Contributions from Bridgman's high-pressure laboratory at Harvard are always of interest. In the present article he discusses flow, and the shearing stresses necessary to produce flow under high pressure for a wide variety of substances. Rigidity, or the resistance to shearing stress, is one of the fundamental constants of the seismologist, and Bridgman here deals with shearing stresses under conditions of great pressure comparable to those under which seismic data are obtained. The experiments were made at room temperatures, but at pressures as high as 50,000kg./cm.,2 corresponding to those prevailing at depths of 166 km. Few rocks now available for examination at the surface of the earth are likely to have been buried so deep.