WILLIAM SIEMENS, F.R.S. 39 



xjiansive engine. Practically considered it is certainly inferior 

 to both, because one-half of the gross power of the working piston 

 is absorbed by the pump, and the losses by friction and leakage 

 are trebled in consequence. Moreover, it has been found by its 

 earliest promoter, Mr. Stirling, of Dundee, that the working of & 

 tight piston, in a highly -heated cylinder, is attended with almost 

 insurmountable difficulty. 



The most essential difference between the steam-engine and the 

 air-engine is, that in the former, the unproductive heat is expended 

 in the boiler, where it becomes latent, in effecting increase of 

 volume without displacement of the piston, whereas, in the latter, 

 it presents itself as free heat at the exhaust port. 



Mr. Stirling, and after him Captain Ericsson, in taking advan- 

 tage of this circumstance in favour of the air-engine, employed 

 the free lost heat to warm the fresh air on entering the reservoir 

 from the pump. 



Ericsson constructed an engine on this plan, in 1833, which, 

 according to received accounts, worked with considerable economy 

 of fuel, but failed, in consequence of a defective heating apparatus, 

 and the continual derangement of the working piston in a heated 

 cylinder. 



The apparatus he employed for recovering the lost heat re- 

 sembled a locomotive boiler, through the tubes of which the cold 

 air passed, while the heated and expanded air circulated in the 

 opposite direction, through the intervening spaces. He termed 

 this apparatus the " regenerator," because he supposed, that by 

 its means the same heat might be used perpetually over and over 

 again, to produce motive power. 



Stirling, of Dundee, patented an air-engine in 181 G, and 

 improvements in 1827 and in 1840. He constructed one of 

 these machines, an account of which he brought before the 

 Institution in 1845.* In it he had combined several important 

 advantages over former attempts, namely 



1st. The hot working piston was dispensed with. 



2nd. The working pressure was increased, by using the same 

 body of highly-compressed air over and over again. 



* Vide Minutes of Proceedings of the Institution of Civil Engineers for 1845, 

 Vol. IV. p. 348. 



