plumb-line

A device comprised of a piece of string weighted down by a plumb-bob (a heavy object, often made of lead, and attached to the bottom of the line) used for measuring the depth of water, or the verticality of an upright surface.

// The path runs straight as a **plumb-line**, worn smooth by feet and baked brick-hard by July, between the green rows of laidby cotton, to the cottonhouse in the center of the field, where it turns and circles the cottonhouse at four soft right angles and goes on across the field again, worn so by feet in fading precision. //

--AILD (3)