Tests of permeability with low hydraulic gradients

The rate of flow of water through any given permeable material containing only small openings varies directly as the hydraulic gradient. This is essentially the law announced by Darcy in 1856 as a result of his experiments with the flow of water through sand. The principle as applied to flow through capillary tubes was, however, announced by Poiseuille In 1843. Since 1856 many careful investigations have been made of the flow of water and other fluids through permeable materials, and the results of these investigations have thoroughly demonstrated the essential accuracy of Darcy's law for a wide range of hydraulic gradients. In the tests that have been made in the Hydrologic Laboratory of the United States Geological Survey on about 1000 water-bearing materials it has been customary to determine the permeability of each sample under several heads, and thus even the routine work in our Laboratory furnishes a great amount of evidence supporting Darcy's law. [For a brief historical statement of this subject, with references, see U.S. Geol. Sur. Water-Supply Paper 638, pp. 131–134.]