MOTIVE POWER OF HEAT. 195 



57. To obtain some notion of the economy which 

 has actually been obtained, we may take the al- 

 leged performances of the best Cornish engines, 

 aud some other interesting practical cases, as ex- 

 amples.* 



(1) The engine of the Fowey Consols mine was 

 reported, in 1845, to have given 125,089,000 foot- 

 pounds of effect, for the consumption of one 

 bushel or 94 Ibs. of coals. Now the average amount 

 evaporated from Cornish boilers, by one pound of 

 coal, is 8 Ibs. of steam ; and hence for each 

 pound of steam evaporated 156,556 foot-pounds of 

 work are produced. 



The pressure of the saturated steam in the boiler 

 may be taken as 3J atmospheres;! and, conse- 



hence the steam-engine worked in this most advantageous 

 way has in reality the very fault that Watt found in New- 

 comen's engine. This defect is partially remedied by 

 Hornblower's system of using a separate expansion cylin- 

 der, an arrangement the advantages of which did not 

 escape Caruot's notice, although they have not been recog- 

 nized extensively among practical engineers, until within 

 the last few years. 



* I am indebted to the kindness of Professor Gordon of 

 Glasgow for the information regarding the various cases 

 given in the text. 



f In different Cornish engines, the pressure in the boiler 



