'Spirits shall be deemed to be at,proof if the volume of the ethyl alcohol contained therein made up to the volume of the spirits with distilled water has a weight equal to that of twelve-thirteenths of a volume of distilled water equal to the volume of the spirits, the volume of each liquid being computed as at fifty-one degrees Fahrenheit'.
In other words. proof spirit meant that the spirit at a temperature of 51°F. weighed exactly twelve- thirteenths of a volume of distilled water equal to the volume of the spirit. It was, in fact. a mixture of spirit and water of a strength of 57.1% of spirit by volume and 42.9% of water.
© SWA 1995