These tests were conducted at the Watertown Arsenal by the Army Ordnance Department in a series of tests on thick-walled cylinders. To develop the required hydraulic pressures an 18:1 intensifier was used in a series with a high pressure water pump. A Crosby test gage was used on the low side of the intensifier and readings were corrected from calibration of the intensifier. Pressure was also checked by the electrical resistance of a manganin coil in the high pressure line. Elastic material: The relation of combined stress in elastic material is in accordance with the strain theory as proposed by St. Venant. Plastic material: 1. The plastic (or converted) strain theory with m = 2 is the correct combined stress relation for plastic materials. 2. The true stress-strain diagram or true stress-true elongation (reduction of area) diagram should be used. Residual stresses: Residual stresses are caused by the differences of the permanent stretch given a fiber and the residual strain remaining in this fiber after removing the load. Failures of cylinders: The properties of the material and dimensions of the cylinder determine the radius where failure begins. / M.S.