Spring balance with inscribed dial, by the Dairy Supply Supply Company Limited, Park Royal, London, NW10, England, c. 1930.

Whereas the Indian measures start with a weight which is converted into a capacity for a certain substance, this spring balance converts a capacity into a weight for a certain substance. Because milk is slightly denser than water, the weight of four gallons is 41 pounds, not 40 pounds. The balance would have been used by dairies to measure volumes of milk to regulate sales.