September 24, 1908J 



A^A TURE 



regeneration as applied to steam engines, although his 

 work on the air engine was still to come. All were dis- 

 tinguished men in their day, and their opinions may be 

 taken as representing the very best scientific knowledge 

 ol the leading engineers of the day. The first of the 

 Tapers to which I refer is called " Description of Stirling's 

 Improved Air Engine," by James Stirling, M.Inst.C.E. 

 It was read on June to, 1845, with Sir John Rennie, the 

 President of the Institution, in the chair. The engine 

 described was the later form of the well-known Stirling 

 air engine, invented by the Rev. Dr. Stirling, a Scottish 

 clergyman, in the year 1815. The development considered 

 was the invention of the reader of the Paper, a brother 

 of Dr. Stirling. The main improvement consisted in the 

 use of air at a greater density than the atmosphere, and 

 the engine at that date had so far succeeded that two had 

 been used at the Dundee Foundry Company's works — one 

 giving about 21 horse-power and the other about 45 horse- 

 power. Practically, therefore, some success had been 

 attained. Mr. Stirling claimed that the 21-horse engine 

 consumed 50 lb. of coal per hour, which is about 2^ lb. 

 per horse-power per hour. This was an extraordinarily 

 good result for the time. At present, however, we are 

 not interested in the practical result, but only in the 

 opinions of the engineers of the day as to the fundamental 

 principles of heat engines. 



It is clear from the Paper that the theory of the 

 regenerator was entirely misunderstood. It was imagined 

 that with a perfect regenerator no heat would be required 

 to perform work. This is evident from Mr. Stirling's 

 answer to .Sir George Cayley. Sir George Cayley described 

 his engine, which was of the internal-combustion type, 

 acting with solid fuel under constant pressure, and showed 

 that, ow-ing to dust and heat in the cylinder and valves, 

 his experiments proved abortive. He stated, however, that 

 his engine had consumed b\ lb. of coke — equal to 9 lb. of 

 coal — per horse-power. To this Mr. Stirling answered : 

 " It must be remarked that Sir George Cayley, in follow- 

 ing an entirely diff'erent object, had overlooked the great 

 leading principle of repeatedly using the same heat," and 

 " he was of opinion that, except on that principle, the air 

 could not be economically used as a moving power." 

 .\nother speaker, -Mr. Cottam, said : " It was evident that, 

 if it was practicable to arrive at the theoretical condition 

 of the absorption of all the caloric by the thin lamina; 

 during the upward passage of the air and the giving it 

 out again during the downward passage, there would not 

 be any loss of heat." Mr. Robert Stephenson did not 

 appear to understand Stirling's air engine at all, because 

 he made the following remarks : *' He understood the 

 process to consist of heating the air in a vessel, whence it 

 ascended to the cylinder between numerous thin laminae, 

 by which the caloric was absorbed, to be again given out 

 to the descending air. Now it appeared to him that, 

 though the ascending process was natural and easy, the 

 reverse action would require a certain expenditure of 

 power, in the depression of the plunger." This remark 

 clearly showed that Stephenson, notwithstanding his 

 eminence as an engineer, at that date had not appreciated 

 the essential conditions of the hot-air engine. 



In the year 1853 the subject of the air engine again 

 came up before the Institution of Civil Engineers, interest 

 being excited evidently by the building of the large engines 

 of the hot-air ship Ericsson in America, the engines having 

 air cylinders of no less than 14 feet diameter. Four Papers 

 were read in this year : " On the Use of Heated .Air as a 

 Motive Power," by Benjamin Cheverton ; " On the Caloric 

 Engine," by Charles Manby ; "On the Principle of the 

 Caloric Air Heated Engine," by James Leslie, M.Inst.C.E. : 

 and " On the Conversion of Heat into Mechanical Effect," 

 by Charles William Siemens. .A.M.I.C.E. 



Cheverton evidently considers, from his Paper referring 

 to Stirling and Ericsson, that " Both parties also rest the 

 efficiency of their engines on the repeated use of caloric. 

