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YOU can check the alcohol content of your liquor by adding a litre of it to gunpowder. If the powder can still be ignited, then the liquor is at least 100 proof. This is one of more than 300 units of measurement covered in Ian Whitelaw’s whistle-stop tour of the history of measurement, a history that traces the evolution of human understanding. From the very limits of what is measurable, such as the Planck time (10^-43 seconds) to the confusing way of measuring shotgun barrels so that a 20-gauge gun is smaller than a 12-gauge one, this is…