  The Kelvin force in a layer of magnetic fluid subjected to a homogeneous
magnetic field and local heating is studied. The study is motivated by the
question about the corresponding Kelvin force density [M. Liu, Phys. Rev.
Lett., 2762 (2000)]. It is shown that the usual and the newly proposed
formulation of the Kelvin force are entirely equivalent. It is only when
approximations are introduced that differences arise.
