When a metal is pulled beyond its yield point, the total extension is partly a plastic permanent set and partly elastic in the sense that some of the strain disappears on the removal of the stress. But this elasticity differs from the primitive elasticity of the material in that the stress plotted against the strain during the unloading period gives a curved line instead of a straight one. On immediately reloading the material, the stress-strain line is again curved, but in the opposite direction; so that, during the unloading and reloading, the stress-strain curve forms a loop of what may be termed mechanical hysteresis. Bauschinger was the first to observe that the limit of proportional elasticity could be lowered by overstraining the metal; and that, by resting the test-piece for a period of 50 hours, the elastic limit was restored to a value sometimes above its initial or primitive value.