Original Purpose The writer's first conception of an instrument for quickly measuring the resilience of rubber samples arose in connection with the basic idea for and development of the present A.S.T.M. hardness tester, in an endeavor to provide a reproducible means of measuring and expressing those properties of rubber compounds superficially apparent to the average user. It seemed that if the resistance to pressure (indenting or flexing) and the pressure, or force, of recovery, of a sample were duplicated, the article produced would “feel” the same as the sample, and under normal conditions of use would act mechanically the same. Nature of Problem This problem of measurement, as is generally known, is complicated by the time-hysteresis characteristic of vulcanized rubber compounds, arising through a dual nature, and causing them to manifest both elastic and plastic properties. Any instrument which applies load momentarily measures the almost purely elastic properties, while one which applies a more or less sustained load measures elastic properties modified by the material's partially plastic nature. These ideas are expressed from commercial and industrial viewpoints without regard to their possible deviations from the strict definitions accepted in physical science.