  The classical Gill's stability problem for the stationary and parallel
buoyant flow in a vertical porous slab with impermeable and isothermal
boundaries kept at different temperatures is reconsidered in a different
perspective. A three-layer slab is studied instead of a homogeneous slab as in
Gill's problem. The three layers have a symmetric configuration where the two
external layers have a high thermal conductivity, while the core layer has a
much lower conductivity. A simplified model is set up where the thermal
conductivity ratio between the external layers and the internal core is assumed
as infinite. It is shown that a flow instability in the sandwiched porous slab
may arise with a sufficiently large Rayleigh number. It is also demonstrated
that this instability coincides with that predicted in a previous analysis for
a homogeneous porous layer with permeable boundaries, by considering the
limiting case where the permeability of the external layers is much larger than
that of the core layer.
