OF GASES AND VAPOURS. 215 
generate a force, and the mechanical action which is obtained ac- 
cording to theory from | kilogram. of steam which had formed at 
t° Cent. amounts, when no expansion occurs, but a perfect con- 
densation takes place, to 
374 (33 + 0°1394 , t) = 12300 + 52°1.¢%. . . (5.) 
This power consequently increases very rapidly for engines 
which work with high temperatures. It is measured as before 
by kilogrammes, which are raised 1 metre. 
The coefficient 374, which now occurs, is the mechanical force 
of the unit of heat (No. 8). 
39. An entire condensation never occurs; if it takes place at 
t, degrees, it should be deducted from the above amount of 
power, which would have been yielded by the formation of a vo- 
lume of steam of ¢, degrees equal to that then existing. 
If p be the pressure and ¢ the density of the steam at ¢ degrees, 
and p, the pressure of the steam at ¢, degrees, then the action 
which is obtained by the formation of 1 kilogramme of steam in 
the first case is 2, which, with the value of e in the formula (5.), 
No. 18, again gives the value just found, 12300+52°1.¢. Upon 
the other side of the piston the pressure p, must be forced just as 
far back, which requires the power ee x (12300 + 52:1 2), 
there consequently remains for the duty of the engine, 
{1 = Pu} (12800 52D a. , 6.) 
The relation a might likewise be expressed with the aid of 
the formulz previously given for ¢ and 4. 
For a low pressure engine ¢ is about 107; p = 950, and for 
t, = 50°, p, = 89; with this the theoretical duty for every kilo- 
gramme of water converted into steam becomes equal to 162005™, 
For a one-horse power (75‘™ in 1 sec.) there must consequently 
only be 17 kilogrammes of water converted into steam in one 
hour, and therefore about 23 kilogrammes of coals should be 
consumed ; while usually with such engines from 5 to 6 kilo- 
grammes of coals are required for the hour and horse power, so 
that the theoretical effect here calculated is about twice as great 
as that obtained in reality. 
But the duty which may be obtained according to theory from 
640 units of heat at the maximum, amounts to 640 x 374 
