A more thorough explanation:

GRAINAGE

Grainage is a historical duty that was imposed on aliens who imported salt into London. The duty was equal to one-twentieth of the amount of salt imported.

For example, if an alien imported 100 pounds of salt into London, they would have to pay a grainage duty of 5 pounds.

Another example would be if an alien imported 500 pounds of salt, they would have to pay a grainage duty of 25 pounds.

These examples illustrate how grainage was calculated based on the amount of salt imported by an alien into London. The duty was imposed to generate revenue for the government and to protect local salt producers from foreign competition.