  A piston is introduced into a spherical lune Casimir cavity turning it into
two adjacent lunes separated by the (hemispherical) piston. On the basis of
zeta function regularisation, the vacuum energy of the arrangement is finite
for conformal propagation in space-time. For even spheres this energy is
independent of the angle of the lune. For odd dimensions it is shown that for
all Neumann, or all Dirichlet, boundary conditions the piston is attracted or
repelled by the nearest wall if d=3,7,... or if d=1,5,..., respectively. For
hybrid N-D conditions these requirements are switched. If a mass is added,
divergences arise which render the model suspect. The analysis, however, is
relatively straightforward and involves the Barnes zeta function. The extension
to finite temperatures is made and it is shown that for the 3,7,... series of
odd spheres, the repulsion by the walls continues but that, above a certain
temperature, the free energy acquires two minima symmetrically placed about the
mid point.
