  Consider hard balls in a bounded rotating drum. If there is no gravitation
then there is no Fermi acceleration, i.e., the energy of the balls remains
bounded forever. If there is gravitation, Fermi acceleration may arise. A
number of explicit formulas for the system without gravitation are given. Some
of these are based on an explicit realization, which we derive, of the
well-known microcanonical ensemble measure.
