818 TRANSACTIONS OF SKCTlON O. 



papers by eminent men of the day, supply an accurate measure of the knowledge 

 possessed by the engineer of the principles of action of his heat engines. Many 

 distinguished names occur in these papers and discussions, including James 

 Stirling, llobert Stephenson, Sir George Cayley, Charles Manby, James Leslie, 

 C. W. Siemens, Hawksley, Pole, W. G. Armstrong (afterwards Lord Armstrong), 

 Edward Woods, E. A. Cowper, D. K. Clark, Benjamin Cheverton, Goldsworthy 

 Gurney, George P. Bidder, Professor Faraday, Isambard K. Brunei, Captain 

 Fitzroy, and F. Braithwaite. At the date of the later of these discussions 

 Brunei had already designed the 'Great Eastern,' in 1852, with its engines of 

 11,000 h.p. Armstrong was a Fellow of the Eoyal Society, and had started the 

 Elswick Works and invented the Armstrong gun. llobert Stephenson was at the 

 height of his fame. He was then a Member of Parliament, President of the 

 Institution of Civil Engineers, and a Fellow of the Royal Society. Siemens was 

 a young man, but was busy on the regenerative furnace ; had considered 

 regeneration as applied to steam engines, although his work on the air engine 

 was still to come. All Avere distinguished men in their day, and their opinions 

 may be taken as representing the very best scientific knowledge of the leading 

 engineers of the day. The first of the papers 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 10, 1845, with Sir John Kennie, the l^resident 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 clergy- 

 man, 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 h.p. and the other about 45 h.p. I'racti- 

 cally, therefore, some success had been attained. Mr. Stirling claimed that the 

 21-horse engine consumed 50 lb. of coal per hour, which is about 2i 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 lundamental principles of heat 

 engines. 



It is clear from the paper that the theory of the regenerator was entirely mis- 

 understood. 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 lype, acting with solid fuel under constant pressure, and 

 showed that, owing to dust and heat in the cylinder and valves, his experiments 

 proved abortive. He stated, however, that his engine had consumed 6j lb. of 

 coke — equal to 91b. of coal — per horse-power. To this Mr. Stirling answered : 'It 

 must be remarked that Sir George Cayley, in following an entirely dilierent 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.' Another 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 laminte during the upward passage of the air and the 

 giving it out again during the downward passage, there Avould 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 laminte, 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 aclion would require a certain expendi- 

 ture 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 engiue again came up before the 

 Institution of Civil Engineers, interest being excited evidently by the building of 



