What does "steelyard" mean?

Steelyard: A form of balance in which the body to be weighed is suspended from the shorter arm of a lever, which turns on a fulcrum, and a counterpoise is caused to slide upon the longer arm to produce equilibrium, its place upon this arm (which is notched or graduated) indicating the weight; a Roman balance; -- very commonly used also in the plural form, steelyards.