 Although they viewed additivity as the hallmark of measurement, most early measurement theorists acknowledged that additivity is not necessary for measuring. Other magnitudes exist that admit of ordering from smaller to greater, but whose ratios and/or differences cannot currently be determined except through their relations to other, fundamentally measurable magnitudes. Examples are temperature, which may be measured by determining the volume of a mercury column, and density, which may be measured as the ratio of mass and volume. Such indirect determination came to be called “derived” measurement and the relevant magnitudes “derived magnitudes” (Campbell 1920: 275–7).