THEORY OF THE STEAM ENGINE. CXXXIIlI. 
proportions of the cylinder. It has to be remembered that steam 
engine design is an art as well as a science, and that, as is usually 
the case, the art is always in advance of the science. 
3. A century’s progress in the design of the steam engine, with 
‘its remarkable accumulation of knowledge, experience, and skill, 
has only produced this result, that when we put 100 Ibs. of coal 
into the boiler, from about 90 of them we obtain no return in 
the way of useful work. Poor though this result may appear, 
itis really a remarkably good one, when we remember that at 
the beginning of the century the engines of James Watt were 
probably not giving the equivalent in work of more than IIb. of 
coal for every 100 burnt in the primitive boilers of those days, 
The following distribution of heat for every 100 heat units con- 
tained in the steam supplied to the cylinder may be taken as 
representing an average case of an unjacketed simple engine :— 
Waste by external conduction and radiation 5°5 per cent. 
Waste by internal conduction (cylinder con- 
densation) ... on ie Op 
Waste—thermodynamic dis ies a2 
Waste by friction... re ca ee eee 
Useful work ...  ... stk 10.4 
The wastes, it will be seen, group themselves under three heads: 
A. The thermodynamic waste. 
B. The mechanical waste- 
©. The thermal waste. 
A. With regard to the first of these wastes,—the thermo- 
Aftitinio thers 3 is no difficulty whatever in accurately computing 
its amount. 
If 7, be the absolute temperature at which heat is supplied to 
& heat engine, and 7’, that at which it is rejected, then the maxi- 
Mum efliciency which it is possible to obtain is given by the 
equation 
E= 
ois Ee ‘h 3 
T, : 
Which shews the greatest possible fraction of a given quantity of 
