822 TRANSACTIONS OF SECTION G. 



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 o 

 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 17, 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, imderstood Caruot's reasoning on the Camot cycle, or Thomson's 

 deductions fi'om Carnot. He was under the impression that heat added in any 

 way to a working fluid, raising the temperature, 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 temperature to the original temperature before 

 heat addition, yet complete conversion of the entire mechanical equivalent was 

 impossible. When so able a man as Siemens had at this stage only reached 

 partial enlightenment, it was evident that much hard work and clear thinking 

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

 obtained. The data for such a theory was accumulating; and one of the most 

 interesting circumstances connected with these Institution of Civil Engineers 

 papers was a communication from M. llegnault to Colonel Sabine, Treasurer of 

 the lloyal Society, dated April 1853, which was read at the meeting, in Avhich 

 liegnault stated that 



' He was about to publish immediately a series of elaborate experimental 

 researches on various subjects connected with the effects of heat on elastic fluids, 

 the results of which would solve many questions long in dispute, and by means 

 of which engineers might accurately calculate the effect of a given amount of 

 fuel, in whatever way it was applied. JM. liegnault communicated in anticipation 

 that he had arrived at the number 0'237 ibr the specific heat of air at constant 

 pressure, and at 0'475 for that of steam under atmospheric elasticity, the specific 

 iieat of water being taken in each case as unity.' 



True to his word, liegnault produced his admirable investigations, and suc- 

 ceeded in solving many problems ; but he did not settle the questions to the 

 extent he had hoped. Even at the present time doubt arises as to the very 

 values he gave for the specific heat of air and steam. The problem proved much 

 more dillicult than he had anticipated, and for modern engine purposes it cannot 

 be considered as wholly solved now — fifty-five years later. 



This description of the position of the hot-air engine, as shown by the opinions 

 of eminent engineers, is most useful as proving how much practical men were 

 in need of the work of Thomson and Joule. It is not surprising that, of all the 

 engineers present, Siemens appeared to be alone in thoroughly grasping the new 

 ideas. Thomson's own conversion from the material theory of heat to the dyna- 

 mical theory was not complete until 1851, and although he had then succeeded 

 in reconciling the ideas of .Joule and Carnot, it is not to be wondered at that 

 engineers two years later had not quite succeeded in grasping the combination of 

 the two laws. This combination, however, supplied engineers with a new and 

 accurate standard of measurement for studying and improving upon their heat 

 engines, and they were by no means slow in grasping the help thus oiTered them 

 by the abstract scientific man. The broad laws of thermodynamics have placed 

 the theory of the heat engine in a position of certainty, which was much needed. 

 It would be a mistake to assume, however, that even the determination of the 

 mechanical equivalent of heat and the second law of thermodynamics expressed 

 in terms of an absolute thermometric scale had solved all the difliculties of the 

 engineer desiring to determine the efficiency of his heat engines. Thomson 



