Meaning of DUTY in English
( n. )
The efficiency of an engine, especially a steam pumping engine, as measured by work done by a certain quantity of fuel; usually, the number of pounds of water lifted one foot by one bushel of coal (94 lbs. old standard), or by 1 cwt. (112 lbs., England, or 100 lbs., United States).
Webster's Revised Unabridged English Dictionary. Пересмотренный полный словарь английского языка Webster. 2012