  To the student of thermodynamics the most difficult subject is entropy. In
this paper we examine the actual, practical application of entropy to two
simple systems, the homogeneous slab with fixed boundary values of the
temperature, and an isolated atmosphere in the presence of the static
gravitational field. The first gives valuable insight into the nature of
entropy that is subsequently applied to the second system.
  It is a basic tenet of thermodynamics that the equilibrium of an extended,
homogeneous and isolated system is characterized by a uniform temperature
distribution and it is a strongly held belief that this remains true in the
presence of gravity. We find that this is consistent with the equations of
extended thermodynamics but that entropy enters in an essential way. The
principle of equivalence takes on a new aspect.
