174 Prof. Barnard on the comparative Expenditure of Heat 
the resistance of the supply cylinders. But to secure the same 
power from the same mass of air between the same limits of tem- 
perature, on this principle, we must employ a degree of expan- 
sion which will produce precisely the same negative pressure at 
the close of the stroke, which the compression cylinders create. 
If we work without negative pressure, we do only what can be 
done in the other form of engine by lengthening the cut-off. 
And if, in this one, we use air previously condensed, as Stirling 
did, we do only what Ericsson is doing now. Moreover, if we 
allow no negative pressure, the range of temperature through 
which we work, must be very limited. 
It is true that the pressure, per square inch of piston surface, 
will, other things being equal, be in favor of the engine without 
compression cylinder. But it is a fallacious conclusion to infer 
that therefore the effective power of the engine will be increased. 
If we put a to represent the area of the piston, then the lengthof 
Fy 

the stroke will be ; whereas in the other form the stroke 
/ 
is ~, and this larger motion is a full compensation, other things 
being equal, for the less mean pressure. On these accounts, and 
on the large scale at least, apparently insurmountable one. — 
It is perhaps worth considering, whether a more serviceable 
engine than has yet been invented, might not be made by = 
a to a certain extent, the two principles. If two supply 
