The kilogram and three other metric units are about to be revamped

THROUGHOUT MUCHAD of human history, man has been the measure of many, if not all, things. Lengths were divided up into feet, palms, spans and smaller units derived from the human hand. Other measures were equally idiosyncratic. Mediterranean traders for centuries used the weight of grains of wheat or barley to define their units of mass. The Roman libra, forerunner of the pound, was 1,728 siliqua (carats), each the weight of a carob seed (possibly because they were thought, erroneously, to be less variable in mass than the seeds of other species).The sizes of similarly named units could also differ. The (king’s foot), used in France for nearly 1,000 years after its introduction by Charlemagne in around 790, was, at 32.5cm, around a centimetre shorter than the Belgic foot, used in England until 1300. The talent was the mass of water required to fill an amphora (approximately 28kg), but Greek, Egyptian and Babylonian versions varied from one another by a few kilos. Nor was there agreement on such things within countries. In France, where there was no unified measurement system at the national level, the situation was particularly dire. The (league), for example, varied from just over 3km in the north to nearly 6km in the south.