"it was Mrs Grose who first brought up the plumb"
A hang plumb
Creative Commons Attribution Share AlikeA hang plumb - Credit: Dayna Bateman

A plumb line, which consists of a line with a weight on one end, is used by mariners to determine the depth of water.

This metaphorical "depth sounding" forms part of a broad tapestry of nautical imagery: the governess describes confronting Miles as “reaching port”, whilst exercising emotional self-control is “clutching the helm”. Indeed, her early impression of Bly is as “a great drifting ship” of which she is “at the helm”. The governess uses this line of imagery to convince us – and herself – that she is steering events, but the notion of a ship drifting through unknown depths hints that the water symbolizes her own fraught psyche, an element which she is unable to control or chart.