  We address a problem of Casimir apparatus in dense medium and weak
gravitational field. The falling of the apparatus has to be governed by the
equivalence principle, with proper account for contributions to the weight of
the apparatus from its material part and from distorted quantum fields. We
discuss general expression for the corresponding force in metric with
cylindrical symmetry. By way of example we compute explicit expression for
Archimedes force, acting on the Casimir apparatus of finite size, immersed into
thermal bath of free scalar field. It is shown that besides universal term,
proportional to the volume of the apparatus, there are non-universal quantum
corrections, depending on the boundary conditions.
