  We provide a simple and clear verification of the physical need for
temperature gradients in equilibrium states when gravitational fields are
present. Our argument will be built in a completely kinematic manner, in terms
of the gravitational red-shift/blue-shift of light, together with a
relativistic extension of Maxwell's two column argument. We conclude by showing
that it is the universality of the gravitational interaction (the uniqueness of
free-fall) that ultimately permits Tolman's equilibrium temperature gradients
without any violation of the laws of thermodynamics.
