Hilgardia [VOIJ. 12, No.6 considered as a pressure potential due to the differential pressures on either side of the liquid-gas interface in the menisci of the water films. They further showed it to be directly measurable, over a certain range of potential, by measuring the negative hydrostatic pressures within the water films of soil moisture. The instrument used for these direct measurements was called a capillary potentiometer, but is now called a tensiometer (19), and consists of a porous absorbing element, an adaptation of the Livingston Auto Irrigator, to which a manometer is attached. When the capillary potentiometer was filled with water and the porous absorbing element was embedded in the moist soil whose capillary potential was to be measured, water transfer took place between the porous element and the soil until, at equilibrium, the pressure of the water inside the potentiometer was equal to the pressure in the soil-moisture films. This pressure was read directly on the manometer. In the application of the dynamic concept to soil moisture studies, the velocity of flow of water through the soil is considered to be proportional to the total water-moving force. A conductivity factor, variously called capillary conductivity, capillary transmission constant, conductivity, and permeability, has been used to express this proportionality (3, 7, 8, 10, 16). The term "permeability" is adopted in this paper. Many data on the permeability of soils in saturated flow or, with the pore spaces entirely filled with water, are available in papers of the U. S. Geological Survey, the American Geophysical Union, and in engineering papers. Slichter (24) made theoretical calculations for the flow of underground water under pressure, in which it was assumed that the velocity of flow was proportional to the pressure gradient. There are relatively few published data on soil permeability in unsaturated flow, however. Such data as have been reported were derived from experimental results on relatively small quantities of soil through which flow was induced by artificially maintaining differential pressures in the moisture films on either side of the sample. Richards (17,18,20) has published data on three soils, including capillary potential as a function of water content, permeability as a function of water content and capillary potential, and permeability of a peat soil as a function of capillary potential. The evaluation of the movement of water through unsaturated soil is important in many practical problems, such as: the drainage of land, of road subgrades, and of all structural foundations and pavements laid on the ground; the contribution of a water table to the water supply of plants; the loss of water from a soil surface by evaporation; and the upward translocation and concentration of soluble salts in the soil.