# Theory of Heat (Maxwell, 4th edition)/Chapter 8

CHAPTER VIII.
ON HEAT ENGINES.
Hitherto the only use we have made of the indicator diagram is to explain the relation between the volume and the pressure of a substance placed in certain thermal conditions. The condition that the temperature is constant gave us the isothermal lines, and the condition that no communication of heat takes place gave us the adiabatic lines. We have now to consider the application of the same method to the measurements of quantities of heat and quantities of mechanical work.
At p. 102 it was shown that if the pencil of the indicator moves from B to C, this shows that the volume of the substance has increased from O $b$ to O $c$, under a pressure which was originally B $b$ and finally C $c$.
The work done by the pressure of the substance against the piston during this motion is represented by the area B $c$ C $b$, and since the volume increases during the process, it is the substance which does the work on the piston, and not the piston which does the work on the substance.
In heat engines of ordinary construction, such as steam engines and air engines, the form of the path described by the pencil depends on the mechanical arrangements of the engine, such as the opening and shutting of the valves which admit or carry off the steam.
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