  In our derivation of the second law of thermodynamics from the relation of
adiabatic accessibility of equilibrium states we stressed the importance of
being able to scale a system's size without changing its intrinsic properties.
This leaves open the question of defining the entropy of macroscopic, but
unscalable systems, such as gravitating bodies or systems where surface effects
are important. We show here how the problem can be overcome, in principle, with
the aid of an `entropy meter'. An entropy meter can also be used to determine
entropy functions for non-equilibrium states and mesoscopic systems.
