  An alternative, scalar theory of gravitation has been proposed, based on a
mechanism/interpretation of gravity as being a pressure force: Archimedes'
thrust. In it, the gravitational field affects the physical standards of space
and time, but motion is governed by an extension of the relativistic form of
Newton's second law. This implies Einstein's geodesic motion for free particles
only in a constant gravitational field. In this work, equations governing the
dynamics of a continuous medium subjected to gravitational and
non-gravitational forces are derived. Then, the case where the
non-gravitational force is the Lorentz force is investigated. The gravitational
modification of Maxwell's equations is obtained under the requirement that a
charged continuous medium, subjected to the Lorentz force, obeys the equation
derived for continuum dynamics under external forces. These Maxwell equations
are shown to be consistent with the dynamics of a "free" photon, and thus with
the geometrical optics of this theory. However, these equations do not imply
local charge conservation, except for a constant gravitational field.
  Keywords: Alternative theories of gravitation; preferred reference frame;
curved spacetime; Maxwell equations; charge conservation
