THROUGHOUT MUCH 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).
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