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THE SCIENTIFIC PAPERS OF 



or 60 foot-lbs., making a total of 150 foot-lbs., which nearly 

 equals the performance of the best Cornish engines. 



The following table is intended to convey a more distinct idea 

 of the comparative merits of the different steam and air engines 

 referred to. 



The statements of the actual performance of air-engines must 

 be considered as only rough approximations, as it is not possible 

 to calculate losses of heat, with any degree of precision. The 

 actual performance of the " Ericsson " engine may be deemed too 

 low, considering its theoretical superiority ; but the author con- 

 siders the disproportion to be fully accounted for, by the extra- 

 ordinary losses of useful effect, arising from the exposure of the 

 heated cylinder, and the low working pressure. 



In Stirling's engine, the heated cylinder is closed and sur- 

 rounded by flues and brickwork, in consequence of which, its 

 economical effect is thought to be equal to that of Ericsson's 

 engine, although it is in theory inferior. 



Thirdly. " On the necessary characteristics of a perfect engine." 



In the first part of this paper it has been shown, that an engine 

 would be theoretically perfect, if all the heat applied to the elastic 

 medium was consumed in its expansion behind a working piston 

 (or its substitutes, such as a disc, a flexible bag, &c.) leaving no 

 portion of it to be thrown into a condenser, or into the atmo- 

 sphere. In the second part, several actual engines have been 

 examined, with a view to test their degree of theoretical and 

 practical perfection. Such an inquiry should have for its end, 



