An apparatus is described for investigating the behavior of soil‐water and clay‐water pastes while flowing through narrow glass tubes. The nature of the curves relating rate of flow to shearing stress is discussed. It is found necessary to introduce a new constant into the flow equation, without which various serious errors occur in estimating the viscosity of very thin pastes, and the mobility of thicker ones.While the author is at present unable to present any complete equation or series of equations to relate rate‐of‐flow to shearing stress independent of capillary dimensions for the whole range of stress investigated, yet certain constants of the material can be obtained having independent dimensions which are closely related to the properties of the soil as measured in the field and in the laboratory.