Definition:Gallon (Imperial)/Historical Note

Historical Note on Imperial Gallon
Prior to $1824$, there existed a number of different gallons throughout the British Empire.

The imperial gallon was defined in $1824$ as a volume close to the ale gallon.

Its original definition was the volume of $10$ pounds (approximately $4.5$ kilograms) of distilled water, weighed in air with brass weights with the barometer standing at $30$ inches of mercury ($102$ kilopascals) at a temperature of $62$ degrees Fahrenheit ($17$ degrees Celsius).

In $1963$, the imperial gallon was redefined as the volume of $10$ pounds of distilled water of density $0 \cdotp 998859$ grams per millilitre weighed in air of density $0 \cdotp 001217$ grams per millilitre against weights of density $8 \cdotp 136$ grams per millilitre.

This works out to be $4.546096$ litres or $277.4198$ cubic inches.

The imperial gallon was redefined once again in $1985$ to be exactly $4 \cdotp 54609$ litres by the Weights and Measures Act.