(i) A law describing the rate of flow of water through saturated porous media. (Named for Henry Darcy of Paris, who formulated it in 1856 from extensive work on the flow of water through sand philtre beds. ) As formulated by Darcy, the law is Q = KS(H + e)/e where Q is the volume of water passed in unit time, S is the area of the bed, e is the thickness of the bed, H is the depth of water on top of the bed, and "K is a coefficient dependent on the nature of the sand," and for cases "when the pressure under the philtre is equal to the weight of the atmosphere. " (ii) generalisation for three dimensions: The rate of viscous flow of water in isotropic porous media is proportional to, and in the direction of, the hydraulic gradient. (iii) generalisation for other fluids: The rate of viscous flow of homogenous fluids through isotropic porous media is proportional to, and in the direction of, the driving force.