CARNOT’S THEORY OF THE MOTIVE POWER OF HEAT. O71 
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 Fomwey 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} lbs. 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 34 atmo- 
spheres ;+ 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, “*, or 253 foot-pounds is the 
amount of work produced for each unit of heat transmitted through the Fowey Con- 
solsengine. 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 2 of its theoretical duty, or 
574 per cent. 
(2.) The best duty on record, as performed by an engine at work (not for 
merely experimental purposes), 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. According to “ Wart’s law,” its temperature would then be the same as (actually a 
little above, as Recnautt has shewn) that of the condenser, and hence the steam-engine worked in 
this most advantageous way, has in reality the very fault that Warr found in Newcomen’s engine. 
This defect is partially remedied by Hornsiower’s system of using a separate expansion cylinder, 
an arrangement, the advantages of which did not escape Carnot’s notice, although they have not been 
recognised extensively among practical engineers, until within the last few years. 
* I am indebted to the kindness of Professor Gorpon of Glasgow, for the information regard- 
ing the various cases given in the text. 
+ In different Cornish engines, the pressure in the boiler is from 23 to 5 atmospheres; and, 
therefore, as we find from Reanautr’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 34 temperatures. 
{ 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 from 100° to 0°. 
