  We compute the drag force on a sphere settling slowly in a quiescent,
linearly stratified fluid. Stratification can significantly enhance the drag
experienced by the settling particle. The magnitude of this effect depends on
whether fluid-density transport around the settling particle is due to
diffusion, to advection by the disturbance flow caused by the particle, or due
to both. It therefore matters how efficiently the fluid disturbance is
convected away from the particle by fluid-inertial terms. When these terms
dominate, the Oseen drag force must be recovered. We compute by perturbation
theory how the Oseen drag is modified by diffusion and stratification. Our
results are in good agreement with recent direct-numerical simulation studies
of the problem at small Reynolds numbers and large (but not too large) Froude
numbers.
