The onset of convection in an anisotropic porous layer inclined at a smallangle from the horizontal

We consider the onset of convection in a porous layer heated from below. Th e layer is anisotropic with respect to both its permeability and diffusivit y and is inclined at a small angle to the horizontal. The aim of this work is to determine analytically by how much the critical Rayleigh number varie s when the layer first ceases to be horizontal. For nearly isotropic layers we show that rolls may be either longitudinal or transverse depending on t he nondimensional parameters; this is unlike the case of O(1) inclinations the results of which are presented in a companion paper. (C) 2001 Elsevier Science Ltd.