This article argues that, by appealing to technological factors, one can compare different innovations, even different economies, over time. An application is made to the development of steam engines and turbines over their history from 1700 to 2000, for which it is shown that four types of physical variables–temperatures, pressures, thermal efficiencies and power ratings–provide common measures of succeeding devices. Such measures can be incorporated in a technological analogue of an input-output system. In principle, an entire economy could be represented in technological form; and, since scientific variables are invariant through time, its evolution could be depicted in quantitative terms.