MR. MACQUORN RANKINE ON THERMO-DYNAMICS. 
171 
Absolute Theoretical Maximum , being the same for every 
perfect Thermo-dynamic Engine working between the 
.. . . 140° — 40 
same limits ot temperature, 
140° + 272|° 
Deductions : — 
For raising the temperature of the feed-water from 40 
to 140° Centigrade 
For stopping the expansive working at 3*07 times the 
initial volume instead of 32 times 
Reduced Efficiency and Effect 
Efficiency. 
Effect per pound of coal 
in foot-pounds. 
0-2424 
1,454,400 
0-0235 
141,000 
465,600 
0-0776 
0-1011 
606,600 
0-1413 
847,800 
The last of these quantities corresponds to a consumption of about 2-34 lbs. of coal 
per indicated horse-power per hour. 
The conditions of the preceding investigation are very nearly fulfilled in steam- 
engines with valves and steam-passages so large, and a velocity of piston so mode- 
rate, that the pressure in the cylinder during the admission of the steam is nearly the 
same with that in the boiler. 
In many steam-engines, however, the steam is more or less “ wire-drawn” ; that is 
to say, it has to rush through the passages with a velocity, to produce which there is 
required a considerable excess of pressure in the boiler above that in the cylinder. 
The power developed during the expansion of the steam from the pressure in the 
boiler to that in the cylinder is not altogether lost; for, as already stated in article 
(45.), it is expended in agitating the particles of the steam, and is ultimately con- 
verted into heat by friction, so that the steam begins its action on the piston in a 
super-heated state ; and both its initial pressure and its expansive action are greater 
than those of steam of saturation of the same density. The numerical relations of 
the temperature, pressure, and density of super-heated steam are not yet known with 
sufficient precision to constitute the groundwork of a system of exact formulae repre- 
senting its action. Some general theorems, however, will be proved in the sequel, 
respecting super-heated vapours, which may be found useful when the necessary ex- 
perimental data have been obtained. 
Calculation and experiment concur to prove, that in Cornish single-acting engines, 
the initial pressure of the steam in the cylinders is very much less than the maximum 
pressure in the boilers ; generally, indeed, less than one-half*. It is doubtful, how- 
ever, whether this arises altogether from wire-drawing in the steam-passages and 
valves ; for when it is considered, that in such engines, even at their greatest speed, 
the steam-valve remains shut during nearly the whole of each stroke, being opened 
during a small portion of the stroke only, it may be regarded as probable, that the 
* See Mr. Pole’s work on the Cornish Engine, and article 36 of a paper on the Mechanical Action of Heat, 
Trans. Roy. Soc. Edinb. vol. xx. 
