 

 conclusion that the regenerator involves a fallacy, and he 

 concludes : " There appears to be at present so much 

 doubt of the utility of the regenerator that it would be 

 \vise to abandon its use. for a time, and by trials with a 

 more simple form of caloric engine establish the fact either 

 of the superiority or of the inferiority of heated air in 

 comparison with steam as a motive power." Mr. Leslie, 

 on the contrary, in his Paper upholds vigorously the 

 accuracy of the principle of the regenerator or economiser. 

 He conies to the conclusion that it is based on true prin- 

 ciples and is attended in practice with real economy of 

 heat, and consequently of fuel. In this conclusion he is 

 doubtless correct ; the regenerator is useful and does 

 economise heat. But Leslie goes much further than 

 this ; he appears to support Stirling in the fallacy that the 

 regenerator may be made indefinitely useful. Stirling 

 states : — 



" And thus it 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 

 (i.e. beyond the series). It is evident also that if the series 

 had been composed of twenty points, or bodies, having a 

 difference of temperature of five degrees, the air might be 

 heated and cooled twenty times at no greater expense of 

 caloric. 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. 

 But let no mathematician be alarmed with the idea of a 

 perpetual motion, or the creation of power. There are 

 many enemies to contend with in the air engine besides 

 friction, which alone prevents perpetuity in some 

 mechanical motions. We have no means, without con- 

 suming a part of our power, of applying the air so closely 

 to the apparatus as to make it absolutely assume the 

 temperature of the bodies to which it is applied. There 

 is, therefore, a loss in the very act of heating and cooling." 

 Leslie comes to the conclusion that Stirling is right, 

 hut that an air engine without a regenerator would be a 

 much less effective and economical application of heat than 

 the steam engine. Leslie gives some interesting par- 

 ticulars of the later air engines of James Stirling. He 

 states that an engine of 45 horse-power was started in 

 March, 1S43, at the Dundee Foundry; that in December, 

 1845 — two years and nine months after starting — one air- 

 vessel gave way, and in May, 1846. another failed, and in 

 January, 1847. a third failed. This information was sup- 

 plied to him by Mr. David Mudie, one of the lessees of 

 the foundry. 



We now come to Siemens' Paper " On the Conversion 

 of Heat into Mechanical Effect," and for the first time 

 wp find the engineer guided by an intelligible principle. 

 Siemens discussed the material theory of heat, and accepted 

 unreservedly the dynamical theory, for which he gives a 

 large measure of credit to Joule. This is the first of the 

 Institution Papers in which I find the name of Joule. 

 Siemens mentions Carnot, Clapeyron, Holtzman of Mann- 

 heim, Joule. Helmholtz, Mever, Rankine, and Prof. 

 Thomson. Curiouslv enough, althotigh Siemens mentions 

 Carnot and the other philosophers who dealt with the 

 Carnot principle, including Thomson, he does not appear 

 at this date — May 17, 1853 — to have realised himself the 

 effect of the la%v of Carnot upon the theory of the heat 

 engine. He clearly appreciated the first law, and gives 

 the mechanical equivalent of heat as determined bv 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 Fahrenheit heat- 

 unit emnloyed. This is clear from a Table found on p. 33 

 oT the Paper, which I reproduce : — 



XO. 2030, VOL. 78] 



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 platx- 

 until the original temperature is reached, then he has a 

 perfect engine indicating the full result of Joule'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 hea'ting surface, occasioned apparently from misappre- 

 hension 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 important expenditure of heat, which should have 

 been provided for." 



Siemens, in the discussion, rightly upheld 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 useful, 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 

 onlv, but it was certain also that its intensitv 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 de- 

 scribed by both. Mr. Edward Woods certainly understood 

 Siemens to have given 772 foot-pounds as the efficiency 

 of an ideal heat engine, because he stated that this showed 

 there was still great room for improvement in engines. 

 Mr. E. A. Cowper had clear ideas ; he said : — 



" Steam, or gases, in expanding, and so giving out 

 power, lost heat. Part of the sensible heat became latent 

 in the production of power, and this heat could only be 

 recovered by expending the power already produced in 

 again condensing the steam back to its original bulk, when 

 the latent heat again became sensible." 



This discussion, then, puts us in the position of engineers 

 at the date of the last Meeting referred to — May i7. 1853. 

 Of all the distinguished engineers who spoke, Siemens 

 alone had thoroughly apprehended the value of Joule's 

 results and understood the full bearing of the mechanical 

 equivalent of heat. He had not, however, understood 

 Cnrnot's reasoniner on the Carnot cycle, or Thonison's 

 deductions from Carnot. He was under the impression 

 that heat added in anv way to a workiner fiuid. raisinr^ 

 the temoerature, could be entirely converted into work by 

 a sufficient expansion. He had not appreciated that, even 

 if expansion be carried far enough to reduce the tempera- 

 ture to the original temperature before heat addition, ^-et 

 complete con\-ersion of the entire mechanical equivalent 

 was impossible. \\'lien so able a man as .Siemens had nt 

 this stai?e only reached partial enlightenment, it was 

 evident that rnuch hard work and clear thinking required 

 to be done before a well-founded theory of heat motive- 



