The coefficients of the Darcy and Forchheimer terms are presented and analysed as functions of the porosity. Before this, we briefly review the development of theories for flow in a saturated porous medium. Thermal convection problems in a porous medium are also revisited with a view to bringing out the structure of the dependence on the porosity as a physical effect determining when convective overturning will occur. Finally, it is proved that the solution to flow in a Forchheimer porous material depends continuously on changes in the porosity.