  A general stability condition for plasma-vacuum systems with resistive walls
is derived by using the Frieman Rotenberg lagrangian stability formulation
[Rev. Mod. Phys. 32, 898 (1960)]. It is shown that the Lyapunov stability limit
for external modes does not depend upon the gyroscopic term but upon the sign
of the perturbed potential energy only. In the absence of dissipation in the
plasma such as viscosity, it is expected that the flow cannot stabilize the
system.
