This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
The standard, legal wine gallon contains 231 cubic inches. All casks and containers are ganged in wine gallons' by United States customs and internal revenue officers. The number of "proof gallons" represented by a given number of wine gallons of spirits varies directly with the amount of alcohol in the spirit. When the hydrometer, testing at 60° F., shows 100°, the spirit is said to be "proof". At this point a wine gallon represents a "proof gallon". If, however, the spirit is stronger, a wine gallon represents more than a "proof gallon;" that is, a wine gallon, if diluted with water until the hydrometer showed 100° at 60° F,, would have been increased in volume to more than a wine gallon. A spirit testing 130 contains 30 per cent, more alcohol than proof spirit; hence one gallon would produce 1.30 gallons of proof spirit if diluted.
In the like manner a spirit testing below, or "under proof," contains less "proof gallons" than it measures in wine gallons. Example: Spirit testing 80° is "20 under proof". Each wine gallon represents 80 per cent. (0.80) " proof gallon". From these principles is deduced the following rule for converting wine gallons to *' proof gallons:"
Multiply the number of wine gallons by the proof indicated by the hydrometer, and point off two decimal places; the result indicates the number of "proof gallons".