Total mass in 1D hydrostatic slab, normalized by the initial mass (M(t)/M(t = 0), y-axis), as function of time (x-axis, in τ_s), integrated with the standard scheme, using either historical (dashed lines) or free-float (solid lines) boundary conditions and different meshes (colors; number of grid cells). The well-balanced scheme maintains M(t)/M(t = 0) = 1 to machine precision in all cases.