What is "plumming"

Plumming \Plum"ming\, n. [See Plumb.] (Min.) The operation of finding, by means of a mine dial, the place where to sink an air shaft, or to bring an adit to the work, or to find which way the lode inclines.

n. (context mining English) The operation of finding, by means of a mine dial (compass), the place where to sink an air shaft, or to bring an adit to the work, or to find which way the lode inclines. vb. (present participle of plum English)