This was the type of chain used by early surveyors to measure distances -- this is where the unit "a chain" comes from. A chain is 66 feet long, 80 chains make a mile. A "rod" or "pole" is 1/4 of a chain, or 16-1/2 feet long. Thus "40 rods" is 10 chains, or 1/8 of a mile.
Modern "chains" are actually steel tapes graduated in chains, and are used by foresters to retrace old survey lines. From the Leonard's Mills Forest and Logging Museum.