The plasticity of metals has been the subject of many recent papers but, owing to the complexity of the subject, there is but little agreement between different researches. Attempts to extract simple generalisations from the very complex phenomena have been made chiefly in two directions (1) Engineers have used test bars of certain specially simple form, such as uniform round bars which they have subjected to twisting or tension, and they have found the effect on their test of varying physical conditions. (2) Mathematicians have assumed an ideal plastic material and have given it properties which may or may not be possessed by some real material. They have then analysed the distributions of stress and strain when this ideal material is subjected to given external forces or distortions.