 They contend that in recovering from the ejected hot air 

 the caloric which gave it superior tension, and employing 

 it in heating the injected air, ' it is made to operate over 

 and over again.' Mr. Ericsson aspires to embody a new 

 principle in motive mechanics — no less, to use his own 

 words, than ' that the production of mechanical force by 



NO. 2030, VOL. 78] 



heat is unaccompanied by the loss of heat,' except such as. 

 arises from radiation, or other practically unavoidable, 

 waste." Cheverton rejects this idea, but, strangely 

 enough, does not appear aware of the work either of 

 Carnot or of Joule. He comes to the conclusion, however, 

 that ** caloric, doubtless, is in all its aspects a manifesta- 

 tion of force, and unquestionably, as a mechanical agent, 

 of a dynamic force, and therefore is directly amenable to 

 the third law of motion." He appears to think that heat 

 is accompanied with molecular activity, but is puzzled by 

 what he accepts to be a fact, that in the steam engine the 

 whole of the heat of the steam as it comes from the boiler 

 is found in the condenser. With regard to the steam, he 

 says : " Undoubtedly, in respect to the materiality of 

 caloric, if it be material, it is transferred intact to the 

 condenser, yet in its passage it may have parted with 

 force, which it cannot communicate again." He comes to 

 the conclusion that the change may take place, not in the 

 quantity, but in the intensity of heat. Here he resembles 

 Carnot ; but it appears to liim impossible to arrive at any 

 useful theory of the heat engine, because he states : 

 " . . . for every investigation leads to the conclusion that 

 the effect of caloric is independent at least of the chemical, 

 if not also of the physical, constitution of bodies. But 

 economy of fuel is a different question from the economy 

 of caloric ; it is altogether a practical matter, and can only 

 be determined by experiment ; for this, and, indeed, most 

 other points of practice, are too intractable to come within 

 the grasp of the most powerful calculus." In the dis- 

 cussion a communication was read from Sir George Cayley, 

 in the course of which he states, with regard to the re- 

 generator : " There can exist no doubt of the effective 

 re-application of heat to an almost unlimited extent by this 

 beautiful invention, due originally to Mr. Stirling, and 

 now carried out to a greater extent by Captain Ericsson." 

 Sir George Cayley discussed the difficulties of Ericsson's 

 engine, but he accepts the principle that heat mav give 

 work and yet be used over and over again practically un- 

 diminished. Armstrong did not express himself u'pon the 

 theory at all, but he was doubtful as to the advantage 

 of the air engine compared with the steam engine, although 

 he believed that it was practicable to recover and use over 

 again a large proportion of the heat applied, and he 

 thought the balance of economy, so far as heat was con- 

 cerned, would be found in favour of air. Siemens agreed 

 to some extent in the advantages of a regenerator, but 

 he showed clearly tliat expansion doing work was accom- 

 panied by a diminution of temperature, and stated that 

 this heat had to b^ replaced by the fire. Bidder was of 

 opinion " that no theoretical advantage was obtained in 

 using heated air instead of vaporised water as a motive 

 power, and it was incapable of being applied practically 

 with as much convenience." It is most interesting to note 

 that Dr. Faraday joined in this discussion. He said very 

 little, and I will give his remarks complete. Dr. Faraday 

 said : " Twenty years ago he had directed his attention 

 to this question, and from theoretical views he had been 

 induced to hope for the successful employment of heated' 

 air as a motive power ; but even then he saw enough to 

 discourage his sanguine expectation, and he had, with 

 some diffidence, ventured to express his conviction of the 

 almost unconquerable practical difficulties surrounding the 

 case, and of the fallacy of the presumed advantages of the 

 regenerator. He still retained his doubts as to the success 

 of the innovation, and feared the eventual results, even 

 of Captain Ericsson's spirited and ingenious efforts." 

 Brunei considered the use of the regenerator to be an 

 entire fallacy, and did not believe that the power derived 

 from the expansion of air by heat could be used effectively, 

 and then be recovered and used again. Mr. Hawksley 

 considered that the machine involved a mechanical fallacy 

 and that the regenerator produced no mechanical effect 

 whatever. Mr. Rendel was the President at the Meeting 

 which dealt with Mr. Cheverton 's Paper, and, in view of 

 the great differences of opinion on tlie subject, he stated 

 that " he would not have the Meeting arrive at a hasty 

 or erroneous conclusion on the question of this engine, 

 and he therefore suggested that Mr. Siemens should draw 

 up a Paper on the subject, and that the Members should 

 collect, for a future Meeting, all the information within 



