CARNOT S THEORY OF THE MOTIVE POWER OF HEAT. 571 



57. To obtain some notion of the economy which has actually been obtained, 

 we may take the alleged performances of the best Cornish engines, and some 

 other interesting practica cases as examples. ' 



(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 lbs. of 

 coals. Now, the average amount evaporated from Cornish boilers, by one pound 

 of coal, is 8ilbs. of steam ; and hence, for each pound of steam evaporated 156,566 

 foot-pounds of work are produced. 



The pressure of the saturated steam in the boiler may be taken as 3i- atmo- 

 spheres ;t and, consequently, the temperature of the water will be 140". Now 

 (Regnault, end of Memoire X.), the latent heat of a pound of saturated steam at 

 140° is 508, and since, to compensate for each pound of steam removed from the 

 boiler in the working of the engine, a pound of water, at the temperature of the 

 condenser, which may be estimated at 30", is introduced from the hot well ; it 

 follows that 618 units of heat are introduced to the boiler for each pound of water 

 evaporated. But the work produced, for each pound of water evaporated, was 

 found above to be 156,556 foot-pounds. Hence, J^, or 253 foot-pounds is the 

 amount of work produced for each unit of heat transmitted through the Fowey Con- 

 sols engine. Now, in Table II., we find"583'0 as the theoretical effect due to a unit 

 descending from 140' to 0", and 143 as the effect due to a unit descending from 

 30" to 0°. The difference of these numbers, or 440, | is the number of foot-pounds 

 of work that a perfect engine with its boiler at 140', and its condenser at 30" 

 would produce for each unit of heat transmitted. Hence, the Fowey Consols en- 

 gine, during the experiments reported on, performed '^ of its theoretical duty, or 

 57^ per cent. 



(2.) The best duty on record, as performed by an engine at work (not for 

 merely experimental pm-poses), is that of Taylor's engine, at the United mines, 

 which, in 1840, worked regularly, for several months, at the rate of 98,000,000 foot- 

 pounds for each bushel of coals burned. This is ^, or -784 of the experimental 



steam, before leaving the cylinder, to expand until its pressure is the same as that of the vapour in 

 the condenser. AccordiBg to " Watt's law," its temperature would then be the same as (actually a 

 little above, as Regnault has shewn) that of the condenser, and hence the steam-engine worked in 

 this most advantageous way, has in reality the very fault that Watt found in Newcomen's engine. 

 This defect is partially remedied by Hornblowek's system of using a separate expansion cylinder, 

 an arrangement, the advantages of which did not escape Caknot's notice, although they have not been 

 recognised extensively among practical engineei's, vmtil within the last few years. 



* I am indebted to the kindness of Professor Gordon of Glasgow, for the information regard- 

 ing the various cases given in the text. 



■j- In different Cornish engines, the pressure in the boiler is from 2^ to 5 atmospheres ; and, 

 therefore, as we find from Regnault's table of the pressure of saturated steam, the temperature of 

 the water in the boiler must, in all of them, lie between 128° and 152°. For the better class of 

 engines, the average temperature of the water in the boiler may be estimated at 140°, the corre- 

 sponding pressure of steam being 3 J temperatures. 



J This number agrees very closely with the number corresponding to the fall from 100° to 0°, 

 given in Table II. Hence, the fall from 140° to 30° of the scale of the air-thermometer is equiva- 

 lent, with reference to motive power, to the fall fi-om 100° to 0°. 



