In the ordinary testing of mild steel the yield point is indicated by the sudden extension of the specimen and the accompanying drop of the beam of the testing machine which occur at this point. This fact would suggest that some reduction of the stress in the specimen under test takes place, and autographic records of tests furnish evidence of the existence of a region immediately after yield in which the relation of stress to strain is of a complicated nature, and in which the intensity of the stress is less than that which caused the yield. It has been shown by Sir Alfred Ewing and Dr. Rosenhain that the yield phenomenon is due to the formation of planes of cleavage in the crystals, along which sliding takes place. The fact that a greater stress is required to initiate the sliding movement than to maintain it has a direct bearing upon the stress distribution on surfaces subjected to non-uniform stresses when yield in any part takes place, and the object of the experiments here described was to determine the minimum stress in mild steel during the transition from the elastic to the plastic state. In the usual methods of obtaining autographic records the effect is considerably obscured by the inertia of the loading appliances, and the impossibility of limiting the extension to the degree required, so that the relation of the recorded load to the strain in the region under consideration is, in these circumstances, to a great extent fictitious. Sir Alexander B. W. Kennedy, some 27 years ago, described a method whereby inertia effects were eliminated, and obtained, as a result, a reduction of stress immediately after yield of about 16 per cent. Since the experiments here described by the present writers were carried out, Prof. W. E. Dalby has published an account of an optical autographic recorder, in which the measurement of the load is effected in a manner similar to that adopted by Kennedy, and in tests carried out on mild steel has observed a reduction of stress of about 13 per cent.